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Continuous seismic reflection profiling in the Strait of Georgia, British Columbia Tiffin, Donald Lloyd 1969

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CONTINUOUS SEISMIC REFLECTION PROFILING IN THE STRAIT OF GEORGIA, BRITISH COLUMBIA b y DONALD LLOYD TIFFIN B.A.Sc., University of B r i t i s h Columbia, 1965 A THESIS SUBMITTED IN PARTIAL 'FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Geophysics In s t i t u t e of Oceanography We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1969 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depa r t m e n t The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada Date _J_ i i ABSTRACT Approximately 790 kilometers of continuous seismic r e f l e c t i o n data were obtained with a 5000 joule Sparker i n the S t r a i t of Georgia, southwestern B r i t i s h Columbia. The S t r a i t i s a geological boundary between Upper Cretaceous Nanaimo Group rocks of the Vancouver Island area and Late Cretaceous-Early Tertiary continental rocks found i n scattered outcrops on the southern mainland. Coast Intrusives form mountains on the mainland northeast of the S t r a i t . The Fraser River has b u i l t a large submarine delta across the S t r a i t and i s the main source of Recent sediments. Deposition i s occurring mainly on the delta front and i n deep basins to the northwest. In.the basin adjacent to the delta, f l a t - l y i n g bottomset beds average about 200 meters i n thick-ness. An older layer of bottomset beds i n ' t h i s basin overlies bedrock and extends under the present foreset beds. Thinner sedimentary layers of possible hemipelagic o r i g i n o verlie Pleistocene banks and ridges along the mainland north of the delta. No s i g n i f i c a n t amounts of Recent sediment are presently accumulating i n the S t r a i t south of the delta. Erosion of possible Late Pleistocene d e l t a i c sediments has deepened the S t r a i t i n that area. Pleistocene deposits of probable d r i f t , t i l l and i n t e r -g l a c i a l sediments occur mainly along the northeast side of the i i i S t r a i t . One extensive s t r a t i f i e d deposit, possibly correlated with exposed Pleistocene deposits on nearby shorelines, may reach 550 meters i n thickness. Below the Pleistocene, s t r a t i -f i e d r e f l e c t o r s , suspected to be Late Cretaceous-Early Te r t i a r y bedrock, unconformably overlie Coast Intrusive bedrock along the mainland shore. The r e f l e c t o r s dip seaward at 8 degrees or more. Along the southwest Island coast Upper. Cretaceous bedrock dips into the S t r a i t . Deformation, most severe i n the south, decreases northward. Dips of bedrock r e f l e c t o r s become less i n mid-Strait before disappearing under delta deposits toward the mainland. Some s y n c l i n a l and a n t i c l i n a l f o l d i n g occurs near mid-Strait. i v TABLE OF CONTENTS Ab s t r a c t Table of Contents L i s t of Figures Acknowle dgement s CHAPTER Page I . INTRODUCTION 1 Purpose and Extent of the I n v e s t i g a t i o n 1 E a r l y H i s t o r y 4 Regional Geology 7 Previous Work i n the S t r a i t of Georgia 12 Previous G e o l o g i c a l Studies 12 Previous Geophysical Studies •' 15 I I . CONTINUOUS SEISMIC PROFILING TECHNIQUE 17 Advantages of CSP Methods Compared w i t h High E x p l o s i v e Methods 17 D e s c r i p t i o n of the Continuous Seismic P r o f i l i n g Equipment 21 Shipboard I n s t a l l a t i o n 25 N a v i g a t i o n 27 The Continuous Seismic Record 27 Paper Type 28 Timing Marks 29 S i g n a l Processing 30 R e s o l u t i o n of R e f l e c t i n g Horizons 32 Reverberations, .Multiples and In t e r f e r e n c e 35 E f f e c t s of Topographical Roughness and P o i n t R e f l e c t o r s . 37 Apparent and True Slopes 39 Nature of Seismic R e f l e c t o r s ^0 V CHAPTER Page I I I . MORPHOLOGICAL SUBDIVISION OF THE STRAIT OF GEORGIA STUDY AREA 45 Terminology of Shelf- and Slope-Like Features' 46 Subdivisions of the Study Area 48 IV. SEISMIC DATA 51 The Fraser Delta Area 55 The Northwestern Basins 66 Ballenas Basin 67 Malaspina Basin 70 Sediment Thickness and Rate of Deposition 72 Turbidites and Slumps i n the Basin Sediments Jh Compaction of Unconsolidated Sediments 78 Elevated Area of Ridges 79 Unconsolidated Sediments 82 Pleistocene Sediments 85 Bedrock 92 Roberts Swell and the Nearby Mainland Shelf 95 The Roberts Swell Unit - 96 Thickness of the Roberts Swell Unit 103 Age and Source 104 Pleistocene Sediments 106 Boundary Basin and Alden Ridge 113 Sediments- ' 114 Bedrock Under Boundary Basin 118 The Island Slope 121 ! Unconsolidated Sediments 122 , Bedrock 124 CHAPTER Page V. SPECIAL INVESTIGATIONS AND OBSERVATIONS 131 Jones Deep 131 Trineomali Trough , 1 3 6 Anomolous H i l l s of the Fraser Delta 139 Sea Level Changes i n the S t r a i t of Georgia 142 Faulting and Tectonic Movements 145 VI. CONCLUSIONS 150 Use of the Continuous Seismic Reflection P r o f i l e r In B r i t i s h Columbian Coastal Waters 150 Summary of Geology and Structure under the S t r a i t of Georgia • 152 Bedrock Structures 152 Pleistocene Geology 156 Recent Sediments 159 REFERENCES . 162 APPENDIX 170 v i i LIST OF FIGURES FIGURE Page 1 . Location of the S t r a i t of Georgia i n Southwestern B r i t i s h Columbia 5 2. D i s t r i b u t i o n of Bedrock Geology i n the Region of the Southern S t r a i t of Georgia 9 3. Block Diagram of Continuous Seismic P r o f i l e r 22 4 . Signal Generating Equipment I n s t a l l e d Aboard CNAV Saint Anthony 24 5. Bathymetry of the S t r a i t of Georgia Study Area back pocket • 6 . A Continuous Seismic P r o f i l e Extending Northeast from Nearshore Gabriola Island 129 7. Echograms over Jones Deep 133 8. Continuous Seismic P r o f i l e over Jones Deep 134 9. Fault C r i t e r i a from CSP Records 147 10. D i s t r i b u t i o n of Bedrock and Pleistocene Geology on the Sea Floor and Below Recent Sediments back pocket 1 1 . Thickness and D i s t r i b u t i o n of Recent Sediments on the Sea Floor back , - 5 * . \>W» * ^ pocket v i i i ACKNOWLEDGEMENTS In a project requiring the use of specialized equipment, ship time, l o g i s t i c a l and technical support, many people are involved. Thanks are due to a l l those who gave so generously their help, advice and encouragement. Of these, Dr. M.J. Keen of Dalhousie University and Dr. Gene Rusnak of United States Geological Survey deserve thanks for lending their Sparker equipment without which th i s study could not have been made. . The interest and assistance of Dr. G.L. Pickard of the I n s t i t u t e of Oceanography and Dr. W.H. Mathews of the Department of Geology i s warmly appreciated. Dr. R.B. E l l i s , Department of Geophysics, suffered several days at sea to help gather data and come to terms with the marine environment. His encouragement and f i n a n c i a l assistance to begin t h i s program helped to make i t po s s i b l e . S h e l l Canada Limited kindly donated seismic data i n the study area. British-American O i l Company also gave technical support. I wish to thank most wholeheartedly Dr. J.W. Murray, Department of Geology, for his assistance, both technical and f i n a n c i a l , i n a l l phases of the work. Dr. R.L. Chase made many h e l p f u l suggestions and his interest and encourage-ment i s appreciated. This work was supported by Geological Survey of Canada, Contract 68-IU. CHAPTER I INTRODUCTION I. PURPOSE AND EXTENT OF THE INVESTIGATION With the advent of new tools of geophysics and geology i t has become possible to study i n d e t a i l the sub-sea structure o f f the continents and under the deep oceans. One such t o o l i s the Continuous Seismic P r o f i l e r (abbrevi-ated to CSP) (Ewing and Tirey, 196I;' Hersey, 1963) . This instrument operates on the same p r i n c i p l e as a conventional ship-board echo sounder: intensive sound pulses are generated at a fix e d r e p e t i t i o n rate and echoes are recorded i n the l i s t e n i n g i n t e r v a l between pulses. I t d i f f e r s from echo sounders, however, i n that operating frequencies are lower and sound energy i s much greater i n order to give s i g n i f i -cant penetration into the sediments and rock, under the sea f l o o r . On the P a c i f i c coast of Canada, no investigation into the use and value of the continuous seismic p r o f i l e r was undertaken by a Canadian s c i e n t i f i c i n s t i t u t i o n p r i o r to 1966, although exploration companies had used this instrument i n B r i t i s h Columbia waters for at lea s t three years pre-viously. However, most of their r e s u l t s are c o n f i d e n t i a l . Since the entire area of the B r i t i s h Columbia coast was uninvestigated by this technique, at least i n the name of research, i t was desirable i n a p i l o t project to determine the effectiveness of the instrument and to learn i t s charac-t e r i s t i c s and idiosyncracies i n the channels and f i o r d s as well as i n more open waters of B r i t i s h Columbia. I t was also desirable to combine this with a preliminary investigation of an area of geological importance. Thus the purpose of this investigation has been not only ( 1 ) to use and evaluate the continuous seismic p r o f i l e r on th i s coast, but also ( 2 ) to use i t i n an attempt to solve problems of undersea geology. An additional objective has been.to present the information gained on the use of CSP i n such a way that geologists and non-technical personnel could p r o f i t from i t . The area of the investigation was, i n part, determined by the time at which the equipment was available. I t was possible to obtain a continuous seismic p r o f i l e r only during the winter months. These months on the North P a c i f i c coast are cold, foggy, and often stormy. Good seismic records are not made under these conditions. I t was therefore decided to remain i n the r e l a t i v e l y sheltered waters of the S t r a i t of Georgia between the mainland and Vancouver Island where, i f gales did a r i s e , the work could be continued i n the i n l e t s which run into the S t r a i t . Another important factor i n the choice of lo c a t i o n was the geological interest i n the S t r a i t of Georgia. 3 Several d i f f e r e n t geological provinces border on the S t r a i t and their effects on the geomorphology of the S t r a i t were not known. A variety of geological situations could occur under i t . These could possibly be recorded and resolved by con-tinuous seismic p r o f i l i n g . The Fraser River delta, for instance, extends into the S t r a i t . Seismic p r o f i l e s through this feature could help to understand i t s e f f e c t upon sedi-mentation i n the area. Coast Range intrusives extend under the S t r a i t from the northeast while Upper Cretaceous bedrock continues under the S t r a i t from the southwest. Mesozoic and Paleozoic rocks also occur at various points around the perimeter. Pleistocene uplands are common i n surrounding areas. It i s desirable to know the d i s t r i b u t i o n of these various bedrock and sediment units on the sea f l o o r r e l a t i v e to each other. Studies i n progress i n adjacent i n l e t s might also benefit by seismic p r o f i l e s down those i n l e t s . Available ship time l i m i t e d the area of study to that part of the central and southern S t r a i t of Georgia extending from Ballenas Islands i n the north to Patos Island i n the >• south. This i s an area of some 3770 square kilometers over which topography, r e l i e f and sedimentation vary considerably. Steep-sided and irregular ridges support near sea-level peaks while deep, f l a t - f l o o r e d basins with water depths exceeding 420 meters occur nearby. Near the Fraser River Delta active sedimentation stretches into the S t r a i t , while 4 i n other areas r e l i c t surfaces of 'Pleistocene and pre-Pleistocene rock and sediment are exposed on the sea f l o o r . Figure 1 shows the study area i n r e l a t i o n to the whole of the' S t r a i t of Georgia and southwestern B r i t i s h Columbia. I I . EARLY HISTORY History records the f i r s t European to v i s i t the waters of the present S t r a i t of Georgia as a Spaniard, Alferez Quimper who, i n June of 1790> s a i l e d through the S t r a i t of Juan de Fuca and investigated i t s eastern end. Although he did not proceed far into or beyond the San Juan Islands, he reported an extensive t r a c t of water l y i n g to'the northwest. One year l a t e r another Spaniard, Jose Maria Narvaez, under Lt. Francisco. E l i z a , entered the S t r a i t of Georgia and explored as far north as 50°N latitude.. E l i z a named many of the islands and gave to the S t r a i t the name 'Gran Canal de Nuestra Senora del Rosario l a Marinera'. Fortunately th i s name did not survive. The following year, 1792, a chart of the southern S t r a i t was drawn by the Spanish explorers Galiano and Valdes. At the same time, the English explorer and cartographer Captain George Vancouver entered the S t r a i t i n search of a 'northwest passage' through the North American continent. He also proceeded to chart the area, and i n doing so, met the Spaniards near Point Grey on Burrard Peninsula. Vancouver's chart of the area shows the excellence of the work of this fine seaman. The complete area from the southern l i m i t s of Puget Sound to Smith's Inle t , north of Cape Caution, a lin e a r distance of 670 kilometers, was i n c l u -ded. With a l l i n l e t s and f i o r d s , a coastline of several thousand kilometers was surveyed i n fine d e t a i l i n just four months time. In honour of his king, George I I I , Captain Vancouver gave the name 'Gulf of Georgia' to the extensive region from Queen Charlotte S t r a i t to Puget Sound. From this comes the name 'Gulf Islands' given i n the present day to the Canadian islands off the southeast coast of Vancouver Island. The 'Gulf of Georgia', since Vancouver's time, has been reduced i n extent to the body of water bounded on the south by the San Juan Islands and on the north by Desolation Sound and Discovery Passage. The name on recent charts has also been changed to the S t r a i t of Georgia. Neither Captain Vancouver nor the Spanish explorers discovered the Fraser River although the Spaniards i n p a r t i c u l a r were looking for such a feature, having heard from the Indians that a large r i v e r existed. Vancouver twice'crossed the de l t a front no more than eight kilometers from the r i v e r mouth. Obviously not knowing the physiography of deltas, and i n spite of abundant evidence i n the form of logs and stumps aground on the mud f l a t s , he did not recognize that a r i v e r discharged there. In f a c t , he emphatically denied the p o s s i b i l i t y of the r i v e r to the 7 Spaniards upon their meeting at Point Grey i n 1792, probably not more than six kilometers from the north arm of that r i v e r ' It remained for Simon Fraser i n 1808 to discover the mouth of the r i v e r by following i t from inland. Fraser, however, was beaten back from the r i v e r mouth by h o s t i l e Indians of the Musqueam tribe before he could explore further in the region. I I I . REGIONAL GEOLOGY The S t r a i t of Georgia, or c o l l o q u i a l l y , Georgia S t r a i t , i s a semi-enclosed body of t i d a l marine water separ-ating southern Vancouver Island from the mainland of B r i t i s h Columbia (Figure 1) . The S t r a i t extends 232 kilometers i n a northwest-southeast d i r e c t i o n , from the American San Juan Islands i n the south to Quadra Island i n the north. The width varies from seventeen and one half kilometers between Texada Island and Vancouver Island, to t h i r t y - f i v e kilometers between Point Grey, near Vancouver, and Valdes Island i n the Gulf Islands. The average width i s approximately twenty- . eight kilometers. The S t r a i t of Georgia i s a submerged portion of the Coastal Trough, a regional topographical low extending from Puget Sound i n Washington, USA, to Dixon Entrance, north of the Queen Charlotte Islands (Holland, 1964, p. 3 2 ) . This topographic low, known i n the S t r a i t of Georgia region as the Georgia Depression, i s surrounded on each side by high mountains and underlain by thousands of meters of late Cretaceous, Tertiary and Quaternary sediments. The S t r a i t i t s e l f marks a boundary between exposures of important geological formations of the Georgia Depression. Figure 2 describes the regional geology of the area surrounding the southern S t r a i t of Georgia. Along the southeastern coast of Vancouver Island, including the associated offshore Gulf Islands, a s t r i p of low-lying country forms an unsubmerged part of the Georgia Depression. I t i s l a r g e l y underlain by Upper Cretaceous marine sedimentary rocks of the Nanaimo Group (Clapp, 1912, 1913, 1914; Usher, 1952) . North of the town of Nanaimo an arch of older volcanic and intrusive rocks i s exposed on Vancouver Island which appears to separate the lowland into two basins,.the Nanaimo Basin i n the south and the Comox Basin i n the north .(Figure 2 ) . Across the S t r a i t on the mainland, no outcrops of Nanaimo Group sedimentary rocks occur. Instead, the Coast Range north of Burrard I n l e t forms high mountains which r i s e above a narrow lowland s t r i p along the S t r a i t . Where i t e x i s t s , this lowland, c a l l e d the Georgia Lowland, i s underlain mainly by g r a n i t i c rocks of the Coast Range intrusives (LeRoy, 1908; Bacon, 1957; Mathews, 1958; Phemister, 1945; Armstrong, i960 a; Roddick, 1965) . Older rocks occur sporadically along the mainland LEGEND >7 o E3E RECENT and PLEISTOCENE Alluvium, drift, interglacial sediments, etc-TERTIARY SEDIMENTS and VOLCANICS Includes Chuckanut. Kitsilano. Burrard and Sooke Formations. Metchosin Volcanics and Garibaldi Group. I UPPER CRETACEOUS SEDIMENTS Mainly Nanaimo Group. PRE-UPPER CRETACEOUS SEDIMENTS. METASEDIMENTS. IGNEOUS and VOLCANIC ROCKS. Includes Vancouver Group. Bowen Island Group. Jarvis Group. Leech R'ver Formation. Sicker Group. Gambier Group. San Juan Group. Ma lariat ItVcon/cs. COAST RANGE INTRUSIONS and pest-tectonic plutons of Vancouver Island and elsewhere. Compiled fn>~n. Gedogicc! Survey of Canadai Map 1069A. Victoria-Vancouver Sheet !9S7;x Map 49-19(3. Alberni Area; Geologic Map of British Columbia. 1963. J.E.MuUer. C.S.C.. unpublished maps. 1967. Geologic Map of a part of Southwestern Briiish Columbia and adjacent Washington. From Geoiogica! Society Field Trips, i960. Published by Vancouver Discussion Club. Washington Geological Sur/ey. Map of San Juan Islands FIGURE 2 Distribution of Bedrock Geology in the Region of the Southern Strait of Georgia. British Columbia and Washington. 10 north and northwest of Burrard I n l e t . South of Burrard Inlet, a re-entrant, occupied mainly by the lower Fraser River v a l l e y , i s almost encompassed by mountains. Called the Fraser Lowlands, the re-entrant i s open only to the west where the Fraser River delta i s b u i l d i n g into the S t r a i t of Georgia. Tertiary rocks dip off the mountain front into the 'Whatcom Basin 1 (Newcomb, et a l , 19^9; Hopkins, 1966) which underlies the Fraser Lowland between Vancouver and Bellingham. Out-crops of these rocks occur at several points on the north and south shore of Burrard Inlet and at various locations surround-ing the Fraser Lowlands (Figure 2 ). Under the Tertiary f i l l of the Whatcom Basin, deep wells ( R i c h f i e l d Pure Sunnyside; R i c h f i e l d Pure Pt. Roberts) reveal the presence of thick sections of Cretaceous sedimentary rocks of continental o r i g i n i n d i c a t i n g that the Whatcom Basin may be, i n part, analagous to the Nanaimo Basin. These sediments probably overlay deeper g r a n i t i c basement (White and Savage, 1965) . No Tertiary rocks similar to those found i n the Whatcom Basin occur on the eastern half of Vancouver Island. In f a c t , the known Tertiary sedimentary rocks on Vancouver Island are outcrops of l i m i t e d extent along Juan de Fuca S t r a i t and the western coast of the is l a n d . However, the upper part of the Gabriola Formation of the Nanaimo Group i s not f o s s i l i f e r o u s and could be Early Tertiary i n age (J.E. Muller, Geological Survey of Canada, personal communication). 11 The S t r a i t of Georgia region has undergone several episodes of g l a c i a t i o n during which ice f i l l e d the S t r a i t to a depth of 1 , 5 0 0 meters ( G l a c i a l Map of Canada, 1 9 5 8 ). Sub-sidence of the land r e l a t i v e to the sea due to ice loading was as much as 230 meters or more (Armstrong and Brown, 1 9 5 4 ) . Ice sculptured rock surfaces and l e f t i n i t s path varied deposits of d r i f t and t i l l . At least two g l a c i a l episodes have been noted on Vancouver Island (Fyles, 1963) and at le a s t three major g l a c i a l advances are known from the Vancouver area (Armstrong, 1 9 5 6 , p. 4 ) . A late readvance of Co r d i l l e r a n ice into the Fraser Lowland, the Sumas g l a c i a t i o n , resulted i n a v a l l e y glacier which, although not extending to the present day S t r a i t of Georgia, deposited d r i f t i n the Fraser Lowland (Armstrong, i 9 6 0 b). Between g l a c i a l episodes, non-glacial periods brought erosion and deposition, the effects of which are shown over wide areas about the S t r a i t . Thick i n t e r g l a c i a l sediments are common on several islands i n the north-eastern part of the S t r a i t (Bancroft, 1913; McConnell, 1914) as well as i n the Fraser Lowland (Armstrong and Brown, 1954; Johnston, 1923; Easterbrook, 1963) and on Vancouver Island's east coast, mainly northwest of Nanoose Bay (Fyles, 1 9 6 3 ; Halstead and T r e i c h e l , 1 9 6 6 ). The termin-ology of Armstrong, et a l , (1965) w i l l be used throughout th i s thesis i n naming Late Pleistocene events. 12 The Fraser River, with a mean annual discharge of 2 8 , 5 0 0 cubic meters per second, i s by far the most important r i v e r flowing into Georgia S t r a i t . Other r i v e r s , of lesser importance, mainly discharge.into the heads of long i n l e t s which connect to the S t r a i t and, except for the Fraser, only minor streams enter d i r e c t l y into the S t r a i t . Since Pleistocene time, the Fraser River has b u i l t a large sub-aerial and submarine delta into the S t r a i t of Georgia. The sub-aerial part extends from near the c i t y of New Westminster to the sea, a distance of twenty-four k i l o -meters. T i d a l f l a t s up to eight kilometers wide occur o f f the sub-aerial part. Beyond these, the submarine delta front continues for another f i f t e e n to twenty kilometers under the S t r a i t . I t i s estimated that the present delta has required less than 1 1 , 0 0 0 years but more than 7 , 3 0 0 years to b u i l d to i t s present state (Mathews'and Shepard, 1 9 6 2 ). I t i s now advancing at an average rate of eight and one half meters per year into the S t r a i t at the ninety meter l e v e l (op. c i t . ) . IV. PREVIOUS WORK IN THE STRAIT OF GEORGIA Previous Geological Studies Interest i n the geology of the area about the S t r a i t of Georgia began shortly after the establishment of the f i r s t community at the present s i t e of V i c t o r i a i n 1 8 4 3 . Indians, observing the s e t t l e r s interest i n 'black stones', or coal, pointed out the presence of a small deposit near Beaver Harbour on northern Vancouver Island. This was worked for a short time i n 1849 before being replaced by development of the Nanaimo coal workings i n 1 8 5 2 , also discovered i n i t i a l l y by Indians. Thus the f i r s t geology of the area was done by Indians. In 1 8 5 6 , the discovery of gold along the Fraser River and i t s t r i b u t a r i e s attracted a great i n f l u x of prospectors and miners to the area. When the known gold deposits were depleted many prospectors searched the coastal areas for precious minerals as indicated by mineral claims i n the area dating back to the very early years. The prospect of mineral resources brought the land areas to the early attention of professional geologists. However, i t was not u n t i l 1921 that a marine geological investigation i n the S t r a i t of Georgia was published. In that year, Johnston (1921) reported on sediments of the Fraser Delta including the r i v e r mouth and adjacent S t r a i t . Waldichuk (1954) gave a generalized account of the character of bottom sediments i n the S t r a i t of Georgia using one hundred bottom samples and data from the hydrographic charts. His work showed s u r f i c i a l sediments consist mainly of soft, s i l t y clays and clays, with l o c a l sand and gravel patches implying the presence of non-depositional s i t e s . 14 This information was l a t e r incorporated into a larger volume (Waldichuk, 1957) on the general oceanography of the region. Mathews and Shepard (1962) conducted a hydrographic and bottom sampling program o f f the Fraser River delta between Point Grey and Point Roberts. They showed sandy sediments predominate to the south of the present r i v e r mouth whereas s i l t predominates to the north. The absence of muds and the o r i g i n of the sands were attributed to checking of coarse bottom transported sediments by the fl o o d tide entering the r i v e r . With the south-setting ebb t i d e , the coarser materials would be released and d i s t r i b u t e d to the south. Another explanation, of r e l i c t sands, although an admitted p o s s i b i l i t y , was thought to be less l i k e l y . A topographic ridge to the northwest of the main r i v e r mouth, c a l l e d Fraser Ridge i n this thesis, was suspected to have a Tertiary or Cretaceous core. Sampling indicated soft mud covered the ridge (op. c i t . , p. 1425) although bottom currents are high south of the r i v e r (Pickard, 1956). Their data also showed anamalous h i l l s with a r e l i e f of twenty or more meters occurred on the d e l t a front off the r i v e r mouth. Mayers (1968), u t i l i z i n g some of the CSP data of t h i s study, did further studies on t h i s h i l l topography. Cockbain (1963a) studied the area from Sand Heads to Ballenas Islands using a Precision Depth Recorder (PDR) and a 12 kiloHertz Edo echo sounder. The 900 kilometers of echo 15 sounding track obtained were used to divide the area into a number of physiographic sub-divisions, some of which, are retained i n t h i s thesis. Penetration of the sound waves of up to t h i r t y - f i v e meters into the sediments was obtained in favourable areas. These records exhibited layering i n the unconsolidated sediments under the f l a t f l o o r s of the basins. Turbidity currents were suggested as a possible mechanism of sediment transport to these s i t e s . Cockbain concluded that topographical differences between the mainland and Vancouver Island possibly r e f l e c t the d i f f e r e n t geology of the areas. He placed the boundary between s t r u c t u r a l regions at the boundary between submerged topographical areas. Cockbain (1963b) also described b r i e f l y the general sediment d i s t r i -bution, i n the S t r a i t as a by-product of his research on the foraminifera of the region. Previous Geophysical Studies L i t t l e geophysical work has been done i n the S t r a i t . Milne and White ( i960) published the f i r s t deep c r u s t a l r e f r a c t i o n seismic survey of the region including the S t r a i t near Vancouver Island. Later, White (1962) conducted further r e f r a c t i o n studies along a p r o f i l e down the northwestern side of the S t r a i t of Georgia. Average P-wave v e l o c i t i e s were computed for a two and a three layer model. He also reported on gravity measurements^ along the east coast of Vancouver Island. 16 A single gravity track has been recorded along the axis of Georgia S t r a i t (Delingher, et a l , 1966). Walcott (1967) recorded many additional gravity stations i n the area. A l l previous gravity data have been compiled into a Bouguer anomaly map of western B r i t i s h , Columbia (Stacey, et a l , 1969) . White and Savage (1965) published r e s u l t s of re f r a c -tion work including unpublished work mentioned in the above references along with.additional data. T i f f i n and Murray (1966) gave a preliminary report on the re s u l t s of the present study in d i c a t i n g the s t r u c t u r a l significance of the work. Tseng (1968) used new a n a l y t i c a l techniques (time-term methods) to amplify, improve and enlarge upon the seismic data obtained by Milne and White ( i960) and White (1962) . I t i s shown that i n the southern S t r a i t of Georgia there i s a marked thickening of low v e l o c i t y sediments with average P-wave v e l o c i t y of 4.5 kilometers per second. CHAPTER II CONTINUOUS SEISMIC PROFILING TECHNIQUE I. ADVANTAGES OF CSP METHODS COMPARED WITH HIGH EXPLOSIVE METHODS Continuous seismic r e f l e c t i o n p r o f i l e s , that i s , seismic data records which, when l a i d side by side, make up a composite and continuous record, can be obtained by conven-t i o n a l explosive seismic techniques, but i t i s d i f f i c u l t and dangerous. Large quantities of explosives must be loaded and carried aboard ship. At sea, consecutive explosive charges are required to be detonated i n the water, generally at close inter v a l s of time. This has been done (Ewing and Tirey, 1 9 6 l ; O f f i c e r , 1955; Paitson, et a l , 1964; Savit, et a l , 1964) but i t i s d i f f i c u l t to perform and even more d i f f i c u l t to maintain for long periods of time without undue s t r a i n and hazard to personnel, not to mention the high cost of explosives. These factors severely li m i t e d the use of continuous p r o f i l i n g techniques before the advent of a non-explosive seismic source. Non-explosive continuous seismic p r o f i l i n g equipment was developed i n the late 1950's (McLure, et a l , 1958; Beckman, et a l , 1959; Hersey, et a l , 196I5 Shor, et a l , 1963) . At that time graphic recorders were i n common use with echo sounders and i t was apparent on these records that echo•sound-18 ing could penetrate the sea f l o o r to provide p r o f i l e s of shallow underlying bedrock (Smith, et a l , 1954; Smith, 1958). However, echo sounding equipment was neither s u f f i c i e n t l y powerful nor able to operate at low enough frequencies to be useful i n seismic operations. Consequently, the CSP method had to r e l y upon d i f f e r e n t techniques to create a sound pulse-, p r i n c i p a l l y either e l e c t r i c a l discharge, gas explosion or discharge of a i r under water. It i s possible to generate a constantly repeatable discharge i n the water every few seconds with any of these techniques. The number of operators required i s small com-pared to a conventional seismic crew and very l i t t l e hazard exists to personnel. One person can attend the equipment for hours at a time without fatigue. An additional great advan-tage i s that only one ship i s required and this does not have to be a large one. . The methods of producing acoustical pulses without high explosives also enjoy the advantage of creating no i l l e f f e c t s to f i s h . They may therefore be used where explosives are prohibited by f i s h e r i e s protection laws. In high explosive seismic work, noise and reverber-ation generated i n the sea water by the explosion and bubble pulse may o b l i t e r a t e a r r i v a l s from sub-bottom structure. With the non-explosive p r o f i l i n g equipment, a less energetic sound wave generates much less noise and reverberation and primary echoes from sea bottom and immediate sub-bottom r e f l e c t o r s 19 are usually d i s t i n c t on the record. Because the sound pulse may be generated many times per minute, r e f l e c t o r s of limited areal extent or s t r u c t u r a l features of a small scale can r e f l e c t several sound pulses during the time of a traverse. The chance of detecting and c o r r e c t l y i d e n t i f y i n g such features i s therefore increased many-fold. A survey ship commonly travels at a speed of six knots or more and f i r e s at a r e p e t i t i o n rate of four seconds or l e s s . A sound pulse is•thus emitted about every f o r t y feet of t r a v e l . With such a shot density there i s comparatively l i t t l e chance of any s i g n i f i c a n t shallow geological structure escaping detection. Although each outgoing pulse contains less energy than that produced by high explosives, the greater number of r e f l e c -tions, even though weak, permit a more r e l i a b l e . i n t e r p r e t a t i o n of otherwise questionable features. Even where the signal-to-noise r a t i o i s low, coherence on the record of a large number of 'in-phase r e f l e c t i o n s allows them to be detected against the background of random noise. Thus the greater density of information as well as i t s greater d e t a i l on a CSP record often makes interpretation simpler and more certa i n than on most explosive r e f l e c t i o n seismic records. The main drawback to the CSP method has been i t s low energy output which, with higher frequency content, l i m i t s i t s penetration c a p a b i l i t i e s . This i s presently being remedied i n more expensive equipment. I t i s now possible to 20 shoot CSP surveys using very high power. For instance, 160,000 joule 1Sparker' r e f l e c t i o n seismic equipment has been used for ocean surveys by private exploration companies (World Petroleum, March, 1968). A 2,000 cubic inch a i r gun has been used successfully i n c r u s t a l r e f r a c t i o n studies by s c i e n t i s t s of A t l a n t i c Oceanographic Laboratory, Dartmouth, Nova Scotia, over shot-detector distances exceeding one hundred kilometers. This gun can also be used for continuous r e f l e c t i o n p r o f i l i n g . The energies and spectrums of these sources are equivalent to that of several pounds of explo-sives. I t should not be assumed, however, that CSP i s d i s -placing explosive seismic surveys. .On the contrary, they are complimentary. CSP methods are useful to delineate near surface features i n preliminary surveys. Large areas can be covered i n r e l a t i v e l y short time. Smaller areas can then be i n t e l l i g e n t l y selected for more costly detailed high explosive deep seismic r e f l e c t i o n studies. Even here, CSP i s useful to define the short-range sub-surface often l o s t i n the deeper penetrating lower frequency high explosive work. The higher frequency output of the CSP adds to i t s a b i l i t y to detect this d e t a i l . Of course CSP, as described here, can only be used i n areas covered by water. Continuous seismic p r o f i l i n g equipment does not, by i t s e l f , give information on seismic v e l o c i t i e s . Wide angle r e f l e c t i o n or r e f r a c t i o n p r o f i l e s from which this information i s calculated can be obtained (Le Pichon, et a l , 1 9 6 8 ; Houtz, et a l , 1 9 6 8 ) with additional equipment such as sono-buoys and radio receivers. Thus seismic r e f r a c t i o n shots are s t i l l a useful part of a survey operation. I I . DESCRIPTION OF THE CONTINUOUS SEISMIC PROFILING EQUIPMENT The continuous seismic p r o f i l i n g equipment used i n this study was a 5 , 0 0 0 . joule 'Sparker' manufactured by Edgerton, Germeshausen and Grier, on loan to UBC for the month of January, 1 9 6 6 , by Dr. M. J . Keen, Department of Geology, Dalhousie University, Halifax, Nova Scotia. I t i s c a l l e d 'Sparker' because i t makes use of an e l e c t r i c a l d i s -charge or spark to create, a sound pulse i n the water (Hersey, et a l , 1 9 6 1 ) . The sparker system can be described i n two sections: signal generating equipment, and receiving and recording equipment. A block diagram of the major components of both sections i s shown i n Figure 3. The signal generating equipment consists of power supplies, capacitor banks, a tri g g e r i n g device and a trans-ducer. E l e c t r i c a l energy at 1 1 0 or 2 2 0 volts A-C from the ship's mains or from an a u x i l i a r y generator i s transformed to four thousand volts and r e c t i f i e d i n the power supply units. The high voltage D-C energy i s then stored i n HYDROPHONE ARRAY AMPLIFIER FIL TER GRAPHIC RECORDER TAPE RECORDER* v . TRIGGER RECEIVING and RECORDING EQUIPMENT TRANSDUCER (SOUND SOURCE) ENERGY STORAGE [CAPACITORS) ENERGY SOURCE (GENERATOR) S nUND GENERATING EQUIPMENT FIGURE 3. Block diagram of continuous seismic profiler. capacitors u n t i l the instant of f i r i n g . An a i r gap switch between the capacitor banks and the transducer prevents premature leakage. The l a t t e r i s t r a i l e d i n the water over the stern of the ship. On command from the recorder, a trigger signal i n i t i a t e s the breakdown of the a i r gap by i o n i z i n g a path across i t . Once i n existence, a heavy current arcs across the gap u n t i l the capacitors are d i s -charged to the point where they can no longer support the arc. Since this occurs within milliseconds, peak current flow to the transducer reaches several hundred amperes. The heavy conductor required for the passage of these high cur-rents also serves as a towing cable for the transducer. The transducer i s e s s e n t i a l l y an open-ended cable which permits the current to be s h o r t - c i r c u i t e d through the seawater to a nearby ground return. In order to provide a low impedance s h o r t - c i r c u i t path, the cable divides at the transducer into three ends, each of which are brought to a replaceable spark-tip i n a seven foot l i n e a r array c a l l e d a sparkarray. The large energy pulse through the seawater causes the water around the spark-tips to vapourize with explosive r a p i d i t y , creating an intensive sound wave with e s s e n t i a l l y omni-directional c h a r a c t e r i s t i c s . The signal generating equipment i s shown i n operation i n Figure 4. Since energy E = \ CV 2 where E i s .in joules, C i s capacitance i n farads and V i s voltage, i t i s apparent that FIGURE 4. Signal generating equipment installed aboard CNAV Saint Anthony. Equipment consists of two identical power supply units at upper left and lower right, two capacitor banks at lower left and a trigger unit, top centre. A heavy four conductor cable leads from the trigger unit to the sparkarray• 25 by increasing either voltage or capacitance, or both, the amount of stored energy can be varied up to the l i m i t s set by breakdown of the physical components of the system. The equip-ment used for the present survey maintained voltage at a constant 4,000 volts and discrete energy variations were obtained by adding or removing banks of capacitors. Echoes from bottom and sub-bottom r e f l e c t o r s are received by arrays of hydrophones towed in the water, also astern of the ship. For most of this survey, a short, l i n e a r untapered array of 10 hydrophones, connected i n a series-p a r a l l e l arrangement, was used. Later, additional survey l i n e s were recorded using a 20-elenient 100-foot array giving a better signal-to-noise r a t i o . Signals from the -array, amplified by a GeoSpace Model SA-216 seismic amplifier system, were f i l t e r e d to reduce undesirable noise and recorded on chemically treated paper ori an Alden 419 Precision Graphic Recorder. I I I . SHIPBOARD INSTALLATION Seismic data was obtained i n two cruises. The f i r s t cruise, during which most of the data was collected, took place i n January, 1966 aboard the tug CNAV 'Saint Anthony',, a Canadian Naval A u x i l i a r y deep-sea salvage tug. The continuous seismic p r o f i l i n g equipment was received aboard ship just p r i o r to New Years Day, 1966 and 26 was stowed comfortably i n a small dry cargo hold below the after deck. Transducer and hydrophone cables were led up through a hatch cover and over the stern, one on each side of the ship. The low. after deck was often awash, but the equipment was safe and working conditions comfortable, although getting to and from the temporary lab sometimes presented problems. During this cruise, p r o f i l e s were run i n the S t r a i t of Georgia, Howe Sound, J e r v i s Inlet and Bute I n l e t , the l a t t e r two i n l e t s being to the north of the study area. Winds during the survey ranged from calm to gale force but no time was l o s t due to weather. A second cruise i n June, 1967* made use of a small t h i r t y foot aluminum-hulled launch manned by two people. Sparker equipment for this cruise was supplied by Dr. Gene Rusnak, United States Geological Survey. The equipment, except for recorder and amplifier, was housed i n an open after cockpit and covered with a tarpaulin. A portable 5 kilowatt e l e c t r i c generator supplied operating power. The recording and amplifying equipment was stored i n the small cabin and run from the ship's generator. E l e c t r i c a l noise through the aluminum h u l l presented a problem which was f i n a l l y overcome by reducing the seismic energy output to 1,000 joules. Even at t h i s low output, penetration obtained was almost as good 2 7 as with the f u l l 5 * 0 0 0 joule output (compare Plate VII with Plate V I I I ) . Additional p r o f i l e s were obtained with the use of this launch i n the S t r a i t of Georgia, Howe Sound and, unsuc-c e s s f u l l y , i n P i t t Lake, a t i d a l lake o f f the Fraser River. The l a t t e r was not successful because the natural s a l i n i t y was i n s u f f i c i e n t to support e l e c t r i c a l conduction across the transducer and an a r t i f i c i a l saline environment could not be maintained around i t while being towed through the water. IVo NAVIGATION During the survey positions were obtained by three point radar and p o l a r i s compass bearings by the ship's navi-gating o f f i c e r s . R e l i a b i l i t y , which varies with factors such as distance from target, type of target and crossing angle of the bearings, was weighted at the d i s c r e t i o n of the operator by a system of three numbers, number one being most r e l i a b l e . In most cases, positions are assumed to be correct to within \ kilometer (£ n a u t i c a l mile) or l e s s . Positions i n some areas where good radar targets were lacking, such as off the Fraser Delta, may be. less r e l i a b l e . V. THE CONTINUOUS SEISMIC RECORD The record generated by a p r e c i s i o n graphic recorder as used i n continuous•seismic p r o f i l i n g i s d i s t i n c t l y d i f f e r -28 ent from the type of record usually obtained by other seismic methods. Signal processing i s accomplished during the record-ing so that the f i n a l record i s presented almost at the instant i t i s received. The.procedure i s repetitous, each shot being followed by a l i s t e n i n g i n t e r v a l of one or two seconds then another shot. Since the record paper i s moving out of the recorder, each in d i v i d u a l shot i s printed beside the previous one. The record i s therefore continuously pro-duced as the survey proceeds, and provides up-to-date information on the sub-bottom structure being traversed. In order to appreciate the amount and type of in f o r -mation on the record, i t i s necessary to understand the basic p r i n c i p l e s involved i n the r e f l e c t i o n of sound waves through earth and water, and their c o l l e c t i o n , processing and presentation on a graphic recorder. Paper Type The s p e c i f i c graphic recorder used with the CSP equipment determines whether the record w i l l be produced on wet or dry chemically treated paper. These are not in t e r -changeable on any one recorder. The Alden Precision Graphic Recorder or PGR (Luskin, et a l , 195^; Knott, 1962) used i n this study requires wet paper. Wet paper has more shrinkage and a greater tendency to d i s t o r t than the dry paper type. This can be troublesome as the paper refuses to l i e f l a t . Timing marks printed at the time of recording d i s t o r t with the paper and preserve the i n t e g r i t y of the time domain. Travel times can therefore be accurately read from any part of the record. Wet paper has a wide dynamic range of tone contrast permitting good re s o l u t i o n between strong and weak signals. Marking i s accomplished by e l e c t r i c a l impulses through a wire h e l i x wound on a rot a t i n g cylinder under the record paper. The h e l i x has a p i t c h equal to the width of the record. As the h e l i x revolves, at a speed determined by t h e . v e r t i c a l scale desired, the contact point scans at a constant speed across the paper. Echoes, timing marks or extraneous noise impulses t r a v e l from the h e l i x through the chemically-treated paper to a second electrode on top, leaving a mark- on the paper. The f i r i n g trigger to i n i t i a t e the outgoing sound pulse i s actuated by a mechanical switch at the moment the l e f t hand end of the h e l i x returns to the paper. Thus the output pulse i s synchronized to the zero time mark on the record. Timing Marks . The recorder i s designed to produce a series of c r y s t a l controlled timing marks at set intervals across the record. The f i r s t heavy mark i s manually adjusted to occur a few milliseconds p r i o r to the trigger signal to compensate for towing depths of transducer and hydrophones. This mark then corresponds to the water surface; that i s , zero meters or zero time. In the accompanying plates, l i g h t e r timing marks can be seen at every 50 milliseconds (or 20-fathom Interval at a v e l o c i t y of sound i n sea water of K,800 feet per second). At each 250 milliseconds (or 100-fathom i n t e r -v a l ) , the timing mark i s heavier to f a c i l i t a t e reading the record. As the paper advances out of the recorder, contin-uous timing l i n e s are produced on the record. To provide a time scale i n the horizontal d i r e c t i o n as well as the v e r t i c a l , the timing marks are automatically occulted for a period of approximately t h i r t y seconds every f i v e minutes. These small breaks are not e a s i l y seen i n the record reproductions. Signal Processing Signal processing can take many forms depending on the type of information -required. • Various techniques for multiple reduction and signal enhancement exis t using sophisticated equipment but the simplest and most di r e c t signal processing i s often adequate i n the f i e l d . If the raw information i s tape recorded, sophisticated methods may await a more favourable arrangement i n the laboratory or computer. Since noise i s a problem on ships where e l e c t r i c a l generators and motors as w e l l as propulsion machinery are 3 1 operating, i t i s usual to f i l t e r the incoming signals i n an e f f o r t to increase the signal-to-noise r a t i o . For instance, i f the outgoing pulse does not contain frequencies lower than 80 Hertz (Hz), one can f i l t e r those frequencies and remove low frequency noise without loss of s i g n a l . Much ship noise i s generated at 60 Hz which can be e f f e c t i v e l y removed by t h i s method or by a notch f i l t e r at 60 Hz. S i m i l a r l y , useful high frequency information is l i m i t e d . For frequencies above a few hundred Hertz penetration i s not great i n the sediments. By f i l t e r i n g these high frequencies, noise can be considerably reduced. In practice one must usually trade off some signal for a gain i n the signal-to-noise r a t i o . A one to two octave band-width i s usually found to be s u f f i c i e n t to obtain good information with reasonably low noise. As well as active or passive f i l t e r elements, other components are e f f e c t i v e l y acting as f i l t e r s as w e l l . The hydrophones may respond only over a l i m i t e d range of audio frequencies. The amplifiers may also have a limited pass band. It i s therefore important to 'match' the various components of a system to obtain e f f i c i e n c y while maintaining the desired f i e l d response. The hydrophone array used for most of this survey u t i l i z e d Hall-Sears MP-4 hydrophones with a wide band pass. The seismic amplifier however, a Geospace 111-115, had a pass band from 10 to.300.Hertz (3 db. p o i n t s ) . Because of 32 low frequency noise and the lim i t e d high..frequency range, the f i l t e r s were usually set to pass a band of frequencies between 80 and 300 Hertz. After f i l t e r i n g , the signal i s half-wave r e c t i f i e d before recording. This permits a phase c o r r e l a t i o n on the record, giving a clearer readout of r e f l e c t e d events. I t i s because of r e c t i f i c a t i o n that l i n e s of alternate dark and l i g h t i n t e n s i t y occur on the record. Without r e c t i f i c a t i o n , only v a r i a t i o n i n tone, would s i g n i f y an echo sequence. The half-wave r e c t i f i e d readout i s much superior to the full-wave tone c o r r e l a t i o n . Resolution of Refle c t i n g Horizons Side by side recording of each shot and i t s subse-quent echoes produces a p r o f i l e of the sea bottom and sub-bottom structures. The width of the outgoing sound wave, however, tends to reduce resolution on the cross-section so produced. The discharge of stored energy takes place over a f i n i t e time and the outgoing acoustical pulse i s therefore stretched over a period of many milliseconds. The pulse i s wavelike; that i s , i t consists of a series of compressions and rarefactions. After r e c t i f i c a t i o n i n the recorder, and side by side recording, the r e s u l t i s a series of l i n e s printed on the record corresponding to the compression and 33 r a r e f a c t i o n i n t e r v a l s a r r i v i n g at the hydrophones. The f i r s t s e r i e s of l i n e s o c c u r r i n g at the top on the accompanying record p l a t e s i s due to the wave t r a v e l l i n g d i r e c t l y from transducer to hydrophones, i t s surface r e f l e c t i o n , and the sound waves created'by bubble o s c i l l a t i o n s and t h e i r surface r e f l e c t i o n s . The time and r e l a t i o n s h i p at which these s i g - . n a l s appear depends upon the distance between transducer and hydrophones and t h e i r depth below the water surface. The . complete wave t r a i n appearing f i r s t on the record i s c a l l e d the d i r e c t a r r i v a l . F o l l o w i n g t h i s down the r e c o r d , the sea f l o o r i s represented by a s i m i l a r band of p a r a l l e l l i n e s , f o r the echo i s a close approximation to the output. Sub-bottom r e f l e c t o r s are l i k e w i s e recorded by bands of l i n e s r a t h e r than one d i s -creet and sharp i n t e r f a c e . The f i r s t l i n e of any s e r i e s corresponds to the true two-way t r a v e l time to the r e f l e c t o r . The other l i n e s merely obscure echoes from c l o s e l y spaced r e f l e c t o r s . Thus the r e s o l u t i o n expected from the system i s a f f e c t e d to a large extent by the length of the outgoing pu l s e . This Is apparent i n , f o r example, P l a t e XXIX between p o s i t i o n s D and E, where sub-bottom r e f l e c t i o n s can be followed:toward the sediment-water i n t e r f a c e i n T r i n c o m a l i Trough. The a c t u a l outcrop of the r e f l e c t o r s i s not apparent on the r e c ord because the sea f l o o r r e f l e c t i o n e f f e c t i v e l y 34 obliterates the sub-bottom for several milliseconds below the sediment surface. The pulse length can be influenced to a degree by the towing depth of the transducer and hydrophones. The i n i t i a l outgoing acoustical pulse i s omni-directional, but upward directed energy is r e f l e c t e d from the water surface and re-directed downward, following the d i r e c t pulse i n that d i r e c t i o n at a time determined by the towing depth. The net e f f e c t i s a stretching of the pulse time i f towing depth i s increased. The obvious correction i s to tow the transducer and hydrophones near the surface. However, since penetration of sound i s proportional to the inverse of the frequency (Grant and West, 1 9 6 5 ) , low frequencies, those favoured by deep towing, are desirable for maximum penetration. Ray theory indicates that by towing at a depth of one quarter of the dominant, wavelength desired, interference generated by the d i r e c t and surface r e f l e c t e d waves should be constructive, increasing the outgoing energy at that wavelength. This assumes that the i n i t a l output pulse length i s of s u f f i c i e n t duration for interference to take place. By towing at a depth of twelve feet, for instance, constructive interference should occur for a frequency of 1 0 0 Hertz. The time required for sound waves to t r a v e l to the surface and back to the transducer at that depth i s f i v e milliseconds. The discharge time of stored energy through the transducer i s usually. 35 greater than t h i s , therefore interference w i l l occur at that frequency. However, i f the towing depth i s increased much beyond t h i s , unnecessary pulse stretching may occur. In actual practice, the sea surface i s r a r e l y a smooth r e f l e c t o r so that the depth to the transducer and hydrophones may vary considerably between wave crest and wave trough. For this investigation, towing depth for both hydro-phones and transducer was approximately 3 meters ( 1 0 f e e t ) . The dominant output frequency was approximately 1 2 0 Hertz. The pulse length for 5 , 0 0 0 joules of output energy as measured at the hydrophones for the d i r e c t a r r i v a l , was approximately 4o milliseconds. This i s not compatible with high resolution and the resolution was not expected to be much better than 3 0 meters ( 1 0 0 f e e t ) . However, where the second r e f l e c t o r was p a r t i c u l a r l y strong, i t proved possible to resolve r e f l e c t o r s spaced at about 1 5 meters ( 5 0 feet) or i n some cases l e s s . Reverberations, Multiples and Interference Sound waves are r e f l e c t e d very r e a d i l y from the sea surface due to the large acoustic mismatch between water and a i r . Thus energy r e f l e c t e d upward from the sea bottom i s not only picked up by the hydrophones but i s also r e f l e c t e d back again from the water surface. The r e f l e c t e d energy may be returned again and 'again from the sea fl o o r to be picked up and recorded at each pass. The r e s u l t on the record i s a 36 series of irrelevant but related events which blanket weaker, but desirable, r e f l e c t i o n s from deep sub-bottom horizons. The e f f e c t i s c a l l e d sea bottom reverberation (Backus, 1959) . Each r e f l e c t i o n of the sea bottom on the record i s a multiple of that event. In an area where r e f l e c t i o n c o e f f i c i e n t s are large, many multiples may be present. Three or four bottom multiples are common and up to seven or eight have been observed, although t h i s i s by no means a l i m i t to their number. Reverberation w i l l continue u n t i l the energy i s dispersed or attenuated. Plate XXX i l l u s t r a t e s a record on which three multiples of the sea f l o o r are present. Reverberation i s often stronger than sub-bottom r e f l e c t i o n s and tends to obscure these. Desirable information may therefore be l o s t on the record. In general, information is r e l i a b l e down to at least the f i r s t multiple of the sea bottom. Beyond t h i s , information may be garbled and unrecog-nizable unless from a strong r e f l e c t o r . Much of the ingenuity of signal processing methods devised to date is rela t e d to unscrambling signals from multiples, or repressing the mul-t i p l e s while enhancing meaningful r e f l e c t i o n s . On occasion multiples may occur between pairs of r e f l e c t i n g sub-bottom surfaces, as when a strong r e f l e c t o r l i e s under the sea f l o o r . Internal r e f l e c t i o n s may then take place between sea bottom and sub-bottom r e f l e c t o r . Sub-bottom r e f l e c t e d energy may also become trapped between 37 sea surface and sea f l o o r or between a pair of sub-bottom r e f l e c t o r s . While these events are rarer than a sea fl o o r multiple,.they do occur and one should be aware of the p o s s i b i l i t y of their presence on a record. One of the problems of interpretation i s the i d e n t i f i c a t i o n of true r e f l e c t i o n horizons from those of multiply r e f l e c t e d signals. An example of a multiple r e f l e c t i o n between sea surface and a sub-bottom horizons i s i l l u s t r a t e d i n Plate X, positions E to F. A.sub-bottom horizon at a depth near 0.275 seconds i s repeated at a depth near 0.55 seconds after r e f l e c t i n g o f f i t s e l f as well as off the sea f l o o r . E f f e c t s of Topographic Roughness and Point Reflectors The graphical record i s affected by the geometry and structure of the sea bottom and sub-bottom. Because the transducer output i s omni-directional, echoes w i l l be received from objects on the sea f l o o r ahead or beside the ship. Since the echo from the object has t r a v e l l e d a longer path than the echo from the sea bottom under the ship, i t w i l l appear on the record at a l a t e r time, that i s , below the sea f l o o r . As the ship approaches and passes the point, the echo path decreases. The recorded signal is thus seen to approach and perhaps pass through the sea bottom echo. If d i r e c t l y over the object, the recorded echo w i l l peak above the sea f l o o r then drop off below as the ship moves away. The r e s u l t i n g 38 recorded p r o f i l e i s ' a hyperbola projecting above the sea f l o o r . Such hyperbolic waveforms emanating from point-r e f l e c t o r s or sharply terminating structures are often mistermed 'seismic d i f f r a c t i o n patterns' and are common on CSP records where topography i s rough and ir r e g u l a r , where steep slopes ex i s t near the ship's track, or where sources i n t e r n a l to the sediments are present, such as boulders i n t i l l , or small gravel lenses i n sand or s i l t . Examples of the waveforms from rugged topography are seen i n Plate VIII, near p o s i t i o n C. Hyperbolae become asymptotic to 45° at horizontal distances that are large compared to depth to. the r e f l e c t i n g source. At lesser distances they assume smaller angles, approaching zero degrees when the hydrophones are d i r e c t l y overhead. When such patterns are p r o l i f i c on the record, i t becomes d i f f i c u l t to separate true dipping s t r a t a from segments of these hyperbolae. Even i f the ship does not pass d i r e c t l y over a pinnacle or boulder but to- one side of i t , a hyperbolic pattern may s t i l l appear on the record but i t may or may not break the surface of the sea f l o o r . That i s , a pattern may appear on the record which i s not related to any r e f l e c t o r immediately under the ship's track. Such a r e f l e c t i o n i s termed a 'side-echo'. A r e f l e c t i n g source below the sediment surface which i s under the ship's track may also produce the 39 same type of pattern. Bennett and Savin (1963) used two channel recording to distinguish side-echoes from true sub-bottom r e f l e c t i o n s . The true r e f l e c t o r s showed stronger records on a low frequency channel when compared to the high frequency recording. Hyperbolae caused by surface i r r e g u l a r i t i e s can be a useful c r i t e r i o n to i d e n t i f y a p a r t i c u l a r r e f l e c t i n g sur-face i f roughness i s c h a r a c t e r i s t i c of that surface. However, such patterns obscure the true shape of an object, slope, or surface and thereby place a l i m i t on the a b i l i t y of sonic equipment to define shapes. Apparent and True Slopes As i n echo-sounding, slopes recorded on a CSP p r o f i l e are not true slopes. Aside from the f a c t that the survey may not have followed the f a l l l i n e , horizontal and v e r t i c a l scales are not usually the same,, The v e r t i c a l time scale i s chosen from those available on the graphic recorder to s u i t the depth of water, sub-bottom penetration and the f i r i n g i n t e r v a l . The s p a t i a l equivalent of the v e r t i c a l time scale varies with the v e l o c i t y of sound i n the materials traversed by the sound wave. The horizontal scale depends upon the paper speed and speed of the ship over the bottom. Most commonly, the v e r t i c a l scale i s greater than the h o r i -zontal, causing small angles to be exaggerated. Because 40 transducer and hydrophones are omni-directional, or semi-d i r e c t i o n a l , side-echoes can be received from points on the sea f l o o r (and sub-bottom) which are not d i r e c t l y under the ship. As a slope i s approached, echoes are received from a point tangent to the wave-front before the ship actually arrives over i t . This point i s therefore at a shallower depth than the sea f l o o r under the ship. On the record then, the slope appears to' be less than i t s actual value. The r e a l and apparent recorded slopes are r e l a t e d by the trigonometric function s i n Q = tan 0 where 0 = angle of the true slope and 0 " angle of the recorded slope (Krause, 1962). The error for slopes up to 15° i s small, less than For slopes greater than t h i s , the error may become appreciable. Sub-bottom slopes can be corrected i n the same way but the sound v e l o c i t y i n the overlaying sediments must be known accurately, e s p e c i a l l y i f one end of a slope i s buried deeper i n overburden than the other. In such a case the time d i f f e r e n t i a l due to depth v a r i a t i o n i n the sediments can be important i n determining the r e a l angle. Nature of Seismic Reflectors The usual objective of a continuous seismic survey i s to investigate the sedimentary or rock s t r a t a i n the upper layers of the earth's crust. How r e f l e c t i o n s as recorded on the seismic p r o f i l e s are related to-geological s t r a t a i s a 41 question .fundamental.'to the p r i n c i p l e of seismic methods. Knowledge of the answer assists a confident interpretation of the seismic p r o f i l e s . I t i s a common misconception that acoustic r e f l e c t i o n s occur only at seismic v e l o c i t y d i s c o n t i n u i t i e s . According to wave theory, (see, f or example, O f f i c e r , 1958; Lindsay, i960) acoustic r e f l e c t i o n s occur at a change of acoustic impedance, a quantity defined as the product of density,^, and acoustic v e l o c i t y , Vo .Thus, while v e l o c i t y d i s c o n t i n u i t i e s without a corresponding change i n density w i l l cause r e f l e c -tions, density contrasts with no v e l o c i t y change may also give r i s e to r e f l e c t i o n s . Grain si z e , mineral composition, porosity, degree of l i t h i f i c a t i o n , depth of b u r i a l , as well as other fa c t o r s , a f f e c t the e l a s t i c properties of materials upon which the acoustic impedance depends. V e l o c i t y i s es p e c i a l l y sensitive to changes i n material properties. One has only to glance at a v e l o c i t y log of a well to see how rapi d l y this varies i n a natural s i t u a t i o n . Density and ve l o c i t y may also change r a p i d l y i n the l a t e r a l d i r e c t i o n . Because of the many geological factors a f f e c t i n g both para-meters (Nafe and Drake, 1957; Hamilton, 1959) "the va r i a t i o n of acoustic impedance can be, and i s , a complicated function. However, because the acoustic impedance i s intimately related to the geology, the seismic r e f l e c t i o n method does work we l l . 42 I d e n t i f i c a t i o n of r e f l e c t o r s as s p e c i f i c rock or sediment types cannot be made on the basis of seismic evidence alone. Additional geological control i n the form of borehole logs, sea f l o o r samples i n areas of outcrop (ascertained from the seismic record), and study of l o c a l and regional land geology i s necessary to make more-or-less posit i v e i d e n t i f i c a t i o n possible. However, from past exper-ience i n seismic p r o f i l i n g , the general geological nature of the material traversed may be surmised from the record character i t s e l f . Moore ( i960) states that the strongest sub-bottom r e f l e c t i o n s occur from the sea f l o o r and from bedrock surfaces beneath r e l a t i v e l y soft sediments. In some cases they occur from bedding planes i n contrastingly layered sediments, such as gravel or sand lenses i n clay, etc. Moore also found that s i l t y - s a n d overburdens show acoustic homo-geneity. He interpreted r e f l e c t o r s within a sedimentary layer, a l l p a r a l l e l to the p r o f i l e of the sea f l o o r , as an indication that the present sea f l o o r i s a depositional environment. The r e f l e c t o r s were usually from a material of contrasting texture to the sediment mass, i . e . a sandy layer i n s i l t y clay or gravel i n sand. McLure, et a l , (1958) found that the r e f l e c t i o n record was layered where the sediments contained t h i n sand or s i l t beds and thin s h e l l lenses. One thin sand bed only 0 .4 wavelength thick produced a prominent r e f l e c t i o n over a wide area i n which i t was present. In 4 3 other areas, r e f l e c t i o n s which did not correlate with l i t h o -l o g i c a l composition were shown by laboratory analysis to have a probable c o r r e l a t i o n to changes i n moisture content of the sediments. Beckman, et a l , (1959) showed a dependable cor-r e l a t i o n between the continuous seismic r e f l e c t i o n record and changes i n porosity. The change i n number of hammer blows required to drive a sampler a unit depth into the sedimentary column also correlated with depth to r e f l e c t i n g surfaces. From practical, application then, as well as from theory, strong sub-bottom r e f l e c t i o n s should be generated from such interfaces as occur between unconsolidated and semi-consolidated sediments, or between indurated or compacted bedrock and unconsolidated and semi-consolidated overburden. Where such major changes i n the nature of the material occur, r e f l e c t i o n s should be recorded unless the r e f l e c t i n g surface l i e s so deeply buried that r e f l e c t e d energy i s too greatly attenuated before reaching the hydrophones. Internal r e f l e c -tors within an otherwise uniform layer may represent factors other than l i t h o l o g i c a l changes but should, nevertheless, be Indicative of i n t e r n a l s t r a t i f i c a t i o n of the sediments. Seismic r e f l e c t i o n s on the records should therefore show evidence of such features as bedding planes, truncated layering, f a u l t s , s y n c l i n a l and a n t i c l i n a l structures, uncon-formities and disconformities; i n short, normal and expectable geological structure. Following Curray and Moore (1964,' p. 202) , such structure can be interpreted from 1) l a t e r a l continuity of r e f l e c t o r s , 2) attitude of r e f l e c t o r s with respect to the underlying, overlying and neighbouring str a t a , 3) the nature of the upper contacts of sequences of r e f l e c -tors, and 4) the thickness of the units, both i n d i v i d u a l l y and i n sequence. Thus, as Curray and Moore (op. c i t . ) wrote, r e f l e c -tions arise from "positions of the former sea f l o o r which existed as interfaces for a s u f f i c i e n t l y long time (prior to burial) to develop into acoustic r e f l e c t i n g horizons by changes i n l i t h o l o g y , by dessication under sub-aerial con-ditions or by more complete consolidation. To develop such c h a r a c t e r i s t i c s the f o s s i l surfaces must have formed during changes i n the depositional, oceanographic or sea l e v e l conditions." CHAPTER III MORPHOLOGICAL SUBDIVISION OP THE STRAIT OF GEORGIA STUDY AREA As. i s often the case i n oceanographic studies, no topographic chart of the study area was available. Therefore a contoured, base chart, exhibited as Figure 5 , and found i n the back pocket of this thesis, was prepared by the author from the following data: 1) hydrographic charts of the area, 2) f i e l d sheets of the Canadian Hydrographic Service, and 3) echograms obtained by Cockbain (1962 a) . As with the Hydrographic Service, the echograms were plotted assuming a sound speed i n water of 1,465 meters per second (4,800 feet per second). The echograms obtained by Cockbain covered only the northern part of the present study area from Sand Heads to Ballenas Islands. In the southern part of the area, from Sand Heads to Patos Island, no echograms were obtained. However, more Hydrographic Service f i e l d data sheets were available. The bathymetry of th i s area was drawn mainly on the basis of these f i e l d sheets. As the topography tends to be smoother i n the southern region, no important d e t a i l that could be shown i n the 10-fathom ( 1 fathom = I . 8 3 meters or 6 feet) contour i n t e r v a l was considered to.be l o s t . Where information density was low, as at Gabriola Reefs, contours were omitted. On the basis of having l a t e r 46 returned d i r e c t l y to charted points, the o v e r a l l accuracy of the positions of contours i s considered to be better than 0.5 kilometers. I. TERMINOLOGY OP SHELF- AND SLOPE-LIKE FEATURES Water depths i n the study area reach a maximum of 433 meters (237 fathoms), well beyond the depths normally found on continental shelves. In f a c t , more than 50$ of the area involved l i e s at depths greater than 132 meters (seventy-two fathoms), the world average for the depth of the shelf edge (Shepard, 1963, p. 257). We are therefore dealing with a body of water deeper than continental shelves but not as deep as ocean basins. Table I l i s t s the area and percentage area i n various depth zones. TABLE I DEPTH ZONES IN THE STRAIT OF GEORGIA Square Nautical % of Total Miles Area Total Study Area 1,100 100 Area Deeper than 20 fathoms ' 823 75 Area Deeper than 100 fathoms 477 ^3.5 Area Deeper than 200 fathoms 106.6 10.3 47 Because the area i s shallow compared to deep ocean basins, but deep compared to continental terraces, the usual meaning attached to the nomenclature of the marginal areas of the continents i s not v a l i d here. For instance, i n many cases water depths very near shore may approach or exceed the depths normally associated with outer continental shelf or slope areas. These terms are therefore redefined here i n the spec i a l context of their use i n this thesis. For this purpose a shelf, where i t e x i s t s , i s defined as the area adjacent to the shoreline which extends seaward at a shallow d e c l i v i t y . In most of the study area the shelf i s not wider than two to three kilometers. In some areas, no shelf e x i s t s . The shelf edge occurs at any depth and i s defined as the point where the d e c l i v i t y increases markedly. The slope, for the purpose of this thesis, i s the i n c l i n e d area which connects the shelf to the basin f l o o r s . In areas without a shelf, the slope begins at the shoreline. Where i t is necessary to make reference to a p a r t i c u l a r shelf or slope on one or the other side of the S t r a i t , the words 'mainland' or 'island' w i l l be pre-f i x e d , where 'mainland' refers to the region on the north and east sides of the S t r a i t , including any islands such as Bowen Island, and 'island' refers to the south and west side of the S t r a i t including Vancouver Island and the Gulf Islands. Any further d i s t i n c t i o n w i l l be indicated by the place name. At the Fraser Delta, the terms ' s h e l f and 'slope' are not 48 d i r e c t l y applicable. Instead the terminology of Mathews and Shepard (1962) w i l l be followed. Here, the upper slope of the delta front constitutes the area of steepest decline, averaging from 1^° to. 3 ^ ° . At greater depths, the d e c l i v i t y i s markedly l e s s . These are the lower slopes of the delta. The upper and lower slopes are composed of foreset beds. While Mathews and Shepard did not recognize bottomset beds, i n this thesis they are found to occur at some distance from the r i v e r mouth as basin f i l l . I I . SUBDIVISIONS OP THE STUDY AREA To simplify the presentation of data,•it is convenient to apply names, not only to i n d i v i d u a l topographical features, but to those areas of submarine topography which, from their bathymetry or unique character of geological or geophysical data, may be grouped as a unit under one descriptive term or heading. Previous c l a s s i f i c a t i o n s have been made only i n l o c a l areas or under more general groupings. For instance, Waldichuk (1953) divided the S t r a i t into three areas on the basis of sediment d i s t r i b u t i o n . The southern section i n the region of the Gulf Islands and San Juan Archipelego, including, the' bays and channels, were described as an area of diverse bottom type with l i t t l e deposition and possible bedrock erosion. The central S t r a i t , covering much of the area of thi s thesis, was recognized as dominated by soft muds 4 9 deposited as a.result of Fraser River inflow. He distinguish-ed the S t r a i t north of the present study area by i t s sand, thought to be derived from erosion of shore sediments i n that region. While this c l a s s i f i c a t i o n i s v a l i d as a regional description of f a d e s , i t bears l i t t l e r e l a t i o n to the mor-phological features of the sea f l o o r , or to their geophysical, character. Cockbain (1963 a) subdivided the northern part of the study area into three morphologically d i s t i n c t d i v i s i o n s : the Fraser River Delta i n the southeast; an area of banks and subdued topography along the mainland side of the S t r a i t i n the northeast; and an area of deep basins separated by ridges along the Vancouver Island coast. Although t h i s c l a s s i f i -cation i s lim i t e d , i t can be enlarged, defined more c l e a r l y , and made of use here. On the basis of topography alone, the study area i s e a s i l y d i v i s i b l e into a northwestern region of rugged charac-ter , a central smoo'ther region dominated by the Fraser Delta, and a southern area where the Fraser River has not deposited recent sediments and erosion may be occurring. Topography of the southern area i s intermediate i n ruggedness, being more irregular than the area subdued by a thick cover of r i v e r -supplied sediment but not as irregular as the northern region of banks and basins. However, CSP p r o f i l e s indicate that a further subdivision, based on more than topographical expres-50 sion, i s useful. The character of the seismic•record i s a function, although not a unique function, of the geology of the bedrock and soft sediments through which the seismic sound waves pass. Thus a change i n character of the seismic record may be related to a change i n character of the rocks or sediments traversed. In the S t r a i t of Georgia the record character i s consistent over cer t a i n areas. These areas are usually related to the bottom configuration, i n d i c a t i n g some consistency i n geology, as well as topography. The following subdivision of the S t r a i t i s based on morphology, bathymetry, geological structure and geophysical character. The sub-divisions are shown on Figure 5 . Subdivision of the Study Area 1. Fraser Delta Area 2. Area of Deep Basins 3. Elevated Area of Ridges 4. Roberts Swell and the Nearby Mainland Shelf 5. Boundary Basin and Alden Ridge 6 . Island Slope P S • <y iy j&: "I • - « I %y0/,,::,;:.7^l:v' • % --yy Mm±~(&m •7 \ X s r s ^ -S A N D H E A D S TO B A L L E N A S I S L A N D S S O U N D I N G S IN FATHOMS Prufccoon McrciiDt i 3 y C \ B R l 0 L \ si® "^ yy - \i STRAIT OF GEORGIA TURN POINT TO B A L L E N A S ISLAND INST ITUTE OF O C E A N O G R A P H Y UNIVERSITY OF BRITISH COLUMBIA ryr-..y--y*'<:i-~ yy y^yy^^^ FIGURE 5 Bathymetry of the Strait of Georgia Study Area showing locations of : o S u r v e y Lines and Positions EB Boreholes © Bottom Current Stations (after Pickard.1956) Piston Core Pipe Dredge Sample 4 / \ Insert (left): Areal Subdivisions 1 Fraser Delta Area 2. Area of Deep Basins 3. Elevated Area of Ridges 4. Roberts Swell and the nearby Mainland Shelf 5. Boundary Basm and Alden Ridge 6 Island Slope -<y<\.; mm* • C A L C 0 N A U T I C A L M I L E S C O N T O U R I N T E R V A L S - 10 F A T H O M S C A U T I O N - T H I S C H A R T IS NOT I N T E N D E D FOR N A V I 6 AT 10 N AL U S E . \ -y$r&~y :^y ^-y^ ^ % > % X . CANADA BRITISH C O L U M B I A TURN PT. TO SAND HEADS Sounria . Jos. /. 2 and 3%. j -X&Jtli S O U N O * N O S IN »»TMOM« Colder Sfaftunef a ^7* Richfield Pure fB Sunny side -BOUNDARY . -v - T ; . i :v ' - / ^ ^ ^ ^ ^ ^ ^ r A T t f i r X A I S L A S I • 1 - r i A P Uy* 123* JT f CHAPTER IV SEISMIC DATA The seismiG data gathered during t h i s survey i s presented i n Plates I to XXXIV. Photographs of the o r i g i n a l records are given, along with l i n e drawings showing the geo-physical and geological interpretation of the p r o f i l e s . These should be referred to i n r e l a t i o n to the text. Tracks of the survey l i n e s are indicated on Figure 5. Positions of f i x e s along each l i n e are marked by c i r c l e s and assigned a l e t t e r . On each p r o f i l e of Plates I to XXXIV the equivalent p o s i t i o n Is marked with the same l e t t e r . On the photographs, the times of the o r i g i n a l f i x e s , recorded by v e r t i c a l l i n e s , can be seen on the records below the l e t t e r s . D i f f i c u l t i e s i n presentation of this CSP data are caused by the large quantities of information, the d i v e r s i t y of geology and the desire to be concise, consistent and l o g i c a l . Twenty-six p r o f i l e s , representing 600 kilometers of l i n e , cross the S t r a i t i n a transverse d i r e c t i o n . In addition 126 kilometers of l i n e follow the axis of the S t r a i t . Shorter p r o f i l e s i n other directions make a t o t a l of 790 kilometers of l i n e whose information i s to be presented. Other authors have used various techniques of data presentation. However, rather than each p r o f i l e being presented and explained i n the 52 text, the data i n this study w i l l be presented by area, the areas being those l i s t e d previously under the heading "Morpho-l o g i c a l Subdivision of the S t r a i t of Georgia" and indicated on Figure 5« To make clearer the seismic data, major horizons have been outlined i n heavy l i n e on the line' drawings. These represent units separated by major unconformities corresponding to geologically s i g n i f i c a n t time breaks. Thinner l i n e s repre-sent layers-which can be distinguished and separated within units. The thinnest l i n e s show the shape and attitude of the i n t e r n a l bedding or s t r a t i f i c a t i o n within a layer, with-out intending to give sp e c i a l meaning to any p a r t i c u l a r r e f l e c t o r . Rather i t i s the trend of the in t e r n a l s t r a t i f i -cation which i s s i g n i f i c a n t i n most cases. The geological interpretations of the p r o f i l e s are made on the l i n e drawings. A legend opposite Plate I i d e n t i -f i e s the symbols used. • Because of the difference of v e r t i c a l and horizontal scale, angles on the p r o f i l e s are greatly distorted. Low angles have the appearance of steep slopes. Exaggeration i s useful i n detecting and mapping gentle slopes or small changes i n low-angled slopes, however, steeper slope angles are com-pressed, making i t d i f f i c u l t to measure or distinguish one angle from those around i t with s i m i l a r , but not equal, slopes. The angle at which exaggeration changes to compression depends upon the inverse of the. exaggeration f a c t o r . For an 53-exaggeration factor, of 12, that most common on the p r o f i l e s of this thesis, angles less than 16 degrees are made larger, while those greater than 16 degrees, while s t i l l exaggerated, are compressed or squeezed together. Thus i t becomes v i r t u a l l y impossible to t e l l from an exaggerated record i f a steep slope i s , for example, 60 degrees or 70 degrees. Nomographs on the l i n e drawings show the exaggeration. One d i f f i c u l t y encountered i n th i s thesis was i n separating true s t r a t a dipping at angles or 30 degrees or greater, from hyperbolae caused by point r e f l e c t o r s . The hyperbolae are asymptotic to 45 degrees when the r e f l e c t i n g point source l i e s under the ship's track. Various techniques were used to distinguish between these features. For instance, i f c arried far enough, the r e a l and hyperbolic patterns, i f not at the same angle, must converge and cross. However, most bedding r e f l e c t o r s do not extend far enough for th i s to occur. True bedding planes may change angle, usually becoming less steep with depth. This was a useful parameter but one which did not always occur. When no other method of d i s t i n g - . uishing bedding presented i t s e l f , hyperbolic patterns were drawn to scale complete with the l o c a l exaggeration and used as templates to try to determine the source of the r e f l e c t i o n . The v e r t i c a l exaggeration varies from record to record and, i n f a c t , probably varies over the length of any one record. Variation, was caused mainly, by changes i n ship's 54 speed over the ground which was i n turn caused by variations i n speed and d i r e c t i o n of t i d a l currents encountered i n the S t r a i t . The effects of wind, too, which at times during the survey reached.gale force, also contributed. In one instance the difference between course steered and course made good amounted to 30 degrees as the vessel made leeway i n high wind and heavy t i d e . Even should perfect control of ship's speed be obtained, exaggeration would s t i l l vary because of v a r i -ations i n the velocity-of sound through d i f f e r e n t sediments. The nomograph provided on the l i n e drawings of the p r o f i l e s should therefore, be used to measure sea f l o o r slopes only. Measurement of sub-bottom slopes requires knowledge of sedi-ment v e l o c i t i e s . These are not well known, but sub-bottom slopes and sediment thicknesses have been estimated using v e l o c i t y information from available sources. V e l o c i t i e s i n various sediments are discussed i n the Appendix. The v e r t i -c a l exaggeration i n the p r o f i l e s , calculated for a sound v e l o c i t y of 1,460 meters (4,800 feet) per second, ranges between nine and f i f t e e n , with twelve the most common. One p r o f i l e (Plate •VTII) recorded at a paper speed slower than normal, has an exaggeration of s l i g h t l y more than four. Records obtained from S h e l l Canada Limited have exaggerations of seven to ten. I. THE FRASER DELTA AREA 55 In this thesis, the name 'Fraser Delta' w i l l r efer to the submarine portion only of the Fraser River Delta. • The region termed here the Fraser Delta Area i s that area which i s , i n general, most heavily encroached upon by the advancing sediment front deposited i n the S t r a i t by the Fraser River. I t i s somewhat arbitrarily chosen to represent the del ta region but i t i s not meant to infer that sedimen-ta t i o n from the Fraser River i s n e g l i g i b l e outside of this area. In f a c t , sediments from this source occur throughout much of the study area. However, the area from Point Grey at Vancouver seaward seven kilometers to the west, then southwest to the deepest point i n that d i r e c t i o n , following the axis of the deepest part south to the 4 9 t h p a r a l l e l , then i n a d i r e c t l i n e to Point Roberts, represents the main portion of the foreset beds of the delta where deposition i s heaviest. The Fraser Delta was described by Johnston (1921) and la t e r by Mathews and Shepard ( 1 9 6 2 ). Its active part extends into the S t r a i t on a broad front of more than thirty-seven kilometers from Point Grey to Point Roberts. Extensive t i d a l f l a t s , as much as nine kilometers wide i n places, extend from the sub-aerial d e l t a and dry out at extreme low tide.s. .These f l a t s , c a l l e d Sturgeon Bank north of the .main 56 r i v e r mouth at Sand Heads, and Roberts Bank to the south, are terminated seaward by an upper slope averaging 1 3/4 degrees to 3 | degrees and l o c a l l y steeper (Mathews and Shepard, 1962). This upper slope tends to decrease with depth, merging with a lower,.more gentle slope which contin-ues downward to the adjacent basin depths. Prom Point Grey to Sand Heads, the general trend of the delta front i s north-south, in marked contrast to other features i n the S t r a i t which trend i n the general northwest-southeast d i r e c t i o n of the S t r a i t i t s e l f . South of Sand Heads the delta front sweeps to the southeast and, below 100 meters (55 fathoms), merges with the contours of Roberts Swello Northwest of Sand Heads the only major disruption to the generally smooth contour of the delta front i s a pro-truding ridge top noted by Mathews and Shepard and thought by them to possess a core of Tertiary or Cretaceous bedrock. The peak of this ridge, named here Praser Ridge, reaches to less than 145 meters (80 fathoms) of the water surface. I t i s almost buried by the advancing delta. West of Sand Heads a group of small h i l l s with r e l i e f of about f i f t e e n to t h i r t y meters occurs beyond a depth of 220 meters (120 fathoms). These h i l l s or k n o l l s , located by Mathews and Shepard, are d i s t i n c t l y a feature of d e l t a i c or estuarine sedimentation and, as shown by the continuous 57 seismic p r o f i l e s over the area, are not an expression of buried topography. Mathews and Shepard suspect the h i l l s to be remnants of former landslides, modified by current action and l a t e r sediment. The sub-bottom configuration of the delta front is i l l u s t r a t e d i n eight p r o f i l e s which cross the S t r a i t i n a transverse d i r e c t i o n arid one p r o f i l e along the front of the delta. Two of these p r o f i l e s were recorded by S h e l l Canada Limited and represent I d e n t i c a l track l i n e s : one p r o f i l e recorded the output of a sparker source (Plate XVIII); the second (Plate XIX) was obtained with a high-powered gas exploder source used contemporaneously with the sparker source. Photographic reproductions of the original'.records are shown with l i n e drawing interpretations i n Plates XV to XXIII. The ridges evident on the delta p r o f i l e s w i l l be discussed i n a la t e r section. S i m i l a r l y the bedrock r e f l e c t o r beneath the delta sediments w i l l be discussed with the section on the island slope. Of the slopes measured on the p r o f i l e s of the delta, none, except at minor l o c a l disturbances, exceeds an apparent angle of 2 ^ ° . Upper delta slopes are steepest. Below 220 meters (120 fathoms) the slope angle i s reduced to about 1 ° , and decreases further as water depth increases. Sediments comprising the modern delta front form a thick wedge extending at least f i f t e e n kilometers across the 58 S t r a i t from Sand Heads to the opposing island slope. Since Sand Heads i s i t s e l f eight kilometers from the shore l i n e of the sub a e r i a l delta-.the thick marine delta sediments extend at least twenty-three kilometers into the. S t r a i t . Under the thick eastern part of the wedge, the base of the sediments cannot be seen on the p r o f i l e s . However, as the wedge thins to the west, the pre-delta surface becomes apparent r i s i n g toward Vancouver Island. On this side many high rocky pin-nacles have been buried, or nearly so, by the advancing delta o sediments, some two m i l l i o n cubic .meters (7 x 10° cubic feet) of which are added to. the delta front each year by the Fraser River (Mathews and Shepard, 19^2, p. 1424) . These buried rocky pinnacles form part of a ridge system p a r a l l e l i n g the Gulf Island slope south of Gabriola Reefs. Under the sediment-water interface, the character of,, the CSP records may be r e l a t e d to the composition of the sedi-ments. S i m i l a r i t y i n the character of echograms was used by King (1965) to delineate areas of l i k e sedimentary fac i e s on the sea flo.or o f f Nova Scotia. P r o f i l e s over the Fraser Delta show changes i n character from the steeper upper slopes, above the 185 meters (100 fathoms) contour, to the f l a t t e r lower slopes. The steeper upper slopes are characterized mainly by low seismic penetration and a pattern of random sub-bottom echoes. Lower delta slopes everywhere permit 59 greater penetration and show more organized and s t r a t i f i e d patterns i n the sub-bottom r e f l e c t i o n s . • The proportion of unconsolidated sand i n the sediment has the inverse c h a r a c t e r i s t i c to the seismic penetration (Mathews and Shepard, 1 9 6 2 , Figure 2 ) . On the upper slopes sand content i s high, e s p e c i a l l y south of the r i v e r mouth. On the lower slopes sand content decreases while s i l t and clay fractions assume greater proportions. In other areas where seismic penetration is..very good, as i n Ballenas Basin, samp-l i n g has shown the sea f l o o r i s composed almost completely of s i l t and clay sized p a r t i c l e s . It i s only when the upper delta front i s close that signal losses become excessive. This points to a conclusion that loose unconsolidated sands of the delta front act to reduce penetration of sound waves. Other workers have also found that clean sand prevents seismic penetration and obscures the underlying section (Moore, i 9 6 0 , , p. 1124; McLure, et a l , • 1958; Ostericher, 1 9 6 5 , p. 4 2 ) . Other charac-t e r i s t i c s of the sediment as well as sand content may have the same e f f e c t , but i f so, these are not known at the present time. According to Johnston ( 1 9 2 1 ) , no s t r a t i f i c a t i o n of sediments i s v i s i b l e i n cores from water depths of 90 "to 180 meters (50 to 100 fathoms), but massive bedding may be present. He considered the fore-set beds of the delta are laminated whereas bottom-set bedding lacks lamination. In Johnston's 6o terminology, bottom-set beds occurred at depths greater than 90 meters (50 fathoms). Mathews and Shepard (1962, p. 1427) found only s l i g h t evidence of lamination i n cores taken mainly from the deeper slopes. They disagreed with Johnston's term-inology and preferred to c a l l a l l bedding i n the area of their investigation 'fore-set' beds more i n keeping with the slope exhibited by them. The CSP p r o f i l e s across the delta area show that sediments at the present day water-sediment in t e r -face would a l l come under the term 'fore-set' whereas much deeper below the fore-set beds, older f l a t - l y i n g s t r a t a are probably bottom-set bedding. The modern delta sediments are symbolized by H3 on the l i n e drawing interpretations while the older buried bottom-set beds are given the symbol H-j_. Modern f l a t - l y i n g bottom-set beds also occur .in the deep basin area and are marked HV). The seismic.profiles over the delta, although not expected to resolve thin laminae, give l i t t l e i n d i c a t i o n of. s t r a t i f i c a t i o n i n the upper delta slopes. The presence of cross-lamination or other small-scale s t r a t i f i c a t i o n that cannot be resolved seismically i s not precluded, but large-scale continuous bedding such as i s present over other parts of the study area cannot be seen here. As water depths increase, s t r a t i f i c a t i o n also increases u n t i l , under the lower delta slopes, many sub-bottom r e f l e c t o r s are observed to l i e conformably, or nearly so, below the sediment-water 6 l interface. However, s t r a t i f i c a t i o n does not continue through-out the entire column of delta sediments but ceases as depth of overburden increases. Stronger seismic r e f l e c t i o n s at lower depths can s t i l l be seen marking major horizons and sediment-bedrock contacts. The f a c t that the upper r e f l e c -tions are weak, and do not occur beyond 9° to 120 meters (300 ;' to 400 feet) below the sediment surface, Implies that the sediments are nearly homogeneous and differences i n acoustic impedance are not great. The differences, although possibly caused by variations i n parameters such as density or.porosity (Hamilton, 1959; McLure, et a l , 1958) , are more l i k e l y assoc-iated with depositional bedding, perhaps located below sampling depth. .Furthering this impression i s the f a c t that where r i d -ges have interrupted the smooth transport of sediments, the s t r a t i f i c a t i o n i s also disrupted but tends to follow an ex-pectable pattern of depositional bedding, r i s i n g over ridge tops and down the other side. In f a c t , r e f l e c t o r s i n sedi-ments below the ridge tops end abruptly against the sides of the- ridge, i n d i c a t i n g the ridges ponded sediments u n t i l the ridge was completely buried, at which time the sediments moved over the top. Eventually, external evidence of the buried ridges was destroyed, leaving i n some cases no evidence at a l l , i n others only a hump or small scarp remaining on the sediment surface to mark the b u r i a l . 62 In the area of. anomalous hummocks, or kn o l l s , o ff the Fraser River mouth, s t r a t i f i c a t i o n i n the layer i s severely disturbed, but not i n a random manner. Instead i t ' i s bent up under the knoll s , and down between them. Except where the knolls have been buried by younger sediments, the s t r a t i f i c a t i o n follows the surface topography. Later sedi-mentation has not buried a l l the h i l l s evenly, but tends to bury those h i l l s on the higher slope f i r s t . This i s highly suggestive of b u r i a l through a mechanism of bottom transport of material from a higher delta source rather than through settlement of material i n suspended transport from the water above. The anomalous knolls w i l l be treated i n a l a t e r sec-t i o n . Much deeper under, the delta sediments, every p r o f i l e records a strong r e f l e c t i o n from an almost f l a t - l y i n g horizon at a depth below sea l e v e l of nearly 0.6 seconds t r a v e l time. This horizon marks the top of the Ej_ layer. Below the h o r i -zon, secondary r e f l e c t o r s i n the H^ layer indicate that the layer i s s t r a t i f i e d . The flatness of the % s t r a t i f i c a t i o n throughout the delta area.contrasts with the dipping s t r a t a in the overlying H3 layer. The Ej_ layer can be correlated from record to record under the whole of the western part of the delta where i t i s confined by the basin sides. At Roberts Swell i n the south, i t disappears, probably by wedging out over the Roberts Swell sediments. However, no information 6 3 i s available at the present time to indicate the exact r e l a -tionship between these two sedimentary units. Between the adjacent p r o f i l e s of Plates XXII and XXIV (see Figure 5 for the locations of these p r o f i l e s ) , the H->_ layer disappears. Under Ballenas Basin to the northwest (Plate I) a similar correlatable horizon extends the layer as far as the end of the basin i n that d i r e c t i o n . Plate XVI shows the layer also exists at a s l i g h t l y higher elevation of 0 . 5 5 seconds between Point Grey and the buried extention of McCall Ridge. From there i t can be traced farther to the north deep under the sediments of Queen Charlotte Trench (Plates XII and XIII) , but wedges out against a r i s i n g bedrock s i l l (Plate XIV) before entering Howe Sound. I t i s not known i f i t con-tinues on the other side of the s i l l i n Howe Sound. The H-j_ horizon dips to the northwest at an average of, no more than one part i n one thousand, or about four minutes of arc, beneath Ballenas Basin. A s l i g h t cross-basin t i l t toward Vancouver Island i n the region of the Fraser Delta is probable but may r e f l e c t a 'variation i n speed of sound through the overlaying wedge of modern delta sediments. Reflectors i n t e r n a l to the layer show l i t t l e turn-up on the flanks of the confining bedrock sides. Sediment transport was there-fore mainly along the length of the basin, rather than trans-versely across i t . In f a c t , from the slope of the r e f l e c t o r s , transport was almost c e r t a i n l y from southeast to northwest,. 64 away from the Fraser River region. These sediments are therefore related to the Fraser River drainage system and are probably ancient bottom-set bedding of the Fraser Delta. Because the horizon i s well below Fraser Ridge, even i n i t s buried parts, and because the cross-basin slope i s so s l i g h t , at the time of deposition the sub-aerial delta must have been some distance from i t s present s i t e , perhaps many kilometers to the east or southeast of Fraser Ridge. Near Point Grey, and i n Queen Charlotte Trench on the east side of Fraser Ridge, the horizon l i e s about 8 to 15 meters (25 to 50 feet) above the same horizon west of the ridge. From this i t can be inferred that Fraser Ridge and McCall Ridge are connected near Point Grey and so blocked the former movement of sediments into Ballenas.Basin around the north end of Fraser Ridge. A break i n this ridge system which allowed, sediment transport into the basin may be buried somewhere to the south of the present r i v e r mouth. The thickness of the bottom-set layer i s variable, depending upon the r e l i e f of the underlying bedrock. Under the fore-set beds of the delta i t averages about 80 meters (260 feet) and sometimes exceeds 150 meters (500 f e e t ) . Deposition occurred d i r e c t l y over bedrock and semi-consolidated deposits forming Ballenas Basin f l o o r . In the depositional process, i r r e g u l a r i t i e s of the f l o o r were f i l l e d i n , leaving a r e l a t i v e l y f l a t , smooth surface over which modern fore-set 6 5 bedding has advanced. The ancient bottom-set beds were con-tained by the walls of the basin i n much the same manner as is presently occurring i n the northwestern basin area. East of Fraser Ridge the top of the bottom-set bedding occurs at about 400 meters ( 1 , 3 0 0 feet) below present sea l e v e l . I f the same elevation p e r s i s t s to the sub-aerial delta, bottom-set bedding would only be found i n wells deeper than 400 meters. Few wells anywhere i n the delta penetrate that deeply. Those that do have been too poorly logged to d i f f e r e n t i a t e bedding types. Total thickness or volume of a l l delta sediments can-not be d i r e c t l y calculated because the pre-delta surface i s not detected under the thickest section of the delta. West of Fraser Ridge and Sand Heads, sediments reach a maximum measur-able thickness of at l e a s t 290 meters (950 f e e t ) . At a p o s i t i o n about 3 . 5 kilometers southwest of-Point Grey, measur-able sediment thickness on the upper slope i s approximately 275 meters (9°0 f e e t ) . The indication there i s that the thickness increases much more to the east. With bottom-set bedding at 400 meters, sediments should be at least that thick i n depressions of the sub-delta surface. A d r i l l hole near Steveston at the mouth of the Fraser River i s reported to have penetrated 215 meters (700 feet) of sand and recent delta material before s t r i k i n g a large boulder (Johnston, 1919 , P- 6) but i t i s apparent from Plates XVII, XVIII and XIX, that' the 66 pre-delta topography had considerable r e l i e f . These p r o f i l e s which outline the buried basin under the delta sediments i n d i cate Fraser Ridge at one time stood almost 370 meters ( 1 , 2 0 0 feet) above the pre-delta basin f l o o r . Most of the r e l i e f of the pre-delta surface has been completely buried. Only l o c a l l y , such as at Fraser Ridge, does any evidence of i t ex i s t on the present sea f l o o r . I I . . THE NORTHWESTERN BASINS In the northwestern section of the study area, two deep basins, Ballenas and Malaspina, occupy the western f l o o r of the S t r a i t . Despite the great depth of water, good seismic pene-t r a t i o n of up to 0.25 seconds or more was achieved i n the unconsolidated sediments here. Nine p r o f i l e s , shown i n Plates III to XII, cross Ballenas Basin to the island slope. Plate II passes north of the end of the basin. In addition, Plate I records a p r o f i l e along the length of Ballenas Basin from a point near Gabriola Reefs i n the southeast, to.the end of the basin north of Ballenas Islands. Plates II to "VTI also cross Malaspina Basin but no p r o f i l e was obtained along i t s length. The bedrock sides of the northwestern basins are more d i r e c t l y associated with either the island slope or the ele-vated ridge areas, and they w i l l be discussed i n those • 67 sections. S i m i l a r l y the bedrock f l o o r under the sediments, i s also referred to the same sections. Except for the southwestern part of Ballenas Basin, l i t t l e penetration was achieved i n the basement rock under-l y i n g the sediments. This is hardly surprising since even the western side of Ballenas Basin, which i s bare of sediment, offers l i t t l e penetration, e s p e c i a l l y northwest of Gabriola Reefs. Locally, some bedrock r e f l e c t o r s do occur under the basin sediments but, as there i s l i t t l e continuity to them, their significance i s obscure. . Ballenas Basin Ballenas Basin extends along the base of the island slope from Ballenas Islands i n the northwest to Galiano Island i n the south, a distance of more than s i x t y - f i v e kilometers. Passing east of Gabriola Reefs, the basin turns to the south and.is now terminated by the r i s i n g slope of the Fraser Delta southwest of Sand Heads. The f l a t f l o o r of the basin i s from four to six kilometers i n width and for the most part exceeds 370 meters (200 fathoms) i n depth. The sea f l o o r slopes from the delta toward the northwest, merging smoothly with the f l o o r of the basin i n that d i r e c t i o n . The slope, decreasing continuously from the. delta, becomes almost horizontal i n the deepest part of the basin. With the' exception of the delta area, most of the basin perimeter i s ringed by the r e l a t i v e l y steep sldewalls of the isla n d slope on one side and submerged 68 ridges on the other. At the delta front, fore-set beds have f i l l e d much of the basin from the side and the lower fore-set beds now rest w e l l up the opposing island slope. Near Gabriola Reefs the peak of a li n e a r ridge extends from under the delta sediments, pointing northwest down the middle of the basin. This ridge w i l l be c a l l e d Finger Ridge. It now l i e s almost buried but has sheltered the section of basin on i t s southwest from the f u l l e f f e c t s of delta sedi-•ments. The basin i n the lee of the ridge remains deeper than the side closer to the Fraser River. Two major sedimentary layers can be distinguished i n Ballenas Basin. In the upper layer, many strong i n t e r n a l r e f l e c t o r s dip to the northwest from the Fraser Delta. These sediments are therefore a part of the modern delt a . In the deep basin area the slope decreases to near zero. The upper layer of sediments can therefore be c a l l e d modern bottom-set beds of the delta. On the l i n e drawing interpretations, these pro-delta :sediments are marked Hg. The H 2 layer thins from the delta area to the northwest and some r e f l e c t o r s disappear i n the basin (Plate I ) . Thus, as the distance from the delta increases, fewer r e f l e c t o r s occur i n the sub-bottom sediments. Some, however, continue to the end of the basin. As explained l a t e r , at least some continuous r e f l e c t o r s may be due to turbi d i t e layers i n the basin. 6 9 Below the modern pro-delta sediments, a strong r e f l e c t i n g horizon marks the top of the layer of ancient pro-delta sediments. This layer i s correlated with the layer under the delta area and Is continuous to the end of Ballenas Basin. I t covers and smooths the r e l i e f of the bedrock upon which i t rests leaving i n general a f l a t , uni-form surface similar to the present sea f l o o r of the basin. The s i m i l a r i t y to modern day pro-delta sediments strengthens the assumption that this layer does represent an older epi-sode of the delta. The volume of sediments i n Ballenas Basin can be estimated. The upper layer of lip sediments has an average depth of about 76 meters (250 f e e t ) . Since the basin sides are steep and the base of the layer i s a reasonably plane surface, a polar planimeter was used to measure the area over which these sediments l i e . Measuring to the 366 meter (200 fathom) contour at the delta yields an area of approximately 250 square kilometers. The volume of H2 sediments contained i n Ballenas Basin i s therefore 12 cubic kilometers. If the delta sediments (H3). are included, taking only the volume west of Fraser Ridge below 183 meters (100 fathoms) where the t o t a l sediment column can be seen, the volume of modern sedi-ments tot a l s approximately 60 cubic kilometers. The volume of the HQ_ layer i s more d i f f i c u l t to determine, but a rough estimate can be made, considering the 70 shape of the bedrock f l o o r and varying depth of sediment over i t . The volume of sediment including the H-^  layer under the Fraser Delta area west of Fraser Ridge i s approximately 12 cubic kilometers. Malaspina Basin North of Ballenas Basin, and separated from i t by a high ridge, a second deep basin, Malaspina Basin, enters the study area from Malaspina S t r a i t between the mainland and Texada Island. Only the most southerly part of this basin extends into the study area and no information other than that on n a u t i c a l charts i s as yet available for more than that part shown in Figure 5 . However, this appears to be the deepest section of a long, narrow basin whose depths over a large area mainly exceed 370 meters (200 fathoms). At least one extensive deep occurs i n the basin containing the greatest depth of the whole of the S t r a i t of Georgia, 433 meters (237 fathoms). The deep area may be r e l a t e d in.some way to a high, steep-sided, round knob r i s i n g from the side of the basin. The knob, c a l l e d Round Ridge, co n s t r i c t s the basin to a narrow passageway at that point. The f l o o r of the passageway i s t i l t e d toward the base of the knob with the deepest point occurring at the base. Not.far from Round Ridge, Malaspina Basin joins the central portion of Ballenas Basin. The f l o o r s of both basins are concordant at the confluence but differences are apparent 71 i n the basins themselves (see Plate IV). Whereas Ballenas Basin i s generally f l a t floored, the f l o o r of Malaspina Basin may bow up or sag down i n the centre. I t tends to f l a t t e n i n the north as i t leaves the study area. Malaspina Basin i s , despite i t s deep areas, s l i g h t l y shallower on the average than Ballenas Basin.. Since only a few p r o f i l e s cross Malaspina Basin and no a x i a l p r o f i l e was recorded, less i s known of this basin than Ballenas Basin. Two sedimentary layers are found here. The upper layer i s almost devoid of i n t e r n a l r e f l e c t o r s . A similar r e f l e c t l o n l e s s layer occurs over the nearby elevated ridge area. Prom the absence of i n t e r n a l r e f l e c t o r s and the good transmission of sound through i t , i t has been c a l l e d 1 s e i s m i c a l l y transparent' and given the symbol H t on the l i n e drawings. Sediments i n which no i n t e r n a l r e f l e c t o r s occur are considered to be homogeneous (Moore, i 9 6 0 ). This character-i s t i c does not occur i n the upper layer of Ballenas Basin, and therefore denotes a difference i n the sedimentary history, of the two basins. The sediments are undoubtedly Holocene i n age, possibly hemipelagic, with the most probable source being the Fraser River, although the p o s s i b i l i t y of unknown sources to the north of the study area should not be overlooked. The sediment surface slopes to the southeast as opposed to the sediments i n Ballenas Basin, but t h i s may be due to an i n i t i a l southeast slope on the lower layer. 72 Below the transparent or H t layer, some strong r e f l e c t o r s occur i n the deep sediments. Correlation with similar r e f l e c t o r s i n Ballenas Basin i s d i f f i c u l t because of the number of choices and the lack of an a x i a l p r o f i l e down the basin. The top. of the layer of Ballenas Basin may be represented by the similar strong r e f l e c t i n g horizon i n Malaspina Basin at the base 1of the layer. I n s u f f i c i e n t data prevents making a good c o r r e l a t i o n except, perhaps, near the junction of the two basins. Reflectors i n the lower layer are not continuous throughout the section of basin studied. Deep r e f l e c t o r s i n both Malaspina and Ballenas Basins indicate a central sag, possibly due to sediment compaction with time and depth of b u r i a l , although other factors such as currents cannot be ruled out. The present sea f l o o r i s warped and therefore other forces may be operating to shape i t . Because of uncertainties i n age, source and hist o r y of the deep sediments of Malaspina Basin, they are denoted by the symbol Q, on the l i n e drawings. The p o s s i b i l i t y exists that some deep sediments may be Late Pleistocene i n age. Sediment Thickness and Rate of Deposition Total sediment thickness i n the basins varies with r e l i e f on the bedrock below the f i l l . Maximum thickness may be as much as 260 meters (850 feet)' i n Ballenas Basin at points away from the delta front area, and l e s s , about 230 meters (750 feet) i n Malaspina Basin. The bedrock f l o o r of 73 the l a t t e r basin i s not known to be as deep as that of Ballenas Basin although i f a p r o f i l e was made along i t s axis, deeper areas might be found. Over the length of Ballenas Basin, the average thickness i s approximately 200 meters (650 f e e t ) . Without an a x i a l p r o f i l e i n Malaspina Basin, i t i s d i f f i c u l t to estimate an average there. An average rate of sedimentation can be calculated for Ballenas Basin. However, many variables occur which would seem to make any figure unrepresentative of the true rate. Sediment supply must have varied considerably since Late Pleistocene as the more accessible g l a c i a l deposits were removed and slopes became more stable. Thus the present rate of sedimentation may be less than when the delta was f i r s t b u i l ding. Counteracting t h i s i s a reduction i n distance from the r i v e r mouth as the delta grows. The present p o s i t i o n of the delta allows deposition d i r e c t l y into Ballenas Basin by both suspended and bottom transport. Unfortunately, i t i s not known for how long t h i s has been proceeding. I t may be that, because the delta i s now b u i l d i n g into the basin from a p o s i t i o n along i t s side, the present rate of sedimentation i n the northwestern area i s greater than i t was i n the d i s -tant past when the HQ_ layer was deposited. The age of the modern delta, from Mathews and Shepard ( 1 9 6 2 , p. 1432) i s less than 1 1 , 0 0 0 years, but more than 7 , 3 0 0 years. Taking 1 0 , 0 0 0 years as an acceptable age, and the average t o t a l depth of 74 sediment i n the deep basin area as approximately 200 meters (650 f e e t ) , the average rate of deposition i s found to be two centimeters per year. This rate, calculated for Ballenas Basin, does not apply outside this basin since i t i s a s i t e of rapid deposition. I t may not apply to Malaspina Basin which, p a r t l y because of the c o n s t r i c t i o n between i t and Ballenas Basin, may have a separate depositional h i s t o r y . I t i s int e r e s t i n g to speculate that, i f the same rate were to continue, i n another 1 0 , 0 0 0 years time sediments would reach the 180 meter (100 fathom) contour, f i l l i n g the basins and burying several more ridges. In a Norwegian f i o r d basin, Holtedahl (1965) has estimated sedimentation rates are 1 . 0 centimeter per year. However, no r i v e r s of size comparable to the Fraser discharge into i t . Turbidites and Slumps i n the Basin Sediments The estuarine sediments over much of the study area, and p a r t i c u l a r l y i n the Ballenas Basin, have as their o r i g i n the Fraser River. That this, i s the main source of sediment supply for.the area i s demonstrated by the f a c t that sediments thicken toward the r i v e r mouth and thin away from i t . Other streams no doubt bring material into the S t r a i t but, while their role could.have been more important i n the past, the Fraser River i s now the major contributor. Sediment d i s t r i b u t i o n from the r i v e r mouth must occur i n at least two ways. A suspended, sediment load provides a 75 r a i n of p a r t i c l e s that t r a v e l f a r • i n t o the S t r a i t and set t l e onto the sea bottom. If this was the only method by which sediments were distr i b u t e d , the r e s u l t should be a mantle of sediments covering ridges as. well'as the basins, the thickness being related to the depth of water column overhead. Low f l a t areas such as the c o l between the northwest and south- " 'east parts of Sangster Ridge should be t h i c k l y covered, with a layer at least half as thick as occurs i n the basins on either side of i t . However, the basins have four to six times the thickness of sediment as the low neck of the ridge. Other ridge areas are bare of sediments, as are many of the sidewalls of the basins. On slopes of less than about 10 degrees there may or may not be a sediment cover. Steeper slopes are i n -variably bare but where sedimentary cover i s absent there i s •no disturbance or unevenness. at the base of the slope suggest-ing slumping or s l i d i n g has occurred to remove i t . In f a c t , the basin f l o o r commonly remains f l a t almost to the steep sides and appears to meet the side slope rather abruptly. In some places a trench or moat occurs along the base of the sidewalls, sometimes continuous for distances of several k i l o - . meters. Therefore, s l i d i n g of sediments off the basin sides does not appear to be an important process i n f i l l i n g the basins. It i s d i f f i c u l t to see how sedimentation from sus-pension can lead to f l a t basin f l o o r s or to sidewalls bare of 76 sediment. Of course, d i s p o s i t i o n of material i n suspended transport i s subject to vagaries of currents and currents are, no doubt, important i n d i s t r i b u t i n g fine material over the S t r a i t . I t i s therefore possible that currents have kept the sidewalls bare, e s p e c i a l l y those at low angles that could support sediments but do not. The presence of trenches along the sidewalls may also be due to current action. L i t t l e i s known of sub-surface currents i n the basin area. They may be strong enough to erode trenches or at least to prevent deposition i n them. But even with currents maintaining bare sides and d i s t r i b u t i n g sediments over basin f l o o r s , i t i s doubtful that suspended sediments alone c o u l d - ' f i l l the basins in the f l a t - f l o o r e d manner recorded on the p r o f i l e s of Ballenas Basin. The presence of many strong r e f l e c t o r s that s t a r t . a t the delta and gradually weaken with distance from i t , extend-ing for many kilometers with a low slope and a plane surface (Plate I ) , and which, i n at least some places, appear to be contained within banked sides, a l l tend to support the ex i s t -ence of tu r b i d i t e layers i n the basin. Turbidity currents from the Fraser Delta which dropped their heaviest material f i r s t , near the base of the steep, delta front, could lead to r e f l e c t o r s having strong r e f l e c t i o n c o - e f f i c i e n t s where coarse material i s present, but weakening as the coarser f r a c t i o n thins out with distance from the source. Smaller t u r b i d i t y currents would probably vanish, l i k e some of the r e f l e c t o r s , 7 7 before the end of the basin i s reached. Stronger currents, perhaps from higher on the delta, or charged with a greater load, would p e r s i s t longer and reach the end of the basin. Turbidity currents tend to flow within low levees and leave a f l a t - s u r f a c e d area i n their wake. The p r o f i l e s show areas where i n t e r n a l r e f l e c t o r s terminate at low i n c l i n e s near the basin edges. Good examples occur near p o s i t i o n C on Plate X and p o s i t i o n D on Plate XI. Turbidity currents also leave moats or trenches around obstructions (Hamilton, 1 9 6 7 ) which could explain these features in Ballenas Basin. In an attempt to determine i f tu r b i d i t e s were present i n the upper few meters of the basin sediments, two cores were obtained from the f l a t f l o o r of Ballenas Basin at posi-tions marked i n Figure 5 . No layering was v i s i b l e i n either core and sample analysis did not reveal any systematic trends with depth. However, only three intervals i n each of the seven foot cores were analyzed. I t i s possible, too, that organic a c t i v i t y on the basin f l o o r s may have destroyed any gradation caused by t u r b i d i t e s . Sand content i n the anal-yzed samples was one to seven percent. The presence of that much sand can be explained by bottom transport i n the nature of density or t u r b i d i t y currents,_but some medium and f i n e sand i s also present i n the suspended load of the Fraser River (Johnston, 1 9 2 1 ) . Thus the cores obtained neither confirm nor r e j e c t the t u r b i d i t y current concept i n this basin. 78 In Malaspina Basin the upper 30 to 60 meters (100 to 200 foot) of sediment i s more transparent than s t r a t i f i e d , i n d icating the presence of homogeneous rather than graded sediments. Thus, recent turbidites,are not suspected i n this layer. Plates II to IV, which cross Malaspina Basin, show deeper sub-bottom r e f l e c t o r s similar to those of Ballenas Basin. Turbidity currents therefore may have occurred i n the past. However, as the r e f l e c t o r s slope to the.south, bottom transport i s l i k e l y to have been i n that d i r e c t i o n as w e l l . Since the north end of Malaspina Basin i s unexplored, sources of t u r b i d i t e s i n that d i r e c t i o n are unknown. Compaction of Unconsolidated Sediments Compaction of the sedimentary layers has undoubtedly occurred and should be observable on the seismic p r o f i l e s . Compaction can be recognized by sagging of bedding where i t is not supported by underlying rock (Cone, et a l , 1963). Therefore, the thicker the sediment, the greater should be the degree of compaction. Thus, i f one assumed that a h o r i -zon was o r i g i n a l l y plane to the point where i t abutts the rock sides of a basin, by measuring the difference i n depth presently exhibited by the horizon where i t i s supported by rock edges and i n the c e n t r a l areas where i t i s not, an estimate of compaction can be made. Several d i f f i c u l t i e s are obvious i n attempting such a measurement. The upper layers of sediment tend to smooth out the depressions l e f t by previous compaction. The top layers are therefore poor-indicators of t o t a l compaction.' As well, i n the delta area the present upper layers are fore-set beds and are sloping rather than f l a t - l y i n g . In Ballenas Basin, the top of sedi-ments sometimes rests f a r up the sides of the rock walls or else turns down into a moat at the edges. These are not areas where compaction can be measured with r e l i a b i l i t y . Hyperbolic r e f l e c t i o n s from steep rock sides tend to obscure rock-sediment contacts•of many otherwise suitable s i t e s so that the top of sediments at the sides cannot be seen. Despite the above d i f f i c u l t i e s an estimate of the degree of compaction was made i n Ballenas Basin using the p r o f i l e of Plate I. The northwest end of the lower Ej_ layer i s seen to be draped up the end wall of the basin and over various bedrock humps along the bottom. Subject to the assumption previously made, that the horizon was o r i g i n a l l y plane and horizontal, the H-j_ sediments there have s e t t l e d some 45 meters or about 3° percent of their o r i g i n a l depth of 135 meters i n this area. I I I . ELEVATED AREA OF RIDGES Along the mainland side of the S t r a i t an area eleva-ted above the basin f l o o r s extends from Thormanby Islands i n the northwest to Burrard Peninsula i n the east. Forming an upland upon which are situated two higher ridges, the 8o area increases i n width from 2.\ kilometers near Thormanby Islands to a maximum of 13 kilometers before decreasing again toward Burrard Peninsula. West of Point Grey, Queen Charlotte Trench, 240 meters' (130 fathoms) i n depth, cuts through the elevated area into Howe Sound. However, Burrard Peninsula and Burrard Inlet to the east are sb well related morphologi-c a l l y and geologically to the ridge and bank areas to the west that they are included i n that area, although not speci-f i c a l l y a part of the present study area. The two afore-mentioned ridges extend i n a northwest-southeast d i r e c t i o n along the elevated area. McCall Ridge, nearest the mainland coast, i s longest and covers much of the elevated area. The southeastern toe of this ridge may extend under the Fraser Delta sediments i n a di r e c t i o n p a r a l l e l or sub-parallel to Burrard Peninsula. Halibut Ridge, on the edge of the elevated area, i s also long and narrow, although smaller i n area than McCall Ridge. Both ridges support banks that r i s e to within a few meters of the sea surface. A r e l a t i v e l y f l a t basin area, c a l l e d here Sechelt Basin, extends, along the base of the mainland slope at a . depth of 170 meters (92 fathoms). The flatness of the sea f l o o r marks th i s basin as an area of thick sediments. In the west the basin drops to a terrace l e v e l below 200 meters (110 fathoms) at the end.of McCall Ridge. In the east, a deep canyon i s incised from the basin l e v e l to the f l o o r of Queen ; 8 1 Charlotte .Trench 75 meters (40 fathoms) below. A remarkably deep hole occurs i n the fl o o r of this canyon. Sounded to a depth of 292 meters (160 fathoms) the hole, c a l l e d Jones Deep after a fabled character of the sea, has ext r a o r d i n a r i l y steep sides for an area of rapid deposition. Mean water depths surrounding the hole are 200 meters (110 fathoms). The f a c t that i t i s there, at a l l requires explanation. The hole i s discussed further i n a l a t e r section. Three other ridges e x i s t i n the northwestern S t r a i t . Their d i f f e r e n t topographical outlines suggest at lea s t some basic differences i n the i r structure. Round Ridge, the most westerly on the slope north of Malaspina Basin, i s , as i t s name implies, a round knob which r i s e s to•approximately the same height as- the elevated terrace near i t . Farther east on the same slope, South Ridge, actually two separate peaks of li m i t e d height, projects from the base of the slope near the f l o o r of Ballenas Basin. Sangster Ridge, a broad, nearly f l a t topped ridge separating Malaspina and Ballenas basins, extends i n an almost east-west d i r e c t i o n rather than i n the usual southeast-northwest d i r e c t i o n of the other ridges. A low neck, or c o l , connects the ridge to an eastern section consisting of several small peaks r i s i n g from a low base. 82 Unconsolidated Sediments Plates II to XIV cross the elevated area and should be referred to i n this section. A mantle of unconsolidated sediments covers much of the area. Near the Fraser Delta . the layer i s thick. In f a c t the southernmost part of McCall Ridge (Plate XII) i s completely buried by d e l t a i c sediments. Queen Charlotte Trench i s -also covered with more than 180 meters (600 feet) of similar sediments. Away from the delta, the sedimentary mantle thins except i n the low areas between ridges and i n Sechelt Basin. West of the highest points of McCall and Halibut ridges, ridge sediments are thin or absent over many areas, even where the ridges are f l a t . In Sechelt Basin unconsolidated sediments range i n thickness to 60 meters (200 feet) or more. The basin sediments are f l a t - l y i n g as far northwest as Mission Point, after which they thin con-siderably and show much r e l i e f as the basin drops to a lower terrace l e v e l . Unconsolidated sediments on the ridges and i n Sechelt Basin are mainly of the 'transparent 1 type, marked H^ on the l i n e drawings. Since they become progressively thinner away from the Fraser Delta area, this i s their obvious source. The ridge sediments j o i n l a t e r a l l y with d e l t a i c sediments of the H 2 and layers close to the delta (Plates' XI and XIII). Where this occurs, weak s t r a t i f i c a t i o n i s v i s i b l e i n the layer, possibly indicative of heavier grain-size material 83 closer to the source. The H-^  sediments are seismically homogeneous, probably hemipelagic sediments brought by the Fraser River to the S t r a i t where they have settled from suspension. Some material may come from other r i v e r s and streams entering the S t r a i t , or from i n l e t s such as Howe Sound which have large r i v e r s at their head but, compared to the Fraser R i v e r , t h e s e sources are n e g l i g i b l e . The manner i n which transparent sediments react to sound waves i s worthy of mention. A recent continuous s e i s -mic p r o f i l e by R.. Stacey (Dominion Observatory, personal communication, 19.68) i n which only frequencies less than 80 Hertz were recorded, barely detected the transparent layer, even though i t i s t h i r t y or more meters thick. The p r o f i l e , of Plate X over Sechelt Basin, recording up to 300 Hertz, shows, the transparent layer as a strong si g n a l . However, i t s multiple, which should occur at twice the t r a v e l time or at approximately 0.440 seconds between positions E and F, i s not present a t . a l l . A multiple at 0.5 seconds corresponds to the f i r s t buried.sediment-sediment interface or uncon-formity r e f l e c t e d from the sea f l o o r . This multiple carries the i r r e g u l a r i t i e s of the unconformity as proof of i t s o r i g i n . A multiple of the same interface r e f l e c t e d from i t s e l f rather than the sea bottom l i e s at 0.55 seconds. There i s no multiple of the sea f l o o r . From these facts i t may be deduced that the transparent layer must have a greater 84 r e f l e c t i o n c o e f f i c i e n t for high frequencies, while low f r e -quencies apparently are transmitted through the layer rather than r e f l e c t i n g from i t . Coring indicates the transparent sediments are very soft, with a high percentage of water and possibly a 'soupy' sediment-water interface rather than a f i r m surface. Samples of the blue-grey mud ooze through the fingers with l i t t l e pressure. Corers dropped into the layer return to the surface with the appearance of having been buried at lea s t as deeply as the shackle on top of the core b a r r e l . After drying, samples of these sediments shrink to about one half their o r i g i n a l s i z e . Beneath the transparent layer a strong r e f l e c t o r marks a second unconsolidated layer over the elevated area, marked H on the li n e drawings. I t i s present mainly under Sechelt Basin and i n low depressions such as the valleys between ridges. The layer i s thin and r e l a t i v e l y f l a t sur-faced and, l i k e the lower H-^  layer i n Ballenas Basin, has smoothed out much of the i r r e g u l a r i t i e s of the bedrock sur-face upon which i t r e s t s . Its occurrence, being confined to depressions or low areas, i s somewhat patchy. A similar lower layer buried under more recent sediments i s evident i n many sedimentary pockets on the island slope as w e l l . The general presence of an older layer i n the S t r a i t i n d i -cates some change i n sedimentation of the S t r a i t . No diastem i s known, but a change i n pattern of sedimentation occurred i n recent times when Fraser River sediments overflowed Fraser Ridge d i r e c t l y . i n t o Ballenas Basin. Before that time, sedi-ment transport was apparently v i a a route south of the ridge. By entering at the side of the basin, the path to any point i n the north and western S t r a i t was decreased, thus the sedi-mentation rate, as well as p a r t i c l e size s e t t l i n g i n the area, may have increased. Sediments of -the lower unconsolidated layer are almost always f l a t - l y i n g and found only i n basins or depressions. Perhaps currents at the time of their depo-s i t i o n were strong enough to remove material from areas other than depr e s s ions. Echograms and CSP p r o f i l e s over South Ridge show that, although near the Fraser Delta, the' thick sedimentary cover of nearby areas i s also mainly lacking. No sediments are observed anywhere on Round Ridge while on Sangster Ridge, sediments occur mainly on the low, flat-topped c o l between the main ridge and i t s eastern toe. Although the c o l i s just 30 to 50 meters above the basins on each side, sediments reach only 15. to 30 meters (50 to 100 feet) i n thickness. Most-other areas of the ridge are bare. . Pleistocene Sediments A strong unconformable horizon marks the base of unconsolidated sediments. Beneath this horizon several seismic events, i n what are believed to be Pleistocene sedi-ments, are evident on the p r o f i l e s . The most s t r i k i n g of 86 these underlies Halibut and McCall Ridges and the intervening low area. Several hundred meters of well s t r a t i f i e d r e f l e c -tors, i d e n t i f i e d on the l i n e drawings by the symbol P ^ and given the name McCall Ridge unit, are mainly horizontal or dipping s l i g h t l y to the west with gentle f o l d s . They com-pose a large part of the sub-bottom ridge structure. S t r a t i -f i e d areas are interspersed by areas of d i f f e r i n g seismic character marked P n. These show only limited s t r a t i f i c a t i o n or none whatsoever. Some appear completely chaotic i n structure. Correlation of the P n areas i s not usually possible from record to record because of rapid changes or fadeouts i n both v e r t i c a l and l a t e r a l d i r e c t i o n s , but the wel l s t r a t i f i e d McCall Ridge unit provides a good marker that can be recognized over many p r o f i l e s . While i n d i v i d u a l s t r a t a cannot be correlated, the top and bottom of the unit can be picked with a reasonable degree of certainty. The P ^ unit mainly overlies deeper bedrock whereas areas of n o n - s t r a t i f i e d or chaotic P n r e f l e c t i o n s occur upon or between the s t r a t i f i e d unit. This sequence i s well i l l u s t r a t e d i n Plate VII. Here, s t r a t i f i e d r e f l e c t o r s underlie the terrace area north of Malaspina Basin. In an overlying P n unit, events are either chaotic or unevenly and weakly s t r a t i f i e d . A P n unit rests under the peak of McCall Ridge on this p r o f i l e . Most of the elevated area and the ridges upon this area are underlain by similar sections. The well s t r a t i f i e d McCall Ridge unit 87 can be correlated from p r o f i l e t o ' p r o f i l e from Queen Charlotte Trench to at least, as f a r northwest as the end of McCall Ridge, a distance of 42 kilometers. From there to Thormanby Islands, a distance of f i v e kilometers, bedrock possibly outcrops i n many places with the P ^ unit between. The few p r o f i l e s over the outcrop area are i n s u f f i c i e n t to show exact d e t a i l , but elongate topographic highs along the terrace edge (Plates I I I and IV) are, l i k e Round Ridge and South Thormanby Island, probably of bedrock o r i g i n . The P ^ unit continues to underlie the terrace west of Thormanby Islands as well as a reef which continues to the north of these islands (Plate I I ) . Based on the sea f l o o r contours i n that d i r e c t i o n , i t may extend as far as Bjerre Shoal, seven kilometers beyond the study area. The age of the ridge structure.below the unconsoli-dated sediments but above the bedrock i s almost c e r t a i n l y Pleistocene. Pleistocene ridges trending i n the same di r e c t i o n and having the same general configuration occur on land. Burrard Peninsula, for instance, i s a Pleistocene ridge overlying Tertiary bedrock. I t ends i n sea c l i f f s not f a r from McCall Ridge. Pleistocene t i l l s , d r i f t , out-wash and other g l a c i a l and i n t e r - g l a c i a l features are common throughout the S t r a i t of Georgia region, including some of the shorelines around the study area. Some of these sedi-ments are unstructured, others, such as at Point Grey, are 88 well s t r a t i f i e d and f l a t - l y i n g . P r o f i l e s that pass or terminate near known Pleistocene shorelines .usually record seismic structure with these, c h a r a c t e r i s t i c s . The north end of Thormanby Island nearest the reef i s underlain by a thick sub-aerial section of s t r a t i f i e d Pleistocene sediments. These sediments are well exposed on a 60 meter (200 foot) c l i f f overlooking the reef. Similar sediments occur on Savary Island and other large islands i n the northern S t r a i t of Georgia (Bancroft, -1913) • They are also found on Vancouver Island (Fyles, 19 ^ 3 ) and on the mainland (Johnston, 1923; Burwash, 1 9 l 8 ) . As mentioned previously, they outcrop on c l i f f s at Point Grey, Burrard Peninsula (Armstrong, 1956) not far from the eastern end of McCall Ridge. Their common occurrence and widespread d i s t r i b u t i o n has led to speculation that at one time they completely covered much of the S t r a i t :, of Georgia (Bretz, 1913; McConnell, 1 9 1 4 ) . It i s therefore not surprising to f i n d sediments under the submerged ridges whose morphological and geophysical character i s suggestive of those found under sub-aerial conditions. Dredging and grab sampling i n areas of outcrop on McCall and Halibut Banks have recovered angular to sub-rounded pebbles and cobbles, many with g l a c i a l l y carved s t r i a t i o n s . A P l e i s t o -cene age for the ridges i s therefore a reasonable assumption. As suggested above, i t i s possible that the McCall Ridge unit i s composed at l e a s t i n part, of sediments similar 8 9 to those on Thormanby and other islands. I t probably pre-dates at least the l a t e s t , or Vashon, stage of C o r d i l l e r a n g l a c i a t i o n . The thick overlying chaotic events may then be t i l l or u n s t r a t i f i e d d r i f t from later g l a c i a t i o n . A sequence of t l l l - s t r a t i f i e d i n t e r g l a c i a l s ediments-till occurs over nearby land areas, (Clapp, 191H-; Fyles, 1963) . The ridges may also be much older Pleistocene structures. In f a c t , since Vashon sediments on land are r a r e l y greater than 30 meters (100 feet) thick and commonly less than 13 meters (40 feet) thick, the overlaying material may consist of much older t i l l or d r i f t . ' The thickness of the McCall Ridge unit varies con-siderably over the area of ridges. North of Thormanby Islands, i t approaches 500 meters ( 1 , 6 5 0 feet) i n thickness. If the Pleistocene c l i f f s above sea l e v e l are included as part of this unit, 560 meters ( 1 , 8 5 0 feet) of section i s represented. Toward the southeast, the thickness i s l e s s , but s t i l l considerable. Except for the area south of Thorm-anby Islands where the unit seems to disappear almost e n t i r e l y , the thickness i s nowhere less than 150 meters (500 f e e t ) . Two other occurrences of similar seismic r e f l e c t o r s are t e n t a t i v e l y i d e n t i f i e d as part of the McCall Ridge unit. The f i r s t , on the east side of Lasqueti Island slope, occurs as a thick, though possibly small layer d i r e c t l y across from the same unit o f f Thormanby Islands. In f a c t , the impression 90 gained from the one p r o f i l e over the area, Plate I I , i s that the r e f l e c t o r s were at one time continuous across Malaspina Basin. Individual r e f l e c t o r s , however, cannot be matched across the basin. A second small area on the south side of Sangster Ridge (Plate IV) also represents a thickness of several tens of meters. I t i s not present on adjacent p r o f i l e s from either side and therefore must be li m i t e d i n l a t e r a l extent. At one time, the P ^ layer may have been much more extensive than at present. The truncated s t r a t a on ridge sides, i t s great thickness and occurrence on both sides of Malaspina Basin point to this p o s s i b i l i t y . Although pres-ently found mainly on the mainland side of the study area, the occurrence on the south side of Sangster Ridge suggests that i t may also have existed i n Ballenas Basin. Later erosion, perhaps by g l a c i e r s , may. account for i t s removal. Other ridges are not underlain by the same well-s t r a t i f i e d P ^ r e f l e c t o r s . Some s t r a t i f i c a t i o n does appear at South Ridge but i t i s deformed and not continuous. This ridge may be older than the McCall Ridge unit but r e l a t i o n -ships are not cert a i n because of many unconformities i n the Pleistocene record between the two ridges. '• Round Ridge, l i k e other bedrock areas, has no in t e r -n a l seismic character at a l l . Even a gas-exploder p r o f i l e across i t shows no i n t e r n a l structure. No Pleistocene 91 sediments appear on i t . Its connection with surrounding bedrock areas i s not known but i t i s suspected to be a gr a n i t i c plug or boss similar to those found on Texada Island (McConnell, 1914) . The eastern end of Sangster Ridge and the c o l con-necting i t to the main body of the ridge are, below the unconsolidated sediment.cover, seismically similar to Round Ridge. No coherent i n t e r n a l seismic character i s apparent. However, under the main western part of the ridge, a con-tinuous horizon does appear deep i n the structure, d i v i d i n g i t into upper and lower layers. Few r e f l e c t i o n s occur above or below this horizon and i t is not known i f the horizon marks the top of bedrock or i s an event i n bedrock. The P ^ unit occurring on Sangster Ridge (Plate IV) i s overlain by, and therefore older than, the layer above the horizon and on this basis the upper layer, which includes much of the ridge, has been assigned a Pleistocene age. Dredging on Sangster Ridge has provided no fr e s h l y fragmented bedrock p a r t i c l e s despite the fact that the dredge wire pulled taut several times as the dredge caught on the bottom. Mud, very fine sand, pebbles, some coated with manganese dioxide, and angular cobbles were dredged. The bottom appears to be very irregular judging from the way the dredge was continually snagged as i t dragged along the sea f l o o r . The ridge may be, i n part, morainal i n o r i g i n . . 92 Lateral moraines of glaciers moving southeast on each side of Texada and Lasqueti Islands may have joined as a medial moraine, depositing large quantities of debris on the ridge s i t e . Bedrock Underlying the unconsolidated sediments of Sechelt Basin and the Pleistocene deposits of the ridges, bedrock r e f l e c t o r s dip off the mainland slope toward the central basin. The r e f l e c t o r s are truncated by the erosion surface of the bedrock. S t r a t i f i c a t i o n terminates just offshore a l l along the mainland coastline. The bedrock of the mainland slope i t s e l f i s u n s t r a t i f i e d . The s t r a t i f i e d bedrock i s designated by the symbol B^ and occurs from Queen Charlotte Trench to the end of McCall Ridge southeast of Thormanby Islands. Deep under the Pleistocene units of the elevated ridge area, the s t r a t i f i c a t i o n cannot be seen, but the h o r i -zon marking the top of bedrock can s t i l l be followed toward the l e v e l of bedrock beneath the basins. Whether or not the horizon continues to represent the same event i s unknown. I t i s therefore marked with the l e t t e r 'B'. Although a de f i n i t e contact cannot be seen on the p r o f i l e s , the s t r a t i -f i e d bedrock almost c e r t a i n l y overlies the c r y s t a l l i n e rocks of the mainland slope. These l a t t e r rocks, mainly Coast Range intrusives extending from Howe Sound to the northwest, are either not penetrated by the seismic signal or show no •93 i n t e r n a l r e f l e c t o r s . By projecting the dips of the over-l y i n g B t s t r a t a , an apparently unconformable contact r e s u l t s under Sechelt Basin. In the southeast a previously mentioned canyon occurs at the contact with the mainland rock on one side and truncated s t r a t a on the other (Plate X and Figure 8) . The s t r a t i f i e d bedrock, overlying as i t does probable Coast Range intr u s i v e s , i s possibly related to the Burrard-. K i t s i l a n o Formations of Late Cretaceous-Early Tertiary age near Vancouver. Sub-aerial bedrock exposures of these formations outcrop near K i t s i l a n o and other areas on.Burrard Peninsula. They dip 'to the south at eight to ten degrees and are overlain unconformably by thickening deposits of P l e i s t o -cene age. On parts of the southern slope, these exceed 215 meters (700 feet) i n thickness- (Armstrong, 1956) . Similar conditions are evident under McCall Ridge (Plates VIII, IX and X), with s t r a t a dipping off the Coast Range intrusives and covered by Pleistocene sediments thickening to the south-west. The difference i n structure between the undersea ridge and. the sub-aerial peninsula i s mainly i n the -layer of unconsolidated marine sediments covering much of the former. The s t r a t i f i e d bedrock along the mainland side i s seismically similar to bedrock st r a t a (designated B^) along the Gulf Island slope. Bedrock of both areas shows similar i n t e r n a l s t r a t i f i c a t i o n . How much of the bedrock f l o o r of the S t r a i t of Georgia i s underlain by these r e f l e c t o r s and 94 to where they go on. the Vancouver. Island side west of Gabriola Reef i s not known from the data available. A d i l i g e n t search of the p r o f i l e s gives some ind i c a t i o n of the presence of i n t e r n a l bedrock s t r a t i f i c a t i o n under var-ious parts of Malaspina 'and Ballenas Basin, but lack of continuity prevents p o s i t i v e c o r r e l a t i o n . The bedrock under the. basins may well be T e r t i a r y as i t i s under Whatcom Basin i n the Fraser Lowland. At Fraser Ridge, for example, weak s t r a t i f i c a t i o n under the deepest horizon may represent the top of T e r t i a r y (?) bedrock (Plate XXIII). The buried eastern section of Finger Ridge (Plate XVII) shows- an a n t i -c l i n e evidently formed i n the same strata. The gas exploder p r o f i l e of Plate XIX also indicates an a n t i c l i n e i n bedrock west of Fraser Ridge. However, the record i s d i f f i c u l t to interpret. South Ridge could also be s t r a t i f i e d bedrock but the upper section of at least one, and perhaps both, of the two peaks i s l i k e l y Pleistocene sediments. The lower layer of Sangster Ridge i s possibly bed-rock, but of what age or relationship to other bedrock of the area i s not known. No correlatable s t r a t i f i c a t i o n or other i d e n t i f i c a t i o n i s apparent. The bedrock exposures on the edge of the elevated terrace area north of McCall Ridge may be older rock related to the pre-Upper Cretaceous on South Thormanby Island. Small topographical features oh the 95 nearby elevated terrace area.and on the mainland side of Malaspina Basin are seismically unstructured bedrock. Round Ridge i s similar i n character and no doubt related. Since the eastern toe of Sangster Ridge i s bedrock of the same seismically unpenetrable character (Plate VII), these features may be a continuation of the pre-Upper Cretaceous arch that separates the Nanaimo Lowlands into two basins at Nanoose Bay. More detailed work i n this area may show i f that i s the case. IV. ROBERTS SWELL AND THE NEARBY MAINLAND SHELF South of the 49th p a r a l l e l a smooth dome-shaped topographical high l i e s at., the foot of the mainland slope off Point Roberts. Extending almost f i f t e e n kilometers across the S t r a i t toward the Gulf Islands, this area, c a l l e d Roberts Swell i s , geomorphically and on the basis of other evidence to be presented, not a part of the present Fraser Delta. Its surface i n most places i s smooth and featureless with a gentle slope to the southwest and west. On the south-west side i t i s bounded by the long l i n e a r U-shaped Trincomal Trough, and on the southeast by Boundary Basin. The mainland slope above Roberts Swell d i f f e r s north and south of Point Roberts. To the north the slope angle i s s l i g h t l y less than the upper delta front» However, at Point Roberts the slope steepens- considerably. Water depths drop 96 almost immediately offshore to 110 meters (60 fathoms) with no evidence of a shelf. For several kilometers southeast of Point Roberts the slope i s formed by Roberts Reef, whose western side i s steep and i r r e g u l a r . South of the end of the reef, the mainland slope becomes smooth and regular, angling more gently downward to the f l o o r of Boundary Basin. This slope c h a r a c t e r i s t i c continues as far as Alden Ridge, a structure p a r t l y buried by the mainland slope sediments. Like Roberts Reef, Alden Ridge forms a s t r u c t u r a l b a r r i e r to shelf sediments, ponding them on i t s eastern side, but there i s no evidence to suggest that the ridge i s a part of, or similar to Roberts Reef. A wide shallow bay,'Boundary Bay, occurs to the east of Roberts Peninsula and Roberts Reef, on the mainland shelf. Plates XXIV to XXX show p r o f i l e s over' Roberts Swell, including one gas exploder record obtained by S h e l l Canada Limited. Five p r o f i l e s run transversely across the S t r a i t ; one p r o f i l e , Plate XXX, i s perpendicular to the others, run-ning along the axis of the S t r a i t . The Roberts Swell Unit The p r o f i l e s show the smooth and r e l a t i v e l y f l a t -topped Roberts Swell with prominently steepened sides on the southwest, south, and southeast. In the northwest, the surface of the swell slopes down.at a low angle toward Ballenas Basin and merges with the lower delta slopes. 97 Penetration achieved i s much better than on the upper delta. Instead of the i n t e r n a l roughness and random s t r a t i f i c a t i o n common on southern delta p r o f i l e s i n equiva-lent water depths, the records show a weak but smooth int e r -nal s t r a t i f i c a t i o n under Roberts Swell. The r e f l e c t i o n s mark the Roberts Swell unit, denoted P r s on the l i n e drawings. S t r a t i f i c a t i o n i s mainly conformable, at least under the smooth top, with the sediment-water interface. L i t t l e i n t e r n a l s t r a t i f i c a t i o n i s present at depths greater than t h i r t y to sixty meters below this interface. That energy does penetrate through the sediments i s shown by the under-l y i n g units, marked P s, recorded below the base of the P r s sediments. Lack of strong r e f l e c t i o n s i n the P r g unit i n d i -cates the sediments are almost seismically homogeneous throughout the section. Roberts Swell i s separated from the Gulf Island slope by Trincomali Trough. Sub-surface r e f l e c t o r s underlying the top of the swell i n this region remain conformable to the sediment surface, even as i t curves into the central region of the trough. S t r a t i f i c a t i o n turns down and termin-ates abruptly against the r i s i n g bedrock of the island slope (Plates XXVI, XXVII and XXVIII). Roberts Swell sediments occur under only part of the trough leaving at.least the western half floored by bedrock. The Roberts Swell unit i s not eroded here. 93 Farther south, sub-bottom s t r a t i f i c a t i o n of Roberts Swell i s truncated by the side of Trincomali Trough as i t curves to the east into Boundary Basin (Plate XXIX). In th i s area the trough, which has cut deeply into the sea f l o o r , has established i t s e l f i n Roberts Swell sediments at the bedrock contact. Although the trough f l a t t e n s out to the east and disappears i n Boundary Basin, truncation of bedding remains c h a r a c t e r i s t i c of the southern and south-eastern borders of Roberts Swell (Plate XXX). Here, a scarp formed by the ends of the truncated bedding stands at angles of f i v e degrees or more for heights of 30 to 60 meters (100 to 200 f e e t ) . Internal r e f l e c t o r s of the Swell, which here dip gently to the northwest, are truncated at the scarp or on the top of the swell above the scarp. . The scarp marks the l i m i t of Roberts Swell as a morphological feature, but ;deeper s t r a t i f i c a t i o n i n the Roberts Swell unit can be traced into Boundary Basin to the south. The unit therefore extends beyond the l i m i t s of the swell i n that d i r e c t i o n . The sharp uprise of a bedrock feature under the scarp (Plate XXX) i s due to the ship's track angling up the side of a ridge buried under the eastern part of the swell. The ridge i s shown c l e a r l y i n Plate XXXI. Southeast of Roberts Swell and contiguous with i t , a thickness of approximately 140 meters (450 feet) of s t r a t i -f i e d sediments l i e s at the base of Roberts Reef and extends 99 at l e a s t p a r t way under Boundary B a s i n before t h i n n i n g out and disappearing (see P l a t e XXXI). S t r a t i f i c a t i o n i s mainly h o r i z o n t a l w i t h gentle l o c a l d i p s . These sediments, too, show evidence of some t r u n c a t i o n by the present sea f l o o r . This f a c t , coupled w i t h the smoothness of the sea bottom contours j o i n i n g t h i s area to Roberts S w e l l , and the known extension of Roberts S w e l l type sediments i n t o Boundary B a s i n , r e l a t e these sediments to the P r s u n i t . Despite the p r o x i m i t y of the r i v e r mouth and the presence of overlapping d e l t a sediments, the seismic p r o f i l e s show no evidence Of any d e l t a sediments (H3) o v e r l y i n g the f l a t , smooth top of the s w e l l . P l a t e s XXIV and XXV, over the northern p a r t of t h i s r e g i o n , and P l a t e XXX, the l o n g i t u d i n a l p r o f i l e , do, however, c l e a r l y show upper d e l t a sediments t r a n s g r e s s i n g over t y p i c a l Roberts Swell r e f l e c t o r s at the side nearest the d e l t a . Where the overlap begins, there i s a change not only i n the character of the seismic s i g n a l , but i n the sediment-water i n t e r f a c e as w e l l . The sediment surface changes from rough' d e l t a slope topography to the smooth.top of the. s w e l l . There i s al s o a marked decrease i n the angle of slope at the base of the encroaching d e l t a sediments. The smooth top, w i t h a slope of only about four p a r t s per thousand or f i f t e e n minutes of a r c , i s completely u n c h a r a c t e r i s t i c of the present d e l t a f r o n t . Furthermore, i n t e r n a l r e f l e c t o r s marking s t r a t i f i c a t i o n i n the swell' tend 100 to dip down and pass under the delta sediments. I t i s therefore clear that Roberts Swell sediments are not only d i f f e r e n t seismically from modern del t a i c sediments but, because they underlie the delta, they must represent a pre-modern-delta episode of sedimentation. Recent dredge hauls made up-slope between water depths of 210 to 140 meters (116 to 78 fathoms) on the south-ern marginal scarp of Roberts Swell show these sediments to be dark grey, muddly sands. Many irregular nodules and pele-cypod casts of hard brown c a l c i t i c or phosphoritic material were recovered. Much broken s h e l l material was dredged along with sub-angular cobbles and pebbles of Igneous o r i g i n . Several chunks of poorly consolidated clinkers of sandstone were also found to be embedded with shells and s h e l l f r a g -ments. I f Roberts Swell type sediments are semi-consolidated, that would explain the better seismic penetration achieved there than on the delta where the sands are younger and less consolidated. Past sampling on the swell has provided the r e s u l t s l i s t e d i n Table I I , with an average composition of glacio-marine d r i f t of the Fraser Valley for comparison. The sediments have a high sand and clay content. Mathews and Shepard (1962, p. 1492) postulated that the high i n c i -dence of sand south of the Fraser River may be due to either r e l i c t sands, or to a gating action of bottom currents i n the r i v e r by t i d e s . The incoming flo o d t i d e , which sends a 101 saline wedge of marine water under the fresh r i v e r water, was thought to check the bottom current and .thus the bottom transport jof coarse grained material so that i t does not reach the r i v e r mouth during the flood stage. During ebb ti d e , the coarse grained material was suspected to"flow f r e e l y from the r i v e r to the south. But with this explana-ti o n sediment deposition should be heavy south of the r i v e r . The seismic records indicate the opposite i s true. No Fraser River sediments are s e t t l i n g upon the top of Roberts Swell. TABLE II • SEDIMENT GRAIN-SIZE ANALYSIS I0UBC Data Report No. 20, 1962 Station Latitude Longitude . Depth . Sand S i l t Clay Median No. (Fathoms) % % . % Diameter (phi units) 3. 48 5 1 . 5 123 0 7 . 0 87 42 3.9 . 19 4 . 3 5 48 5 5 - 7 123 1 3 . 1 77 52 • 34 14 • 3 . 9 5 167 48 5 5 - 5 123 0 6 . 8 65. 58 26 16 3 . 5 171 48 5 9 - 7 123 1 2 . 2 63 59 18 23 3 . 4 5 Glaciomarine d r i f t of the Lower Fraser Valley (After Armstrong > 40 1 9 5 7 , p. 4) 50 10 I t i s known that t i d a l currents . are strong over the swell. Table I I I , from Pickard ( 1 9 5 6 ) , l i s t s current'.meas-urements taken with an Ekman current meter suspended sixteen inches above the bottom i n a l i n e from Roberts Peninsula to Galiano Island. Highest current v e l o c i t i e s occur along the 102 • mainland, slope, but currents at other points over the swell are also high. TABLE III ' BOTTOM CURRENT MEASUREMENTS ACROSS ROBERTS SWELL Gabriola Island to Roberts Peninsula Bottom Currents (knots) Latitude Longitude Depth Maximum Mean (Fathoms) Flood Ebb Flood Ebb D 4 8 ° 5 5 . 4 1 2 3 ° 2 3 . 5 23 0 . 3 0 . 3 5 0 . 1 0 . 1 5 E 48 5 5 - 7 123 2 1 . 8 84 0 . 7 0 . 6 0 . 2 5 0 . 15 F 48 5 6 . 3 123 1 9 . 9 105 0 . 8 0 . 4 0 . 3 5 0 . 1 5 G 48 5 7 . 6 123 1 4 . 5 78 0 . 4 0 . 6 0 . 1 5 0 . 2 H 48 5 9 . 3 123 0 8 . 4 54 0 . 9 5 0 . 7 0 . 5 0 . 3 J 48 5 9 . 6 123 0 7 . 0 23 0 . 3 0 . 3 5 0 . 1 5 0 . 1 5 (After Pickard, 1956) Surface c i r c u l a t i o n i n the S t r a i t i s , i n general, counter-clockwise with a predominant northward movement along the mainland side (Waldichuk and Tabata, 1955)• I f the same c i r c u l a t i o n holds near the bottom of the S t r a i t , main sediment transport should be to the north along the coast, with a consequent reduction i n sedimentation i n the Roberts Swell area. This i s i n agreement with the contin-uous seismic data. . The high percentage of coarse f r a c t i o n found on Roberts Swell and to the north could be due to winnowing by t i d a l currents r e s u l t i n g i n a lag deposit of coarser material being l e f t behind while the l i g h t e r material i s swept farther away. That erosion does, or did, occur at Roberts Swell demon-103 strated by the presence of truncated bedding on the sea f l o o r , pointed out previously at the southeastern margin of • the. swell. With the current v e l o c i t i e s measured, erosion of the surface of the swell i s possible and may s t i l l be occurring. Sand and coarse material from the swell could then be swept north along the mainland slope by a counter- 1 clockwise current. This would account for heavy sand concentrations south of the main r i v e r mouth. Thickness of.the Roberts Swell unit. Only one pro-f i l e , the gas exploder record of Plate XXVII, penetrates to the base of Roberts Swell sediments over a complete traverse. On t h i s p r o f i l e , greater s t r a t i f i c a t i o n i s observed through the depth of sediments because of. the greater output power of this equipment. The configuration of the basin containing the sediments i s well marked. Most surprising i s the extreme thickness of the Roberts Swell unit. Assuming a v e l o c i t y of 1 , 8 0 0 meters per second ( 6 , 0 0 0 feet per second), perhaps a conservative estimate, a maximum thickness of 455 meters ( 1 , 5 0 0 feet) i s obtained. An average thickness of 390 meters ( 1 , 2 5 0 feet) i s maintained over a distance of eight kilometers or more across the S t r a i t . An area of more than 100 square kilometers i s covered by the sediments. The t o t a l volume 104 may therefore be of the order of 40 cubic kilometers, not including those sediments outside Roberts Swell which are i d e n t i f i e d with the P^. unit. Age and source. The Roberts Swell unit overlies and therefore post-dates both Roberts Peninsula and Roberts Reef. Sea c l i f f s on Roberts•Peninsula are composed i n part of sedi-ments of the Olympia I n t e r g l a c i a l episode immediately pro-ceeding the l a s t major g l a c i a t i o n . The Roberts Swell unit i s therefore younger than the Olympia i n t e r g l a c i a l . A pre-modern delta age i s imposed by the present seismic evidence and confirmed by the f i n d i n g of angular cobbles with g l a c i a l s t r i a t i o n s i n dredge hauls. Cobbles of the size and nature found could only have arrived i n that location by ice trans-portation. Whether the cobbles are part of a'glacio-marine t i l l or a cobble pavement from d r i f t ice i s not immediately apparent. The absence of g l a c i a l features on the surface of the smooth swell' and within i t s i n t e r n a l structure,, as opposed to the irregular character of Pleistocene structures nearby, suggests that deposition occurred after the departure of the l a t e s t C o r d i l l e r a n ice sheet from the S t r a i t . That i s , the sediments are most probably post-Vashon, but pre-modern-delta, i n age. Fl o a t i n g sea ice must have been abundant during the l a t e r Sumas Stade of v a l l e y g l a c i a t i o n which may account for the cobbles over the swell. Carbon-14 dates on weathered-appearing shells dredged with the cobbles 105 gave ages of l,845±l6o years B.P. and 6 ,4 l5±280 years B.P. The sediments, and the scarp i t s e l f , must be older than t h i s . The surface slope of sediments and the dip of int e r -nal s t r a t i f i c a t i o n indicate the source d i r e c t i o n was to the southeast, possibly i n the Boundary Bay area. Glacio-marine and marine deposits of post-Vashon age are found i n that area and throughout the Fraser Valley (Armstrong, 1956, 1957, I960;. Easterbrook, 1963)• The Roberts Swell sediments are possibly related to the same post-Vashon events. I t i s also possible to speculate that the Roberts Swell unit may' be part of a former delta of the Fraser River. Armstrong ( i 9 6 0 ) suggests that the present course of that r i v e r was developed i n the post-Sumas Stade. Before that time the r i v e r i s believed to have passed through the Sumas Valley south of i t s present channel to enter the S t r a i t i n the Bellingham Bay area where the Nooksack River now di s -charges. If thi s i s so, Roberts Swell sediments could represent the northern part of a previous delta. A seismic l i n e p a r a l l e l i n g the axis of the S t r a i t but six or eight kilometers west of the present l i n e would help to resolve th i s problem by providing information on the relationship of the swell to the lower and deeper sediments of the present delta, e s p e c i a l l y to the ancient bottom-set beds (HQ_) that occur d i r e c t l y upon the bedrock f l o o r . i o 6 Pleistocene Sediments Roberts Peninsula Is known to be composed of post-g l a c i a l , g l a c i a l , and non-glacial sediments of Pleistocene age. A deep well ( R i c h f i e l d Pure Point Roberts) near English B l u f f penetrated through 258 meters (847 feet) of Pleistocene sediments before reaching Tertiary (Miocene?) at 199 meters (652 feet) below sea l e v e l . Therefore, near Roberts Penin-sula a Pleistocene unit would be expected to overlie T e r t i a r y bedrock i n shallow water. At the closest approach of the survey ship to Roberts Peninsula, near Point Roberts (Plate XXVIII), the sea f l o o r i s an unconformable surface over which no d e l t a i c or Roberts Swell type sediments occur. Reflectors are c h a r a c t e r i s t i c s of those which i n other areas are known or suspected to be Pleistocene d r i f t deposits of the chaotic or n o n - s t r a t i f i e d P n type. Therefore i t i s reasonable to infer that, near Roberts Peninsula, the sea f l o o r i s composed of Pleistocene d r i f t and t i l l deposits similar to those found in c l i f f s along the shore. By co r r e l a t i o n between p r o f i l e s and by the r e f l e c t i o n character on the records, the same sediments can be extended to include Roberts Reef. These are designated by the symbol P n. Evidence of Pleistocene age would be strengthened i f a Pleistocene-Tertiary contact, probably represented by an unconformity near 200 meters (650 feet) below sea l e v e l , could be found.. Only three records show s u f f i c i e n t of the Roberts Peninsula area to be 107 able to look for such d e t a i l . From these p r o f i l e s (Plates' XXVII, XXVIII and XXIX) no d e f i n i t e conclusion can be drawn. A r e f l e c t o r does appear at about the proper depth, but i t i s not c e r t a i n i f i t i s actually the Pleistocene-Tertiary boundary or an.internal event i n the Pleistocene. The deep record provided by the gas exploder p r o f i l e of Plate XXVII i s too cluttered by multiples to show a d e f i n i t e horizon. The presence or p o s i t i o n of Tertiary bedrock i s therefore questionable in. the seismic records of the Roberts Peninsula area. Mainland Shelf Sediments Information provided by the seismic p r o f i l e s on the sub-bottom sediments of the Boundary Bay-Alden Bank mainland shelf area i s i n t e r e s t i n g but not conclusive. Plates XXXI, XXXII and XXXIII i l l u s t r a t e the sub-bottom character of this mainland shelf area. Referring to these p r o f i l e s , i t i s evident that the shelf i s underlain by at least two sedimen-tary layers, and perhaps three. The geological h i s t o r y of the layers, i s not r e a d i l y apparent. Thick unconsolidated sediments under the mainland shelf extend at l e a s t as f a r south as Alden Bank, the southern l i m i t of the study area. Shelf sediments are contained east of Alden Bank and Roberts Reef structures and overlie their lower parts. S t r a t i f i -cation i s w e l l defined and, i n the main, conformable although 108 at least one shallow unconformity is present under the shelf on the most southern.proflie (Plate XXXIII). Upper layer of shelf sediments. The upper layer of sediments, marked i n the l i n e drawings, covers most of the shelf area. In the Boundary Bay area, where i t rests con-formably over the layer below, i t i s thick, probably exceed-ing 110 meters (3^0 f e e t ) , but thins' toward the outer shelf edge by truncation and thinning out of bedding (Plate XXXII). A few kilometers farther south, near Alden Ridge, (Plate XXXIII), the upper layer i s no longer f l a t - l y i n g , but dips toward the coast to rest unconformably upon the layer below. Although thinner than i n Boundary Bay, sea bottom contours indicate i t i s probably the same' sedimentary unit. On this p r o f i l e i t s thickest section i s only 36 meters (120 feet) and thins toward the coast as the layer below r i s e s to the sea f l o o r . The sub-aerial Praser Delta includes the north shore of Boundary Bay east of Roberts Peninsula. Johnston ( 1 9 2 3 , p. 40) reports a well just north of the bay penetrated about 122 meters (400 feet)'of Recent delta deposits before reaching g l a c i a l t i l l . A l l u v i a l sands, s i l t s , s i l t y sands and s i l t y clays were also found i n more recent boreholes d r i l l e d i n shallow water i n Boundary Bay just north of the International Border. One borehole passed through 105 meters (344 feet) of delta sediments without meeting pre-delta material (Borehole 109 No. 2 , Golder, Brawner and Associates). The horizon marking the base of the upper layer i n Plate XXXII subtends the mainland slope at a depth of 130 meters (430 feet) below.sea l e v e l , and can be traced to the northeast as a horizontal r e f l e c t o r for at least 5 . 5 kilometers (three miles) into the bay. If i t can be extrapolated another 13 kilometers (eight miles) to the s i t e of the boreholes i t becomes clear that the upper layer under the shelf i s a part of the modern delta of the Fraser River. Farther south on the shelf the upper layer may represent Recent Fraser delta sediments thinning out i n that d i r e c t i o n . Middle layer of shelf sediments. Below the H3 layer, a layer designated by the l e t t e r 'Q1 underlies the shelf and slope. F l a t - l y i n g , conformable s t r a t i f i c a t i o n i n the layer is truncated by the mainland slope between Roberts Reef and Alden Ridge. A strong r e f l e c t o r marking i t s upper surface terminates at the edge of the mainland shelf (Plate XXXII) just at the top of the slope. East of Alden Ridge the layer thickens toward the coast and may outcrop on the sea f l o o r near the eastern end of Plate XXXIII, about two kilometers from Point Whitehorn. Several speculations are possible for the o r i g i n of this layer. Near Point Whitehorn coastal uplands are com-posed of Late Pleistocene t i l l and glacio-marine d r i f t (Easterbrook, 1963) which rest unconformably upon an irregular 110 and eroded surface of thick pre-Vashon marine sediments, the Cherry Point s i l t s . This erosion surface appears at the base of sea c l i f f s south of Point Whitehorn and not more than f i v e kilometers from the s i t e of the p r o f i l e of Plate XXXIII. This suggests that the middle layer may be correlated with the pre-Vashon Cherry Point s i l t s under the erosion sur-face. However, since on shore these are overlain by t i l l and a t i l l layer cannot be i d e n t i f i e d on the seismic records (there are no chaotic r e f l e c t o r s o r ' d i f f r a c t i o n patterns usually c h a r a c t e r i s t i c of t i l l ) , t h i s i s u n l i k e l y . Also, basic differences i n the p r o f i l e s of the Pleistocene sedi-ments recorded near Roberts Peninsula which are both Vashon and pre-Vashon i n age (Johnston, 1923) and those of the middle layer recorded near Point Whitehorn, tend to support an opin-ion that the middle layer here i s not pre-Vashon. That the layer represents l a t e r Vashon glacio-marine deposits i s also not w e l l supported. The Vashon glacio-marine d r i f t of the area i s , according to Easterbrook (p. 1475) u n s t r a t i f i e d pebbly, sandy clay reaching thicknesses of only 20 meters (70 f e e t ) . The middle layer on the adjacent shelf i s as much as. 150 meters.(500 feet) thick. I f the lowest shelf layer, which i s seismically similar and conformable to the middle layer, i s included, the combined thickness probably exceeds 200 meters (650 f e e t ) . I f the upland deposits did increase in thickness to. seaward, reaching the amount required by the I l l seismic evidence, then the l a t t e r p o s s i b i l i t y may be more acceptable. The well north of Boundary Bay reported by Johnston encountered clay and boulders, apparently d r i f t , below 122 meters (400 f e e t ) . Below 146 meters (480 f e e t ) , blue clay was met, with successive deeper layers of clay and boulders. If the sediments overlying the middle layer are Recent delta material as found i n the Boundary Bay well down to 122 meters (400 f e e t ) , then the middle layer i n the Boundary Bay area could be the d r i f t recorded by Johnston as l y i n g under those sediments. Since the upland areas along the nearby coast are separated by deep troughs now f i l l e d with several hundred feet of p o s t - g l a c i a l alluvium (Easterbrook, 19^3, P- 1 4 7 5 ) , another p o s s i b i l i t y may be that the shelf sediments are the seaward extension of th i s alluvium. Thus the middle layer as well as the upper one, e s p e c i a l l y i n the Boundary Bay region, could be p o s t - g l a c i a l sediments. On the p r o f i l e of Plate XXXII, the middle layer extends into Boundary Basin and overlies sub-bottom units of suspected Pleistocene age (P n) i n that area. Roberts Swell type sediments also overlie the same units. The Roberts Swell unit i s found at depths below sea l e v e l equivalent to the middle and lower layers of the mainland shelf. Both Roberts Swell sediments and the middle layer are truncated 1 1 2 by the sea f l o o r . These sediments are not d i r e c t l y cor-relatable on the seismic p r o f i l e s but, from the above data, the inference is that they are related and may be parts of a common unit. If they are, the lower layers of the mainland shelf may then also be part of a former delta of the Fraser River when i t discharged i n this area. More detailed seismic p r o f i l i n g i n this area would resolve this more f u l l y . The iowest layer of sheIf sediments. L i t t l e seismic information i s available on the lowest layer. The d i s t i n c -t i o n between i t and the layer above i s based upon the presence of a strong conformable horizon between them and thus the d i s t i n c t i o n may be more apparent than r e a l . The marker horizon could be an exceptionally strong i n t e r n a l r e f l e c t o r within a single depositional unit. The layer i s therefore not designated separately from the one above i t i n the l i n e drawing interpretations. The layer l i e s over and therefore post-dates Alden Ridge. Its upper marker i s f l a t - l y i n g and the layer seems to appear only on the east side of the ridge. Reflectors at the same depths to the west of the ridge are mainly d i f f e r e n t i n seismic character, e s p e c i a l l y the deeper ones. Thus the source of these sediments must have been to the east. Due to the thickness of overlying sediments and noise and reverberation on the records the base of the layer i s not clear... An i n d e f i n i t e r e f l e c t o r i n Plate XXXIII at 0 .34 113 seconds, equivalent to about three hundred meters (1,000 feet) below sea l e v e l perhaps marks the base. In nearby coastal regions bedrock below Pleistocene occurs at depths near this value. V. BOUNDARY BASIN AND ALDEN RIDGE The southern end of the S t r a i t of Georgia i s unique in the study area. Few linear trends are present i n the s t r u c t u r a l geometry of the sea f l o o r , nor are there any f l a t - f l o o r e d areas as i n the area of northwestern basins. The triangular shaped Boundary Basin i s bounded on i t s three sides by Roberts Swell, the mainland slope, and the island slope. The f l o o r of the basin i s broadly irregular i n contrast to.other basins whose f l o o r s have been smoothed by sediment deposition. Trincomali Trough enters the basin at the western apex of the triangle as a narrow U-shaped trough. As the basin f l o o r widens, the trough curves to the east and north, following the edge of Roberts Swell and changing to a V-shaped canyon incised into- the basin f l o o r . Before reaching the central basin, i t broadens out, f l a t t e n s , and disappears. The deepest area of Boundary Basin occurs at Boundary Pass where' a depth of 269 meters (147 fathoms) i s recorded (Canadian Hydrographic Chart No. 3^50) . Alden Ridge, on the mainland slope east of Boundary Basin, r i s e s to within a few meters of the surface of the 114 S t r a i t at Alden Bank. Water depths east of the bank are less than 75 meters (40 fathoms) whereas to the west they f a l l sharply to more than 180 meters (100 fathoms) i n Boundary Basin. The sub-bottom character i s also unlike that of other basins in the study area. Seismic penetration i s , i n general, no more than one quarter second. Reflectors, including what appear to be major horizons, are not always continuous. Gaps in r e f l e c t o r s make correlations uncertain, as do rapid changes i n sub-bottom character from one p r o f i l e to the next. Even the p r o f i l e along the axis, of the S t r a i t (Plate XXXIV) does not give much assistance i n interpretation. Despite these, problems, two, and possibly three, sub-bottom r e f l e c -t i o n c h a r a c t e r i s t i c s of the basin indicate the compound nature of the region. Sediments An upper, h o r i z o n t a l l y s t r a t i f i e d layer p e r s i s t s over much of the basin f l o o r . Plate XXXIV, which i s continuous with Plate XXX along the axis of the S t r a i t , shows the layer continues under Roberts Swell, i d e n t i f y i n g the upper sediments with those of Roberts Swell unit and extending that layer to Alden Ridge i n the south. S t r a t i f i c a t i o n i s mainly horizontal but dips gently northwest, near Alden Ridge. A thicker sec-t i o n of the unit i s banked against Alden Ridge forming a 115 flat-topped terrace. Sediments under the terrace are trun-cated by the sea f l o o r . S t r a t i f i c a t i o n becomes less d i s t i n c t i n the mid-basin area. Truncation by the sea f l o o r of horizontal r e f l e c t o r s i n the P r s unit occurs on the mainland slope, at Roberts Reef, and at the southern margin of Roberts Swell, as well as at Alden Ridge. These occurrences, at very similar water depths, almost surround Boundary Basin and suggest that at one time the Roberts Swell type sediments were continuous over the basin. Thus, i f the basin were r e f i l l e d with these sediments to a depth of about 130 meters (70 fathoms), the r e s u l t would j o i n the truncated fragments into one i n t e g r a l unit. Further support for this conclusion can be reasoned from evidence of the source d i r e c t i o n of the P r s sediments. On Roberts Swell the source d i r e c t i o n was southeast, an im p o s s i b i l i t y i f Boundary Basin existed at the time of depo-s i t i o n , but s a t i s f a c t o r y i f the basin was f i l l e d with sedi-ments . Considerable erosion must have occurred since depo-s i t i o n to create Boundary Basin. Up to 90 meters (300 feet) of sediments may have been removed from the sea f l o o r . To change from a depositional state to an erosional state also attests to a large change i n marine conditions after emplace-ment. Sub-aerial erosion requires emergence of l 80 to 220 116 meters (600 to 800 f e e t ) , an amount not noticeably supported by other evidence i n the area. Rather, as no deposition i s presently occurring i n the basin, i t seems, more l o g i c a l to accredit the erosion to marine forces, possibly scour caused by large t i d a l currents entering the area from the channels to the south. Currents i n Boundary Pass reach a v e l o c i t y of 2.5 meters per second. A depression at the pass, most notice-able i n Figure 5 , may be indicative of the power of marine erosion i n that area. • Currents are also strong over Roberts Swell, as pointed out i n Table I I I . At a time during or shortly after the recession of Vashon g l a c i a t i o n from the S t r a i t , the sea stood at a con-siderably higher l e v e l r e l a t i v e to the land. Various authors estimate r e l a t i v e sea l e v e l was 180 to 300 meters (600 to 1 , 0 0 0 feet) higher than at present (Easterbrook, 1 9 6 3 ; Armstrong and Brown, 1954; Armstrong, 1956). The effects of t i d a l currents on the sea f l o o r could therefore be vastly d i f f e r e n t with that much additional water overhead. T i d a l v e l o c i t i e s i n the enlarged passes, as well as i n the S t r a i t , would be l e s s . Sediment deposition may have occurred at such a time, with waning g l a c i a t i o n providing an abundance of detritus to b u i l d up.large marine or glacio-marine deposits. As the land rose i n i s o s t a t i c adjustment to the melted ice load, currents along the sea f l o o r no doubt' increased i n v e l o c i t y and l i k e l y i n turbulence as well, with increased 117 scouring e f f e c t . P o s t - g l a c i a l emergence may have exceeded the p o s t - g l a c i a l transgression of the sea so that r e l a t i v e sea l e v e l was for a time, lower than at present (see Easterbrook,' 1963, p. 1480). . During a low stage of sea l e v e l , erosional effects could increase to the point where sea f l o o r sediments i n the southern S t r a i t were vigorously removed. Later eustatic sea l e v e l r i s e s overtaking i s o s t a t i c emergence would then bring about the present state. The base of the P r s layer i n central Boundary Basin i s d i f f i c u l t to determine. Plate XXXIV, the a x i a l p r o f i l e , indicates the base l i e s near 0.5 seconds below sea l e v e l , a r e s u l t e n t i r e l y compatible with that of the gas exploder record of Plate XXVTI over Roberts Swell. However, d i f f i -c u l t i e s i n interpretation arise just northwest of Alden Ridge. The a x i a l p r o f i l e of Plate XXXIV indicates mainly s t r a t i f i e d material i n the. sub-bottom while transverse p r o f i l e s indicate large areas of n o n - s t r a t i f i e d sediments. The areas cannot be defined sharply on any single p r o f i l e because the charac-t e r i s t i c s tend to grade into one another i n d i s t i n c t l y rather than making a sharp contact. The areas are therefore i d e n t i -f i e d only by their seismic character and marked P n or P s accordingly. Since few continuous r e f l e c t o r s occur i n these deposits, the general impression i s that the deep sub-bottom i s either almost homogeneous or completely random and chaotic 118 i n nature. Sub-bottom of this type appears to be confined to the southern and.- southwestern part of Boundary Basin west of Alden Ridge. The material overlies the side of Alden Ridge in Plate XXXII, but may form part of the ridge top i n Plate XXXIII. The two p r o f i l e s are not clear in this res-pect. An unknown horizon, perhaps.bedrock, appears under the layer. To the west near Boundary Pass (Plate XXXII), a thick s t r a t i f i e d section, probably sedimentary, underlies the layer, but overlies bedrock of the island slope. I t i s not c e rtain what this section represents either. During several periods of g l a c i a t i o n vast ice sheets occupied the S t r a i t of Georgia and surrounding regions. U n s t r a t i f i e d and se m i - s t r a t i f i e d t i l l sheets were deposited over wide areas. Undoubtedly the southern S t r a i t did not escape, and a l l these unknown deposits may be remnants of t i l l , although they are d i f f e r e n t i n seismic•character from t i l l i n the northern part of the study area. West of Alden Ridge the thickness of t his t i l l (?) i s greater than 200 meters (600 f e e t ) . Bedrock Under Boundary Basin The nature of the deepest horizon under Boundary Basin i s d i f f i c u l t to interpret i n view of the li m i t e d i n f o r -mation. The depth of the horizon, generally at more than 0.5' seconds below sea l e v e l , puts i t at the general depth of the bedrock f l o o r of Ballenas Basin to the north. I t i s 119 therefore most probable that the horizon, marks pre-Pleistocene bedrock i n the area. Reflections from the suspected bedrock horizon are of two c h a r a c t e r i s t i c s . In the northern basin the horizon truncates str a t a , marked B, which have an apparent dip to the southeast (Plate XXXIV, p o s i t i o n C). This s t r a t a i s no doubt correlatable with similar s t r a t a folded into a s y n c l i n a l ridge near Roberts Swell (Plate XXXI, po s i t i o n C). The top of the buried ridge is- about 275 meters ( 9 ° ° feet) below sea -l e v e l . Bedrock from the Gulf Island side (marked B^) cannot be traced d i r e c t l y across to the s t r a t i f i e d bedrock under Boundary Basin but, from Plate XXXI, i t rests-at the same depth and has similar seismic c h a r a c t e r i s t i c s . The nearest deep well i s R i c h f i e l d Pure Point Roberts on Roberts Peninsula, about f i f t e e n kilometers away. Tertiary i s i d e n t i f i e d there at about 200 meters.(650 feet) below sea l e v e l . The Tertiary rocks are more than 1,800 meters (6,000 feet) thick at the w e l l s i t e and overlie similar Cretaceous rocks. The deep s t r a t i f i e d layer i d e n t i f i e d with bedrock may therefore be • either Late Cretaceous rocks similar to those on the Gulf Islands or T e r t i a r y formations as found under the Fraser Lowland. In the southern Boundary Basin area the character of the bedrock horizon changes. The recorded horizon i s marked by a rugged and irregular surface which i s persistant through-1 2 0 out the southern area. No penetration occurs through the rough surface. Where the change i n bedrock r e f l e c t i n g c h a r a c t e r i s t i c occurs, there i s also a change i n depth to the horizon. The presence of two d i f f e r e n t bedrock r e f l e c t i o n s at di f f e r e n t depths may serve to mark a change i n bedrock l i t h -ology i n the southern S t r a i t . If this i s so, the s t r a t i f i e d unit i s most probably the younger. Plate XXXIV (between positions B and C) shows both types of horizon and indicates their obscure re l a t i o n s h i p , although i n no p r o f i l e i s It any clearer. An unconformable contact between the'two charac-t e r i s t i c bedrock layers i s suggested. In the same plate, bedrock under Boundary Basin merges into a multiple of the sea bottom which prevents i t from being traced under Alden Ridge. Thus the relationship of structures under the ridge to those in the nearby basin i s not known. The transverse p r o f i l e of Plate XXXII suggests that the core of the ridge i s bedrock, whereas the p r o f i l e of Plate XXXIII seems to require the ridge to be composed of se m i - s t r a t i f i e d and n o n - s t r a t i f i e d P n material. Plate XXXIV shows conclusively that at le a s t some of the ridge i s under-l a i n by s t r a t i f i e d materials tentatively i d e n t i f i e d as P g type sediments, but a bedrock interpretation i s not ruled out. 121 VI. THE ISLAND SLOPE The island slope has two major trends i n the study area. The junction of these occurs at Gabriola Reefs east of Gabriola Island. A change i n slope morphology also occurs at this point. Along the Gulf Islands south of Gabriola Reefs the island slope i s broken by a system of southeast trending p a r a l l e l ridges. Each successive ridge crest i s lower toward the S t r a i t . In the southern area the ridges are clos e l y spaced. Some, near the Gulf Island shores, break the surface of the water as rocky reefs and i s l e t s . Tumbo Island i s a sub-aerial extension of one of these. Locally, ridges are steep-sided with angles up to 23 degrees measured from the seismic p r o f i l e s . Steeper angles may, and probably do, occur. The average steepness of the Gulf Island slope i s , of course, less than 23 degrees. Farther north the ridges diverge from the coastline and become lower and less d i s t i n c t . However, they continue to form the island slope as far north as Gabriola Reefs where the trend of the slope turns abruptly westward. There, the ridges' stop. North of Gabriola Reefs the Vancouver Island slope assumes a west-northwesterly trend. In contrast to the Gulf Island slopes, the slope here i s r e l a t i v e l y smooth, dropping quickly from the narrow shelf to the 365 meter (200 fathom) depths of the adjacent' basin. Slopes as steep as 25 degrees 122 occur i n lower, oversteepened portions. The average d e c l i -v i t y i s approximately ten degrees, steeper than the average slope i n the south. Just north of Gabriola Island a shallow ridge angles across the slope forming a valley, or wide canyon, behind i t . Entrance Island- is a sub-aerial part of the ridge. The val l e y i s apparently suspended, or 'hanging', 180 meters (100 fathoms) above the sediments of the deep basin and 360 meters above the buried bedrock, perhaps indicative of the ef f e c t of g l a c i a l erosion i n the S t r a i t . Most of the transverse p r o f i l e s across the S t r a i t show some of the island slope but only a few approach close enough to shore to show the upper part. This over-caution on the part of the ship's navigators could not be avoided but, judging from the quality of those few records which did reach the upper slope, the loss i s not too s i g n i f i c a n t . Unconsolidated Sediments Thick sediments occur on the island slope only In the area adjacent to the Fraser Delta. The lower slope across from the Fraser Delta i s t e r r a c e - l i k e , the bedrock surface being at such a low d e c l i v i t y that delta sediments (H^) can rest t h i c k l y upon i t . Plates XVIII to XXII show the terrace-l i k e slope with many pockets of sediment trapped i n basins between bedrock ridges which r i s e above the general terrace l e v e l . A f l a t horizontal r e f l e c t o r near the base of sediments 123 i n most pockets probably correlates with the top of the H-|_ layer under the modern delta f a c i e s . Sediments i n some pockets reach 75 meters (245 feet) thick. Away from the delta, s u r p r i s i n g l y l i t t l e sediments occur i n depressions and valleys on the slope. The sea f l o o r is mainly composed of exposed bedrock. Northwest of Gabriola Reefs, l i t t l e sediment overlies the comparatively steep-sided bedrock slope. Sediments occur only on a few places which are s u f f i c i e n t l y low-angled. A thin wedge of Ballenas Basin sediments extends up some of the less steep lower slopes. Others are apparently too steep to r e t a i n sediments. Thin sediments also cover the f l o o r of the hanging.valley near Gabriola Island. Near Neck Point sediments i n a second v a l l e y , which may also be a hanging v a l l e y , f i l l a small, semi-enclosed basin and may have overflowed onto, the f l o o r of Ballenas Basin ninety meters ( f i f t y fathoms) below. Echograms of Ballenas Basin f l o o r below the valley show irregular humps on the otherwise f l a t f l o o r which can be interpreted as landslide features from the va l l e y above. This i s the only known occurrence of such features i n the study area. In one area and possibly two at the base of the island slope, possible Pleistocene sediments occur. Near the northwest end of Roberts Swell on Plates XXIV, XXV and XXVI, s t r a t i f i e d r e f l e c t o r s (P s) ov e r l i e bedrock but underlie 124 Roberts Swell sediments. Both top and bottom of the P s unit are unconformable. The strata are nearly horizontal, and underlie most, i f not a l l , of the western margin of Roberts Swell. A second area i s recorded only on the gas exploder p r o f i l e of Plate XIX. L i t t l e i n t e r n a l s t r a t i f i c a t i o n is evident and the layer i s marked P n (?). A small topographi-c a l expression on the sea f l o o r (Figure 5 and Plate XIX, p o s i t i o n B) probably defines the limited extent of the area involved. Bedrock The island slope has two d i f f e r i n g topographical c h a r a c t e r i s t i c s which allow a natural d i v i s i o n of the slope into a northern and southern part. The change i n topographi-c a l character occurs at Gabriola Reefs and i s matched by a change i n seismic character of the rock underlying the slope. Along the Gulf Island slope the offshore ridges are composed of bedrock which, from their location, are almost c e r t a i n l y part of the. thick Upper Cretaceous Nanaimo Group rocks. They have been marked B^ i n the l i n e drawings. Bed-rock r i s e s from under the delta sediments toward the outcrop areas of ridges higher on the slope. S t r a t i f i c a t i o n i n the bedrock, c h a r a c t e r i s t i c of the Gulf Island slope, dips steeply to the east, but decreases i n dip under the mid-Strait area. Strata i s truncated on the slope and at the ridges, but 125 becomes more conformable to the bedrock surface under mid-S t r a i t . A gentle a n t i c l i n e i n bedrock occurs under mid-S t r a i t i n Plate XXI. Similar truncated s t r a t a dip to east and west off Finger Ridge (Plate XVII) suggesting that this ridge may also be, at least i n part, a breached bedrock a n t i c l i n e . Under the ridges on the slope many hyperbolic patterns occur, perhaps r e s u l t i n g from r e f l e c t i o n s o f f weathered or upthrust edges of truncated bedrock str a t a . These cover and ob l i t e r a t e or prevent I d e n t i f i c a t i o n of r e a l s t r a t a which, on the nearby Gulf Islands, dip between 15 and 30 degrees to the east (J. Muller, unpublished map). Hyper-bolae become asymptotic to 45 degrees when r e f l e c t e d from points under the ship's track. With a record exaggeration of 1 2 , the difference i n angles of 30 and 45 degrees as recorded on the p r o f i l e s i s only 3 degrees. With only short segments of s t r a t a recorded, these are not e a s i l y resolved. Thus, on the slope area, uncertainty remains i f some of the observed r e f l e c t i o n s are hyperbolae or dipping strata. • The southernmost slope (Plates XXIV to XXXIII) narrows and becomes steeper. Because of hyperbolic r e f l e c t i o n pat-terns i t i s impossible to pick out s t r a t i f i c a t i o n i n most places. Since the slope i s steep, the s t r a t i f i c a t i o n , i f present, i s likewise probably steep. South of Boundary Pass 126 (Plate XXXIII) no recognizable penetration into the bedrock of the island slope i s achieved. The general appearance of the offshore ridges and the truncated s t r a t a (where present on the slopes) give the impression that the slope was formed by a series of f a u l t s which dropped the eastern side to a lower l e v e l r e l a t i v e to the west. Muller (Geological Survey of Canada, unpublished map) records many f a u l t s i n the coastal area of the Gulf Islands and has interpreted them i n terms of block t i l t i n g . The•topographical character of the Vancouver Island slope north of Gabriola Reefs i s d i f f e r e n t from the Gulf Island slope to the south. No ridges occur as off the Gulf Islands. Instead, the slope i s , in general, steep and con-tinuous. The lower part commonly shows oversteepening below the surface of the Ballenas Basin sediments. This i s not present south of Gabriola Reefs. Seismically the slope north of Gabriola Reefs i s characterized by a lack of r e f l e c t o r s under the bedrock sur-face (Plates I I to XII, XVI and XVII). Because of this apparent lack of seismic penetration, l i t t l e information on the bedrock character can be deduced. Even a gas exploder p r o f i l e (Plate VI), despite greater power, gives no i n t e r n a l r e f l e c t i o n s from under the bedrock slope. The d i f f e r e n t seismic character of the island slope north of.Gabriola Reefs to that i n the south implies a d i f f e r -127 ence i n physical properties upon which seismic r e f l e c t i o n s depend. Regional bedrock geology (Figure 2) shows the Gulf Island area i s completely underlain along the S t r a i t by thick Nanaimo Group rocks of Upper Cretaceous age. The fringe of islands along the S t r a i t , including Gabriola Island west of Gabriola Reefs, i s composed of rocks of the Gabriola Formation, the youngest or top member, of the Nanaimo Group. Depths of Upper Cretaceous rocks are estimated by Clapp (1914) to average 2 , 0 5 0 meters ( 6 , 7 6 0 feet) i n the Nanaimo area while on small offshore islands he estimated 3 , 0 0 0 meters ( 1 0 , 0 0 0 f e e t ) . Northwest of Gabriola Island a high arch of pre-Upper Cretaceous rocks i n the area of Nanoose Bay separates the Upper Cretaceous geosyncline into two basins, of which only the southern of the two, Nanaimo Basin, i s i n the study area. A change i n seismic character could be expected when going from Upper Cretaceous such as at Gabriola Island to the older section. The Upper Cretaceous sandstone, conglomerates and shales can be expected to have d i f f e r e n t r e f l e c t i o n q u a l i t i e s than the pre-Cretaceous rocks of altered basic volcanics, py r o - c l a s t i c s and older meta-sediments. Thus a change could be expected i n the p r o f i l e s taken to the west of Gabriola Island compared to those along the Gulf Islands. The seismic records at Gabriola Island are, however, l i t t l e d i f f e r e n t . from those near Nanoose Bay or Neck Point where pre-Cretaceous 128 rocks outcrop on the coast. L i t t l e sub-bottom penetration was obtained i n either area. Penetration into bedrock begins south of Gabriola Reefs where the change i n topographic character and trend also occur. To be sure that such was indeed the case, a short p r o f i l e was obtained s t a r t i n g from near shore at mid-Gabriola Island and extending across the slope and into Ballenas Basin. The r e s u l t s , shown i n Figure 6 , confirm that the record generated off Gabriola Island i s more c h a r a c t e r i s t i c of the Vancouver Island slope than the Gulf Island slope to the south. Why seismic penetration should increase south of Gabriola Reefs instead of north of Gabriola Island i s not known. Lithology of rocks underlying the slope i s not expected to change there. Where the change is expected, the seismic character of the record i s not affected. A possible cause of this behaviour may be the steepness of the topo-graphy, which, northwest of Gabriola Reefs, becomes generally steeper than the area to the south. Of a l l the p r o f i l e s over the island slope area, only two continue through i n t e r - i s l a n d passages into areas known to be surrounded by Upper Cretaceous rocks. Because of hyperbolic r e f l e c t i o n s and reverberation, these p r o f i l e s (Plates XVIII and XIX, XXVI and XXVII) are not conclusive i n determining i f r e f l e c t i o n s c h a r a c t e r i s t i c of these rocks are FIGURE 6 . A continuous seismic profile extending northeast from near shore Gabriola Island. Length is 8 kilometers. Positions on Figure 5. 130 continuous onto the lower slope. If they are, the lower slopes would almost c e r t a i n l y be part of the youngest of the Nanaimo Group formations. Strata on the island slope, i f projected up dip toward the Gulf Island would, i n the absence of f a u l t i n g , o v erlie the islands by upwards of 300 meters (1,000 f e e t ) . CHAPTER V SPECIAL INVESTIGATIONS AND OBSERVATIONS During the course of the study several small features became apparent on the seismic p r o f i l e s that do not add greatly to the general view of the S t r a i t of Georgia sub-structure but which are, nevertheless, int e r e s t i n g . The present chapter w i l l deal with these. I. JONES DEEP One of the most puzzling features on the bathymetric chart of Figure 5 i s a deep hole, c a l l e d Jones Deep, on the fl o o r of a submarine canyon leading from Sechelt Basin to Queen Charlotte Trench. Co-ordinates for the deep are Latitude 49°20'N,. Longitude 123°28'.8W. The canyon, cut into bedrock at the base of the mainland slope at the contact between the coastal rocks and the s t r a t i f i e d offshore bedrock, is now f i l l e d with unconsolidated sediments of the 'trans-parent' type to a depth of several tens of meters. The elongate cone-shaped hole penetrates through.the sediment to, or near, the bedrock base of the canyon which l i e s at a depth of 310 meters (170 fathoms) below sea l e v e l . This depth i s 165 meters (90 fathoms) below the general l e v e l of . surrounding sediments, and 46 meters (25 fathoms) deeper than the sediment f l o o r of nearby Queen Charlotte Trench. The 132 top of the hole i s about one kilometer wide. Because of the spread of sound waves from the echo sounder.the width of the hole at the bottom i s not known, but at or near bedrock the hole i s not l i k e l y to exceed a few meters i n diameter. Side slopes average about 16 degrees but may steepen toward the bottom. Dredging i n the hole obtained only soft blue-grey mud, t y p i c a l of areas of transparent sediments. Echograms over the hole are shown i n Figure 7 . Seismic p r o f i l e s of Plates XI, XIV and Figure 8 show the bedrock canyon with i t s covering of thick, unconsolidated f i l l . Explanations for the existence of a deep hole i n unconsolidated sediments must also account for i t s maintenance in an area of rapid sedimentation. Recent f a u l t movement under the canyon, for instance, could r e s u l t i n a deep c l e f t but i t does not provide a mechanism for maintaining the hole. Also, nearby p r o f i l e s provide no evidence, of recent motion i n the surrounding rock or sediments. Fault motion would not be l i k e l y to leave soft muds at the high angles observed on the sides. Currents i n the water column may prevent sedimentation or even erode the sea bottom i n certain areas, for instance, i n Boundary Basin. Some ridge areas may be kept clear of sediment cover by'currents, but i t i s doubtful that.currents could excavate such a deep, steep-sided hole. Larger holes or trenches do occur i n areas where persistent heavy, tur-7 KM FIGURE 7 Echograms over Jones. Deep showing depth and configur at ion of the hole. 01 Uj 5: U J U J 160 1 KM 200 240 280 SOUTHEAST NORTHWEST FIGURE 8. Cont inuous seismic profile over Jones Deep showing depth of unconsolidated sediment surrounding the area. bulent currents are known. Such currents may, i n time, even erode bedrock. A depression at Boundary Pass near Patos Island where t i d a l currents often reach 1.5 to 2 . 5 meters per second ( 3 to 5 knots) .may be related to high currents, but over Jones Deep no strong currents are known to e x i s t . T i d a l current models suggest t i d a l streams do move north along the mainland coast (P. Crean, P a c i f i c Oceanographic Group, personal communication) but indicate that currents are very small and decrease to near zero not far to the northwest of Jones Deep. The p o s s i b i l i t y that the hole may be a r e l i c of previous topography that has not yet been f i l l e d i n , i s not l i k e l y e ither. Sedimentation rates i n the area are high as proven by the depth of sediments on the seismic p r o f i l e s . Queen Charlotte Trench i s covered by at least l 8 0 meters (600. feet) of unconsolidated sediments. McCall Ridge near the hole has a covering of 36 meters (120 feet) and the canyon i t s e l f j u st south of the hole i s covered by more than 60 meters (200 feet) of sediment. Figure 8 shows the depth of sediment around the hole. The sides are steep for normal depositional slopes i n the area. A more l i k e l y explanation may be that the hole i s . kept open by a fresh water outlet of a deep aquifer under the canyon. The contact between rock types would be a l i k e l y place for such an outlet. Fresh water, being less dense 136 than s a l t water, r i s e s toward the sea surface. C i r c u l a t i o n set up by the r i s i n g water column may be s u f f i c i e n t to prevent sedimentation over the hole and thus permit the present equilibrium to e x i s t . There are no known aquifers of this type reported i n the S t r a i t of Georgia, and the Coast Range g r a n i t i c rock and altered bedrock of Bowen Island and other nearby islands i s dense and s o l i d , not normally assoc-iated with subterranean water movements. Deep fractures and j o i n t i n g are common in the rock but some doubt remains that water could be c a r r i e d from the land to the depth and d i s t -ance required. Mr. Chris Burton, geologist at B r i t a n n i a , Mine on Howe Sound, a deep mine located i n similar rock material, has indicated (personal communication) that ground water c i r c u l a t i o n i n deep areas of the mine i s n e g l i g i b l e . None of the proposed explanations are e n t i r e l y s a t i s f a c t o r y from a l l aspects but, i n the author's opinion, the l a t t e r s t i l l remains the most pl a u s i b l e . S a l i n i t y tests for brackish water should be made over the hole. I I . TRINCOMALI TROUGH A trough, or v a l l e y , at the base of the Gulf Island slope connects the southern end of Ballenas Basin to Boundary Basin. It has been named here, Trincomali Trough. The trough i s more than 18 kilometers i n length but, over most of i t s course, i t r a r e l y exceeds 1.5 kilometers i n width at 137 the 180 meter (100 fathom) contour. The f l o o r slopes from north to south with s i l l depth at about 187 meters (102 fathoms) at the north end. Cross-sections of the trough are shown i n Plates XXIV to, XXIX. The configuration changes from north to south, influenced by the underlying bedrock struc-ture of the Gulf Island slope and by more recent deposition. In the north the trough i s f l a t - f l o o r e d and broad. Bedrock of the islan d slope, forming the west side, continues under the trough, buried by. up to 100 meters (300 feet) of sedi-ments which probably post-date Roberts Swell. The east side was, at an e a r l i e r stage, formed by a low Pleistocene ridge. At the time of deposition of the Roberts Swell sediments this was buried to a depth of 30 or more meters. The sediments of Roberts Swell-now make up the gentle east r i s e of the trough as i t enters Ballenas Basin. The central trough area (Plates XXVI to XXVIII) d i f f e r s i n configuration from either end. It i s U-shaped with a much thinner sedimentary layer over the bedrock. Roberts Swell sediments form a high eastern wall and the buried-Pleistocene ridge i s much deeper, well below the bed-rock base of the trough and v i s i b l e only i n the gas exploder p r o f i l e of Plate XXVII. Toward the south end, shallow bedrock of the island slope extends farther to the east, fo r c i n g Trincomali Trough to curve away from the island side. The trough i s incised 138 into the contact between the bedrock and softer Roberts Swell sediments, forming a V-shaped canyon cut to. a depth of nearly 90 meters (50 fathoms) below the l e v e l of Roberts Swell. No sediments cover the f l o o r of this part of the southern trough but t h i n layers of sediment, possibly related to Roberts Swell, rest on the island slope to the west. The present variations i n shape of Trincomali Trough over i t s length are probably r e l a t i v e l y recent. At an e a r l i e r stage before the southern trough was formed i t sloped i n the reverse d i r e c t i o n . Sediments i n Boundary Basin were once more extensive and f i l l e d , the Basin to a higher l e v e l . The trough i n the south did not ex i s t at i t s present depth or shape, but was much shallower. The truncation of Roberts Swell strata prove that the southern trough was cut after deposition of these ' sediments. In the central section, the conformability of Roberts Swell s t r a t a under the' side of the trough show that the trough existed here in i t s present state when Roberts Swell sediments were deposited and has changed but l i t t l e since. In the north, the trough was much deeper before thick sediments were deposited on i t s f l o o r . Thus the early slope of the trough was from south to north. Deposition i n the north end and erosion, i n the south changed the slope to i t s present north to south d i r e c t i o n . Whether erosion i s continuing or not i s unknown. However i t i s clear that deposition i s not occurring i n the southern or central part 139 of the trough. Any sediment transported into those parts must be carried completely through i t . Since no Recent sediments are accumulating i n Boundary Basin, i t i s doubtful that sediments are carried to the south unless carried beyond the southern S t r a i t . Movement i n the opposite d i r e c t i o n from Boundary Basin may occur since large accumulations of sand are found at the north side of Roberts Swell, but these may be from the Swell i t s e l f . A program of sediment sampling through Trincomali Trough and around i t s ends may provide further information on sediment movements and sources i n the southern S t r a i t . I I I . ANOMALOUS HILLS OP THE FRASER DELTA Approximately nine kilometers downslope from the mouth of the Fraser River, between depths of 220 to 33° meters (120 to 180 fathoms) an area about 5 i kilometers wide and 9 kilometers long has been described by Mathews and Shepard (1962) as consisting of hummocky topography. Pro-f i l e s over the area are shown i n Plates XX to XXIII. The hummocks are actually small h i l l s or knolls with about 15 to 30 meters of r e l i e f from trough to peak. In cross-section on the continuous seismic p r o f i l e s they have the appearance, of being wavelike undulations i n the sub-surface delta material rather than random h i l l s or h i l l o c k s deposited upon that surface. Although crest-to-crest wave-140 length varies, i t i s normally 600 to 750 meters ( 2 , 0 0 0 to 2 , 5 0 0 f e e t ) . The p r o f i l e and wavelength downslope i s similar to that along slope. The knolls therefore appear to be symmetrical in both directions. I t should be pointed out, however, that a s u f f i c i e n t l y f i n e g r i d of pr e c i s e l y con-t r o l l e d echo sounding p r o f i l e s has not been established to permit an accurate description and evaluation of the complete h i l l region. Some h i l l s may have other shapes. Mathews and Shepard suspect the area to be the remains of former landslides modified by current action and more recent sedimentation. Lying as i t does below the area of maximum sedimentation at the r i v e r mouth, this suspicion is j u s t i f i e d . The upper slopes near the r i v e r mouth are r e l a t i v e l y steep and cut by deep g u l l i e s comparable to land-slid e g u l l i e s off the M i s s i s s i p p i Delta. Slope angles of four degrees or more occur above the 90 meter (50 fathom) contour and l o c a l l y much steeper slopes may be present (see Mathews and Shepard, 1 9 6 2 , Figure 3 ) . Below 90 meters water depth, but above the anomalous h i l l area, the slope angle decreases but i s s t i l l 1 .5 degrees. Thus slides from near the r i v e r mouth could have s l i d into the present area of-.the anomalous h i l l s . This assumption i s , at least i n part, borne out by the seismi c ' p r o f i l e s . The additional information not available to Mathews and Shepard' i s on the thickness of the slide layer and i t s i n t e r n a l structure.. 1 4 1 The seismic p r o f i l e s show sub-bottom r e f l e c t o r s remain continuous under several h i l l s . These sub-surface r e f l e c t o r s undulate with the crests and troughs of the sur-face. The depth of movement can thus be traced to at least 7 5 to 9 0 meters ( 2 5 0 to 3 0 0 feet) under delta sediments before the r e f l e c t o r s weaken and disappear. Beneath the h i l l areathe next deeper major horizon, about 1 2 0 meters ( 4 0 0 feet) below the sediment-water interface, has also buckled. Deeper s t i l l , a horizon which has been i d e n t i f i e d previously as the top of ancient bottomset bedding has also been d i s t -urbed. Thus movement may have occurred i n a layer several scores of meters thick. Instead of many Individual s l i d e s , the seismic records imply one massive s l i d e layer which has moved en masse down the slope over a deep glide plane. At the toe of the sl i d e less competent sediments under the h i l l area have flexured and buckled under the stresses imposed by the mass above. At some places i n the h i l l area the layer appears to have fractured with the upslope end g l i d i n g over the lower part (Plate XXI). The appearance on the record of wavelike h i l l s i s therefore l i k e l y due to fle x u r i n g . Some h i l l s under which the in t e r n a l r e f l e c t o r s are steep and di s -continuous are apparently fractured blocks which, i n s l i d i n g , have rotated backward leaving i n t e r n a l layering exposed. Beyond the sl i d e area, the thickness of the undisturbed sedi-ments consistent with the sl i d e layer measures about 76 meters 142 (250 f e e t ) . I f a layer of this o r i g i n a l thickness took part i n the s l i d e , the e f f e c t of crumpling at the toe has thick-ened the layer by 20 percent or more. Later sedimentation has f i l l e d i n and subdued the s l i d e topography. I t i s interesting to note that troughs higher on the slope have been f i l l e d whereas the lower ones have not. If sedimentation has not advanced as far as the lower slopes, then the sli d e i s probably not old. Assuming the sedimentation rate of one foot per year given by Mathews and Shepard ( 1 9 6 2 , p. 1423) for the area close to the r i v e r mouth, the s l i d e could not be less than about sixty years old. However, that rate may be excessive for- the distance of the knolls from the r i v e r mouth. A better estimate may be made by following the top of the sli d e layer upslope and measuring the thickness of undisturbed recent delta sediment over i t . There, close to the r i v e r mouth, the assumed sedi-ment rate w i l l be more accurate. The sediment cover, approxi-mately 49 meters (160 feet) thick, indicates the s l i d e probably occurred some 160 years ago. IV. SEA LEVEL CHANGES IN THE STRAIT OP GEORGIA The geological record of the area about the S t r a i t of Georgia, including the Nanaimo Lowland and Fraser Lowland, gives ample evidence of former higher stands of sea l e v e l (Armstrong, 1959; Easterbrook, 1963; Fyles, 1963)• Lower 143 stands of sea l e v e l are more d i f f i c u l t to show. The evidence may now be covered with water, may also be buried under more recent sedimentation, or may be completely removed by erosion. The general drowned aspect of the B r i t i s h Columbia coastline and the absence of d r i f t deposits on the outer coast are c i t e d by Peacock (1935) as evidence for former lower r e l a t i v e sea l e v e l s . Plates III A, and VII A, of Holland (1964) i l l u s t r a t e the drowned appearance of much of the coastal area. Fyles (1963) admits to the p o s s i b i l i t y ' that land stood higher to the sea than i t does today since post-Vashon time. In northern Washington, Easterbrook ( 1 9 6 3 , p. l48l) found trenching i n Cherry Point s i l t s and clays below present beach l e v e l and presumed that r e l a t i v e sea l e v e l had, at one time, been lower. Radio-carbon dating of shells from the upper Cherry Point s i l t gives an age of 3 8 , 0 0 0 years B.P., or pre-Olympia I n t e r g l a c i a t i o n . Pleistocene upland areas i n the Fraser Valley are separated by deep troughs now covered i n several hundred feet of sediment (Armstrong, 1 9 5 6 , p. 5 ; Easterbrook, 1 9 6 3 , p. 1475)« Since the alluvium f i l l e d valleys are presently not much above sea l e v e l , the troughs may have been eroded during a time when sea l e v e l stood hun-dreds of feet below i t s present l e v e l . Three ways i n which r e l a t i v e sea l e v e l changes occur are: 1) by eustatic adjustments of the world ocean l e v e l s , 2) by i s o s t a t i c adjustments of land areas due to Pleistocene 144 ice loading, and 3) by l o c a l or regional tectonic movements. A l l three have operated i n the S t r a i t of Georgia area. Curray (1961) has shown eustatic changes of sea l e v e l over the Late Pleistocene Period reached a maximum at about 2 0 , 0 0 0 years B.P. Sea l e v e l then was 360 feet below i t s present l e v e l . At the same time, the S t r a i t of Georgia area was approaching the end of a non-glacial episode, the Olympia Interglaciation' (Armstrong, et a l , 1967 , p. 3 2 4 ) . ' The area had been r e l a t i v e l y ice-free for perhaps 1 5 , 0 0 0 years or more during which time i n t e r g l a c i a l sediments had been depos-it e d throughout much of the area. Thus no ice load depressed the land, emergence from previous g l a c i a l episodes probably was complete, or nearing completion, and sea l e v e l was con-siderably lower. What tectonic movement was occurring, i f any, i s not known. The P a c i f i c coastal b e l t is generally conceded to be a youthful area which, despite the present drowned appearance of the coastline, i s undergoing u p l i f t . The net r e s u l t of the pre-Vashon r e l a t i v e movements may have been that sea l e v e l i n the area was considerably lower than i t i s at the present time. If the s t r a t i f i e d sediments presently found under McCall and Halibut Ridges were more extensive i n the northern study area as suggested by the remnants of these deposits scattered about that area, i t i s possible that they were, at least i n part, removed by sub-aerial erosion before the' onset 145 ,-of. the Vashon ice sheet. However, sub-aerial erosion could not account for the present depths of the S t r a i t of Georgia since these are well below the s i l l depths of the S t r a i t . Also i t is believed that, since the Vashon ice sheet over-rode much of the e a r l i e r unconsolidated deposits on land, i t s erosive a c t i v i t y was li m i t e d (Johnston, 1923). But • g l a c i a l erosion may have been much greater i n the S t r a i t , the main channel for ice movement. Evidence for lower sea l e v e l i n the S t r a i t of Georgia i s not generally present i n the areas of the S t r a i t covered in this survey. The general l e v e l of the elevated terrace area may be indicative of a long stand .of the- sea near 110 fathoms. West of Thormanby Islands i t stands near 90 fathoms (Plate II) and i s cut i n what are probably pre-Vashon P l e i s t o -cene sediments. However, no consistent terrace depths are observed throughout.the S t r a i t area. I t i s possible that evidence could be found closer to shore where the ship could not go. Hydrographic charts do show reefs and shoals at various depths, usually less than f i v e fathoms, along the coasts and near the islands. V. FAULTING AND TECTONIC MOVEMENTS I d e n t i f i c a t i o n of f a u l t s on seismic records i s often d i f f i c u l t and sometimes tenuous. Several c r i t e r i a for f a u l t i d e n t i f i c a t i o n from conventional seismic records are noted 146 i n the l i t e r a t u r e (Campbell, 1965). The density of i n f o r -mation provided by the continuous seismic method make some c r i t e r i a more useful than others. Correlation of seismic events across a v e r t i c a l displacement, or even a zone of disruption, may indicate f a u l t i n g , e s p e c i a l l y i f the sea bottom too, has been displaced and forms a scarp. A l i n e drawing of a f a u l t from another area i d e n t i f i e d i n this way is given i n Figure 9 ( A )• Horizontal as well as v e r t i c a l displacement on a f a u l t may be.recognized by a repeated pattern of r e f l e c t i o n s . Such a f a u l t i s shown in the l i n e drawing of Figure 9(b) from an actual seismic record. Local disruptions of s t r a t i f i c a t i o n i n which no c o r r e l a t i o n can be made across the disrupted area may be indicative of f a u l t i n g but other causes can give similar effects on the record. Further proof i s then needed to make positive i d e n t i f i c a t i o n . Also indicative of f a u l t i n g , but not proof, i s a v e r t i c a l displacement of the sea f l o o r when no r e f l e c t o r , or l i t t l e penetration, i s present. Because of exaggeration usually present on CSP records, angles appear much steeper than they actually are, and these steep-looking slopes can e a s i l y be confused as a f a u l t scarp. Topographical expression as a f a u l t c r i t e r i a must therefore be used with caution. No f a u l t s have been well i d e n t i f i e d from the seismic p r o f i l e s over the study area. Nevertheless i t i s suspected that f a u l t s do occur. In the Nanaimo Basin area major f a u l t s f Sea floor \scarp sub-bottom reflectors fault (a) Normal fault in plane reflectors. (b) Repeated pattern of seismic reflectors indicates fault, with lateral movement. FIGURE 9. Fault criteria appearing on CSP records. 147 have been reported by Buckham (1947), Clapp (1912, 1914), Muller (personal communication)', and others. B e l l (1967) postulates a f a u l t extending along the mainland side of the S t r a i t of Georgia. On the basis of topographic character alone, the en echelon step-like ridges of the Gulf Island slope could be assumed to have f a u l t o r i g i n s . The seismic p r o f i l e s , by penetrating the sediment f i l l e d troughs between the ridges and exposing their bases as well as other buried crests, further this impression but by no means prove i t . The ends of bedding planes are truncated on the steeper island side of the ridge. Thus some ridges at least are cuesta-like. Like cuestas, they may have f a u l t s p a r a l l e l l i n g them. Earth-quake a c t i v i t y i s common in the Gulf Islands (Milne, 1963) near th i s area. The most westerly p r o f i l e beyond the end of Ballenas Basin, Plate I I , shows what can be interpreted as f a u l t blocks southeast of Sangster Island. The blocks are t i l t e d toward Lasqueti Island. Bolson-like features formed by the t i l t e d blocks have been f i l l e d by l a t e r sediments or t i l l . Muller (personal, communication) has recorded the presence of large t i l t e d f a u l t blocks on Texada Island nearby. White and Savage (1965) note the p o s s i b i l i t y of major f a u l t i n g not far to the north of this area.. I. Leaf I48 omitted i n page numbering. 149 Other steep-sided slopes of islands and ridges, such as Ballenas Islands, may have f a u l t o r i g ins, but no d e f i n i t e seismic information ..is available i n the way of proof. FIGURE 11 . THICKNESS AND DISTRIBUTION CF RECENT SEDIMENTS ON THE SEA FLOOR LEGEND 0 to /5 meters thickness 15 to 40 meters 40 to 90 meters Above 80 meters Modern Foreset Sediments Modem Bottomset Sediments Seismically Transparent Sediments I N S T I T U T E O F O C E A N O G R A P H Y U N I V E R S I T Y OF B R I T I S H C O L U M B I A C O N T O U R I N T E R V A L S - 10 F A T H O M S C A U T I O N - T H I S C H A R T IS NOT I N T E N D E D F O * N A V I G A T I O N A L USE ,1 s , to ft / ^ r . — B. v .V.' i t I 6». f*2JLs\ v -.V jC/-? ' •. o J r J — r . ' V - > »l/ SAND^HEADS " - \ ^ S ^ ^ V . . , BALLENAS ISLANDS - - i ^ T ^ ^ ^ ^ - fc^SS^1 ^ ^ X ^ ; X ^ A B ^ ' W FIGURE 10. Distribution of bedrock and Pliestocene geology on the sea floor and below Recent sediments. Bedrock: LEGEND Unclassified Pliestocene: Assumed pre-Upper Cretaceous or Coast Range intrusives Assumed Upper Cretaceous Assumed Late Cretaceous-Eor'y Tertiary Probable drift or till McCall Ridge unit Roberts Swell unit ^tim^^mmm mm STRAIT OF GEORGIA T U R N P O I N T TO B A L L E N A S I S L A N D INSTITUTE O F O C E A N O G R A P H Y U N I V E R S I T Y OF B R I T I S H C O L U M B I A =_ M M 1 - — i» — - 4 M • » t C A L I 0 M U T l C U H I L I S C O N T O U R I N T E R V A L S - 10 F A T H O M S CAOTIOH- THIS CM*«T It NOT INTINOIO fON NAVI«*TlONAL USE / — 1 f f n r • j a r • ^  r a . . / - U . . LT L_ 1 W ? I I I T I t M C O L O M B I A TURN PT. TO SAND HEADS RiChficid P^rc IB SunrfsfdT -A i v ' **%.'. r • • • • i f ' " 1 • • r. /*;" *v -:. 1 • I i .9- o , . VJ.-.V. BOUNDARY. . » f ? # i V » ^ — * 5 ^ 1 CHAPTER VI CONCLUSIONS I. USE OF THE CONTINUOUS REFLECTION PROFILER IN BRITISH COLUMBIAN COASTAL WATERS The- continuous seismic p r o f i l e r i s an invaluable t o o l for the geophysicist or geologist interested i n studying the upper c r u s t a l structure of the sea f l o o r off B r i t i s h Columbia. While excellent penetration can be obtained i n most soft unconsolidated or semi-consolidated sediments, bedrock pene-t r a t i o n depends upon the'character of the rock, the degree of smoothness, and the slope of i t s upper boundary. Rough and irregular boundaries scatter sound energy and r e f l e c t hyper-b o l i c patterns which obscure structure below the boundary. However large scale features on the bedrock surface can often be recognized. In areas where sound penetration gives mean-in g f u l r e s u l t s i n bedrock, much useful information such as dir e c t i o n and degree of st r u c t u r a l dip, s y n c l i n a l and a n t i -c l i n a l axes, and perhaps evidence of f a u l t i n g , can be obtained. Because sound v e l o c i t y is commonly much greater i n indurated rock, apparent low penetration i n the time domain may be great i n terms of actual depth. Continuous seismic p r o f i l i n g i s extremely useful where a mantle of t i l l a n d . r e l ict Pleistocene or Recent•sedi-151 merit obscures the e a r l i e r surface and i t s nature to most other methods of study. From record character i t i s possible to distinguish various sedimentary f a c i e s . Soft unconsolidated sediments can be recognized overlying areas of semi-consolidated sedi-ments or bedrock. Where deep echoes are weak, i t may be d i f f i c u l t to separate bedrock from semi-consolidated or other sediments but, i n many cases, the.top of bedrock can be traced from other areas where' i t i s more de f i n i t e and e a s i l y recog-nized. In f a c t , one great advantage of continuous seismic p r o f i l i n g i s i t s a b i l i t y to locate outcrop areas where sampling can be used to maximum advantage while the extent of the sampled layer can be traced much farther under the overburden. Like a l l geophysical methods, the CSP method i s most useful i n association with other techniques or measurements which provide additional information. Accurate s t r u c t u r a l information i s available only i f v e l o c i t i e s are obtained, for example, by r e f r a c t i o n or wide-angle r e f l e c t i o n methods using CSP equipment with sonobuoys (LePichon, et a l , 1968; Houtz, et a l , 1968) . Gravity and magnetic.measurements provide further useful information on bedrock structures.. Sampling of the sea bottom i s indispensible to a CSP study i n order to determine the nature of the material underlying the sea f l o o r . 152 I I . SUMMARY OF GEOLOGY AND STRUCTURE UNDER THE STRAIT OF GEORGIA Bedrock Structures In the study area of the S t r a i t of Georgia, sedi-mentary areas have been outlined down to the bedrock surface, and i n some areas, s t r u c t u r a l features i n the bedrock i t s e l f have.been delineated. Under the S t r a i t bedrock f a l l s into two general categories: seismically opaque, and seismically s t r a t i f i e d . Unfortunately, many rock types such as Coast Range granodiorites, volcanic and basic rock, pre-Upper Cretaceous meta-sediments and some bedrock of Upper Cretaceous age are apparently opaque. . These rocks cannot be distinguished one from the other i n this area by their seismic character. Where te n t a t i v e l y i d e n t i f i e d , the i d e n t i f i c a t i o n i s due to proximity of shoreline outcrops to the recording s i t e . The d i s t r i b u t i o n of bedrock and Pleistocene deposits, under the sea f l o o r of the S t r a i t i s shown i n Figure 10, back pocket. Bedrock showing seismic s t r a t i f i c a t i o n i s present under several parts of the S t r a i t , the two main areas being along the Gulf Island slope south of Gabriola Reefs (marked B k ) , and i n the McCall Ridge and Sechelt Basin area along the northern mainland (marked B^). Along the northern mainland, between the north and south ends of McCall Ridge, the str a t a dip to the southwest at angles of approximately 5 to 10 degrees or 90 to 180 meters 153 per kilometer. The top of bedding i s found about 245 meters (800 feet) below sea l e v e l at a distance of two to three kilometers offshore. Onshore, Coast Range granodiorites dominate the coastline with a few older meta-sedimentary o u t l i e r s occurring near Howe Sound. The mainland slope between the shoreline and truncated s t r a t a offshore i s doubt-less composed of granodiorite or the older meta-sedimentary rocks near Bowen Island. The truncated s t r a t a overlies these rocks and dips under thick Pleistocene deposits farther o f f -shore. The stratigraphic p o s i t i o n and even the dip i s similar to that of Late Cretaceous and Early Tertiary rocks r e s t i n g unconformably on g r a n i t i c rocks of the Coast Mountains and underlying Burrard Peninsula at Vancouver, not far distant. On th i s basis the section of s t r a t i f i e d bedrock near the mainland coast has been, assigned to the Late Cretaceous-Early Tertiary Period. What happens to these rocks farther under Georgia S t r a i t has not been ascertained. The s t r a t a dis-appear under McCall or Halibut Ridges and, except for minor s t r a t i f i c a t i o n appearing along the a x i a l p r o f i l e of Ballenas Basin, (Plate I) n o • s t r a t i f i e d bedrock occurs to the opposing Vancouver Island slope. Bedrock along the coast of Vancouver Island opposite the suspected.Late Cretaceous-Early Tertiary area ranges from Upper Cretaceous at Gabriola Island to pre-Upper Cretaceous sedimentary and volcanic rocks north of Nanaimo, a l l seismi-154 c a l l y opaque. If the dip of the Tertiary rocks along the mainland side remained at 10 degrees into Ballenas Basin, a thickness of greater than 3 3 4 0 0 meters ( 1 1 , 0 0 0 feet) should be present at the base of the Vancouver Island slope. I t is doubtful that such i s the case but the presence of even some s t r a t i f i c a t i o n i n the bedrock under Ballenas Basin i s sug-gestive of the presence of a rock type more c h a r a c t e r i s t i c of the northeast side of the S t r a i t . Thus, i t i s possible that the Vancouver Island slope marks a f a u l t scarp with the basin side down. Ter t i a r y rocks may be i n f a u l t contact with older units on Vancouver Island. If such a f a u l t should e x i s t , i t i s most c e r t a i n l y a major one. Lack of good bed-rock r e f l e c t o r s under the basin i n the c r o s s - p r o f i l e s prevents more positive interpretation. S t r a t i f i c a t i o n along the Gulf Island slope does not appear to be continuous with the s t r a t i f i e d unit along the northern mainland, but may be r e l a t e d . In this area the s t r a t a dip to the east and can be followed a short way under the Fraser Delta area where the dip decreases and the s t r a t a are deformed into gentle s y n c l i n a l and a n t i c l i n a l structures. This s t r a t i f i e d bedrock i s , by i t s p o s i t i o n and s t r u c t u r a l r e l a t i o n s h i p to the Gulf Island slope, associated with the Upper Cretaceous formations on the Gulf Islands. However, because the s t r a t a obviously remain near the surface and may even r i s e under Fraser Ridge, a r e l a t i o n s h i p with the Late 155 Cretaceous-Early Tertiary bedrock under the Fraser Lowland area i s possible (Crickmay and Pocock, 1963). Therefore, unless a large f a u l t runs down the S t r a i t under the Fraser Delta as postulated by B e l l (1967) , the lower part at least of the Burrard Formation on the mainland and the Upper Cretaceous Gabriola Formation on the Gulf Islands may be contiguous. Muller (Geological Survey of Canada, personal communication), believes the Gabriola Formation may be, at least i n part, T e r t i a r y . Under the southern Gulf I&land slope bedrock has a steeper northeasterly dip than bedrock under the island slope to the northwest. The dip continues steep to-'the point where-bedrock disappears under Roberts. Swell. Beyond there, only tops of bedrock are seen i n some p r o f i l e s . Small areas of bedrock s t r a t i f i c a t i o n appear under Boundary Basin but because they are not continuous with the island slope or other areas where the geology i s known, their relationships are not clear. From the depth below sea l e v e l i t i s thought that they may be part of Tertiary bedrock as found i n boreholes i n the Fraser Lowlands, but once again, the proximity of island slope bedrock of probable Upper Cretaceous age make even this uncertain. Deformation appears to be more intense i n the bedrock under Boundary Basin than elsewhere on the island slope. In other areas of the S t r a i t , , 156 the top of bedrock i s recorded but l i t t l e or no penetration i s achieved. Although evidence is not clear,.the p r o f i l e s suggest that the S t r a i t of Georgia area may have undergone a flexure with upwarping..of the Coast Ranges and depression of the basin axis close to the Vancouver Island side. The S t r a i t of Georgia i s consistently deeper on this side. The isla n d side may then have fractured along the Vancouver Island slope. I f the system of ridges along the. Gulf Island slope i s f a u l t controlled, their convergence i n the south indicates a more concentrated f a u l t zone i n that area. Before the deposition of Pleistocene sediments, the S t r a i t of Georgia was a long v a l l e y floored by bedrock which sloped gently off the Coast Range Mountains toward Vancouver Island. The v a l l e y extended from north of the study area to the San Juan Archipelago and probably included the re-entrant now occupied by the.Fraser Lowlands, much of which was devel-oped during the Pleistocene. R e l i e f on the valley f l o o r was l i k e l y considerable as evidenced by the existence of breached a n t i c l i n e s and bedrock ridges now buried i n more recent sedi-ments. Pleistocene Geology During the Pleistocene large deposits of t i l l and inter-g l a c i a l sediments were l e f t i n the S t r a i t . The d i s t r i b u t i o n of these deposits•is shown i n Figure 10. In the northern 157 study area they tend to range along the mainland side of the S t r a i t whereas i n the area south of the Fraser Delta, Pleistocene deposits occur across the S t r a i t . At least two areas where extensive deposits can be traced as a single unit have been found. McCall and Halibut Ridges, although under-l a i n by bedrock, consist mainly of thick sections of P l e i s t o -cene. These undersea ridges are comparable i n magnitude and structure to the Pleistocene ridges of Burrard Peninsula and Burnaby Mountain at Vancouver. The McCall Ridge unit i s recognized from near Point Grey to at least as far as the north end of North Thormanby Island with perhaps a short break south of South Thormanby Island. I t may be as much as 560 meters (1,800 feet) thick i n places. Bedding, i n the unit i s f l a t or exhibits only low dips. Truncation of the beds suggests erosion has occurred. The unit may have covered much more of the S t r a i t than the area presently occupied. South of the Fraser Delta, the second large sedi-mentary deposit, the Roberts Swell unit of probable Late Pleistocene age, covers a large part of the southern S t r a i t of Georgia including Roberts Swell, the mainland shelf, most of Boundary Basin and at least the northwestern part of Alden Ridge. This unit extends close to the base of the Gulf Island slope. Older Pleistocene deposits occur below i t along Trincomali Trough and near Roberts Peninsula. Under Boundary Basin truncation of this unit around the perimeter 158 of the basin indicate that i t once existed to a greater depth, but has since been eroded i n Late Pleistocene to Recent time. As the area i s presently one of non-deposition, and r e l a t i v e l y high v e l o c i t y t i d a l currents occur, erosion may s t i l l be active. Other Pleistocene deposits which occur are not con-tinuous over a large area or cannot be correlated from p r o f i l e to p r o f i l e . Many ridges i n the S t r a i t are topped by P l e i s t o -cene sediments. However, the bases of at least some ridges are bedrock. Only Round Ridge i s completely bedrock, possibly a boss of Coast Range g r a n i t i c material. Sangster Ridge i s , i f not wholly Pleistocene, at least i n part composed of sedi-ments of that age. Other ridges such as Finger Ridge, South Ridge and Fraser Ridge, also appear to be capped by Pleistocene sediments. No Pleistocene i s found along the Vancouver Island slope. G l a c i a l erosion may have been p a r t i c u l a r l y powerful on the Vancouver Island side i f the present suspended valleys i n the bedrock of the slope are actually 'hanging' v a l l e y s . The oversteepened lower slopes of Ballenas Basin also point to heavy g l a c i a l erosion. I t i s possible that most of the volume of ice movement to the southeast through the S t r a i t was concentrated on the Vancouver Island side through Ballenas Basin. No Pleistocene deposits are recognized i n the basin. Basin f i l l i s Recent. 159 The p r o f i l e of Malaspina Basin i s V-shaped with sides less steeply i n c l i n e d than in Ballenas Basin i n d i - ' eating less g l a c i a l erosion and perhaps less ice movement. The Pleistocene deposits are probably t i l l , d r i f t and i n t e r - g l a c i a l i n o r i g i n . On the continuous seismic records, u n s t r a t i f i e d , i r r e g u l a r l y s t r a t i f i e d or chaotic r e f l e c t i o n c h a r a c t e r i s t i c s , often broken by many unconform-i t i e s , are thought to be evidence of t i l l or d r i f t materials, whereas s t r a t i f i e d material i s i n t e r - g l a c i a l or g l a c i a l . Several g l a c i a l episodes occurred i n the S t r a i t but, because of the rapid l a t e r a l as well as v e r t i c a l changes i n the Pleistocene sediments, the number of separate' g l a c i a l occur-ances cannot be determined from these records. I f the McCall Ridge unit i s accepted as a single i n t e r - g l a c i a l formation, then i t seems l i k e l y that i t . separates at least two g l a c i a l episodes above and below i t . However, more than one g l a c i a l episode may have occurred pr i o r to, or since, deposition of the McCall Ridge unit. Recent Sediments Recent sediments i n the S t r a i t have as their p r i n c i p a l source, the Fraser River. Sediment movement i s mainly to the north from the delta and l i t t l e , i f any, sedimentation i s presently occurring i n the S t r a i t south of the delta. D i s t -r i b u t i o n and thickness of Recent sediments i s shown i n i 6 o Figure 1 1 , back pocket. The modern Fraser Delta, which i s probably no.more than 1 0 , 0 0 0 years i n age, has extended across the S t r a i t to the Gulf Island slope, overlying the north side of Roberts Swell, but not occurring over the top of the Swell. . L i t t l e Recent sedimentation i s found i n Boundary Basin to the south. No southward sediment trans-port appears to be occurring except perhaps that which con-tinues beyond the study area. Sediments brought to the S t r a i t by the Fraser River are now deposited on the delta front or d i s t r i b u t e d to the north and northwest mainly into the basin areas. The main sediment catchment area i s Ballenas Basin where sediments exceed 180 meters (600 feet) In average thickness even at distances of 50 kilometers from the Fraser River. Sechelt Basin and Malaspina Basin also receive considerable quantities of Recent sediments. Recent.sediments also cover ridge areas along the mainland north of Burrard Inlet and are thick near the Fraser Delta, but they t h i n r a p i d l y and become spotty as the d i s -tance from the delta increases. In many areas, slopes into the basins are free of Recent sediment cover even though their angle i s low enough to support such cover. Since no slump or s l i d e deposits are found under the sides i t i s sus-pected that currents are responsible for sweeping the side-walls clear. 161 The transparent seismic quality of sediments over the ridge tops and i n some basin areas i s i n contrast with s t r a t i f i c a t i o n appearing i n sediments i n Ballenas Basin. The s t r a t i f i c a t i o n i n the l a t t e r basin may be due i n part to t u r b i d i t y currents o r i g i n a t i n g on the Fraser Delta front. A major change i n Recent sedimentation i s marked by a strong r e f l e c t i n g horizon i n thick sediments over Ballenas Basin and other smaller and l o c a l basin areas. The sediments below the r e f l e c t o r are suspected to be ancient pre-delta beds from a time before the delta foreset beds extended over-top McCall-Fraser Ridges. The Fraser River may have entered the S t r a i t farther to the south and east than i t presently does. As the delta b u i l t seaward-toward i t s present s i t e , sediments began to s p i l l over the sides of the ridges perhaps causing the change i n sedimentation which created the seismic discontinuity noted over a wide area. Few Recent sediments occur along the Vancouver Island slope except l o c a l l y i n pockets or depressions i n the bedrock. The f l o o r of the hanging v a l l e y north of Gabriola Island i s only t h i n l y covered. REFERENCES Armstrong, J.E., 1956: S u r f i c i a l geology of the Vancouver area, B r i t i s h Columbia. Geol. Survey Canada, Paper 5 5 - 4 , l 6 pp. Armstrong, J.E., 1957: S u r f i c i a l geology of New Westminster map-area, B r i t i s h Columbia. Geol. Survey Canada, Paper 5 7 - 5 , 25 pp. Armstrong, J.E., 1960a: Geology of the coast mountains, North Vancouver, B r i t i s h Columbia. "Guidebook for Geological F i e l d Trips i n Southwestern B r i t i s h Columbia." Geological Discussion Club, Vancouver, 1 5 - 2 5 . Armstrong, J.E., 1960b: S u r f i c i a l geology of Sumas map-area, B.C. Geol. Survey Canada, Paper 5 9 - 9 * 27 pp. Armstrong, J.E., and W.L. Brown, 1954: Late Wisconsin marine d r i f t and associated sediments of the lower • Fraser/Valley, B r i t i s h Columbia. Geol. Soc. America B u l l . , 6 5 , 3 4 9 - 3 6 4 . Armstrong, J.E., D.R. Crandell, D.J. Easterbrook and J.B. Noble, 1965: Late Pleistocene stratigraphy and chronology i n southwestern B r i t i s h Columbia and northwestern Washington. Geol. Soc. America B u l l . , 7 6 , 3 2 1 - 3 3 0 . Bacon, W.R. Bancroft, J.j 1957: Geology of lower J e r v i s I n l e t , B r i t i s h Columbia. B r i t i s h Columbia Dept. of Mines, B u l l . 3 9 , 45 pp. L., 1913: Geology of the coast and islands between the S t r a i t of. Georgia and Queen Charlotte Sound, B r i t i s h Columbia. Geol. Survey Canada, Mem. 2 3 , 146 pp. Beckmen, W.C., A.C. Roberts and B. Luskin, 1959: Sub-bottom depth recorder. Geophysics, 2 4 , 7 4 9 - 7 6 0 . B e l l , G.L., 1967: Big p o t e n t i a l o i l basins offshore from west coast. Oilweek, June 2 7 , 1 9 6 7 3 P« 4 8 . 163 Bennett, Lee C. J r . , and S.M. Savin, 1963: The natural history of the Hardangerfjord. 6 . Studies of the sediments of parts of the Ytre Samlafjord with the continuous seismic p r o f i l e r . Sarsia, 1 4 , 7 9 - 9 4 . Backus, M.M., 1959: Water reverberations--their nature and elimination. Geophysics, 2 4 , 2 3 3 - 2 6 1 . Bretz, J.H., 1913: Glaciation of the Puget Sound region. Wash. Geol. Survey B u l l . , 8 , 2 4 4 p p . Buckham, A.P., 1947: The Nanaimo coal f i e l d . Trans Canada Inst. Mining Met., 5 0 , 4 6 0 - 4 7 2 . Burwash, E.M.J., 1918: Geology of Vancouver and v i c i n i t y . University of Chicago Press, Chicago, 111., 106 pp. Campbell, Francis F., 1965: Fault c r i t e r i a . Geophysics, 3 0 , 9 7 6 - 9 9 7 . Clapp, C.H., 1912: Southern Vancouver Island. Geol. Survey Canada, Mem.. 1 3 , 208 pp. Clapp, C.H., 1913: Geology of the V i c t o r i a and Sanich map area., Vancouver Island, B r i t i s h Columbia. Geol. Survey Canada, Mem. 3 6 , 208 pp. Clapp, C.H., 1914: Geology of the Nanaimo map area. Geol. Survey Canada, Mem. 51, 133 pp. Clark, S.P. J r . , 1966: Handbook of Physical Constants. Geol, Soc. America, Mem. 9 7 . Cockbain, A.E., 1963a: Submarine topography and sediment thickness i n the southern S t r a i t of Georgia. Inst. Oceanography, Univ. of B r i t i s h Columbia, Manuscript Report No. 14, 8 pp. Cockbain, A.E., 1963b: D i s t r i b u t i o n of foraminifera i n Juan de Fuca and Georgia S t r a i t s , B r i t i s h Columbia, Canada. Contributions from the Cushman Foundation for Foraminiferal Research, 1 4 , Part 2 , 37-57- • Cone, R.A., N.S. N e i d e l l and K.E. Kenyon, 1963: The natural hi s t o r y of the Hardangerfjord. 5 . Studies of the deep water sediments with the- continuous seismic p r o f i l e r . 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Survey Canada, Mem. 1 3 5 , 87 pp. 166 King, L.H., 1965: Use of a conventional echo-sounder and textual analysis i n delineating sedimentary f a c i e s . Bedford Inst. of Oceanography, Report 65-14, 27 pp. Knott, S.T., 1962: Use of precision graphic recorders i n oceanography. Marine Sciences Inst., 1 , 251-262. Krause, D.C., 1962: Interpretation of echo-sounder p r o f i l e s . International Hydrographic Review, 3 9 , 65-123. LePichon, X., J . Ewing and R.E. Houtz, 1968: Deep sea sedi-ment v e l o c i t y determination made while r e f l e c -t i o n p r o f i l i n g . Jour, Geophys. Research, 7 3 , 2597-2614. LeRoy, O.E., 1908: Preliminary report on a portion of the main coast of B r i t i s h Columbia and adjacent islands included i n the New Westminster and Nanaimo d i s t r i c t s . Geol. Survey Canada, 56 pp. Lindsay, R.B., i 9 6 0 : Mechanical r a d i a t i o n . International Series i n Pure and Applied Physics. McGraw-H i l l Book Co., Toronto, 415 pp. Luskin, B., B.C. 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Milne, 'W.G., and W.R.-H. White, i 9 6 0 : A seismic survey i n the v i c i n i t y of Vancouver Island, B.C. Pub. Dom, Obs., 2 4 , 1 4 5 - 1 5 4 . Moore, D.G., i 9 6 0 : Acoustic r e f l e c t i o n studies of the con-t i n e n t a l shelf and slope off southern C a l i f o r n i a . Geol. Soc. America B u l l . , 7 1 , 1121-1136. Nafe, J.E., and C L . Drake, 1957: V a r i a t i o n with depth i n shallow and deep water marine sediments of porosity, density and the v e l o c i t i e s of compressional and shear waves. Geophysics, 2 2 , 5 2 3 - 5 5 2 . Newcomb, R.C, J.E. Sceva and 0 . Stromme, 1949: Ground water resources of western Whatcom Country. U.S. Geol. Survey, open f i l e report, 135 pp. O f f i c e r , C.B., 1955: A deep sea seismic r e f l e c t i o n p r o f i l e . Geophysics, 2 0 , 2 7 0 - 2 8 2 . O f f i c e r , C.B., 1958: ' Introduction to the theory of sound transmission. McGraw-Hill Book Co., 458 pp. Ostericher, Charles J r . , 1965: Bottom and sub-bottom investigation of Penobscot Bay, Maine, 1959 . U.S. Navy Oceanographic O f f i c e , Tech. Report •TR-173. Paitson, L., C.H. Savit, D.M. Blue and W.A. Knox, 1964: Reflection survey at Barracuda f a u l t . Geophysics, 29, 9 4 1 - 9 5 C Peacock, M.A., 1935: Fiordland of B r i t i s h Columbia. Geol. Soc. America B u l l . , 46, 6 3 3 -696. Phemister, T.C., 1945: The coast range batholoth near Vancouver, B.C. Quart. Jour., Geol. Soc. • London, 1 0 1 , pts. 1 and 2 , 37-88. Pickard, G.L . , ' 1 9 5 6 : Surface and bottom currents i n the S t r a i t of Georgia. Jour. Fisheries Research Board Canada, 1 3 , 5 8 1 - 5 9 0 . 168 Roddick, J.A., 1965: Vancouver North, Coquitlam and P i t t Lake map-area, B.C. Geol. Survey Canada, Mem. 3 3 5 . Savit, C.H., W.A. Knox, D.M. Blue and L. Paitson, 1964: Reflection and v e l o c i t y p r o f i l e s at the Outer .Ridge, Puerto Rico. Jour. Geophys. Research, 6 9 , 7 0 1 - 7 1 9 . Shepard, F.P., 1963: Submarine geology. 2nd Ed. Harper's Geoscience Series. Harper and Row, Publishers New York. 557 pp. Shor, G.G. J r . , D.G. Moore and W.B. Huckaby, 1963: Deep sea tests of a new non-explosive r e f l e c t i o n pro-f i l e r . Jour. Geophys. Research, 6 8 , 1 5 6 7 - 1 5 7 1 . Smith, ¥ . 0 . , 1958: Recent underwater surveys using low frequency sound to locate shallow bedrock. Geol. Soc. America B u l l . , 6 9 , 6 9 - 9 7 . Smith, ¥ . 0 . , C.B. Cumming and J.E. Upson, 1954: Mapping ) bedrock with low frequency sound. Geol. Soc. ' America B u l l e , 6 5 , (Abstract), 1 3 0 7 . Stacey, R.A., L.E. Stephens, R.V. Cooper andB.G. Brule, 1969: Gravity measurements - i n B r i t i s h Columbia. Gravity Map Series, Map No. 8 8 . Dom. Obs., Ottawa. T i f f i n , D.L., and J.W. Murray, 1966: Mapping offshore shelf with continuous seismic. Oilweek, 17 , No. 3 8 , 4 9 - 5 1 . Tseng, K.H., 1968: A new model for the crust i n the v i c i n i t y of Vancouver Island, B.C. Unpublished M.Sc. thesis. Dept. of Geophysics, Univ. of B r i t i s h Columbia. Usher, J.L., 1952: Ammonite faunas of the Upper Cretaceous rocks of Vancouver Island, B r i t i s h Columbia. Geol. Survey Canada B u l l . 2 1 , 182 pp. Walcott, R.I., 1967: Bouguer anomoly map of southwestern B r i t i s h Columbia. Inst. of Earth Sciences, S c i e n t i f i c Report No. 15 , . Univ. of B r i t i s h Columbia. 1 6 9 Waldichuk, M., 1952: Oceanography of the S t r a i t of Georgia. 1. S a l i n i t y d i s t r i b u t i o n . Progress Reports of the P a c i f i c Coast Stations., Fisheries Research Board Canada, 9 3 , 2 6 - 2 9 . Waldichuk, M., 1953: Oceanography of the S t r a i t of Georgia. 2. Temperature d i s t r i b u t i o n . Progress Reports of the P a c i f i c Coast Stations, Fisheries Research Board Canada, 9 4 , 1 9 - 2 3 . Waldichuk, M., 1954: Oceanography of the S t r a i t of Georgia. 3- Character of the bottom. Progress Reports of the P a c i f i c Coast Stations, Fisheries Research Board Canada, 9 5 , 5 9 - 6 3 -Waldichuk, M., and S. Tabata, 1955: Oceanography of the S t r a i t of Georgia V. Surface Currents. Progress Reports of the P a c i f i c Coast Stations, Fisheries Research Board Canada, 1 0 4 , 3 0 - 3 3 -Waldichuk, M., 1956: Oceanography of the S t r a i t of Georgia. ) 6 . Fresh water budget. Progress Reports of 1 the P a c i f i c Coast Stations, Fisheries Research Board Canada, 1 0 7 , 2 4 - 2 7 . Waldichuk, M., 1957: Physical oceanography of the S t r a i t of Georgia, B r i t i s h Columbia. Jour. F i s h . Research Board Canada, 1 4 , 3 2 1 - 4 8 6 . White, W.H., 1959:' Cord i l l e r a n Tectonics i n B r i t i s h Columbia. Amer, Assoc. Pet. Geol., B u l l . 4 3 , 6 0 - 1 0 0 . White, W.R.H., and J.C. Savage, 1965: A seismic r e f r a c t i o n and gravity study of the earth's crust i n B r i t i s h Columbia. Seis. Soc. Amer., B u l l . 5 5 , 4 6 3 - 4 8 6 . White, W.R.H., 1962: The structure of the earth's crust i n the v i c i n i t y of Vancouver Island as ascertained by seismic and gravity observations. Ph.D. thesis. Dept. of Geophysics, Univ. of B r i t i s h Columbia. 97 pp. APPENDIX SOUND VELOCITIES IK WATER, -SEDIMENTS AND ROCKS OF THE THESIS AREA In order to convert the time data recorded on the seismic p r o f i l e s to depth data, the v e l o c i t y of sound waves over the t r a v e l path of the signa l must be accurately known. This knowledge permits the correct determination, not only of depth, but of thickness, shape, slope and dip as we l l . I t i s possible that reconstructed structures may d i f f e r s i g n i f i c a n t l y from the o r i g i n a l record when converted from time to depth. However, sound v e l o c i t i e s may. vary with l a t e r a l p o s i t i o n as we l l as with depth, and a l l variations must be known to construct a proper sub-surface model. Sound V e l o c i t y i n S t r a i t of Georgia Water Under normal oceanic conditions, the speed of sound i n sea water varies between the l i m i t s of 1,400 to 1,560 meters per second (4,600 to 5*100 f e e t per second).. For accurate measurements, corrections must therefore be made to account f o r these v a r i a t i o n s . Since variations are a function of the i n s i t u s a l i n i t y , temperature and pressure d i s t r i b u -t i o n , these parameters provide the necessary means f o r an accurate correction to the sea water depths. O f f i c e r (1958* P» 19) gives the following approximate correction factors f o r 171 water d i f f e r i n g from a standard sea water: an increase of 3 . 1 3 meters per second per Centigrade degree ( 5 . 7 feet per second per Fahrenheit degree); an increase of 1 . 3 4 meters per second ( 4 . 2 7 feet per second)- per one thousandth part increase i n s a l i n i t y ; an increase of I . 8 2 meters per second per 100 meter depth increment. In the S t r a i t of Georgia,' a high i n f l u x of fresh water from p r e c i p i t a t i o n and r i v e r runoff leads to the form-ation of a brackish surface layer over much of the S t r a i t (Waldichuk, 1952). During spring freshet, surface s a l i n i t y varies l a t e r a l l y from two parts per thousand near the Fraser River mouth 1 to twenty-six parts per thousand'in the southern S t r a i t . During the months of December to A p r i l when r i v e r discharge i s at a winter minimum, surface s a l i n i t y varies from twenty to t h i r t y parts per thousand over the same area. Below 30 meters (100 feet) i n depth, the seasonal v a r i a t i o n i s n e g l i g i b l y small. Temperature variations follow a similar pattern. In summer the upper brackish layer i s warm and a strong, shallow thermocline e x i s t s . Winter temperatures are more uniform i n both l a t e r a l and v e r t i c a l directions (Waldichuk, 1 9 5 3 ) • The e f f e c t of these variations on the speed of sound can be assessed for the survey area. Assuming a two layer model from the s a l i n i t y and temperature data of Waldichuk can -approximate the winter conditions i n the S t r a i t , since'most 172 of the survey was carried out i n January, the following data are obtained. The. surface layer i s 15 meters (50 feet) thick with an average temperature of 6.1°C and s a l i n i t y at 26 parts per thousand. The bottom layer i s 366 meters ( 1 , 2 0 0 feet) thick with temperature of 7-2°C and s a l i n i t y at 30 parts per thousand. Lateral and v e r t i c a l variations at that time are assumed to.have been near the yearly mini-mum, thus a single two layer model w i l l t y p i f y the whole area. Using tables of the U.S. Naval Hydrographic O f f i c e , Publication Number 6 l 4 , the respective layer v e l o c i t i e s are 1,460^meters ( 4 , 7 8 5 .feet) per second and 1 ,475 meters ( 4 , 8 4 0 feet)' per second. As the sound wave traverses a much thicker lower layer, i t i s apparent that true water depth i s deeper than the records indicate since they are calibr a t e d for a sound speed of 1 ,465 meters ( 4 , 8 0 0 feet) per second. The difference, however,.is much less than one percent and i s therefore n e g l i g i b l e . Sound V e l o c i t i e s i n Unconsolidated Sediments Data for sound v e l o c i t i e s i n sediments of the study area i s lacking for most of the units encountered. Deep wells d r i l l e d by o i l companies have been logged only i n their lower depth intervals while those intervals within the range of the continuous seismic p r o f i l i n g equipment are not logged. An attempt was made to measure sound v e l o c i t y i n unconsolidated beach sands both at Point Grey and at Boundary 173 Bay. Hammer seismic equipment was used at low tide when the sand had not yet dried out. Results varied over i n d i v i d u a l hammer blows but gave a mean v e l o c i t y i n wet, coarse sand of 1 ,525 meters ( 5 , 0 0 0 feet) per second. For comparison, a test i n loose, dry sand farther up the beach gave a v e l o c i t y of 457 meters ( 1 , 5 0 0 feet) per second. In wet, fin e sand and s i l t at Boundary Bay the average v e l o c i t y was 1 ,370 to 1 , 4 0 0 meters ( 4 , 5 0 0 to 4 , 6 0 0 feet) per second although i n d i v i d u a l attempts ranged over much wider l i m i t s . Clark's "Handbook of Physical Constants" (1966) quotes a v e l o c i t y range of 500 to 2 , 0 0 0 meters ( 1 , 6 4 0 to 6 , 6 0 0 feet) per second i n alluvium at shallow depth. For a depth in alluvium of 2 , 0 0 0 . meters ( 6 , 6 0 0 f e e t ) , the v e l o c i t y range quoted i s 3 , 0 0 0 to 3 , 5 0 0 meters ( 9 , 8 0 0 to 1 1 , 5 0 0 feet) per second. A r e f r a c t i o n shot i n alluvium of the Salmon River delta on northern Vancouver Island (White, 1962) gave a v e l o c i t y measurement of I . 5 8 kilometers ( 5 , 2 0 0 feet) per second. The alluvium i s described (p.65) as "ri v e r deposited s i l t " . The private consulting f i r m of Swan Wooster-CBA (a j o i n t venture) made available (personal communication, 1969) r e s u l t s of seismic v e l o c i t y measurements i n three boreholes i n Vancouver Harbour just east of F i r s t Narrows. The bore-holes penetrated through coarse d e l t a i c and g l a c i a l materials • 174 and of the three, two bottomed i n Tertiary sandstone. The v e l o c i t y was found to range from 1 ,110 to 1 ,830 meters ( 3 , 6 5 0 to 6 , 0 0 0 feet) per second f o r sound i n d e l t a i c sedi-ments. The material i s composed of f i n e , medium and coarse sands, sand and gravel, s i l t y sands, s i l t s , s h e l l s , and organic material, described as ranging from compact to dense. The higher v e l o c i t i e s appear i n the denser material. From the above information, the v e l o c i t y of sound i n the upper Fraser Delta i s most l i k e l y to be near 1 ,525 meters ( 5 , 0 0 0 feet) per second. Since this v e l o c i t y i s close to that of water, sediment thickness read d i r e c t l y off the pro-f i l e s at the water v e l o c i t y w i l l err on the low side by approximately No v e l o c i t y measurements are available for the lower delta front sediments or those of the deep basins. O i l companies which have worked i n similar sedimentary frame-works assume a v e l o c i t y of 1 ,830 meters ( 6 , 0 0 0 feet) per second ( P . F u l l e r , S h e l l Canada Ltd., personal communication, . 1 9 6 9 ) . As th i s v e l o c i t y i s apparently not based.upon measure-ment, i t s v a l i d i t y i s questionable. Since recent hemi-pelagic sedimentation is involved at these s i t e s , data from deep sea and continental shelf measurements may be applicable. Laboratory measurements made by Hamilton (1965) on deep sea clayey s i l t s and s i l t y clays, when corrected to i n s i t u values, ranged from 1 , 5 0 0 to 1 ,555 meters ( 4 , 9 0 0 to 5 , 1 0 0 175 to 1 ,555 meters ( 4 , 9 0 0 to 5 , 1 0 0 feet) per second for depths of b u r i a l to 140 meters (460 f e e t ) . In s i t u measurements on continental shelf sediments (Hamilton, et a l , 1956) gave v e l o c i t i e s . o f 1 ,750 meters ( 5 , 7 5 0 feet) per second for coarse sands decreasing to 1 , 6 8 0 meters ( 5 , 5 2 0 feet) per second for fine sands, and dropping to 1 , 4 6 0 meters ( 4 , 8 0 0 feet) per second for f i n e r sandy-silt and c l a y e y - s i l t material. Thus average v e l o c i t i e s for the lower delta front may be near 1 ,585 meters ( 5 , 2 0 0 feet) per second while for the clayey muds of the basins, a v e l o c i t y closer to that of water i s not unreasonable. In f a c t , the basin sediments may be, i n the surface layer at le a s t , high porosity sediments, i n which case sound v e l o c i t y may be less than that of water (Hamilton, 1956). However, as v e l o c i t y increases with depth of b u r i a l , and basin sediments reach several hundred meters thickness, an average velocity..of 1 ,585 meters ( 5 , 2 0 0 feet) per second for these sediments as well has been assumed. Thus the thick-ness as read from the p r o f i l e s has been increased by approx-imately eight percent. The hemi-pelagic sediments on the ridge tops and sides may have similar sound v e l o c i t i e s . V e l o c i t i e s i n Pleistocene d r i f t and t i l l and in t e r -t i l l sediments are not known. The Vancouver Harbour bore-holes which passed through g l a c i a l materials give a range of v e l o c i t i e s of 1 ,220 to 2 , 7 4 0 meters ( 4 , 0 0 0 to 9 , 0 0 0 feet) per second. A similar, range was obtained i n Pleistocene sediments 176 from two boreholes logged i n the Fraser Valley ( R i c h f i e l d Pure Abbotsford; R i c h f i e l d Pure Sunnyside) .- In the study area, the v a r i a t i o n of character of the seismic record over • Pleistocene deposits suggests similar variations, i n the sediments. Thus no single v e l o c i t y i s l i k e l y to be suitable for a l l cases. However, a v e l o c i t y one-third greater than the v e l o c i t y of sound i n sea water, 1 ,950 meters ( 6 , 4 0 0 feet) per second, i s probably a r e a l i s t i c average about which the true v e l o c i t y w i l l vary. This value has been used for measure-ments of Pleistocene sediment thickness i n this thesis. Sound V e l o c i t i e s i n Bedrock Bedrock v e l o c i t i e s have been determined for some rock units near the study area (White and Savage, 1965), but since l i t t l e bedrock penetration was achieved i n this survey, only v e l o c i t i e s i n the Whatcom Basin sediments and Upper Cretaceous Nanaimo Group formations are required. The two boreholes i n Vancouver Harbour which terminated i n Tertiary sandstones indicate v e l o c i t i e s of 20fo and 2 , 6 6 0 meters ( 6 , 8 0 0 and 8 , 7 5 0 feet) per second i n these rocks. Two other wildcat wells reaching Tertiary sandstones i n Whatcom Basin for which v e l o c i t y logs are available ( R i c h f i e l d Pure Abbotsford; R i c h f i e l d Pure Sunnyside) give r e s u l t s ranging from 3 , 3 6 0 to 4 , 7 3 0 meters ( 1 1 , 0 0 0 to 1 5 , 5 0 0 feet) per second. Since the; l a t t e r two logs were recorded deep within the wells, the depth of b u r i a l may be a factor i n the increased v e l o c i t i e s 177 obtained. For this thesis, a v e l o c i t y of 2 , 5 6 0 meters ( 8 , 4 0 0 feet) per second was adopted for Tertiary bedrock. Refraction shots i n the S t r a i t of Georgia have pro-vided an average compressional wave v e l o c i t y of 4 . 0 5 k i l o -meters ( 1 3 , 3 0 0 feet) per second for Upper Cretaceous Nanaimo Group rocks (Milne and White, i 9 6 0 ) . Wells that penetrated Upper Cretaceous rocks ( R i c h f i e l d Pure Abbotsford; R i c h f i e l d Pure Sunnyside; R i c h f i e l d Pure Point Roberts) give sonic v e l o c i t i e s ranging from 4 . 0 5 to 6 . 1 kilometers ( 1 3 , 3 0 0 to 2 0 , 0 0 0 feet) per second. Because the value of 4 . 0 5 kilometers per second was obtained on the western side of the S t r a i t as a working value, i t i s used for sound v e l o c i t y i n Upper Cre-taceous rocks i n t h i s study. L E G E N D S ^ : HOLOCENE SEDIMENTS PLEISTOCENE SEDIMENTS UNDIFFERENTIATED P mr McCALL RIDGE:-UNIT »f TRANSPARENT TYPE Prs ROBERTS SWELL UNIT H3 MODERN DELTA Ps S TRA TIFIED UNDIFFERENTIA TED "2 MODERN PRO-DELTA . . Pn NON-STRATIFED OR CHAOTIC H t ANCIENT PRO-DELTA P UNDIFFERENTIATED . . 0 PLEISTOCENE  A N % R HOLOCENE SEDIMENTS . BEDROCK . Bi INFERRED TERTIARY: MAY INCLUDE SOME UPPER CRETACEOUS B, INFERRED UPPER CRETACEOUS. MAY INCLUDE. SOME TERTIARY k B. PROBABLE COAST RANGE INTRUSIVE. VOLCANIC OR PRE-UPPER CRETACEOUS METASEDIMENTARY ROCK . . . . B UNDIFFERENTIATED BEDROCK LEGEND HOLOCENE SEDIMENTS Hj UNDIFFEREN TIA TED H 3 TRANSPARENT TYPE MODERN DELTA H2 MODERN PRO-DELTA Hj ANCIENT PRO-DELTA Q PLEISTOCENE SEDIMENTS F^r McCALL RIDGE UNIT Prs ROBERTS SWELL UNIT f] STRATIFIED UNDIFFERENTIATED D NON-STRATIFIED OR CHAOTIC P UNDIFFERENTIATED PLEISTOCENE A N % R HOLOCENE SEDIMENTS BEDROCK Bf INFERRED TERTIARY. MAY INCLUDE SOME UPPER CRETACEOUS B, INFERRED UPPER CRETACEOUS. MAY INCLUDE SOME TERTIARY B. PROBABLE COAST RANGE INTRUSIVE, VOLCANIC OR PRE-UPPER J 3 CRETACEOUS M ETAS ED IMEN TARY ROCK ' UNDIFFERENTIA TED BEDROCK LENGTH 46 KILOMETERS POSITIONS ON FIGURE 5 LEGEND OPPOSITE PLATE I PLATE I o- CH Vancouver . Island Slope Lasqueti Island LENGTH 25 KILOMETERS POSITIONS' ON FIGURE LEGEND OPPOSITE PLATE I PLATE II Elevated Area Malaspina Basin Songster Ridge Ballenas Ballenas •0 O — —> .*/•.':• •-. • >• r-ry - 1/-.' > • *s 500-= 3 -1/2 (00--3/4 r S profile location LENGTH 22 KILOMETERS POSITIONS ON FIGURE 5 LEGEND OPPOSITE PLATE I PLATE III REFACTION TIME (SECONDS). .REFLECTION TIME (SECONDS),., REFLECTION TIME (SECONDS) REFLECTION TIME (SECONDS) v ., Qj O 800-2OG-W0^ISS^1, B: ^j-.^-V.-V^-v-r^:- V:-: — 33 7200-• GAS EXPLODER PROFILE BY SHELL CANADA LIMITED. LENGTH 31 KILOMETERS POSITIONS ON FIGURE 5 LEGEND OPPOSITE PLATE J PLATE VI REFLECTION TIME ' (SECONDS) 9 § S REFLECTION ' TIME (SECONDS) SUJOLf 1 D l SJ3;3UJ Hid 30 V31VM i o o <\ SUJOUjDJ SJ3J3UJ Hld30 V31VM Island Ballenas Basin Halibut Ridge McCall Ridge Sochelt Basin . Mainland Slope PLA TEX REFLECTION TIME (SECONDS) REFLECTION TIME (SECONDS) rsi ^ >j- Cs, REFLECTION TIME (SECONDS) REFLECTION ' TIME (SECONDS). 3 g Uj —J SUJOLj )DJ S7^; ^  uj HJd3Q U31VM CD SUJO (J)DJ o o S J 3 f -iOJ Hld3Q U31VM i o o to ..i .t.v.>.A«i:i.u:to. J A i n Pi N location }\ . LENGTH 15 KILOMETERS POSITIONS ON FIGURE 5 LEGEND OPPOSITE PLATE I PLATE XIII Uj o -c PLATE XIV 0- 0-f 3: A. Uj Q: 2 w o 100 400-200] 300 A 0- 0 -1— Ballenas Basin 1 . Ridge 0. o 100-200-\ 400-300-location \\ ^>%^$> LENGTH 15 KILOMETERS P0S/T/0NS ON FIGURE ' LEGEND OPPOSITE PLATE I SPARKER PROFILE BY SHELL CANADA LIMITED PLATE XV REFLECTION TIME (SECONDS) "2 "5-a c I I -o 03 Hi O REFLECTION TIME (SECONDS) 5 3 £ Hid30 U31VM H1H30 U31VM j ) REFLECTDN TIME (SECONDS) . REFLECT/ON TIME (SECONDS) ^ ^ ^ ^ S i o SLU0U1DJ i ' . & ,SJ3)dUI Hid 30 U31YM SJ3J3UJ Hld30 t/31VM • i REFLECTION TIME (SECONDS) REFLECTION TIME (SECONDS)  : ' Q Uj 800-.1000-5CO#0£§ GAS EXPLODER PROFILE BY SHELL CANADA LIMITED. • LENGTH 29 KILOMETERS. POSITIONS ON FIGURE-. . ' LEGEND OPPOSITE PLATE "'/. PLATE XIX uj to 0-200-01 100A V) <u o o ^200 1 600-300 H iOOA 3: 200-P-9 2C0 5 ioo-% im,y.:: 600-— i s K:;-NViV'--".fV•'••• "• •-•••'•'•;••••'•'• V'^ '•'••• :r•••'••>•• V(vf"^f»U>TO'-'V--v»}7'VM^ ^0" 8 Profile I Location LENGTH 21 KILOMETERS POSITIONS ON FIGURE LEGEND OPPOSITE PLATE I PLATE XX • j Profile y Location LENGTH 16 KILOMETERS; POSITIONS ON FIGURE' • LEGEND OPPOSITE PLATE I PLATE XXII 0- 0-f Roberts Swell Fruser Delta Fraser Ridge prcfile I'cc a ' i c n L£MTW 37 KILOMETERS POSITIONS ON FIGURE. LEGEND OPPOSITE PLATE I PLATE XX Ul 0- Of 100-, ^ Si *200^ U J ^ -f 500-30M 600-700* Ch (A O 100-200-500-600-709-• . i m o o o LENGTH 13 KILOMETERS POSITIONS ON FIGURE LEGEND OPPOSITE PLATE 1. PLATEXXIV I 0 1) rn p C D Co ^ o :> .. Co o 3 § o Co rrj 1 C C D O co WATER DEPTH meters fathoms WATER DEPTH meters fa 1 horns 8 fe-ll m m i m I (S0N0D3S). 3HI1 ' N0/1DJ1J3U . (SONQOJSj. 3HI1 NQIlDJWd 12G0-GAS EXPLODER PROFILE BY SHELL CANADA LIMITED LENGTH 17 KILOMETERS POSITIONS ON FIGURE LEGEND OPPOSITE PLATE I PLATE XXVII REFLECT I ON TIME (SECONDS) REFLECT/ON TIME (SECONDS) 5 £ ^ REFLECTION TIME (SECONDS) REFLECTION TIME (SECONDS) 0. SJ3)3(JU Hid3 0 U31VM 2 SJ9 I 3 UJ Hid 30 d 31 VM Lu 200 c o i 1> 2C0 ^ABpun.dcry:,: - Basin.. L.;.J:™.L.. . Roberts Swell r rns ;r =0 1/2 Y.. : . ' * ~ ' . ' r T i j " . i ' . r , ' . ' ' j • : r9 § D 3: lifts . 200-WZ '.Y;^ !^  ^ LENGTH 27 KILOMETERS POSITIONS ON FIGURE LEGEND OPPOSITE' PLATE I PLATE XXX REFLECTION TIME (SECONDS) REFLECTION TIME (SECONDS) SJ3)3UJ HldBO U31 VM SJ3)3UJ H1330 d 31 VM t 

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