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Structure, evolution, and petroleum potential of the Queen Charlotte Basin Lyatsky, Henry 1992

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STRUCTURE, EVOLUTION, ANDPETROLEUM POTENTIALOF THEQUEEN CHARLOTTE BASINbyHENRY LYATSKYB.Sc., University of Calgary, 1985M.Sc., University of Calgary, 1988A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIESDEPARTMENT OF GEOLOGICAL SCIENCESWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIANovember 1992© Henry Lyatsky, 1992In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.Department of  - The University of British ColumbiaVancouver, CanadaDate^( zDE-6 (2/88)iiABSTRACTThis study focuses on the structure and evolution ofthe Tertiary Queen Charlotte sedimentary basin and itspetroleum potential. The study is based on integratedgeological basin analysis, which involves compiling allrelevant geological, geophysical, and geochemical data;interpreting them using a multidisciplinary approach; andcreating a petroleum-exploration model for the basin.The Queen Charlotte Basin lies offshore, and itsgeologic structure was interpreted from gravity, magnetic,and seismic data. Potential-field trend patterns aresimilar to physiographic lineaments. Regional faults arethe Kitkatla, Principe Laredo/Banks Island, and Sandspit.Their crosscutting relationships, lack of strike-slipdisplacements, and presence of throughgoing Oligocenedikes, all preclude substantial strike-slip movements inthe basin during the Neogene. The origin of the basin bylarge extension and strike-slip faulting is ruled out; amodel based on block tectonics is offered instead.The similarity of orthogonal, Tertiary fault networkswithin the basin with Mesozoic or older networks on itsflanks, suggests the inheritance of the main structuralcharacteristics of the basin from pre-Tertiary time. Themosaic of fault-bounded blocks was formed no later than theLate Jurassic; since then, block faulting and structuralinversion have occurred repeatedly. Tertiary extension wasiiitoo small to obliterate the older fault pattern.Large plutons lie beneath eastern Queen CharlotteSound, on trend with the Neogene Anahim volcanic belt onthe mainland. These features reflect the passage of theregion over a mantle hot spot in the Miocene, which mayhave also caused the renewal of tectonic activity. Igneousheating and hydrothermal-fluid circulation occurredlocally, whereas burial-related conductive heating wasregional in scale.Petroleum potential is associated chiefly withMesozoic rocks beneath Queen Charlotte Sound. Source rocksoccur in the Upper Triassic-Lower Jurassic interval, andreservoirs in the Cretaceous units. Tertiary rocks form aregional seal.The Hecate Strait area lacks thick Mesozoic rocks, butthe Queen Charlotte Sound area contains Mesozoic source andreservoir rocks, overlain by a Tertiary seal. TheCretaceous stratigraphic interval, which may contain largeblock structures, should be the primary target for futurepetroleum activity.ivTABLE OF CONTENTSABSTRACT ^TABLE OF CONTENTS ^  ivLIST OF TABLES  viiLIST OF FIGURES  viiiLIST OF PLATES PREFACE ^  xiCHAPTER 1. INTRODUCTION ^  11.1. Location and definition of the Queen CharlotteBasin ^  11.2. Overview of the available data ^  71.2.1. Geologic mapping and drillhole data ^ 71.2.2. Seismic data ^  101.2.3. Potential-field data  111.2.4. Utility of geophysical data in thestudy area  131.3. Objectives of the present study ^  131.4. Research methodology ^  15CHAPTER 2. GEOLOGIC SETTING  192.1. Pre-Tertiary stratigraphic record onshore ^ 192.2. Inferred stratigraphy offshore ^  322.3. Principal geological stages of regionalevolution ^  362.3.1. Paleozoic interval  362.3.2. Late Triassic to Early Jurassic interval ^ 372.3.3. Jurassic episode of tectonism ^ 372.3.4. Late Jurassic to Late Cretaceousinterval ^  392.3.5. Latest Cretaceous and Tertiary tectonism ^ 402.4. Tectonic setting of the study area ^ 42CHAPTER 3. TERTIARY STRATIGRAPHY OF THE QUEENCHARLOTTE BASIN FILL AND CORRELATIONOF SEISMIC EVENTS ^  483.1. Lithology of the Queen Charlotte Basin filland difficulties with its stratigraphiccorrelations ^  483.2. Difficulties with seismic stratigraphiccorrelations  533.2.1. Reflection coefficients ^  533.2.2. Acoustic basement vs. geologic basement ^ 553.2.3. Seismic transmission losses in the QueenCharlotte Basin ^  563.2.4. Pitfalls in seismic interpretation causedby transmission losses ^  603.2.5. Example of interpretation of seismicdata ^  613.3. Implications for research methodology ^ 68VCHAPTER 4. REVIEW OF PRINCIPAL GEODYNAMICAL MODELSOF FORMATION OF NON-COMPRESSIONALSEDIMENTARY BASINS ON CONTINENTAL CRUST ^ 704.1. Modeling of basin evolution - general statement ^ 704.2. Stretching (pure-shear) model of basinformation ^  714.2.1. Formulation ^  714.2.2. Points of contention ^  754.2.2.1. Thickness of continentalplates  754.2.2.2. Lateral rheologicalinhomogeneitiesin the continental lithosphere ^ 794.2.2.3. Mechanism of continentalrifting ^  814.2.2.4. Assumption of conductiveheating  834.3. Simple-shear model  854.3.1. Formulation ^  854.3.2. Points of contention ^  864.4. The plume model  864.4.1. Formulation  864.4.2. Points of contention  894.5. Discussion ^  90CHAPTER 5. PETROPHYSICAL PROPERTIES OF ROCKS ANDPROCESSING OF MAGNETIC AND GRAVITY DATA ^ 925.1. Petrophysical properties of rocks in the studyarea ^  925.1.1. Rock densities ^  925.1.2. Rock magnetization  945.2. Processing of magnetic and gravity data ^ 965.2.1. Reduction of gravity data ^  965.2.2. Horizontal-gradient magnetic vector map ... 1045.2.3. Shadowgrams   1105.2.4. Upward continuation of potential-fielddata ^  1105.2.5. Selection of data-processing results forinterpretation ^  124CHAPTER 6. LINEAR TRENDS IN POTENTIAL-FIELD MAPSAND PHYSIOGRAPHIC LINEAMENTS ^ 1266.1. Lineaments in magnetic maps ^  1266.1.1. Interpretation of unfiltered total-fieldaeromagnetic data  1266.1.2. Interpretation of upward-continuedmagnetic data ^  1366.2. Lineaments in gravity maps ^  1396.3. Interpretation of geologic structure frompotential-field lineament patterns ^ 1426.4. Patterns of physiographic lineaments  1446.4.1. Construction of the map  1446.4.2. Description of physiographic lineamentpatterns ^  146vi6.5. Similarities of lineament trends ^ 148CHAPTER 7. STRUCTURE OF THE QUEEN CHARLOTTE BASIN ^ 1507.1. Structural trends on the rim of the QueenCharlotte Basin ^  1507.1.1. Vancouver Island ^  1507.1.2. Queen Charlotte Islands ^  1517.1.3. British Columbia mainland  1527.2. Timing and sense of motion of the principalfault systems in the Tertiary ^  1557.3. Regional fault networks and structure of theQueen Charlotte Basin ^  1627.3.1. Structure of the basin rim  1627.3.2. Fault-related structural characteristicsof the Queen Charlotte Basin fill ^ 164CHAPTER 8. GEOLOGIC EVOLUTION OF THE QUEEN CHARLOTTEBASIN ^  1668.1. The amount of Cenozoic extension and the role ofblock faulting in the evolution of the QueenCharlotte Basin  1668.2. Mechanism of Queen Charlotte Basin subsidence ^ 1788.3. Influence of tectonics on the distribution ofsedimentary rocks in the Queen Charlotte Basin ^ 1868.4. Variations of fault-block tectonics in theQueen Charlotte Basin ^  191CHAPTER 9. PETROLEUM POTENTIAL OF THE QUEENCHARLOTTE BASIN AREA ^  1959.1. Previous exploration ^  1959.2. Source rocks ^  1979.2.1. Upper Triassic and Jurassic ^ 1979.2.2. Upper Jurassic and Cretaceous  2029.2.3. Tertiary  2029.3. Reservoir rocks ^  2029.3.1. Mesozoic  2029.3.2. Tertiary  2039.4. Oil seeps ^  2049.5. Regional and local seal ^  2049.6. Thermal maturation of rocks  2059.7. Timing of oil generation and entrapment ^ 2079.8. Petroleum prospects in the study area  2099.8.1. General statement ^  2099.8.2. Hecate Strait  2099.8.3. Queen Charlotte Sound  2109.9. Thermal aspects of geodynamical evolution ofthe Cenozoic Queen Charlotte Basin ^ 212CHAPTER 10. SUMMARY AND CONCLUSIONS  216REFERENCES ^  221viiLIST OF TABLESTable 2-I. Generalized stratigraphic column forQueen Charlotte Islands and adjacent areas of theInsular Belt ^  24Table 2-II. Main events in the structural historyof Queen Charlotte Islands ^  26Table 2-III. Generalized stratigraphic column fornorthern Vancouver Island  27Table 9-I. Type of organic matter and TOC contentof the Upper Triassic-Lower Jurassic, UpperJurassic-Upper Cretaceous, and Tertiary sedimentaryrocks in the Queen Charlotte Basin area ^ 198Table 9-II. Generalized stratigraphic columnsfor Queen Charlotte Islands and northern VancouverIsland ^  200viiiLIST OF FIGURESFigure 1-1. Region of study ^  2Figure 1-2a. Morphogeological belts of theCanadian Cordillera ^  4Figure 1-2b. Distribution of metamorphic andplutonic rocks in the Canadian Cordillera ^ 5Figure 1-3. Location of drillholes and 1988 seismicreflection profiles in the study area  9Figure 1-4. Ship tracks used in acquisition ofgravity and magnetic data off the west coast of Canadaprior to 1982 ^  12Figure 2-1. Geologic map of Queen Charlotte Islands ^ 21Figure 2-2. Geologic map of Vancouver Island ^ 23Figure 2-3. Eastward-migrating strandlinesdelimiting the eastern margin of the Cretaceousshallow marine basin on the site of QueenCharlotte Islands ^  34Figure 2-4. Terrane map of the Canadian Cordillera ^ 43Figure 3-1. Attenuation of seismic as a functionof the number of igneous sheets and reflectioncoefficient at contacts ^  59Figure 3-2. Segment of seismic reflection Line 1 ^ 62Figure 3-3. A structural interpretation of theseismic profile in Fig. 3-2  63Figure 4-1. Variations in thickness of Earth'stectosphere ^  77Figure 5-1. Gravity map of the Queen CharlotteBasin area (Bouguer on land, free-air offshore) ^ 98Figure 5-2. Enhanced isostatic gravity map of theQueen Charlotte Basin area ^  101Figure 5-3. Total-field magnetic anomaly map of theQueen Charlotte Basin area  105Figure 5-4. Aeromagnetic horizontal-gradient vectormap of the Queen Charlotte Basin area ^ in pocketFigure 5-5. Shadowgram of the total-field magneticdata from the Queen Charlotte Basin area. "Light-source" inclination 30 ° , declination 045° ^ 111Figure 5-6. Shadowgram of the total-field magneticdata from the Queen Charlotte Basin area. "Light-source" inclination 30 ° , declination 090° ^ 113Figure 5-7. Shadowgram of the total-field magneticdata from the Queen Charlotte Basin area. "Light-source" inclination 3e, declination 135° ^ 115Figure 5-8. Gravity data from the Queen CharlotteBasin area (Bouguer on land, free-air offshore)upward continued to 5 km ^  117Figure 5-9. Gravity data from the Queen CharlotteBasin area (Bouguer on land, free-air offshore)upward continued to 20 km  119Figure 5-10. Total-field magnetic data from theQueen Charlotte Basin area upward continued to(a) 5 km and (b) 20 km ^  121ixFigure 6-la. Patterns of lineaments in magneticmaps from the Queen Charlotte Basin area ^ 127Figure 6-lb. Examples of magnetic lineaments ^ 128Figure 6-2. Patterns of lineaments in gravitymaps from the Queen Charlotte Basin area  129Figure 6-3. Structure of the Queen Charlotte Basinas interpreted by Dietrich et al. (1992) fromseismic reflection data ^  131Figure 6-4. Major physiographic lineaments in theQueen Charlotte Basin area  135Figure 7-1. Northeastern end of seismic reflectionLine 5 showing the Neogene structural depression westof Banks Island. A: data; B: structuralinterpretation ^  159Figure 8-1. Segment of seismic reflectionLine 1. A: data; B: structural interpretation ^ 168Figure 8-2. Neogene faults disrupting shallowlevels of the fill of the Queen Charlotte Basin,illustrated in seismic reflection Line 1. A: data;B: structural interpretation ^  170Figure 8-3. Effects of compressional inversion ofan extensional fault in Hecate Strait, illustratedin seismic reflection Line 7. A: data;B: structural interpretation ^  172Figure 8-4. Schematic cross-sections across thenorthern part of the study area  192xLIST OF PLATESPlate I. Generalized geologic map of thestudy area  ^250xiFOREWORDI started my study of the Queen Charlotte Basin in1988, at the University of Victoria and the GeologicalSurvey of Canada's Pacific Geoscience Centre, supervised byRoy Hyndman and George Spence. In 1990, I was offered anopportunity to participate in the Queen Charlotte BasinFrontier Geoscience Program directly, by coming to theGSC's Cordilleran Division and the University of BritishColumbia in Vancouver. I am deeply grateful to DirkTempelman-Kluit, Bob Thompson, and Jim Haggart for givingme that chance, and to Dick Chase for supervising the studyat the UBC. Jim Murray has offered a lot of useful advice.I would like to thank Jim Haggart, Bob Thompson, GlennWoodsworth, Cathie Hickson, and Dirk Tempelman-Kluit fortheir warm support, encouragement, and friendship. I havelearned a lot about Cordilleran geology while working withthe GSC's Vancouver office, which has provided a collegialand cheerful research environment. Discussions with JimHaggart, Glenn Woodsworth, Bob Thompson, Jim Monger, Petervan der Heyden, John Luternauer, and many others haveproved extremely valuable.Besides the committee members, I thank the followingscientists for critically reading all or parts of thisdissertation and providing many useful comments: GlennWoodsworth, Bob Thompson, Dirk Tempelman-Kluit, CathieHickson, Don Lawton, Peter Lewis, Jim Dietrich, RoyxiiHyndinan, Ralph Currie, George Spence, and Jake Hudson.Discussions with Bob Crosson, Tomiya Watanabe, John MacRae,John Hole, and Art Calderwood have also been valuable.The data used in this study were provided by the GSC,and I appreciate the help of Ralph Currie and Dave Seemannin obtaining the potential-field maps. Additional datawere provided, sometimes in advance of publication or on aconfidential basis, by Cathie Hickson, Glenn Woodsworth,Jim Dietrich, Kristin Rohr, Roger Higgs, John Hole, KevinNahm, Linda McCulloch-Smith, Tark Hamilton, Charle Gamba,and George Spence.Geophysical data processing was carried out using thecomputer facilities at the Cordilleran and Geophysicsdivisions of the GSC, where I received much help from ArtHaynes and Warner Miles. Larry Sobczak offered advice onthe use of gravity reductions. Financial and technicalsupport for this study was provided by the GSC, NSERC, theUniversity of Victoria, and the UBC. The diagrams weredrafted by Tonia Oliveric and Brian Sawyer.Brian Mahoney shared my office, bringing the muchappreciated chaos and decadence into things (in ways thatneedn't be explained here), and teaching me how to occupythe most office space with the least amount of material.Last, but not least, I thank my parents, Vadim andRegina Lyatsky, for supporting me always and in every way.-1-CHAPTER 1. INTRODUCTION1.1. Location and definition of the Queen Charlotte BasinThe Queen Charlotte sedimentary basin (Fig. 1-1) is aprominent tectonic feature occupying an area of about500x200 km in the western Canadian Cordillera. The basinis located near the boundary between two large Cordillerangeologic and physiographic zones (Plate I in pocket, andFig. 1-2): the Coast Belt on the mainland, distinguished bya rugged relief and elevations up to 5000 m, and theInsular Belt. The latter is largely covered by sea, but itcontains prominent land areas such as Vancouver Island andQueen Charlotte Islands, with elevations up to 2000 m.The Tertiary Queen Charlotte Basin is located in alow-lying part of the Insular Belt known as the HecateDepression (Holland, 1964), which also includes the GeorgiaStrait area to the south. Most of the basin now lies belowsea level and is covered by the waters of Queen CharlotteSound, Hecate Strait, and Dixon Entrance, which are open tothe Pacific Ocean. Water depth is shallow, rarelyexceeding 200 m (e.g., Chase et al., 1975).Parts of the Queen Charlotte Basin fill are exposed onGraham Island in the northern part of the Queen Charlottearchipelago.Although the Queen Charlotte Basin is commonlyconsidered to be Tertiary in age (Shouldice, 1971), avariety of other definitions have also been advanced.-3-Figure 1-1. Region of study. Shaded area is the QueenCharlotte Basin. Heavy black lines indicate major faults.BIFS - Banks Inland fault system; CSF - Clarence StraitFault; GCF - Grenville Channel Fault; HIF - Holberg InletFault; KFS - Kitkatla fault system; LIF - Louscoone InletFault; PLFS - Principe Laredo fault system; QCFS - QueenCharlotte fault system; RSF - Rennell Sound Fault; SFS -Sandspit fault system. Inset shows location of drillholesand new seismic reflection profiles (see also Fig. 1-3).Modified after Lyatsky and Haggart (1992).AFigure 1-2a. Morphogeological belts of the CanadianCordillera (after Monger and Journeay, 1992). greenschistfadesamphibolitefaciesgranitic rock^b^blueschist faciesFigure 1-2b. Distribution of metamorphic and plutonicrocks in the Canadian Cordillera (after Monger andJourneay, 1992).-6-Stacey (1975) implicitly included in the basin the UpperCretaceous sedimentary rocks he suspected are presentbeneath Tertiary strata offshore. Thompson et al. (1991)suggested the Queen Charlotte Basin comprises all thepost-Middle Jurassic rocks in the study area, based onfundamental similarities in sedimentary lithologies andstructural style.To avoid confusion, I have followed the long-standingconvention that the Queen Charlotte Basin comprises onlythe thick Tertiary sedimentary succession onshore andoffshore (Shouldice, 1971; Yorath and Chase, 1981; P. Lewiset al., 1991a). The Neogene Skonun Formation makes up mostof the basin fill. It intertongues with and in many areasis floored by the volcanic rocks of the Upper Oligocene-Miocene Masset Formation. Paleogene sedimentary rocks arefound locally on Queen Charlotte Islands, but theirrelationship to the Queen Charlotte Basin is uncertain.The structure and evolution of the basin should not beconsidered in isolation from the underlying, older geologicformations and pre-existing structures. For the purposesof petroleum-potential assessment, Upper Triassic-LowerJurassic, as well as Upper Jurassic-Upper Cretaceous, rocksespecially need to be taken into account because they are,respectively, the principal source of hydrocarbons and theprimary reservoir (Chapter 9; Lyatsky and Haggart, 1992).-7--1.2. Overview of the available data 1.2.1. Geologic mapping and drillhole dataComprehensive summaries of the regional geology ofdifferent parts of the study area were presented by Brew etal. (1966), Sutherland Brown (1966, 1968), Shouldice (1971,1973), Chase et al. (1975), Jeletzky (1976), Muller(1977), Berg et al. (1978), Yorath and Chase (1981), andHutchison (1982). Although these reports were based on alimited data set, they contain information which provedsignificant for subsequent work.In the last several years, much new geologicalinformation has been collected on Queen Charlotte Islandsand the mainland, especially as a result of the detailedmapping conducted by the Geological Survey of Canadathrough the Queen Charlotte Basin Frontier GeoscienceProgram. Early results of this work on Queen CharlotteIslands have been summarized by Thompson et al. (1991) andP. Lewis et al. (1991a-b).Understanding of the geology of the mainland areas hasbeen improved considerably as a result of recentcontributions by Crawford et al. (1987), Rusmore andWoodsworth (1991), Monger (1991), Monger and Journeay(1992), and van der Heyden (1989, 1992). In southeastAlaska, knowledge of regional geology is now also moredetailed, largely due to new mapping reported by Gehrels etal. (1987), Gehrels and Saleeby (1987a-b), Gehrels (1990),and Brew et al. (1991).-8-Northern Vancouver Island, in contrast, is surveyed inless detail, reconnaissance mapping having been carried outmostly in the 1960s and 1970s (Jeletzky, 1976; Muller,1977, 1980; Muller et al., 1974, 1981). Additional mappinghas been conducted recently, but only on a local scale(Massey and Friday, 1989).Thus, most of the information about the onshoregeology of the Queen Charlotte Basin rim now originatesfrom Queen Charlotte Islands and some parts of themainland. The predominance of references to these areas inthis dissertation is an artifact of data availability.Information about the offshore geology is obtainedmainly from eight widely spaced petroleum-exploration wellsdrilled in the 1960s (Fig. 1-3; Shouldice, 1971). Mostapparently failed to penetrate rocks older than Miocene,but Cretaceous strata were identified in the Sockeye E-66and Tyee N-39 wells (as summarized by Haggart, 1991).Young (1981) reported pre-Tertiary ages from these wellsfor the rocks he considered volcanic, but new petrologicalanalyses suggest these rocks are reworked volcaniclasticmaterial which had a Mesozoic source (Leslie, 1989), andHickson (1991, p. 321) has stated that some of the K-Ardates reported by Young "are of dubious analyticalquality". Detailed biostratigraphic information for thewells is lacking even for the interval drilled (Chapter 3).As a result of recent research, the onshore geology ofthe study area is known better, and the data are moreti1`)^\r)te/C20^100^0Kilometres0Figure 1-3. Location of drillholes (Shouldice, 1971) and1988 seismic reflection profiles (Rohr and Dietrich, 1990)in the study area.-10-detailed and diverse, than ever before. Given theavailability of this composite data set, a new analysis ofthe structure and evolution of the Queen Charlotte Basin,to provide a fresh assessment of the hydrocarbon potentialof the region, is timely.1.2.2. Seismic dataBoth reflection and refraction seismic surveys wereundertaken in the study area over the last several years.Seismic reflection coverage is limited. About 1,000km of modern 40-fold marine data were acquired in 1988 inQueen Charlotte Sound, Hecate Strait, and Dixon Entrance(Fig. 1-3; Rohr et al., 1989; Rohr and Dietrich, 1990,1991, 1992; Dietrich et al., 1992). These profiles, aswell as older petroleum-industry data (Shouldice, 1971), aU.S. Geological Survey seismic line (Snavely et al., 1980),and single-channel profiles, were used by the above-notedauthors to identify structural depressions and fault trendswithin the Queen Charlotte Basin. Unfortunately, high-quality seismic reflection coverage is sparse (Fig. 1-3),and deep levels of the basin are imaged poorly due tosignal-penetration problems (Chapter 3; Lyatsky, 1991b).Seismic refraction profiles provide coarse constraintson the crustal structure in the study area. Results ofprevious experiments were discussed by Johnson (1972),McMechan and Spence (1983), Clowes and Gens-Lenartowicz(1985), Pike (1986), Mackie et al. (1989), and Drew and-11-Clowes (1990). A regional refraction survey was conductedin the Queen Charlotte Basin in 1988 in conjunction withthe reflection survey; its specifications and preliminaryresults have been presented by Rohr et al. (1989), Yuan(1990), Spence et al. (1991), and Hole et al. (1992).Interpretation of seismic data offshore is constrainedby the information from the eight wells drilled in the1960s (Fig. 1-3; Shouldice, 1971). In the present study,the seismic data were used in conjunction with magnetic andgravity maps to interpret the structure of the QueenCharlotte Basin.1.2.3. Potential-field dataMagnetic maps in the study area prior to the FrontierGeoscience Program were based on relatively widely spacedlines acquired by ship (Fig. 1-4; Currie et al., 1983a-b).As part of the program, high-quality, modern aeromagneticdata (Currie and Teskey, 1988) were collected with aflight-line spacing of 1.5 to 2.5 km and mean terrainclearance of 305 or 610 m. Total-field data were correctedfor the International Geomagnetic Reference Field andgridded at an 812.8-m interval (i.e. two samples per mile).They are available in digital form from the GeophysicalData Centre of the Geological Survey of Canada, in Ottawa.The gravity coverage is irregular. Station spacingvaries across the study area and is about 10 km on average.The best coverage is found offshore (Fig. 1-4), whereasFigure 1-4. Ship tracks used in acquisition of gravity andmagnetic data off the west coast of Canada prior to 1982.After Currie et al. (1983a).-13-land areas are covered unevenly and generally in lessdetail (Currie et al., 1983a; Sweeney and Seemann, 1991).The final accuracy of readings is about ±2 mGal (1 mGal =10-5 m/s 2 ).Although satisfactory in general, the gravity coverageis sparse in most parts of the region, as compared with theaeromagnetic coverage. Hence, the short-wavelength anomalycomponent is reduced in gravity maps. The distribution ofrecording stations where the gravity data were collectedpermitted these data to be gridded at a 2-km interval.1.2.4. Utility of geophysical data in the study areaGravity and magnetic maps from the Queen CharlotteBasin area reflect lateral variations in rock density andmagnetization. These data were used to identify largebasement blocks, plutons, and fault networks onshore andoffshore. Local control on the interpretation of geologicstructure offshore is provided by seismic reflectionprofiles. In turn, gravity and magnetic data permit a morerealistic interpretation of seismic reflection lines acrossthe basin. Thus, the various data sets are complementary.1.3. Objectives of the present studyAn earlier model for the origin of the Tertiary QueenCharlotte Basin was presented by Yorath and Hyndman (1983)and updated by Hyndman and Hamilton (1991). These workersdeveloped their model by fitting the available data for the-14-region to the theoretical model of basin formation byrifting put forward by McKenzie (1978). However, newinformation from geologic mapping on the rim of the basinonshore has proven incompatible with this model.Results of field mapping in the land areas surroundingthe basin on the east (Crawford et al., 1987; Woodsworth,1988, 1991; Rusmore and Woodsworth, 1991; van der Heyden,1989, 1992) and on the west (Thompson et al., 1991; P.Lewis et al., 1991a-b) cannot be reconciled with theprevious interpretation of the Queen Charlotte Basin as aproduct of McKenzie-style rifting. None of these studiesdocuments significant Tertiary extension, nor attendantlarge-scale strike-slip faulting, in the region surroundingthe basin. The emergence of new constraints on theinterpretation of geologic evolution of the westernCanadian Cordillera thus necessitates a revision of theolder model of basin formation.The main objective of this study was to gather andevaluate all the available relevant data, process them asnecessary, and interpret them to develop a geological modelfor the evolution of the Queen Charlotte Basin consistentwith the geology of the surrounding areas. This modelwould meet all the available constraints, and it would be areliable foundation for future petroleum-geology researchand hydrocarbon exploration in the region.To meet these principal objectives, the author set outto complete the following:-15-(1) to gather and interpret all the available data requiredfor a multidisciplinary basin analysis;(2) to evaluate the applicability of the existingtheoretical models of sedimentary-basin evolution tothe Queen Charlotte Basin;(3) to produce a model for the structure and evolution ofthe Queen Charlotte Basin incorporating the newinformation;(4) to generate a petroleum-exploration model for the studyarea that could serve as a guide for future oil search.1.4. Research methodologyA comprehensive interpretation of geological,geochemical, and geophysical data was undertaken for theQueen Charlotte Basin. Integrated geological basinanalysis (Lyatsky and Lyatsky, 1990) involves(1) the reliance on hard data to constrain interpretationsof basin evolution, and(2) the incorporation of all available data sets into amultidisciplinary study.This view of geological basin analysis is broader than thatoften proposed elsewhere. For example, Potter andPettijohn (1977) and Kleinspehn and Paola (1988) haveapparently underrated the importance of geophysical data insedimentary-basin analysis, devoting little space to thatsubject. Where geophysical techniques are utilized (e.g.,Berkhout, 1989), they are commonly restricted to seismic-16-reflection methods only.On the other hand, Green (1985) included economicevaluation of petroleum prospects into the scope of basinanalysis. Although economics are indispensable for theultimate success of hydrocarbon-exploration ventures, Iprefer to limit the disciplines used in the geologicalbasin analysis to the natural sciences.The integrated geological analysis of the QueenCharlotte Basin includes an examination of the availabledata as a first step. Particular attention is paid to thegeological information provided by field mapping anddrilling (Chapters 2 and 3). The applicability of moderntheoretical models of basin evolution to the QueenCharlotte Basin is critically reviewed in the lightof their applicability to the study area (Chapter 4).Analysis of the geological information constrains theinterpretation of geophysical data (Chapters 3, 5, 6, 7,and 8). Such constraints are necessitated by the non-uniqueness common in geophysical interpretation.Complications arise because different geological featurescan be characterized by similar geophysical signatures(e.g., de Stadelhofen and Juillard, 1987; Blakely andConnard, 1989; Simpson and Jachens, 1989). In the presentstudy, the non-uniqueness is reduced by maximizing thecalibration of geophysical interpretation by geologicalobservations, and by joint consideration of as manygeophysical data sets (gravity, magnetic, seismic) as are-17-available (Chapters 6, 7, and 8).Integrated interpretation of the geological andgeophysical data yields a comprehensive picture of thestructure of the Queen Charlotte Basin (Chapter 7). Thisalso makes it possible to infer the presence and origin ofmany features in the basin and to constrain the geologicalmodel of basin evolution (Chapter 8).The next step is to correlate this model withgeochemical and geothermal data in order to establishadditional controls on the evolution of the basin and toassess the hydrocarbon potential of the region. It thenbecomes possible to prioritize areas of explorationinterest and even suggest potential drilling targets(Chapter 9).These practical results of the present study areintended to reduce the risk inherent in hydrocarbonexploration and to assist subsequent workers in planningmore detailed research and exploration programs.Practice shows that it is sometimes possible to erectrealistic quantitative models of basin evolution in mature,well-explored sedimentary provinces, where the pertinentparameters are reliably constrained by data (see the reviewin Chapter 4). However, quantification of evolution modelsfor the Queen Charlotte Basin is not considered justifiableat present due to a shortage of such constraining data,although it may become feasible at a future date when morewell data are collected.-18-I have therefore restricted the use of quantitativetechniques to the evaluation of the quality of seismicreflection data (Chapter 3) and to the processing ofmagnetic and gravity data (Chapter 5) from the region.Such a use of numerical techniques is subordinated to thepurposes of geological basin analysis, which is the subjectof the present research.Most of the results of this study have appeared inprint elsewhere (Lyatsky, 1991a-b; Lyatsky et al., 1992;Lyatsky and Haggart, 1992), but that material is repeatedhere for completeness.-19-CHAPTER 2. GEOLOGIC SETTING2.1. Pre-Tertiary stratigraphic record onshoreGeologic maps of Queen Charlotte and Vancouver islandsare shown in Figs. 2-1 and 2-2, respectively, and aregional summary map is presented in Plate I (in pocket).The geology of Queen Charlotte Islands is well known as aresult of recent mapping. The stratigraphic succession andstructural evolution of this part of the Cordillera issummarized in Tables 2-I to 2-111; the broad geologicsimilarity of Queen Charlotte and Vancouver islands isreadily apparent. Stratigraphy and structural evolution ofother land areas on the rim of the Queen Charlotte Basinare less well-known due to geologic complexity and lack ofdetailed mapping.The oldest dated rocks in the study area are EarlyCambrian. They are part of a pluton recognized recently onLong and Dall islands in southern southeast Alaska (Fig.1-1). The rocks intruded by this pluton are suspected tobe Precambrian (Gehrels, 1990). A complex assemblage ofyounger Paleozoic rocks of various lithologies is alsofound in southeast Alaska (Gehrels and Saleeby, 1987a-b;Gehrels et al., 1987; Brew et al., 1991).In the Canadian part of the study area, youngerPaleozoic rocks have been identified on the flanks of theQueen Charlotte Basin. They have been described onVancouver Island (Muller, 1977, 1980; Massey and Friday,-20-Plate I (in pocket). Generalized geologic map of the studyarea. Heavy lines are faults identified or confirmed inthis study. The dashed line represents the minimumwestward extent of the plutons inferred to lie beneatheastern Queen Charlotte Sound (Chapter 6). The dottedpattern indicates the area of prime petroleum-explorationinterest (Chapter 9). D - approximate location of Tertiarydepocenters (Chapters 6, 7, and 8). Onshore geology afterWheeler and McFeely (1991).133°^ 132°^ 131°DIXON1771 Quaternary alluvium, tillTertiary Masser Formation andunnamed sedimentaryand volcanic unitsCretaceous Longarm Formationand Queen Charlotte Group^ Jurassic Yakoun Groupni Jurassic and Triassic Kunga^ and Maws Groups,Karmutsen FormationTertiary and Jurassic intrusive^ rocksENTRANCEA^/ / / / / /,/,'• MASSET FORMATION VENT AREAS1 Juskatla Mtn.2 lronside Mtn.3 Seal Inletiii:::iii//1;,;\ HECATE• PALEOGENE SEDIMENTARY ROCKS1 Long Inlet2 Yakoun Lake3 Hip pa Island4 Port LouistpGSC-22-Figure 2-1. Geologic map of Queen Charlotte Islands.LIF - Louscoone Inlet Fault; DCF - Dawson Cove Fault.DCF is roughly coincident with the Rennell Sound Fault ofSutherland Brown (1968). Modified from P. Lewis et al.(1991a).- 2 -LATE AND (?) MIDDLETRIASSICPARSON BAY FORMATIONQUATSINO FORMATIONKARMUTSEN FORMATIONI^ -4 SICKER GROUP PALEOZOICLEGENDCARmANAH GROUP^MIDDLE TERTIARYC ATFA CE INTRUSIONSIV V V V VYvv , METCHOSIN VOLCANICSV VVVVBONANZA GROUP^EARLY JURASSICVANCOUVER GROUPFigure 2-2. Geologic map ofVancouver Island (after Mulleret al., 1981).MILES0^20^40NANAIMO GROUPQUEEN CHARLOTTE GROUPKYUOUOT GROUPLEECH RIVER FORMATIONPACIFIC RIM COMPLEX777 ISLAND INTRUSIONSer.,T7EARLY TO MIDDLETERTIARYEARLY TERTIARYLATE CRETACEOUSLATE JURASSICTOEARLY CRETACEOUSEARLY AND (?) MIDDLEJURASSICVvE,2..1METAMORPHIC COMPLEXES^JURASSIC AND OLDERV ..:41■”\GEOLOGIC TIMESTRATIGRAPHY PLUTONSUnit/Lithology Thickness U — Pb AGE K-Ar AGEHOLOCENE AGE(Ma) QUATERNARY SEDIMENTSPLEISTOCENEUAND VOLCANIC DEPOSITS0Ni0>—W.,8PLIOCENE1.65.2 SKONUN^sandstone— —KANO PLUTONIC SUITE23.3MASSETMIOCENE mudsfone,^col..FM.basolt/rhyolite f o^FORMATION^coalpyroclasfics<5000m000mLa<F__ 0LIGOCENEp . . 2,4 -.—....^.^•^.(..) ccLA15.o EOCENE35,4 UNNAMEDVOLCANICbasalt, ondesite^UNNAMED^bYk ^•flows.^volconi-^SED^ENTARY ROCKS^C • I ..sandstone,..^. ••..^.^.^.^.^.^.40 o•-•-•461— ROCKSPALEOCENE56.566.0clastic^tong.°MAASTRICHTIAN74.0',CAMPANIAN HHUNNAMED SEDIMENTARY^dark grey shaleL., SANTONIAN83.086.6ROCKS^minor sandstoneCONIACIAN gildgri bc^" flV/ ^88.5TURONIANU O XONNA FORMATION conglomerate, sondstone, minor shaleDOL..1,larbldhltmudsfone^sandstone,CENOMANIAN90.597.0C,< ALBIAN O=. HAIDA FORMATIONU1cc0 APTIAN112' BARREMIAN124 LtlNLARME.1 132 s. p•HAUTERIVIAN"" m 0,135 Z ' -.EoVALANGINIAN140g Sv) m 17 amBERRIASIAN PROBABLY 0.`146 c13 ZOF< •145-7.TiTHONIAN z,ce m 147^`,152155,^ .,7 -Ico a.1581537-7,-KIMMERIDGIANOXFORDIAN,^,0 .^.SKI' GATE CHANNEL BEDS^son stone.^—'NI 157• •••.0cn ..__-.) CALLOVIAN 161,/#^.,'^,0' ^.4'.^/ 9/ 7'^749^ /7 / ^,r/z,o/ ,i7 /4,4 • •^•^• 166* -MORESBY GROUP^fciorn.gerveoricz ict=zie. siltstone < 600 m•168BATHONIAN171,-,_,.l'jMV)V)'"ccrLLI00—2166174g lo'a 0f- g'e^/.//Ad/^HictificiA 1 0 Wsi^fidie,^i.907-^A/ a •---172BAJOCIAN YAKOUN^GROUP^andesitic flows, flow breccias, tuff,volcanic^sandst one.^sillslone,^cgl.. < 700 m -.1< WAALENIANM W/ iiniiimiiiiiiiimmireariiiiiew AMP 4 .,'E .E 0,—> ,- vs 1^ s.178 tnc7 0 . 60 0a uTOARCIANPLIENSBACHIAN187 O FANNIN FORMATION  Zo csal194 1-Z 7..,c -,(n 0-SINEMURIANCREEKHETTANGIAN204 thin-bedded silistons208NORIAN PERIL FORMATION^dlan223 KCARNIANT^bd dd 1235rminorKARMUTSEN FORMATION basalt,^limestone^and^argillile .••UmLADINIANV)Ln<238 :.1•j^ ,PANISIAN241 /cc1--SPATHIANSmITHIANP.4 DIENERIANGRIESBACHIAN ' d245O1,i0L., PERMIAN UNNAMED CARBONATE—CHERT UNIT-25-Table 2-I. Generalized stratigraphic column for QueenCharlotte Islands and adjacent areas of the Insular Belt.Shading indicates hiati. Updated from Thompson et al.(1991), P. Lewis et al. (1991a), and Haggart (1992a).-2Table 2-11. Main events in the structural history of QueenCharlotte Islands. Updated from Thompson et al. (1991) andP. Lewis et al. (1991a).GEOLOGIC TIME,STRUCTURAL DEFORMATION; r HOLOCENE AGENo)U5 ig PLEISTOCENE1.6 BLOCK FAULTING;)_cci.PLIOCENEo5.2SEVERAL EPISODES OFEXTENSION ANDIMIOCENEEzwoNa<.7',.23.3 AND COMPRESSION.. OUGOCENEewI __,,,0235.456.5EOCENEPALEOCENE65.0 NORTHEAST—VERGING14AASTRICHTIAN74.0 COMPRESSION; REACTIVATION OFCAMPANIANia83.0 OLD BLOCK FAULTSSANTONIANI'86.5CONIACIAN88.5TURONIAN0= 90.5CENOMANIAN0w 97.00<( ALBIAN1—LaiCeC.) rAPTIAN112124BARREMIAN—'ce4 132HAUTE RIVIAN"1135VALANG I N IAN140BERRIASIAN146 iw TITHONIANo 3 KIM ME RI DGIAN 152 BLOCK FAULTING155OXFORDIANONI 157CALIOVIAN0V)LAj (1) tja 161BATHON IANM VI___,. 00 166 SOUTHWEST—VERGINGBAJOCIAN"""CC--)2 174 I CONTRACTIONAL FOLDINGAND THRUSTINGAALENIANTOARC IAN178ti187PLJENSBACHIANcr<L.,194SINEMURIAN204HETTANGIAN208La NORIAN 223CARNIANC.)f/1235238LA DIN IA NANISIAN04 241SPATH IA NFC1-r SmITHIAN-JCVDIENERIANWGRIE S BAC HIAN245c_)araoL.,_...iccLPERMIANFormation (Lithology)^Intrusives>-cc<1:CCWI-Neo. ? ^?^?Volcanics and SedimentsPaleo-,,^\Catface IntrusionsNANAIMO GROUP^! sandstone, siltstone, conglomerate)UD ?^' ?^?0Lt.(0 QUEEN CHARLOTTE GROUP (sandstone, conglomerate, shale)<1—wce L0 LONGARM FM.^(sandstone, conglomerate, siltstone)cn< M \_Island IntrusionsCCM ‘;' . . -,L HARBLEDOWN FM^ BONANZA VOLCANICS(siltstone, shale, tuff) (intermediate compositions)PARSON BAY FM^(calcareous siltstone, shale)O U QUATSINO FM^(limestone)(.7)(0< KARMUTSEN FM^(basalt, pillow lava)E ----,...----?WIWWWWW■0110 ?"01.****/~~•••••Wo•••? .e.w...".4.~.."...........I..MLPaleozoic Carbonates and Volcanics-28-Table 2-111. Generalized stratigraphic column for northernVancouver Island. Compiled from Muller et al. (1974) andJeletzky (1976). Knowledge of northern Vancouver Islandgeology is sketchy compared to that for Queen CharlotteIslands, therefore, this column contains less detail thandoes Table 2-I.-29-1989; Andrew et al., 1991) and Queen Charlotte Islands(Hesthammer et al., 1991) on the south and west, and alongthe mainland coast of Hecate Strait (Woodsworth andOrchard, 1985) on the east.The ages and stratigraphic relationships of thePaleozoic rocks are obscure. These rocks include a varietyof stratified igneous and sedimentary units and plutons,variously metamorphosed and deformed. Hesthammer et al.(1991) have suggested stratigraphic equivalence of latePaleozoic rocks on Vancouver and Queen Charlotte islands.The Mesozoic sedimentary succession is floored bythick, massive flood basalts of the Upper TriassicKarmutsen Formation. These rocks are ubiquitous on QueenCharlotte and Vancouver islands (Sutherland Brown, 1968;Muller, 1977), and they have also been recognized onBonilla Island in eastern Hecate Strait (Roddick, 1970;Woodsworth, 1988).At first glance, the Karmutsen Formation basaltsappear monotonous. However, detailed studies have revealedmany local variations. Both submarine and subaerial flowshave been documented, and their geochemistry is diverseacross the study area (Barker et al., 1989; Andrew andGodwin, 1989a; P. Lewis et al., 1991a; Richards et al.,1991). The total thickness of the Karmutsen Formation onQueen Charlotte Islands exceeds 4500 m (Table 2-I).The Karmutsen volcanics are conformably overlain byUpper Triassic to Lower Jurassic fossiliferous sedimentary-30-strata consisting of limestone, marine shale, and sandstone(Jeletzky, 1976; Muller, 1977; Cameron and Tipper, 1985;Thompson et al., 1991). These rocks have been studied indetail on Queen Charlotte Islands, where they have beensubdivided into two groups, the Kunga and the Maude,comprising several formations (Table 2-I). The SadlerLimestone and the Peril Formation of the Kunga Group arelargely platform carbonates, whereas the younger SandilandsFormation and the Maude Group are shale, sandstone, andminor tuff. The Sandilands Formation locally containsvolcanic rocks as well (J.W. Haggart, pers. comm., 1992).On northern Vancouver Island, Upper Triassicsedimentary rocks are also found. In contrast to QueenCharlotte Islands, however, the Lower Jurassic strata arepartly sedimentary and partly volcanic. Flows, tuffs, andbreccias of the volcanic component are usually assigned tothe Bonanza Group, widespread on Vancouver Island (Table2-III; Muller et al., 1974; Jeletzky, 1976).Accumulation of the Upper Triassic and Lower Jurassicsediments apparently took place in a single, large, marinebasin which may have extended across much of the westernCordillera (Tipper and Richards, 1976; Jeletzky, 1976;Cameron and Tipper, 1985; Desrochers, 1989). The basin wasshallow and tectonically quiescent, especially in thenorthern part of the study area. Quiescence persistedlonger on Queen Charlotte Islands than on Vancouver Island.Cumulative thickness of the sedimentary rocks of the Kunga-31-and Maude groups exceeds 1000 m.On Queen Charlotte Islands, this sequence is overlainunconformably by the Middle Jurassic Bajocian Yakoun Group,which consists of volcanic and associated epiclasticsedimentary rocks (Cameron and Tipper, 1985; Thompson etal., 1991). Another unconformity separates the YakounGroup from the volcanic-derived clastic rocks of theoverlying Moresby Group and younger, Upper Jurassic,clastic strata (Cameron and Tipper, 1985; Haggart, 1992a).Thickness of the Yakoun and Moresby groups varies butapparently does not exceed several hundred meters.The Cretaceous system on Queen Charlotte Islandscomprises the Longarm Formation and the Queen CharlotteGroup. These units are widespread and are characterized bycoarse- to fine-grained marine clastic rocks; localoccurrences of volcanic rocks have also been noted. Theclastic rocks are primarily lithic wackes, locallyincluding subarkoses and arenites (Fogarassy and Barnes,1989, 1991; Haggart et al., 1989; Haggart, 1991, 1992a).The cumulative maximum thickness of the Cretaceous strataon Queen Charlotte Islands is about 3000 m.During Cretaceous time, sedimentation took placeprincipally in shelf-depth marine environments (Haggart,1986, 1989, 1991, 1992a; Haggart and Gamba, 1990; Gamba,1992). The conformable and laterally continuous nature ofthe Cretaceous strata attests to their deposition in atectonically quiescent setting. This basin was smaller-32-than the one which existed in Late Triassic and EarlyJurassic time. It occupied the site of the present-dayQueen Charlotte Islands and northern Vancouver Island andlikely the intervening parts of Queen Charlotte Sound(Lyatsky and Haggart, 1992; see below).Another sedimentary basin began to form in LateCretaceous time on southern and central Vancouver Island,where some 4000 m of conglomerate, sandstone, mudstone, andcoal of the Nanaimo Group were deposited (Table 2-III;Muller, 1977; Mustard, 1991). These rocks are representedonly locally on northern Vancouver Island, and someequivalents are found on Queen Charlotte Islands (Haggartand Higgs, 1989; Haggart, 1991).2.2. Inferred stratigraphy offshore Little is known directly about the rocks beneathHecate Strait and Queen Charlotte Sound. Drilling hasshown the Tertiary sedimentary package is about 5000 mthick, but most wells failed to penetrate strata older thanMiocene (Shouldice, 1971, 1973; Higgs, 1991; Haggart,1991). Nevertheless, inferences about the underlying rockscan be made by projecting geologic relationships observedonshore and by interpreting geophysical data.The Upper Triassic and Lower Jurassic rocks wereprobably deposited throughout the present-day Hecate Straitand Queen Charlotte Sound. Basalts of the KarmutsenFormation are recognized on both flanks of the Queen-33-Charlotte Basin: on Vancouver, Queen Charlotte, and Bonillaislands (Fig. 1-1). They likely continue beneath HecateStrait and Queen Charlotte Sound.Stratigraphic equivalence of the overlying UpperTriassic and Lower Jurassic sedimentary rocks has beendocumented for Queen Charlotte Islands, Vancouver Island,and many small islands in Queen Charlotte Strait (Jeletzky,1976; Desrochers, 1989). This indicates a wide regionaldistribution of these rocks. On-strike projection of Kungaand Maude Group strata on southeastern Queen CharlotteIslands also suggests these rocks extend beneath QueenCharlotte Sound.Facies analysis of Cretaceous rocks on Queen CharlotteIslands, as well as paleocurrent and provenance studies,indicates that a landmass existed during the Cretaceous onthe site of the present-day Coast Mountains and HecateStrait. Subaerial exposure and erosion of pre-Cretaceousrocks in these areas provided abundant clastic detrituswhich was shed into a marine basin on the site of QueenCharlotte Islands and northern Vancouver Island (Yorath andChase, 1981; Yagishita, 1985; Higgs, 1990; Thompson et al.,1991). Haggart (1991) has proposed a paleogeographic modelfor the Cretaceous basin, according to which much of theHecate Strait area was uplifted during Cretaceous time(Fig. 2-3).Cretaceous rocks on northern Vancouver Island (Table2-111) are similar in lithology to those on Queen Charlotte33 132°^ 1.DI , ONENTRANCErti0 HECATEf4)44.?____,,,\,--,^!^''...\..Ay. 4*:;;\ ‘ STRAIT7NA)A1.0,53'CMORESBY ,0^ISLAND 1'44,4,0km.452"S CFigure 2-3. Eastward-migrating strandlines delimiting theeastern margin of the Cretaceous shallow marine basin onthe site of Queen Charlotte Islands (modified from Haggart,1991). The area of Hecate Strait was uplifted during muchof Cretaceous time.-35-Islands (Jeletzky, 1976; Muller et al., 1974, 1981), andbroadly comparable transgressive patterns have been notedin both areas (Haggart, 1991, 1992b). Non-marineCretaceous deposits were penetrated by the Sockeye E-66 andTyee N-39 wells in Hecate Strait. This suggests apaleogeographic scenario where the area of the present-dayQueen Charlotte Sound was a part of the same sedimentarybasin that included Queen Charlotte Islands and northernVancouver Island; this basin thus extended along the entirewestern part of the study area.These stratigraphic correlations suggest that bothUpper Triassic-Lower Jurassic and Cretaceous sedimentaryrocks are present in abundance beneath Queen CharlotteSound. In contrast, presence of thick Mesozoic sedimentarystrata beneath Hecate Strait is in doubt.The Cenozoic fill of the Queen Charlotte Basin isexposed locally on Queen Charlotte Islands, especially onGraham Island. The Tertiary succession contains Paleogeneand Neogene clastic and volcanic rocks (Timms, 1989;Haggart et al., 1990; White, 1990; Hickson, 1988, 1989,1991; Higgs, 1991). Neogene sediments of the SkonunFormation make up most of the basin fill offshore(Shouldice, 1971; Higgs, 1991), but older sedimentary rocksare likely present underneath.Interpretation of gravity data (Stacey, 1975) andseismic refraction profiles (Clowes and Gens-Lenartowicz,1985) suggests that rocks characterized by low densities-36-and low seismic velocities, similar to those found in theNeogene Skonun Formation, exist beneath Queen CharlotteSound at levels deeper than those reached by drilling.Some of these rocks are probably Tertiary in age. Theyseem to be concentrated in fault-bounded structuraldepressions (Lyatsky, 1991a; Rohr and Dietrich, 1992),although the existing seismic reflection data do not permitan unequivocal determination of the thickness of theCenozoic Queen Charlotte Basin fill because of the limiteddepth of signal penetration (Lyatsky, 1991b; see Chapters 3and 8 for details).The inferred distribution of Mesozoic sedimentaryrocks below the Queen Charlotte Basin offshore is alsoconsistent with the suspected presence of low-density,low-velocity rock material at deep levels beneath QueenCharlotte Sound.2.3. Principal geological stages of regional evolution2.3.1. Paleozoic intervalSeveral episodes of tectonism occurred before theextrusion of the Upper Triassic Karmutsen Formationbasalts. In some areas on Vancouver Island and insoutheast Alaska, Paleozoic rocks are metamorphosed. Theyare usually characterized by a structural patternconsiderably more complex than that observed in youngerrocks (Sutherland Brown, 1966; Brew et al., 1966; Muller,1977, 1980; Woodsworth and Orchard, 1985; Gehrels and-37-Saleeby, 1987a-b; Gehrels et al., 1987; Massey and Friday,1989; Gehrels, 1990).2.3.2. Late Triassic to Early Jurassic intervalBasalts of the Karmutsen Formation covered the studyarea in the Late Triassic. These basalts are overlainconformably, with a gradational contact, by Upper Triassicto Lower Jurassic carbonates and clastic sediments.Such a stratigraphic record reflects regional tectonicquiescence in the study area (Cameron and Tipper, 1985),and similar conditions existed at that time elsewhere inthe western Canadian Cordillera (Tipper and Richards,1976). Only minor tectonism occurred in the latestTriassic on northern Vancouver Island (Jeletzky, 1976).2.3.3. Jurassic episode of tectonismQuiescent sedimentation was interrupted by the onsetof regional tectonic compression, magmatism, andmetamorphism in the Early Jurassic on Vancouver Island(Muller et al., 1974, 1981; Jeletzky, 1976) and in theMiddle Jurassic on Queen Charlotte Islands (Cameron andTipper, 1985; Thompson et al., 1991). Metamorphism ofrocks as young as Early Jurassic, followed by uplift,produced the Westcoast Crystalline Complex, a long, narrowbelt of high-grade metamorphic rocks stretching along thewestern coast of Vancouver Island (Fig. 2-2; Muller, 1977;Isachsen, 1987).-38-Regional metamorphism elsewhere in the region was lessextensive, however, and in most areas pre-Middle Jurassicrocks are not metamorphosed at all. Equivalents of theWestcoast Crystalline Complex have not been recognized onQueen Charlotte Islands.On Vancouver Island, magmatism associated with theEarly/Middle Jurassic orogenic episode is represented bythe calc-alkaline basalts of the Bonanza Group, which islikely comagmatic with the intermediate-composition IslandIntrusions (Table 2-III; Muller, 1977; Andrew and Godwin,1989b; Andrew et al., 1991). On Queen Charlotte Islands,intermediate syn-orogenic and post-orogenic Jurassicmagmatism is indicated by the Bajocian Yakoun Groupvolcanics and the Bathonian to Oxfordian San Christoval andBurnaby Island plutonic suites (Tables 2-I and 2-II;Anderson and Reichenbach, 1991; Thompson et al., 1991).Tectonic compression occurred on Queen CharlotteIslands in the Bajocian. It caused up to 50% shortening onGraham Island and northern Moresby Island, accomodated bysouthwest-verging folds and thrust faults (Thompson et al.,1991; P. Lewis et al., 1991a). As a result, a pronouncedangular unconformity is found at the base of post-EarlyJurassic rocks.On southern Moresby Island, in contrast, rocks weredeformed less, and the Middle Jurassic unconformity istherefore less apparent (Lewis, 1991). The character ofMiddle Jurassic tectonism was thus non-uniform across Queen-39--Charlotte Islands.As was noted by Thompson et al. (1991), timing ofJurassic tectonism and magmatism in the Insular Belt fitsinto a regional eastward-younging trend of migration oforogenic activity, which was also manifested on themainland. In the Coast Belt, field mapping and radiometricage dating indicate an eastward-younging trend ofemplacement, uplift, and cooling of plutons, from LateJurassic to Early Tertiary time (Hutchison, 1982; van derHeyden, 1989, 1992). Consistent with this trend, the areaalong the eastern coast of Hecate Strait was uplifted inthe Late Jurassic and Early Cretaceous.2.3.4. Late Jurassic to Late Cretaceous intervalOn Queen Charlotte Islands, Late Jurassic time wasmarked by the initiation of high-angle normal faulting(Table 2-11). This contrasts with the contractionalfolding and thrust faulting that characterized MiddleJurassic tectonism. Since Late Jurassic time, episodicblock faulting has been the predominant structural style inthe Queen Charlotte Basin area.Dip-slip movements along several large faults involvedrocks of the Middle Jurassic Bathonian-Callovian MoresbyGroup on Queen Charlotte Islands, and verticaldisplacements apparently exceeded 1000 m (P. Lewis et al.,1991a-b; Thompson et al., 1991). The prominent RennellSound)and likely Sandspit,fault systems (Figs. 1-1 and 2-1)-40-were active, as were a number of smaller faults.The vertical block movements controlled sedimentationlocally in the Late Jurassic, seemingly restricting it tofault-bounded structural depressions (Thompson et al.,1991; Haggart, 1991, 1992a). As tectonic activitysubsided, sedimentation spread across much of the studyarea, and a large basin was established in the western partof the region. During Cretaceous time, the strandlines ofthis basin gradually migrated eastward across QueenCharlotte Islands (Fig. 2-3) and northern Vancouver Island(Haggart, 1991, 1992b). Clastic sedimentation on QueenCharlotte Islands continued with few interruptions untilthe Late Cretaceous Campanian or Maastrichtian.2.3.5. Latest Cretaceous and Tertiary tectonismA new period of tectonism in the region occurredduring the latest Cretaceous and early Tertiary. Manyolder high-angle faults on Queen Charlotte Islands werereactivated, with a predominantly reverse sense ofdisplacement (Thompson et al., 1991). These movementsaccommodated a total shortening of about 10%, as determinedby studies of structures in the Cretaceous strata (P. Lewiset al., 1991a-b). Igneous activity took place in thePaleogene on Queen Charlotte and Vancouver islands (Tables2-I and 2-III; Muller, 1977; Andrew and Godwin, 1989c;Anderson and Reichenbach, 1991).Through Tertiary time, deformation on Queen Charlotte-41-Islands continued to be dominated by block faulting. Manyhigh-angle faults were active several times, with differentsenses of motion. Many of them were reactivated olderstructures (Indrelid, 1991). Below I argue that in theoffshore areas a similar deformation style is found, and Iestimate the total Cenozoic tectonic extension in the QueenCharlotte Basin at a maximum of 10% (Chapter 8).No significant Tertiary strike-slip movements arerecognized across the major faults in the Insular Belt; thesame is true for the major faults in the adjacent part ofthe Coast Belt (e.g., Brew and Ford, 1978). Scissor-typemovements did occur locally across the southern part of theLouscoone Inlet Fault on Moresby Island in the Tertiary(Lewis, 1991; Figs. 1-1 and 2-1). These movements wererelated to local extensional faulting and northward tiltingof separate, small crustal blocks on the east side of thefault, but they die out towards northern and southernMoresby Island. Both dextral and sinistral kinematicindicators are recognized in the fault zone, but the timingof these movements is poorly constrained (J.W. Haggart,pers. comm., 1992).No consistent regional tilt of Queen Charlotte Islandshas been revealed by field mapping (Thompson et al., 1991;Hickson, 1991). Neither is a regional tilt apparent fromgeophysical data offshore (see Chapters 6, 7, and 8 for adescription of the Queen Charlotte Basin structure).At present, Queen Charlotte and Vancouver islands are-42-uplifted, while the basin is confined mostly to HecateStrait and Queen Charlotte Sound. North of 52 ° N, in theQueen Charlotte Islands-Hecate Strait area, this representsa complete tectonic inversion since Cretaceous time.2.4. Tectonic setting of the study areaSeismic reflection and gravity data suggest thecrystalline crust beneath Queen Charlotte Sound is thinnerthan that beneath the adjacent areas, and a localizedshallowing of the Moho occurs (Stacey and Stephens, 1969;McMechan and Spence, 1983; Mackie et al., 1989; Yuan, 1990;Drew and Clowes, 1990; Sweeney and Seemann, 1991; Spence etal., 1991). The thickness of the crystalline crust beneaththe Tertiary sediments in Queen Charlotte Sound isestimated to be as little as 18 km, with the crust-mantleboundary located at 23 km. In contrast, the Moho depth isabout 25 km beneath Queen Charlotte Islands, up to 30 kmbeneath Hecate Strait, more than 30 km beneath the CoastMountains, and up to 38 km beneath Vancouver Island.The Queen Charlotte Basin is thought to lie atop theWrangell terrane, or Wrangellia (Fig. 2-4; Jones et al.,1977; Berg et al., 1978, 1988; McMillan, 1991; Gabrielse etal., 1991; Wheeler et al., 1991; Monger and Journeay,1992). Wrangellia is conventionally defined by thepresence of a distinctive stratigraphic assemblagecomprising massive Upper Triassic basalt and overlyinglimestone and clastic rocks (Jones et al., 1977); this rock500 kmTERRANES IN CANADIAN CORDILLERAaccreted terranes shown by (A);pencratonic terranes by (P);terranes with oceanic basementbut cratonic detritus by (A and P);oceanic, marginal basin, andcontinental margin accretionarycomplexes in boldfaceAX ALEXANDER (A)BR BRIDGE RIVER (A)CA CASSIAR (P)CC CACHE CREEK (A)CD CADWALLANDER (A)CG CHUGACH (A and P)CH CHILLIWACK and HARRISON (A)CR CRESCENT (A and P)HO HO (A and P)JF JUAN DE FUCA OCEANIC PLATEKO KOOTENAY (P)MO MONASHEE (P)MT METHOW (A)NS NISLING (P)OZ OZETTE (A and P)PA PACIFIC OCEANIC PLATEPA PACIFIC RIM (A and P)ON OUESNEL (A)SH SHUKSAN (A and P)SM SLIDE MOUNTAIN (A)ST STIKINE (A)TU TAKU (A)WR WRANGELLIA (A)YA YAKUTAT (A and P)YT YUKON-TANANA (A and P)Figure 2-4. Terrane map of the Canadian Cordillera (afterMonger and Journeay, 1992).-44-package is underlain by Paleozoic sedimentary and volcanicstrata (Monger, 1991). In the study area, these upperPaleozoic and especially lower Mesozoic rocks arewidespread on Queen Charlotte and Vancouver islands, andthey are assigned to Wrangellia by definition.Monger et al. (1982) proposed that Wrangellia dockedwith North America in the Cretaceous. This contention hasbeen challenged by Brew and Ford (1983) and van der Heyden(1989, 1992). The last author favors a considerablyearlier time of accretion, possibly Middle Jurassic.Until recently, Wrangellia was viewed as beingdistinct from the Alexander terrane, which is recognized insouthern southeast Alaska and along the eastern coast ofHecate Strait (Woodsworth and Orchard, 1985; Gehrels andSaleeby, 1987b; Brew et al., 1991; Gabrielse et al., 1991;Wheeler et al., 1991). A complex and laterally variableassemblage of Paleozoic and Mesozoic rocks characterizesthe Alexander terrane. Its tectonic evolution is knownsketchily (Gehrels and Saleeby, 1987a; Samson et al., 1989,1990). Data from Alaska suggest the Alexander and Wrangellterranes have formed a single tectonic entity since atleast the Middle Pennsylvanian (Gardner et al., 1988).The Coast Belt, which flanks the study area on theeast, is made up largely of massive, Late Jurassic to earlyTertiary granitoid plutons and their high-grade metamorphicroots; variously metamorphosed pendants of the volcano-sedimentary country rocks are present locally (Woodsworth,-45-1979; Hutchison, 1982; Woodsworth et al., 1983; Crawford etal., 1987; van der Heyden, 1989, 1992; Gabrielse et al.,1991). The Coast Belt is considered to be a Cretaceouscompressional orogen, flanked by thrust faults andforedeeps (Rusmore and Woodsworth, 1991; Monger, 1991).The vast plutons of the Coast Belt mask the boundarybetween the Alexander-Wrangellia terrane on the west, andthe Stikine terrane, or Stikinia, on the east (Monger etal., 1982; Crawford et al., 1987). The latter is a largeterrane in the interior of the Canadian Cordillera, in theIntermontane Belt (Monger et al., 1982; Gabrielse et al.,1991). The relationship between these three terranes isstill uncertain. Van der Heyden (1989, 1992) proposed theyformed a single Alexander-Wrangellia-Stikinia "megaterrane"with no internal tectonic sutures, which docked with NorthAmerica in the Middle Jurassic.New information suggests that most of the QueenCharlotte Basin is underlain by only one terrane,Wrangellia, whose characteristic Karmutsen Formation basaltcrops out on both sides of the basin (Woodsworth, 1988).Tectonic differences between Mesozoic terranes, and theiraccretion history, are of little importance to this study,which deals chiefly with the much younger, Cenozoicevolution of the region.West of the study area lies the present-daycontinental margin of North America. It is coincident withthe Queen Charlotte fault system (Fig. 1-1), along which-46-dextral displacement is occurring at present (Engebretsonet al., 1985; Stock and Molnar, 1988; Riddihough andHyndman, 1989; DeMets et al., 1990; Hyndman and Hamilton,1991). This fault system is the plate boundary between theNorth American continent and the oceanic Pacific plate(Mackie et al., 1989; Bêrube et al., 1989; Riddihough andHyndman, 1989; Lyatsky et al., 1991). The Queen Charlottefault system truncates all geologic trends on western QueenCharlotte Islands (Sutherland Brown, 1968). Possibleinfluences of Cenozoic plate interactions west of the studyarea on the evolution of the Queen Charlotte Basin arediscussed in Chapter 8.Prior to the Frontier Geoscience Program, the QueenCharlotte Basin was thought to have been formed by rifting(sensu McKenzie, 1978) in Tertiary time (Yorath and Chase,1981; Yorath and Hyndman, 1983; Hyndman and Hamilton,1991). In a broadly similar interpretation, Rohr andDietrich (1992) and Dietrich et al. (1992) proposed thatthe basin formed largely as a result of distributed dextralstrike-slip faulting in Hecate Strait, with accompanyingtectonic extension in Queen Charlotte Sound and compressionin Dixon Entrance.These interpretations are inconsistent with the newgeological and geophysical data collected onshore andoffshore (Thompson et al., 1991; Lyatsky, 1991a), as isdiscussed in the present dissertation.I propose a geologic model of the formation of the-47-Queen Charlotte Basin by block tectonic activity similar tothat in many continental areas worldwide (see Chapter 8).I suggest the subsidence of the basin was controlled bycrustal blocks bounded by steep faults. The block mosaicwas inherited from Mesozoic or earlier time. Relativemovements of the blocks occurred again in the Tertiary dueto fluctuations in the regional stress field, possiblyinduced by the passage of the region over a mantle hotspot, orogenic mobility of the Coast Belt, or slightchanges in plate interactions to the west. The basinitself was not disrupted by appreciable strike-slipfaulting, based on cross-cutting relationships anddisplacement histories of major faults onshore and offshore(Thompson et al., 1991; Woodsworth, 1991; Lyatsky, 1991a).In the following chapters, I present evidence for thefault-block model of the Queen Charlotte Basin evolution.The evidence comes from a combined interpretation ofvarious geological and geophysical data sets. I offer acomprehensive interpretation of basin origin and structure,as well as a new hydrocarbon-exploration model for theregion.-48-CHAPTER 3. TERTIARY STRATIGRAPHYOF THE QUEEN CHARLOTTE BASIN FILLAND CORRELATION OF SEISMIC EVENTS3.1. Lithology of the Queen Charlotte Basin fill and difficulties with its stratigraphic correlations The Queen Charlotte Basin contains about 5000 m ofsediments. Because most of the basin is concealed beneaththe waters of Hecate Strait and Queen Charlotte Sound,knowledge of its stratigraphy is limited. Informationabout the fill of the Queen Charlotte Basin is gained fromoutcrops and drillholes.Three main stratigraphic units make up the Tertiarysuccession in the study area (Sutherland Brown, 1968;Shouldice, 1971; Leslie, 1989; Lewis, 1990; White, 1990,1991; Hickson, 1988, 1989, 1991; Thompson et al., 1991;Woodsworth, 1991; Higgs, 1991). From base to top, they are(Table 2-1):(1) an unnamed succession of volcanic flows, mudstone, andsandstone of Paleogene and early Neogene age;(2) the Upper Oligocene-Miocene Masset Formation, composedmainly of volcanic rocks; and(3) the Neogene Skonun Formation, comprising mostlymudstone and sandstone with volcanic and volcaniclasticinterbeds.On Queen Charlotte Islands, the unnamed Paleogeneclastic and volcanic rocks are found locally, whereas-49-volcanics of the Masset Formation are widespread. Semi-consolidated clastic rocks of the Skonun Formation arerestricted mostly to the offshore areas, where theyconstitute most of the fill of the Queen Charlotte Basin.Drillhole penetrations offshore and outcrop mapping onshoreindicate the Skonun Formation is interlayered mudstone,sandstone, and minor coal, with interbeds of volcanic flowsand volcaniclastic rocks and possibly sills.Rohr and Dietrich (1991, 1992) have suggested thatigneous layers in the Tertiary strata offshore are confinedlargely to the deeper parts of structural depressions, i.e.to lower stratigraphic levels. However, field work onQueen Charlotte Islands and on small islands in easternHecate Strait shows that Tertiary sedimentary and igneousor volcaniclastic rocks are interlayered at variousstratigraphic levels (Cameron and Hamilton, 1988; Hickson,1991; Haggart et al., 1990; Woodsworth, 1991; Higgs, 1991).On Graham Island, some of the igneous layers interbeddedwith the basin sediments are 3 to 8 Ma in age (Timms, 1989;White, 1991) and thus occur near the top of the Tertiarysuccession. Neogene volcanic rocks are also found locallyon northern Vancouver Island (Armstrong et al., 1985).Some volcanic and volcaniclastic layers onshore andoffshore are tens or hundreds of meters thick (Shouldice,1971; Patterson, 1989; Woodsworth, 1991). Individualigneous layers are not extensive laterally, however, andhence are not reliable markers for stratigraphic-50-correlations in the Queen Charlotte Basin.To hinder correlations further, many faults are foundin the basin. The faults were active at different times,in some cases more than once, and they disrupt differentlevels of the basin fill (Lyatsky, 1991a-b; Rohr andDietrich, 1990, 1991, 1992; Chapter 8). Seismicityindicates that some of the faults are still active (Rogerset al., 1988; Bêrube et al., 1989; R.B. Horner, pers.comm., 1989). Because of vertical movements of numerousblocks, many local unconformities developed in the basin(Shouldice, 1971), which also complicates correlations.Additional stratigraphic complexity arises because offacies variations which are common in the Queen CharlotteBasin. Regional facies changes have long been recognizedin the Skonun Formation across the basin, based ondrillhole and outcrop information: whereas marinesedimentary rocks predominate beneath Queen CharlotteSound, non-marine deposits are more typical in the northernpart of the basin. Local facies variations also occur inthe Tertiary sedimentary rocks, but they are less welldefined (Shouldice, 1973; Patterson, 1989; Higgs, 1991).Chronostratigraphic correlations of Tertiary rocks inthe Queen Charlotte Basin are based largely on pollen andforaminifera (Hopkins, 1981; Patterson, 1989; White, 1991).The poor resolution inherent in these techniques, coupledwith limited sampling offshore, permits only very coarseage determinations. The biostratigraphic intervals thus-51-dated are hundreds or even thousands of meters thick, andthey represent time periods of millions of years duration.The exact boundaries of these intervals are also uncertain.Despite these complications, detailed well-to-wellcorrelation of Neogene strata offshore was attemptedrecently by Higgs (1991), based on wireline logs and rocklithologies. He proposed a subdivision of these stratainto two major stratigraphic "units" - syn-rift and post-rift - corresponding to the two principal stages of basinevolution in McKenzie's (1978) theoretical model offormation of sedimentary basins by stretching of thelithosphere. Higgs assumed a priori that such ageodynamical mechanism was responsible for the creation ofthe Queen Charlotte Basin.Higgs arbitrarily associated extensional faultactivity in the basin with the deposition of only the lower(syn-rift) "unit", which he suggested comprises laterallydiscontinuous lithofacies. In contrast, he assumed theoverlying "unit" was deposited in a quiescent post-riftsetting and is therefore characterized by regionallycontinuous facies. In particular, he suggested thatsandstone lenses only 10 to 30 m in thickness, reflected indrillhole logs, can be correlated across the basin.I argue that Higgs' (1991) model of two-phase basinfill (syn-rift/post-rift) is unsupported by the data andthat his detailed stratigraphic correlations are unfounded.Higgs has failed to provide objective criteria that would-52-permit an unequivocal differentiation between his two"units" based on lithology. His identification of the"unit" boundary is artificial because no fundamentaldifferences are apparent between the Tertiary rocks in thetwo stratigraphic "units", based on the available outcropand well-core descriptions and the present writer'sexamination of the core. Furthermore, the quality ofwireline logs from the wells is degraded due to sidewallcaving of the semi-consolidated sediments (Hopkins, 1981;Patterson, 1989).Log-based well-to-well correlations of lithofaciesmay, of course, be reliable in structurally uncomplicatedand mature sedimentary basins where drillhole control istight. The Queen Charlotte Basin meets neither of theserequirements, and detailed stratigraphic correlations overlarge distances are thus speculative and may be misleading.Numerous local unconformities and facies changes arefound at various stratigraphic levels in the QueenCharlotte Basin, as established by drilling in the 1960s(Shouldice, 1971, 1973). No new data exist to suggest thisconclusion should be reconsidered. Higgs himselfinterpreted his upper "unit" to have been deposited indelta-plain and tidal-shelf environments, or fartheroffshore but usually still above the storm wave base. Suchdeposits are, of course, unlikely to contain thin bedscoherent for tens or hundreds of kilometers.Mindful of these conditions, Shouldice (1971, 1973)-53-suggested that well control in the Queen Charlotte Basin isinadequate for detailed stratigraphic correlations.Subsequently, more detailed studies of well data confirmedthis conclusion, and Patterson (1989, p. 230) cautionedthat "due to structural and lithological complexities inthe region, a correlation between the Murrelet L-15,Harlequin D-86, and Osprey D-36 wells is not possible usingmechanical logs or lithotypes".Higgs (1991) failed to provide specific justificationfor his regional correlation of the sandstone lenses andother rocks in the basin. In the absence of detailedoutcrop or well control, it is possible, even likely, thatthe thin sandstone beds are discontinuous laterally, beinga recurrent lithofacies which formed in response to localfluctuations in paleoenvironment.I conclude that the well data from the Queen CharlotteBasin are too sparse and generalized to permit detailedstratigraphic correlation of Neogene rocks offshore. Noregional markers are apparent in the basin.3.2. Difficulties with seismic stratigraphic correlations 3.2.1. Reflection coefficientsDirect information on seismic velocities and densitiesof rocks in the Queen Charlotte Basin area is derived fromwell logs and outcrops, but the amount of such data islimited (Chapter 5). Additional, less reliable constraintson these parameters are obtained indirectly, by processing-54-and modeling of reflection and refraction seismic andgravity data (Young, 1981; Clowes and Gens-Lenartowicz,1985; Pike, 1986; Mackie et al., 1989; Yuan, 1990; Rohr andDietrich, 1990, 1991, 1992; Sweeney and Seemann, 1991;Spence et al., 1991; Hole et al., 1992).Seismic reflection and refraction surveys do notinvariably yield an identical or unique velocity structureof the subsurface (Berry and Mair, 1980). For example,apparent velocities derived from refraction data may behigh due to acoustic anisotropy of the rock mass, as wassuggested by, among others, Davis and Clowes (1986) andPavlenkova (1989). This is especially true where igneousflows and sills are interlayered with sedimentary rocks(Davis, 1982), as is common in the Queen Charlotte Basin.The amplitude reflection coefficient at a geologiccontact is defined by the contrast in acoustic impedanceacross the contact. Acoustic impedance, Z, is a product ofseismic velocity and rock density. If Z 1 and Z 2 representacoustic impedances of the media of incidence andtransmission, respectively, the normal-incidence reflectioncoefficient, RC, is given by (e.g., Sheriff and Geldart,1982, p. 67)RC = (Z; -Z 1 )/(Z 2+Z1 .)^ (1)The available information about rock properties in theQueen Charlotte Basin, incorporated into equation (1),-55-indicates that reflection coefficients for P-waves atsedimentary-igneous contacts in the basin may exceed 0.2.The principal contributing factor is velocity variationsacross interfaces (31-1 km/s for Tertiary sedimentary rocksvs. 5±1 km/s for igneous rocks), whereas density variationsare smaller (2,300±200 kg/m3 for sedimentary rocks vs.2,700 kg/m 3 or more for igneous rocks). Because theseismic energy transmitted across an interface is less thanthe incident energy, an abundance of strongly reflectivesedimentary-igneous contacts in the Tertiary section canproduce significant transmission losses in the downgoingand upgoing seismic signal. Such losses should be takeninto account in the interpretation of seismic reflectiondata from the study area.3.2.2. Acoustic basement vs. geologic basementOne major difficulty in seismic interpretation in theQueen Charlotte Basin is distinguishing between thegeologic and acoustic basement in seismic sections.Sheriff (1984) defined acoustic basement as "the deepestmore-or-less continuous seismic reflector; often anunconformity below which seismic energy returns are poor orabsent". Thus, acoustic basement is defined, notgeologically, but geophysically. Its position in a seismicsection is influenced by such factors as power of seismicenergy source, surface or sea-floor conditions, signalattenuation during propagation, as well as data-recording,-56--processing, and -plotting parameters. The definition ofacoustic basement is also a function of basin stratigraphyand interpreter's bias.Furthermore, unlike the geologic basement of asedimentary basin, the acoustic basement may or may notbe marked by a single interface. Its correlation withspecific unconformities (or the crystalline basement) issubjective in the absence of independent knowledge of basinstratigraphy.The present study suggests that acoustic basement inthe seismic sections from the Queen Charlotte Basin occursat various stratigraphic levels within the basin fill. Itdoes not correspond to a single, continuous, isochronousgeologic surface.3.2.3. Seismic transmission losses in the Queen CharlotteBasinThe abundance of strongly reflective sedimentary-igneous contacts in the Queen Charlotte Basin may cause asevere decay of seismic signal during its propagationthrough the rock mass. To evaluate signal degradationquantitatively, I computed transmission losses forone-dimensional layered earth models containing only twolithologies with contrasting acoustic impedances.In general, where N reflective interfaces occur abovethe target, the effective reflection-attenuation factor, F,is defined by the equation (Al-Sadi, 1980):N^ -57-F = RT 11 (1-R i)^ (2)J=1where RT is the reflection coefficient at the targetreflector, and Rj is the reflection coefficient atinterface J above the target. The term (1-Ri) defines thetwo-way transmission coefficient at interface J. Normalincidence is assumed throughout the section, and allcontacts are taken to be welded (sensu Krebes, 1987).All beds are assumed to be sufficiently thick topermit the application of definitions of reflection andtransmission coefficients at a half-space boundary. Thisis justified because the dominant wavelength in the first 2s traveltime in the seismic sections is 50 to 100 m,comparable to the thickness of many igneous layers.If the number of igneous layers above the target is Xand if each layer contributes two reflective surfaces (topand base), then the number of interfaces where transmissionlosses would occur is N=2X. If reflection coefficients atall interfaces above the target have the same absolutevalue, R, equation (2) simplifies toF = R T (1-R 2 ) 2X = RTP^ (3)The parameter, P, which represents the decay of the arrivalfrom the target due to transmission losses, isP = F/R T = (i_R 2) 2X^ (4)-58-i.e. the product of two-way transmission coefficients atall interfaces above the target. With large transmissionlosses, P decreases dramatically, and the seismic image ofthe target degrades.Equation (4) was used to compute signal decay for Rranging from 0.10 to 0.26 and X from 1 to 17. Thisaccounts for a wide range of possible stratigraphicsituations in the Queen Charlotte Basin.Roksandic (1985), who reported the results of atheoretical investigation of acoustic-signal decay in astratified rock mass, relied on synthetic-seismogramtechniques to compute and illustrate transmission losses.This approach was avoided in the present study, however,because of the large number of earth models considered.Instead, results of the calculations were contoured on anR-X grid, and they are presented in Fig. 3-1.This diagram shows that the value of P dependsstrongly on the layered earth model selected to representbasin stratigraphy. Transmission losses are small and P islarge for low values of R and X, i.e. for models with fewstrongly reflective interfaces. As R and X increase,however, transmission losses grow. In a seismic section,this could degrade the signal-to-noise ratio at largetraveltimes, potentially raising the acoustic basement tohigher stratigraphic levels and complicating seismicstratigraphic correlations.2-4-6-Xmmw0 8 -zE1--U)LL0 10-mwmMm 0z 12-c?14-16-REFLECTION COEFFICIENT (R).10^.14^.18^.22^.26Figure 3-1. Attenuation of seismic signal, P, as afunction of the number of igneous sheets, X, and reflectioncoefficient at contacts, R, based on equation (4) in text.The parameter, P, whose vaules are contoured, is a measureof transmission losses. From Lyatsky (1991b).-60-3.2.4. Pitfalls in seismic interpretation caused bytransmission lossesIn addition to igneous layers, coal seams within theQueen Charlotte Basin also cause high-amplitude reflections(Rohr and Dietrich, 1991). This is expected, as coal seamsgenerally have extremely low acoustic impedance (Lyatskyand Lawton, 1988; Lawton and Lyatsky, 1991). Transmissionlosses occur also at mudstone-sandstone contacts.The number of strongly reflective interfaces in theQueen Charlotte Basin varies laterally. The number ofigneous sheets changes from drillhole to drillhole, rangingfrom none to more than 10. As a consequence, the qualityof seismic images of the basin fill may vary: deep parts ofthe Tertiary section are imaged better where the seismicsignal suffers fewer transmission losses at shallow levels.Scattering of upgoing and downgoing seismic energy due tointernal inhomogeneities and surface roughness of igneouslayers may further reduce the signal-to-noise ratio atlarge traveltimes. Energy scattering occurs also where thebasin fill is disrupted by faults.The high reflection coefficients at sedimentary-igneous and other contacts, combined with constructiveinterference effects such as thin-bed tuning (Widess, 1973)of reflections from igneous layers, may produce seismicimages of the basin with a stratified section at shorttraveltimes, separated by one or several high-amplitudeevents from a lower section containing few coherent-61-reflections. An unwary observed may form a falseimpression that geologic basement has been imaged, and hecould confuse the the acoustic basement with the floor ofthe entire sediment-filled basin.Of course, not all seismic energy is dissipated in theQueen Charlotte Basin fill. Some low-frequency arrivalsare apparent at traveltimes up to 8 s. However, theirdominant wavelength is hundreds of meters, and such signalis of little interest for imaging the Tertiary rocks.Because the number and stratigraphic setting ofTertiary igneous layers varies across the basin, seismicstratigraphic correlations are complicated, and theacoustic basement in reflection profiles can represent adiachronous set of geologic surfaces within the basin.Direct correlation of seismic events along reflectionprofiles may therefore lead to a false picture of thestructure and stratigraphy of the Queen Charlotte Basin.3.2.5. Example of interpretation of seismic dataAbout 1000 km of modern multichannel seismicreflection data are available from the Queen CharlotteBasin (Fig. 1-3). In this section, I present a detailedinterpretation of one of the profiles (Figs. 3-2, 3-3)to illustrate the difficulties which may be encountered incorrelating seismic events and interpreting thedisplacements across faults. I conclude that the availableseismic data are not characterized by signal penetrationD832B33252F.=> 2CC>- 3401110 \kmEiw 1P-_,w> 2<CCF-> 3-<40I—Figure 3-3. A structural interpretation of the seismicprofile in Fig. 3-2, showing the main faults (white linesStratigraphic horizons are not picked because ofuncertainties in their correlation (from Lyatsky, 1991b).-64-and density of subsurface coverage sufficient to permit areliable, detailed interpretation of the Queen CharlotteBasin structure and stratigraphy. The basin is apparentlytoo complex, both stratigraphically and structurally, toallow for high-quality imaging of its fill using seismicreflection techniques.Unmigrated data are presented in Figs. 3-2 and 3-3. Iused such data for structural interpretation because theycontain diffractions that help locate faults at largetraveltimes. Migrated profiles were also examined, inconjunction with the unmigrated ones.In the southeastern part of the profile in Fig. 3-2,acoustic basement occurs at 700 to 800 ms. The appearanceof raised basement is likely caused by an abundance ofshallow igneous rocks in this area, as supported by theinterpretation of seismic refraction data by Clowes andGens-Lenartowicz (1985). These authors inferred massive,high-velocity volcanic rocks at a depth of only about 1 km.The massive nature of these rocks is not proven, however,as Davis (1982) has cautioned that seismic refractiontechniques do not always permit unambiguous discriminationbetween massive volcanics and those interstratified withsedimentary rocks: high refraction velocities maycharacterize both situations. Nonetheless, magneticanomalies over this part of the profile have an amplitudeof some 500 nT and a substantial short-wavelength component(Chapter 5), consistent with a shallow igneous source.-65-In contrast to the southeastern part of the seismicsection, coherent reflections are observed to thetraveltime of 2000 ms or more near the Harlequin D-86 well,where shallow igneous layers are lacking. Dominantlithologies in this well are interbedded sandstone andmudstone (Patterson, 1989), which presumably produce theobserved reflections. Seismic refraction data of Clowesand Gens-Lenartowicz (1985) also show an increasedthickness of low-velocity material in this area.A structural interpretation of this seismic profile isshown in Fig. 3-3. If the potential diachronism of theacoustic basement is ignored, northwest-side-down faultswith a cumulative throw corresponding to as much as 2000 mstwo-way traveltime may be required to explain the seismicreflection pattern between shot points (SP) B33000 andB33250. Northwest-side-down faults undoubtedly occur inthe area, but a displacement corresponding to 2000 ms isunlikely. Reflections at traveltimes 1000 ms or less areminimally affected, and they do not exhibit the amount ofdrape that may be expected over faults with large dip-slipoffsets. Moreover, some of the faults, such as the onenear SP B32890, seem to accomodate southeast-side-downdeformation. The bathymetric depression between SP B32500and B32950 (Mitchell's Trough of Luternauer and Murray,1983) causes a velocity pull-down, but its amount is smalland its boundary at SP B32950 does not coincide with thelocation of the above-noted southeast-side-down fault.-66-Northwest-side-down displacement is thought to characterizethe faults at SP B33020 and B33160.The high-amplitude seismic event which represents theacoustic basement in the southeast loses amplitude towardthe Harlequin D-86 well, where no igneous layers wereintersected above 3135 m (Patterson, 1989). Dissipation ofthe igneous-related event coincides with strengthening ofseismic reflections at traveltimes up to 2000 ms and withdecay of magnetic anomalies, whose short-wavelengthcomponent is reduced and whose amplitudes drop to about 100to 200 nT near the well (Chapter 5).I propose, therefore, that the apparent northwestwardthickening of the Tertiary sequence in the seismic sectionmay be caused partly by improvement in seismic-energypenetration due to disappearance of shallow reflectiveinterfaces associated with igneous rocks. If this is true,the acoustic basement in the profile is not an isochronousgeologic surface. Some of the large-traveltime reflectionsnear the Harlequin D-86 well may be correlative with theweak "sub-basement" events to the southeast.The inferred volcanic rocks on the southeast side ofthe profile may be underlain by a large plutonic feedersystem (Fig. 3-3). This would explain the high seismicvelocities of rocks beneath the shallow volcanics, firstdetermined by Clowes and Gens-Lenartowicz (1985) to beabout 5 km/s based on an unreversed seismic refractionprofile. Processing of multichannel seismic reflection-67-data by Rohr and Dietrich (1990) showed that rocks at 800ms traveltime have interval velocities of about 3 km/s,rapidly increasing to 5 km/s at a traveltime of 2 s.Presence of voluminous igneous rocks may also account forthe persistence of the magnetic high in that area when thedata are upward continued to 20 km (Chapter 5) and even 40km (Teskey et al., 1989a). A gravity high of 20 to 30 mGalabove the inferred igneous system may reflect densitycontrasts with the surrounding sedimentary rocks.The suggested igneous complex may be connected withthe large Neogene plutons beneath eastern Queen CharlotteSound inferred from potential-field data (Chapter 6; PlateI in pocket). If the "sub-basement" seismic events beneathMitchell's Trough (Fig. 3-3) are primary reflections, theymay be interpreted as sills in the sediments housing apluton, or as layers within the igneous complex.Sagging of the heavy igneous system could have causedstructural inversion and southeast-side-down faulting, andthis may have led to the formation of Mitchell's Trough,which was later modified by Quaternary ice (Chase et al.,1975; Luternauer and Murray, 1983; Luternauer et al.,1989). Yorath and Chase (1981) speculated that evolutionof large bathymetric features in Queen Charlotte Sound mayhave been influenced by movements on older faultsunderneath. I note that the northeast trend of Mitchell'sTrough coincides with a regional fault network in theInsular Belt and with the orientation of other-68-physiographic features (Chapter 6; Lyatsky, 1991a; Peacock,1935), which may indeed suggest some structural control onits evolution.3.3. Implications for research methodologyAlthough knowledge of the geology of the study areahas improved as a result of recent field mapping, directinformation about the offshore area is still derived fromjust eight old drillholes (Fig. 1-3). No reliable,detailed interpretation of basin stratigraphy is possiblebased on the well data, because the biostratigraphiccontrol and the quality of well logs are poor.Although the new seismic reflection profiles helpinterpret the structure of the Queen Charlotte Basin, theyare widely spaced, and correlation of faults between themis ambiguous. Seismic stratigraphic correlations are alsocomplicated, even along a single profile. Existence ofigneous layers within the Tertiary basin fill influencesthe position of acoustic basement in seismic sections.Even where shallow igneous rocks are lacking, substantialdecay of the signal-to-noise ratio is observed attraveltimes exceeding 2000 ms, and acoustic basement is notalways represented by a single, prominent seismic event.Thus, mapping of basin-fill thickness, stratigraphy,and structure from seismic reflection data alone is not areliable technique in the study area.The complications encountered in the interpretation of-69-the Queen Charlotte Basin stratigraphy and structure fromthe drillhole and seismic data necessitate a more robustapproach to the study of this basin. Fortunately, gravityand magnetic data offer the coverage required fordelineation of fault networks and crustal blocks in theregion (Lyatsky, 1991a). In Chapters 6, 7, and 8, avariety of geological and geophysical data sets areintegrated to constrain the interpretation of the structureand evolution of the Queen Charlotte Basin, and to evaluateits petroleum potential.-70-CHAPTER 4. REVIEW OF PRINCIPAL GEODYNAMICAL MODELSOF FORMATION OF NON-COMPRESSIONAL SEDIMENTARY BASINSON CONTINENTAL CRUST4.1. Modeling of basin evolution - general statementTwo new trends in the analysis of sedimentary basinshave emerged in the last twenty years. One trend is todevelop generalized geodynamical schemes of basinevolution, and the other is to quantify the models of basinformation. These research orientations arose from ourgrowing knowledge of the geology of various areas anddeeper understanding of the principles of tectonics.Generalized basin-formation models may allow broadinferences about the evolution of a sedimentary basin. Onthe other hand, they sometimes ignore the basin'speculiarities. Some quantitative methods of data analysisand process modeling are useful in the studies of mature,densely drilled sedimentary basins, but their applicationto poorly studied frontier areas may be unwarranted due tolack of constraints on the input parameters.In the present chapter, three most often-citednon-compressional models of basin formation are discussed:the stretching (pure-shear or rift) model, the detachment(simple-shear) model, and the mantle-plume/phase-changemodel. Their applicability to the study of the QueenCharlotte Basin is also evaluated.-71-4.2. Stretching (pure-shear) model of basin formation4.2.1. FormulationThe stretching model of basin formation is invokedmore frequently than others, and it has been applied to theQueen Charlotte Basin (Yorath and Hyndman, 1983). Thismodel is discussed in detail in this chapter, and itsadvantages and pitfalls are pointed out. Its applicabilityto the study of the Queen Charlotte Basin is assessed inChapter 9, based on the discussion presented here coupledwith the geological and gephysical constraints establishedfor the region in Chapters 6, 7, and 8.The stretching model is attractive because it offersan elegant apparatus for quantitative modeling of thermaland subsidence histories of extensional basins. Thisapproach is based on the ideas formulated originally byMcKenzie (1978), who presented a theoretical model of thethermo-mechanical response of the continental lithosphereto instantaneous stretching and thinning. In this model,stretching results in passive upwelling of the underlyinghot asthenosphere, as well as rifting and rapid isostaticsubsidence of the upper levels of the crust; the volume ofthe crust is assumed to be conserved at this stage.Asthenospheric upwelling creates a positive heat-flowanomaly. Its conductive dissipation during the post-riftstage of basin evolution allows the lithosphere to regainits original thickness by addition of material from below,but the thickness of the crystalline crust is not thereby-72-restored. Dissipation of the thermal anomaly results inslow, broadly distributed subsidence of the affected area,and a basin forms as sediments fill the depression.Isostasy requires that the sedimentary load double or eventriple the depth to the floor of the basin.This model is strictly one-dimensional. It assumesAiry isostasy, a laterally homogeneous lithosphere, and nolateral heat conduction. Further, no convective heattransfer is assumed to occur within the basin.The amount of initial, rapid subsidence, denoted byMcKenzie (1978) as S I , can be computed if thermal-expansioncoefficients, initial thicknesses, densities, etc. areknown for the continental crust, mantle lithosphere, andasthenosphere. Knowledge of the amount of tectonicextension is also required. The value of SI is negative,representing subsidence as a first response to stretching,if the initial crustal thickness exceeds 18 km. The normalthickness of the continental crust is about double thisvalue, therefore, the model predicts subsidence to be theusual initial response to rifting.Second-stage, thermal subsidence, denoted S T , is slow,and its amount and duration are computed by solving theconductive heat-flow equation in one dimension. Themagnitude of the thermal anomaly is presumed to be relatedto the amount of tectonic extension during the first stage.Thermal history of a basin can be inferred from a study ofthermal maturation of its fill, and its subsidence history-73-can be inferred if the stratigraphy of the basin is known.A peculiarity of the McKenzie (1978) model is that it mayrequire the lithospheric-stretching parameter, a, to haveextremely high values, up to 4 or 5 in some cases.This model is oversimplified, and many attempts havebeen made to refine the modeling procedure by incorporatingadditional factors such as dissimilar rheologies ofdifferent lithospheric layers, finite rates of initialextension, flexural strength of the lithosphere, etc. Forexample, Jarvis and McKenzie (1980) examined the effects offinite stretching rates on the extensional models of basinformation. They concluded that long-lived stretching wouldproduce a greater initial subsidence than would theinstantaneous stretching of McKenzie (1978), especially ifthe duration and amount of extension are considerable.Cochran (1983) also proposed that a long-lasting stretchingepisode would produce increased synrift subsidence, butpost-rift subsidence would be reduced. He furthersuggested that lateral heat conduction would acceleratesubsidence at the early post-rift stage.Royden and Keen (1980) advanced a basin-formationmodel with the continental crust and mantle lithospherehaving different stretching parameters. This requires theformation of a structural detachment at the base of thecrust. These authors also attempted to incorporate theeffects of radiogenic heat production within thesupracrustal sedimentary layer into their calculations.-74-Watts et al. (1982) considered the role oflithospheric flexural strength in basin evolution. Theysuggested that Airy isostasy is an oversimplification ofreality, and its application may lead to underestimation ofA values. These authors also examined the complicationsarising from the hypothetical increase in the lithosphere'sflexural strength caused by post-rift cooling. Incontrast, Beaumont (1978) proposed that the strength of aflexed lithosphere may decrease with time due to thelithosphere's inelasticity.Le Pichon and Sibuet (1981), and later White et al.(1986), took a more practical approach, attempting torelate estimated fault geometries and tilts of individualfault blocks in a stretched zone to the amount ofextension, as is discussed also in Chapter 8. Bellingerand Sclater (1983) sought to explain the commonly observeduplift of rift flanks by taking into account the effects oflithospheric flexure; like some of the other workers, theyfavored the idea of a dissimilar amount of extension atdifferent levels in the lithosphere. Others have notedthat extensional basin-formation models are complicatedfurther by the possibility that dense, mantle-derivedigneous material may be injected into the extendedcontinental crust, also causing subsidence (Sclater et al.,1980; Royden et al., 1980).A result of these and other modifications to thestretching model of McKenzie (1978) has been a greater-75-dependence of the output on a priori assumptions, as wellas an increased sophistication of the mathematicalapparatus. Constraining the model parameters has thusbecome even more critical than before. Besides, aconsiderable loss of generality has occurred, caused by thereplacement of constants in McKenzie's (1978) equationswith parameter-dependent functions, and by the expansion ofthe model beyond one dimension. Nevertheless, thestretching models are still attractive because of theirtheoretical elegance, ease of implementation on a computer,and ability to relate mathematically the assumed thermalhistory and the burial history of an extensional basin.4.2.2. Points of contention4.2.2.1. Thickness of continental platesMcKenzie (1978) and many other workers assumed alaterally homogeneous lithosphere 125 km thick. Indeed,the base of the lithosphere in continental regions is ofteninferred to lie a depth of 100 to 150 km (Bonatti, 1987).However, large deviations from this range are common. Forexample, flexural modeling of the Late Cretaceous andTertiary subsidence of the Western Canada foreland basinshows the effective thickness of the North American plateto change from just 38 km beneath the eastern Cordillera,to more than 200 km beneath the Canadian Shield (Wu, 1991).In southern Africa, the lithosphere of cratonic regions isup to 190 km thick, decreasing to only 140 km under the-76-surrounding mobile belts, based on studies of distributionand petrology of kimberlite pipes and diamonds (Boyd andGurney, 1986; Haggerty, 1986).Even deeper continental roots are suggested by seismicstudies of the earth's interior. The work of Jordan (1979)and Anderson and Dziewonski (1984) shows that high-velocity(low-temperature?) zones beneath the continents extend todepths of up to 400 km (Fig. 4-1). Shear-wave-splittingstudies of teleseismic arrivals indicate that, in theArchean Superior Province of the Canadian Shield, velocityanisotropy trends exhibited by the upper mantle to a depthof 200 to 250 km are similar to surface geologic trends;this indicates the continental root likely formedsimultaneously with the continent (Silver and Chan, 1988).Jordan (1979) also considered the deep roots to bestationary with respect to the continents, thusconstituting integral parts of continental plates.Therefore, the idea that plates are simply largelithospheric blocks of constant thickness is misleading,and Morgan's (1968) term tectosphere is more appropriate(Jordan, 1979). Thickness, rheology, and other mechanicalproperties of continental roots are poorly constrained, andtheir effect on the response of the tectosphere tostretching is hard to predict. It is likely, however, thatthese roots are more rigid than the asthenosphere proper.Moreover, the pre-rift thickness of stretched lithospheremay be difficult to determine, and the value of 125 km-78-Figure 4-1. Variations in thickness of Earth'stectosphere, shown in a generalized diagram (modified afterJordan, 1979). Thicknesses up to 400 km have beenencountered beneath Archean cratons, whereas the oceanictectosphere is only between 100 and 150 km thick. Valuesbetween these two extremes are associated with Proterozoicand Phanerozoic continental regions.-79-should not be accepted uncritically. Lateral rheological inhomogeneities in thecontinental lithosphereA laterally homogeneous continental lithosphere isassumed in many stretching models, following McKenzie(1978). This assumption is weakened by the recognition oflateral variations in lithospheric thickness. Complexityoccurs also within the continental crust and upper mantle,where abrupt lateral variations in the velocity structurehave been revealed by deep seismic surveys. An examplefrom Scandinavia was presented by Clowes et al. (1987), anda comprehensive discussion of the structure of the crustwas provided by Meissner (1989). Lateral variations inseismic velocities may be associated with inhomogeneitiesof rheological properties of the lithosphere.The assumption of uniform distribution of tectonicextension across a basin is also inherent to manystretching models. This, however, is hard to reconcilewith the existence of lateral inhomogeneities within thelithosphere. According to Illies (1981, p. 263), a"specific peculiarity of the continental crust is itsnearly infinite memory of all stages of previous tectonicdeformations. The fabric of faults, joints, metamorphismor magmatism survive mostly as relics during subsequenttectonic revolutions. The fluctuating regional stressconditions of successive orogenic events create-80-structures that are often posthumously reactivatedfeatures of older strain generations..."Largely for this reason, extensional strain is usuallyconcentrated in relatively narrow rift valleys whose"progression follows old weakness zones".In general, evolution of an extensional basin islikely to be strongly influenced by pre-existing regionalstructures, as Swanson (1986) and Grant (1987) havedemonstrated for basins along the east coast of the U.S.and Canada. I contend the same is true for the QueenCharlotte Basin, where pre-Tertiary fault networks werereactivated in the Cenozoic and thus played a critical rolein basin evolution. These networks were not obliterated byTertiary tectonism, which suggests the amount of extensionwas very small (Chapters 7 and 8).Moreover, the assumption of conservation of crustalvolume during rifting is also questionable (Ziegler, 1992;Rosendahl et al., 1992). Rifting may cause intrusion intothe lower crust of mantle-derived material, giving theaffected part of the crust mantle-like properties (i.e.high density and seismic velocity). Thus, estimates ofbasin extension from geophysically determined variations inthe Moho depth may be grossly exaggerated. Mechanism of continental riftingA rift is defined as (Bates and Jackson, 1980, p. 538)"a long, narrow continental trough that is bounded by-81-normal faults; a graben of regional extent. It marks thezone along which the entire thickness of the lithospherehas ruptured under extension."Such a feature may be produced by domal uplift (Bott, 1976,1980) and/or localized extensional necking (Freund, 1967)of the lithosphere. Faults within rift zones commonly havea single dominant orientation, ideally perpendicular to thedirection of the horizontal extensional principal stress.Indeed, presence of such a structural fabric may be used asa test for the existence of a rift (Chapters 6 and 7). Thelocation and orientation of a rift may also be affected bypre-existing fault networks and crustal weakness zones.In the stretching models of basin formation (e.g.,McKenzie, 1978), rifting is defined more loosely, andtectonic extension is sometimes assumed to be distributedover a wide area. The combination of a high A value and abroad rift zone may imply greater lateral movements of thebounding crustal blocks than does the first definition. Awide rift, like a narrow one, is expected to becharacterized by numerous, subparallel, normal faults.Inherent to many stretching models of basin formationis the assumption that rifting occurs passively, due toshallow-seated extensional stresses supplied from afar.This assumption is weak for two reasons. First, moststretching models "in their present form... cannot explainuplift associated with rifting" (Morgan and Baker, 1983, p.3). Second, "mysterious stretching forces far away from-82-the area of the action" (Meissner and KSpnick, 1988, p. 7)are invoked as a driving mechanism of passive extension;the origin of these forces is sometimes speculative.It is not surprising, then, that Steckler and Watts(1980), who applied a stretching model to the continentalmargin off southern France, found it unsatisfactory. Theyconcluded (p. 428-429):"passive heating resulting from stretching cannot fullyaccount for the observed tectonic subsidence of themargin. Thus, other mechanisms of heating the margin arerequired. ...Continental rifting may be caused by deepseated processes in the mantle which result in an activeheating of the lithosphere."Sclater et al. (1980) proposed that lithosphericthinning may be accomplished by melting and erosion of thelithosphere from below. Meissner and KOpnick (1988)suggested that rifting may be caused by introduction intothe subcrustal zone of hot material from lower levels inthe earth. Bonatti (1987) considered diapiric upwelling oflight asthenospheric material to lithospheric levels to bea common cause of rifting: such upwelling causes doming ofthe crust, which may in turn produce a rift (Bott, 1976,1980). This successfully explains flank uplifts associatedwith many rift zones. Non-extensional thinning of thecrust is also possible (Ziegler, 1992). Localizedasthenospheric upwelling is consistent with the existenceof hot spots (Vink et al., 1985). Significantly, such a-83-mechanism allows for a rift to be created without largetectonic extension; this point is discussed in Section 4.4.It is no less important that basin-forming subsidencemay be induced by geodynamical mechanisms that form norifts at all. For example, Beaumont (1978) considered sixpotential basin-forming processes:(1) thermal uplift, followed by erosion and thermalcontraction;(2) isostatic adjustment of a graben;(3) intrusion of dense material into the crust;(4) phase changes at depth;(5) spreading of continental plates; and(6) necking of the lithosphere.It is possible for some of these processes (the first,third, and fourth) to operate without producing a rift, aswas discussed, for example, by Bott (1976, 1980). High-grade metamorphic reactions and phase transformations atdepth can induce changes in the density of the lower-crustal material. Speculatively, compaction of thatmaterial results in epeirogenic regional subsidence whosecauses have no relationship to rifting (Section 4.4;Artuyshkov and Baer, 1989). I contend that the QueenCharlotte Basin evolved without a McKenzie-type rift beingformed (Chapter 6, 7, and 8; Lyatsky, 1991a). Assumption of conductive heatingThe McKenzie (1978) model of basin formation assumes-84-that heating of the basin is purely conductive, the sourceof heat being the upwelled asthenosphere beneath the rift.In fact, the assumption of conductive heating underlies theentire mathematical apparatus which relates the organicmaturation of the basin fill to tectonic extension.However, this assumption does not hold everywhere.Summer and Verosub (1989) noted that introduction ofmagmas into the basin fill may elevate the level of organicmaturation of the surrounding sediments. Other workers(e.g., Gretener, 1981; Lerche, 1989; Land, 1991; Person andGarven, 1992) have stressed the importance of circulationof hydrothermal fluids in the distribution of heat througha basin. The effects of groundwater flow on the level oforganic maturation of sediments may be profound, sometimesvertically shifting the position of the oil window in thesection by hundreds or even thousands of meters (Person andGarven, 1992). Migration of hydrothermal fluids in a basincan be channeled by faults, bedding planes, and otherconduits (Lerche, 1989; Bredehoeft et al., 1992).Results of the present study (Chapter 9) indicate thatthe Queen Charlotte Basin was heated in part convectively,by igneous and hydrothermal activity. The assumption ofpurely conductive heating, upon which many stretchingmodels of basin formation are founded, thus breaks down.This renders the pure-shear models irrelevant for the studyof the Queen Charlotte Basin.-85-4.3. Simple-shear model 4.3.1. FormulationMany of the fundamental assumptions of the pure-shearmodels are plagued by shortcomings, uncertainties, andlimitations, and they are incompatible with the geology ofsome basins. Moreover, this mechanism is not the only onecapable of producing depressions on the earth's surface andthus sedimentary basins (Sections and 4.4).For these reasons, a number of workers have turned toalternative models of basin and rift formation. A model ofcrustal extension based on simple shear was offered byWernicke (1981, 1984). Unlike pure shear, simple shearwould produce an asymmetric rift bounded on one side by acrustal- or lithospheric-scale normal detachment faultwhich dips gently towards the rift zone. As the hangingwall slides down the fault plane, a rift valley and/or asedimentary basin is formed in the resulting depression.Wernicke's detachment differs from that of Royden and Keen(1980) in that it cuts across crustal or lithosphericlayers and reaches the surface of the earth.The simple-shear model is compatible with both activeand passive mechanisms of rifting, and it accounts easilyfor the formation of elongated, localized rift valleys andflank uplifts. Like the pure-shear model, the simple-shearmodel is sometimes viewed as an end-member of a broadfamily of models of evolution of rifts and basins. Ingeneral, simple shear characterizes the upper crust, while-86 -examples of pure shear are usually found in the ductilelower crust (Meissner and Wever, 1992).4.3.2. Points of contentionThe simple-shear model has a number of drawbacks. Onedrawback, the implausibility of the confinement of trans-lithospheric strain to a single fault zone as required bythe model, was noted by Wernicke (1981, 1984) himself.Furthermore, low-angle, trans-lithospheric faultswould likely be detectable with seismic techniques, yetsuch faults have proved elisive in reflection profiles.Recent work in the North Sea and on the east coast ofCanada has shown that many large-scale normal faults onceinterpreted as basin-bounding decollements do not flattenout but instead remain steep at considerable depths in thecrust (Kusznir et al., 1991; Roberts and Yielding, 1991).4.4. The plume model 4.4.1. FormulationAs was noted above, thinning and subsidence of thecrust and formation of sedimentary basins do notnecessarily require a large amount of lithospheric andcrustal extension. Basins can be formed with very littlestretching, and subsidence may occur as an aftermath of theactivity of mantle plumes. Metamorphic reactions at depthmay also be responsible for the formation of regionaldepressions in the continental crust.-87-Bott (1976, 1980) stressed that crustal uplift drivenfrom below may be a critical factor in rift development,and "the tensile stress may be partly or wholly produced bythe associated domal uplift". In this model, formation oflocalized rifts is accomplished by downward sliding ofblocks of the brittle upper crust along steep faults whichdip towards the rift valley. At depth, these blockmovements are dissipated by ductile deformation of thelower crust. Only a small amount of tectonic extensionaccompanies the faulting.Houseman and England (1986) were among the few workersto seek reconciliation between the stretching models andthe idea of rift formation by mantle plumes. They assumeduniformly thin (125 km) continental plates, however, andtheir subdivision of the lithosphere into five layers withcontrasting rheological parameters seems arbitrary.Artyushkov and Baer (1984, 1985, 1986a-c, 1989)attempted to formulate a comprehensive model of basinformation that would combine the positive aspects of themodels discussed above. They also considered causativegeodynamical processes, stressing the role of metamorphicreactions at the base of the crust. These authors, likemany others, considered a common driving mechanism of riftformation to be the upward propagation of a hot, low-density plume from deep levels in the earth. When such aplume impinges on the base of the lithosphere, it producesa bulge on the surface of the earth, where volcanism and-88-rifting may take place. The plume also thins thelithosphere by eroding it from below, creates a positivethermal anomaly, and possibly causes phase transformationsand metamorphic reactions at the base of the crust.In the plume model, lithospheric thinning and initialcrustal uplift may be followed by rapid, isostaticsubsidence analogous to S/ of McKenzie (1978) but requiringconsiderably less tectonic extension. The next stage ofbasin evolution, somewhat similar to McKenzie's ST, ischaracterized by more-broadly distributed and gradualsubsidence.Unlike the stretching model, however, the plume modeldoes not invoke passive extension to account for thethermal anomaly, and it does not imply a clear, simplerelationship between the geodynamical parameters involved.A deep rift or sedimentary basin can be formed with e<1.2,i.e. without significant extension.The subsidence mechanism of Artyushkov and Baer allowsfor a sedimentary basin to be formed even without rifting.Gradual phase changes or metamorphic reactions at depth mayalter the buoyancy of the crust, thus causing epeirogenicsubsidence of broad areas while avoiding the S I ("syn-rift") stage entirely. The subsidence thus caused may belong-lived, and it may bear no relationship whatever tocontinental rifting.The models of basin subsidence by plume activityand/or metamorphic reactions at depth have the advantage of-89-being flexible enough not to require significant tectonicextension for all sedimentary basins. This may be relevantto the study of the Queen Charlotte Basin area, which ischaracterized by a small amount of extension and whichappears to have passed over a mantle hot spot in theMiocene (Chapters 6 and 9).4.4.2. Points of contentionThe model of Artyushkov and Baer (1989) invokesvarious geodynamical processes, phase transformations, andmetamorphic reactions at depth, which are speculative. Forexample, mantle plumes are assumed to erode the crust frombelow, in part by transforming the lower-crustal materialinto heavy, compact eclogite or garnet granulite, whichthen either detaches and sinks into the upwelledasthenosphere, or remains attached and depresses the crustin the affected area. In this model, epeirogenicsubsidence of broad areas may also be explained partly bycompaction of rocks in the lower crust by eclogitization.Our understanding of lower-crustal metamorphism andgeodynamical processes is very poor (e.g., Meissner, 1989;Meissner and Weyer, 1992). In a speculative manner,Haggerty (1986) cited occurrences of diamonds witheclogitic inclusions as evidence for the presence ofeclogite near the lithosphere-asthenosphere boundary, butthere is little reliable information about the amount ofeclogite in the lower crust and mantle lithosphere. Large--90-scale eclogitization of the base of the crust by heatingcan be hypothesized, but it has yet tp be documented innature. Furthermore, its tendency to compact the lowercrust would be counteracted at least partly by thermaldilatation of heated rocks.The model of Artyushkov and Baer has the addeddrawback of being generalized to the degree that itspredictive power is degraded. Based on these workers'premises, the discussion of basin-formation mechanisms canbe little more than a speculative footnote to a basin-analysis study.4.5. DiscussionEach of the basin-formation models discussed above hasits own advantages and strengths. Each may seem consistentwith a variety of geological and geophysical observationsin any particular area. However, these models rely onnumerous assumptions about the dynamical behavior of theearth, or even about the geology of the basin, which arenot everywhere tenable or testable based on available data.In poorly studied frontier basins, the problem of non-uniqueness of evolution models arises because many of theobservations may be explicable in a variety of ways whenhard geological constraints are lacking.It is critical, then, to recognize that models arejust that - models. As such, they should not be confusedwith knowledge, or their implications with evidence. In-91-many basin-formation models, initial assumptions arerecycled to be incorporated into the solutions. Theapplication of a wrong model of basin evolution may lead toresults that are unsupported by the data. Because of theuncertainties inherent in many basin-formation models, andthe scarcity of information about the geology of the QueenCharlotte Basin, I chose to avoid the model-based approachin the present study. The data-based approach was employedinstead, as discussed in detail in Chapter 1.I do not wish to imply, however, that modelingtechniques are unacceptable in principle for all aspects ofthe study of the Queen Charlotte Basin. Within theconceptual framework of the evolution of the basin advancedin the present dissertation, it may someday becomepossible, for example, to employ geochemical or geophysicalmodeling locally to study organic maturation of sediments,as well as basin structure and stratigraphy. Such work mayhave to await the collection of additional, detailedgeological and geophysical data.-92-CHAPTER 5. PETROPHYSICAL PROPERTIES OF ROCKS ANDPROCESSING OF MAGNETIC AND GRAVITY DATA5.1. Petrophysical properties of rocks in the study area5.1.1. Rock densitiesInformation about rock densities in the study area wasobtained from previously published sources (Stacey andStephens, 1969; Stacey, 1975; Currie and Muller, 1976;Anderson and Greig, 1989; Sweeney and Seemann, 1991;Dehler, 1991) and new measurements made by R.G. Anderson(pers. comm., 1990) and C.J. Hickson (pers. comm., 1991).Density of Neogene rocks offshore was determined by thewriter from well logs, although these picks were not alwaysreliable due to sidewall caving. The data are scarce,however, and the density values presented below should beconsidered only approximate.A low density (1,800 to 2,500 kg/m 3 ) characterizessediments of the Neogene Skonun Formation in the QueenCharlotte Basin. The highest values in this range areencountered in southern Hecate Strait, in the area of theSockeye B-10 and E-66 and Murrelet L-15 wells. In alldrillholes, density of Neogene sediments increasesdownward, reflecting sediment compaction.Cretaceous sedimentary rocks have a density of about2,700 kg/m3 . On Queen Charlotte Islands, Tertiary (MassetFormation) and Middle Jurassic (Yakoun Group) volcanicrocks have a density of 2,650 kg/m 3 , whereas Lower Jurassic-93-Bonanza volcanics on Vancouver Island are usually heavier(2,700 to 2,800 kg/m3- ). Lower Jurassic clastic rocks onQueen Charlotte Islands are relatively light, 2,640 kg/m 3or less. Upper Triassic Kunga Group limestones have adensity of at least 2,760 kg/m 3 .These values contrast with those for Upper TriassicKarmutsen Formation volcanics (2,880 kg/m 3 on Queen3Charlotte Islands and about 2,950 kg/m on VancouverIsland), and with those for the underlying Paleozoic rocks(2,730 to about 2,900 kg/m 3 , depending on lithology andgrade of metamorphism of rocks).Thus, the greatest density contrasts are expectedamong three main categories of volcano-sedimentarysupracrustal rocks in the study area:(1) Neogene clastic rocks of the Skonun Formation, whosedensity is 1,800 to 2,500 kg/m 3 ;(2) the Upper Triassic to Tertiary volcanic and sedimentarystrata above the Karmutsen Formation basalt but belowthe Skonun Formation clastics - their density is about2,700 kg/m 3 ; and(3) Upper Triassic Karmutsen Formation volcanics andunderlying older rocks - their density is variable butis generally about 2,900 kg/m 3 .Bulk densities of most plutonic rocks in the Insularand Coast belts vary between 2,600 kg/m 3 for granite and2,820 kg/m 3 for diorite; granodiorite is the most commonlithology. On Queen Charlotte Islands, Tertiary plutonic-94--rocks usually have densities of 2,400 to 2,800 kg/m 3 , andJurassic plutonic rocks, 2,500 to 2,800 kg/m 3 ; the valuesof 2,600 to 2,700 kg/m 3 are the norm. This range overlapswith categories 2 and 3 of stratified sedimentary andvolcanic rocks, and significant density contrasts betweenplutons and their country rocks may thus be lacking. Someof the large plutons are likely zoned internally, which mayproduce local density contrasts. In an adjacent area onthe mainland, Hutchison (1982) showed that various elementsof the Ecstall and Smith Island plutons have densitiesvarying between 2,610 and 2,950 kg/m 3 . Dike rocks in thestudy area are insignificant volumetrically (Souther andJessop, 1991), and their densities on Queen CharlotteIslands are similar to those of other igneous rocks.5.1.2. Rock magnetizationTotal magnetization is the rock property that relatesa magnetic anomaly to its geologic source (Reynolds et al.,1990). Magnetic properties of rocks are a function of theabundance of certain minerals within these rocks, and theprincipal magnetic mineral in the study area is thought tobe magnetite (Coles and Currie, 1977).Magnetite is usually associated with igneous rocks,and the distribution of these rocks largely controls themagnetic anomaly pattern in the Queen Charlotte Basin area.Magnetic susceptibilities of some of the volcanic rocks onVancouver Island were measured by Currie and Muller (1976).-95-The Karmutsen Formation basalt is characterized by valuesfrom 40x10 -6 to 2,000x10 -6 emu (500x10 -6 to 25,000x10 -6 SIunits), and Bonanza Group volcanics have a similar range ofsusceptibilities. In contrast, Paleozoic Sicker Groupvolcanics are associated with values rarely exceeding100x10-6 emu (1,250x10 -6 SI units).Volcanic rocks of the Karmutsen Formation and theBonanza Group on Vancouver Island carry some remanentmagnetization (Yole and Irving, 1980; Irving and Yole,1987). On Queen Charlotte Islands, rocks of the Karmutsenand Masset formations, as well as Paleogene plutons, alsohave remanent magnetization (Hicken and Irving, 1977; Wynneand Hamilton, 1989). However, no systematic studies ofmagnetic properties of rocks have been conducted in thestudy area. In general, remanent magnetization of volcanicrocks tends to be appreciable (Reynolds et al., 1990).Most plutonic rocks in the Insular Belt are I-type(Anderson and Reichenbach, 1991). Such rocks are usuallymagnetic (Reynolds et al., 1990), but no susceptibilitymeasurements are available for the study area. Localvariations in magnetite content may be produced by internalzoning of large plutons.Interpretation of magnetic anomalies may becomplicated further because the abundance of magnetitewithin a single rock unit may vary due to both primary andsecondary causes, the latter including metamorphism andmineralization. Coles and Currie (1977) and Young (1981)-96-have suggested that contact metamorphism may produce localincreases in magnetite content and hence magneticsusceptibility of rocks in the Insular Belt. Such effectshave been documented in the study area even where thoserocks are normally non-magnetic limestones (Eastwood, 1965;Sutherland Brown, 1968; Carson et al., 1971).In clastic rocks, reworked fragments of magneticminerals may be concentrated locally, which alsocomplicates the interpretation of magnetic anomalies.Still, clastic rocks of the Skonun Formation, whichconstitute much of the fill of the Tertiary Queen CharlotteBasin, are generally non-magnetic.5.2. Processing of magnetic and gravity data5.2.1. Reduction of gravity dataA gravity anomaly is the difference between theobserved gravity field from the theoretical field computedfor an idealized, "gravitating, rotating, spheroidal Earth"(Goodacre et al., 1987a). Mathematically, an anomaly isthe difference between the measured gravity value and theone thus computed for a given location. The theoreticalvalue is normally computed taking into account the latitudeand elevation of the recording station.Anomalies are caused by density contrasts located atvarious depths in the earth:(1) at subcrustal levels,(2) in the crystalline crust, and-97-(3) in the volcano-sedimentary supracrustal cover.Measured gravity values are affected also by topography andrecording-site elevation and latitude.My objective was to extract from the observed datathose anomalies which reflect geologically significantdensity contrasts at crustal and supracrustal levels.The dependence of gravity values on the distance fromthe center of the Earth, hence on elevation, is accountedfor by the free-air reduction (Goodacre et al., 1987b).The Bouguer reduction (Goodacre et al., 1987c) takes intoaccount the attraction of the rock mass, assumed to be ahorizontal slab, density 2,670 kg/m, between the recordingstation and the sea level. The map in Fig. 5-1 combinesfree-air gravity offshore and Bouguer gravity on land.The Airy model of isostasy, which is typically assumedfor the earth's crust, requires that areas of positivetopography, if in equilibrium, be underlain by a crust ofincreased thickness; conversely, the areas of negativetopography must be underlain by a crust thinner thannormal. Gravitational attraction of crustal roots andantiroots is accounted for by the isostatic reduction(Simpson et al., 1986; Goodacre et al., 1987d; Simpson andJachens, 1989). An isostatic map was generated for thestudy area, but it proved to be of limited use forinterpretation and was not reproduced in this dissertation.Isostatic maps may still contain anomalies other thanthose sourced by intracrustal or supracrustal density5550 - 5545 5040 - 4535 - 4030 - 3525 - 3020 - 2515 -^2010 -^155 -^100 -^5-5 -^0-10- -5- 15 - -10- 20 - -15- 25 - -20-30 - -25- 35 - -30- 40 - -35- 45 - -40- 50 - -45- 55 - -50-60 - -55-65 - -60- 70 - -65-75 - -70- 80 - -75-85 - -80- 90 - -85-95 - -90-100 - -95- 105 - -100-110 - -105- 115--110<^-115UNDEFINEDMILLIGALS*6,00-99-Figure 5-1. Gravity map of the Queen Charlotte Basin area(Bouguer reduction on land, free-air offshore), displayedas a color/contour plot.-100-contrasts. These components of the gravity field,undesirable for the purposes of the present study, may becaused by crustal flexure, local variations in heat flow,etc. Their wavelengths usually exceed those of geologicfeatures of interest. Many such anomalies are correlativewith topography, and they can be attenuated by the enhancedisostatic reduction developed recently by Sobczak andHalpenny (1990).The algorithm used by these authors relies on a least-squares procedure to linearly correlate isostatic gravityvalues in a map area with topography onshore and imaginaryrock-equivalent topography offshore. The latter iscomputed by replacing the mass of sea water (density 1,030kg/m3 ) with an equivalent mass of rock (density 2,670kg/m3 ), and adding the thickness of the simulated rocklayer to the existing bathymetry. The topography-anomalyrelationship is used to correct the isostatic data and thusproduce an enhanced isostatic anomaly map (Fig. 5-2).The anomalies in the enhanced isostatic map are, intheory, caused largely by intracrustal and supracrustalsources, with other influences minimized. To obtain aregionally representative topography-anomaly correlation,the computations were performed on a quadrant much largerthan the study area, defined by diagonally opposite pointsat 55 ° N/126° W and 49 ° N/140° W.A shortcoming of the enhanced isostatic map is thatthe linear anomalies apparent in the free-air map are—co( —-102-Figure 5-2. Enhanced isostatic gravity map of the QueenCharlotte Basin area, displayed as a color/contour plot.The algorithm used to generate this map was outlined bySobczak and Halpenny (1990). Negative anomalies over theQueen Charlotte Basin represent structural depressions, asdiscussed in the text. Their locations are similar tothose determined from seismic reflection data (Fig. 6-3).Color code as in Fig. 5-1.-103-poorly expressed, a side effect of the dependence of datareductions on topography. In Queen Charlotte Sound andsouthern Hecate Strait, for example, many bathymetricfeatures trend northeast (Chase et al., 1975), oblique tothe predominantly easterly and northerly free-air anomalytrends. Data reductions thus cause interference, smoothinglineaments and giving local anomalies a more rounded shape.The enhanced isostatic gravity map is nevertheless usefulfor locating large, sediment-filled depressions in theQueen Charlotte Basin, whereas the free-air/Bouguer mapsare most suitable for identifying regional fault networks.In general, the interpretation of anomalies resultingfrom gravity reductions discussed above requires caution.Complications may arise because these procedures rely onseveral assumptions at various stages of data processing.1. The geoid is accurately represented by the referenceellipsoid in the study area.2. Isostatic compensation is one-dimensional, sensu Airy.3. Mantle density is constant beneath the study area.4. A horizontal slab of uniform density fairly representsthe rock mass between the sea level and the recordinglevel, and the water mass between the sea level and thesea floor.5. Lower-crustal roots and antiroots are fairlyapproximated by plane masses at a depth of 30 km.6. Crustal flexure produces a linear relationship betweentopography and isostatic-anomaly values.-104-Fortunately, the errors arising from variations on theseassumptions are usually small (Simpson et al., 1986;Goodacre et al., 1987a-d).The terrain correction is sometimes also applied togravity data. However, it is not required in the studyarea because the Queen Charlotte Basin is located on thecontinental shelf, where variations in bathymetry are onlytens to low hundreds of meters. Previously, Stacey andStephens (1969) and Stacey (1975) applied a terraincorrection to gravity values for land stations onshore andfor the underwater measurements in fiords, but comparisonof their maps with those generated in the present studyreveals only small differences, usually no more than 10mGal even on land. These differences are local, and theyprobably result largely from the lack of detail in the20-year-old maps of Stacey and Stephens (1969) and Stacey(1975), which are based on relatively widely spaced gravitystations and are contoured at a 10-mGal interval.5.2.2. Horizontal-gradient magnetic vector mapIn addition to the gravity maps, total-field magneticdata (Fig. 5 -3) were used in this study. To enhance short-wavelength features in both gravity and magnetic data,horizontal-gradient vector maps were generated for theQueen Charlotte Basin area. The map based on thetotal-field aeromagnetic data was published previously byLyatsky et al. (1992) on a 1:1,000,000 scale and is-600 -450 -300 -150^0^150^300^450^600Magnetic Anomaly nT-133° -132°^-13V^-130°^-129°-106-Figure 5 -3. Total-field magnetic anomaly map of the QueenCharlotte Basin area, displayed as a color plot. Heavyblack lines indicate locations of seismic profiles (as inFig. 1-2). Computer graphics courtesy J.A. Hole.-107-reproduced in Fig. 5-4 in pocket.Horizontal-gradient maps reflect lateral variations inthe magnitude of a potential field; abrupt variations areemphasized. The map shown here differs from conventionalhorizontal-gradient maps in both computational procedureand presentation of results. One standard technique(Sharpton et al., 1987; Goodacre et al., 1987e) relies onleast-squares fitting of a first-order surface to a windowof 5x5 potential-field values. The slope of this plane isa scalar quantity considered to represent the magnitude ofthe horizontal gradient of the potential field at the pointin the center of the window. Contouring and/or colorcoding these gradient values can be used to produce maps.The new technique used in the construction of the mappresented here (Fig. 5-4 in pocket) was discussed byLyatsky et al. (1990, 1992) and Thurston (1991). Adetailed description of the algorithm is enclosed at theback of this dissertation. The procedure involves least-squares fitting of a third-order surface to a window of 5x5potential-field values. The use of a higher-order surfaceallows for a more realistic representation of the fieldwithin the window. On the other hand, the third order islow enough for the best-fit surface not to be greatlyaffected by any spurious data points. I avoided the use ofsecond-order surfaces because absence of inflections mayreduce the quality of the fit.The horizontal gradient of the third-order surface-108-Figure 5-4 (in pocket). Aeromagnetic horizontal-gradientvector map of the Queen Charlotte Basin area. This map isalso available as Geological Survey of Canada Open File2436 (Lyatsky et al., 1992). Arrow length: lmm map lengthrepresents 80 nT/km.-109-fitted to the window of grid points is computed at thecentral point of the window, and both its magnitude anddirection are recorded. The gradient, treated as a vector,is displayed on a map as an arrow whose orientationrepresents the direction of the gradient and whose lengthis proportional to the gradient's magnitude. After someexperimentation, Lyatsky et al. (1992) chose 1 mm maplength to correspond to 80 nT/km for their 1:1,000,000scale map. The arrows point "downhill", i.e. away fromlocal maxima in the magnetic field.Reduction of magnetic data to the pole is sometimesundertaken before horizontal-gradient calculations areperformed, to eliminate the effects of non-verticalinclination of the anomaly-inducing geomagnetic field. Intheory, results of such a reduction simulate the appearanceof the anomalies if the area were located at the magneticpole, where the geomagnetic field is vertical. This data-processing step was omitted here for two reasons. First,it pole is based on the assumption that all magneticanomalies are inductive in origin and that remanence doesnot contribute. In reality, however, rocks commonly carrya substantial remanent magnetization (Reynolds et al.,1990). Second, the study area is located at a highmagnetic latitude, and the inclination of the geomagneticfield is 72 0 to 73 0 (Hewitt and Haines, 1991). Predictably,reduction to the pole in this region has only a smalleffect on the character of the magnetic map (Dehler, 1991).-110-5.2.3. ShadowgramsTo emphasize short-wavelength magnetic anomalies in adifferent fashion, side-lighting was applied to these datausing the numerical technique of Broome et al. (1985) andTeskey et al. (1989b). In this procedure, a potential-field map is viewed as representing a topography, which isilluminated by the Sun. Brightness of illumination of apixel is proportional to the cosine of the angle ofincidence of the imaginary light ray on the pixel. In thepresent study, the declination of "the Sun" was 045(northeast; Fig. 5-5), 090 ° (east; Fig. 5-6), and 135 °(southeast; Fig. 5-7), and the inclination was 30 ° . Theuse of different declinations helps enhance lineaments withdifferent orientations. Shadowgrams, or shaded-reliefmaps, emphasize short-wavelength anomalies and lineamentsat the cost of a loss of amplitude information.This technique was applied to the gravity data aswell, and several shadowgrams were generated. However, thegravity coverage proved too sparse to make theseshadowgrams useful for interpretation, and the gravityshaded-relief maps are not reproduced in this dissertation.5.2.4. Upward continuation of potential-field dataTo investigate large-scale geologic features in thestudy area, Bouguer/free-air gravity (Fig. 5-1) and total-field aeromagnetic (Fig. 5-3) data were upward continued to5 and 20 km (Figs. 5-8 to 5-10). This procedure simulates( ( --112-Figure 5-5. Shadowgram of the total-field magnetic datafrom the Queen Charlotte Basin area. The algorithm used togenerate this map was outlined by Broome et al. (1985)."Light-source" inclination 30 ° , declination 045 °(northeast). From Lyatsky (1991a).- ( ( 3--114-Figure 5-6. Shadowgram of the total-field magnetic datafrom the Queen Charlotte Basin area. "Light-source"inclination 30° , declination 090 ° (east). From Lyatsky(1991a).-( ( L _-116-Figure 5-7. Shadowgram of the total-field magnetic datafrom the Queen Charlotte Basin area. "Light-source"inclination 30 ° , declination 135° (southeast). FromLyatsky (1991a).(-118-Figure 5-8. Gravity data from the Queen Charlotte Basinarea (Bouguer on land, free-air offshore) upward continuedto 5 km. The yellow band corresponds to amplitudes between0 and 5 mGal. The contour interval is 5 mGal.-120-Figure 5-9. Gravity data from the Queen Charlotte Basinarea (Bouguer on land, free-air offshore) upward continuedto 20 km. Plotting parameters as in Fig. 5-8.(2(----122-Figure 5-10. Total-field magnetic data from the QueenCharlotte Basin area upward continued (a) 5 km and (b) 20km. Positive anomalies are colored red, negative anomaliesgreen. The yellow band represents amplitudes between -25and 0 nT.-123-the appearance of the potential-field data recorded at acertain elevation above the ground or sea level.The theory behind upward continuation was discussed byGrant and West (1965), Briusov (1980), Blakely and Connard(1989), and Teskey et al. (1989a). No sources or sinks ofthe potential field are assumed to exist between the realrecording level and the nominal elevation to which the dataare upward continued. In the Queen Charlotte Basin area,this assumption is justified because no rocks protrudeabove the sea level offshore. The topographic elevationson Queen Charlotte and Vancouver islands are usually in thehundreds of meters (Chapter 1), significantly less than thelevel of upward continuation.The procedure involves filtering the input dataon the basis of anomaly wavelength. All wavelengths areattentated, but short-wavelength anomalies are attenuatedmore. If the potential field is measured on a horizontalplane and is desired on a higher horizontal plane, upwardcontinuation is given by (Blakely and Connard, 1989):F[h(x,Y)] = F[h(x,y)]exp(-k4z)^Az>0,^(5 )where k is the anomaly wavenumber (the quantity k/2n is theinverse of wavelength), Az is the distance of upwardcontinuation, F[h(x,y)] is the potential field in theFourier domain, and F[h (x,y)] is the upward-continuedpotential field in the Fourier domain.-124-Local, short-wavelength anomalies, which would not beobserved at an elevated recording level, are suppressed.Broad anomalies of shallow origin are not excluded, andmost features in upward-continued data are caused byrelatively large-scale sources at various depths in thesubsurface.5.2.5. Selection of data-processing results forinterpretationBecause the Queen Charlotte Basin is poorly exploredand its structure is not known well, it was unclear at theoutset of this study which data-processing techniques wouldproduce results most useful for geologic interpretation.Thus, a decision was made to take an experimental approachand apply a variety of processing techniques to both thegravity and magnetic data. Only those maps were selectedfor the final presentation which could be most readilyinterpreted based on the existing geologic constraints.The potential-field maps whose geologic meaning is lessexplicit, or which contain information available from othersources, are not included in this dissertation.The gravity data in the study area are generallysparser than the aeromagnetic coverage. Thus, theirshort-wavelength content is reduced, and shadowgrams andgradient maps generated for the gravity data failed toreveal anomaly patterns not apparent otherwise. On theother hand, the long-wavelength anomaly component is-125-prominent in both the magnetic and the gravity data, andupward continuation of both data sets produced results ofconsiderable geological interest (Chapter 6).Gravity maps in both the Bouguer/free-air and theenhanced isostatic reductions are reproduced here becausethey reveal different aspects of basin structure. The bestresults are obtained when these maps are consideredjointly, and seismic reflection profiles are used for localcalibration. This permits one to locate Tertiarystructural depressions (depocenters) in the Queen CharlotteBasin and to identify their bounding faults. Aninterpretation of the magnetic and gravity data ispresented in Chapters 6, 7, and 8, which follow.-126-CHAPTER 6. LINEAR TRENDS IN POTENTIAL-FIELD MAPSAND PHYSIOGRAPHIC LINEAMENTS6.1. Lineaments in magnetic maps 6.1.1. Interpretation of unfiltered total-fieldaeromagnetic dataThe main magnetic lineaments discussed in this chapterare summarized in Fig. 6-1, and the gravity lineaments inFig. 6-2. The principal fault systems in the region areshown in Fig. 1-1 and in Plate I (in pocket).Sources of most magnetic anomalies in Hecate Straitare found in the top 4 km of rock material in thesubsurface (Teskey, 1989). This suggests the anomaliesreflect the distribution of igneous rocks at supracrustallevels, and magnetic lineaments associated with faults areaccentuated where the , igneous rocks are disrupted.The most prominent of the linear magnetic anomalies isfound in the northeast corner of the map area (Figs. 5-3 to5-7; M1 in Fig. 6-1). It has a west-northwesterlyorientation and extends across the northern part of HecateStrait into central Dixon Entrance. This anomaly cutsacross the magnetic lineaments that run in a north-northwesterly direction (M2 in Fig. 6-la), with oneintersection occurring at 54 °10'N/131° 30'W, north of BanksIsland. Based on the alignment of this magnetic lineamentwith faults mapped onshore, I have previously interpretedthe west-northwesterly anomaly (M2) as the offshore0\0o^ Figure 6-1a. Patterns of lineaments in magnetic maps fromthe Queen Charlotte Basin area. Codes M1 to M12 indicatespecific features referred to in the text. The rosediagram illustrates orientations of the lineamentspresented, with a 15 ° interval.Figure 6-lb. Examples of magnetic lineaments in thehorizontal-gradient vector map (from Fig. 5-4 in pocket)for the northern part of the study area. Lineament codesas in Fig. 6-1a.;4'001I d*4e,3 v dpgsw oID44, \Anew 1Cy 444,h ^fl•3.L.L071iVHaNnN33170^• •1VIIVZVISIEIV^_s----,- -1_-_,,_^..116,31 (:^SNNVO ,,,,,--'_.5.,,.^c\C'''' ,,c--,,,^< " SI 1.1.1d .^--- Fv---- -, _^....) cl- zi--- -s.r-g,0 -^s ,'■ . SI '^C ,^ ''-^\--•1.1111011111 : 1. -----21 ) V111N301ele•°3HN V 3 ° °MTh---9ssg:^ 136: :'--- _^/'k.. -S1 S311VM.., ,,.---.--1 AO 3ON1Ud`61e, fc,' sr37•111 PIVHVUOi t'SIMV~.Figure 6-2. Patterns of lineaments in gravity maps fromthe Queen Charlotte Basin area. Codes G1 to G4 indicatespecific features referred to in the text. The dashed lineis the boundary between the northern and southern domains,for which separate rose diagrams were generated (interval15 ° ). The Queen Charlotte fault system is not reflected inthe rose diagrams because it lies outside the basin.9e,1110111Di.141o .741111.11.-130-continuation of the Kitkatla fault system (Fig. 1-1;Lyatsky, 1991a). The north-northwesterly anomaly ineastern Hecate Strait and Dixon Entrance (M3 in Fig. 6-la)likely represents a link between the Principe Laredo/BanksIsland fault system in eastern Hecate Strait and theClarence Strait Fault in southeast Alaska.The character of the Kitkatla and Principe Laredo/Banks Island magnetic lineaments indicates that both thesefault systems are broad and many-stranded. The same issuggested by seismic reflection data (Fig. 6-3). Onnortheasternmost Graham Island, epicenters of someearthquakes fall on a line parallel to, but a fewkilometers south of, the inferred offshore position of theKitkatla fault system (Rogers et al., 1988; R.B. Horner,pers. comm., 1989); the sense of movement is not known.Seismic reflection data from northern Hecate Strait (Rohrand Dietrich, 1990, 1992; Dietrich et al., 1992) indicateNeogene dip-slip movements on elements of this system.Queen Charlotte Islands contain numerous magneticlineaments with various orientations. Easterly and west-northwesterly trends, common on Graham Island (M4 in Fig.6-1), are probably related to fracture patterns within theTertiary Masset volcanics (Hickson, 1988, 1989, 1991).Such trends, among others, are prominent in the fracture-controlled drainage pattern on Graham Island (Alley andThomson, 1978). On the other hand, examination ofvertical-derivative magnetic maps suggests that someWELL LOCATIONSOS - OSPREY^M MURRELETJ-1-1. NORMAL FAULT........S - SOCKEYET - TYEEC - COHOH - HARLEQUINA - AUKLETTH - TOW HILLREVERSE FAULT Y STRIKE-SLIP FAULTFOLDISOPACH (km) OF TERTIARY BASIN FILL8ANDSPI T FAULTGRAHAM ISLAHG-132-Figure 6-3. Structure of the Queen Charlotte Basin asinterpreted by Dietrich et al. (1992) from seismicreflection data. MR - Moresby Ridge. Note locations ofstructural depressions: they match those determined by thewriter from the gravity data (Fig. 5-2). However, thepresent writer's mapping of regional fault systems andnetworks differs from that suggested here. Magnetic dataindicate the Kitkatla and Principe Laredo/Banks Islandfault systems intersect and continue northwest offshore(Fig. 1-1). Northeasterly and easterly structures arecommon across the Queen Charlotte Basin, based on theinterpretation of potential-field data, but such trends arerepresented poorly in the map shown here. The geologicaland geophysical evidence discussed in the text precludeslarge Tertiary strike-slip movements in the basin onshoreand offshore. The orthogonal fault networks in the QueenCharlotte Basin delineate a mosaic of crustal blocksinherited from Mesozoic or older time (Chapters 7 and 8).-133-anomaly aliasing has occurred in this region due to thewide spacing of east-west flight lines, which could havecontributed to the abundance of east-west trends.North-northwesterly trends are associated locally withthe Sandspit fault system (M5 in Fig. 6-1). The magneticlineament related to this system is lost over northernGraham Island, but it reappears in Dixon Entrance (M6).The west-northwesterly lineament at about 53 °N/132 ° W(M7 in Fig. 6-1) corresponds to the Rennell Sound Fault(Fig. 1-1). This anomaly is restricted to Queen CharlotteIslands, and no offshore extension is apparent. TheRennell Sound magnetic anomaly (M7) intersects a north-south lineament at 1313 40'W (M8, see Fig. 6-lb).The abundance of west-northwesterly magneticlineaments across the study area suggests the Rennell SoundFault may be part of a pervasive regional fault network.Faint west-northwest trends occur on Graham Island (M4) andnear the eastern margin of the map area, south of PrincessRoyal Island, and they are subparallel. Since the absenceof short-wavelength anomalies in the offshore region maybe due to increased depth to source, continuity ofcausative geologic structures cannot be ruled out.Eastern Queen Charlotte Sound excepted, the magneticfield is more subdued over the Queen Charlotte Basinoffshore than over land areas. This likely reflectsvariations in depth to source, although variations in rockcomposition may also contribute. In Hecate Strait, weak,-134-broad, northeast-trending magnetic lineaments are observedbetween 53° N and 54° N (M9; Fig. 6-1), roughly on trendwith some of the physiographic lineaments (Fig. 6-4) andTertiary dikes on the mainland. This anomaly crosscuts theKitkatla and Principe Laredo lineaments at about 5i N,without offset (Figs. 5-3 to 5-7, and 6-1b). Anothernortheast-trending magnetic anomaly is found at 52° 30'N/13f W (M10). It coincides with a similarly orientedbathymetric feature and with the Moresby Ridge, the lattera structural culmination within the Queen Charlotte Basin(Fig. 6-3). The strong magnetic signature of the ridgeindicates it is composed at least partly of igneous rocks(Young, 1981). A prominent north-south magnetic lineamentis seen at 53 °N/131 ° W (M11). In Queen Charlotte Sound,north-south and northwest trends are found. The latter areconfined largely to the southernmost part of QueenCharlotte Sound (M12) and are a continuation of thenorthern Vancouver Island magnetic anomaly pattern, whichis likely related to the distribution of structural blocks,volcanic rocks, and plutons (compare Figs. 2-2 and 5-3).Areas of extensive plutonism on Queen CharlotteIslands (Fig. 2-1; Anderson and Reichenbach, 1991)correspond roughly to localized, positive magneticanomalies (Fig. 5-3). The correlation between outcrops ofplutonic rocks and magnetic anomalies is imperfect,probably because the distribution of intrusive material atdepth differs from that at the surface. An interestingFigure 6-4. Major physiographic lineaments in the QueenCharlotte Basin area (from Lyatsky, 1991a). Explanation inSection 6.4.-136-case is the pluton on northwestern Moresby Island, at53 ° 10'N/132 0 30'W, which is not associated with a magneticanomaly. This may be a result of cancelation of various(induced, remanent) magnetization vectors in these rocks,or absence of magnetic minerals.Shadowgrams (Figs. 5-5 to 5-7) indicate that pluton-related anomalies on Queen Charlotte Islands have asubstantial short-wavelength component. The horizontal-gradient vector map (Fig. 5-4 in pocket, Fig. 6-lb) showssuch domains to be rather shapeless clusters of discreteglobular-shaped features of various sizes.6.1.2. Interpretation of upward-continued magnetic dataThe magnetic data were upward continued to 5 km (Fig.5-10a) and 20 km (Fig. 5-10b), and results of upwardcontinuation to 40 km were presented elsewhere by Teskey etal. (1989a). Examination of these maps reveals that thenumerous, short-wavelength anomalies in eastern QueenCharlotte Sound merge into several large highs. Variousexplanations of this geophysical phenomenon are possible.1. The shape of these anomalies results from a loss oflateral resolution of the data due to filtering, andtheir source is shallow. However, shallow-seatedvolcanic rocks on northern Graham Island and elsewherein the region fail to produce such prominent magnetichighs.2. An uplifted crustal block is the cause of the-137-anomalies as it brings magnetic rocks closer to thesurface. However, large uplifted blocks elsewhere inthe study area are associated with prominent free-air/Bouguer gravity highs (Stacey and Stephens, 1969;Lyatsky, 1991a), and no such high is found in easternQueen Charlotte Sound (Fig. 5-9).3. The magnetic-anomaly sources merge at depth to form alarger geologic feature. The anomalies may then beproduced by large, deep-seated plutons with multipleapophyses and structures at shallow depths.I favor the third interpretation. Many prominent magnetichighs elsewhere in the region are associated with largeplutonic bodies. The localized highs over Queen CharlotteIslands (Fig. 5-10b) correspond to areas of Jurassic andTertiary plutonism (Fig. 2-1), as mapped by SutherlandBrown (1968) and Anderson and Reichenbach (1991). Thepositive anomaly south of Porcher Island (Fig. 1-1) iscaused by an Early Cretaceous pluton whose elements areexposed onshore (Woodsworth, 1991). The localized magnetichigh on the northern edge of the map area, at longitude1322W, is apparently related to plutonic rocks on southernPrince of Wales Island, which were described by Buddingtonand Chapin (1929), Brew et al. (1966), and Gehrels andSaleeby (1987a). Early Jurassic plutons (the IslandIntrusions: Chapter 2) on northern Vancouver Island werestudied by Jeletzky (1976), who concluded they probablymerge at depth. In the U.S. part of the Insular Belt, to-138-the north, many granitoid plutons are also associated withmagnetic anomalies (Brew et al., 1991).The age of the inferred plutons in eastern QueenCharlotte Sound (see Plate I in pocket) is uncertain.Granodiorites of an unknown but likely Early Cretaceous age(G.J. Woodsworth, pers. comm., 1990) are exposed on a smallisland in the southern part of the area of the magneticanomalies. The northern part of this magnetic domain,however, lies on trend with the Neogene Anahim belt ofvolcanic centers and hypabyssal plutons on the mainland(Fig. 1-1), which suggests that at least some of theanomaly-causing plutons in Queen Charlotte Sound are young.The lack of associated gravity anomalies can be accountedfor if the density of intermediate-composition plutonicrocks is similar to that of country rocks (Section 5.1).Upward continuation of the magnetic data to 20 km(Fig. 5-10b) results in removal of all the north-northwesterly linear trends except those in the vicinity ofthe Queen Charlotte fault system. Most west-northwesterlytrends are also attenuated. The lineaments representingnorthwestward extensions of the Principe Laredo andKitkatla fault systems are fragmented in the 5-km map (Fig.5-10a) and lost in the 20-km map (Fig. 5-10b). Themagnetic lineament associated with the Rennell Sound Faultis absent in both upward-continued maps.The filtering used in upward continuation passed threeof the major trends discussed earlier: northerly, easterly,-139-and northeasterly. The north-south trends are found inQueen Charlotte Sound, at 5030'N/129° 30'W (Fig. 5-10b).East-west trends are present locally in many parts of themap area. Especially prominent is the northeasterly trendrepresented by a north-side-down gradient zone east of thesouthern tip of Moresby Island that forms the northern edgeof the positive magnetic domain in Queen Charlotte Sound(M13 in Fig. 6-1). I interpret this to indicate truncationof the inferred plutons in that area by a northeast-trending fault at depth. This magnetic lineament is not asprominent in the horizontal-gradient vector map (Fig. 5-4in pocket) or in the shadowgrams (Figs. 5-5 to 5-7), whereit is crosscut by local northerly and north-northwesterlytrends. The lineament and the magnetic highs it boundslose definition in western Queen Charlotte Sound (Fig.5-10b). Elevated heat-flow values were reported from thatarea by Hyndman et al. (1982), so the loss of definitionmay be caused in part by a shallower Curie isotherm. Areduced amount of plutonic rocks may also cause thedissipation of the anomalies.6.2. Lineaments in gravity mapsStacey and Stephens (1969) and Stacey (1975), whocarried out the early gravity work in the study area, notedthat large, positive gravity anomalies, such as the oneover Queen Charlotte Islands (Fig. 5-1), are associatedwith uplifted crustal blocks in the Insular Belt. The-140-positive anomalies are missing over much of the westernpart of Queen Charlotte Sound, where no uplifted blocks arefound. They are observed over northern Vancouver Islandonly locally (Dehler, 1991), possibly due to variations incrustal thickness in that area. The Queen Charlotte faultsystem is expressed as a down-to-the-west gradient zone.The main gravity lineaments in the Queen CharlotteBasin area are presented in Fig. 6-2. The western boundaryof the Hecate Depression was shown by Stacey and Stephens(1969) to coincide with the Sandspit fault system (Fig.1-1), which is marked by an east-side-down gravity gradientzone (G1 in Fig. 6-2; Sweeney and Seemann, 1991); thisgradient zone is expressed especially well in upward-continued gravity maps (Figs. 5-8, 5-9). The magneticlineament associated with this structure is discontinuousover much of Graham Island, where it fails to disrupt themagnetic anomalies related to the Masset volcanics (Fig.5-3). Field mapping also shows that no large faultstrending in this direction disrupt these volcanics onnorth-central Graham Island (Hickson, 1988, 1991). Incontrast, the gravity gradient zone continues beyond thatarea into western Dixon Entrance (Figs. 5-1, 5-9; G1 inFig. 6-2), where it is associated with east-side-downnormal faults imaged seismically by Snavely et al. (1980).These results are consistent with the findings of Thompsonet al. (1991), who suggested that the Sandspit fault systemis a long-lived block-bounding structure whose elements-141-have been repeatedly reactivated since Jurassic time butwhich lacks Cenozoic strike-slip displacements.North-northwest gravity trends are found throughoutthe map area (Figs. 5-1, 6-2). They are associated withregional fault systems, but they are also present on asmaller scale within the Queen Charlotte Basin, especiallyits northern half.Over northern Vancouver Island and southern QueenCharlotte Sound, west-northwesterly and easterly trendsoccur. Stacey and Stephens (1969) and Stacey (1974)related the easterly trends to the prominent east-westphysiographic lineaments on the mainland at latitude 51IN(Fig. 6-4). The core of the southern Queen Charlotte Soundgravity high is also elongated in the east-west direction(G2 in Fig. 6-2).Northerly and easterly gravity trends are prominent inthe south and center of the Hecate Depression (G3), andnorth-northwesterly and northeasterly trends occur in thenorth and center (G4). No sharp boundary between thesedomains is observed, and a substantial overlap occurs.Consistent with this pattern, results of interpretation ofseismic reflection data show that north-northwesterlystructures predominate in Hecate Strait and northerly onesin Queen Charlotte Sound (Fig. 6-3).The regional coverage afforded by the potential-fieldmaps allows a more realistic delineation of fault networks(as represented by gravity and magnetic lineaments) in the-142-Queen Charlotte Basin than do the seismic profiles.Northeasterly and easterly trends are identifiable in thefree-air/Bouguer gravity map, but they were delineatedpoorly by the seismic data, and their wide distribution wasthus underestimated by the seismic interpreters (Rohr andDietrich, 1991, 1992; Dietrich et al., 1992).Positive gravity anomalies over Queen CharlotteIslands, northern Vancouver Island, and the southernmostQueen Charlotte Sound persist when the data are upwardcontinued to 5 and 20 km (Figs. 5-8, 5-9). The broad,east-side-down gradient zone representing the Sandspitfault system continues southward along the eastern coast ofQueen Charlotte Islands. Over the Hecate Depression, thegravity field is generally subdued. North-northwesterlytrends are faint, but east-side-down gradient zones withthis orientation are apparent over the mainland (Figs. 5-8,5-9), where crustal-scale faults trending north-northwestform the dominant structural grain (Hutchison, 1982; vander Heyden, 1989, 1992).6.3. Interpretation of geologic structure from potential-field lineament patters Potential-field maps highlight regional structuralfeatures. Although geologic sources of individualanomalies cannot be identified everywhere, useful insightsare gained from examination of regional lineament fabrics.Networks of geophysical lineaments in the Queen Charlotte-143-Basin area are similar to geological lineament networks,suggesting a causal relationship (Chapter 7).The Kitkatla and Principe Laredo/Banks Island faultsystems intersect and continue northwest offshore (Fig.1-1). The Sandspit fault system extends across GrahamIsland into Dixon Entrance, but it does not disrupt theTertiary Masset volcanics. Evidence exists for largeplutons beneath Queen Charlotte Sound, and their alignmentwith the Anahim belt on the mainland suggests a Neogenetime of emplacement; older plutons may also be present.The interpretation of potential-field data presentedhere leads to several conclusions of regional geologicsignificance.1. No single, linear structural grain exists in theQueen Charlotte Basin, and numerous, apparently orthogonallineament patterns are found instead. Such a structuralconfiguration is incompatible with a rift origin of thebasin because rifts are typically characterized by well-defined, sub-parallel fault orientations (Illies, 1981).2. Potential-field anomaly patterns in the QueenCharlotte Basin are logically interpretable as related tothe distribution of fault-bounded blocks in the crust.3. Some of the localized magnetic anomalies reflectthe presence of large plutons onshore and offshore. Suchplutons, of a probable Neogene age, are postulated beneatheastern Queen Charlotte Sound.These results render inapplicable the existing-144-quantitative theoretical models of basin formation (Chapter4), as will be discussed in the subsequent chapters.Interpretation of potential-field data suggests thatmovements of fault-bounded crustal blocks played asignificant role in the evolution of the Queen CharlotteBasin, consistent with the geologic history of the flankingareas onshore (Thompson et al., 1991; Jeletzky, 1976;Muller, 1977; Hutchison, 1982; van der Heyden, 1989, 1992).A broadly similar situation apparently exists in southeastAlaska, based on interpretation of gravity, magnetic, andgeological data (Brew et al., 1991).The existence of plutons beneath Queen Charlotte Soundcannot be verified by geological observations. Likewise,numerical modeling of geophysical anomalies cannot becalled upon to verify their presence because their shape,density, and magnetization are unknown. This wouldcritically undermine the uniqueness of quantitative models,making them misleading in this situation.6.4. Patterns of physiographic lineaments 6.4.1. Construction of the mapGeomorphologic lineaments are commonly used to studyfault networks in continental regions (e.g., Misra et al.,1991; Deffontaines et al., 1992). Figure 6-4 showsprominent physiographic lineaments in the study area. Thedata base consisted of many topographic and bathymetricmaps and airphotos, scales 1:2,000,000 to 1:50,000.-145-Satellite photographs of parts of the area were presentedpreviously by Roddick (1981).The origin of structural control on topography isdiscussed in numerous textbooks (e.g., Press and Siever,1978; Smythe et al., 1978), to which the reader isreferred. In southeast Alaska, such structural control onphysiography was noted by Buddington and Chapin (1929). Onnorthern Vancouver Island, influence of geologic structureon topography is considerable (Muller et al., 1974). Inthe Coast Mountains, Quaternary glaciation resulted largelyin the sculpting of the pre-existing topography (Baer,1973; Claque, 1984) so as to deepen the valleys. It thusaccentuated the distribution of major lineaments. Manyvalleys and fiords on the mainland coincide with regionalfaults (Fig. 1-1, and Plate I in pocket).In contrast, north-south glacial lineations onnortheastern Graham Island (Sutherland Brown, 1968; Clagueet al., 1982) are not controlled structurally. In HecateStrait and Queen Charlotte Sound, recent sedimentarybedforms and glacial scouring of the sea floor (Barrie andBornhold, 1989) are also unrelated to the underlyingstructures. However, the amplitude of such physiographicfeatures is small on the submerged shelf and the onshoreplains of northeastern Graham Island, and such featureswere not plotted in Fig. 6-4. The larger bathymetriclineaments in Queen Charlotte Sound, on the other hand, arelikely controlled by the underlying faults (Chapter 3).-146-Following in part the example of Peacock (1935), Iconstructed the map in Fig. 6-4 by reproducing lineargeomorphologic features on land and sea floor, includingrivers, lakes, intermontane valleys, coastlines, and sea-floor scarps, as straight lines on a base map. Toemphasize large-scale, regional fabrics and eliminate localeffects, short features with uncommon orientations weresubsequently removed. Relatively long lineaments (tens ofkilometers or more) or those which seemed to possessregionally persistent trends, were retained. No rosediagram was generated for this map because the data set hadundergone several steps of filtering by mental process andthus is entirely qualitative.6.4.2. Description of physiographic lineament patternsThree paired sets of physiographic orientations arefound in Fig. 6-4:(1) northerly and easterly;(2) north-northwesterly and east-northeasterly tonortheasterly; and(3) west-northwesterly and north-northeasterly.The first two of these pairs were noted by Peacock (1935),who studied regional topographic lineament fabrics on thewest coast of Canada based on a more restricted data set.1. North-south physiographic trends are widespreadexcept on Queen Charlotte Islands. They are apparent eastof Prince Rupert, associated with fiords and intermontane-147-valleys. They persist offshore into southern Hecate Straitand northern Queen Charlotte Sound, where some of the sea-floor scarps on the continental shelf are oriented north-south. On Princess Royal Island and near the city ofKitimat, fiords and valleys also have this orientation, andan extension southward, into Queen Charlotte Sound, isapparent. The southeastern shoreline of Prince of WalesIsland in the northern part of the region also trendsnorth-south.East-west lineaments occur throughout the study area.They are prominent in the form of fiords on the mainland atabout 51 ° N, east of Queen Charlotte Strait, where Stacey(1974) related their origin to localized tectonic activityin the Neogene. However, that interpretation is weakenedby the wide regional distribution of the east-west trends.At 53IN, for example, east-west lineaments appear on QueenCharlotte Islands (near Sandspit) and on the mainland.Another such lineament, made up of fiords and valleys, isfound on Prince of Wales Island.2. North-northwesterly lineaments occur in severalareas on the flanks of the Queen Charlotte Basin. The mostprominent ones are associated with the Queen Charlottefault system, represented by the orientation of the westerncoast of Queen Charlotte Islands and large scarps on theadjacent Pacific Ocean floor. Other north-northwesterlytrends are associated with fiords surrounding Pitt Island.Distinctive east-northeasterly trends are prominent on-148-the mainland south and east of Prince Rupert. At latitude54o N to 55 0N, near the city of Terrace, they are associatedwith the Skeena River valley. Similar trends can be foundnorth of Prince Rupert, on northeastern and central GrahamIsland, and on the mainland between 5i N and 51) N (nearBella Bella and across Princess Royal Island).3. A less common physiographic trend is west-northwest. It is prominent east and south of PrinceRupert, where shorelines and fiords have this orientation.North of Banks Island, this trend is associated with thetrace of the Kitkatla fault system. The Kitkatlatopographic lineament continues with some interruptionsacross the Coast Mountains, where it is represented byintermontane valleys, and seems to merge with the YalakomFault (Lyatsky, 1991a). The geologic causes of thiscontinuity are uncertain, but the linkage of these faultswas tentatively proposed by Tipper and Richards (1976).West-northwesterly lineaments are found also between PrinceRupert and Kitimat. A parallel trend occurs on QueenCharlotte Islands (Fig. 6-4), coincident with the trace ofthe Rennell Sound Fault.Another set of physiographic lineaments has a north-northeasterly orientation. These trends occur on themainland, in the Terrace-Kitimat area.6.5. Similarities of lineament trends Many magnetic, gravity, topographic, and bathymetric-149-lineaments have similar orientations. A number of them areassociated with recognized fault systems, but the extent ofthese fault systems offshore was not previously known.Correlation of lineaments in potential-field maps withgeology, topography, and bathymetry has allowed me to mapfault systems with confidence across the region.Elsewhere, various authors (e.g., Wellman, 1985;Maughan and Perry, 1986; Anfiloff, 1988; Murray et al.,1989; Ameen, 1992) have successfully related linear-anomalydomains in gravity and magnetic maps, topography, andgeologic structure to regional structural fabrics and, inparticular, to the distribution of crustal blocks.-150-CHAPTER 7. STRUCTURE OF THE QUEEN CHARLOTTE BASIN7.1. Structural trends on the rim of the Queen CharlotteBasin 7.1.1. Vancouver IslandThe deformational history of northern Vancouver Islandis obscure. The Paleozoic core of this region is orientednorth-northwest (Muller, 1977, 1980), and Massey and Friday(1989) have described north-northwest-trending folds of alikely Late Devonian age. Local Paleozoic fold axes seemto be more northerly than those found in the Mesozoicstrata which are, as a rule, deformed into large, open,northwest-trending folds (Sutherland Brown, 1966).Mesozoic faults on Vancouver Island have variousorientations. North-northwesterly trends are ubiquitous,but other orientations also occur. East-northeasterlyfaults cut elements of the Upper Triassic KarmutsenFormation (Muller et al., 1974). Muller et al. (1981)reported north-striking faults that cut rocks as young asEarly Jurassic. Cretaceous rocks seem to be disruptedmostly by north- and northwest-striking faults (Muller etal., 1974). The age of initiation of these structures isuncertain, but northeasterly faults cut Eocene and youngerrocks (Muller et al., 1974, 1981).Jeletzky (1976) proposed that west-northwest-trendingregional structures controlled the distribution of LateTriassic to Middle Jurassic sedimentary facies on northern-151-Vancouver Island; these structures may be a product of LateTriassic Rhaetian deformation. Jeletzky suggested thenortheasterly trend may also have been prominent in theCretaceous, in the form of the Brooks Peninsula Upliftwhich extended across northern Vancouver Island, althoughthe exact orientation and shape of this feature areuncertain. The Neogene Alert Bay volcanic belt (Armstronget al., 1985) apparently follows one such fault system.7.1.2. Queen Charlotte IslandsOn Queen Charlotte Islands, Sutherland Brown (1966,1968) noted that the regional structural grain is north-northwest. He described three other trends as well:northerly, northeasterly, and easterly. Many of the north-northwesterly structures are the block-bounding regionalfaults, such as the Sandspit fault system, mapped recentlyby Thompson et al. (1991) and P. Lewis et al. (1991a-b)..These faults have been active episodically since LateJurassic time with a dip-slip sense of motion.A study of the age, orientation, and distribution ofdikes on southern Queen Charlotte Islands (Souther, 1989;Souther and Jessop, 1991) showed that many dikes areconcentrated in distinct swarms, separated by areas ofscarce or randomly oriented dikes. A number of north-trending swarms were defined on southern Moresby Island.To the north, other swarms trend east-northeast to north-northeast. The age of most dikes is Middle Eocene to Early-152-Miocene. Souther (1989) associated the ordered dike swarmswith zones of crustal weakness separating rigid blocks.Faults and folds with a northeasterly trend exist onGraham Island (Sutherland Brown, 1968), but the regionaltiming of their origin is unknown. On northwestern GrahamIsland, outcrop-scale faults striking 060° were active inthe Late Triassic or Early Jurassic as a part of a largerpattern of deformation (Lewis and Ross, 1989), but thesmall scale of these structures complicates correlationswith regional fault networks. On central Graham Island,parallel faults were encountered by Hesthammer (1990) andIndrelid (1990). Northeast-trending structures are alsofound in younger rocks (Sutherland Brown, 1968; SutherlandBrown et al., 1983).Steep north-striking faults occur on Queen CharlotteIslands (Sutherland Brown, 1968). Hickson (1989, 1991)showed that they cut volcanic rocks of the Tertiary MassetFormation on Graham Island. On Moresby Island, however,the north-striking faults mapped by Taite (1990, 1991) wereapparently active in both Jurassic and Tertiary time. Theearly Middle Jurassic Rennell Sound Fault (Fig. 1-1) has awest-northwesterly orientation, oblique to the predominantnorth-northwest structural grain on Queen Charlotte Islands(Thompson et al., 1991).7.1.3. British Columbia mainlandRegional geologic descriptions of the Coast Plutonic-153-Complex east of Hecate Strait were recently provided byHutchison (1982), Woodsworth et al. (1983), Crawford et al.(1987), and van der Heyden (1989, 1992). The principalgeologic trend in the Coast Belt, as in the Insular Belt,is north-northwest, represented by large block-boundingfaults in that region. Other trends are also present.In the area of the Skeena River, Roddick and Hutchison(1972) noted northeast-trending ductile shear zones inrocks they considered to be late Paleozoic. Subsequentwork (Crawford et al., 1987) has cast doubt on the age ofthe protolith of the Central Gneiss Complex: many of theserocks are likely Mesozoic in age. Nonetheless, northeast-trending minor ductile shear zones, of Paleogene age, arecommon in the core of the Coast Plutonic Complex betweenlatitudes 53° N and 55 ° N (Roddick, 1970; Hutchison, 1982;Woodsworth et al., 1985; Heah, 1990, 1991).Douglas (1986) suggested that older northeast-trendingfaults may have influenced rock deformation in the SkeenaRiver area in the Late Cretaceous. He speculated thatstructural variations possibly reflect heterogeneitiescaused in part by pre-existing fabrics. The Skeena Valleyhas a consistent northeasterly trend, cutting across theentire Coast Belt, although Claque (1984) found no obviousstructural control on its orientation.Northeasterly faults are not confined to the studyarea, and they occur in many other parts in the Cordillera.The fault-bounded, northeast-trending Skeena Arch in the-154-Intermontane Belt, whose northern flank lines up with theSkeena River lineament, was active throughout the Jurassic(Tipper and Richards, 1976; Tipper, 1984). Field mappingshows that in Middle and Late Jurassic time, the SkeenaArch was the shoreline along the southern margin of theBowser Basin. This northeast-trending strandline probablycontinued westward into what is now the core of the CoastBelt. Although the western extent of the Skeena Arch isuncertain, it may have in some fashion controlled east-northeast-trending deformation in the Central GneissComplex (Hutchison, 1982). In Early to mid-Cretaceoustime, the arch was inactive, but it was rejuvenated later.An east-northeasterly line of Neogene volcanic centersat about 52 °N (the Anahim belt; Fig. 1-1) was interpretedby Bevier et al. (1979) and Souther (1986) as a product ofthe passage of North America over a mantle hot spot, or asan incipient rift. The hot-spot hypothesis is at leastconsistent with the general eastward-younging ages ofvolcanic centers in the belt. On the other hand, the east-northeast trend is not unique to this region as it appearsalso in other parts of the Coast Belt. Some of the largeplutons inferred beneath eastern Queen Charlotte Sound lieon trend with the Anahim belt and may form its westwardcontinuation. If this is true, the plutons were emplacedin the Neogene (Chapter 6). Such a young age hasimplications for Cenozoic heating of the Queen CharlotteBasin (Lyatsky and Haggart, 1992; Chapter 9).-155-Smaller-scale igneous trends are also found on themainland. Northeast-trending lamprophyre dikes of Mioceneage north of Prince Rupert (Smith, 1973) may have beencontrolled by older structures (Hutchison, 1982). Baer(1973) noted north-striking dikes of presumed Miocene toPleistocene age between 52 °N and 51) N. Tertiary(?) dikesin the Prince Rupert area have northerly, northeasterly,and easterly orientations (Hutchison, 1982).Of particular significance to the structural analysisof the Queen Charlotte Basin are the northeast-trendingbasaltic dikes on Porcher Island, described by Woodsworth(1991). The age of these dikes is 26 Ma. They crosscutall fabrics on the west-northwest-trending Kitkatla faultsystem and are not offset themselves; the significance ofthis relationship is discussed in the following section.7.2. Timing and sense of motion on the principal faultsystems in the Tertiary Field mapping and geophysical studies have shown thatno regionally significant strike-slip faulting occurred onnorthern and central Queen Charlotte Islands during theTertiary (Chapter 6; Thompson et al., 1991; Lyatsky, 1991a;P. Lewis et al., 1991a-b). The Cretaceous strandlinesidentified by Haggart (1991) persist across the islandswithout detectable lateral offset (Fig. 2-3), and they arenot disrupted by the Rennell Sound Fault or other largefaults. The Upper Oligocene to Miocene Masset Formation-156-volcanics on northern Graham Island are not displaced bystrike-slip faulting either.On northeastern Graham Island, drilling has shown thatMasset volcanics are covered by about 2000 m of NeogeneSkonun Formation sediments (Shouldice, 1971). Thus, thisarea was a Tertiary depocenter, previously unrecognized(Fig. 6-3) due in part to lack of seismic coverage. Thedepocenter is associated with a pronounced, localized,negative gravity anomaly (Figs. 5-1, 5-2). Orientation ofthe bounding gradient zones in the Bouguer/free-air mapsuggests this depocenter is a depression bounded on thewest by north- and possibly northeast-striking normalfaults. Presence of such east-side-down faults on thewestern flank of the depression is confirmed by fieldmapping (Hickson, 1991). The north-south faults are local,and they are unrelated to the north-northwest-trendingSandspit fault system, which lies to the west, beneathMasset Formation volcanics.A large, west-northwest-trending fault system cutsacross northern Hecate Strait and central Dixon Entrance(Fig. 1-1). Yorath and Chase (1981) interpreted it as acontinuation of the Principe Laredo/Banks Island faultsystem, which trends north-northwest along the east side ofHecate Strait. The north-northwesterly Clarence StraitFault in southeast Alaska continues south into DixonEntrance (Stacey and Stephens, 1969; Johnson, 1972), andYorath and Chase (1981) and Wheeler and McFeely (1991)-157-linked it with the Kitkatla fault system, whose orientationon the mainland British Columbia is west-northwest.Results of this study indicate the Principe Laredo/Banks Island and Kitkatla fault systems indeed continuenorthwest offshore, but in a different fashion (Fig. 1-1),as they intersect west of Porcher Island. The PrincipeLaredo/Banks Island fault system trends north-northwest,likely merging with the Clarence Strait Fault. Thepronounced, west-northwesterly magnetic lineament thatcrosses northern Hecate Strait represents the continuationof the Kitkatla fault system. It trends obliquely tonorth-northwesterly and northeasterly structures andpotential-field anomalies in Hecate Strait, and it is notoffset laterally across any of these features.The displacement history of these regional faults isconstrained by field mapping onshore. In southeast Alaska,Gehrels et al. (1987) noted an apparent dislocation ofoutcrops of Ordovician and Silurian rocks across ClarenceStrait and speculated that the fault may accommodate about15 km of dextral offset; the time of the presumed movementis uncertain. On the mainland of British Columbia, thePrincipe Laredo/Banks Island and Kitkatla fault systemsbound large crustal blocks in the Coast Belt, distinguishedby different ages of emplacement, uplift, and cooling ofplutons. These steep faults were active with a dip-slipsense of movement in Late Jurassic and Early Cretaceoustime, but there are no indications of Mesozoic or Cenozoic-158-strike-slip movements in excess of a few kilometers (Baer,1973; Hutchison, 1982; van der Heyden, 1989, 1992).The northwest-trending Banks Island fault systemoffshore is associated with a small, apparently Tertiary-aged depression seen in seismic reflection data (Figs. 6-3,7-1). The free-air/Bouguer gravity map (Fig. 5-1) shows asquare negative anomaly west of Banks Island, bounded bynorthwesterly and northeasterly gradient zones. The UpperTriassic Karmutsen Formation crops out on Bonilla Island(Fig. 1-1; Roddick, 1970; Woodsworth, 1988) to thenorthwest, on trend with the Banks Island fault system.This also suggests the structural depression has a limitedextent in that direction. Dip-slip Tertiary displacementscharacterize the Banks Island system onshore (G.J.Woodsworth, pers. comm., 1990).The Kitkatla fault system does not accommodate Neogenestrike-slip movements, because Woodsworth (1991) hasestablished that its structural fabrics are cut by LateOligocene (26 Ma), northeast-trending dikes on PorcherIsland. Plunges of slickensides in both the Kitkatla andPrincipe Laredo/Banks Island fault systems are steep (G.J.Woodsworth, pers.. comm., 1990), which indicates the latestpulse of movement was dip-slip.Seismic reflection profiles across the Kitkatla faultsystem offshore show dip-slip displacements involvingsediments which are apparently Neogene in age (Rohr andDietrich, 1990, 1992; Dietrich et al., 1992). No strike-Figure 7-1. Northeastern end of seismic reflection Line 5(see inset for location) showing the Neogene structuraldepression west of Banks Island. A: data; B: structuralinterpretation.(Figure 7-lb.-161-slip indicators are evident in the seismic profiles. Ingeneral, even where such indicators seem obvious, strike-slip movements may be difficult to ascertain, as Harding(1990) cautioned that interpretation of such indicatorsfrom seismic sections is commonly ambiguous. Instead,presence or absence of wrench faulting can be establishedby geologic field mapping and investigation of regionalstructural relationships.The Kitkatla fault system traverses northern HecateStrait and central Dixon Entrance. Since this fault systemitself does not accommodate Neogene strike-slip movements,I conclude that none of the faults it may intersect innorthern Hecate Strait and Dixon Entrance could have beenactive in the strike-slip sense in the Neogene.The same is true of areas to the south: no regionalstrike-slip displacements occurred in post-Middle Jurassictime on Queen Charlotte Islands (Thompson et al., 1991; P.Lewis et al., 1991a-b). On Vancouver Island and in theStrait of Georgia, regional strike-slip faulting is notindicated by either the geological (Muller, 1977) or thegeophysical (White and Clowes, 1984) data.Thus, significant Cenozoic strike-slip movements arelacking on the rim of the Queen Charlotte Basin: onVancouver Island, the mainland, northern Hecate Strait, andQueen Charlotte Islands. Therefore, tectonic modelsinvolving considerable north-south Cenozoic extension inQueen Charlotte Sound or southern Hecate Strait, and/or-162-strike-slip faulting in northern Hecate Strait, encountersevere space problems because none of the faults in theQueen Charlotte Basin and the surrounding areas accommodatethe large lateral dislocations required by such models.7.3. Principal fault networks in the Queen Charlotte Basin7.3.1. Structure of the basin rimIn the Insular Belt, the fault networks describedabove have influenced patterns of sedimentation and faciesdistribution since at least the Late Jurassic (Thompson etal., 1991), and possibly even the Late Triassic (Jeletzky,1976). In the western Coast Belt, large fault systems suchas the Kitkatla and Principe Laredo/Banks Islandaccommodated dip-slip movements of crustal blocks since theLate Jurassic (Hutchison, 1982; van der Heyden, 1989,1992). Evidence for appreciable strike-slip displacementsacross these faults is lacking.Field mapping on Queen Charlotte Islands has shownthat no horizontal rotations of large crustal blocks havetaken place since at least the Middle Jurassic (P. Lewis etal., 1991b). On northern Vancouver Island, such rotationswere proposed by Irving and Yole (1987) based onpaleomagnetic data from the Early Jurassic Bonanza Groupvolcanics. This suggestion conflicts with the availablegeological information as no faults exist to accommodaterotations of crustal blocks in that region (Muller et al.,1974, 1981; Jeletzky, 1976), and Irving and Yole (1987) did-163-not specifically propose such faults. The Queen Charlottefault system excepted, only minor, localized strike-slipfaulting has been documented in the study area, acrossparts of the Louscoone Inlet Fault on southern MoresbyIsland (Lewis, 1991; J.W. Haggart, pers. comm., 1992).Gravity and magnetic data and field mapping in thestudy area suggest continuity and wide areal distributionof geological, geophysical, and physiographic lineaments.Many of the causative geologic structures onshoreoriginated in the Mesozoic or earlier and have been activeepisodically since (Jeletzky, 1976; Thompson et al., 1991;P. Lewis et al., 1991a-b). I conclude that the similarityof Tertiary fault networks in the Queen Charlotte Basinwith pre-Tertiary networks in the surrounding areasindicates inheritance of the main structural features inthe basin from Mesozoic or older structures.If large horizontal tectonic displacements occurred inthe study area in the Cenozoic, and if the accommodatingregional strike-slip faults are lacking onshore, suchfaults must be located offshore. However, the continuityand overlap of fault networks onshore and offshore arguesagainst a fundamental difference in the structure of theQueen Charlotte Basin and the surrounding regions. Thecontinuity of the Kitkatla fault system across northernHecate Strait, established based on magnetic data (Figs.1-1, 5-3, and 6-1; also Lyatsky, 1991a), is a compellingargument against Neogene strike-slip movements in this area-164-because, as noted above, this fault system is lockedonshore by Late Oligocene dikes (Woodsworth, 1991).The predominance in the study area of persistent,orthogonal fault networks inherited from pre-Neogene timeis strong evidence against the model of formation of theQueen Charlotte Basin by McKenzie-style (1978) rifting inthe Cenozoic, advocated by Yorath and Hyndman (1983) andHyndman and Hamilton (1991). Rifts accommodating largeextension are characterized by a single, dominantorientation of faults (Chapter 4; Illies, 1981), but anytectonic extension that affected the study area in theTertiary was evidently too small to impose a significantjuvenile overprint and obliterate the block-relatedstructural patterns. On the other hand, continuity of theKitkatla fault system across the northern part of the QueenCharlotte Basin invalidates the model of basin formation byen-echelon strike-slip faulting proposed by Rohr andDietrich (1992): that model requires dextral movements onnorthwest-trending faults in Hecate Strait, but suchmovements are precluded by the data presented here.7.3.2. Fault-related structural characteristics of theQueen Charlotte Basin fillSeismic reflection data show the Queen Charlotte Basinis cut by numerous faults which disrupt all levels of theTertiary basin fill (Shouldice, 1971; Rohr and Dietrich,1990, 1992; Lyatsky, 1991b; Lyatsky and Haggart, 1992).-165-Geological studies onshore (Thompson et al., 1991) andgeophysical investigations offshore indicate that manyTertiary faults are reactivated older features, and thatstructural inversion was common. Thus, the study of theolder structural patterns in the region helps interpret theevolution of the Cenozoic Queen Charlotte Basin.Block movements and high-angle faulting have dominatedthe structural style in the study area onshore since atleast the Late Jurassic, and a similar mosaic of fault-bounded blocks has now been found offshore. The high-angle faults were active more than once, and they delimitcrustal blocks which form a regional tectonic pattern.Such a style of tectonism is known as Germanotype (Stille,1924; de Sitter, 1956), and it is a common characteristicof many intraplate continental regions (Ziegler, 1987).The presence of fault-related structures is a criticalfactor in the formation of large oil accumulations in manyfrontier sedimentary basins. Reactivation of old, high-angle faults may produce rollover anticlines, as well asblock-related stamp and drape structures, in the overlyingrocks (e.g., Cohen et al., 1990). The significance of thefault-block structural style for hydrocarbon exploration inthe study area is discussed in Chapter 9.-166-CHAPTER 8. GEOLOGIC EVOLUTIONOF THE QUEEN CHARLOTTE BASIN8.1. The amount of Cenozoic extension and the role of block faultingin the evolution of the Queen Charlotte Basin Orientations of many faults in the Tertiary QueenCharlotte Basin are similar to the trends of pre-Tertiarystructures mapped onshore on the rim of the basin. Adominant linear structural grain is lacking in the basin,and orthogonal fault networks are observed instead. Manyof the faults beneath Queen Charlotte Sound and HecateStrait are old structures reactivated by the fluctuatingregional stress field. I propose that the pre-Tertiaryfault-block mosaic largely determined the distribution ofdepocenters in the Cenozoic.A series of northward-tilted blocks is found locallyonshore along the east coast of southern Moresby Island.This domain is bounded on the west by the Louscoone InletFault (Figs. 1-1, 2-1), whose southern part locallyaccommodates scissor-type Tertiary movements (Lewis, 1991).This fault may perhaps be regarded as an element of theprominent dip-slip Sandspit fault system, but the SandspitFault proper loses definition off southern Moresby Island,based on seismic and potential-field data (Young and Chase,1977; Young, 1981).Many fault-bounded blocks in the Queen Charlotte Basin-167-are large and are neither tilted nor internally deformed(Fig. 3-3). Some relatively large blocks are discerniblytilted but are still little disturbed internally (Fig.8-1). In contrast, other areas contain numerous, small,tilted blocks (Fig. 8-2) similar to those encountered onsouthern Moresby Island. These are the three main types ofstructure found in the basin.Seismic reflection profiles offshore show numerousNeogene normal faults which accommodate differentialsubsidence and tilting of blocks (Figs. 3-3, 7-1, 8-1, 8-2,and 8-3). Steep, normal faults predominate. Movements onsome of the faults in Hecate Strait were compressionallyinverted in late Cenozoic time (Fig. 8-3).Reflecting differential subsidence, uplift, or tiltingof blocks, unconformities in the basin are mostly local andcannot be correlated basin-wide with confidence (Shouldice,1971). In some parts of the basin (Fig. 8-1), faults andunconformities are seen in seismic sections at traveltimesexceeding 1 s (Fig. 8-1), wherereas elsewhere normal faultsoffshore also disrupt very shallow strata (Fig. 8-2).Neogene tectonism in the Queen Charlotte Basinoccurred in many pulses and continues today, as evidencedby seismicity (Rogers et al., 1988; Bêrubê et al., 1989;R.B. Horner, pers. comm., 1989).It has been proposed (White et al., 1986) that, ingeneral, if a normal fault flattens out at depth, theamount of tectonic extension it accommodates may be greaterNWB31950^B31850 B31750^B31650^B31550^B31450oppowmpyloggrompopoompomFoommommeiggpootoilmemonuompoquipmpleam owimFigure 8-1. Segment of seismic reflectionLine 1 (Rohr and Dietrich, 1990). Notesubsidence and tilting of fault-boundedblocks. Location of the profile is givenin the box. A: data; B: structuralinterpretation (from Lyatsky and Haggart,1992; white lines represent faults).NW831650^831550^B31450q""A"""19,4 '.4",101 :I0,04 ,1041[1,4,19 1 ,4011 ,0""41 ,011!"0" ,"44 , triAimiplim1.41 ,01 mr14 ,000.6.4•-41; 41—FL.-JCCB31950 B31850^B3175010.. 14444 ,044014 ,! 4, 1."4ap,'177eZC7' ,14M1C1,..7;Z:ea").14„4.<03I-10 kmFigure 8 - 1b.3e1040..SWB31350B35050^B34950Ih:NOMMMOOM . . ,, . ,,,, J , ! ,,,,SONIMONNINNO.„74.—.....;, :iitlig4;,-,;,,A;...oget' —, ',^• —,----,;—C^'^.01iiiWNWB35150SEB34750^B34650^B34550Cl)Ui2—JLLI- 2>-50crI —Figure 8-2. Neogene faults disruptingshallow levels of the fill of the QueenCharlotte Basin, illustrated in seismicreflection Line 1 (location in box).A: data; B: structural interpretation(white lines represent faults).110 kmFigure 8-2b.SEB34650^B34550B35150 B35050^B34950MINKOMPIVW1WW B34850^B34750,•^NJa^t! ^I^I. ^• I^'• •^;il^I. , I^IINW1300 ^1400^1500^1600Figure 8-3. Effects of compressionalinversion of an extensional fault inHecate Strait, illustrated in seismicreflection Line 7 (location in box).A: data; B: structural interpretation(white lines represent faults).1 300^1400,, 16001500$^ 1700 1800U)^":: ,;;......: .. 7 *1::... ti. ...,7, ..,,.-,..^',,,-.70.„, ,,,,r.,, ,t, ;;.„,,,,.. .; !,,,......• ,' ,,--3-...;=..,  ,,,,"^.....- ;• ; . w. ^',.$ 5',40!z .-,-ims!!:;,.. . .,?”.• 4.1;;;;.'7.,:; c^, ', ..r",,..,'!"."-rr: ".1-;:.1,4,:,-rr.;;, ,......,m 1 oe....„.L.L.1^ro,...;..,.;i:^'?^ r....'... :::::....,, :i+-..-- 7,^.,,....7.,^. 1— --^'-'-- - rte' ^,r'.:::.%, ......4;.;""'''!",'.' Pfe'.....- ..^,;,..,,..-^-- --,; :.,--- ;..,--.:,^•^-,....r .:-. ?: ,`.:... .2r,— 1—^',.:"..i.z.... • ' 1._-, .s;.04.1e--.,;—1L11N'rri:+gket,Figure 8 - 3b.1-174-than that estimated from the throw at shallow depths.However, in their reconstructions of fault movements, Whiteet al. (1986) disregarded the effects of differentialsediment compaction and ascribed syndepositional listricfaulting to tectonic extension only. Thus, they ignoredthe processes of growth faulting, where the slip on anormal fault is perpetuated by sedimentary loading andcompaction on the downthrown side (Gretener, 1986).Moreover, Kusznir et al. (1991) and Roberts andYielding (1991) have demonstrated that flattening of normalfaults does not always occur and cannot be assumed as arule. These workers showed that many large faults onceassumed to be listric in fact remain planar and steeplydipping to considerable depths in the crust. In general,crustal-penetrating high-angle faults are common in manyareas worldwide. In some cases, curvature of fault planesmay be required where hanging-wall blocks are tilted; suchmovements may be dissipated within the crust (Gretener,1986). In the Queen Charlotte Basin, flattening of high-angle faults, at least within the basin fill, is generallynot suggested by the seismic data (Figs. 3-3, 7-1, 8-1,8-2, 8-3). Many faults penetrate into the basement, and aconsiderable amount of "hidden" extension is unlikely.The amount of extension accommodated by normal faultsoffshore was estimated by the present writer andindependently by Hyndman and Hamilton (1991), by restoringthe displacements on the faults in seismic sections. Where-175-fault geometry and displacement are well constrained, thistechnique can be used to obtain a measure of lengthening ofthe section. In the absence of a dense seismic coverage,such estimates can only be very approximate. Additional,severe complications, arising due to the lack of regionalseismic stratigraphic markers and the uncertainties in theseismic detection of the geologic basement in the QueenCharlotte Basin (Chapter 3), preclude exact restoration offault offsets. For these reasons, the estimates of basinextension based on fault displacements are qualitative.My restorations of dip-slip displacements on faultsimaged seismically in the Queen Charlotte Basin offshoresuggest they account for tectonic extension of no more than10% (maximum 5% in Queen Charlotte Sound and 15% in HecateStrait).Similar results of structural reconstructions onseismic sections were reported by Hyndman and Hamilton(1991), who concluded that only a small amount of extensionis apparent in the seismic profiles. However, theyspeculated that considerable, additional Tertiary extensionwas accommodated by intrusion of dikes.This suggestion is negated by the results of fieldmapping. Dike swarms are abundant only in selected areas;they are volumetrically insignificant on a regional scale,and their contribution to crustal extension across thestudy area is negligible. On southern Queen CharlotteIslands, for example, Cenozoic dikes account for crustal-176-extension of 3% to 4% locally (Souther and Jessop, 1991).Along the eastern margin of Hecate Strait, dikesaccommodate "less than a few percent", maybe less than0.1%, Tertiary extension (Woodsworth, 1991, p. 334).Thus, I conclude that the total resolvable extensionin the Queen Charlotte Basin in the Cenozoic was no morethan 10% and was accommodated by normal faults.Based on the data presented, I contend that manyTertiary structural depressions in the Queen CharlotteBasin are associated with downdropped polygonal blocks(Plate I in pocket). The associated minor extension wasaccommodated by the block-bounding faults that wereinherited in part from earlier time. Cenozoic extensionalfaults were formed largely by reactivation of pre-existingfault networks, and local complexity occurs due to presenceof splays, antithetic faults, etc. in the basin fill.High-angle faults dominated the post-Middle Jurassicstructural evolution of the study area onshore (Jeletzky,1976; Muller, 1977; Thompson et al., 1991). These faultslargely controlled the folding patterns, as folds arecommonly associated with the steep faults. Detached foldsand low-angle thrust faults formed in some areas onnorthern and central Queen Charlotte Islands in the MiddleJurassic, but younger structures of this type are rare andinsignificant on a regional scale (Thompson et al., 1991;P. Lewis et al., 1991a-b). Many large faults on theislands were active more than once, with various sense of-177-movement, in the Mesozoic and Cenozoic. Seismic profilesoffshore, especially in Queen Charlotte Sound, show manyhigh-angle Neogene faults bounding downdropped and upliftedblocks which are relatively undisturbed internally, anddetached folds and low-angle faults are uncommon.Fault-block tectonism, originally termed Germanotype(Stille, 1924; de Sitter, 1956), was recently interpretedin terms of plate tectonics by Ziegler (1987). Germanotypefault-block tectonics is associated with the regionaldeformation of continental intraplate areas, where strainis restricted largely to narrow zones in a fracturedcrystalline crust (see also Anfiloff, 1988). The majorhigh-angle faults which bound rigid crustal blocks mayaccommodate alternating pulses of uplift, subsidence, andtilting, in response to fluctuations of the stress field.These faults propagate from the basement upward through thesupracrustal volcano-sedimentary cover by reactivation.Large-scale vertical block movements may be dissipated atdepth by ductile flow in the upper mantle (Wezel, 1985) orin the lower crust (Dohr, 1989).Structural inversion results from changes of the tiltor the sense of vertical movement of a block (Cohen et al.,1990). Sources of the causative stresses may be proximalor distal (Grant, 1987), acting horizontally (Anfiloff,1988) or vertically (Cooper, 1990). Movements of a blockmay occur repeatedly during its evolution, with varyingsense of displacement on the bounding faults. Depending on-178-the momentarily prevailing stress field, block-boundingfaults may accommodate a limited lengthening or shorteningof the section, as well as local strike-slip displacements.Such a tectonic style may be continent-wide orrelatively local. It was recently described on variousscales in Germany (Betz et al., 1987), Sweden (Norling andBergstr8m, 1987), the Middle East (Cohen et al., 1990;Ameen, 1992), Australia (Wellman, 1985; Anfiloff, 1988),offshore Newfoundland (Grant, 1987), and elsewhere.8.2. Mechanism of Queen Charlotte Basin subsidence Subsidence of the Queen Charlotte Basin was previouslyexplained in terms of the McKenzie (1978) stretching(rifting) model by Yorath and Chase (1981), Yorath andHyndman (1983), and Hyndman and Hamilton (1991). However,this explanation conflicts with the new geological andgeophysical data from the study area, as discussed above.The amount of extension required by the rift model issubstantially greater than that allowed by the newgeological and geophysical constraints onshore andoffshore. Significant lateral movements of crustal blockssurrounding the basin have now been disproved (Thompson etal., 1991; Haggart, 1991; Hickson, 1991; P. Lewis et al.,1991a-b; Chapters 2, 6, and 7 of this dissertation).Another model of the Queen Charlotte Basin evolutionwas offered by Rohr and Dietrich (1992) and Dietrich et al.(1992). These authors suggest that subsidence of the Queen-179-Charlotte Basin occurred in response to dextral strike-slipfaulting distributed into the basin area from the QueenCharlotte fault system to the west. The presumedintrabasinal wrench faults strike north-northwest and arelocated in Hecate Strait; significant Tertiary extension inQueen Charlotte Sound and compression in Dixon Entrance arealso proposed in this model.Like the rift model of Yorath and Hyndman (1983), thisnew interpretation is not consistent with the structure ofthe study area. It is invalidated by the absence of largestrike-slip faults onshore and by the continuity of theKitkatla fault system across northern Hecate Strait, whichprecludes appreciable strike-slip faulting in the Neogene.Alternative causes of basin subsidence, consistentwith the presently available information, thus need to beconsidered. Such a geologic model is offered below.Since Late Jurassic time, formation of sedimentarybasins in the Insular Belt has coincided with periods ofuplift and denudation of the adjacent orogens. Thedevelopment of the late Mesozoic sedimentary basin on thesite of Queen Charlotte Islands (Haggart, 1991, 1992a) wascoeval with long-lived uplift of the mainland and theHecate Strait area (Hutchison, 1982; Crawford et al., 1987;van der Heyden, 1989, 1992). Formation of the GeorgiaBasin and deposition of the Upper Cretaceous Nanaimo Groupon eastern Vancouver Island was contemporaneous with upliftof the Coast Belt and the central Cascade Mountains-180-(Monger, 1991). The timing of initiation of the subsidenceof the Tertiary Queen Charlotte Basin is constrainedpoorly, but Parrish (1983) noted that post-Eocene uplift ofthe Coast Belt north of latitude 51 °N did coincide withsubsidence of the Queen Charlotte Basin.The geodynamical relationship between uplift andsubsidence in the Coast and Insular belts is unclear. TheCoast Belt, composed largely of granitoid plutons, has beenundergoing uplift intermittenly since Late Jurassic time.Its Cretaceous and Early Tertiary evolution wascharacterized by regional uplift related to orogeniccompression (Monger et al., 1982; Crawford et al., 1987;Rusmore and Woodsworth, 1991; Monger, 1991; Monger andJourneay, 1992). Its post-Eocene uplift, in contrast,proceeded in an extensional environment (Parrish, 1983).Nonetheless, periods of subsidence in the Insular Beltaccompanied all these uplift episodes in the Coast Belt,regardless of their syn-orogenic or post-orogenic,compressional or extensional nature.The Cretaceous subsidence in the Insular Belt isthought to have taken place in a forearc setting (Haggart,1991). The subduction zone presumably lay somewhere to thewest, and the continental magmatic arc was the evolvingCoast Plutonic Complex to the east. The formation of theCoast Belt was accompanied by tectono-magmatic thickeningof the crust (Hutchison, 1982; Crawford et al., 1987; vander Heyden, 1989, 1992; Rusmore and Woodsworth, 1991;-181-Monger, 1991; Monger and Journeay, 1992). Tectoniccompression in Cretaceous time is reflected in the presenceof outward-verging thrust faults on both flanks of theCoast Belt. Additional thickening of the crust wasaccomplished by emplacement of low-density granitoids. Therelatively quiescent subsidence in the Insular Belt duringthe late Mesozoic and early Cenozoic may have been partly aflexural response to the loading of the crust in theadjacent Coast Belt.Uplift of the Coast Belt since the Late Eocene, in thelast 40 Ma, may have been driven in part by isostaticupwelling of the previously thickened, granitoid-richcrust, and by externally supplied stresses. The uplift wascontrolled by regional, steep faults (e.g., Brew and Ford,1978). However, the influence of these processes on theevolution of the Queen Charlotte Basin cannot be assessedbased on the present state of knowledge of the tectonichistory of the region, and it is not apparent from thestratigraphic and structural characteristics of the basin.A shallow Moho is found beneath the basin in the QueenCharlotte Sound area, where the crystalline-crustalthickness is reduced (Section 2.4). If the shallow Mohoand thin crystalline crust were caused entirely by Tertiaryextension, the amount of that extension could be estimatedat more than 100%, depending on the pre-rift crustalthickness assumed. However, such estimates of extensionare precluded by all the other data from the region.-182-The study area was affected by many tectonic episodes,and the assumption that the reduced thickness of thecrystalline crust beneath Queen Charlotte Sound is purely aresult of Cenozoic tectonism is questionable. Regionalvariations in crystalline-crustal thickness in the studyarea may be inherited from early Tertiary, Mesozoic, orearlier time. Shallow Moho is restricted to only the QueenCharlotte Sound area, while the whole basin is compensatedisostatically (Section 8.3). This suggests the raised Mohomay be a pre-Tertiary feature. If the reduced crustalthickness beneath Queen Charlotte Sound is a long-livedphenomenon, it would predispose that area to subsidence andbasin formation.If the crustal-thickness variations were produced atleast in part in the Tertiary, several geodynamicalprocesses may have contributed. In a study of sedimentarybasins in Germany, Dohr (1989) resurrected the old ideathat non-compressional updoming of the continental crustmay be caused by the migration of ductile lower-crustalmaterial into the zone of uplift from the surroundingareas. In an analogous manner, a growing salt pillow drawsmaterial from the proximal parts of the salt bed. Thisproduces uplift over the pillow and subsidence over itsflanks. In both cases, the flow occurs within a restricteddepth range (the salt bed or the lower crust) ) and this maylead to necking.In the Queen Charlotte Basin area, at least north of-183-51U N, such a situation may have existed in the last 40 Ma,after regional compression had ended. The rising CoastBelt, with its low-density, granitoid-rich crust, may havebeen the equivalent of the pillow, and the Queen CharlotteBasin a product of flank subsidence. This model isspeculative because mechanics of lower-crustal deformationare unclear (Meissner, 1989; Meissner and Weyer, 1992).In the Tertiary, the study area is thought to havepassed over a mantle hot spot, as evidenced by the presenceof the northeast-younging Neogene Anahim volcanic belt onthe mainland (Fig. 1-1; Bevier et al., 1979; Souther,1986). The activity of this mantle plume may have resultedin the thinning of the lithosphere or even the crust frombelow, followed by isostatic subsidence of the affectedarea. The hot spot may have generated the stressesresponsible for the renewal of block movements in the QueenCharlotte Basin. The intrusion of plutons postulatedbeneath Queen Charlotte Sound may also be related to theactivity of the hot spot. Dissipation of any positivethermal anomalies would have caused additional subsidencein that area. However, knowledge of influence of mantleplumes on the tectonics of sedimentary basins is imperfect(Chapter 4), and this discussion is also speculative.Other factors may have affected the evolution of theCenozoic Queen Charlotte Basin. Tectonic extension,estimated here at no more than 10%, was likely responsiblefor some of the mechanical crustal thinning and subsidence.-184-A combination of minor extension and hot-spot activity mayhave caused the intrusion of mantle-derived material intothe lower crust beneath Queen Charlotte Sound, producing ashallowing of the geophysically determined Moho (Ziegler,1992; Rosendahl et al., 1992). Indeed, seismic refractiondata suggest that a layer of high-velocity material ispresent at mid-crustal levels beneath Queen Charlotte Sound(Yuan, 1990), consistent with the intrusion hypothesis.The Tertiary intraplate stress field in the study areamight have also been influenced by external forces, plateinteractions to the west being one possible cause.However, the exact nature of Cenozoic plate interactions atthe western Canadian continental margin, as well as theireffect on the regional structural evolution in post-Eocenetime, is poorly understood.Strike-slip plate movements are generally thought tohave prevailed along this segment of the continental marginof western North America in the last 43 Ma (Engebretson etal., 1985; Stock and Molnar, 1988; Riddihough and Hyndman,1989; DeMets et al., 1990; Hyndman and Hamilton, 1991).Transpression characterizes the margin at present, andearthquakes with reverse-fault solutions occur in the QueenCharlotte fault system and on Queen Charlotte Islands(Bêrube et al., 1989).However, the influence of Cenozoic plate interactionson the structural evolution of the Queen Charlotte Basincannot be quantitatively evaluated. Past episodes of-185-transtension and transpression cannot be resolved alongthis segment of the margin based on the existing plate-interaction models (J.M. Stock, written communication toR.I. Thompson, 1990). Major sources of uncertainty areerrors in plate reconstructions and the lack of knowledgeof the past orientation of the continental margin.Basement-controlled block faulting similar to that inthe Queen Charlotte Basin has been described in detailelsewhere. Its influence on the evolution of many Mesozoicand Cenozoic sedimentary basins in northern Europe is wellestablished (Ziegler, ed., 1987). Such a tectonic stylehas also been proposed for the Phanerozoic evolution of thewestern Plains region in the U.S., from Arizona to NorthDakota (Maughan and Perry, 1986). The intraplate stressesthat caused reactivation of old faults in the basement wereinduced by relative movements of large (larger than thebasins) terranes across major faults, sutures, and orogeniczones. Sources of intracrustal stresses were external tothe basins, and strike-slip displacements within the basinswere minimal. Similarly, Pacific-North America plateinteractions across the Queen Charlotte fault system in theCenozoic may have contributed stresses that affectedbasement blocks in the Queen Charlotte Basin, but the basinitself was not disrupted by regional strike-slip faults.In contrast, intrabasinal wrench faulting played acontrolling role in the evolution of many areas lying attips of large rift systems which are propagating incipient-186-plate boundaries, e.g., the Imperial Valley in southernCalifornia and the Gulf of Aqaba in the Middle East (Ben-Avraham, 1985). Within these basins, lateral dislocationsof crustal blocks were apparently significant, related totectonic movements on a large scale: formation of theImperial Valley and some of the basins to the north isthought to be a result of the opening of the Gulf ofCalifornia, and the Gulf of Aqaba is a branch of the RedSea rift. Therefore, contrary to Rohr and Dietrich (1992),the Queen Charlotte Basin and the Cenozoic basins insouthwestern California are not tectonically analogous.8.3. Influence of tectonics on the distribution ofsedimentary rocks in the Queen Charlotte BasinThe Queen Charlotte Basin is compensated isostaticallyon a regional scale (Stacey, 1975), as evidenced by thefact that both free-air and enhanced isostatic gravityvalues usually remain within 30 mGal from zero (Figs. 5-1,5-2). Yuan (1990), who investigated crustal structurebeneath Queen Charlotte Sound, agreed that isostaticcompensation of the basin exists. He suggested, however,that this situation represents the terminal stage of basin-forming Tertiary rifting, accomplished by post-riftisostatic re-equilibration of the area.This suggestion is ill-founded. In theory, isostaticequilibrium could have easily been maintained in the studyarea without tectonic movements in the Tertiary or at any-187-other time, and hence the present-day compensation of thecrust cannot be used as positive evidence for tectonism.Isostatic equilibrium is the global norm, not an anomaloussituation, and it may be disturbed when tectonic movementstake place. It is only a lack of compensation that can beinterpreted as an indication of tectonism, but the QueenCharlotte Basin is in isostatic equilibrium.A shallow Moho is found beneath Queen Charlotte Sound(Section 2.4). One may be tempted to associate thisphenomenon with large Cenozoic extension which led to theformation of the Queen Charlotte Basin. However, such aconclusion cannot be drawn except in isolation from theregional geologic framework, and it would be misleading fortwo reasons. First, significant Tertiary extension isprecluded by the other data from the region, and theevidence against such models is damning. Second, theshallow Moho underlies only the southern part of the basin.Moreover, variations in Moho depth and crustalthickness occur in many continental tectonic settings, andtheir relationships to specific types of tectonism are notunique. Possible origins of such variations in the studyarea were mentioned above (Section 8.2).For the isostatic equilibrium to be maintained in theQueen Charlotte Sound area, gravitational attraction of theraised mantle must be compensated. Much of thecompensating mass deficiency seems to occur at shallowlevels, above Karmutsen Formation basalts. Gravity data-188-(Stacey, 1975) and seismic refraction data (Clowes andGens-Lenartowicz, 1985) imply the presence of low-density,low-velocity material beneath Queen Charlotte Sound atlevels deeper than those drilled. The identity of thismaterial may be two-fold. First, regional stratigraphiccorrelations suggest that western Queen Charlotte Sound isunderlain by a greater thickness of low-density Mesozoicsedimentary rocks than is Hecate Strait (Chapter 2; Lyatskyand Haggart, 1992). Second, large, sediment-filled,Cenozoic structural depressions are also found in thesouthern part of the Queen Charlotte Basin.Cenozoic depocenters are represented by localizedgravity lows in the enhanced isostatic map (green colors inFig. 5-2). Their locations are consistent with thosedetermined from seismic reflection data (Fig. 6-3; Dietrichet al., 1992). Two large gravity lows occur in thesouthern part of the basin, one in Queen Charlotte Soundand the other in southern Hecate Strait. Numerous (butmore localized) negative anomalies are found in central andnorthern Hecate Strait. The structural depressions inwestern Hecate Strait off central Queen Charlotte Islands,although apparent in seismic reflection profiles (Fig.6 -3), are expressed less distinctly in the enhancedisostatic gravity map than are those to the north and tothe south. A possible explanation is local variations inthe effective density (sensu Litinsky, 1989) of the QueenCharlotte Basin fill, caused by density variations in the-189-Skonun Formation sediments or by presence of relativelyheavy volcanic rocks interlayered with the sediments.The southernmost of the two large enhanced isostaticgravity lows is found in Queen Charlotte Sound, between0^0^0^ 051 N and 52 N, 128 30'W and 129 W, where north-south andeast-west fault trends are expressed in the free-air maps(Figs. 5-1, 6-2) and, to a certain extent, in seismicreflection data (Fig. 6-3). The other large enhancedisostatic low, in southern Hecate Strait (Fig. 5-2),between 51° 30'N and 52 °30'N, 1301) W and 131 W, alsoindicates the presence of a fault-bounded depression (seeFig. 6-3). Its eastern boundary coincides with a prominentnorth-trending free-air lineament at longitude 13eW (Figs.5-1, 6-2), which persists when the data are upwardcontinued to as much as 20 km (Fig. 5-9). This lineamentis probably caused by a large fault system, partly apparentin seismic data (Fig. 6-3) and included in the regionalfault map in Fig. 1-1. The western boundary of thisstructural depression is marked by an interplay ofnortheasterly and northwesterly free-air trends offsouthern Moresby Island (Figs. 5-1, 6-2). Another, small,isolated structural depression occurs at 5110'N/129 ° 30'W(Figs. 5-2, 6-3; Plate I in pocket).The reduced enhanced isostatic gravity values inwestern Queen Charlotte Sound may have a three-fold cause:(1) large depth and areal extent of Cenozoic structuraldepressions;-190-(2) variations in density of the Tertiary Queen CharlotteBasin fill; and(3) abundance of Mesozoic sedimentary rocks underneath.The comparatively elevated enhanced isostatic valuesin eastern Queen Charlotte Sound (Fig. 5-2) likely reflectthinning of Skonun Formation sediments, as is confirmed bythe seismic and drillhole data (Fig. 6-3; Shouldice, 1973;Higgs, 1991; Rohr and Dietrich, 1992). High-amplitudemagnetic anomalies are also found above eastern QueenCharlotte Sound, and some of them are probably caused bylarge plutons of an intermediate composition (Chapter 6).Structural differences between Queen Charlotte Soundand Hecate Strait arise because fault patterns in the twoareas are dissimilar. North-south and east-west faultnetworks characterize the Queen Charlotte Sound area, whilethe Hecate Strait area is dominated by north-northwesterlyand northeasterly trends (Chapter 6). Faults are spacedcloser in Hecate Strait than in Queen Charlotte Sound(compare Figs. 3-3, 8-1, 8-2, and 8-3); as a result, thesizes of crustal blocks and hence of Tertiary depocentersare larger in the southern part of the Queen CharlotteBasin. This is reflected in the reduced area (but notnecessarily amplitude) of negative anomalies in theenhanced isostatic gravity map (Fig. 5-2). Seismic datashow that blocks and structural depressions are areallysmaller in Hecate Strait, although some of them are up to6000 m deep (Fig. 6 -3; Dietrich et al., 1992). Structural-191-inversion occurred in that part of the Queen CharlotteBasin in late Cenozoic time (Fig. 8-3).The Hecate Strait area was thus tectonically moreactive during the Cenozoic than the Queen Charlotte Soundarea. Stratigraphic differences between the northern andsouthern parts of the basin are also profound: HecateStrait is underlain mostly by non-marine Tertiary deposits,while marine sediments predominate beneath Queen CharlotteSound (Shouldice, 1971, 1973; Higgs, 1991).8.4. Variations of fault-block tectonics in the QueenCharlotte Basin The geologic model of the structure and evolution ofthe Queen Charlotte Basin presented in this dissertation isconsistent with the available data. In this model, thebasin evolved under the influence of a pre-existing fault-block mosaic in the basement (Fig. 8-4). The distributionof crustal blocks of various sizes, and the response ofthis mosaic to the fluctuating regional stress field,controlled the structure and evolution of the basin.The uplift history of the principal crustal blocks inFig. 8-4 was discussed in Chapter 2. In the EarlyJurassic, much of the region was occupied by a broad,tectonically quiescent marine basin. In Cretaceous time,the Hecate Strait area was uplifted, while the QueenCharlotte Islands area was depressed, receiving sediments.These block movements were inverted in the Cenozoic: Insular Belt (—Coast Belt —'... 0° O...0 ° 0:0 ° 0°°0 ° 0"0 ° 0"0 0 00^ 0 00:00 ; 00^00^00^00^0000.'000.'000.'00-00 00 0000 000 0 0000 0 00 0 00 0 00^0^0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0000 0  00. 0 00.00 00 0. .0 00 0 .0^00VVVVVVVVVV V VVVV V VV VVVVVVVV V VVV"VVVVVVVVVVVVVVVVVVVVVx^VVVVVVVVVVVV 'VVVVVVVVVVVVVVVVVVVVVV \VVVVVVVVVVVV VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV V VVVV V VVVVVVVVVVVVVVVVVVV VVVVVVV VVVVVVV'VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV'VVVVVV"VVSFSInsular Belt Coast Belt —VVVVQueen Charlotte Islandsx X XX^X X X Xxxxxx X XVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV' 'VSFS■,•VVVVVVVVVVVVVVVVVVHecate StraitVVVVVVV VV VVVVVVVVVVVVVVVV VVVVVVVVVVVV V V'V V V VPLFSV VVVV IVVVVVVVVVVVVVVVVVVVVVVVV VVVVVVVVVVVVVVs,VVVVVVVVVVVVVVV'VVV'VVVVVVVV V VV V VVV V VVV V VVVV V VVVVVV VV VVVVV'VN'V ,VVVVVVVVV,VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV VVVVVVVVVVVVVVVVVVCoast Mts.VV'VVVVVVVV VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV VVVVVVVV"VVVV , 'Insular Belt Coast Belt —PLFSr/ir r' rr'VVVVVV V VV VVVV VV VV VVV VVVVVV V VVVVVV V VVV VV VVVVVVVVVVVVV VV VVVVV VVVVVVVV VV VV VV VVV V VVV VVVVVVVV VVVVVVVVVVVV VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVFigure 8-4. Schematic cross-sections through the northernpart of the study area (previous page), illustrating theevolution of the top 5 km of the crust, for Early Jurassictime (a), mid-Cretaceous time (b), and the present (c).SFS - Sandspit fault system; PLFS - Principe Laredo faultsystem. The legend is on this page. No cross-sectionshave been generated for the southern part of the study areadue to a shortage of geologic constraints.IVVVVVV VIv v vvv v vVVVVVVVV V V V V Tertiary sedimentary and igneous rocksQueen Charlotte Gp. & Longarm Fm.Kunga & Maude groupsKarmutsen Fm.••X X X X XX X X X Xplutonic rocksirdro r^)1 metamorphic rocks-194-presently the islands are uplifted and the Hecate Straitarea depressed.The Queen Charlotte Basin can be subdivided into twoprincipal domains based on structure and geologic history.In Hecate Strait, most faults strike north-northwest andnortheast. The faults are spaced closely, and as a result,depressed blocks and Tertiary depocenters are comparativelysmall. Neogene sedimentation took place in a continentalto marginal marine environment; this indicates that ratesof subsidence generally did not exceed those ofsedimentation, despite the active faulting.In Queen Charlotte Sound, in contrast, larger blockspredominate. They are bounded mostly by north-south andeast-west faults. Neogene sedimentation took place inmarine environments, suggesting that subsidence was fasterthan sediment deposition.Regardless of the domain differences, movement ofbasement blocks was the controlling influence on theevolution of the Queen Charlotte Basin. The followingchapter contains a discussion of the hydrocarbon potentialof this region based on the new tectonic interpretation.-195-CHAPTER 9. PETROLEUM POTENTIALOF THE QUEEN CHARLOTTE BASIN AREA9.1. Previous explorationSince the 1950s, many sedimentary basins oncontinental shelves and marginal plains worldwide have beenactively explored because of the oil discoveries in theGulf of Mexico and on the North Slope of Alaska. Theactivity in the study area has been a part of this effort.Petroleum exploration has been undertaken severaltimes in the study area (Fig. 1-2) in the last severaldecades, unfortunately without success (Haimila andProcter, 1982). The main reason for exploration interestwas simply the existence of numerous, readily apparent oilseeps on Queen Charlotte Islands (e.g., Sutherland Brown,1968; Cameron and Tipper, 1981).In the 1960s, several exploration wells were drilledon Queen Charlotte Islands onshore and in Hecate Strait andQueen Charlotte Sound offshore. No discoveries were made,but oil staining of Neogene sediments was encountered inthe Sockeye B-10 well in western Hecate Strait. Thesedrillholes provided, for the first and only time, directinformation on the fill of the Queen Charlotte Basinoffshore, and the results of this exploration program werediscussed by Shouldice (1971, 1973). The maximum totaldepth reached was in excess of 4500 m.^The sedimentaryand volcanic rocks penetrated do not appear to be older-196-than Neogene; the exceptions are the Sockeye E-66 and TyeeN-39 wells, where non-marine Cretaceous sedimentary rocksare recognized (Haggart, 1991).The 1960s drilling program was preceded by theacquisition of multichannel seismic reflection data.Unfortunately, the quality of these data was poor, andsubsequent work showed that, as a result, some of the wellshad been drilled into what later proved to be structuraldepressions (Fig. 3-3).At that time and after, most workers assumed the mainexploration target in the study area offshore was theTertiary sedimentary fill of the Queen Charlotte Basin(Shouldice, 1971; Yorath and Cameron, 1982; Higgs, 1991),although Haimila and Procter (1982) and Yorath and Cameron(1982) also speculated that the underlying Mesozoic rockscould be prospective. The Hecate Strait area was theprincipal focus of exploration offshore (Fig. 1-2).No drilling was undertaken in the study area in the1970s and 1980s due to the imposition of an explorationmoratorium in 1972. This moratorium has been extended andis still in effect. As a consequence, the Queen CharlotteBasin has remained poorly explored. Nevertheless, renewedinterest in the region is expected when the moratorium islifted. Evaluation of economic resources in this area isalso important because of the ongoing Canada-U.S. maritime-boundary dispute in Dixon Entrance (van Zandt, 1976; J.A.S.Fogarassy, unpublished report, 1991).-197-The evaluation of the hydrocarbon potential of theregion based on the lithosphere-stretching model offormation of the Queen Charlotte Basin (Yorath and Hyndman,1983) is discouraging. If this model is correct, pre-Neogene rocks were overheated, and their oil destroyed, inthe Miocene. In addition, exploration has already shownthat the Tertiary rocks offshore are not prospective.The main practical objective of the Geological Surveyof Canada's Frontier Geoscience Program has been to assessanew the hydrocarbon potential of the Queen Charlotte Basinarea. Such an assessment is presented in this chapter, andhas also been summarized by Lyatsky and Haggart (1992). Iconclude that the principal exploration target in the studyarea should be the Mesozoic (Cretaceous) rocks beneathQueen Charlotte Sound.9.2. Source rocks 9.2.1. Upper Triassic and Lower JurassicThree main types of hydrocarbon-producing organicmatter are presently distinguished (Tissot et al., 1974).Type I kerogen produces oil and is commonly associated withmarine black shales. Type III organic matter mostly yieldsgas, and it is generally found in continental deposits.Kerogens of intermediate composition are assigned to TypeII. Various types of kerogen are found in the sedimentaryrocks in the study area (Table 9-I).Paleozoic rocks, being strongly deformed and variously-198-Table 9-I. Type of organic matter and total-organic-carbon (TOC) content of the Upper Triassic-Lower Jurassic,Upper Jurassic-Upper Cretaceous, and Tertiary sedimentaryrocks in the Queen Charlotte Basin area. The UpperTriassic-Lower Jurassic sequence contains the principalsource rocks in the region. Summarized by Lyatsky andHaggart (1992) from Macauley (1983, 1984), Macauley et al.(1985), Bustin and Macauley (1988), Vellutini et al.(1990), and Bustin et al. (1990).SUCCESSION KEROGEN TYPE TOCT II and III up to 2.5% locallyUJ - UK III lowUTr - LJ II and I up to 11% locally-199-metamorphosed (Chapter 2), are not prospective from thepetroleum-exploration standpoint. Neither are thewidespread, thick, massive basalts of the KarmutsenFormation. I therefore concur with Procter et al. (1984)that the Karmutsen volcanics (Table 9-11) form the economicbasement in this region.Of considerable interest are Upper Triassic and LowerJurassic limestone and shale of the Kunga and Maude groups(Table 9-11). These rocks are hundreds of meters thick.They accumulated in a shallow marine environment (Jeletzky,1976; Cameron and Tipper, 1985; Thompson et al., 1991), ina basin which apparently extended across most of the studyarea (Chapter 2).Organic geochemistry of the Upper Triassic and LowerJurassic sedimentary rocks was studied on Queen CharlotteIslands by Macauley (1983, 1984), Macauley et al. (1985),Bustin and Macauley (1988), and Vellutini et al. (1990).They determined that the Kunga and Maude groups arecharacterized by Type I and II organic matter (Table 9-I).Intervals within these source rocks have total organiccarbon (TOC) content up to 11% (Vellutini et al., 1990).Oil shales are also present, although in minor amounts.In contrast, sedimentary rocks of the Middle JurassicYakoun and Moresby groups (Table 2-I) are volcanic-derived(Cameron and Tipper, 1985; Thompson et al., 1991) and arecharacterized by low TOC values. They contribute little tothe petroleum prospects in the study area.GEOLOGICTIME QUEEN CHARLOTTE ISLANDS NORTHERN VANCOUVER ISLANDo0N0>..cC<I::cl)zVolcanic andSedimentary Stratar3.--SCONUNMASSETFM.^FM.•2 ++++++LijZ LiicE Unnamed Volcanic/ ++++++ ++++++o - JTti Sedimentary Strata ++++++Ciia_D• A• +al9co';'c)^a)A fu)cco 00 -as 02i-o LE.'•NANAIMO GROUPAy Ay /,QUEENCHARLOTTE0GROUPUnnamed UpperJurassic-Lower r(.35 Cretaceous StrataN ++++++0U)/ PP / g / 4++++++++++++cp MORESBY GP.++++++-i-++++++++++++YAKOUN GP.4 13 — u)c al a) A4530Iii >7n_ 20)._ u)>.,-- 0_0 -C 10toc071;_J 'E C =c13 ° 0 (LE c -5z as , -77) C'_ _43ac- -,-)cnD<EC1— 8A A•R^• Unnamed Carbonate-w-J<CL,2 — ,..caCr CI ''''Luz‹CL cc ‘...Chert Unit; Volcanics (?)Carbonates andVolcanics-201-Table 9-11. Generalized stratigraphic columns for QueenCharlotte Islands and northern Vancouver Island. Threemajor sedimentary successions - Upper Triassic to LowerJurassic, Upper Jurassic to Upper Cretaceous, and Tertiary- define the petroleum potential of the Queen CharlotteBasin area. After Lyatsky and Haggart (1992). Detaileddescription of the stratigraphy of Queen Charlotte Islandsand northern Vancouver Island is presented in Chapter 2 andsummarized in Tables 2-I and 2-111.-202-9.2.2. Upper Jurassic and CretaceousClastic rocks of the Upper Jurassic to UpperCretaceous sequence are widespread on Queen CharlotteIslands and northern Vancouver Island (Table 9-II; Mulleret al., 1974; Haggart, 1991, 1992a). Vellutini et al.(1990) have shown this succession is characterized by TypeIII organic matter, with a low average TOC. An exceptionis the Haida Formation, which locally contains up to 5.3%of Type III organic matter.9.2.3. TertiarySediments of the Neogene Skonun Formation contain TypeII and III organic matter, with TOC up to 2.5% locally(Bustin et al., 1990; Vellutini et al., 1990). Theirsource-rock potential has been described as only poor tomoderate (Bustin et al., 1990).9.3. Reservoir rocks 9.3.1. MesozoicReservoir potential of the Mesozoic rocks in the studyarea is variable. The Upper Triassic-Lower Jurassicsedimentary succession on Queen Charlotte Islands lacksreservoir lithofacies (Cameron and Tipper, 1981). Thesestrata are unlikely to contain substantial oilaccumulations unless heretofore unrecognized porouslithofacies characterize these rocks in the offshore areas.Rocks with good reservoir properties have been-203-identified within the basal part of the Cretaceoussuccession. On Queen Charlotte Islands, these rockscontain texturally mature quartz and feldspar and lack anargillaceous component. In some areas, they exhibitporosity in excess of 15% (Fogarassy and Barnes, 1989,1991). The porosity is largely secondary, produced bydissolution of carbonate cement.These potential reservoirs are variable in thicknessbut are typically 10 to 50 m thick. On eastern QueenCharlotte Islands, however, both thickness and porosity ofthis facies increase to the southeast, towards QueenCharlotte Sound and southern Hecate Strait (Fogarassy andBarnes, 1989, 1991; J.W. Haggart, pers. comm., 1992). Inaddition, other Cretaceous sedimentary rocks on QueenCharlotte Islands offer minor reservoir potential(Fogarassy and Barnes, 1991).9.3.2. TertiaryThe Neogene Skonun Formation contains numerous,discontinuous sandstone lenses both onshore and offshore;porous sandstone is found in the top 2500 m. However, thereservoir potential of the Neogene rocks is degraded fortwo reasons. First, their permeability is generally lowdue to blockage of fluid conduits by products of feldspardecomposition (Shouldice, 1971). Second, the lateralextent of individual sandstone beds in the Skonun Formationis limited, and they are commonly separated by mudstone.-204-9.4. Oil seeps Bitumen and seeps of liquid oil are found in outcropsof Mesozoic and Cenozoic rocks on Queen Charlotte Islands(Sutherland Brown, 1968; Cameron and Tipper, 1981, 1985).Geochemical analysis of the organic matter from some of theseeps confirms that the principal oil source is the rocksof the Upper Triassic to Lower Jurassic Kunga and Maudegroups, although kerogens from younger rocks have alsocontributed (Hamilton and Cameron, 1989). On northernVancouver Island, pyrobitumen found at one locality wasconsidered by Fleming (1983) to be from a Triassic source.Oil stains of Neogene sediments in the Sockeye B-10 well inHecate Strait were apparently also sourced from the UpperTriassic-Lower Jurassic stratigraphic interval (L.R.Snowdon and M.G. Fowler, in Thompson et al., 1991).9.5. Regional and local seal The low permeability of the Skonun Formation sedimentssuggests they may form a regional seal offshore.Especially impermeable are the sediments found at depthsexceeding 3000 m.Existence of ubiquitous oil seeps and stains indicatesthat a pulse of oil migration occurred recently, and Ispeculate the likely conduits are the numerous Neogenefaults found onshore and offshore. If so, the caprockquality of the Tertiary strata may be degraded.Significantly, the Skonun Formation offshore, where-205-drilled, was not found to be overpressured (Shouldice,1971), which suggests the interval tested is not capped byan impermeable seal.9.6. Thermal maturation of rocks Thermal maturation of rocks in sedimentary basins overlarge areas is commonly accomplished by burial-relatedconductive heating (e.g., Vassoyevich et al., 1969;Gretener, 1981). With normal geothermal gradients, rocksmay enter the oil window at burial depths of as little as 2or 3 km (Tissot et al., 1975; Perrodon, 1983).In addition, heat may be distributed through a basinconvectively, by injection of magmas and circulation ofhydrothermal fluids (Chapter 4). Fluid circulation isespecially common, driven by compactional and diageneticdehydration of sediments or by magmatism, and large-scalefaults often serve as conduits for groundwater flow(Gretener, 1978; Lerche, 1989; Summer and Verosub, 1989;Land, 1991; Bredehoeft et al., 1992). Circulation ofhydrothermal fluids in a basin may have a profound effecton the organic maturity of sediments locally, and theposition of the oil window may be shifted by thousands ofmeters vertically (Person and Garven, 1992).In the Queen Charlotte Basin area, cumulativethickness of rocks above the organic-rich Upper Triassicand Lower Jurassic strata is several kilometers (Table2-I). Oil generation from the source rocks is thus-206-expected offshore, where these rocks are buried beneathseveral kilometers of Cretaceous and Tertiary strata.Convective heating of rocks has also been documentedin the region. On Queen Charlotte Islands, studies ofMesozoic and Cenozoic sedimentary rocks have shown that twomajor, discrete pulses of heating influenced theirevolution. This heating was local, induced by magmatism inthe Jurassic and Tertiary (Tables 2-I and 9-11; Vellutiniand Bustin, 1990; Souther and Jessop, 1991; Orchard andForster, 1991). It placed some rocks into or even abovethe oil window. Other rocks were affected minimally,however, and the oil seeps may reflect local, not regional,oil generation and expulsion (Snowdon et al., 1989).On northern Vancouver Island, Jurassic and Tertiaryplutonism has also been documented (Tables 2-111 and 9-11;Jeletzky, 1976; Muller, 1977; Andrew and Godwin, 1989a-c;Andrew et al., 1991). The occurrence of presumablyTriassic-sourced dead oil (pyrobitumen) in the vicinity ofa Jurassic dike (Fleming, 1983) indicates that igneousactivity influenced thermal maturation of source rocks inthat region as well.Hydrothermal activity was also responsible for heatingthe study area in the Tertiary, and it continues today.Active hot springs are found on Queen Charlotte Islands(Souther, 1976), and field observations suggest suchactivity was extensive earlier, in the Neogene (J.G.Souther, pers. comm., 1991). Migration of hydrothermal-207-fluids may be controlled by faults, as is evidenced onQueen Charlotte Islands by the association of the MioceneCinola epithermal gold deposit with the Sandspit faultsystem (Christie, 1989) and of the active hot springs withthe Louscoone Inlet Fault.Convective heating likely affected the fill of theQueen Charlotte Basin offshore. It has been suggested thata hot spot passed underneath Queen Charlotte Sound in theNeogene; the same hot spot created the northeast-trendingAnahim volcanic belt on the mainland (Fig. 1-1; Bevier etal., 1979; Souther, 1986). The large plutons beneath QueenCharlotte Sound (Chapter 6; Lyatsky, 1991a) lie on trendwith the Anahim Belt, and they may have been generated bythe same hot spot. This suggests a Neogene age of theiremplacement. If this is true, southern Queen CharlotteBasin was heated locally but significantly in the Neogene.Elevated modern heat-flow values have been reportedlocally from the floor of the continental slope west ofQueen Charlotte Sound (Hyndman et al., 1982). They areassociated with volcanism at the Tuzo Wilson Knolls (Chase,1977; Cousens et al., 1985), and thus have little bearingon the thermal history of the Queen Charlotte Basin.9.7. Timing of oil generation and entrapmentBoth migration of oil and formation of structures(potential traps) likely occurred in the study area inseveral pulses. Jurassic magmatism in the western part of-208-the area, on Vancouver and Queen Charlotte islands (Table9-11), likely resulted in local oil generation. MiddleJurassic folding, as well as Late Jurassic high-anglenormal faulting (Table 2-11), created structures favorablefor oil entrapment. However, no reservoirs were yetavailable in the region.In the latest Cretaceous, burial-related oilgeneration may have occurred. By that time, elements ofthe Upper Triassic-Lower Jurassic succession in the westernpart of the study area had been covered by about 2000 m ofyounger sediments. Thus, source rocks may have been at aburial depth sufficient for oil generation. Inversion ofold, high-angle faults, which occurred in Late Cretaceousand Early Tertiary time, likely produced structuressuitable for entrapment of hydrocarbons. The extent of thereservoirs at that time is uncertain, however.During the Tertiary, another pulse of oil generationlikely resulted from burial of source rocks beneath thethick fill of the Queen Charlotte Basin. Mesozoic rockswere buried beneath about 5000 m of Cenozoic deposits,which would allow them to produce oil or thermogenic gas.In addition, plutonism onshore and offshore during Tertiarytime also heated some rocks sufficiently to allow them togenerate oil. On the other hand, plutonism may have ruinedthe source potential of Mesozoic rocks in some areas.Studies onshore (Thompson et al., 1991) and offshore(Lyatsky, 1991a-b; Rohr and Dietrich, 1992) show that-209-Neogene time was characterized by extensive faulting. Ifoil migration postdates these structures, and if alarge-scale reservoir already existed, then conditions werefavorable for hydrocarbon accumulation at that time.However, the quality of the Tertiary caprock may be reduceddue to faulting, allowing any accumulated oil to escape.9.8. Petroleum prospects in the study area 9.8.1. General statementIn this section, the information presented above iscombined with the knowledge of distribution of the majorMesozoic and Cenozoic rock units onshore and offshore(Chapter 2). This has allowed me to assess petroleumprospects in the region, prioritize areas of explorationinterest, and suggest possible exploration targets.9.8.2. Hecate StraitBeneath Hecate Strait, the Tertiary Queen CharlotteBasin fill seems to be underlain by partly eroded UpperTriassic and Lower Jurassic rocks, whereas Cretaceousstrata are probably absent due to non-deposition. Tertiarymagmatism was probably less extensive here than in QueenCharlotte Sound. Oil staining of Neogene sediments in theSockeye B-10 well indicates that some of the Upper Triassicand Lower Jurassic source rocks have produced oil;hydrocarbon generation from Tertiary kerogen is alsolikely. Numerous juvenile faults disrupt the Neogene rocks-210-and probably the underlying older rocks as well. Localanticlines may be hydrocarbon traps, provided theirformation predates the migration of oil.On the other hand, the Neogene faulting may havedegraded the caprock quality of the Skonun Formation. Moreproblematic, there is no reason to suspect that the stratabeneath Hecate Strait contain a regional-scale reservoir.I conclude, therefore, that petroleum prospects areprobably low in this area.9.8.3. Queen Charlotte SoundI infer that western Queen Charlotte Sound may befavorable for petroleum exploration (Plate I in pocket).This area is underlain by a thicker sedimentary sequence ofUpper Triassic-Lower Jurassic and Cretaceous rocks than isHecate Strait (Chapter 2). Thus, petroleum source rocksand potential reservoirs are probably more abundant.All three major stratigraphic packages seem to besuperimposed beneath Queen Charlotte Sound. The UpperTriassic-Lower Jurassic, Cretaceous, and Tertiarysedimentary successions appear to form a source-reservoir-seal sequence favorable for regional oil generation andaccumulation. Additional reservoirs are found at the upperTertiary levels, but they are not overlain by caprock, anddrilling into these strata has been unsuccessful.The reduced thickness of source rocks on northernVancouver Island casts doubt on the prospectivity of-211-southwestern Queen Charlotte Sound. On the other hand,migration may have distributed oil across the area.Both conductive and convective heating affected thisarea, and oil generation probably occurred several times.Even if the plutons beneath eastern Queen Charlotte Soundwere not emplaced in the Neogene, the timing of plutonismacross the region suggests they are likely post-EarlyJurassic. In this case, they would have still been able tooverheat the Upper Triassic-Lower Jurassic source rocks.If the plutons are Neogene, then parts of the Tertiarysuccession may have also been affected. The regional scaleof organic maturation of source rocks and oil generation isthus uncertain for some of the thermal events.The post-Middle Jurassic structural evolution of theQueen Charlotte Basin area was controlled largely by anetwork of high-angle faults. Structural inversionoccurred repeatedly, and a fault-block tectonic regimepredominated across the study area. Hydrocarbon traps insuch a setting are expected to be related mostly to drapeand stamp structures, as well as roll-over anticlines.It is not known whether reservoirs are sufficientlywell developed beneath Queen Charlotte Sound, but theCretaceous sequence, which contains such facies on QueenCharlotte Islands, is a possible exploration target. SomeCenozoic strata may also be oil-bearing.Adverse factors include the possibility that theNeogene structures postdate oil migration and that some of-212-the older oil accumulations were destroyed by Neogenefaulting and magmatism. Igneous-related heating may haveruined the source potential of the Upper Triassic and LowerJurassic rocks in some areas.9.9. Thermal aspects of geodynamical evolution of the Cenozoic Queen Charlotte Basin The petroleum potential of a region is a function ofits geologic evolution. Thus, it is important that theassessment of petroleum prospects of the study area bebased on a realistic model of its geologic history.If the rift model of the evolution of the QueenCharlotte Basin (Yorath and Hyndman, 1983; Hyndman andHamilton, 1991) were correct, Mesozoic rocks beneath thebasin would likely be overmature due to conductive heatingin the Tertiary. However, this model is invalidated by newinformation about the structure of the region. First,Thompson et al. (1991) noted a fundamental discrepancybetween the structure of northern and central QueenCharlotte Islands observed in the field, and the structurepresumed in the rift model (Yorath and Chase, 1981).Second, I have concluded above that none of the regional,orthogonal fault networks onshore and offshore wereproduced by Neogene rifting (Chapters 6, 7, and 8).Instead, the fault-block mosaic was largely inherited frompre-Tertiary time, and the old faults were rejuvenated inthe Cenozoic. However, the rift model of formation of the-213-Queen Charlotte Basin is based largely on thermal, notstructural, considerations, and these are discussed below.The Neogene thermal history of the basin wasinterpreted by Yorath and Hyndman (1983) in terms of purelyconductive (burial-related), regional heating presumablycaused by stretching of continental lithosphere (riftingsensu McKenzie, 1978), with resultant basin development.The application of the McKenzie (1978) model to the QueenCharlotte Basin was justified by reference to the highvitrinite-reflectance values obtained from cuttings ofNeogene sediments in the offshore wells, especially thosein the southern part of the basin. Although a largescatter characterizes these data, an elevated Mioceneregional heat flow (in excess of 120 mW/m 2, more thandouble the present-day values) was inferred based ontime-temperature-index (TTI) calculations. This method, asused by Yorath and Hyndman (1983), relates the vitrinitereflectance of sediments to the integral of their thermalhistory over time. The abnormally high Miocene heat flowwas interpreted to have been conductive and regional,resulting from basin-forming continental rifting in QueenCharlotte Sound and southern Hecate Strait. Extension wasestimated to have been large, by a factor of about 3.3.I argue that these vitrinite-reflectance data shouldnot be used for basin-formation modeling without strongreservations. Caution is advisable for four reasons.First, rapid bit penetration during drilling, as well-214-as caving of poorly consolidated Neogene sediments, hascaused considerable mixing of the borehole cuttings(Hopkins, 1981). Extensive sidewall caving in the offshorewells is also apparent from well caliper logs, which wereexamined by the present writer: sudden and irregularincreases in hole diameter reflect caving.Second, the sediments in contact with the numerousvolcanic flows and sills seem to be thermally altered.These sediments have uncommonly high vitrinite-reflectancevalues; mixing would have caused such cuttings tocontaminate the data for the underlying strata. In fact,caving is common above and below the igneous sheets.Third, recycled vitrinite is found in the basin fill,at least locally (White, 1991). It appears that SkonunFormation sediments were derived in part from areascontaining rocks whose erosion contributed thermally matureclasts to the fill of the Queen Charlotte Basin.Fourth, inferences about the thermal history of rocksbased on vitrinite-reflectance data are sometimesunreliable. They require area-specific calibration ofparameters such as kerogen type, especially where the TTImethod is used (Waples et al., 1992a-b). Thus, reliance onvitrinite-reflectance data and TTI calculations is notalways justified in frontier basins.These four observations suggest the estimate ofMiocene regional heat flow presented by Yorath and Hyndman(1983) is perhaps exaggerated. Therein may lie one reason-215-for these workers' high estimate of tectonic extension inthe study area.Furthermore, distribution of heat through the QueenCharlotte Basin in the Tertiary was both conductive andconvective. If the basin fill was indeed overheated in theTertiary, this may have in part been a result of localized,convective effects related to magmatism. However, the riftmodel of basin formation does neglects to consider theTertiary magmatism and hydrothermal circulation in thestudy area. This would introduce additional error into thecalculations, also leading to excessive estimates ofCenozoic extension.The above discussion illustrates a shortcoming ofgeodynamical models of basin formation which assume purelyconductive heat transfer. Although the thermal and burialhistory of a sedimentary basin are usually interrelated ina complex manner (Lerche, 1989), many models of basinformation nonetheless simply relate the inferred heating ofa basin to the amount of the assumed-causative tectonicstretching of the continental lithosphere (Chapter 4).Such models, which ignore the convective component of basinheating, produce exaggerated estimates of tectonicextension. In fact, numerous workers, including Royden etal. (1980), Summer and Verosub (1989), and Person andGarven (1992), have pointed out that in those basins wherefluid migration and magmatism have been extensive, many ofthe stretching models cannot be applied realistically.-216-CHAPTER 10. SUMMARY AND CONCLUSIONSA large volume of new geological and geophysical datahas been collected in the study area in the last severalyears. In the present dissertation and relatedpublications (Lyatsky, 1991a-b; Lyatsky et al., 1992;Lyatsky and Haggart, 1992), I have combined different datasets in an integrated geological analysis of the TertiaryQueen Charlotte sedimentary basin.As a result, a new concept of Queen Charlotte Basinstructure, evolution, and hydrocarbon potential hasemerged, which differs in many respects from the previousmodels. However, the concept of basin evolution presentedhere is consistent with all available information. Itreflects the present state of knowledge of the geology ofthe basin rim (Chapter 2), as elucidated recently throughfield mapping under the Geological Survey of Canada's QueenCharlotte Basin Frontier Geoscience Program.In the present study, magnetic and gravity data wereprocessed to highlight both regional and local anomalies(Chapter 5). A new technique of horizontal-gradientcalculation and display was utilized, along with a numberof more conventional data-processing techniques.Patterns of linear anomalies in the potential-fieldmaps were compared with regional physiographic trends andfault networks onshore. This permitted me to delineateorthogonal fault networks in the study area. The-217-similarity of Tertiary fault patterns offshore with pre-Tertiary patterns in land areas suggests the structure ofthe Queen Charlotte Basin was largely inherited fromMesozoic or possibly older time. Tectonic extension in theCenozoic, estimated to have been 10% at maximum, was notsufficient to obliterate the old fault networks and imposea juvenile structural overprint (Chapters 6, 7, and 8).Examination of magnetic and gravity data as well asseismic profiles has also allowed me to map severalregional fault systems across the study area, both withinand outside the Queen Charlotte Basin. The Sandspit faultsystem continues across Graham Island and Dixon Entrance,but it does not disrupt Tertiary rocks on northern GrahamIsland. The Kitkatla and Principe Laredo/Banks Islandfault systems intersect near Porcher Island and continuenorthwest across Hecate Strait and Dixon Entrance.This configuration, coupled with constraints ondisplacement history provided by field mapping onshore,precludes Neogene strike-slip displacements across faultsin Hecate Strait. Only a small amount of Tertiary tectonicextension is possible in the Queen Charlotte Basin becausenone of the major faults in the region accomodate the largelateral dislocations of crustal blocks that significantbasin extension would require (Chapter 7).Conventional quantitative models of basin evolutionhave been summarized, and their applicability to the QueenCharlotte Basin evaluated (Chapter 4). I have shown that-218-the model involving rifting and extension beyond a few percent cannot be applied realistically to the study areabecause the regional structure such a model predicts is notconsistent with the available geological and geophysicalconstraints.Structural evolution of the study area since at leastthe Late Jurassic was controlled by repeated movements ofcrustal blocks separated by high-angle faults. Such astructural style was described in land areas on the rim ofthe Queen Charlotte Basin by Jeletzky (1976), Muller(1977), and more recently by Thompson et al. (1991).High-angle faults have also been imaged seismically in thebasin offshore, where similar fault networks are found(Chapters 3, 6, 7, and 8).Combining the geological and potential-fieldgeophysical data helped me identify large, previouslyunrecognized, plutons beneath Queen Charlotte Sound.Regional geologic correlations indicate these plutons arepost-Early Jurassic, likely Neogene, in age. They may be aproduct of the passage of the Anahim hot spot beneath thestudy area in the Miocene.Because of a high level of igneous and hydrothermalactivity, Cenozoic heating of the Queen Charlotte Basin wasboth conductive and convective. The convective componentof basin heating resulted from magmatism and circulation ofhydrothermal fluids. This further reduces the relevance ofgeodynamical models of basin formation which assume the-219-heating to be purely conductive (Chapter 9). Of course,conductive heating of the lower levels of the Tertiarybasin fill has also occurred, caused by burial of the rocksbeneath younger deposits.Based on the available data, I was able to reassessthe hydrocarbon potential of the region. The inferreddistribution of various sedimentary formations offshoresuggests the Queen Charlotte Sound area is underlain by(1) the Upper Triassic and Lower Jurassic carbonate andshale sequence which contains oil-prone source rocks;(2) the Cretaceous clastic sequence containing poroussandstone which may be a reservoir of regionalsignificance; and(3) the Tertiary rocks which are a regional seal but mayalso be a secondary oil source and reservoir locally.I consider Queen Charlotte Sound to be the area ofprime petroleum-exploration interest, and the Cretaceousstratigraphic interval is the primary target. Drape andstamp structures are the most likely traps.On the other hand, igneous heating may have partlydestroyed the source potential of the Upper Triassic andLower Jurassic rocks, and oil generation may have been onlylocal. Besides, presence of a reliable regional reservoirin the Cretaceous interval is still unconfirmed. Anadditional uncertainty is the timing of oil migration,which may predate the extensive faulting and structureformation in the Neogene. This faulting has apparently-220-reduced the caprock quality of the Tertiary strata in someareas offshore, and it may have disrupted older oil , and gaspools. Nevertheless, in spite of these possible drawbacks,western Queen Charlotte Sound is likely prospective for oilexploration.The new model put forward for the structure andevolution of the Queen Charlotte Basin area is based on thenew data collected recently by numerous scientists from theGeological Survey of Canada, the University of BritishColumbia, the University of Victoria, and otherinstitutions. The results of my study are consistent withthe new knowledge of the geology of the British Columbiamainland and Queen Charlotte Islands, as well as therecently collected geophysical data. 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