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The effect of deformational mechanisms on the permeability of Upper Paleozoic limestone, dolostone and… Hammack, Janet L. 1989

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THE EFFECT OF DEFORMATIONAL MECHANISMS ON THE PERMEABILITY OF UPPER PALEOZOIC LIMESTONE, DOLOSTONE AND SANDSTONE NEAR OVERFOLD MOUNTAIN, 55 KILOMETRES SOUTHEAST OF FERNIE, BRITISH COLUMBIA by JANET L. HAMMACK B.Sc, UNIVERSITY OF BRITISH COLUMBIA, 1985 BA., UNIVERSITY OF COLORADO, 1978 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES GEOLOGICAL SCIENCES We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA JUNE 1989 © JANET L. HAMMACK B.Sc, BA., 1989 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) ABSTRACT O v e r f o l d Mountain i s l o c a t e d approximately 55 km southeast o f F e r n i e B r i t i s h Columbia i n the MacDonald Range of the Rocky Mountains. In the a r e a , l i m e s t o n e s , dolostones and sandstones o f the Upper C a r b o n i f e r o u s Rundle Group and Rocky Mountain Group have been thrown i n t o a s e r i e s of n o r t h w e s t e r l y t r e n d i n g f o l d s on the hanging w a l l of the Lewis T h r u s t s h e e t . T h i s study f o c u s e s on the d e f o r m a t i o n a l mechanisms which have l e d t o the development of one of these megascopic s t r u c t u r e s , w i t h an emphasis on the r o l e of p e r m e a b i l i t y b e f o r e , d u r i n g and a f t e r d e f o r m a t i o n . D e f o r m a t i o n a l mechanisms which have been a c t i v e near O v e r f o l d Mountain i n c l u d e s o l u t i o n p r o c e s s e s (pressure s o l u t i o n and h y d r a u l i c f r a c t u r i n g ) , shear f r a c t u r i n g , and i n t r a g r a n u l a r mechanisms (mechanical t w i n n i n g , d i s l o c a t i o n g l i d e , and m i c r o f r a c t u r i n g ) . How s t r a i n i s p a r t i t i o n e d between t h e s e mechanisms i s l a r g e l y governed by the p e r m e a b i l i t y o f the u n i t . P e r m e a b i l i t y i s of primary importance i n the d e t e r m i n a t i o n of how a rock w i l l respond i n a n o n h y d r o s t a t i c s t r e s s f i e l d a t low temperatures (<0.5 Tm). In the study a r e a , carbonate rocks w i t h a h i g h i n i t i a l p e r m e a b i l i t y have accommodated s t r a i n by p r e s s u r e s o l u t i o n . Carbonates and sandstones of low i n i t i a l p e r m e a b i l i t y have accommodated s t r a i n by shear f r a c t u r i n g and i n t r a g r a n u l a r mechanisms. i i Finite permeability in the carbonates and sandstones of the study area, has been altered as a result of these deformational mechanisms. Units which had a high permeability prior to deformation have had their permeability blocked by pressure solution. Units which had a low permeability prior to deformation, have developed microfractures which have increased the finite permeability. This latter phenomenon is well illustrated in the dolostone units studied, both of which have a very well developed fracture porosity. i i i TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS iv LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS xi 1.0. INTRODUCTION 1 1.1. LOCATION AND ACCESS 2 1.2. PREVIOUS STUDIES 4 2.0. GEOLOGIC SETTING 6 2.1. STRATIGRAPHIC FRAMEWORK 6 2.1.1. LOCAL STRATIGRAPHY 9 2.2. STRUCTURAL FRAMEWORK 13 2.2.1. LOCAL STRUCTURAL GEOLOGY 14 3.0. DEFORMATIONAL MECHANISMS - A BACKGROUND 16 3.1. INTRAGRANULAR PROCESSES 17 3.1.1. CALCITE 20 3.1.2. DOLOMITE 24 3.1.3. QUARTZ 28 3.2. INTERGRANULAR PROCESSES 2 9 3.2.1. FRACTURES 29 3.2.1.1. HYDRAULIC FRACTURES 30 3.2.2. PRESSURE SOLUTION 38 iv 4.0. METHOD OF DATA COLLECTION AND ANALYSIS 57 4.1. EXTENSION FRACTURES 57 4.2. SHEAR FRACTURES AND FAULTS 59 4.3. SEMI-BRITTLE SHEAR ZONES.. 61 4.4. STYLOLITE SEAMS 61 4.5. DATA COLLECTED - MICROSCOPIC SCALE 65 4.5.1. EXTENSION FRACTURES AND STYLOLITES 65 4.5.2. TWIN LAMELLAE IN CALCITE 65 4.5.3. DEFORMATION LAMELLAE IN QUARTZ 66 5.0. UNIT DESCRIPTION AND OBSERVATION 67 5.1. LITHOLOGIC UNITS 67 6.0. OBSERVATIONS ON THE MEGASCOPIC SCALE - HINGE STYLE..79 6.1. HINGE STYLE CONTROLS 80 6.2. HINGE STYLE OBSERVATIONS 81 7.0. STRUCTURAL ANALYSIS, STRAIN PARTITIONING AND PERMEABILITY HISTORY 89 7.1. UNIT 1 - ROCKY MOUNTAIN GROUP 89 7.1.1. EXTENSION FRACTURES 89 7.1.2. SHEAR FRACTURES 9 6 7.1.3. SEMI-BRITTLE SHEAR ZONES 98 7.1.4. PRESSURE SOLUTION 98 7.1.5. DEFORMATION LAMELLAE IN QUARTZ 103 7.1.6. TWINNING IN CALCITE 103 7.1.7. SUMMARY - UNIT 1 103 7.2. UNITS 2 THROUGH 6 - UPPER CARNARVON MEMBER 106 7.2.1. EXTENSION FRACTURES 108 7.2.2. SHEAR FRACTURES AND FAULTS 117 v 7.2.3. SEMI-BRITTLE SHEAR ZONES 120 7.2.4. PRESSURE SOLUTION 122 7.2.5. TWINNING IN CALCITE AND DOLOMITE 134 7.3. UNIT 7 - MARSTON MEMBER 138 7.3.1. PRESSURE SOLUTION 139 7.3.2. EXTENSION FRACTURES 141 7.3.3. TWINNING IN CALCITE 146 7.4. UNIT 8 - LOOMIS MEMBER 148 7.4.1. EXTENSION FRACTURES 148 7.4.2. SHEAR FRACTURES 151 7.4.3. SEMI-BRITTLE SHEAR ZONES 154 7.4.4. PRESSURE SOLUTION 154 7.4.5. CALCITE TWIN LAMELLAE 158 8.0. CONCLUSIONS 160 vi LIST OF TABLES Page Table 1 - Densities of mesoscopic and microscopic structures within Unit 1 104 v i i LIST OF FIGURES Page Figure 1 - Location Map 3 2 - Structural provinces of the Rocky Mountains....? 3 - Photograph of study area showing division of Mount Head Formation Members 10 4 - Stratigraphic column of the Upper Carboniferous rock units studied 11 5 - Stereonet plot showing bedding orientations in the anticline and syncline 15 6 - Photomicrograph showing twin lamellae in biosparite 19 7 - Crystal structure of twinned calcite 21 8 - Crystal structure of calcite 21 9 - Photomicrograph showing the low density of twin lamellae in packstone 23 10 - Crystal structure of dolomite 25 11 - Photomicrograph showing the rhombic shape of the dolomite crystals 27 12 - Mohr circle model of hydraulic fracturing 32 13 - Mode of development of syntaxial fractures....36 14 - Mode of development of antitaxial fractures... 37 15 - Photomicrograph showing the straight to concavo-convex contacts between guartz grains 4 0 16 - Morphology of columnar and ridge stylolites...43 17 - Stress control on fluid movement along stylolites seams 43 18 - Continuum between columnar stylolites and stylolites with lineations 44 19 - SEM micrograph showing lineations on columnar stylolites 4 6 2 0 - SEM micrograph showing the alignment of kaolinite flakes along stylolite columns...47 21 - Photomicrograph showing the concentration of dolomite rhombs within a stylolite seam in a slightly dolomitic packstone 48 2 2 - Photomicrograph showing the concentration of quartz grains within a stylolite in sandy dolostone 49 2 3 - SEM micrograph showing the alignment of i l l i t e flakes in a stylolite seam in sandy dolostone 50 24 - Crystal structure of clay minerals 53 2 5 - Three orthogonal axes described by a folded surface 58 26 - Kinematic axes described by a slickensided shear fracture 60 v i i i Page Figure 27 - Kinematic axes described by a semi-brittle shear zone 62 28 - Kinematic axes described by columnar and ridge stylolites 63 29 - Photomicrograph of Unit 1 68 3 0 - Photomicrograph of Unit 2 69 31 - Photomicrograph of Unit 3 71 32 - Photomicrograph of Unit 4 72 3 3 - Photomicrograph of Unit 5 73 3 4 - Photomicrograph of Unit 6 74 35 - Photomicrograph of Unit 7 76 3 6 - Photomicrograph of Unit 8 78 3 7 - Photograph showing the geometry of the syncline hinge in Unit 1 82 38 - Photograph showing the geometry of the anticline hinge in Units 2 and 3 83 39 - Photograph showing the geometry of the anticline hinge in Units 4 and 5 85 40 - Photograph showing the geometry of the anticline hinge in Unit 7... 87 41 - Photograph showing the geometry of the anticline hinge in Unit 8 88 42 - Photomicrograph showing the two stages of cementation in Unit 1 91 43 - Stereonet plots of the mesoscopic and microscopic extension fractures in Unit 1 92 44 - Stereonet plots of shear fractures within Unit 1 in the syncline 97 45 - Kinematic analysis of semi-brittle shear zones in Unit 1 99 4 6 - Stereonet plots of poles to flattened contacts between quartz grains in Unit 1 100 47 - Stereonet plots and kinematic analysis of stylolites within the Etherington Formation, in the limbs of the syncline 102 48 - Orientations of fractures and stylolites formed during compaction and early stages of folding 110 49 - Extension fracture densities and bulk dilation for Units 3 and 5 112 50 - Extension fracture densities and bulk dilation for Units 2, 4, and 6 112 51 - Photomicrograph showing the fluorescence within microfractures in thin sections impregnated with fluorescent dye for Unit 3..114 52 - Photomicrograph showing the fluorescence within microfractures in thin sections impregnated with fluorescent dye for Unit 5..115 ix Page Figure 53 - Photomicrograph showing the fluorescence within microfractures in thin sections impregnated with fluorescent dye for Unit 4..116 54 - Shear geometries formed during folding of the anticline 118 55 - Normal fault orientations formed after folding of the anticline 119 56 - Semi-brittle shear zone geometries found in the anticline 121 57 - Rotation of a bedding parallel stylolite relative to the principal stresses from nucleation through fold closure 125 58 - Poles to stylolites within Units 2 through 6, contoured for stylolite suture amplitude 12 6 59 - Kinematic analysis of stylolite seams within Units 2 through 6 128 60 - Stylolite densities and suture amplitudes for Units 2 through 6 129 61 - Densities of microfractures and two phases of stylolites in Units 3 and 5 131 62 - Densities of microfractures and two phases of stylolites in Units 2, 4 and 6 131 63 - Variations in the densities of calcite twin lamellae in Units 2, 4 and 6 135 64 - Variations in the densities of dolomite twin lamellae in Units 3 and 5 13 6 65 - Average linear densities of microfractures and veins in Unit 7 142 66 - Method of calculation of bulk dilation 144 67 - Bulk dilation from veins and stylolites within Unit 7 145 68 - Average linear densities of calcite twin lamellae within Unit 7 147 69 - Photomicrograph showing fluorescence along grain boundaries and calcite twin lamellae in Unit 8 from thin sections impregnated fluorescent dye 150 70 - Bulk dilation from veins and stylolites within Unit 8 152 71 - Stereonet plots and kinematic analysis of shear fractures within Unit 8 153 72 - Stereonet plots and kinematic analysis of stylolites within Unit 8 155 73 - Densities of three sets of stylolites within Unit 8 157 74 - Average linear densities of calcite twin lamellae and microfractures within Unit 8....159 75 - Map of geology and sample locations in the area near Overfold Mountain 171 x ACKNOWLEDGEMENTS I would l i k e t o thank Dr. J.V. Ross and Dr. W.C. Barnes f o r t h e i r p a t i e n c e and support throughout t h i s s t u d y . S p e c i a l thanks i s due t o Dr. Barnes f o r h i s s u g g e s t i o n t h a t I pursue a graduate degree. Funding f o r t h i s t h e s i s was pr o v i d e d by a grant from S h e l l Canada. I am g r a t e f u l t o the S h e l l g e o l o g i s t s f o r t h e i r i n t e r e s t and s u g g e s t i o n s . In p a r t i c u l a r , thanks i s due t o C h a r l i e Bruce, John R u e l l e , Bob McMechan, and Pete Gordy. I would a l s o l i k e t o thank t e c h n i c i a n Yvonne Douma f o r her speed and e x p e r t i s e i n the p r e p a r a t i o n o f the numerous t h i n - s e c t i o n s r e q u i r e d f o r t h i s s t udy. I am g r a t e f u l t o t e c h n i c i a n s Bryon C r a n s t o n , Gord Hodge, and Marc Baker who have a l l gone out of t h e i r way t o h e l p d u r i n g my many hours o f need. Very s p e c i a l thanks i s due t o my f i e l d a s s i s t a n t , L o u i s e Maddison, who d e l i n e a t e d the s t r a t i g r a p h y o f the study area as p a r t o f her Honours t h e s i s , and whose a b l e a s s i s t a n c e i n the f i e l d was i n v a l u a b l e t o the completion o f t h i s s t u d y . I would a l s o l i k e t o acknowledge the geology women's hockey team, Steve S i b b i c k , Myra Keep, and M i c h e l l e Lamberson, each of whom have c o n t r i b u t e d t o the r e t e n t i o n o f my s a n i t y . x i 1 1. INTRODUCTION The Canadian Rocky Mountains represent the easternmost p a r t of the Canadian C o r d i l l e r a . T h i s mountain b e l t has a s t r u c t u r a l s t y l e which i s unique w i t h i n the Canadian C o r d i l l e r a , c o n s i s t i n g of l a r g e , i m b r i c a t e t h r u s t f a u l t s and a s s o c i a t e d f o l d s which have shortened the rocks of the b e l t by more than 200 km. T h i s study focusses on the d e f o r m a t i o n a l mechanisms which have l e d t o the development of one of these megascopic s t r u c t u r e s . S e v e r a l i n t i m a t e l y r e l a t e d f a c t o r s have c o n t r i b u t e d to the s t y l e o f deformation found i n the Canadian Rocky Mountains. Of foremost importance, however, i s the a n i s o t r o p i c nature of the sedimentary package. Throughout the r e g i o n the s t r a t i g r a p h i c sequence commonly i n c l u d e s i n t e r b e d d e d l i m e s t o n e s , d o l o s t o n e s , s h a l e s and sandstones, each of which behaves v e r y d i f f e r e n t l y i n a d e v i a t o r i c s t r e s s f i e l d . The v i s c o s i t y c o n t r a s t and abrupt c o n t a c t s between each of these l i t h o l o g i e s has f a c i l i t a t e d the development of f l e x u r a l - s l i p f o l d s and bedding p a r a l l e l s h e a r . W i t h i n the study a r e a , the d e f o r m a t i o n a l mechanisms of primary importance are s o l u t i o n processes ( h y d r a u l i c f r a c t u r i n g and p r e s s u r e s o l u t i o n ) , shear f r a c t u r i n g , and i n t r a g r a n u l a r mechanisms (mechanical twinning and d i s l o c a t i o n g l i d e ) . Some of these mechanisms have i n c r e a s e d the p e r m e a b i l i t y of the rock d u r i n g deformation, enhancing 2 f l u i d flow and p o s s i b l y hydrocarbon m i g r a t i o n . Remnant s t r u c t u r e s c r e a t e d by these mechanisms, i n p a r t i c u l a r h y d r a u l i c f r a c t u r e s and s t y l o l i t e seams, a c t as kinematic i n d i c a t o r s which d e p i c t not o n l y a s t r a i n h i s t o r y of the rock, but a l s o a h i s t o r y of the p e r m e a b i l i t y over the deformation i n t e r v a l . An e f f o r t i s made i n t h i s study to u n r a v e l the d e f o r m a t i o n a l and p e r m e a b i l i t y h i s t o r i e s of d i s t i n c t l i t h o l o g i e s w i t h i n a megascopic f o l d p a i r i n s o u t h e a s t e r n B r i t i s h Columbia. T h i s has been c a r r i e d out by the d e t a i l e d a n a l y s i s of the s i z e , d e n s i t y , and c r o s s - c u t t i n g r e l a t i o n s h i p s of the remnant s t r u c t u r e s on both the m i c r o s c o p i c and mesoscopic s c a l e s . 1 . 1 . L O C A T I O N A M D A C C E S S O v e r f o l d Mountain i s l o c a t e d between Lodgepole Creek and Bighorn Creek, approximately 55 km southeast of F e r n i e B r i t i s h Columbia i n the MacDonald Range of the Canadian Rocky Mountains ( L a t . 4 9° 137 N, Long. 1 1 5° 47' W; F i g u r e 1 ) . The study area i s a c c e s s i b l e by two-wheel-drive v e h i c l e v i a l o g g i n g roads t o t h e head of a w e l l maintained horse t r a i l which may be f o l l o w e d f o r the remaining 5 km t o the base camp. F i e l d work was conducted between J u l y 2 and August 30, 1986. Figure 1 - Location map for study area. 4 1.2. PREVIOUS STDDIES G e o l o g i c o b s e r v a t i o n s of the Canadian Rocky Mountains were f i r s t p u b l i s h e d by James Hector (1863), a member of Captain P a l l i s e r ' s e x p e d i t i o n . Soon a f t e r , p i o n e e r i n g work by Dawson (1875, 1886), Dawson and McConnell (1885), W i l l i s (1902) and Daly (1912) began t o r e v e a l the f o l d and t h r u s t nature o f the b e l t as w e l l as a b a s i c s t r a t i g r a p h i c sequence. P i o n e e r i n g s p i r i t gave way t o monetary a s p i r a t i o n s with the d i s c o v e r y o f gas and o i l i n the Turner V a l l e y i n 1913. At t h i s t i m e , the se a r c h f o r s t r u c t u r a l t r a p s and p r o d u c t i v e beds began, b r i n g i n g a deluge of petroleum g e o l o g i s t s i n t o the a r e a , each adding t o the wealth o f knowledge of both the s t r u c t u r e and the s t r a t i g r a p h y of the f o l d and t h r u s t b e l t . S e v e r a l major r e g i o n a l s t u d i e s have c e n t r e d on the Rocky Mountains o f southeastern B r i t i s h Columbia. B a l l y and others (1966) p r e s e n t e d a landmark study which pr o v i d e d an understanding o f t h e extent and nature o f the f a u l t systems t h e r e . T h i s study a l s o showed the important v a r i a t i o n s i n s t r u c t u r a l s t y l e , from the Rocky Mountain Trench t o the west, through t o the Front Ranges. Important i n s i g h t i n t o the t e c t o n i c s i g n i f i c a n c e of the s t r u c t u r e s o f t h e f o l d and t h r u s t b e l t has been g i v e n by P r i c e (1959, 1962, 1965) who has mapped the Rocky Mountains of s o u t h e a s t e r n B r i t i s h Columbia i n s i g n i f i c a n t d e t a i l . From t h i s mapping, i n f e r e n c e s have been made as t o 5 the s t r u c t u r a l c o n t r o l s on the deformation seen t h e r e ( P r i c e , 1973; P r i c e and Mountjoy, 1970). Only a minor e f f o r t has been made t o r e l a t e mesoscopic and m i c r o s c o p i c s t r u c t u r e s t o the megascopic s t r u c t u r e s i n the Rocky Mountains. In a notable e x c e p t i o n , P r i c e (1967) found a good c o r r e l a t i o n between the mesoscopic and megascopic f a b r i c s i n the southern Rocky Mountains, an area which i n c l u d e s O v e r f o l d Mountain. There have, however, been many important s t u d i e s of mesoscopic and m i c r o s c o p i c s t r u c t u r e s i n o t h e r , s i m i l a r , s t r u c t u r a l p r o v i n c e s . Some re s e a r c h e r s have r e l a t e d mesoscopic f r a c t u r e s and s t y l o l i t e s t o megascopic f o l d s (Groshong, 1975) and t o f a u l t s (Friedman, 1969; R i s p o l i , 1981; Hancock, 1985). Others have used these s t r u c t u r e s , as w e l l as c a l c i t e t w i n l a m e l l a e , t o estimate the f i n i t e s t r a i n w i t h i n major s t r u c t u r e s (Friedman and Heard, 1974). A r e l a t i o n s h i p between the development o f these mesoscopic and m i c r o s c o p i c s t r u c t u r e s and f l u i d s w i t h i n the rock d u r i n g deformation has been suggested by P h i l l i p s (1972, 1974), A l v a r e z and others (1976), Beach (1977), K e r r i c h (1978), G e i s e r and Sansone (1981), and E t h e r i d g e and Vernon (1983) as w e l l as o t h e r s . Many of th e s e r e s e a r c h e r s agree t h a t p r e s s u r e s o l u t i o n and h y d r a u l i c f r a c t u r i n g not only r e q u i r e f l u i d s f o r development, but may a l s o enhance the bulk f l u i d flow through the rock . 6 2. GEOLOGIC SETTING The Canadian Rocky Mountain f o l d and t h r u s t b e l t has l o n g been r e c o g n i z e d as a major source of petroleum, n a t u r a l gas and c o a l . Because o f the economic s i g n i f i c a n c e of the a r e a , e x t e n s i v e g e o l o g i c a l and g e o p h y s i c a l data have been accumulated, making t h i s one o f the most s t u d i e d and b e s t understood s t r u c t u r a l p r o v i n c e s of i t s k i n d i n the w o r l d . The keys t o the s t y l e of deformation w i t h i n the f o l d and t h r u s t b e l t l i e f i r s t l y i n the s t r o n g l y a n i s o t r o p i c n ature of the sedimentary l a y e r s i n v o l v e d , and s e c o n d l y i n the c o n t r a s t i n competency between the c r y s t a l l i n e basement and t h i s o v e r l y i n g sedimentary c o v e r . V a r i a t i o n s i n t h i c k n e s s and l i t h o l o g y a c r o s s the b e l t have a l s o had a s u b s t a n t i a l e f f e c t on t h e s t y l e of deformation, a f a c t o r which has prompted the d i v i s i o n of the f o l d and t h r u s t b e l t i n t o f o u r zones: the F o o t h i l l s , F r ont Ranges, Main Ranges and Western Ranges ( F i g u r e 2; B a l l y and o t h e r s , 1966). 2 . 1 . STRATIGRAPHIC FRAMEWORK The sedimentary sequence o f the Rocky Mountain f o l d and t h r u s t b e l t may be d i v i d e d i n t o two d i s t i n c t packages. O l d e s t of these i s a s u c c e s s i o n of d i s t i n c t l y bedded m i o g e o c l i n a l sediments which range i n age from Late Precambrian ( H e l i k i a n ) t o L a t e J u r a s s i c . T h i s package i s o v e r l a i n by a L a t e s t J u r a s s i c t o E a r l y T e r t i a r y c l a s t i c wedge sequence. The m i o g e o c l i n a l sediments onlap the metamorphic and i n t r u s i v e r o c k s o f the e a r l i e r Precambrian Figure 2 - Structural provinces of the southern Canadian Rocky Mountains. After Bally and others (1966). 8 basement, which were c o n s o l i d a t e d d u r i n g the Hudsonian orogeny (1,700 Ma), and r e p r e s e n t a westward e x t e n s i o n of the Canadian S h i e l d . Sedimentary rocks found at O v e r f o l d Mountain are Upper P a l e o z o i c i n age and belong to the widespread m i o g e o c l i n a l s u c c e s s i o n . R e g i o n a l l y , the m i o g e o c l i n a l s u c c e s s i o n i s c h a r a c t e r i z e d by interbedded l i m e s t o n e s , d o l o s t o n e s , sandstones and s h a l e s which were d e p o s i t e d i n t o a shallow, warm, marine environment. The e l a s t i c s i n t h i s s u c c e s s i o n were p r o v i d e d by the emergent c r a t o n i c p l a t f o r m to the e a s t . The t h i c k n e s s of the m i o g e o c l i n a l package v a r i e s from about 2,000 m, e a s t of the Front Ranges t o more than 12,000 m i n the western Rocky Mountains ( P r i c e and Mountjoy, 1970). In t h e Late J u r a s s i c , t e c t o n i c p r o c e s s e s i n the western C o r d i l l e r a caused the development of a foredeep b a s i n i n the area and t h e subsequent d e p o s i t i o n of a c l a s t i c wedge sequence ( B a l l y and o t h e r s , 1966). T h i s sequence c o n s i s t s of marine sediments and non-marine e l a s t i c s which were shed from the newly emergent rocks t o the west. F o s s i l evidence has shown t h a t the e a r l i e s t i n f l u x o f e l a s t i c s i n t o the b a s i n o c c u r r e d i n the L a t e s t J u r a s s i c and continued i n p u l s e s throughout the Cretaceous and Paleocene (Wheeler, 1966). During t h i s t i me, ongoing t e c t o n i c processes caused the m i g r a t i o n of the foredeep b a s i n toward the n o r t h e a s t , where the e l a s t i c s are seen t o i n t e r f i n g e r w i t h , and o v e r l a p , marine sediments. Remnants of the wedge are b e s t p r e s e r v e d i n the Rocky Mountain F o o t h i l l s , where a 9 maximum t h i c k n e s s of 6,500 m i s reached ( P r i c e and Mountjoy, 1970). 2.1.1. L O C A L S T R A T I G R A P H Y Sedimentary rocks exposed i n an o v e r t u r n e d , asymmetric f o l d p a i r found near O v e r f o l d Mountain belong t o the Lower Ca r b o n i f e r o u s Mount Head and E t h e r i n g t o n Formations (Rundle Group) and the o v e r l y i n g Upper C a r b o n i f e r o u s Rocky Mountain Group. In the study a r e a , l i m e s t o n e s , d o l o s t o n e s , and shales of the Mount Head Formation are v e r y w e l l exposed along a n o r t h w e s t e r l y t r e n d i n g r i d g e which f o l l o w s the hinge of the a n t i c l i n e ( F i g u r e 3 ) . Dolostones and arenaceous dolostones of the o v e r l y i n g E t h e r i n g t o n Formation and sandstones and arenaceous dolostones of t h e Rocky Mountain Group are exposed a t the hinge and limbs of the s y n c l i n e . The Mount Head Formation i s widespread al o n g the f o l d and t h r u s t b e l t and has been d i v i d e d i n t o seven members on the b a s i s o f l i t h o l o g i c and f a u n a l o b s e r v a t i o n s (Macqueen and Bamber, 1968). The fauna w i t h i n the Mount Head Formation of the study area have been p l a c e d w i t h i n the zonal assemblage of the uppermost t h r e e members of t h i s f o r m a t i o n , d e p o s i t e d i n the Late V i s e a n (Maddison, 1987; F i g u r e 4 ) . The o l d e s t of these u n i t s i s the Loomis Member which c o n s i s t s of thick-bedded t o massive pelmatozoan and o o l i t i c g r a i n s t o n e s . These rocks are o v e r l a i n by the t h i n to medium-bedded l i m e s t o n e , f i n e l y c r y s t a l l i n e dolostone and shale of the Marston Member, which i n t u r n i s o v e r l a i n by Figure 3 - Photograph illustrating the outcrop pattern of three Members of the Mount Head Formation in the anticline within the study area. The Etherington Formation outcrops in the syncline, along the valley floor. H o 11 ROCKY MOUNTAIN GROUP Calcareous quartz arenite ETHERINGTON FORMATION Interbedded dolostone and sandy dolostone MOUNT HEAD FORMATION CARNARVON MEMBER Interbedded limestone and dolostone MARSTON MEMBER Interbedded limestone, dolostone and shale LOOMIS MEMBER Massive limestone Figure 4 - Simple stratigraphic column of the Upper Carboniferous units studied near Overfold Mountain. Tic marks measure 50 m. 12 t h i c k t o thin-bedded s k e l e t a l and o o l i t i c g r a i n s t o n e and lime mudstone of the Carnarvon Member. The c o n t a c t of the Mount Head Formation with the o v e r l y i n g E t h e r i n g t o n Formation i s a r e g i o n a l d i s c o n f o r m i t y (Douglas, 1958). No obvious s u r f a c e of unconformity was seen i n the study area and the c o n t a c t was taken as the top of the uppermost limestone u n i t . In the study a r e a , the E t h e r i n g t o n Formation i s composed of medium-bedded d o l o s t o n e s , arenaceous d o l o s t o n e s , c a l c a r e o u s quartz a r e n i t e s and s h a l e s . Brachiopods are common i n some u n i t s , and the arenaceous u n i t s are l o c a l l y d i s t i n c t i v e l y c r o s s -l a minated. T h i s f o r m a t i o n i s exposed on the overturned limb of the f o l d s where i t reaches a t h i c k n e s s of approximately 60 m. The c o n t a c t of the E t h e r i n g t o n Formation w i t h the o v e r l y i n g Rocky Mountain Group i s a l s o b e l i e v e d t o be a r e g i o n a l d i s c o n f o r m i t y (Douglas, 1958). S i n c e no unconformable s u r f a c e was observed i n the study a r e a , the c o n t a c t was taken as the uppermost d o l o m i t i c u n i t . The Rocky Mountain Group c o n s i s t s of very c l e a n l i g h t - g r e y massive q u a r t z a r e n i t e with c a l c a r e o u s cement. T h i s u n i t i s w e l l exposed a t t h e hinge and limbs of the s y n c l i n e and has a minimum t h i c k n e s s of 100 m. 13 2.2 S T R U C T U R A L FRAMEWORK On the r e g i o n a l s c a l e , the Rocky Mountains are seen to comprise a wedge of sediments which t a p e r s toward the n o r t h e a s t . These sediments have been shortened by n e a r l y 2 00 km by the development of l a r g e , l i s t r i c t h r u s t f a u l t s ( P r i c e , 1965). Displacement v a r i e s between i n d i v i d u a l f a u l t s , but the l a r g e s t t h r u s t sheets are known t o have been t r a n s p o r t e d as much as tens of k i l o m e t r e s ( B a l l y and o t h e r s , 1966). These f a u l t s g e n e r a l l y have a sout h w e s t e r l y d i p , and have t h r u s t o l d e r rock onto younger as the hanging w a l l was d i s p l a c e d toward the no r t h e a s t or e a s t ( P r i c e , 1965). The w e l l developed sedimentary l a y e r i n g has imparted an a n i s o t r o p y which has c o n t r o l l e d the s t y l e o f deformation throughout the Canadian Rocky Mountains. Because of the abrupt changes i n l i t h o l o g y between i n d i v i d u a l beds and the c o r r e s p o n d i n g l y abrupt changes i n mechanical p r o p e r t i e s , t e c t o n i c s h o r t e n i n g has p r i n c i p a l l y been taken up by l a y e r p a r a l l e l s l i p ( P r i c e , 1973). T h i s c o n d i t i o n has r e s u l t e d i n the development of f l e x u r a l s l i p f o l d s and t h r u s t f a u l t s . The t h r u s t f a u l t s u r f a c e s are c h a r a c t e r i z e d by broad i n t e r v a l s o f s l i p p a r a l l e l t o bedding, t y p i c a l l y a l o n g or w i t h i n l e s s competent l a y e r s such as s h a l e s , connected by sh o r t i n t e r v a l s o f s l i p (ramps) ac r o s s the more competent l a y e r s (Douglas, 1958). These t h r u s t f a u l t s do not extend down i n t o the c r y s t a l l i n e basement r o c k s ; i n s t e a d the 14 basement s u r f a c e r e p r e s e n t s a r e g i o n a l decollement along which the t h i n sedimentary cover was l i t e r a l l y "scraped" o f f . The basement rocks were l e f t r e l a t i v e l y u n d i s t u r b e d by t h i s p r o c e s s ( B a l l y and o t h e r s , 1966). 2 . 2 . 1 . L O C A L S T R U C T U R E O v e r f o l d Mountain e x i s t s as a s u r f a c e e x p r e s s i o n of one of a s e r i e s of n o r t h w e s t e r l y t r e n d i n g f o l d s which have formed on the hanging w a l l of the Lewis t h r u s t s h e e t . In the a r e a , the r e s i s t a n t P a l e o z o i c limestones and d o l o s t o n e s , exposed i n the hinges of a n t i c l i n e s , form sharp r i d g e s and peaks. Less r e s i s t a n t Upper P a l e o z o i c and younger sandstones and s h a l e s are exposed i n the hinges of s y n c l i n e s , the axes of which t y p i c a l l y f a l l a long v a l l e y f l o o r s . The a n t i c l i n e and s y n c l i n e i n "the study area each have one limb which d i p s g e n t l y t o moderately toward the southwest and have a shared limb which i s v e r t i c a l t o overturned ( F i g u r e 5 ) . F o l d axes (plunge 10° toward 155°) of the two f o l d s are p a r a l l e l . The a x i a l plane of the s y n c l i n e (148/55 SW) d i p s somewhat more s t e e p l y than t h a t of the a n t i c l i n e (145/45 S W ) . The o r i e n t a t i o n of these f o l d s t r u c t u r e s f o l l o w s the n o r t h w e s t e r l y r e g i o n a l t r e n d . •\ • •\ Q \ ° c g p ' a Syncline Anticline Figure 5 - Poles to bedding in the syncline and anticline studied near Overfold Mountain. Great circle represensts the axial plane. Solid dot represents the fold axis. 16 3. DEFORMATIONAL MECHANISMS - A BACKGROUND D e f o r m a t i o n a l processes which have taken p l a c e at O v e r f o l d Mountain are evidenced on both the mesoscopic and m i c r o s c o p i c s c a l e . On the mesoscopic s c a l e , s t r a i n i s documented by e x t e n s i o n f r a c t u r e s , shear f r a c t u r e s and s t y l o l i t e seams. These f e a t u r e s a r i s e from i n t e r g r a n u l a r p r o c e s s e s , t h a t i s , the d e f o r m a t i o n a l mechanisms have operated between i n d i v i d u a l g r a i n s . On the m i c r o s c o p i c s c a l e , s m a l l e r v e r s i o n s of the mesoscopic s t r u c t u r e s are seen, as w e l l as s t r u c t u r e s r e s u l t i n g from i n t r a g r a n u l a r p r o c e s s e s . I n t r a g r a n u l a r processes operate w i t h i n g r a i n s and cause d i s t o r t i o n s , or s l i p , w i t h i n the c r y s t a l l a t t i c e . I n t r a g r a n u l a r mechanisms: 1. P o l y s y n t h e t i c t w i n n i n g . 2. T r a n s l a t i o n g l i d i n g ( s l i p ) . 3. Deformation l a m e l l a e development. 4. E x t e n s i o n f r a c t u r i n g . I n t e r g r a n u l a r mechanisms: 1. R o c k - f l u i d i n t e r a c t i o n s . a . H y d r a u l i c f r a c t u r i n g . b. P r e s s u r e s o l u t i o n . 1) S t y l o l i t e seams i n c a r b o n a t e s . 2) F l a t t o concavo-convex c o n t a c t s between quartz g r a i n s i n sandstones. 2. Shear f r a c t u r i n g . 3. F a u l t s . A l l o f the aforementioned d e f o r m a t i o n a l mechanisms have been u t i l i z e d t o some degree throughout the study a r e a . L i t h o l o g y , however, p l a y s a dominant r o l e i n the d e t e r m i n a t i o n of how s t r a i n i s p a r t i t i o n e d between these mechanisms. F a c t o r s which i n f l u e n c e the way i n which a rock deforms i n c l u d e : framework and matrix mineralogy, abundance of m a t r i x , g r a i n s i z e , p e r m e a b i l i t y and degree o f cementation. For example, the d i f f e r e n c e i n the chemical composition and l a t t i c e s t r u c t u r e s of c a l c i t e , dolomite and quartz cause profound d i f f e r e n c e s i n t h e i r s t r e n g t h and s o l u b i l i t y . Two rocks composed of unequal amounts of these minerals w i l l thus d i f f e r i n t h e i r response t o non-h y d r o s t a t i c s t r e s s . D i f f e r e n t l i t h o l o g i c u n i t s from the same s e c t i o n of the f o l d may t h e r e f o r e r e v e a l a s i m i l a r s t r a i n h i s t o r y , but' have a r r i v e d at t h e i r f i n a l p o i n t of f i n i t e s t r a i n v i a d i f f e r e n t pathways. 3 . 1 INTRAGRANULAR PROCESSES I n t r a g r a n u l a r deformation r e s u l t s i n the accommodation of s t r a i n by p r o c e s s e s t h a t take p l a c e w i t h i n the c r y s t a l l a t t i c e o f the g r a i n . The i n t r a g r a n u l a r d e f o r m a t i o n a l mechanisms t h a t have been o p e r a t i v e i n the rocks o f the study area i n c l u d e : 1. G l i d e mechanisms - D u c t i l e regime. a. P o l y s y n t h e t i c t w i n n i n g . 1) C a l c i t e . 2) Dolomite - Minor. b. T r a n s l a t i o n g l i d e . 1) Dolomite. 2) C a l c i t e - Minor. c. Deformation l a m e l l a e development. 1) Q u a r t z . 2. E x t e n s i o n f r a c t u r i n g - B r i t t l e regime. 1) C a l c i t e . 2) Dolomite. 3) Q u a r t z . P o l y s y n t h e t i c twins are abundant i n the c a l c i t e g r a i n s and cement w i t h i n the limestones of the study a r e a . These 18 twins are p a r t i c u l a r l y w e l l developed i n the b i o s p a r i t e u n i t s ( U n i t s 6 and 8; F i g u r e 6). Dolomite c r y s t a l s , on the o t h e r hand, do not show much de f o r m a t i o n a l t w i n n i n g , a r e f l e c t i o n of the s t r o n g e r s t r u c t u r e of dolomite c r y s t a l s . The mechanism of t r a n s l a t i o n g l i d e , or s l i p , has probably been o p e r a t i v e i n the c a l c i t e and dolomite g r a i n s . S i n c e s l i p r e s u l t s i n a p e r f e c t l a t t i c e s t r u c t u r e , the amount of s t r a i n accommodated by t h i s mechanism cannot be measured u n l e s s the o r i g i n a l g r a i n shape i s known. Quartz g r a i n s c o n t a i n i n g deformation l a m e l l a e are found i n the sandstone and sandy dolostone u n i t s . They are not abundant, only about 2 0 cm"2, t h i s i n d i c a t e s t h a t t h i s mechanism has not p l a y e d a v e r y l a r g e r o l e i n the accommodation of the s t r a i n induced by f o l d i n g . I n t r a g r a n u l a r e x t e n s i o n f r a c t u r e s are found i n a l l l i t h o l o g i e s . In c a l c i t e and d o l o m i t e , these f r a c t u r e s o f t e n f o l l o w twin or cleavage p l a n e s but are t y p i c a l l y random. Quartz appears to have p r e f e r e n t i a l l y accommodated s t r a i n by i n t r a g r a n u l a r f r a c t u r i n g . The process by which f r a c t u r e s are i n i t i a t e d and propagated w i l l be covered i n more d e t a i l i n the s e c t i o n on f r a c t u r i n g . Each m i n e r a l and each aggregate of m i n e r a l s i s thus seen t o accommodate s t r a i n by a d i f f e r e n t mechanism. How a rock responds to an a p p l i e d s t r e s s i s an e x p r e s s i o n of the c r y s t a l chemistry of t h e m i n e r a l s w i t h i n i t . C r y s t a l s have behaved d u c t i l e y where the l a t t i c e s t r u c t u r e has allowed 19 Figure 6 - Photomicrograph illustrating the abundant twin lamellae within the grains and cement of the biosparite unit, Unit 8. Long edge of photograph measures 0.3 mm. 20 i t . In t h e absence of d u c t i l e mechanisms, c r y s t a l s u l t i m a t e l y f a i l by f r a c t u r e . The f o u n d a t i o n of d u c t i l i t y l i e s i n the a b i l i t y of a m a t e r i a l t o flow, without l o s s of cohesion or s t r e n g t h , when s t r e s s e d beyond i t s e l a s t i c l i m i t . D u c t i l e s t r a i n i n c r y s t a l s r e s u l t s from g l i d i n g along one or more g l i d e p l a n e s . G l i d e p l a n e s , {T}, are t y p i c a l l y planes of c l o s e l y packed atoms w i t h i n the c r y s t a l l a t t i c e . The d i r e c t i o n of g l i d e a l o n g t h i s p l a n e , <t>, i s governed by the arrangement of atoms. The two i n t r a c r y s t a l l i n e g l i d e mechanisms o p e r a t i v e i n the carbonates r e s u l t from the m i g r a t i o n of a d i s l o c a t i o n along a g l i d e p l a n e . The f i r s t mechanism, t w i n n i n g , l e a d s t o the d i s t o r t i o n o f the c r y s t a l l a t t i c e on one s i d e o f the plane such t h a t the d i s t o r t e d l a t t i c e has a twin r e l a t i o n s h i p with the u n d i s t o r t e d l a t t i c e (Figure 7). T h i s r e s u l t s i n l a t t i c e i m p e r f e c t i o n s along the twin p l a n e , a f a c t o r which may cause these p l a n e s t o be the l o c u s of l a t e r f r a c t u r i n g . The second g l i d e mechanism, t r a n s l a t i o n g l i d e o r s l i p , occurs by the m i g r a t i o n of a d i s l o c a t i o n along a g l i d e p l a n e , and w i l l not r e s u l t i n the accumulation of d i s l o c a t i o n s along a p l a n e . 3 . 1 . 1 . CALCITE The s t r u c t u r e of c a l c i t e i s known t o c o n s i s t of l a y e r s of Ca2 + i o n s a l t e r n a t i n g with l a y e r s of C032' groups. T h i s l a y e r e d s t r u c t u r e p r o v i d e s many planes of c l o s e l y packed ions a l o n g which g l i d e may occur ( F i g u r e 8 ) . C a l c i t e forms p o l y s y n t h e t i c twins very e a s i l y (low s t r e s s and low 21 Calcium \^ Carbonate Figure 7 - Idealized crystal structure of twinned calcite. Twin plane is {0012}. After Higgs and Handin (1959). Figure 8 - Idealized crystal lattice structure of undeformed calcite. See Figure 7 for legend. 22 temperature; N i c o l a s and P o i r i e r , 1976). By f a r the most common twin i s formed p a r a l l e l t o the {0112} planes (the e - p l a n e s ) . C a l c i t e twinning r e s u l t s i n simple shear p a r a l l e l t o the twin p l a n e , i n an amount p r o p o r t i o n a l to the t h i c k n e s s of the twinned m a t e r i a l . Many r e s e a r c h e r s have used these twins t o a s s i g n s t r e s s c o n f i g u r a t i o n s t o deformed limestones (Turner and Wiess, 1963). In t h i s study the d e n s i t y of the p o l y s y n t h e t i c twins have been used as a guide to the r e l a t i v e amount of s t r a i n t h a t has been accommodated by t h i s mechanism. Twin l a m e l l a e are best developed i n the g r a i n s t o n e u n i t s such as U n i t 8 (Figure 6) and appear t o have been l e s s important i n the accommodation of s t r a i n i n the g r a i n s of packstone and wackestone u n i t s such as U n i t s 7 and 2 (Figure 9 and 34). The i n v e r s e r e l a t i o n s h i p between the d e n s i t y o f twin l a m e l l a e and percentage of m a t r i x r e s u l t s from two p r o c e s s e s . 1. In the u n i t s w i t h f i n e - g r a i n e d m a t r i x , s t r a i n has been accommodated by i n t r a g r a n u l a r deformation of the m a t r i x g r a i n s i n s t e a d of the framework g r a i n s . 2. In the u n i t s with f i n e - g r a i n e d m a t r i x , s t r a i n has been accommodated by p r e s s u r e s o l u t i o n , a process which i s b e l i e v e d to take p l a c e a t s t r e s s e s lower than the c r i t i c a l s t r e s s f o r t w i n n i n g . The r a t i o n a l e f o r the l a t t e r mechanism i s p r e s e n t e d i n the s e c t i o n on p r e s s u r e s o l u t i o n . The f o u n d a t i o n of the f i r s t mechanism l i e s i n the e f f e c t of g r a i n s i z e on the ease with which c a l c i t e deforms by i n t r a - c r y s t a l l i n e g l i d e mechanisms. The concept of a g r a i n s i z e e f f e c t r e l i e s on the theory t h a t the s u r f a c e of a g r a i n w i l l be a source of d e f e c t s w i t h i n 23 Figure 9 - Photomicrograph illustrating the low density of twin lamellae within the grains of skeletal packstone (Unit 7). Long edge of photograph measures 0.06 mm. 24 the g r a i n . These d e f e c t s w i l l have l i t t l e e f f e c t on the s t r e n g t h o f the c r y s t a l i f the g r a i n i s l a r g e , but l i k e l y i s very important when d e a l i n g w i t h g r a i n s i n the m i c r i t i c s i z e range ( l e s s than 2 um). Experimental evidence shows t h a t at room temperature, c a l c i t e i s 10 t o 20 times s t r o n g e r f o r the mechanism of t r a n s l a t i o n g l i d e than i t i s f o r e-twinning (Turner and Wiess, 1963). T h i s e f f e c t i s reduced with i n c r e a s i n g temperature though not s u f f i c i e n t l y t o make t r a n s l a t i o n g l i d e an important d e f o r m a t i o n a l mechanism i n the c a l c i t e from the study a r e a . C a l c i t e has behaved b r i t t l e y on the i n t r a - c r y s t a l l i n e l e v e l by the development o f e x t e n s i o n f r a c t u r e s w i t h i n g r a i n s . F r a c t u r i n g i n c a l c i t e may occur a t any p o i n t i n the c r y s t a l l a t t i c e t h a t c o n t a i n s a c o n c e n t r a t i o n of d i s l o c a t i o n s . As mentioned, twin p l a n e s , g r a i n boundaries and t h e i r i n t e r s e c t i o n s r e p r e s e n t areas i n the c r y s t a l where t h i s s i t u a t i o n may a r i s e , and where f r a c t u r i n g i s commonly i n i t i a t e d . 3.1.2. D O L O M I T E L i k e c a l c i t e , dolomite c o n s i s t s of l a y e r s o f carbonate groups a l t e r n a t i n g with l a y e r s of c a t i o n s . There i s no s o l i d s o l u t i o n between c a l c i t e and d o l o m i t e , due t o the l a r g e d i f f e r e n c e i n i o n i c r a d i i between the two c a t i o n s , Mg2+ (0.065 nm) and Ca2 + (0.099 nm) . Layers o f Mg2+ c o n s i s t e n t l y a l t e r n a t e w i t h l a y e r s o f Ca2 + ( F i g u r e 10). The d i f f e r e n c e i n the s i z e o f the two c a t i o n s makes e-twinning 25 Carbonate Figure 10 - Idealized crystal structure of undeformed dolomite. 26 i m p o s s i b l e and r e s u l t s i n a c r y s t a l s t r u c t u r e which i s much s t r o n g e r than the c a l c i t e s t r u c t u r e . Dolomite does not behave d u c t i l e y as e a s i l y as c a l c i t e . When i t does, i t i s known to deform by t r a n s l a t i o n g l i d e p a r a l l e l t o {0001} (the c-plane) and, to a l e s s e r e x t e n t , by twin g l i d i n g along the f - p l a n e {0221}. Other r e s e a r c h e r s have shown t h a t under a c o n f i n i n g p r e s s u r e of 500 MPa, dolomite w i l l undergo t r a n s l a t i o n g l i d e a t a l l temperatures, p r o v i d e d the c r y s t a l i s s u i t a b l y o r i e n t e d (approximately the same requirements f o r t r a n s l a t i o n i n c a l c i t e ) . I f not s u i t a b l y o r i e n t e d f o r t r a n s l a t i o n , the dolomite c r y s t a l s f a i l b r i t t l e y i f below 400°C. Above 400°C the c r y s t a l deforms by twin g l i d e . Another c o n t r a s t between dolomite and c a l c i t e i s t h a t dolomite becomes s t r o n g e r w i t h i n c r e a s i n g temperature between 25°C and 400°C, o p p o s i t e t o the response i n c a l c i t e (Higgs and Handin, 1959) . As f o r c a l c i t e , f r a c t u r i n g i n dolomite w i l l most e a s i l y be i n i t i a t e d where d e f e c t s are c o n c e n t r a t e d . A g a i n , as f o r c a l c i t e , twin planes and g r a i n boundaries r e p r e s e n t two areas where t h i s c o n d i t i o n i s met. Twin pl a n e s are r a r e i n the d o l o m i t e i n the study a r e a . The g r a i n boundaries are thus the main l o c u s f o r the i n i t i a t i o n o f f r a c t u r e s . T h i s e f f e c t i s enhanced by the rhombic shape of the g r a i n s . The sharp edges and p o i n t s of the dolomite rhombs are very e f f e c t i v e s t r e s s c o n c e n t r a t o r s ( F i g u r e 11), a f a c t o r which i s r e s p o n s i b l e f o r the widespread m i c r o f r a c t u r i n g i n the 27 Figure 11 - Photomicrograph illustrating the rhombic shape of dolomite crystals within the dolostone units studied near Overfold Mountain. Long edge of photograph measures 0.02 mm. 28 d o l o s t o n e s of the study a r e a , and i s probably a l s o r e s p o n s i b l e f o r the h i g h f r a c t u r e p e r m e a b i l i t y found i n the d o l o s t o n e s of some producing hydrocarbon r e s e r v o i r s . 3.1.3. QUARTZ The quartz s t r u c t u r e c o n s i s t s of one S i4 + i o n c o v a l e n t l y bonded i n t e t r a h e d r a l c o o r d i n a t i o n w i t h f o u r 02" i o n s . Each 02" i o n i s c o o r d i n a t e d with two S i4 + i o n s . As a r e s u l t of these c o v a l e n t bonds, quartz i s very s t r o n g and does not r e a d i l y deform by i n t r a - c r y s t a l l i n e g l i d e mechanisms. In f a c t , experimental evidence has shown dry quartz t o have a s t r e n g t h almost equal, to i t s t h e o r e t i c a l s t r e n g t h a t 500°C ( G r i g g s , 1967). T h i s unusual s t r e n g t h i s due t o the d i f f i c u l t t a s k of b r e a k i n g the s t r o n g c o v a l e n t Si-O bonds. T h i s t a s k i s made much e a s i e r by the a d d i t i o n of water t o the c r y s t a l l a t t i c e . Water leads t o the h y d r o l y s i s of the S i - 0 bonds and the weakening of the l a t t i c e ( G r i g g s , 1967). The thus formed Si-OH HO-Si b r i d g e s are e a s i l y broken, a l l o w i n g the m i g r a t i o n of d i s l o c a t i o n s . H y d r o l y t i c weakening of quartz i s known t o be important i n encouraging a l l d e f o r m a t i o n a l mechanisms i n v o l v i n g q uartz ( i . e . d i s s o l u t i o n , c r y s t a l l i z a t i o n , f r a c t u r i n g ) . The o n l y d i r e c t evidence of i n t r a - c r y s t a l l i n e g l i d e mechanisms w i t h i n the quartz a r e n i t e of the study area i s the occurrence of deformation l a m e l l a e . Deformation l a m e l l a e are narrow p l a n e r f e a t u r e s along which the c r y s t a l l a t t i c e has been d i s r u p t e d . These l a m e l l a e have been i d e n t i f i e d by many r e s e a r c h e r s i n e x p e r i m e n t a l l y and 29 naturally deformed quartzites, and have been the subject of intense investigation (Christie et a l . , 1964; McLaren et al . , 1970; Christie and Ardell, 1974). Lamellae in experimentally deformed quartz form along the basal plane of the quartz structure ({0001}). Lamellae in naturally deformed quartz have a more random orientation and are found to form at angles up to 30° from the basal plane (Heard and Carter, 1968). 3.2. INTERGRANULAR PROCESSES Intergranular deformation results in the accommodation of strain by processes that take place between grains. The structures resulting from intergranular deformational mechanisms are: 1. Hydraulic fractures. 2. Shear fractures. 3. Pressure solution features. a. Stylolite seams in carbonates. b. Flattened contacts in sandstones. The development of each of these structures is closely tied to the permeability of the rock at the time of its formation. The reasons for this will be covered in detail for each structure. 3.2.1. FRACTURES Excellent reviews of the development of current theories on the brittle deformation of rocks have been given by Nadai (1950), Paterson (1978) and Jaeger and Cook (1976). Some of the earliest experimental work centred on brittle failure was done in 177 3 by C. A. Coulomb. This work was 30 later modified in the late 19— century and early 20— century by researchers such as Tresca (1868) and Mohr (1901, 1914). One of the most noteworthy advances in fracture theory was made by A. A. Griffith in 1920. Griffith determined that the strength of a material is dependent upon the existence of tiny cracks (Griffith's cracks) within grains and at grain boundaries. He also postulated that a crack will propagate by extension when stress is applied, provided i t is in an orientation which will allow i t to overcome the tensile strength of the material. His theory further encompassed the effect of pore pressure on crack propagation. Griffith's theory has been extensively applied and modified since its inception. Of interest to this study is the effect of fluids within the cracks, a component which can increase the rate of crack growth by two mechanisms; first by the increase in fluid pressure (Paterson, 1978; p. 86) and second by the effect of stress corrosion (Atkinson, 1982; Atkinson and Meredith, 1987). 3.2.1.1. HYDRAULIC FRACTURES Hydraulic fractures are extension fractures which form in response to high pore pressures (as described by Griffith's theory of failure). When fluid within a rock is compressed, i t exerts a force which is normal to a l l surfaces. This results in a reduction of the effective stresses by the pore pressure value. 31 Minimum Compressive S t r e s s Intermediate Compressive S t r e s s Maximum Compressive S t r e s s Pore Pressure Note: Compressive s t r e s s i s n e g a t i v e . The r e s u l t o f t h i s p r o c e s s i s d i s p l a y e d d i a g r a m m a t i c a l l y on a Mohr diagram ( F i g u r e 12). When the e f f e c t i v e s t r e s s e s are p l o t t e d , i t i s seen t h a t i f the pore p r e s s u r e i s h i g h enough, and the d e v i a t o r i c s t r e s s low enough, the c i r c l e w i l l become tangent t o the f a i l u r e envelope i n the f i e l d of t e n s i l e f a i l u r e . F r a c t u r e s formed by t h i s p r o c e s s w i l l always propagate i n a d i r e c t i o n p a r a l l e l t o the maximum compressive s t r e s s d i r e c t i o n , i n a p l a n e which c o n t a i n s the i n t e r m e d i a t e p r i n c i p a l s t r e s s and i s normal t o t h e minimum compressive s t r e s s d i r e c t i o n . Knowledge o f f r a c t u r e o r i e n t a t i o n thus g i v e s i n s i g h t i n t o the s t r e s s c o n f i g u r a t i o n a t the time o f i t s f o r m a t i o n . H y d r a u l i c f r a c t u r e s o r i g i n a t e a t t h e m i c r o s c o p i c l e v e l by the p r o p a g a t i o n and i n t e r c o n n e c t i o n o f t i n y G r i f f i t h ' s c r a c k s . These t i n y c r a c k s are b e l i e v e d t o develop where d i s l o c a t i o n s w i t h i n the c r y s t a l l a t t i c e " p i l e up", e i t h e r w i t h i n t h e c r y s t a l (along cleavages o r t w i n p l a n e s ) , or a t g r a i n b oundaries (Atk i n s o n and M e r e d i t h , 1987). The sharp t i p s of t h e s e s l i t - l i k e c r a c k s are e f f i c i e n t s t r e s s c o n c e n t r a t o r s . Water w i t h i n a c r a c k r e a c t s e a s i l y w i t h t h i s Hydraulic Fractures - The Mohr Circle Model 01 (J 3 -P Compressive Field 01 "P Tensile Field -'Confining Pressure P-Pore Pressure ( 5 3 - M a x i m u m Compressive Stress Figure 12 - Mohr circle representation of the effect of pore pressure in the determination of the effective stresses acting on a rock in a deviatoric stress field. Compressive stress is negative. CO to 33 stressed material causing the weakening of the material at the tip and the propagation of the crack. This process, known as stress corrosion, can cause crack propagation at stresses lower than the critical value for the material in question. Two conditions will cause a fracture to remain microscopic in size: 1. The rock is depleted in fluids so that pore pressure cannot build-up. 2. The microfracture becomes rotated with respect to the stress axes such that i t is no longer in a favorable position to propagate. If conditions allow, microfractures will connect to form larger extension fractures. Mesoscopic fractures are produced by the cyclic re-opening and f i l l i n g of smaller cracks, a process termed "crack-seal" deformation (Ramsay, 1980). Repetitions of this process can lead to the development of large mesoscopic fractures. Two growth geometries, syntaxial and antitaxial, have been described for the crystallization of material within hydraulic fractures (Durney and Ramsay, 1973; Cox and Etheridge, 1983; Ramsay, 1980). Both are recognized in the study area on the mesoscopic and microscopic scale. Syntaxial fractures are characterized by a fracture f i l l i n g which has a mineralogy that is common in the host rock. This suggests that the fluid from which the vein material crystallized had not travelled far, and thus has implications as to the permeability of the rock at the time of fracture formation. The crystal growth within the 34 fracture is seen to be fibrous. Fibre growth is often in optical continuity with a host grain in the wallrock and is perpendicular to the fracture wall (Figure 13). The wall is irregular, and appears to follow grain boundaries, indicating that these areas of weakness have guided the fracture development. Later episodes of cracking have taken place along the centre of the fracture, leading to a break in the optical continuity of the fibre across the vein, as well as the development of a crystalline f i l l that is free of wallrock fragments. Earliest crystallization in syntaxial fractures occurs along the walls of the fracture, latest crystallization occurs at the centre of the fracture. The fracture f i l l i n g in antitaxial fractures is often of a mineral species that is foreign to the host rock such as calcite veins formed in quartz arenite. This indicates a relatively large distance of fluid transport, which in turn suggest that the host rock was relatively permeable at the time of fracture formation. As with syntaxial fractures, mineral growth is fibrous, and fibres (if straight) grow perpendicular to the vein walls. Unlike syntaxial fractures, however, fibres are in optical continuity across the fracture (Figure 14). Later cracking occurs between the wall and the f i l l due to the mechanical differences between the two materials. This causes the inclusion of wall rock fragments within the crystalline fracture f i l l . Earliest crystallization in antitaxial fractures occurs along the 35 f r a c t u r e c e n t r e , l a t e s t c r y s t a l l i z a t i o n o ccurs along the f r a c t u r e w a l l . 36 Figure 13 - Mode of development of syntaxial fractures. After Durney and Ramsey, 1973. 37 Wallrock fragments are included with the new crystal growth New material crystallizes at the fracture boundary Figure 14 - Mode of development of antitaxial fractures. After Durney and Ramsey, 1973. 38 3.2.2. PRESSURE SOLUTION Pressure solution was first studied as a deformational mechanism by Sorby (1858, 1863). Sorby (1858) observed the morphology of cleavage structures and was the first to suggest mineral migration along cleavage planes into associated hydraulic fractures. He also attempted one of the fi r s t studies dealing with strain estimation from deformed oolites (Sorby, 1908). In this study, Sorby showed that material was preferentially dissolved along some grain contacts and that the orientation of these contacts could be used for the determination of the stress configuration at the time of dissolution. The much quoted experimental work by Riecke (1894, 1912) involved the effect of non-hydrostatic stress on solubility. He showed that, given two crystals in a solution with which they are in equilibrium, i f a stress is applied to one crystal i t will dissolve and new mineral growth will occur on the unstressed crystal. Further experimental work by Correns (1949) determined that growth and dissolution of non-hydrostatically stressed crystals is regulated by stress, temperature, type of material, and crystallographic orientation. This principle was first applied to geologic processes by Sorby (1863) and Spring (1888), and has been exercised extensively in the development of current theory on pressure solution. Fuchs (1894) determined that stylolite seams were formed as a result of pressure solution. In his study, 39 Fuchs observed two types of stylolite, diagenetic and tectonic. Diagenetic stylolites result from the stresses associated with sediment loading and thus form parallel to bedding. Tectonic stylolites form oblique to bedding and are a result of tectonic loading. The first half of the 2 0— century proved to be a time of l i t t l e innovative work in the area of pressure solution. Most work done during this period merely elaborated that done previously (Wright, 1906; Knopf, 1933). In the 1960's, a new interest in pressure solution phenomena was spawned by investigators such as Flinn (1965), Rast (1965), and Ramsay (1967). Since this time, there has been a great deal of research concentrated on the importance of pressure solution phenomena in both the diagenetic and the tectonic realms. Pressure solution is seen to occur to some extent in all of the lithologies in the study area. In carbonates, i t is one of the most important mechanisms for the accommodation of strain. Typically, sandstones do not undergo pressure solution to the extent that carbonates do and, in the study area, no mesoscopic fabric is developed. Dissolution in the sandstones from the study area is demonstrated only on the microscopic scale as straight or concavo-convex contacts between quartz grains (Figure 15). This contrast between lithologies is mainly due to the difference in solubility between the two materials, carbonates being far more soluble than quartz in the acidic environment of the stylolite seam. 40 Figure 15 - Photomicrograph illustrating the straight and concavo-convex contacts between quartz grains in Unit 1. Long edge of photograph measures 0.06 mm. 41 When a granular substance is subjected to compression, a considerable stress concentration occurs at grain contacts. The resultant stresses at the grain contacts may be many times greater than the overall stress placed upon the substance. According to Riecke's principle, when a solvent acts upon an elastic material in a compressive stress field, the material is dissolved most easily at the points of stress concentration. Pressure solution is thus most readily initiated at grain contacts which are subjected to the highest normal stress. The orientation of the stylolite seam is governed by the direction of the specific permeability. Ideally, in a homogeneous, isotropic medium, stylolite seams form in a plane normal to the maximum compressive stress direction. Sedimentary rocks are rarely isotropic or homogeneous, however, due to variations in grain size, degree of cementation, and mineralogy, as well as the presence of pre-existing fractures and stylolites. These features affect the permeability of the rock and will tend to cause a deviation of the seam orientation from the ideal (Nelson, 1983). The toothlike appearance of stylolites in cross-section results from the presence of columns or ridges along the seam (Figure 16). These two distinct stylolite morphologies, columns and ridges, may evolve in different environments; the shape of the structure being governed by the path of least resistance for fluid movement. Columnar 42 s t y l o l i t e s might then be favoured under c o n d i t i o n s where the magnitudes of the minimum and i n t e r m e d i a t e p r i n c i p a l s t r e s s e s are e q u a l , and f l u i d can move i n a l l d i r e c t i o n s p a r a l l e l t o the s t y l o l i t e seam (F i g u r e 17). T h i s s t r e s s c o n f i g u r a t i o n would have e x i s t e d d u r i n g sediment l o a d i n g . During f o l d i n g , the magnitudes of the p r i n c i p a l s t r e s s e s are unequal. Under these c o n d i t i o n s , i f the sediments are r e l a t i v e l y homogeneous, f l u i d movement would be favoured i n the d i r e c t i o n of minimum s t r a i n along the seam (Figure 17). T h i s p r o c e s s c o u l d produce s t y l o l i t e s w i t h the r i d g e morphology commonly seen i n the study a r e a . The l i n e a t i o n formed by the c r e s t of the r i d g e s would p a r a l l e l the d i r e c t i o n of minimum s t r a i n . C l a y f l a k e s w i t h i n both of these s t y l o l i t e morphologies p a r a l l e l the s t y l o l i t e seam. "Ridge" s t y l o l i t e s w i t h i n the study area d i f f e r from s i m i l a r s t r u c t u r e s d e s c r i b e d by o t h e r authors (Hancock; 1985). Hancock (1985) d e s c r i b e s s t y l o l i t e s w i t h a r i d g e morphology which have formed i n continuum with columnar s t y l o l i t e s ( F igure 18). The l i n e a t i o n a s s o c i a t e d w i t h these r i d g e s p a r a l l e l s the maximum compressive s t r e s s . L i n e a t i o n s s i m i l a r t o t h a t d e s c r i b e d by Hancock (1985) are seen on the m i c r o s c o p i c s c a l e i n a columnar s t y l o l i t e found eas t of t h e study a r e a , w i t h i n the A l t y n Formation. A scanning e l e c t r o n microscope image of t h i s s t y l o l i t e r e v e a l s l i n e a t i o n s along the column which p a r a l l e l the maximum compressive s t r e s s d i r e c t i o n ( F i g u r e 19). T h i s l i n e a t i o n appears t o be the d i r e c t r e s u l t of c l a y c a t a l y z e d COLUMNAR STYLOLITES RIDGE STYLOLITES Figure 16 - Morphology of columnar and ridge stylolites. 63 6i Direction of fluid movement Figure 17 - Direction of fluid movement along a stylolite seam. During compaction, intermediate and niinimum principal stresses are equal, fluid moves freely in all directions. During deformation, intermediate and minimum stresses are unequal, fluid moves most easily in the direction of the minimum stress. 44 lineation subaarallel to direction of maximum compression oblique columns Figure 18 - Continuum between columnar stylolites and stylolites with lineations as described by Hancock (1985). 45 d i s s o l u t i o n along the s i d e s o f the column. K a o l i n i t e f l a k e s along the s i d e s of the columns are ordered i n t o "rods" ( F i g u r e 20) such t h a t the f l a k e s p a r a l l e l the seam, and the c l a y rods are normal t o the seam ( p a r a l l e l t o the maximum compression). S t r i a t i o n s p a r a l l e l these c l a y r o d s , and even mimic the hexagonal shape of the k a o l i n i t e f l a k e s . T h i s suggests t h a t the s t r i a t i o n s have been "carved" out of the limestone by the a c i d i c c l a y edges. I l l i t e i s the common c l a y m i n e r a l found w i t h i n the s t y l o l i t e s near O v e r f o l d Mountain. U n l i k e k a o l i n i t e , i l l i t e f l a k e s do not become ordered i n t o r o d s . I n s t e a d , the e l a s t i c i t y of i l l i t e a l l o w s i t t o bend around the s t y l o l i t e columns. No s t r i a t i o n s normal t o the s t y l o l i t e seams were found i n s t y l o l i t e s c o n t a i n i n g i l l i t e , j u s t as no columnar s t y l o l i t e s were found. I t i s p o s s i b l e then, t h a t c l a y m ineralogy, as w e l l as f l u i d movement, may have p l a y e d a major r o l e i n the d e t e r m i n a t i o n of the morphology o f the s t y l o l i t e . As d i s s o l u t i o n p r o g r e s s e s , a selvage of i n s o l u b l e r e s i d u e i s b u i l t up. Depending upon the l i t h o l o g y p r e s e n t , t h i s r e s i d u e may c o n s i s t of q u a r t z , c l a y s , kerogen, bitumen, p y r i t e , oxides and/or d o l o m i t e . Dolomite appears t o be l e s s s o l u b l e than c a l c i t e under c o n d i t i o n s of p r e s s u r e s o l u t i o n . Limestone u n i t s which are s l i g h t l y d o l o m i t i c tend t o have a c o n c e n t r a t i o n of dolomite rhombs w i t h i n s t y l o l i t e seams ( F i g u r e 21). P y r i t e and i r o n oxide are seen t o accumulate i n seams w i t h i n d o l o s t o n e s as i s quartz ( F i g u r e 22). 46 Figure 19 - S E M micrograph illustratinglineations found on columnar stylolites from the Altyn Formation. These lineations parallel the stylolite columns and developed parallel to the maximum compressive stress direction. 47 Figure 20 - S E M micrograph illustrating the alignment of kaolinite flakes along stylolite columns. The "carving" action of kaolinite is suggested by the hexagonal shape of the striations on the carbonate. K - Kaolinite; C - Carbonate. Photomicrograph illustrating the concentration of dolomite rhombs in stylolite seams within a slightly dolomitic packstone (Unit 7). Long edge of photograph measures 0.3 mm. Figure 22 - Photomicrograph illustrating the concentration of quartz grains, opaque minerals (pyrite and iron oxide), and orgamcs within a stylolite seam in a sandy dolostone from the Etherington Formation. Long edge of photograph measures 0.3 mm. 50 Bitumen appears t o be p r e s e n t i n most seams, i n d i c a t i n g t h a t the seams were a c t i v e d u r i n g o r a f t e r hydrocarbon m i g r a t i o n . The r o l e of c l a y m i n e r a l s i n the c a t a l y s i s of p r e s s u r e s o l u t i o n has been g i v e n much a t t e n t i o n (Heald, 1956; Weyl, 1959; De Boer, 1975; E n g e l d e r and Marshak, 1985). Reasons f o r t h i s phenomenon are s t i l l not c l e a r , but i t i s suggested t h a t the o r i e n t e d c l a y f l a k e s a c t as a d i f f u s i o n pathway alon g which f l u i d s can m i g r a t e (Weyl, 1959; G e i s e r and Sansone, 1981). The a b i l i t y o f c l a y m i n e r a l s t o a c t as Lewis a c i d s may a l s o a i d i n t h e d i s s o l u t i o n of carbonate m i n e r a l s ( G o l d s t e i n ; 1982, 1983). An SEM micrograph of c l a y i n a s t y l o l i t e seam w i t h i n a sandy dolostone from the E t h e r i n g t o n Formation ( F i g u r e 23) shows the alignment of p l a t y p h y l l o s i l i c a t e s s u r r o u n d i n g euhedral q u a r t z . I t can be seen here t h a t c a l c i t e has been p r e f e r e n t i a l l y d i s s o l v e d . The e u h e d r a l nature of the q u a r t z suggests t h a t quartz has a c t u a l l y p r e c i p i t a t e d as an overgrowth on an e x i s t i n g q uartz g r a i n w i t h i n the s t y l o l i t e seam. T h i s t h e o r y i s supported by the q u a r t z i n the s u r r o u n d i n g rock being w e l l rounded and l a c k i n g overgrowths. A f t e r removal from th e g r a i n s u r f a c e s , d i s s o l v e d i o n s d i f f u s e away from the p o i n t o f d i s s o l u t i o n and p r e c i p i t a t e i n a reas o f lower ( h y d r o s t a t i c ) s t r e s s . I f the system i s c l o s e d , t h e m a t e r i a l w i l l p r e c i p i t a t e i n nearby e x t e n s i o n f r a c t u r e s o r may d i f f u s e i n t o t h e pore spaces of t h e r o c k . I f the system i s open ( G e i s e r and Sansone, 1981), some o f the m a t e r i a l may be t r a n s p o r t e d out o f the r o c k . 51 Figure 23 - S E M micrograph illustrating the alignment of illite flakes (I) parallel to a stylolite seam within sandy dolostone from the Etherington Formation. Calcite (C) has been preferentially dissolved along the seam, whereas quartz (Q) appears to have precipitated within the seam as indicated by the euhedral crystal form. 52 The a b i l i t y o f c l a y t o c a t a l y z e the pr o c e s s o f pre s s u r e s o l u t i o n i s i n h e r e n t i n the c r y s t a l l o g r a p h i c s t r u c t u r e of the p h y l l o s i l i c a t e m i n e r a l s . The most common p h y l l o s i l i c a t e m i n e r a l s found i n the s t y l o l i t e seams a t O v e r f o l d Mountain are muscovite and c h l o r i t e . The muscovite and c h l o r i t e have l i k e l y formed by the r e c r y s t a l l i z a t i o n o f s m e c t i t e c l a y , a t r a n s i t i o n which t y p i c a l l y occurs at depths of b u r i a l l e s s than 4 km (Weaver, 1959) . The common s t r u c t u r e of these l a y e r e d m i n e r a l s i s a v a r i a t i o n on the 2:1 p h y l l o s i l i c a t e s t r u c t u r e ( F i g u r e 24). Each of the l a y e r s o f the p h y l l o s i l i c a t e i s made up of one sheet of o c t a h e d r a l l y c o o r d i n a t e d c a t i o n s ( A l3 +, F e3 +, Fe2 + and Mg2+) , sandwiched between two sheets o f t e t r a h e d r a l l y c o o r d i n a t e d c a t i o n s ( S i4 + and A l3 +) . S u b s t i t u t i o n between c a t i o n s o f unequal v a l e n c e s t a t e i n these sheets c r e a t e s a charge d e f i c i t which r e s u l t s i n a n e g a t i v e s u r f a c e charge on the l a y e r s ( P a u l i n g , 1930). T h i s s u r f a c e charge i s balanced by the c a t i o n s w i t h i n the i n t e r l a y e r . Each of the p h y l l o s i l i c a t e m i n e r a l s d i f f e r s i n t h e i r degree o f isomorphous s u b s t i t u t i o n and consequently i n the amount of s u r f a c e charge on the i n t e r l a y e r . Each m i n e r a l a l s o d i f f e r s i n the manner i n which t h i s charge i s n e u t r a l i z e d . The u n i t c e l l o f muscovite has a l a y e r charge of -2 e". T h i s charge i s l a r g e l y due t o the s u b s t i t u t i o n o f A l3 + f o r S i4 + i n the t e t r a h e d r a l sheet and i s n e u t r a l i z e d by potassium i o n s (K+) w i t h i n the i n t e r l a y e r ( F i g u r e 24). Potassium i s e s p e c i a l l y w e l l s u i t e d t o the muscovite 53 < - Interlayer Potassium ions MUSCOVITE ^ # 1 < - Interlayer hydroxide sheet CHLORITE H20 H20 H20 H20 H20 H20 H 2 O C + H 2 O H 2 O C + H 2 O <-Interlayer hydrated cations H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O SMECTITE Tetrahedrally coordinated cations (Si^  + , A l 3 + ) Octahedrally coordinated cations (Al 3 + , Fe 3 + , Fe^+, Mg2+) Figure 24 - Crystal structure of muscovite, chlorite, and smectite. 54 s t r u c t u r e i n t h a t i t f i t s very snugly i n t o the space c r e a t e d by the s i x c o o r d i n a t e d t e t r a h e d r a adjacent t o the i n t e r l a y e r . As a r e s u l t of t h i s snug f i t , and the r e l a t i v e l y low bond s t r e n g t h between K+ and H20, muscovite has a r e l a t i v e l y low c a t i o n exchange c a p a c i t y and would l i k e l y not be as important i n a i d i n g d i f f u s i o n d u r i n g p r e s s u r e s o l u t i o n as o t h e r , more r e a c t i v e c l a y m i n e r a l s . The c h l o r i t e u n i t c e l l has a l a y e r charge s i m i l a r t o t h a t o f muscovite (Dixon and Weed, 1977; p.342) again due t o the s u b s t i t u t i o n of t r i v a l e n t c a t i o n s f o r S i4 + i n the t e t r a h e d r a l s h e e t . T h i s n e g a t i v e charge i s n e u t r a l i z e d by a p o s i t i v e l y charged i n t e r l a y e r hydroxide sheet ( F i g u r e 2 4 ) . The c a t i o n exchange c a p a c i t y of c h l o r i t e i s r e l a t i v e l y low, i n d i c a t i n g t h a t i t i s probably of l i t t l e importance i n exchange r e a c t i o n s . C h l o r i t e and muscovite are a common product of the r e c r y s t a l l i z a t i o n of s m e c t i t e s . The smectite s t r u c t u r e i s w e l l s u i t e d t o pressure s o l u t i o n t h e o r i e s which i n f e r t h a t c l a y m i n e r a l s have p r o v i d e d a pathway f o r f l u i d movement and the d i f f u s i o n of ions through the system (Weyl, 1959; G e i s e r and Sansone, 1981). The 2:1 l a y e r s t r u c t u r e i s s i m i l a r t o muscovite and c h l o r i t e , but isomorphous s u b s t i t u t i o n i n both the o c t a h e d r a l and t e t r a h e d r a l sheets r e s u l t s i n a somewhat lower l a y e r charge (-0.66 e" per u n i t c e l l ) . T h i s charge i s n e u t r a l i z e d by hydrated c a t i o n s w i t h i n the i n t e r l a y e r ( F i g u r e 2 4 ) . These hydrated c a t i o n s are r e a d i l y exchanged, and d i f f u s i o n of c a t i o n s along t h i s i n t e r l a y e r occurs 55 relatively easily. The low layer charge also allows smectite layers to be pulled apart under small tensile stresses, a factor which may aid bulk fluid flow along the interlayer. Broken bonds on the edges of the clay structure cause the exposure of OH" groups from the tetrahedral and octahedral sheets. These OH" groups readily donate their protons, causing the clay to behave as a Lewis acid. This may in turn increase the dissolution rate of the carbonate minerals according to the reaction: CaC03 + 2H+ -> Ca2+ + C02 + H20 The negative charge at the broken edges may serve to attract cations toward the interlayer and thus may be effective in catalyzing the cation exchange reactions, >a factor which would increase the rate of diffusion and push the above reaction toward calcite dissolution. The edge charge created at the broken edges is known as "pH-dependent charge" due to the effect of pH on the magnitude of charge. Low pH conditions will inhibit the dissociation of the OH" groups (Dixon and Weed, 1977; p. 314) and decrease the rate of diffusion of cations. The catalysis of calcite dissolution by clays thus occurs by two processes. First, by the locally acidic conditions created at the broken smectite edges. Second, by the increased rate of diffusion of the products of dissolution away from the pressure solution site. 56 The presence of euhedral quartz w i t h i n the s t y l o l i t e seam may be e x p l a i n e d by p r e c i p i t a t i o n of quartz as overgrowths d u r i n g s t y l o l i t e f o r m a t i o n . The l o c a l l y a c i d i c c o n d i t i o n s w i t h i n the s t y l o l i t e seam may be r e s p o n s i b l e f o r quartz p r e c i p i t a t i o n , as quartz s o l u b i l i t y decreases w i t h a decrease i n pH. A s l i g h t l y d i f f e r e n t c o n d i t i o n i s i n v o l v e d i n the development of p r e s s u r e s o l u t i o n i n quartz a r e n i t e . Since the s o l u b i l i t y of quartz i s low below a pH of approximately 9, the a c i d i c nature of c l a y m i n erals would be expected to i n h i b i t p r e s s u r e s o l u t i o n . I n v e s t i g a t o r s have found however, t h a t c l a y m i n e r a l s do enhance p r e s s u r e s o l u t i o n i n q u a r t z i t e s (Heald, 1956; Weyl, 1959; De boer, 1977; Wanless, 1979; Marshak and E n g e l d e r , 1985; Engelder and Marshak, 1985). T h i s enhancement i s l i k e l y mechanical i n o r i g i n , the c l a y s p r o v i d i n g a f l u i d f i l m f o r the d i f f u s i o n of i o n s . U n l i k e c a l c i t e , the d i s s o l u t i o n of quartz i s known to be a slow p r o c e s s , even under i d e a l c o n d i t i o n s (high pH and h i g h temperature). T h i s f a c t o r , as w e l l as the e f f e c t of c l a y s , would e x p l a i n the c o n t r a s t i n the response of the two m i n e r a l s t o p r e s s u r e s o l u t i o n . 57 4 . METHOD OF DATA COLLECTION AND ANALYSIS A folded surface i s defined by three orthogonal axes; a, b, and c; where c i s perpendicular to bedding and b p a r a l l e l s the axis of folding (Figure 25; Hancock, 1985; P r i c e , 1967). S t y l o l i t e s , shear fractures, and s e m i - b r i t t l e shear zones each define t h e i r own set of kinematic axes (a, b, and c). These minor axes have a unique r e l a t i o n s h i p to the megascopic reference axes, and are the basis i n the .determination of the s t r a i n history induced i n the rock as folding progressed. The method of analysis of each of these structures follows. F i e l d work included the observation and measurement of mesoscopic structures found in each l i t h o l o g i c u n i t . Several stations along the limbs and hinge regions of the folds were chosen so that any variations i n s t r a i n and deformational mechanism might be determined. Oriented samples for th i n section analysis of microstructures were also c o l l e c t e d at each s t a t i o n . Station l o c a t i o n s , outcrop patterns of l i t h o l o g i c u n i t s , and s t r u c t u r a l data have been plotted on a 1:5000 topographic map (Map 1). 4 . 1 . EXTENSION FRACTURES The close r e l a t i o n s h i p between hydraulic f r a c t u r i n g and the permeability of the rock has been discussed i n Section 3.2.1. The magnitude of each fra c t u r i n g episode may be used as an indicator of the r e l a t i v e permeability of the Figure 25 - Three orthogonal axes described by a folded surface. The c - axis is perpendicular to bedding; the b - axis parallels the axis of folding. After Hancock (1985), and Price (1967). 59 rock over that i n t e r v a l . In other words, a higher density of fractures suggests greater permeability. Fractures with d i f f e r e n t orientations often cross-cut each other, revealing t h e i r r e l a t i v e ages . This information can be u t i l i z e d , along with the kinematic information inherent i n each fracture, i n the development of a kinematic hi s t o r y of the rock as well as a permeability history. At the outcrop, fractures were grouped into sets on the basis of p a r a l l e l i s m and cross-cutting relationships. The density of the fractures i n each set was then noted as well as the thickness, type, and habit of material f i l l i n g the fractures. Where possible, extension fractures were categorized as syntaxial or a n t i t a x i a l . 4 . 2 . SHEAR FRACTURES AND FAULTS The c l a s s i f i c a t i o n of shear fractures and fa u l t s was based on the presence of slickenside s t r i a e or v i s i b l e o f f s e t . The orientation of the fracture surface and, where possible, the slickenside s t r i a t i o n s were recorded. A slickensided fracture represents a unique set of kinematic axes (Figure 26). a-axis - The l i n e of the slickenside s t r i a e . b-axis - The axis of rotation of the s l i p , the l i n e which i s perpendicular to the slickenside s t r i a e i n the plane of the fracture. I f shearing i s concordant with f o l d development, t h i s axis w i l l p a r a l l e l the megascopic f o l d axis. c-axis - The l i n e normal to the fracture surface. Together, the c-axis and a-axis define the plane of deformation (the ac plane of the fold) Slickenside striae P , « n e of the shear fracture 26 This data, in conjunction with other kinematic indicators is an invaluable tool in the analysis of the kinematics of the rock. 4.3. SEMI-BRITTLE SHEAR ZONES In this context, shear zones are zones of en echelon, often sigmoidal, extension fractures which form within zones parallel to the theoretical shear direction. The sense of shear on these zones may be determined by the orientation of the fractures with respect to the zone (Figure 27). The kinematic axes defined by shear zones parallel those defined by slickensided shear fractures. The c-axis is the normal to the zone, the a-axis lies in the plane of the zone, in the direction of shear. The line of intersection between the shear zone and the associated extension fractures represents an axis of rotation, or 6-axis, for the zone. If shear zone development was concordant with fold development, this axis of rotation should parallel the megascopic fold axis. 4 . 4 . STYLOLITE SEAMS Stylolites describe a plane of dissolution, the normal to which represents a kinematic axis of compression (c-axis; Figure 28). The stylolites within the study area commonly have a ridge morphology, these ridges may also be used as kinematic indicators. The ridge crests appear to parallel the direction of fluid movement along the stylolite seam, this direction parallels the direction of minimum 62 B) c Line of intersection of the shear zone and the extension fractures Figure 2 7 - A) An array of shear zones within limestone (Unit 2 ) . B ) Kinematic axes described by a semi-brittle shear zone. The c-axis is the normal to the zone; the 6-axis is the line of intersection between the zone and the extension fractures within the zone; the a-axis is the direction of shear on the zone. c COLUMNAR STYLOLITES c RIDGE STYLOLITES Figure 28 - Kinematic axes described by columnar and ridge stylolites. The c-axis is normal to the stylolite seam, or parallel to the stylolite column when columns are oblique to the seam; the a-axis parallels the ridge crests in ridge stylolites. 64 compression ( d i r e c t i o n of minimum s t r a i n ) along the seam at the time of s t y l o l i t e development (kinematic a - a x i s ) . A t h i r d k i n e m a t i c a x i s i s represented by the l i n e p e r p e n d i c u l a r t o the r i d g e c r e s t s w i t h i n the plane of the s t y l o l i t e (/?-axis) . T h i s a x i s i s expected t o p a r a l l e l the megascopic f o l d a x i s i f concordant with f o l d i n g . On the mesoscopic s c a l e , s t y l o l i t e seams were separated i n t o s e t s based on t h e i r o r i e n t a t i o n and c r o s s - c u t t i n g r e l a t i o n s h i p s . D e n s i t i e s of s t y l o l i t e s e t s were measured and, where p o s s i b l e , the age r e l a t i o n s h i p s between s t y l o l i t e s and f r a c t u r e s were noted. The amplitude of the s t y l o l i t e r i d g e s and the t h i c k n e s s of the i n s o l u b l e r e s i d u e w i t h i n the seams were recorded t o a i d i n the d e t e r m i n a t i o n of the amount of m a t e r i a l removed by d i s s o l u t i o n . 65 4.5. DATA COLLECTED - MICROSCOPIC SCALE 4.5.1. EXTENSION FRACTURES AND PRESSURE SOLUTION S t r u c t u r e s v i s i b l e on the mesoscopic s c a l e , i n p a r t i c u l a r e x t e n s i o n f r a c t u r e s and s t y l o l i t e seams, are a l s o found on the m i c r o s c o p i c s c a l e . The o r i e n t a t i o n of each of these m i c r o s t r u c t u r e s was determined by u n i v e r s a l stage a n a l y s i s of t h r e e mutually p e r p e n d i c u l a r o r i e n t e d t h i n s e c t i o n s of each sample. M i c r o s t r u c t u r e s were grouped i n t o s e t s on the b a s i s of o r i e n t a t i o n and c r o s s - c u t t i n g r e l a t i o n s h i p s . The l i n e a r d e n s i t y , width, and type of f i l l were noted f o r each s e t . U n i t 1, the quartz a r e n i t e u n i t , showed some f l a t t e n i n g of quartz g r a i n s due t o pre s s u r e s o l u t i o n . The o r i e n t a t i o n s of the f l a t t e n e d c o n t a c t s were measured on the u n i v e r s a l s t a g e . U n f i l l e d m i c r o f r a c t u r e s were analyzed by f l u o r e s c e n c e microscopy. Rock s l a b s were impregnated under vacuum wit h a low v i s c o s i t y epoxy r e s i n (Epotek 301) c o n t a i n i n g the f l u o r e s c e n t dye Rhodamine B. Thi n s e c t i o n s of the s l a b s were then viewed i n blue l i g h t . M i c r o f r a c t u r e s were e a s i l y r e c o g n i z e d due t o the f l u o r e s c e n c e of the dye w i t h i n them. 4.5.2. TWIN LAMELLAE IN CARBONATES Where p o s s i b l e , the d e n s i t y of twin l a m e l l a e i n c a l c i t e and dolomite g r a i n s and cement was examined. Although t h i s method i s not a q u a n t i t a t i v e i n d i c a t o r of s t r a i n w i t h i n the rock, i t does give an indication of the importance of this mechanism in the deformation of different lithologic units. 4.5.3. DEFORMATION LAMELLAE IN QUARTZ In the units containing quartz grains, the percentage of quartz grains containing deformation lamellae was determined. This was done as an estimation of the importance of this mechanism in the accommodation of strain during folding. 67 5. UNIT DESCRIPTIONS AND OBSERVATIONS V a r i a t i o n i n l i t h o l o g y both between and w i t h i n i n d i v i d u a l f ormations has allowed ample o p p o r t u n i t y f o r the s e l e c t i o n o f d i s t i n c t l i t h o l o g i c u n i t s . The m i o g e o c l i n a l sediments found near O v e r f o l d Mountain i n c l u d e l i m e s t o n e , d o l o s t o n e , sandstone and s h a l e , each w i t h a unique g r a i n s i z e , bedding t h i c k n e s s , and d i a g e n e t i c h i s t o r y . Chosen f o r t h i s study were seven limestone u n i t s , two dolostone u n i t s , and one sandstone u n i t . 5.1. LITHOLOGIC UNITS Rocky Mountain Group U n i t 1 - Quartz a r e n i t e . Tan and p a l e grey weathering (5Y8/1), l i g h t t o medium grey, (N7), f i n e t o medium g r a i n e d c a l c a r e o u s q uartz a r e n i t e ; massive t o medium bedded wi t h minor t h i n c h e r t y beds; U n i t 1 has a moderate t o low r e s i s t a n c e t o weathering; t h i c k n e s s i s approximately 100 m. Two stages of cementation are seen i n t h i n -s e c t i o n (Figure 29). E a r l y q uartz overgrowths have been f o l l o w e d by a l a t e r , s p a r r y c a l c i t e cement which i s seen t o f i l l pore spaces and r e p l a c e quartz g r a i n s as w e l l as overgrowths. Mount Head Formation - Lower Carnarvon Member U n i t 2 - Limestone. Medium grey weathering (N8), medium brownish-grey (5Y4/1), p a r t i a l l y m i c r i t i z e d s k e l e t a l g r a i n s t o n e ( F i g u r e 3 0); c o n t a i n s r u s t y 68 Figure 29 - Photomicrograph of Unit 1, calcareous quartz arenite. Two stages of cementation are seen. Early quartz overgrowths (O) are replaced by sparry calcite (C). Long edge of photograph measures 0.06 mm. Figure 30 - Photomicrograph of partially micritized skeletal grainstone of Unit 2. Point contacts are common between micritized grains. Cement is sparry calcite. Long edge of photograph measures 0.06 mm. 70 weathering medium dark grey chert nodules up to 2 m in length; abundant large brachiopods and corals. This unit has a high resistance to weathering, and forms c l i f f s . Thickness is approximately 9 m. Unit 3 - Dolostone. Light grey, tan and orange weathering (5Y6/1), medium-dark grey (5Y4/1), massive, finely crystalline dolostone (Figure 31); contains abundant white to dark-grey chert nodules. Unit 3 has a low resistance to weathering and forms a recessive trench between Units 3 and 4. Thickness is approximately 6 m. Unit 4 - Limestone. Medium grey weathering (N7), medium grey-brown (5Y3/2), massive lime mudstone to wackestone (Figure 32); tan weathering, white to medium-grey weathering elongate chert nodules up to 10 cm. in length are common. Unit 4 has a great resistance to weathering and forms c l i f f s . Thickness'is approximately 2 m. Unit 5 - Dolostone. Light grey-tan and orange weathering (5Y6/1), dark grey (5Y5/2), massive finely crystalline dolostone (Figure 33). Unit 5 has a low resistance to weathering as did Unit 3. Thickness is approximately 4 m. Unit 6 - Limestone. Light olive-grey weathering (N5), dark brown to dark grey (N2), raicritized skeletal grainstone (Figure 34); grain size ranges from 71 Figure 31 - Photomicrograph of the finely crystalline dolostone of Unit 3. Dark grains are fine grained framboidal pyrite. Long edge of photograph measures 0.06 mm. Figure 32 - Photomicrograph of skeletal wackestone of Unit 4. Skeletal fragments are sparry and are suspended in a micritic matrix. Long edge of photograph measures 0.3 mm. 73 Figure 33 - Photomicrograph of the finely crystalline dolostone of Unit 5. Dark grains are fine grained framboidal pyrite. Long edge of photograph measures 0.06 mm. Figure 34 - Photomicrograph of the micritized skeletal grainstone of Unit 6. Dark areas are micritized skeletal fragments. Light areas are sparry calcite cement. Long edge of photograph measures 0.3 mm. 75 less than 10 um to 5 mm; modal grain size is approximately 0.5 mm. Unit 6 is highly resistant to weathering and is one of the main c l i f f forming units in the study area. Thickness is approximately 20 m. Mount Head Formation - Marston Member Unit 7 - Limestone. Medium grey-brown weathering (N5), dark grey (N2), coarse grained skeletal packstone to grainstone; contains abundant fossils including rugosen corals, crinoids, brachiopods and bryozoans; this unit is poorly sorted and has a modal grain size of approximately 2 mm; isolated dolomite crystals are found within the matrix of packstone samples; shaly beds occur near the stratigraphic base; this unit is moderately resistant to weathering and has a thickness of approximately 15 m. The percentage of matrix in this unit is seen to increase toward the northeast in the map area. The most southwesterly exposure of the unit is at the hinge of the anticline where it is a skeletal grainstone (Figure 35A). To the northeast, along the fold limbs, the unit is seen to be a packstone (Figure 3 5B). Mount Head Formation - Loomis Member Unit 8 - Limestone. Medium grey weathering (N5), medium grey (N5), massive, oolitic and skeletal Figure 35 - Photomicrographs from Unit 7. A) At the hinge: Unit 7 is a skeletal grainstone with sparry calcite cement and point contacts between grains. B) In the limbs, northeast of the hinge, Unit 7 is a skeletal packstone with sutured contacts between grains. Long edge of photograph is 0.3 mm for (A) and 0.06 mm for (B). 77 grainstone (Figure 3 6); grains are typically partially micritized; the grain size mode is approximately 2 mm and grains are moderately well sorted; calcite cement is sparry and the point contacts between grains suggests that cementation occurred during early diagenesis, before significant compaction. This unit is highly resistant to weathering; the combined thickness of Units 9 and 10 is greater than 30 m. 78 Figure 36 - Photomicrograph of the oolitic and skeletal grainstone of Unit 8. Long edge of photograph measures 0.3 mm. 79 6. OBSERVATIONS ON THE MEGASCOPIC SCALE - HINGE STYLE Lithologic units were chosen on the basis of mechanical properties as well as lithology. Each unit is mechanically unique, not only due to lithologic differences, but also due to physical properties such as bed thickness. While one unit was selected because i t exhibited an unusual hinge style, another was chosen due to an unusual high or low density of certain mesoscopic structures. These features are controlled by the deformational processes which have acted on the rock at the granular scale, but are inextricably linked to the mechanical integrity of the entire package of folded rock. The unique mechanical properties of each of these lithotypes is displayed throughout the study area on a l l scales of observation. On the megascopic scale, variations in mechanical properties are most readily viewed at the hinges of folds, where the most strain has been accommodated. Each unit is seen to have a unique hinge style (Section 5.2.1.). When the rocks are observed on the mesoscopic scale, fractures and stylolites are seen to be common. Some units show high densities of these structures whereas other, adjacent units, do not, indicating that the two units have accommodated strain in different ways. In other words, the strain has been partitioned differently between the two units. This strain partitioning is best observed on the microscopic scale, where i t can be seen that each unit is distinct in the way in which strain has been partitioned between 80 i n t r a g r a n u l a r and i n t e r g r a n u l a r mechanisms. A comparison of the d e n s i t i e s of the m i c r o s c o p i c and mesoscopic s t r u c t u r e s a i d s i n the d e t e r m i n a t i o n of the mechanisms of deformation t h a t have caused the f o l d i n g of each u n i t . 6.1 HINGE STYLE CONTROLS In the study a r e a , hinge s t y l e appears t o be governed by a few important p r o p e r t i e s of the i n d i v i d u a l u n i t s as w e l l as p r o p e r t i e s of nearby u n i t s . 1. The mineralogy of the u n i t . Some u n i t s behave r e l a t i v e l y d u c t i l e y and are able to bend around the hinge without a s i g n i f i c a n t l o s s of c o h e s i o n . T h i s category i n c l u d e s most of the limestone u n i t s (2, 6, 7, and 8 ) . Other u n i t s l a c k d u c t i l i t y and are more l i k e l y t o show s h e a r i n g a t the h i n g e . T h i s category i n c l u d e s the d olostone and sandstone u n i t s (Unit 1, 3 and 5 ) . 2. The t h i c k n e s s and massiveness of the u n i t . T h i c k massive u n i t s have no i n t e r n a l s l i p s u r f a c e s and o f t e n have broad, open h i n g e s . These u n i t s appear t o e x e r t c o n t r o l over the shape of the f o l d , w h ile o t h e r , t h i n n e r u n i t s conform t o the shape p r e s c r i b e d by these t h i c k , c o n t r o l l i n g u n i t s . T h i s category i n c l u d e s some of the l i m e s t o n e u n i t s (Unit 6 and 8) . 3. P r o x i m i t y t o nearby t h i c k , massive u n i t s . T h i s category i n c l u d e s limestones and d o l o s t o n e s of moderate t h i c k n e s s . U n i t s o v e r l y i n g massive u n i t s w i l l e i t h e r f o l l o w the open hinge s t y l e (Unit 7) or w i l l form a t i g h t e r hinge by s l i p 81 along the bedding surfaces or shaly beds (Unit 2, 3, 4 and 5). 6.2 HINGE STYLE - OBSERVATIONS UNIT 1 - Quartz arenite. The hinge of Unit 1 has a close chevron geometry with an interlimb angle of approximately 70° at the hinge (Figure 37). There is a great deal of shearing at this hinge, more so than at the hinges of the carbonate units. Limited exposure of this unit has not allowed thickness measurement. Units 2 through 6 represent a continuous stratigraphic sequence.: UNIT 2 - Limestone. Unit 2 has an open, circular hinge with an interlimb angle of approximately 90°. No loss of cohesion is apparent at this hinge (Figure 38). There has been no significant change in the thickness of this unit throughout the fold. UNIT 3 - Dolostone. Unit 3 is sandwiched between Units 2 and 4 and consequently has a fold geometry which is intermediate between the two. The interlimb angle is approximately 80° at the hinge. This unit shows much more fracturing than Unit 2 likely due to lithologic differences (Figure 38). There has been no significant change in thickness of this unit throughout the fold. Figure 37 - Close chevron geometry of the hinge of Unit 1. 84 UNIT 4 - Limestone. The hinge region of Unit 4 has a close chevron geometry with an interlimb angle of 65°. There is a great deal of shear fracturing along the hinge, likely due to the tight geometry. The tightness of this fold hinge is related to the decollement at the base of Unit 5 (Figure 39). There has been no significant change in the thickness of this unit throughout the fold. UNIT 5 - Dolostone. At the base of Unit 5 is a shale bed which has acted as a surface of decollement between Unit 5 and Unit 6. The hinge of Unit 5 has a close chevron geometry with an interlimb angle of 60°. The shale bed is discontinuous within the hinge suggesting that i t has been locally thinned and thickened to adjust to the room problem caused by folding (Figure 39). There is a thickening of this unit at the hinge, mainly due to shear fracturing. Unit 6 - Limestone. Unit 6 is a thick, massive unit with an open circular hinge. The interlimb angle is approximately 90° at the hinge. As mentioned, the top of this unit is a surface of decollement, and the units above have significantly tighter hinges. There has been no apparent change in the thickness of this unit throughout the fold. 86 Unit 7 - Limestone. Unit 7 is part of the well-bedded Marston Member sediments. The interbedded shales of this Member and the well bedded nature of the sediments has added to the ease of folding. In general the units of this Member conform to the dictates of the thick controlling units (Unit 8 and Unit 6). The hinge at Unit 7 is broad and has an open circular geometry with an interlimb angle of approximately 105° (Figure 40). There has been no significant change in the thickness of this unit throughout the fold. Unit 8 - Limestone. Unit 8 belongs to the very thick and massive Loomis Member. This unit has exerted a large amount of control over the fold style of the anticline. The hinge is broad and has an open, circular geometry with an interlimb angle of approximately 110° (Figure 43). There has been no significant change in the thickness of this unit throughout the fold. 87 88 Figure 41 - Open, circular geometry of hinge of Unit 8. 89 7. STRUCTURAL ANALYSIS. STRAIN PARTITIONING AND  PERMEABILITY HISTORY 7.1. UNIT 1 - ROCKY MOUNTAIN FORMATION: QUARTZ ARENITE U n i t 1 i s unique i n two r e s p e c t s . F i r s t , i t i s the only l i t h o l o g i c u n i t chosen which crops out at the hinge of the s y n c l i n e w i t h i n the study a r e a . Second, i t i s the only s i l i c i c l a s t i c l i t h o l o g y chosen f o r t h i s s t u d y . The d e f o r m a t i o n a l mechanisms which have accommodated f o l d i n g i n t h i s sandstone u n i t a r e , f o r the most p a r t , the same as those u t i l i z e d i n the f o l d i n g of the c a r b o n a t e s . I t i s the way i n which the s t r a i n has been p a r t i t i o n e d between mechanisms t h a t d i f f e r s , and p r o v i d e s a b a s i s f o r comparison. H y d r a u l i c and shear f r a c t u r e s are the p r e v a l e n t mesoscopic s t r u c t u r e s seen i n U n i t 1. S l i c k e n s i d e s are commonly found on shear f r a c t u r e s u r f a c e s . Only one s t y l o l i t e seam was observed, d i s p l a y i n g the r e l a t i v e unimportance of p r e s s u r e s o l u t i o n as a d e f o r m a t i o n a l mechanism i n t h i s u n i t . The most common s t r u c t u r e s observed on the m i c r o s c o p i c s c a l e are m i c r o f r a c t u r e s , deformation l a m e l l a e w i t h i n quartz g r a i n s , and f l a t t e n e d t o concavo-convex g r a i n c o n t a c t s r e s u l t i n g from p r e s s u r e s o l u t i o n . 7.1.1. EXTENSION FRACTURES Mesoscopic e x t e n s i o n f r a c t u r e s w i t h i n U n i t 1 are commonly u n f i l l e d . When f r a c t u r e f i l l i n g i s observed i t i s t y p i c a l l y composed of c a l c i t e or q u a r t z , u s u a l l y not b o t h . 90 These f i l l e d f r a c t u r e s are r a r e l y seen t o c r o s s - c u t each o t h e r , but when they do, quartz f i l l e d f r a c t u r e s are seen t o pre-date c a l c i t e f i l l e d f r a c t u r e s . T h i s suggests t h a t t h e r e was a change i n the f l u i d c omposition over the deformation i n t e r v a l from a s i l i c a r i c h f l u i d , r e f l e c t i n g the comp o s i t i o n of the surrounding r o c k , t o a carbonate r i c h f l u i d which may have o r i g i n a t e d from the u n d e r l y i n g carbonate r o c k s . The quartz f i l l e d f r a c t u r e s tend t o have a s y n t a x i a l form, whereas the c a l c i t e f i l l e d f r a c t u r e s have an a n t i t a x i a l form. Change i n f l u i d c omposition over the deformation i n t e r v a l i s supported by the apparent cementation h i s t o r y o f the sandstone. An e a r l y quartz cement forms overgrowths on quartz g r a i n s . Both the g r a i n s and the overgrowths have deformation l a m e l l a e , i n d i c a t i n g t h a t t h i s cement was p r e -d e f o r m a t i o n a l . The quartz cement has been p a r t i a l l y r e p l a c e d by a sp a r r y c a l c i t e cement which i s v i r t u a l l y u n s t r a i n e d , suggesting t h a t i t s emplacement was post-d e f o r m a t i o n a l (Figure 42) . The shortage o f c r o s s - c u t t i n g r e l a t i o n s h i p s i n U n i t 1 has made the d e t e r m i n a t i o n of the t i m i n g o f the d i f f e r e n t e pisodes o f ex t e n s i o n f r a c t u r i n g i m p o s s i b l e . However, some a s s e r t i o n s can be made from f r a c t u r e o r i e n t a t i o n and d e n s i t y . F i r s t , t h e r e i s one f r a c t u r e group which i s found i n a l l p a r t s of the s y n c l i n e as w e l l as the a n t i c l i n e ( F i g u r e 4 3 ) . T h i s group has an o r i e n t a t i o n which i s approximately normal t o the megascopic f o l d a x i s . Due t o 91 Figure 42 - Photomicrograph of the two stages of cementation in Unit 1. Post deformational sparry calcite cement (C) locally replaces early quartz overgrowths (O). Long edge of photograph measures 0.06 mm. A) Poles to Mesoscopic Extension Fractures Contour interval = 3 sigma (Kamb Method) Figure 43 - Stereonet plots of mesoscopic and microscopic extension fractures within Unit 1. Solid great circle represents bedding; dashed great circle represents the ac-plane of the syncline. 93 the f o r t u i t o u s o r i e n t a t i o n , t h i s f r a c t u r e has not been r o t a t e d from i t s o r i g i n a l o r i e n t a t i o n , r e g a r d l e s s of the t i m i n g of i t s development. D i r e c t c o r r e l a t i o n s can thus be made between the f o l d l i m b s . C r o s s - c u t t i n g r e l a t i o n s h i p s i n U n i t s 2 through 8 show t h a t t h i s f r a c t u r e s e t formed i n two p u l s e s . The f i r s t p u l s e was e a r l y i n the k i nematic h i s t o r y of the r o c k , p r e d a t i n g most f r a c t u r e s e t s throughout the study a r e a . These f r a c t u r e s are quartz f i l l e d i n U n i t 1 and may be p r e - t e c t o n i c i n o r i g i n or may have formed a t a v e r y e a r l y s t a g e of f o l d i n g . A second p u l s e o c c u r r e d l a t e i n the f o l d h i s t o r y , these f r a c t u r e s are among the youngest i n the study a r e a . F r a c t u r e s w i t h t h i s o r i e n t a t i o n are common i n b u c k l e f o l d s and are b e l i e v e d t o r e p r e s e n t f o l d a x i s p a r a l l e l e x t e n s i o n r e s u l t i n g from the room problem c r e a t e d d u r i n g f o l d i n g . M e s o f r a c t u r e s e t s which are p e r p e n d i c u l a r t o bedding are found a t most l o c a t i o n s i n the f o l d . I t i s p o s s i b l e t h a t some o f these f r a c t u r e s were formed d u r i n g sedimentary l o a d i n g , p r i o r t o d e f o r m a t i o n , although the obvious decrease i n t h e s e f r a c t u r e s toward the f o l d hinge would tend t o suggest t h a t most were s y n - d e f o r m a t i o n a l . Another mesoscopic f r a c t u r e s e t of i n t e r e s t i s sub-p a r a l l e l t o bedding ( F i g u r e 4 3 ) . I f these f r a c t u r e s are c o n cordant, then they were formed p r i o r t o , or a t an e a r l y stage o f t h e f o l d i n g . There i s l i t t l e v a r i a t i o n i n the d e n s i t i e s o f mesoscopic f r a c t u r e s between the f o l d l i m b s . There i s 94 however a d i s t i n c t decrease i n the d e n s i t i e s of these s t r u c t u r e s i n a l l o r i e n t a t i o n s at the f o l d h i n g e . There are s e v e r a l p o s s i b l e e x p l a n a t i o n s f o r t h i s phenomenon. 1. The p e r m e a b i l i t y of the sandstone was low. Under t h i s c o n d i t i o n , f l u i d s c o u l d not e n t e r the r o c k , and pore p r e s s u r e would not b u i l d up i n c r a c k s ( s t r a i n h a r d e n i n g ) . As a r e s u l t , shear f a i l u r e i n s t e a d of t e n s i l e f a i l u r e would o c c u r . 2. The f l u i d content of the sandstone was low. There would be no pore p r e s s u r e e f f e c t on the s t r e s s e d sandstone and thus very l i t t l e t e n s i l e d e formation. 3 . The d i f f e r e n t i a l s t r e s s was too h i g h a t the h i n g e . T h i s would r e s u l t i n the development of shear f r a c t u r e s at pore p r e s s u r e s lower than those r e q u i r e d f o r t e n s i l e f a i l u r e . b) M i c r o s c o p i c S c a l e M i c r o f r a c t u r e s i n U n i t 1 are very s h o r t , t y p i c a l l y o n l y c u t t i n g through one or two quartz g r a i n s . Where the f r a c t u r e s have not been healed by quartz they are u n f i l l e d . M i c r o f r a c t u r e s w i t h i n the quartz g r a i n s t y p i c a l l y have n u c l e a t e d a t g r a i n boundaries and cut d i r e c t l y through g r a i n s , although commonly they have propagated along the c o n t a c t s between g r a i n s and quartz overgrowths. In g e n e r a l t h e r e i s a very good c o r r e l a t i o n between the m i c r o f r a c t u r e s and the m e s o f r a c t u r e s . The v a r i a t i o n i n the o r i e n t a t i o n s of m i c r o f r a c t u r e s i n the d i f f e r e n t p a r t s o f the f o l d i s i l l u s t r a t e d i n the s t e r e o n e t s i n F i g u r e 4 3B. Three t r e n d s are r e a d i l y v i s i b l e i n the f o l d . 1. A l l p a r t s of the f o l d show the f r a c t u r e s e t which i s sub-normal t o the megascopic f o l d a x i s . The d e n s i t y of t h i s f r a c t u r e s e t decreases s l i g h t l y toward the f o l d h i n g e . 95 2. There i s a group of m i c r o f r a c t u r e s s u b - p a r a l l e l t o bedding and a group of m i c r o f r a c t u r e s p e r p e n d i c u l a r t o bedding i n the f o l d l i m b s . These f r a c t u r e s are not found i n the h i n g e . 3. There i s a h i g h d e n s i t y of m i c r o f r a c t u r e s which are p e r p e n d i c u l a r t o bedding and p a r a l l e l t o the f o l d a x i s at the h i n g e , p o s s i b l y due t o e x t e n s i o n a l s t r a i n induced by the bending of the beds. T h i s f r a c t u r e i s a l s o found on the mesoscopic s c a l e , only at the h i n g e . The v a r i a t i o n i n the o r i e n t a t i o n s of m i c r o f r a c t u r e s i n the s y n c l i n e suggests t h a t the e x t e n s i o n a l d i r e c t i o n at the time of f o l d i n g v a r i e d throughout the f o l d . T h i s o b s e r v a t i o n i s i n accordance with computer s i m u l a t i o n s of f o l d i n g by D i e t e r i c h (1970) . The t o t a l d e n s i t y of m i c r o f r a c t u r e s w i t h i n U n i t 1 i s f a i r l y c o n s t a n t throughout the f o l d . There i s however, a v a r i a t i o n i n the d e n s i t y of i n d i v i d u a l f r a c t u r e s e t s ( F i g u r e 43B). T h i s suggests t h a t m i c r o f r a c t u r e development took p l a c e a t a constant r a t e throughout the f o l d , and t h a t m i c r o f r a c t u r e o r i e n t a t i o n s were governed by the l o c a l s t r e s s c o n f i g u r a t i o n . In g e n e r a l , the c o r r e l a t i o n between the m i c r o f r a c t u r e s and m e s ofractures found i n the sandstone u n i t i s b e t t e r than t h a t found i n the carbonate u n i t s . M i c r o f r a c t u r e o r i e n t a t i o n data from t h i s u n i t a l s o show l e s s s c a t t e r than t h a t from the carbonate u n i t s . Both of these o b s e r v a t i o n s are l i k e l y due t o the l a c k of cleavage and twin planes i n the q u a r t z g r a i n s , s t r u c t u r e s which are known t o a c t as l o c i f o r f r a c t u r i n g i n the carbonates (Atkinson and M e r e d i t h , 1987). M i c r o f r a c t u r e s found i n quartz may thus 96 g i v e a b e t t e r r e p r e s e n t a t i o n of the t r u e s t r e s s c o n f i g u r a t i o n a t the time of t h e i r f o r m a t i o n . 7.1.2. SHEAR FRACTURES Shear f r a c t u r i n g appears to have been the most important mechanism f o r the accommodation of s t r a i n w i t h i n U n i t 1. The d e n s i t y of shear f r a c t u r e s i n t h i s u n i t i s c o n s i d e r a b l y h i g h e r than t h a t of the u n d e r l y i n g carbonate u n i t s , l i k e l y due t o the g r e a t e r p l a s t i c i t y of the c a r b o n a t e s . The importance of t h i s d e f o r m a t i o n a l mechanism i s i l l u s t r a t e d by the i n c r e a s e i n the d e n s i t y of c o a x i a l shear f r a c t u r e s w i t h p r o x i m i t y t o the s y n c l i n a l hinge A n a l y s i s of s l i c k e n s i d e d shear s u r f a c e s r e v e a l s two o r i e n t a t i o n s f o r the a x i s of s l i p ( 6 -axis). The predominant c a l c u l a t e d fo-axis i s approximately p a r a l l e l t o the megascopic f o l d a x i s , as expected. In the overturned l i m b , a second 6-axis has been c a l c u l a t e d which i s approximately 9 0 ° from the megascopic f o l d a x i s . S l i c k e n s i d e s on the l a t t e r shear s u r f a c e s approximately p a r a l l e l the megascopic f o l d a x i s and are non-coaxial with f o l d i n g ( F i g u r e 44). For reasons t h a t are not c l e a r , there i s an i n c r e a s e i n the abundance of n o n - c o a x i a l shear s u r f a c e s with d i s t a n c e from the hinge ( F i g u r e 44). The presence of a n o n - c o a x i a l phase of deformation i s a l s o demonstrated by the p r e s s u r e s o l u t i o n c o n t a c t s between quartz g r a i n s i n t h i s u n i t , as w e l l as by the s t y l o l i t e seams found i n the u n d e r l y i n g E t h e r i n g t o n Formation ( S e c t i o n 7.1.4) and Mount Head Formation ( S e c t i o n 7 . 2 - 7 . 4 ) . a) Poles to shear fractures. Contour interval = 3 sigma (Kamb method) -Ar- Pole to slickensided shear fracture (c-axis), with slip linear X Slickenside striae (a-axis) O Axis of slip determined by striae (7>-axis) • Megascopic fold axis Figure 44 - Mesoscopic shear fractures within Unit 1. Dashed great circle represents the megascopic ac plane, Solid great circle represents bedding. 98 7 . 1 . 3 . SEMI-BRITTLE SHEAR ZONES Shear zones are not abundant i n U n i t 1 . Where they are observed ( l o c a t i o n s D and G from F i g u r e 4 3 ) , they f o l l o w the same shear t r e n d s observed i n the nearby shear f r a c t u r e s ( F i g u r e 4 5 ) . 1 . L o c a t i o n D: two shear zones are found which d e f i n e an a x i s o f r o t a t i o n (6-axis) p a r a l l e l t o the megascopic f o l d a x i s . T h i s i s the same shear geometry d e f i n e d by the shear f r a c t u r e s at t h i s l o c a t i o n . 2. L o c a t i o n G: t h r e e shear zones d e f i n e an a x i s o f r o t a t i o n (6-axis) which i s n o n - c o a x i a l w i t h f o l d i n g . T h i s a x i s p a r a l l e l s the a x i s o f r o t a t i o n determined by the shear f r a c t u r e s a t t h i s l o c a t i o n . No c o a x i a l shear zones or shear f r a c t u r e s were found here. 7 .1.4. PRESSURE SOLUTION Pressure s o l u t i o n has been a c t i v e throughout t h i s quartz a r e n i t e u n i t , a l b e i t of r e l a t i v e l y l i t t l e importance i n the accommodation of s t r a i n d u r i n g f o l d i n g . L i t t l e evidence o f p r e s s u r e s o l u t i o n i s v i s i b l e on the mesoscopic s c a l e . On the m i c r o s c o p i c s c a l e however, g r a i n c o n t a c t s are seen t o have been f l a t t e n e d by pr e s s u r e s o l u t i o n . T h i s has a f f e c t e d both g r a i n s and quartz overgrowths, showing t h a t d i s s o l u t i o n o c c u r r e d a f t e r quartz cementation. C a l c i t e cement, on the oth e r hand, post-dates p r e s s u r e s o l u t i o n . Subhedral c a l c i t e c r y s t a l s are o f t e n found a l o n g f l a t t e n e d g r a i n c o n t a c t s . A crude f a b r i c has developed by the f l a t t e n i n g o f the g r a i n s . In the f o l d limbs the f a b r i c i s ve r y p o o r l y 99 G Pole to the shear zone (c-axis) Slip linear of the shear zone, arrow gives the sense of shear on the upper surface of the zone. Axis of rotation of the shear zone (b-axis), represents the line of intersection of the shear zone and the enclosed extension fractures. Direction of shear on zone (c-axis), represents the line normal to the 6-axis in the plane of the shear zone. Megascopic fold axis Figure 45 - Equal area projection of the mesoscopic shear zones within Unit 1. The great circle represents the megascopic ac plane. A D G - Poles to flattened contacts between quartz grains. Equal area projection, contour interval equal to 3 sigma (Kamb method). Dashed great circle represents the ac plane of the megascopic fold. Solid great circle represents bedding. For locations see Figure 7-2. 101 developed and p a r a l l e l s bedding. T h i s suggests t h a t i t s o r i g i n may be p r e - t e c t o n i c compaction ( F i g u r e 46; L o c a t i o n A and G). At the h i n g e , however, the f a b r i c i s o b l i q u e t o bedding and much b e t t e r developed. Here i t d e f i n e s an event of n o n - c o a x i a l compression which p a r a l l e l s the megascopic f o l d a x i s ( F i g u r e 46; L o c a t i o n D). The presence of t h i s f a b r i c a t the hinge suggests t h a t i t formed a f t e r f o l d development, and t h a t the t e c t o n i c a l l y enhanced p e r m e a b i l i t y at the hinge caused pressure s o l u t i o n t o be l o c a l i z e d t h e r e . Carbonate rocks belonging to the u n d e r l y i n g E t h e r i n g t o n Formation are exposed along the limbs of the s y n c l i n e w i t h i n the study a r e a . Although these rocks are not among the l i t h o l o g i c u n i t s chosen f o r t h i s study, s t r u c t u r a l data was c o l l e c t e d which g i v e s some i n s i g h t i n t o the d e f o r m a t i o n a l h i s t o r y of the t h i s f o l d . The carbonates w i t h i n t h i s Formation h e r e , developed s t y l o l i t e seams f a r more r e a d i l y than the sandstone of U n i t 1. A n a l y s i s of these seams, and the c o r r e s p o n d i n g s t y l o l i t e r i d g e s , supports the deduction t h a t t h e r e has been a p o s t - f o l d i n g event of compression sub-p a r a l l e l t o the megascopic f o l d a x i s . There are two w e l l developed s e t s of s t y l o l i t e s found i n both limbs of the s y n c l i n e ( F i g u r e 47). The f i r s t , and most abundant, i s p a r a l l e l t o bedding. A second group l i e s i n a plane which i s sub-normal t o the megascopic f o l d a x i s . Ridges a l o n g the f i r s t s e t of s t y l o l i t e s d e f i n e s a 6-axis which p a r a l l e l s the megascopic f o l d a x i s ( F i g u r e 47). P a r a l l e l i s m t o bedding suggests t h a t the s t y l o l i t e Northeast Limb Southwest Limb Pole of the stylolite seam (c-axis) ID Trend and Plunge of the ridge crests along the stylolite seam. Segment of the plane defined by the pole to the stylolite seam and the ridge crests along the seam (ac plane). O Pole to the ac plane defined by the stylolite seam (b-axis) 1<£ Pole to seam with ridges which have formed at an oblique angle to the seam. _ J _ Direction of maximum compression defined by stylolites with oblique ridges. • Megascopic fold axis Figure 47 - Equal area projection of mesoscopic stylolite seams within the Etherington Formation. The dashed great circle represents the megascopic ac plane. The solid great circle represents bedding. Contour interval = 3 sigma (Kamb method). 103 originally nucleated during sediment loading and continued to grow over the deformation interval. The latter set delineates a non-coaxial 6-axis, approximately 90° from the fold axis, sub-parallel to the non-coaxial 6-axis determined by the shear fracture analysis. The fact that the non-coaxial stylolite set parallels the non-coaxial plane of flattening seen at the hinge of Unit 1, suggests that these features are contemporaneous. If so, this stylolite set was formed after fold development and implies that there was a relatively large post-deformational influx of fluid during an interval of fold-axis-parallel compression. 7.1.5. DEFORMATION LAMELLAE IN QUARTZ There is an increase in the number of quartz grains containing deformation lamellae at the hinge of the syncline (Table 1). This indicates that the rocks are more highly strained in the hinge than in the limbs of the fold. Higher strain at the hinge is expected in folds with a chevron geometry such as this. 7.1.6. TWINNING IN CALCITE CEMENT The calcite crystals that form the second stage cement in this unit are virtually untwinned (Figure 42). This indicates that this cement was crystallized after the deformational episode. 7.1.7. SUMMARY - UNIT 1 The obvious increase in the density of coaxial shear fractures at the synclinal hinge shows that this is the most 104 important d e f o r m a t i o n a l mechanism i n the f o l d i n g o f sandstone near O v e r f o l d Mountain. E x t e n s i o n f r a c t u r e s have a marked d e n s i t y decrease a t the hinge r e g i o n on the mesoscopic s c a l e , and no v a r i a t i o n on the m i c r o s c o p i c s c a l e . C o a x i a l p r e s s u r e s o l u t i o n was of minor importance and has l i t t l e o r no v a r i a t i o n a c r o s s the f o l d hinge (Table 1 ) . A l l of t h e s e f a c t o r s suggest a low p o r o s i t y or f l u i d d e f i c i e n c y w i t h i n the sandstone a t t h e time o f f o l d i n g . TABLE 1 - DENSITIES OF MESOSCOPIC AND MICROSCOPIC STRUCTURES E x t e n s i o n f r a c t u r e Mesoscopic1 (per m) M i c r o s c o p i c1 (per cm) NE Limb 145 100 SW Limb 128 165 Hincre 34 151 Shear F r a c t u r e s C o a x i a l (per m2) Non- c o a x i a l (per m2) 24 15 32 5 S t y l o l i t e Seams 0 0 0 F l a t t e n e d G r a i n Contacts C o a x i a l N o n - c o a x i a l Low Low Low Low Low High Deformation Lamellae2 (per cm) 10 12 17 C a l c i t e Twin Lamellae ( i n cement) 0 0 0 1 Combined l i n e a r d e n s i t i e s 2 L i n e a r d e n s i t y o f qu a r t z o f a l l f r a c t u r e s e t s , g r a i n s w i t h l a m e l l a e . I t i s apparent from t h e s t r u c t u r a l data p r e s e n t e d t h a t t h e r e was a major s h i f t i n t h e s t r e s s c o n f i g u r a t i o n a f t e r t h e development of the s y n c l i n e a t O v e r f o l d Mountain. T h i s c o n c l u s i o n has been drawn by t h e s t r e s s a n a l y s i s o f 105 s l i c k e n s i d e d shear f r a c t u r e s , shear zones and f l a t t e n e d c o n t a c t s between quartz g r a i n s , as w e l l as s t y l o l i t e s w i t h i n the u n d e r l y i n g E t h e r i n g t o n Formation. 1. A n a l y s i s of s t y l o l i t e data and f l a t t e n e d g r a i n c o n t a c t s suggests t h a t l a t e i n the d e f o r m a t i o n a l h i s t o r y , the maximum compressive s t r e s s was s u b - p a r a l l e l t o the megascopic f o l d a x i s . The c o r r e s p o n d i n g i n t e r m e d i a t e a x i s o f compression (6-axis) i s determined t o be toward the n o r t h e a s t . 2. A n a l y s i s o f shear f r a c t u r e data d e l i n e a t e s two o r i e n t a t i o n s f o r an a x i s of r o t a t i o n (approximately equal t o the int e r m e d i a t e compressive s t r e s s ) . The most common o r i e n t a t i o n i s p a r a l l e l t o the megascopic f o l d a x i s , as expected. However, at some time(s) i n the d e f o r m a t i o n a l h i s t o r y t h i s a x i s appears t o have switched t o a p o s i t i o n approximately 9 0 ° from t h i s f o l d a x i s , toward the n o r t h e a s t . 3. A n a l y s i s o f e x t e n s i o n f r a c t u r e s i n d i c a t e s t h a t a t s e v e r a l times i n the d e f o r m a t i o n a l h i s t o r y , the minimum compressive s t r e s s was s u b - p a r a l l e l t o the megascopic f o l d a x i s . T h i s phenomena i s c o a x i a l with f o l d i n g and i s commonly observed i n f l e x u r a l - s l i p f o l d s . There appears t o have been an i n f l u x o f f l u i d s i n t o the system a t the time o f the l a t t e r , n o n - c o a x i a l , e p i s o d e . T h i s i s evidenced by the i n c r e a s e d i n c i d e n c e of p r e s s u r e s o l u t i o n , both i n U n i t 1 and i n the u n d e r l y i n g E t h e r i n g t o n Formation c a r b o n a t e s . Pressure s o l u t i o n i n U n i t 1 i s l o c a l i z e d a t the s y n c l i n a l hinge, p o s s i b l y due t o the enhanced p e r m e a b i l i t y t h e r e . T h i s enhanced p e r m e a b i l i t y may a l s o e x p l a i n the absence of n o n - c o a x i a l shear f r a c t u r i n g a t the h i n g e . 106 7.2. UNITS 2 THROUGH 6 - UPPER CARNARVON MEMBER U n i t s 2 through 6 represent a continuous s t r a t i g r a p h i c sequence of interbedded limestone and d o l o s t o n e w i t h i n the Lower Carnarvon Member of the Mount Head Formation. Each of these u n i t s has a unique aspect t h a t has caused i t t o behave d i f f e r e n t l y from the ot h e r s d u r i n g f o l d i n g . Evidence f o r t h i s i s seen throughout the f o l d as a v a r i a t i o n i n the d e n s i t i e s o f f r a c t u r e s , s t y l o l i t e s and shear zones i n adjacent u n i t s . There are two f a c t o r s which appear t o have c o n t r o l l e d the way i n which s t r a i n has been accommodated w i t h i n t h i s sequence. F i r s t i s l i t h o l o g y - limestones simply accommodate s t r a i n d i f f e r e n t l y than d o l o s t o n e s . Second, and more important i n the study a r e a , i s the t h i c k n e s s of the u n i t . T h i c k , massive u n i t s such as U n i t 6 have c o n t r o l l e d the s t y l e o f f o l d i n g o f the t h i n n e r beds above and below. Dolostone u n i t s are seen t o have behaved d i f f e r e n t l y than the ad j a c e n t limestone u n i t s . T h i s i s p r i n c i p a l l y due t o the c o n t r a s t i n the behavior of the m i n e r a l c o n s t i t u e n t s , dolomite and c a l c i t e . C a l c i t e i s more s o l u b l e , and t h e r e f o r e w i l l undergo pr e s s u r e s o l u t i o n more e a s i l y than d o l o m i t e . A l s o , c a l c i t e forms d e f o r m a t i o n a l twin l a m e l l a e much more r e a d i l y than does d o l o m i t e . As a r e s u l t , limestone i s i n h e r e n t l y more d u c t i l e than d o l o s t o n e , a f a c t o r which i s e v i d e n t on a l l s c a l e s o f o b s e r v a t i o n . 107 V e i n d e n s i t i e s may be used q u a l i t a t i v e l y as a guide t o the p e r m e a b i l i t y of the rock a t the time of t h e i r formation ( S e c t i o n 7.2.1). Dolostones are seen t o have a very h i g h d e n s i t y of t h i n c a l c i t e f i l l e d or u n f i l l e d m i c r o f r a c t u r e s . Limestones, on the othe r hand, tend t o have a low d e n s i t y of t h i c k c a l c i t e f i l l e d f r a c t u r e s . T h i s d i s s i m i l a r i t y i s a r e s u l t of the d i f f e r e n t p e r m e a b i l i t i e s of the rock at the time of f r a c t u r e f o r m a t i o n . A hig h f r a c t u r e d e n s i t y marks deformation under c o n d i t i o n s of low p e r m e a b i l i t y . Under these c o n d i t i o n s f l u i d escape i s i m p o s s i b l e , and pore p r e s s u r e w i l l b u i l d up a t many d i s c r e t e p o i n t s w i t h i n the r o c k . M i c r o f r a c t u r e s formed a t these p o i n t s may remain u n f i l l e d , as f l u i d s cannot migrate i n t o them. These u n f i l l e d m i c r o f r a c t u r e s impart a secondary p o r o s i t y t o what would otherwise be a r e l a t i v e l y impermeable rock and may be l a r g e l y r e s p o n s i b l e f o r the f r a c t u r e p o r o s i t y found i n many hydrocarbon r e s e r v o i r s . S t r u c t u r a l t r e n d s found i n the s y n c l i n e are repeated i n the a n t i c l i n e . Two d i s t i n c t phases of deformation are found i n the a n t i c l i n e , a c o a x i a l phase and a n o n - c o a x i a l phase. During the n o n - c o a x i a l phase, the maximum compressive s t r e s s appears t o have been s u b - p a r a l l e l t o the megascopic f o l d a x i s . C r o s s - c u t t i n g r e l a t i o n s h i p s show t h a t s t y l o l i t e s formed d u r i n g the n o n - c o a x i a l phase formed l a t e r than the c o a x i a l s t y l o l i t e s . The h i g h e r d e n s i t y o f n o n - c o a x i a l s t y l o l i t e s at the hinge f u r t h e r e n f orces the b e l i e f t h a t these s t r u c t u r e s are p o s t - f o l d i n g . 108 C r o s s - c u t t i n g r e l a t i o n s h i p s between v e i n and s t y l o l i t e s e t s o f d i f f e r e n t ages suggest t h a t t h e r e may have been s e v e r a l episodes of f l u i d i n f l u x i n t o the deforming system. A p e r i o d o f f l u i d i n f l u x i s marked by the presence of a young, w e l l developed s t r u c t u r e which cu t s an o l d e r , p o o r l y developed, low d e n s i t y s t r u c t u r e . F r a c t u r e s formed d u r i n g these events are c o a x i a l and do not have a c o n s i s t e n t o r i e n t a t i o n . Thus, they cannot be r e l a t e d t o a s i n g l e episode of f l u i d i n f l u x . I n s t e a d , they appear t o have formed c o n t i n u o u s l y over the i n t e r v a l of de f o r m a t i o n . 7 . 2 . 1 . EXTENSION FRACTURES E x t e n s i o n f r a c t u r e s w i t h i n U n i t s 2 through 6 are t y p i c a l l y c a l c i t e f i l l e d . C a l c i t e c r y s t a l s w i t h i n t h e s e f r a c t u r e s have two forms: f i b r o u s and b l o c k y . F i b r o u s c a l c i t e , the growth form, i s a v a l u a b l e kinematic i n d i c a t o r (Chapter 4 ) . U n f o r t u n a t e l y , most of the c a l c i t e w i t h i n the f r a c t u r e s has been r e c r y s t a l l i z e d t o a blocky form which i s u s e l e s s as a kinematic i n d i c a t o r . Where f i b r o u s c a l c i t e i s observed i n v e i n s i t i s p e r p e n d i c u l a r t o the v e i n w a l l and does not show any i n d i c a t i o n of p r o g r e s s i v e s t r a i n (Durney and Ramsay, 1977). Under these c o n d i t i o n s , the p o l e t o the v e i n s u r f a c e r e p r e s e n t s the minimum compressive s t r e s s a t the time o f i t s f o r m a t i o n , and the maximum and i n t e r m e d i a t e p r i n c i p a l s t r e s s e s l i e w i t h i n the plane of the v e i n . S t y l o l i t e s d e l i n e a t e two phases of deformation near O v e r f o l d Mountain ( S e c t i o n 7.2.4.). E x t e n s i o n f r a c t u r i n g a s s o c i a t e d w i t h both of these phases i s expected. 109 U n f o r t u n a t e l y , i t i s im p o s s i b l e t o sepa r a t e the v e i n s a s s o c i a t e d w i t h the l a t e r phase, which was no n - c o a x i a l with f o l d i n g , from those formed i n the l a t e stages of the c o a x i a l phase. During the non-coaxial phase, the maximum compressive s t r e s s was s u b - p a r a l l e l t o the megascopic f o l d a x i s and the in t e r m e d i a t e s t r e s s appears t o have been n e a r l y h o r i z o n t a l and t o the n o r t h e a s t . I f v e i n s were formed d u r i n g t h i s phase they would d i p s h a l l o w l y t o the sout h e a s t , as do many of the v e i n s a s s o c i a t e d w i t h the c o a x i a l phase. C r o s s - c u t t i n g r e l a t i o n s h i p s show no c o n s i s t e n t l a t e stage v e i n w i t h t h i s o r i e n t a t i o n . C r o s s - c u t t i n g r e l a t i o n s h i p s between c o a x i a l v e i n s of d i f f e r e n t o r i e n t a t i o n s d e s c r i b e a complex h i s t o r y of ex t e n s i o n f r a c t u r i n g which l o c a l l y d i f f e r s from the i d e a l model. Some c o n s i s t e n t f r a c t u r i n g i s found however. The e a r l i e s t f r a c t u r e found i n the study area i s p e r p e n d i c u l a r t o bedding and p a r a l l e l the f o l d a x i s ( F i g u r e 48). T h i s v e i n i s b e l i e v e d t o have formed p r i o r t o deformation, due t o sediment l o a d i n g . The e a r l i e s t s t y l o l i t e s found are p e r p e n d i c u l a r t o these v e i n s ( p a r a l l e l t o beddin g ) , and l i k e l y formed a t the same time. The second v e i n found wi t h some c o n s i s t e n c y throughout the study area i s p a r a l l e l t o bedding ( F i g u r e 48). I t i s b e l i e v e d t h a t t h i s v e i n formed d u r i n g c o a x i a l s h o r t e n i n g j u s t p r i o r t o the onset of f o l d i n g . S t y l o l i t e s a s s o c i a t e d w i t h t h i s v e i n are p e r p e n d i c u l a r t o bedding and p a r a l l e l t o the f o l d a x i s . A t h i r d v e i n s e t i s p e r p e n d i c u l a r t o the f o l d a x i s . T h i s s e t 6, 110 Stage 1 - Sediment Loading i i i i i r r r T l I Fractures - Perpendicular to Bedding 6 3 4. Stylolites - Parallel Bedding Stage 2 - The Onset of Tectonic St resses 6a-> T Z L 1 r Fractures - Low Angle to Bedding Stylolites - High Angle to Bedding Previously Formed Fractures/Stylolites Figure 48 - Orientations of fractures and stylolites formed during compaction (Stage 1), and early stages of folding (Stage 2). I l l i s v e r y w e l l developed throughout the f o l d . C r o s s - c u t t i n g r e l a t i o n s h i p s show t h a t t h i s s e t formed c o n t i n u o u s l y over the i n t e r v a l of f o l d i n g . T o t a l f r a c t u r e d e n s i t y and bulk d i l a t i o n f o r m i c r o f r a c t u r e s and mesofractures have been p l o t t e d f o r the d i f f e r e n t u n i t s i n the d i f f e r e n t p a r t s of the f o l d ( F i g u r e 49 and 50). There i s a decrease i n f r a c t u r e d e n s i t y a t the hinge and a c o r r e s p o n d i n g i n c r e a s e i n b u l k d i l a t i o n . T h i s i n f e r s t h a t the p e r m e a b i l i t y of the rocks a t the hinge was h i g h e r than t h a t i n the limbs d u r i n g f o l d development. In rocks of high p e r m e a b i l i t y , f l u i d p r e s s u r e w i l l not b u i l d up and few h y d r a u l i c f r a c t u r e s are n u c l e a t e d . As a r e s u l t , those which are n u c l e a t e d w i l l continue t o propagate due t o s t r e s s c o r r o s i o n a t the f r a c t u r e t i p s ( S e c t i o n 3.2.1.). A l s o as a r e s u l t of h i g h p e r m e a b i l i t y , f l u i d s can r e a d i l y migrate i n t o the f r a c t u r e s as they open and the width of the f r a c t u r e can grow i n p r o p o r t i o n t o i t s l e n g t h . These t h i c k mesofractures are r e s p o n s i b l e f o r the accommodation of a g r e a t d e a l of e x t e n s i o n a l s t r a i n . In comparison, u n f i l l e d m i c r o f r a c t u r e s , such as those commonly found i n the study a r e a , accommodate very l i t t l e permanent s t r a i n . The d i l a t i o n a s s o c i a t e d w i t h these f r a c t u r e s was e l a s t i c , and l a s t e d o n l y as l o n g as the h i g h s t r e s s e s were i n p l a c e . T h i n s e c t i o n s impregnated with f l u o r e s c e n t dye r e v e a l e d no u n f i l l e d m i c r o f r a c t u r e s i n the t h i c k massive lim e s t o n e u n i t s ( U n i t s 2 and 6 ) . The dolostone u n i t s ( U n i t s 3 and 5) are seen t o have an extremely hi g h f r a c t u r e p e r m e a b i l i t y , UNIT 3 UNIT 5 400 CO z 300 LLJ OC 3 h-O < ce ^ 200-z o CO z Ld X 100 N o r t h e a a t L i a b H l n q * S o u t l i v c t a t L l a b / \ / \ / \ / \ / \ 3 4 LOCATION 400 -00 CO z U) D o a ACTU - z cc o u 200-: o ILAT z o Q to - z • Ul UL X 100: : CN CD N a r t h M l t L l B b ,' H l j y j . — I — r — r -3 4 5 LOCATION S o u t h v a s t L i a h LOCATIONS Q ^: : o 3 •M m Mesoscopic fractures (per m) Microscopic fractures (per cm) Bulk dilation (%) Figure 49 - Mesoscopic extension fracture density, microfracture density and bulk dilation plotted against location within the anticline, for two dolostone units within the Lower Carnarvon Member (Units 3 and 5). Bulk dilation has been estimated from the linear densities and thickness of all sets of extension fractures. UNIT 2 UNIT 4 UNIT 6 LOCATION 200 2 1 5 0 u < ioo H z g z I— 5 0 Northaaat Limb —I 1 1— 3 4 5 1 0CAII0N Mesoscopic fractures (per m) Microscopic fractures (per cm) Bulk dilation (%) Figure 50 - Mesoscopic extension fracture density, microfracture density and bulk dilation plotted against location within the anticline, for three limestone units within the Lower Carnarvon Member (Units 2, 4, and 6). Bulk dilation has been estimated from the linear densities and thickness all sets of extension fractures. Fold locations refer to Figure 49. 1 1 4 Figure 51 - Thin section of Unit 3 impregnated with fluorescent dye. Fluorescence reveals fracture porosity along many grain boundaries not visible under plane light. A) Plane light; B) Blue light. Long edge of photograph measures approximately 0.06 mm. Figure 52 - Thin section of Unit 5 impregnated with fluorescent dye. Fluorescence reveals abundant fracture porosity surrounding dolomite crystals and only limited fracture porosity within a remnant calcite clast. A) Plane light; B) Blue light. Long edge of photograph measures 0.06 mm. Figure 53 - Thin section of Unit 4 impregnated with fluorescent dye. Fluorescence reveals fracture porosity along grain boundaries, especially around disseminated dolomite rhombs. A) Plane light; B) Blue light. Long edge of photograph measures 0.06 mm. 117 f l u o r e s c e n c e surrounding n e a r l y every g r a i n ( F i g u r e 51 and 52 ) . U n i t 4, a l s o l i m e s t o n e but t h i n n e r than U n i t s 2 and 6, was found t o have some u n f i l l e d m i c r o f r a c t u r e s , though not n e a r l y t o the ext e n t o f the do l o s t o n e u n i t s ( F i g u r e 5 3 ) . 7.2.2. SHEAR FRACTURES AND FAULTS Evidence f o r bedding p a r a l l e l shear i s found throughout the a n t i c l i n e i n the form o f s l i c k e n s i d e d bedding s u r f a c e s i n t h e f o l d limbs and bedding p a r a l l e l decollement a t the hinge (at the base o f U n i t 5 ) . Shear f r a c t u r e s which are o b l i q u e t o bedding, however, are r a r e , both i n the d o l o s t o n e s and the limestones i n t h e Lower Carnarvon Member. T h i s may r e s u l t from the tendency f o r carbonates t o accommodate s t r a i n by d u c t i l e p r o c e s s e s (twinning and s t y l o l i t e s ) such t h a t the c r i t i c a l s t r e s s f o r s h e a r i n g i s not r e a c h e d . F a u l t s which are o b l i q u e t o bedding, on t h e o t h e r hand, are r e l a t i v e l y abundant. T h i s would suggest t h a t s h e a r i n g a t an angle t o bedding i s r e s t r i c t e d t o zones a l o n g which r e l a t i v e l y l a r g e amounts o f movement have taken p l a c e . Two phases o f f a u l t i n g a re seen i n these u n i t s . A phase which i s c o a x i a l w i t h f o l d i n g i s common i n the ove r t u r n e d limb o f the f o l d ( F i g u r e 5 4 ) . S l i c k e n s i d e s on these f a u l t s u r f a c e s r e v e a l a 6-axis which i s s u b - p a r a l l e l t o the megascopic f o l d a x i s . Normal f a u l t s , which r e p r e s e n t a l a t e r , p o s t - f o l d i n g episode o f e x t e n s i o n , are e v i d e n t i n the v i c i n i t y o f the hinge ( F i g u r e 5 5 ) . These s t r u c t u r e s may have formed contemporaneously w i t h l a t e stage e x t e n s i o n a l 118 Figure 54 - Three common shear geometries for faults and shear fractures formed during folding near Overfold Mountain. Modified after Hancock (1984). 119 Figure 55 - Orientation of post-folding normal faulting near Overfold Mountain. 120 s t r u c t u r e s such as the F l a t h e a d F a u l t , l o c a t e d t o the e a s t . No f a u l t s o r shear f r a c t u r e s were found i n these u n i t s t h a t c o r r e l a t e with the episode of f o l d a x i s p a r a l l e l compression d e l i n e a t e d by s t y l o l i t e s . 7.2.3. SHEAR ZONES S e m i - b r i t t l e shear zones are f a r more common i n the lime s t o n e s than i n the d o l o s t o n e s near O v e r f o l d Mountain. T h i s may be a f u n c t i o n of the r e l a t i v e l y low p e r m e a b i l i t y and low d u c t i l i t y of the dol o s t o n e u n i t s . Both f a c t o r s would cause the do l o s t o n e s t o accommodate shear s t r a i n by b r i t t l e mechanisms ( u n f i l l e d m i c r o f r a c t u r e s and shear f r a c t u r e s ) r a t h e r than s e m i - b r i t t l e mechanisms (shear z o n e s ) . Shear zone o r i e n t a t i o n s ( F i g u r e 56) are c o n s i s t e n t w i t h the o r i e n t a t i o n s of shear f r a c t u r e s and f a u l t s w i t h i n the study area ( F i g u r e 54). A l l of the shear zones found are c o a x i a l w i t h f o l d development and appear t o have formed a t some time between the i n i t i a t i o n of f o l d i n g and the c l o s u r e of the f o l d . These zones are l o c a l l y abundant i n the limbs of the a n t i c l i n e and absent a t the hi n g e . E x t e n s i o n f r a c t u r e s w i t h i n shear zones are e i t h e r p e r p e n d i c u l a r t o the f o l d a x i s ( r a r e ) , or have a f r a c t u r e t o bedding angle between 4 5 ° and 6 0 ° (common). The s t r e s s c o n f i g u r a t i o n d e s c r i b e d by the l a t t e r zones suggests t h a t they are a s s o c i a t e d with bedding p a r a l l e l shear such t h a t the maximum compressive s t r e s s was a t an angle of approximately 4 5 ° t o bedding a t the time o f t h e i r f o r m a t i o n . T h i s would have Figure 56 - Semi-brittle shear zone orientations found within the limbs of the anticline at Overfold Mountain. 122 occurred a t some time a f t e r the onset of f o l d i n g , when the beds had been r o t a t e d t o a p o s i t i o n 4 5 ° from the maximum compressive s t r e s s . In t h i s o r i e n t a t i o n , the bedding planes would have been i n the f i e l d of maximum shear s t r e s s , and a h i g h percentage of s t r a i n would have been accommodated by bedding p a r a l l e l shear. Since bedding p l a n e s at the hinge were never r o t a t e d i n t o a p o s i t i o n of maximum shear s t r e s s , shear zones d i d not form t h e r e . 7.2.4. PRESSURE SOLUTION S t y l o l i t e s w i t h i n the Mount Head Formation have s i m i l a r t rends t o those found i n the o v e r l y i n g E t h e r i n g t o n Formation. As w i t h the o v e r l y i n g r o c k s , two phases of s t y l o l i t e development are seen, a c o a x i a l phase and a non-c o a x i a l phase. In U n i t s 2 through 6, the phase of p r e s s u r e s o l u t i o n which i s c o a x i a l with f o l d i n g i s w e l l developed throughout the f o l d whereas the n o n - c o a x i a l phase i s mainly found i n the overturned limb and the h i n g e . S e v e r a l f a c t o r s are r e s p o n s i b l e f o r the d e n s i t y v a r i a t i o n between s t y l o l i t e s of d i f f e r e n t ages. Most important are l i t h o l o g y , p e r m e a b i l i t y , and the amount of compressional s t r a i n the rock has been r e q u i r e d t o accommodate. An attempt has been made t o determine the importance of t h i s mechanism i n the accommodation of compressional s t r a i n i n the d i f f e r e n t l i t h o l o g i e s , i n d i f f e r e n t p a r t s of the f o l d , and the i m p l i c a t i o n s f o r p e r m e a b i l i t y and f l u i d flow over the deformation i n t e r v a l . 123 The amount of m a t e r i a l removed along a s t y l o l i t e can be estimated from the amplitude o f the s t y l o l i t e s u t u r e s . In some c a s e s , the amplitude o f the sutures can g i v e a b e t t e r estimate of the s t r a i n accommodated by a group of s t y l o l i t e s than the s t y l o l i t e d e n s i t y . T h i s i s p a r t i c u l a r l y t r u e i n l i t h o l o g i e s o f low p e r m e a b i l i t y and low c l a y content such as U n i t s 2 through 6. In rocks such as t h e s e , s t y l o l i t e n u c l e a t i o n i s i n h i b i t e d by the l a c k of a d i f f u s i o n pathway f o r the d i s s o l v e d m a t e r i a l . Once a s t y l o l i t e has n u c l e a t e d and a sm a l l amount of c l a y i s concentrated w i t h i n i t s s e l v a g e , the r a t e o f d i f f u s i o n along the seam i s i n c r e a s e d , and so the r a t e o f d i s s o l u t i o n i s i n c r e a s e d (Engelder and Marshak, 1985). Evidence f o r c l a y c a t a l y s i s of d i s s o l u t i o n i n s t y l o l i t e f ormation i s found throughout the study area as a v a r i a t i o n i n the d e n s i t i e s and o r i e n t a t i o n s of the s t y l o l i t e s o f d i f f e r e n t ages. As the beds are r o t a t e d d u r i n g f o l d i n g , p r e v i o u s l y formed s t y l o l i t e s are moved t o a p o s i t i o n t h a t i s no l o n g e r normal t o the maximum compressive s t r e s s . One might expect t o f i n d younger s t y l o l i t e s which have n u c l e a t e d i n response t o the new s t r e s s f i e l d . In f a c t , t h i s i s r a r e l y the c a s e . Where younger s t y l o l i t e s a re found, they are v e r y p o o r l y developed. There are two p o s s i b l e e x p l a n a t i o n s f o r t h i s phenomenon. F i r s t , i t i s p o s s i b l e t h a t the rock was dry a f t e r the onset of f o l d i n g and t h e r e were no f l u i d s t o i n i t i a t e p r e s s u r e s o l u t i o n . Second, and more l i k e l y , i s t h a t compressive s t r a i n was taken up by the 124 p r e - e x i s t i n g s t y l o l i t e s . T h i s would support the id e a t h a t the a c t i v a t i o n energy r e q u i r e d f o r the d i s s o l u t i o n of carbonates i n the absence o f c l a y i s s i g n i f i c a n t l y h i g h e r than t h a t f o r the r e a c t i o n which has been c a t a l y z e d by even a s m a l l amount of c l a y (Engelder and Marshak, 1985; G e i s e r and Sansone, 1981). As a r e s u l t , rocks w i t h a low p e r m e a b i l i t y and low c l a y content w i l l tend t o have a low s t y l o l i t e d e n s i t y , but the s t y l o l i t e s formed w i l l have a l a r g e s u t u r e amplitude. S i n c e the s t y l o l i t e s u t u r e s found i n the study area have a r i d g e s t r u c t u r e r a t h e r than a columnar s t r u c t u r e , and s i n c e the r i d g e s p a r a l l e l the ac plane o f the f o l d ; the s u t u r e s are p e r p e n d i c u l a r t o the seam and thus t h e r e i s no kinematic marker t o show t h a t the s t y l o l i t e s grew i n an o r i e n t a t i o n which was o b l i q u e t o the maximum compressive s t r e s s ( F i g u r e 57). Suture amplitudes from the s t y l o l i t e s found i n the limbs and hinge o f the a n t i c l i n e have been contoured i n F i g u r e 58. In the overturned l i m b , t h r e e c o n c e n t r a t i o n s o f s t y l o l i t e s with l a r g e s u t u r e s are e v i d e n t . In or d e r o f importance they a re: 1. P a r a l l e l t o bedding. 2. P e r p e n d i c u l a r t o bedding and p a r a l l e l t o the f o l d a x i s . 3. Sub-normal t o the f o l d a x i s . Group 1 and 2 s t y l o l i t e s are c o a x i a l w i t h f o l d i n g . Group 3 i s n o n - c o a x i a l . The most common s t y l o l i t e observed i s p a r a l l e l t o bedding ( F i g u r e 58 and 48). These 125 Figure 57 - Rotation of a bedding parallel stylolite relative to the principal stresses from nucleation through fold closure. Note that the minimum strain direction along the stylolite seam does not vary throughout rotation (B and C). This direction parallels the stylolite ridges as well as the proposed direction of fluid movement. A) Nucleation: a clay selvage developed along a bedding parallel stylolite during compaction. B) During folding: fluid movement along pre-existing clay selvage has begun to carve "ridges" along the stylolite seam. The ridge crests parallel the direction of fluid movement. C) Fold closure: ridge crests still parallel the minimum strain direction. Figure 58 - Poles to stylolites within Units 2 through 6 in the overturned anticline near Overfold Mountain. Contour intervals represent 1 mm of amplitude on stylolite sutures. 1 = Bedding parallel stylolites; 2 = Stylolites parallel with fold axis and perpendicular to bedding; 3 = Stylolites perpendicular to the fold axis H to 127 s t y l o l i t e s are b e l i e v e d t o have n u c l e a t e d d u r i n g e a r l y compaction of the sediments. S t y l o l i t e s p e r p e n d i c u l a r t o bedding and p a r a l l e l t o the f o l d a x i s developed d u r i n g a p e r i o d of l a y e r p a r a l l e l s h o r t e n i n g d u r i n g the i n i t i a l stages of f o l d i n g ( F i g u r e 58 and 48). As the beds were r o t a t e d d u r i n g f o l d i n g , both of these s e t s of c o a x i a l s t y l o l i t e s were a c t i v e and accommodated the compressive s t r a i n r e q u i r e d i n the development of the f o l d ; few new s t y l o l i t e s were n u c l e a t e d because the c r i t i c a l s t r e s s r e q u i r e d f o r n u c l e a t i o n was not reached. T h i s would e x p l a i n the absence of a s t y l o l i t e s e t which i s a x i a l p l a n e r t o the f o l d . A n a l y s i s of s u t u r e o r i e n t a t i o n s of these c o a x i a l s t y l o l i t e s r e v e a l s a kinematic b - a x i s p a r a l l e l t o the megascopic f o l d a x i s ( F i g u r e 59). Group 3 s t y l o l i t e s are n o n - c o a x i a l w i t h r e s p e c t t o the f o l d i n g of the beds. These s t y l o l i t e s are sub-normal t o the f o l d a x i s and r e p r e s e n t a p e r i o d of compression which was s u b - p a r a l l e l t o the f o l d a x i s . C r o s s - c u t t i n g r e l a t i o n s h i p s between c o a x i a l and n o n - c o a x i a l s t y l o l i t e s show the l a t t e r t o be a younger s t r u c t u r e thus the n o n - c o a x i a l phase was p o s t - f o l d i n g . S t y l o l i t e d e n s i t y and suture amplitudes f o r c o a x i a l and n o n - c o a x i a l s t y l o l i t e s have been p l o t t e d i n F i g u r e 60. Of i n t e r e s t i s the i n v e r s e r e l a t i o n s h i p between c o a x i a l and n o n - c o a x i a l s t y l o l i t e s a t most s t a t i o n s . At the h i n g e , U n i t 5 (dolostone) i s seen t o have a h i g h d e n s i t y of c o a x i a l s t y l o l i t e s and no n o n - c o a x i a l s t y l o l i t e s . On the o t h e r Northeast L i m b Hinge Southwest L i m b 0 m O Pole of the stylolite seam (c-axis) Trend and Plunge of the ridge crests along the stylolite seam. Segment of the plane defined by the pole to the stylolite seam and the ridge crests along the seam (ac plane). Pole to the ac plane defined by the stylolite seam (b-axis) Megascopic fold axis Figure 59 - Kinematic axes of stylolite seams in Units 2 through 6 (within the Lower Carnarvon Member of the Mount Head H Formation), in the overturned anticline near Overfold Mountain. ro Stylolite density (per m) Suture amplitude (mm) Figure 60 - Stylolite densities and suture amplitudes for Units 2 through 6, at the hinge of the anticline. A) Stylolites coaxial with folding. B) Stylolites formed post-folding (non-coaxial with folding). 130 hand, U n i t s 3 (dolostone) and 4 (limestone) have a very low d e n s i t y of c o a x i a l s t y l o l i t e s and a h i g h d e n s i t y of non-c o a x i a l s t y l o l i t e s . U n i t s 2 and 6 (both l i m e s t o n e s ) , the c o n t r o l l i n g u n i t s , have in t e r m e d i a t e v a l u e s of b o t h . T h i s i n f e r s t h a t the p e r m e a b i l i t y of the beds were a l t e r e d by the d e f o r m a t i o n a l mechanisms which caused the f o l d i n g , p r i o r t o the second phase of de f o r m a t i o n . In order t o a s c e r t a i n the reasons f o r the v a r i a t i o n s i n p e r m e a b i l i t y , one must c o n s i d e r a l l of the s t r u c t u r e s which may have a f f e c t e d the p e r m e a b i l i t y : v e i n s , s t y l o l i t e s and u n f i l l e d m i c r o f r a c t u r e s . In the d o l o s t o n e u n i t s (Units 3 and 5) t h e r e i s a good c o r r e l a t i o n between m i c r o f r a c t u r e d e n s i t y and n o n - c o a x i a l s t y l o l i t e d e n s i t y ( F i g u r e 61). U n i t 3 has a l a r g e d e n s i t y of u n f i l l e d c o a x i a l m i c r o f r a c t u r e s , s t r u c t u r e s which would form under low p e r m e a b i l i t y c o n d i t i o n s . Under these c o n d i t i o n s , one would expect t o f i n d a low s t y l o l i t e d e n s i t y , as i s the case f o r the c o a x i a l phase. The f r a c t u r e p o r o s i t y imparted i n t o the rock p r i o r t o the n o n - c o a x i a l phase of deformation i s r e f l e c t e d i n the h i g h d e n s i t y of n o n - c o a x i a l s t y l o l i t e s found a t the h i n g e . U n i t 5, on the o t h e r hand, has a lower d e n s i t y of m i c r o f r a c t u r e s , most of which are f i l l e d w i t h c a l c i t e . T h i s u n i t accommodated a g r e a t d e a l of e x t e n s i o n a l s t r a i n , mainly p a r a l l e l t o the f o l d a x i s . The r e l a t i v e l y low d e n s i t y of u n f i l l e d m i c r o f r a c t u r e s i n U n i t 5 suggests t h a t i t had a h i g h p e r m e a b i l i t y at the hinge d u r i n g the c o a x i a l phase of d e f o r m a t i o n , but a low p e r m e a b i l i t y d u r i n g the n o n - c o a x i a l UNIT 3 UNIT 5 LOCATIONS LOCATION LOCATION Microfractures Stylolites coaxial with folding Stylolites developed after folding (x4for Unit 3) Figure 61 - Densities of microfractures (per cm) and two phases of stylolites (per m) at several locations within the anticline for two dolostone units within the Lower Carnarvon Member (Units 3 and 5). Figure 62 - Densities of microfractures (per cm) and two phases of stylolites (per m) at several location within the anticline for three limestone units within the Lower Carnarvon Member (Units 2,4, and 6). Fold locations refer to Figure 61. 133 phase of d e f o r m a t i o n . T h e r e f o r e no n o n - c o a x i a l s t y l o l i t e s were formed. U n i t s 2 and 6 are very s i m i l a r i n c h a r a c t e r . Both are t h i c k , massive limestones t h a t appear t o have had s i m i l a r p e r m e a b i l i t y h i s t o r i e s over the deformation i n t e r v a l . In both of these u n i t s , the shape of the d e n s i t y curves f o r the c o a x i a l and n o n - c o a x i a l s t y l o l i t e s are n e a r l y p a r a l l e l ( F i g u r e 6 2 ) . T h i s suggests t h a t t h e r e was l i t t l e s t r a i n induced change i n p e r m e a b i l i t y d u r i n g f o l d i n g . There i s a f a i r l y good c o r r e l a t i o n between m i c r o f r a c t u r e d e n s i t y and n o n - c o a x i a l s t y l o l i t e d e n s i t y . T h i s suggests t h a t f r a c t u r e p e r m e a b i l i t y , imparted d u r i n g f o l d i n g , may have caused the development of the s t y l o l i t e s which were formed a f t e r f o l d i n g . U n i t 4 i s a r e l a t i v e l y t h i n limestone bed sandwiched between the somewhat s t i f f e r u n i t s ( U n i t s 2, 3, 5, and 6 ) . As a r e s u l t , i t has been f o r c e d t o accommodate these u n i t s , mostly by e x t e n s i o n p a r a l l e l t o the f o l d a x i s . The h i g h d e n s i t y of f i l l e d c o a x i a l m i c r o f r a c t u r e s at the hinge suggests t h a t t h i s u n i t was r e l a t i v e l y impermeable over the i n t e r v a l of f o l d i n g . The very s m a l l amount of compressional s t r a i n accommodated by t h i s u n i t a t the h i n g e , suggests t h a t i t was mainly i n an e x t e n s i o n a l regime and thus may have a c q u i r e d a f r a c t u r e p e r m e a b i l i t y not v i s i b l e by c o n v e n t i o n a l p e t r o g r a p h i c microscopy. T h i s was confirmed by f l u o r e s c e n c e microscopy which shows abundant m i c r o f r a c t u r e s along g r a i n boundaries and twin planes ( F i g u r e 53). 134 I t can thus be observed through two stages of deformation t h a t s t y l o l i t e development was c l o s e l y l i n k e d t o p e r m e a b i l i t y i n the study a r e a . A l s o , i t becomes even more apparent t h a t d e f o r m a t i o n a l mechanisms a f f e c t p e r m e a b i l i t y , by l o w e r i n g the p e r m e a b i l i t y of permeable beds, and i n c r e a s i n g the p e r m e a b i l i t y of impermeable beds. 7.2.5. TWINNING IN CALCITE AND DOLOMITE Twinned C a l c i t e i s found i n the limestone u n i t s throughout the f o l d . The average l i n e a r d e n s i t y of c a l c i t e twins has been determined f o r each s t a t i o n . The d e n s i t y of these twins decreases with d i s t a n c e from the f o l d hinge (Figure 63). Some s c a t t e r of data i s caused by the l o c a l i z e d m i c r i t i z a t i o n of g r a i n s ; i n p a r t i c u l a r , U n i t 2 has been d i a g e n e t i c a l l y m i c r i t i z e d at the h i n g e . The s m a l l g r a i n s i z e o f the m i c r i t e has made twin measurement i m p o s s i b l e thus the d e n s i t y of c a l c i t e twin l a m e l l a e appears t o be low. In f a c t , the d e n s i t y of twin l a m e l l a e i n the m i c r i t e i s probably q u i t e high due t o the h i g h d e n s i t y of d i s l o c a t i o n s a t g r a i n s u r f a c e s and the h i g h s u r f a c e t o volume r a t i o . Dolomite twinning i s r a r e i n the study a r e a ; none i s seen at a l l at most s t a t i o n s . The d o l o s t o n e u n i t s ( U nits 3 and 5) do show some dolomite twinning a t the a n t i c l i n e h i n g e , where the rocks are the most h i g h l y s t r a i n e d . F i g u r e 64 shows the d e n s i t y of the twins i n dolomite as a f u n c t i o n of d i s t a n c e from the a n t i c l i n e h i n g e . I t should be noted t h a t c a l c i t e c r y s t a l s w i t h i n v e i n s i n the d o l o s t o n e s 135 Sample Locations Figure 63 - Variations in the average density of calcite twin lamellae (per cm) as a function of location within the anticline for three limestone units (2, 4, and 6) of the Lower Carnarvon Member. 136 Sample Locations Unit 3 Unit 5 Figure 64 - Variations in the average density of dolomite twin lamellae (per cm) as a function of location within the anticline for two dolostone units (Units 3 and 5) of the Lower Carnarvon Member of the Mount Head Formation. 137 do show t w i n n i n g , the d e n s i t y of which i n c r e a s e s with the age of f r a c t u r e but decreases w i t h d i s t a n c e from the h i n g e . 138 7.3. UNIT 7 - MARSTON MEMBER U n i t 7 i s a massive, v e r y t h i c k packstone w i t h i n the Marston Member o f the Mount Head Formation. M e c h a n i c a l l y , the Marston Member as a whole i s unique i n comparison w i t h the o t h e r members o f t h i s Formation. I t i s composed of i n t e r b e d d e d l i m e s t o n e and d o l o s t o n e , as i s the Carnarvon Member, but the beds are t y p i c a l l y t h i n n e r and t h e r e i s a h i g h e r percentage o f d o l o s t o n e and s h a l e . As a r e s u l t of the l a r g e v i s c o s i t y c o n t r a s t between the beds, most of the s t r a i n a s s o c i a t e d w i t h f o l d i n g has been accommodated by the l e s s competent d o l o s t o n e s and s h a l e s . Rocks of the Marston Member are sandwiched between the v e r y t h i c k and massive limestone of the Loomis Member (Un i t s 9 and 10) and t h e t h i c k limestone u n i t a t the base of the Carnarvon Member (Uni t 6 ) . These massive u n i t s have c o n t r o l l e d the s t y l e o f f o l d i n g i n the more t h i n l y bedded rocks o f the Marston Member. The l i t h o l o g y o f U n i t 7 v a r i e s between outcrops a t the hinge and those o f t h e l i m b s . In the hinge r e g i o n , t h i s u n i t i s a w e l l cemented s k e l e t a l g r a i n s t o n e . P o i n t c o n t a c t s between g r a i n s suggest t h a t cementation was an e a r l y d i a g e n e t i c f e a t u r e , l o n g p r e c e d i n g d e f o r m a t i o n . In the l i m b s , however, the r o c k i s a s k e l e t a l packstone. There i s a g e n e r a l l a c k o f cement, and p r e s s u r e s o l u t i o n has a f f e c t e d a l l g r a i n c o n t a c t s . In the limb a r e a s , U n i t 7 i s r e l a t i v e l y r i c h i n c l a y s i n comparison t o the o t h e r u n i t s chosen f o r t h i s s t u d y . C l a y c a t a l y s i s of p r e s s u r e s o l u t i o n has been 139 o p e r a t i v e , c a u s i n g d i s s o l u t i o n along g r a i n boundaries and s t y l o l i t e f o r m a t i o n . The e f f e c t of c l a y c a t a l y s i s of p r e s s u r e s o l u t i o n has been d i s c u s s e d i n S e c t i o n s 3.2.2. and 7.2.4., and w i l l not be d e t a i l e d f u r t h e r h e r e . Pressure s o l u t i o n i s observed t o be the main d e f o r m a t i o n a l mechanism i n t h i s u n i t . C l a y c a t a l y s i s has, i n f a c t , been so e f f e c t i v e as t o a l l but negate the need f o r o t h e r d e f o r m a t i o n a l mechanisms. In p a r t i c u l a r , t h e r e i s no evidence of shear f r a c t u r e s , f a u l t s or shear zones w i t h i n t h i s u n i t . Veins are found throughout the u n i t but occur at a c o n s i d e r a b l y lower frequency than i n the o t h e r carbonate u n i t s s t u d i e d , s u g g e s t i n g t h a t c l a y c a t a l y s e d p r e s s u r e s o l u t i o n has taken p l a c e under a lower e f f e c t i v e s t r e s s than t h a t r e q u i r e d f o r a l l o t h e r mechanisms. 7.3.1. PRESSURE SOLUTION Although p r e s s u r e s o l u t i o n has been determined t o be the most important d e f o r m a t i o n a l mechanism i n the f o l d i n g of U n i t 7, t h e r e are some exce p t i o n s t h a t need t o be c o n s i d e r e d . In the limbs of the f o l d , almost every g r a i n c o n t a c t i s sutured ( F i g u r e 35B). T h i s i s not the case a t the h i n g e , where the u n i t i s w e l l cemented and g r a i n s o f t e n are seen t o have p o i n t c o n t a c t s (Figure 35A). The l a c k of sutured g r a i n c o n t a c t s a t the hinge suggests t h a t the c a l c i t e cement was emplaced e a r l y i n d i a g e n e s i s , b e f o r e s u f f i c i e n t depth of b u r i a l was reached f o r the onset of p r e s s u r e s o l u t i o n . In the f o l d l i m b s , however, the u b i q u i t o u s p r e s s u r e s o l u t i o n p a r a l l e l t o bedding suggests 140 t h a t the rock had a h i g h p e r m e a b i l i t y d u r i n g compaction and thus s t y l o l i t e s were n u c l e a t e d a t many g r a i n c o n t a c t s . A l l mesoscopic s t y l o l i t e s observed are s u b - p a r a l l e l t o bedding, again emphasizing the importance of a p r e - e x i s t i n g c l a y s e l v a g e i n the c a t a l y s i s of p r e s s u r e s o l u t i o n . S t y l o l i t e s o b l i q u e t o bedding are found on the m i c r o s c o p i c s c a l e o n l y . In the overturned limb of the f o l d , a s e t of m i c r o - s t y l o l i t e s was seen which i s p e r p e n d i c u l a r t o bedding and p a r a l l e l t o the f o l d a x i s . T h i s s e t i s b e l i e v e d t o r e p r e s e n t the episode of s h o r t e n i n g which took p l a c e d u r i n g the i n i t i a l stage of f o l d i n g ( F i g u r e 48). A second s e t of m i c r o - s t y l o l i t e s i s seen a t the hinge o n l y . T h i s s e t i s sub-normal to the f o l d a x i s , and p a r a l l e l s the n o n - c o a x i a l s t y l o l i t e s found i n o t h e r p a r t s of the f o l d . R e l a t i o n s h i p s between the t h r e e s t y l o l i t e s e t s and the p e r m e a b i l i t y of the rock a t the time of t h e i r f o r m a t i o n are s i m i l a r t o those observed i n U n i t s 2 through 6. E a r l y cementation r e s u l t e d i n a low p e r m e a b i l i t y i n the rock which i s now a t the hinge of the f o l d . Few s t y l o l i t e s were developed e i t h e r b e f o r e o r d u r i n g f o l d i n g . The low p e r m e a b i l i t y d u r i n g f o l d development caused m i c r o f r a c t u r i n g which i n c r e a s e d the f i n i t e p e r m e a b i l i t y of the r o c k . F l u i d s moving through the rock along f r a c t u r e s allowed p r e s s u r e s o l u t i o n t o take p l a c e d u r i n g p o s t - f o l d i n g , f o l d - a x i s -p a r a l l e l compression r e s u l t i n g i n the development of micro-s t y l o l i t e s normal t o t h i s a x i s . In the f o l d l i m b s , on the o t h e r hand, t h i s u n i t had a h i g h p e r m e a b i l i t y d u r i n g 141 compaction and the onset of f o l d i n g . T h i s p e r m e a b i l i t y was decreased t o almost n i l d u r i n g f o l d i n g , as p r e s s u r e s o l u t i o n p r o g r e s s i v e l y c l o s e d e x i s t i n g i n t e r g r a n u l a r p o r e s . As a r e s u l t , no l a t e stage n o n - c o a x i a l s t y l o l i t e s are found. 7.3.2. EXTENSION FRACTURES Throughout U n i t 7, e x t e n s i o n f r a c t u r e s have acted as l o c i f o r the d e p o s i t i o n of m a t e r i a l which has been removed along s t y l o l i t e seams. As has been the case elsewhere i n the study a r e a , the importance of h y d r a u l i c f r a c t u r i n g i n the deformation of the f o l d i n g beds i s dependent upon the p e r m e a b i l i t y a t the time of d e f o r m a t i o n . The p e r m e a b i l i t y of U n i t 7 v a r i e d l a t e r a l l y p r i o r t o deformation such t h a t the hinge area had a low p e r m e a b i l i t y and the limbs had a h i g h p e r m e a b i l i t y . As a r e s u l t , the importance of h y d r a u l i c f r a c t u r i n g has v a r i e d throughout the f o l d , not only due t o v a r i a t i o n s i n the amount of s t r a i n accommodated, but a l s o due t o l i t h o l o g i c v a r i a t i o n s . Trends i n the d e n s i t i e s of m i c r o f r a c t u r e s and m e s o f r a c t u r e s (veins) observed i n t h i s u n i t (Figure 65) r e f l e c t the p e r m e a b i l i t y of the r o c k . At the hinge, a h i g h m i c r o f r a c t u r e d e n s i t y , accompanied by a low mesofracture d e n s i t y expresses the low p e r m e a b i l i t y t h e r e . The i n v e r s e i s found a t the l i m b s , where the p e r m e a b i l i t y was probably h i g h d u r i n g f o l d i n g . When c o n s i d e r i n g the s t r a i n i m p l i c a t i o n s of e x t e n s i o n f r a c t u r e s , one must look not only a t the f r a c t u r e d e n s i t i e s , but a l s o a t the amount of e x t e n s i o n accommodated by each LOCATION Figure 65 - Average linear densities of microfractures and mesofractures (veins) within a limestone unit of the Marston Member of the Mount Head Formation (Unit 7). Locations are in the limbs and the hinge of the anticline near Overfold Mountain. 143 f r a c t u r e . Although the d e n s i t y of u n f i l l e d m i c r o f r a c t u r e s i s h i g h i n beds of low p e r m e a b i l i t y , most o f the s t r a i n accommodated by them a t the time o f t h e i r f o r m a t i o n i s e l a s t i c i n n a t u r e , and the amount of permanent d i l a t i o n i s s m a l l . M e s o f r a c t u r e s ( v e i n s ) , on the oth e r hand, t y p i c a l l y o ccur a t a low d e n s i t y , r e l a t i v e t o m i c r o f r a c t u r e s , but can accommodate l a r g e amounts of permanent d i l a t i o n . Bulk d i l a t i o n can be e s t i m a t e d f o r each observed f r a c t u r e s e t i f the average f r a c t u r e width and the l i n e a r d e n s i t y o f each s e t i s known ( F i g u r e 66). By adding the b u l k d i l a t i o n o f a l l o f the f r a c t u r e s e t s observed, one can q u a n t i t a t i v e l y e s t i m a t e a t o t a l b u l k d i l a t i o n accommodated by e x t e n s i o n f r a c t u r i n g . T h i s e s t i m a t e i s u s e f u l i n two ways. F i r s t , i t can be used t o compare the importance o f e x t e n s i o n f r a c t u r i n g between rocks w i t h v a r y i n g degrees o f s t r a i n . I t can a l s o be used as an e s t i m a t e o f the volume i n c r e a s e a s s o c i a t e d w i t h e x t e n s i o n f r a c t u r i n g . I f the deforming system was c l o s e d , then t h i s volume i n c r e a s e s h o u l d be equal and o p p o s i t e t o the volume decrease caused by p r e s s u r e s o l u t i o n (bulk n e g a t i v e d i l a t i o n ) . Bulk d i l a t i o n has been p l o t t e d f o r the mesoscopic and m i c r o s c o p i c f r a c t u r e s and s t y l o l i t e s i n the limb and hinge r e g i o n s o f the a n t i c l i n e ( F i g u r e 67). Negative d i l a t i o n f o r the s t y l o l i t e s i s taken from the l e n g t h o f the s t y l o l i t e " t e e t h " and r e p r e s e n t s the minimum amount of m a t e r i a l removed by p r e s s u r e s o l u t i o n . I f the system was c l o s e d , the b u l k d i l a t i o n e s t i m a t e d f o r the s t y l o l i t e s s h o u l d be equal VEIN SET 1 Total extension from VEIN SET 1 = Original length perpendicular to VEIN SET 1 = 1- W t D x W,D, Bulk dilation from VEIN SET 1 = W D W D Total bulk dilation from all vein sets = 1 1 -1- 2 2 1 - W ^ 1 - W 2 D 2 Figure 66 - Method of calculation of bulk dilation. Figure 67 - Bulk dilation from veins and stylolites within a limestone unit of the Marston Member of the Mount Head Formation (Unit 7). Locations are in the limbs and hinge of the anticline near Overfold Mountain. 146 to or l e s s than t h a t f o r the f r a c t u r e s . Indeed, t h i s i s seen at the hinge and i n the southwest limb of the a n t i c l i n e . In the no r t h e a s t l i m b , however, th e r e appears t o have been a s i g n i f i c a n t volume de c r e a s e . 7.3.3. TWINNING IN CALCITE In g e n e r a l , c a l c i t e twins are l e s s abundant i n U n i t 7 than the o t h e r limestone u n i t s i n t h i s s t u d y . As was observed i n oth e r u n i t s , the d e n s i t y o f c a l c i t e twins decreases w i t h d i s t a n c e from the h i n g e . The d e n s i t y of c a l c i t e twins appears t o be p r o p o r t i o n a l t o m i c r o f r a c t u r e d e n s i t y i n t h i s u n i t (Figure 68). T h i s would be expected inasmuch as the d e n s i t y of both s t r u c t u r e s i s r e l a t e d t o the amount of s t r a i n the rock has been s u b j e c t e d t o and the presence of d i s l o c a t i o n s i n the c a l c i t e c r y s t a l l a t t i c e . An i n v e r s e r e l a t i o n s h i p i s observed between s t y l o l i t e d e n s i t y and c a l c i t e t w i n n i n g . T h i s suggests t h a t c l a y enhanced d i s s o l u t i o n takes p l a c e at a lower s t r e s s than the c r i t i c a l r e s o l v e d shear s t r e s s f o r c a l c i t e t w i n n i n g . There i s a hig h d e n s i t y o f c a l c i t e twins i n the n o r t h e a s t limb of the s y n c l i n e which corresponds t o the h i g h s t r a i n e stimated f o r the s t y l o l i t e s seams. T h i s would suggest t h a t , at some p o i n t i n the deformation, the s t r a i n r a t e may have i n c r e a s e d such t h a t both c a l c i t e twinning and pr e s s u r e s o l u t i o n were u t i l i z e d . I t i s u n c l e a r why t h i s has oc c u r r e d o n l y i n the nort h e a s t l i m b . 147 LOCATION 1 1 2 3 LOCATION MICROFRACTURES CALCITE TWIN LAMELLAE (x10) Figure 68 - Average linear densities of calcite twin lamellae and microfractures (per cm) within a limestone unit of the Marston Member of the Mount Head Formation (Unit 7). Locations are in the limbs and hinge of the anticline near Overfold Mountain. 148 7 . 4 . UNIT 8 - LOOMIS MEMBER The Looirtis Member c o n s i s t s of t h i c k , massive, c l i f f -f orming, grey l i m e s t o n e . The s t i f f n e s s of t h i s Member has caused i t t o have a broad, open hinge geometry, a geometry which i s mimicked by the t h i n l y bedded rocks o f the o v e r l y i n g Marston Member ( i n c l u d i n g U n i t 7 ) . U n i t 8 c o n s i s t s of the uppermost beds of the Loomis Member, at the c o n t a c t w i t h the Marston Member. U n i t 8 i s a w e l l cemented, o o l i t i c g r a i n s t o n e ( F i g u r e 6 and 36). P o i n t c o n t a c t s between g r a i n s suggest t h a t cementation o c c u r r e d very e a r l y i n d i a g e n e s i s , b e f o r e s i g n i f i c a n t compaction. As a r e s u l t of e a r l y cementation, t h i s rock had a ver y low primary p o r o s i t y throughout the de f o r m a t i o n a l h i s t o r y . The s i z e and d e n s i t i e s o f v e i n s , u n f i l l e d m i c r o f r a c t u r e s , and s t y l o l i t e s r e f l e c t the low p o r o s i t y of t h i s u n i t . U n f i l l e d m i c r o f r a c t u r e s i n p a r t i c u l a r , have i n c r e a s e d p e r m e a b i l i t y d u r i n g d e f o r m a t i o n , and have enhanced the f i n i t e p e r m e a b i l i t y . 7 . 4 . 1 . EXTENSION FRACTURES Ex t e n s i o n f r a c t u r i n g has been of l e s s importance i n the accommodation o f s t r a i n i n U n i t 8 than i n the o t h e r u n i t s i n t h i s s t u d y . T h i s i s probably due t o the extremely low p o r o s i t y of t h i s u n i t , a f a c t o r which may have prevented f l u i d s from e n t e r i n g the ro c k , and thus prevented the development of the hig h f l u i d p r e s s u r e s r e q u i r e d f o r h y d r a u l i c f r a c t u r i n g . 149 V e i n s e t s f o l l o w s i m i l a r t r e n d s t o those found elsewhere i n the f o l d . The i n d i v i d u a l v e i n s a re t h i n n e r , however, r e s u l t i n g i n a lower b u l k d i l a t i o n . Two common v e i n s e t s a re found i n t h i s u n i t . The f i r s t i s p a r a l l e l t o bedding, and corresponds t o those v e i n s formed a t the onset of f o l d i n g ( F i g u r e 4 8 ) . T h i s v e i n s e t i s most common a t the hinge and i n the northwest l i m b . The second common v e i n s e t i s p e r p e n d i c u l a r t o the f o l d a x i s and i s c o r r e l a t i v e w i t h s i m i l a r v e i n s found i n a l l u n i t s s t u d i e d . Low p e r m e a b i l i t y d u r i n g deformation has r e s u l t e d i n an abundance o f u n f i l l e d m i c r o f r a c t u r e s i n U n i t 8. Fl u o r e s c e n c e microscopy r e v e a l s t h a t the f r a c t u r e s commonly f o l l o w g r a i n boundaries and c a l c i t e twin l a m e l l a e ( F i g u r e 6 9 ) . The i n t e r c o n n e c t i n g network formed by these m i c r o f r a c t u r e s would g r e a t l y enhance the p e r m e a b i l i t y o f t h i s u n i t . T h i s enhancement i s low i n comparison t o the do l o s t o n e u n i t s ( U n i t s 3 and 5 ) , however, where f r a c t u r e p o r o s i t y i s found a t n e a r l y every g r a i n boundary ( F i g u r e s 51 and 52). The d i s c r e p a n c y i n the abundance o f m i c r o f r a c t u r e s w i t h i n t h e s e two l i t h o l o g i e s i s p a r t l y due t o the mineralogy o f the g r a i n s - c a l c i t e may deform by i n t r a - g r a n u l a r g l i d e mechanisms r a t h e r than f r a c t u r i n g . Of more importance however, i s the shape of the g r a i n s . The sharp p o i n t s and edges o f the dolomite rhombs are v e r y e f f e c t i v e s t r e s s c o n c e n t r a t o r s . F r a c t u r e s are more e a s i l y i n i t i a t e d a t these p o i n t s o f h i g h s t r e s s . No sharp s u r f a c e s e x i s t i n the o o l i t i c g r a i n s t o n e . Figure 69 - Thin section of Unit 8 impregnated with fluorescent dye. Fluorescence reveals fracture porosity along grain boundaries as well as twin planes within grains. A) Plane light; B) Blue light. Long edge of photograph measures 0.06 mm. 151 Bulk d i l a t i o n and b u l k n e g a t i v e d i l a t i o n have been e s t i m a t e d from the v e i n s and s t y l o l i t e s i n the limbs and hinge o f t h e f o l d ( F igure 7 0 ) . S i n c e the curve f o r the b u l k n e g a t i v e d i l a t i o n c o n s i s t e n t l y l i e s w e l l below t h a t f o r the p o s i t i v e d i l a t i o n , i t can be i n f e r r e d t h a t t h i s u n i t has remained a t c o n s t a n t volume throughout f o l d i n g . The l a r g e amount o f p o s i t i v e and n e g a t i v e d i l a t i o n i n the n o r t h e a s t limb of the a n t i c l i n e i n d i c a t e s t h a t these rocks are more h i g h l y s t r a i n e d than rocks i n o t h e r p a r t s of the f o l d . T h i s phenomenon was a l s o found i n the rocks of the Marston Member and the Lower Carnarvon Member. Reasons f o r t h i s are not c l e a r . 7.4.2. SHEAR FRACTURES Shear f r a c t u r i n g i s a dominant d e f o r m a t i o n a l mechanism i n U n i t 8, much more so than the o t h e r limestone u n i t s s t u d i e d . In f a c t , i n many ways t h i s u n i t has behaved more l i k e the d o l o s t o n e u n i t s ( U n i t s 3 and 5) than the o t h e r l i m e s t o n e u n i t s ( U n i ts 2, 4, 6, and 7 ) . T h i s i s a r e s u l t o f the low p o r o s i t y of t h i s u n i t , a f a c t o r which has prevented t h e u t i l i z a t i o n o f the d e f o r m a t i o n a l mechanisms which r e q u i r e f l u i d s : p r e s s u r e s o l u t i o n and h y d r a u l i c f r a c t u r i n g . These mechanisms were v e r y a c t i v e i n the deformation o f t h e l i m e s t o n e s which had a h i g h e r p o r o s i t y , thus s t r a i n was accommodated without shear f a i l u r e . Two phases of deformation a r e d e l i n e a t e d by the shear f r a c t u r e s i n U n i t 8 ( F i g u r e 7 1 ) . T h i s i s i n agreement w i t h shear f r a c t u r e s found throughout the study a r e a . The 152 LOCATIONS Figure 70 - Bulk dilation from veins and stylolites vrithin a limestone unit of the Loomis Member of the Mount Head Formation (Unit 8). Locations are in the limbs and hinge of the anticline near Overfold Mountain. 153 Northeast L i m b Southwest L i m b —Ac— Pole to siickensided shear fracture (c-axis), with slip linear O Axis of slip determined by striae (b-axis) X Slickenside striae (a-axis) • Megascopic fold axis Figure 71 - Equal area projection of poles to shear fractures in a limestone unit within the Loomis Member of the Mount Head Formation (Unit 8). Information is given for the limbs of the overturned anticline near Overfold Mountain. The dashed great circle represents the megascopic ac plane. The solid great circle represents bedding. Contour interval = 3 sigma (Kamb method). 154 e a r l i e s t phase i s c o a x i a l with the f o l d i n g of the beds. Kinematic a n a l y s i s of s l i c k e n s i d e s on these f r a c t u r e s r e v e a l s an a x i s of r o t a t i o n (b-axis) which p a r a l l e l s the megascopic f o l d a x i s . A l a t e r , p o s t - f o l d i n g , phase of shear f r a c t u r i n g i s found i n the overturned limb o f the a n t i c l i n e . Kinematic a n a l y s i s o f s l i c k e n s i d e s on these f r a c t u r e s r e v e a l s an a x i s of r o t a t i o n toward the n o r t h e a s t . 7.4.3. SEMI-BRITTLE SHEAR ZONES No s e m i - b r i t t l e shear zones were found i n t h i s u n i t . T h i s a g a i n i s an e f f e c t of low p e r m e a b i l i t y and p o r o s i t y . Lack of f l u i d i n the rock has prevented the development of h y d r a u l i c e x t e n s i o n f r a c t u r e s , a c r u c i a l element i n the development of shear zones. D i f f e r e n t i a l s t r e s s i s thus al l o w e d t o i n c r e a s e t o a p o i n t of shear f a i l u r e . 7.4.4. PRESSURE SOLUTION As was found i n the o v e r l y i n g u n i t s , s t y l o l i t e s w i t h i n U n i t 8 d e l i n e a t e t h r e e phases of s t y l o l i t e n u c l e a t i o n . The e a r l i e s t s t y l o l i t e s are p a r a l l e l t o bedding and were n u c l e a t e d by compaction d u r i n g sediment l o a d i n g . The second group of s t y l o l i t e s p a r a l l e l the f o l d a x i s and are p e r p e n d i c u l a r t o bedding. These s t y l o l i t e s developed d u r i n g bedding p a r a l l e l s h o r t e n i n g at the onset of f o l d i n g . A n a l y s i s of both of these s t y l o l i t e s e t s r e v e a l a ki n e m a t i c b - a x i s which p a r a l l e l s the megascopic f o l d a x i s ( F i g u r e 72), c o n f i r m i n g t h a t these s t y l o l i t e s are c o a x i a l w i t h f o l d development. The youngest s t y l o l i t e s were formed a f t e r the Northeast L imb Hinge Southwest L i m b Pole of the stylolite seam (c-axis) Trend and Plunge of the ridge crests along the stylolite seam. O Segment of the plane defined by the pole to the stylolite seam and the ridge crests along the seam (ac plane). Pole to the ac plane defined by the stylolite seam (b-axis) Megascopic fold axis Figure 72 - Poles to stylolite seams in a limestone unit within the Loomis Member of the Mount Head Formation (Unit 8). Data is for the overturned anticline near Overfold Mountain. Contour interval equal to 3 sigma (Kamb method). Dashed great circle represents the ac plane of the megascopic fold. Solid great circle represents bedding. 156 f o l d i n g o f the beds and d i p s t e e p l y t o the northwest. A n a l y s i s o f these s t y l o l i t e s r e v e a l s a ki n e m a t i c b - a x i s which p a r a l l e l s the no n - c o a x i a l s t y l o l i t e s found throughout the study a r e a . As mentioned p r e v i o u s l y , the number of the s t y l o l i t e s n u c l e a t e d a t any one time i s a f u n c t i o n o f the p e r m e a b i l i t y of the r o c k . A low p e r m e a b i l i t y a l l o w s the n u c l e a t i o n o f o n l y a few s t y l o l i t e s . In U n i t 8, a h i s t o r y o f low primary p o r o s i t y , and a p r o g r e s s i v e i n c r e a s e i n secondary ( f r a c t u r e ) p o r o s i t y , i s r e f l e c t e d i n the d e n s i t i e s o f s t y l o l i t e s developed b e f o r e , d u r i n g , and a f t e r f o l d i n g . F i g u r e 73 shows the d e n s i t i e s o f the t h r e e s t y l o l i t e s e t s a t v a r i o u s s t a t i o n s i n the limbs and hinge r e g i o n o f the f o l d . Of i n t e r e s t i s the i n v e r s e r e l a t i o n s h i p o f t e n found between t h e d e n s i t y o f one s t y l o l i t e s e t and the d e n s i t y o f the s e t which preceded i t i n t i m e . For i n s t a n c e , where t h e r e i s a low d e n s i t y of s t y l o l i t e s which are c o a x i a l w i t h f o l d i n g ( s t y l o l i t e s 1 and 2 ) , such as i n the n o r t h e a s t f o l d l i m b , t h e r e i s a h i g h d e n s i t y o f the l a t e r , non-c o a x i a l , s t y l o l i t e s ( s t y l o l i t e 3 ) . The h i g h d e n s i t y o f these l a t e r s t y l o l i t e s i n d i c a t e s t h a t they n u c l e a t e d a t many s i t e s , a c o n d i t i o n which would have r e q u i r e d an enhanced p e r m e a b i l i t y . M i c r o f r a c t u r e s formed d u r i n g the f o l d i n g o f the beds are an obvious source o f t h i s secondary p e r m e a b i l i t y . The i n v e r s e r e l a t i o n s h i p between the d e n s i t i e s o f succeeding s t y l o l i t e s e t s i s more obvious, and more c o n f u s i n g , i n t h e hinge r e g i o n , where t h e p e r m e a b i l i t y 157 LOCATIONS STYLOUTE 1 — — STYLOLITE 2 STYLOLITE 3 Stylolite 1 - parallels bedding Stylolite 2 - perpendicular to bedding and parallel to the fold axis Stylolite 3 - perpendicular to the fold axis. Figure 73 - Densities (per m) of three sets of stylolites in a limestone unit from the Loomis Member of the Mount Head Formation (Unit 8). Locations are in the limbs and hinge of the anticline near Overfold Mountain. 158 appears t o have v a r i e d s p a t i a l l y , over a d i s t a n c e of s e v e r a l meters, as w e l l as t e m p o r a l l y . 7 . 4 . 5 . CALCITE TWIN LAMELLAE The abundance of m i c r o f r a c t u r e s and c a l c i t e twin l a m e l l a e decrease p r o p o r t i o n a l l y with d i s t a n c e from the hinge ( F i g u r e 74). T h i s t r e n d i s a l s o found i n the othe r carbonate u n i t s s t u d i e d . Both of these s t r u c t u r e s i n d i c a t e t h a t the hinge r e g i o n has accommodated more s t r a i n on the g r a n u l a r s c a l e than the f o l d l i m b s . One reason f o r t h i s i s t h a t s t r a i n i n the f o l d limbs has been accommodated dominantly by bedding p a r a l l e l shear, a mechanism which has not been important at the hinge due t o the o r i e n t a t i o n of the bedding planes w i t h r e s p e c t t o the maximum and minimum compressive s t r e s s e s . Figure 74 - Average linear densities of calcite twin lamellae and microfractures (per cm) within a limestone unit of the Loomis Member of the Mount Head Formation (Unit 8). Locations are in the limbs and hinge of the anticline near Overfold Mountain. 160 8 . CONCLUSIONS The d e f o r m a t i o n a l mechanisms t h a t have been o p e r a t i v e at O v e r f o l d Mountain i n c l u d e s o l u t i o n p r o c e s s e s ( h y d r a u l i c f r a c t u r i n g and pr e s s u r e s o l u t i o n ) , shear f r a c t u r i n g , and i n t r a g r a n u l a r mechanisms (mechanical t w i n n i n g , d i s l o c a t i o n g l i d e and m i c r o f r a c t u r i n g ) . F l u i d s w i t h i n the rock have r e g u l a t e d which of these mechanisms has been u t i l i z e d i n the accommodation o f s t r a i n . When f l u i d s were p r e s e n t , p r e s s u r e s o l u t i o n and h y d r a u l i c f r a c t u r i n g ensued. These processes appear t o have taken p l a c e at s t r e s s e s lower than those r e q u i r e d f o r shear f r a c t u r i n g and c a l c i t e t w i n n i n g , as t h e r e i s an i n v e r s e r e l a t i o n s h i p between the abundance of s t r u c t u r e s developed by s o l u t i o n p r o c e s s e s and t h a t of those from the o t h e r p r o c e s s e s . When rocks were f l u i d d e f i c i e n t , they commonly f a i l e d by shear and by m i c r o f r a c t u r i n g . The mechanical behavior of each o f the l i t h o l o g i e s s t u d i e d has been governed not on l y by p e r m e a b i l i t y , but a l s o by the mineralogy o f the g r a i n s w i t h i n the u n i t . Mineralogy has c o n t r o l l e d whether an i n d i v i d u a l g r a i n behaved i n a b r i t t l e o r d u c t i l e f a s h i o n . D u c t i l e mechanisms u t i l i z e d i n the rocks o f the study area are p r e s s u r e s o l u t i o n and i n t r a c r y s t a l l i n e g l i d e . M i c r o f r a c t u r e s found along g r a i n boundaries and w i t h i n g r a i n s a re evidence o f b r i t t l e d e f o r m a t i o n . In the study a r e a , the limestones and do l o s t o n e s have deformed i n a s e m i - b r i t t l e manner, u t i l i z i n g both b r i t t l e and d u c t i l e mechanisms. The quartz 1 6 1 a r e n i t e u n i t , on the o t h e r hand, has mainly deformed by b r i t t l e mechanisms; i n t h i s case shear f r a c t u r i n g . C a l c i t e g r a i n s w i t h i n the limestone u n i t s commonly have deformed by twinning and twin g l i d e . Dolomite c r y s t a l s have a l s o formed t w i n s , though not t o the extent of c a l c i t e . Quartz has not deformed s i g n i f i c a n t l y by i n t r a c r y s t a l l i n e g l i d e mechanisms due t o the s t r o n g c o v a l e n t bonds between S i4 + and 0 2 ~ . The ease w i t h which each of these minerals i s d i s s o l v e d by p r e s s u r e s o l u t i o n a l s o v a r i e s . A g a i n , the c o v a l e n t l y bonded atoms i n the quartz l a t t i c e do not d i s s o c i a t e e a s i l y , and s t r a i n accommodated by p r e s s u r e s o l u t i o n i s r e l a t i v e l y minor. The i o n i c bonds i n the carbonate l a t t i c e s , on the o t h e r hand, are r e l a t i v e l y e a s i l y broken and p r e s s u r e s o l u t i o n o c c u r s r e a d i l y . Many of the d e f o r m a t i o n a l mechanisms s t u d i e d have a f f e c t e d the f i n i t e p e r m e a b i l i t y of the carbonates and sandstones. As mentioned, the s o l u t i o n p r o c e s s e s have been most a c t i v e i n rocks w i t h a h i g h i n i t i a l p e r m e a b i l i t y . Pressure s o l u t i o n i n these rocks has served t o c l o s e e x i s t i n g pore spaces and decrease the f i n i t e p e r m e a b i l i t y . The abundant t h i c k v e i n s formed i n these r o c k s , as w e l l as the f i n e g r a i n e d , c l a y r i c h selvage along s t y l o l i t e seams, a l s o serve as e f f e c t i v e b a r r i e r s t o f l u i d movement. The o p p o s i t e e f f e c t i s found i n rocks which had a low i n i t i a l p e r m e a b i l i t y . Abundant m i c r o f r a c t u r e s i n these rocks have served t o i n c r e a s e the f i n i t e p e r m e a b i l i t y . T h i s e f f e c t i s 162 most impress i v e i n the dolostone u n i t s , where m i c r o f r a c t u r i n g i s found along most g r a i n b o u n d a r i e s . The r o l e of f l u i d s i n the deformation of rocks near O v e r f o l d Mountain i s d i s p l a y e d i n the type and d e n s i t y of v e i n s , s t y l o l i t e s , shear f r a c t u r e s , m i c r o f r a c t u r e s and c a l c i t e twin l a m e l l a e . Kinematic a n a l y s i s of these s t r u c t u r e s has d e l i n e a t e d t h r e e phases f o r t h e i r development: p r e - d e f o r m a t i o n a l compaction, f o l d i n g , and p o s t - f o l d i n g . P e r m e a b i l i t y and p o r o s i t y has v a r i e d through each of these phases, ca u s i n g a v a r i a t i o n i n the way i n which s t r a i n has been p a r t i t i o n e d between the aforementioned s t r u c t u r e s . A n a l y s i s of the s t r u c t u r e s developed i n each phase has thus g i v e n i n s i g h t i n t o how and. why the p e r m e a b i l i t y has v a r i e d over the deformation i n t e r v a l of the r o c k . V a r i a t i o n i n the p e r m e a b i l i t y of the carbonate u n i t s d u r i n g the t h r e e phases of deformation i s best documented by the d e n s i t y of s t y l o l i t e s developed d u r i n g each phase ( s t y l o l i t e d e n s i t y i s p r o p o r t i o n a l t o p e r m e a b i l i t y ) . M i c r o f r a c t u r e s formed i n u n i t s of low p e r m e a b i l i t y d u r i n g one phase i n c r e a s e the p e r m e a b i l i t y of the rock d u r i n g the next phase. Enhanced p e r m e a b i l i t y d u r i n g t h i s l a t e r phase i s i n d i c a t e d by the development of a h i g h e r d e n s i t y of s t y l o l i t e s . P e r m e a b i l i t y has t h e r e f o r e governed the way i n which s t r a i n has been p a r t i t i o n e d between the d e f o r m a t i o n a l mechanisms employed throughout the s t r a i n h i s t o r y of the 163 r o c k . 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