THE WnERSITY OF CALGARY

An mte- geophysid investigation of basement conmls on Devonian carbonates in central Alberta, Canada

Darran Jones Edwards

A DISSERTATION SUBMllTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCïOR OF PHlLOSOPHY

DEPARTMENT OF GEOLOGY AND GEOPHYSICS

CALGARY,ALBERTA MAY, 1997

O Darran Jones Edwards 1997 National Libraiy 1+1 ofcmada Acquisitions and Acquisiins et Bibliographie Se~*ces seMces bibliographiques 395 Wellington Street 395, nwr Welhgton WwaON KlAOIYQ OttewaON KtAON4 CaMda Canada

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The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantkd extracts tiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. Abstract

This study examines the role of basement stnrcture and lithology in the deposition of Middle and Upper Devonian carbonates in central Alberta and attempts to test various geophysical methods to help m detemiinmg tbis. This poorly understood aspect of Devonian carbonate sedimentation is assessed hugh a rnultidisciplinary approach incorporating seismic reflection, potentiai-field, and cidihole data

The crystalline basement appears to be characte- by est- or southeast-dipping tbnist faults in east-central Alberta and West- to southwest-inclineci Iayering of possibie lithoIogicd origin to the west. Conventional and horizontal-gradient vector (HGV) potentid-field data seldom detect these proposed major basement contacts. However, a prominent northeast- to north-northeas-trending crusta1 discontinuity, the Snowbird Tectonic Zone, can be tracked throughout the study ares

Basement surface morphology and the degee of sedimentary cover deformation varies across central Alberta Structurai, tithologicai and geomorphological processes probably shaped the topography of the basement surface. In places, basernent faults propagated directly into the sedimentary cover with a near-vertical attitude, Occasionally, dipping intrabasement seismic events may be projected to approxmiately coincide with low-relief irregulaïities on the basement surface. Basement structure can sometimes be indirectly transferred upsection through differential compaction or drape of sediments over such basement irregularities.

This structural variation is similarly reflected in the different modes of basement interaction with Devorim carbonates. For example, kgRiver Formation paîch reefs melian- Givetian) werr initially mted direcrly on raidfault bIocks in north-central Alberta. To the south, channel development, an important influence on Swan Hills Formation (Givetian to Frasnian) pli-orrn morphology, may be tied to mctivated basement faulting. Elsewhere, drape over raised basement topography apparentiy provideci the loci for some Swan Hills and Leduc Formation (Frasnian) mfal buildups. Regionaiiy, very few potential-field lineaments coincide with knom carbonate buildups. Core data suggest that minor tectonic adjustments may have occurred synchronously with Keg River and Swan Hills reef .*. Ill growth, Post-Devonian movement on basement fdts may have enabIed hydroc~nsand doIomitizing fluids to ldyaccess these carbonates-

The possible recognition of subtie geo10gical inhenentancebetween the basement and carbonate sedmienîary cover is of significauce in terms of future hydrocarbon exploration for Devonian carbonate reservoirs. Acknowbdgments

A multidisciplinary project of this nature dksupon a variety of geophysical and geologicai datasets and personnel. 1owe a great deal of gratitude to a large nurnber of people. Firstly, 1 would like to thank my supervisor, Jim Brown, for giving me the opportunity to undertake research mto such a conmversial topic. His technical and moral support over the years are greatly appreciated. This dissertation has ptly benefited in many ways hm comrnents made by Gerry Ross, Don Lawton, Don Gend2;wii.I and Henry Lyatsky. Discussions with Fran Hein and Jim Didch aiso proved fruitful.

The following companies and individuals are thanked for donating seismic data to this project: Compton Resources Ltd; Husky ûii Ltd (Michael Enachescu); International Colin Energy Co. @oug McLachlan); Mobil Oil Canada; Sigma Explorations Ltd (Barry Korchinski); Talisman Energy Ltd (Tom Andrews), and Unocal Canada Ltd (Ken Mitchell). The use of LITHOPROBEseismic data was kuidly made available by Gerry Ross and Emie Kanasewich. Drillhole data were provideci by Digitecb Infoxmation SeNices Ltd and CD Pubco Inc. Gridded magnetic and Bouguer gravity &ta were available thugh the Geological Survey of Canada. Commmwedth Geophysical Geophysical Development Co. (Sudhir Jain) provideci access to additional magnetic data in north-central Alberta. Without these various types of data, this research would not have been possible.

Within the Department of Geology and Geophysics, tecbnïcai support at different stages of this smdy by members of the LITHOPROBESeismic Rocessing Facility (LSPF) and CREWES groups is appreciated. In particular, 1 would like to acknowledge Kns Vasudevan, Rolf Maier, Hugh Geiger, Arie van der Velden, Darren Foltinek and Henry Bland for their assistance. Jeff Thurston is thanked for sbaring his knowledge of the horizontal-gradient vector (HGV)algorithm. The various stafi of the Gallagher Library and main office were also helpfui during my time there, particularly Regina Shedd, Marvel Nash and Marg Westbrook.

Financial support for this project was provideci by an Amoco Canada Graduate Feliowship, an Alberta Minister of Advanced Education Intemational Education Award, LiTHOPROBE Supporthg Geoscience Awards and an NSERC Operathg Grant (to Jim Brown), together with graduate teaching and rtsearch assistantships administered by the Department of Geology and Geophysics.

Over the years, a great number of fiends have successfully manageci to distract me m many ways hmlong &ys and nights spent working on this dissertation. Sunice it to Say, late night discussions wirh fellow students mevitably seemed to involve ferrnented beverages and a pool table. Away hmthe university, the proximity of the mountains made for many memorabIe moments and experiences.

Fmaliy, but by no means least, 1 would like to thank my parents, Lewis and Valmai Edwards, for their constant support and encouragement over the years, and for frequently askuig "have you nearly finkhed?".

1fiope that thïs dissertation will inspire Merstudy in the years to corne. .. Approval page ...... n Abstract ...... in... Acknowledgments ...... v Table of contents ...... vii List of tables ...... x List of figures ...... xi

Chapter 1: Introduction ...... 001 1.1 Background ...... 001 1.2 Dissertation objectives ...... 005 1.3 Methodology ...... 006 1.4 Dissertation outlke ...... 013

Chapter 2: The potential-field horizontal-gradient vector (HGV)method ...... 015 2.1 Utïlïty of magnetic and gravity data in central Alberta ...... 016 2.2 The horizontal gradient of potential-field data ...... 021 2.3 The horizontal-gradient vector method ...... 023 2.4 Synthetic 3-D modelling of potential-field HGV data ...... 036 2.4.1 Prism mode1 ...... 037 2.4.2 Dome mode1 ...... 049 2.4.3 Dipping-layer mode1 ...... -049 2.5 Geological comboraticm of HGV interpretation: an example fiom the Wopmay Orogen ...... 056 2.6 Sumxmy ...... 063

Chapter 3: Regional basement structure of centrai Alberta...... 065 3.1 A heterogeneous basement: regional domains of central Aibe rta...... -066 3.2 Crustal-scde seismic data and basernent structure ...... 078 3.3 Basement stmcture and the sedimentary cover of central Alberta ...... 091

vii Chapter 4: Conmls of modern recf development ...... 0% 4.1 Thecarbonatefactory ...... 096 4.2 The modeni carbonate growth wmdow: the requisite environment ...... 097 4.3 General conmls on carbonate buiidups ...... -10 4.3.1 Antecedent topography ...... 100 4.3.2 Tectanism ...... 103 4-33 Eustatic sea-level changes ...... 104 4.4 Artificial reefs ...... 106 45 Discussion ...... 106

Chapter 5: Basement influences on the Keg River Formation of north-cenûd Alberta . 108 5.1 Geological setting and stratigraphy ...... 111 5.2 Data availability ...... 118 5.3 Peace River Arch tectonism and basement dacetopography ...... 127 5.4 Paleotectonism and Keg River Formation carbonate buildups ...... 144 5.5 3-D scaled physical seismic modelling of a reef-fault block ...... 157 5.6 Future exploration of the Keg River-Senex play ...... 164

Chapter 6: Basement infiuences on the Swan Hills Formation of west-central Alberta .167 6.1 Geological setting and stratigraphy ...... 169 6.2 Data availability ...... 173 6.3 Basement structure and surface topography ...... 179 6.4 Tectonic heredity of the sedimentary cover ...... 203 6.4.1 Faulting in the sedimentary cover ...... 203 6.4.2 Drape folding in the sedimentary cover ...... -206 6.5 Basement control on Swan Hills Formation carbonates: previous work .. -21 1 6.6 Paleotectonism and Swan Hills Formation carbonate buildups ...... -216 6.6.1 Isolated Swan Hills Formation buildups ...... 216 6.6.2 The Swan Hills Formation upper plab...... 225 6.7 Tectonic dolomitization of the Swan Hills Formation ...... -226

Chapter 7: Basement infiuences on the Leduc Formation of east-central Alberta ....-233 7.1 Geological setting and stratigraphy ...... -238 7.2 Data availability ...... 240 7.3 Basement strucaire and surface topography ...... 263 7.4 Smcturai cmtd on Leduc Formation carbonates: previous work ...... 265 7.5 Paleotectonism and Swan Hills Formation carbonate buildups ...... -270 75.1 The Rimbey-Leduc-Meadowbrookreef chab ...... 270 7.5.2 The Bashaw rcef cornplex ...... 292 7.5.3 The soudiem Alberta sblf margin ...... -304 7.6 Discussion ...... -316

Chapter 8: Summary and conclu~m~...... -319 8.1 Limitations of an integrated gbophysical methodology ...... 319 8.2 Stnictural elements of the crystalline basement in central AIberta ...... 320 83 Tectonic inûeribnct of the Phaneromic cover in central Al berta...... -321 8.4 Modes of bastment-Devonian carbonate interaction m central Alberta ...-323 8.4.1 Re-depositid stnictures ...... 323 8.4.2 Syn-dcpositiond stnictures ...... 327 8.4.3 Post-depositional structures ...... 327 8.5 Implicaticms for future hydrocarbon exploration ...... 328 8.6 Future work and recomniendations ...... 329

References ...... 333 List of Tables

Table 1.1 . List of seismic &tasets used in study ...... 009

Table 3-1 . Bibliography of surface and subsurface lineament trends ...... -û9S

Table 5-1 . Acquisition and proîcssing parameters for lines Panny 1 and Pmy2 .... -120 Table 5-2 Acquisition and pmcesshg parameters for 3-D smey ...... -122 Table M. Summary of dimensions md materiais for reef-fault block mode1 ...... 159

Table 61. Acquisition and pracessing parameters for line CCk ...... 174 Table 6-2- Acquisition and processing parameters for line CCk2 ...... -176 Table 63. Aquisition and processing parameters for line Antel ...... 177 Table 0-4. Acquisition and pmessing parameters for htEdsonl ...... 178

Table 7.1 . Acquisition and processing parameters for the CAT seismic data ...... 241' Table 7.2 . Acquisition and prccessing parameters for line RB ...... -245 List of Figures

Figure 1.1 . The Devonian succession in central Alberta ...... 003 Figure 1.2 . Regional map of Upper Devonian reef complexes in central Alberta .....004 Figure 1-3 . Location map of seismic datasets utilized in study ...... 007

Figure 2.1 . Total-field aeromagnetic map of central Alberta ...... 017 Figure 2-2 . Bouguer gravity map of central Albeaa ...... 020 Figure 2.3 . TotaLfield, vertical- and horizontal-gradient profiles over a prhn ...... 022 Figure 2. 4. Calculation of a horizontal-gradient vector ...... 022 Figure 2-5 . Magnetic horizontal-gradient vector (HGV) map of central Alberta ..... 025 Figure 2-6. Bouguer gravity HGV map of centrai Aiberta ...... 026 Figure 2-7 . Scaling of magnetic HGV maps (ploûed dom)...... 028 Figure 2-8. Scaling of Bouguer gravity HGV maps (plotted downbill) ...... 03 1 Figure 2.9 . Magnetic and Bouguer gravity HGV maps plotted uphill ...... 034 Figure 2-10 . Bouguer gravity rnodelling of a prism ...... 038 Figure 2-1 1. Magnetic modelling of a prisrn ...... 040 Figure 2-12 . Magnetic modelling of a prism rotated 45" clockwise ...... 043 Figure 2-13 . Magnetic modelling of a prism rorated 30" clockwise ...... -044 Figure 2-1 4. Magnetic modelling of a prism with reduced relief rotated 30" clockwise -045 Figure 2-15 . Magnetic modeIling of a multiple prism mode1 rotated 30" clockwise .. -046 Figure 2.16 . Magnetic modelling of a flat dome or plate ...... -050 Figure 2-17 . Magnetic modelling of a dome ...... 052 Figure 2-18 . Magnetic modelling of a dipping iayer...... -054 Figure 2.19 . Geological map of the Northern Intemides of the Wapmay ûrogen .... 057 Figure 2-20 . Map of Wopmay Orogen geology and total-field magnetic data ...... -060 Figure 2-21 . Map of total-field magnetic and magnetic HGV &ta ...... 061 Figure 2-22 . Interpreted magnetic linearnents and gealogy of the Wopmay hgen... 062

Figure 3.1 . Regional basement domain map of central Alberta ...... 068 Figure 3.2 . Map of total-field magnetic data and basement domanis in central Alberta -070 Figure 3-3 . Map of magnetic HGV data and basement domains in central Alberia ....073 Figure 3. 4. Map of Bouguer gravity data and basement domains in centra1 Alberîa ...079 Figure 3-5 . Map of gravity HGV data and basernent domains in central Alberta. .... 081 Figure 3-6 . Crustai-mie display of Central Alberta Transect seismic data ...... 084 Figure 3-7 . Migraîed cohemcy-filterd seismic line CCkl ...... 089 Figure 3-8 . Migrated coherency-filtered seismic line Antel ...... 092

Figure 4-1 . Modern carbonate growth window ...... 098 Figure 4-2 . Replicathg the optimum growth couditiolls for a srnaIl coral reef ...... 098 Figure 4-3 . Building blocks of a reef ...... 101 Figure 4.4 . Summary of factors favouring reef developrnent ...... 101

Figure 5-1 . Keg River Formation carbonate buiidups in north-centrai Alberta ...... 109 Figure 5-2 . Stratigraphie chart far the Devonian of the eastern Peace River Arch .... 108 Figure 5-3 . Precambrian basement surface contour map. north-central Alberta ..... -113 Figure 54. Keg River Formation paleogeography ...... 116 Figure 5-5 . Schematic cross.sectim, eastern Peace River Arch ...... -117 Figure 5-6 . Characteristic velocity log and synthetic tie ...... 119 Figure 5-7 . Total-field magnetic data and Keg River buildups. north-centraI Alberta .-123 Figure 5-8 . Total-field magnetic and rnagnetic HGV data, noah-centrd Alberta .... -125 Figure %9 . Magnetic HGV data (plotteci iiphill'). north-centrai Alberta ...... 128 Figure 5-10 . hterpreted magnetic lineaments. notth-centrai Alberta ...... 130 Figure 5-1 1. Bouguer gravity data and Keg River buildups. north-centrai Alberta ... 132 Figure 5-12 . Bouguer gravity and Bouguer gravity HGV data, noah-centrai Alberta . 134 Figure 5-1 3 . Bouguer gravity HGV data @lotted 'upbiLl'). north-central Alberta .... 136 Figure 5-14 . Interpreted Bouguer gravity lineaments. n0rth-cenüa.i Alberta ...... 138 Figure 5-15 . Migrated seismic iines Panny 1 and Panny2 ...... 141 Figure 5-16 . 3-D seismic survey grid ...... -145 Figure 5-17 . Representative inline (#41) section ...... 146 Figure 5-1 8. Representative crossline section ...... 147 Figure 5-19 . Precambrian time structure map ...... 149 Figure 5-20 . 3-D perspective view of the Precambrian basement surface ...... 150 Figure 5-21 . Recambnan fault stmcwe map ...... 151 Figure 5-22 . Near-Keg River Formation (basal anhydrite) time structure map ...... 152 Figure 5-23 . Watt Mountain Formation time structure map ...... -153 Figure 5-24 . Precambrian to near-Keg River Formation isochron map ...... 155 Figure 5-25 . hterpreted Aecambnan thestructure map. northern survey atea ..... 156 Figure 5-26 . Pian view of. and cross-sections across. reef-fauit block mode1 ...... 158 Figure 5-27 . 3-D physical modelling over a rtef-fault block mode1 ...... 160 Figure5-28 . Physical mode1 timeslice at 82011s...... 162 Figure 5-29 . Physical mode1 theslice at 860 ms ...... 163 Figure 5-30 . Types of reef-tàult bIock interaction. eastern Peace River Arch ...... 165

Figure 6-1 . Swan Hilis Formatim paieogeography. west-centrai Alberta ...... 168 Figure 62. Stratigraphy of the Beaverhill Lake Group. Swan Hills area ...... 170 Figure 6-3 . Swan Hills Formation reservoirs in west-central Alberta ...... 172 Figure &4. Synthetic tie down to the Rewnbrian basement (Mobil PR Carson) .... 175 Figure 65. TotaI-field magnetic data and Swan Hills buildups. west-ceniral Alberm .. 180 Figure (35. Total-field magnetic and magnetic HGV data, west-cenaal Alberta ..... 18 1 Figure 6-7 . Magnetic HGV data (plotteci 'uphill'). west-central Alberta ...... 182 Figure 6-8 . Intexpreted magnetic lineaments. west-centrai Alberta ...... 183 Figure 6-9 . Bouguer gravity data and Swan Hills buildups. west-central Alberta .... 184 Figure 6-10 . Bouguer gravity and Bouguer pvity HGV data, west-central Alberta .. 185 Figure 6-1 1 . Bouguer gravity HGV data (plotted 'uphill'). west-central Alberta .....186 Figure 6-12 . Interpreted Bouguer gravity lineaments. west-central Alberta ...... 187 Figure 613. Uninterpreted migrated seismic data fkom Line CCkl ...... 188 Figure 6-14 . Intexpreted migrated seismic data fiom iine CCkl ...... -188 Figure Cl5. Uninterpreted rnigrated seismic data fiom Iine Autel ...... -188 Figure 616. Interpreted migrated seismic data from line Antel ...... -188 Figure 617. ScaM stacked version of line CCkl ...... -195 Figure 618. Scaled rnigrated version of iine CCkl ...... 195 Figure 6-19 . Interpretation of a scaled rnigrated heCCkl ...... 195 Figure 620. Scaled migrated version of line Antel ...... -199 Figure 621. Inteqretation of a scaied migrated line Ante 1 ...... -199 Figure 6-22 . Example of an inclined reflecbr intersecting the Recarnbrian dace. . -199 Figure 623. Migrated seismic data fiom üne Edsonl ...... 204 Figure 6-24 . Line CCkl after flattening on the Ireton Formation ...... -208 Figure 625. Line CCkl after flattening on the Watt Mountain Formation ...... 208 Figure 626. Line Antel after flattening on the SWS marker ...... 212 Figure 6-27 . Interpreted line Antel after flartening on the Watt Mountain Formation . .212 Figure 6-28 . The Swan Hills Formation lower platform. Carson Creek North ...... 218 Figure 6-29 . Stratigraphie cross-section across Carson Creek North ...... 219 Figure 630. Unùitcrpreted migrated seismic data hmline CCk2 ...... -221 Figure 631. InterpretEd migrated seismic datahm heCCk2 ...... -221 Figure (i-32 . DoIomite resemoirs and magnetic HGV heiunents ...... 228 Figure 633- Localized dolomitizatim of the Swan Hilis Formation, Rosevear Field .. 229 Figure 634. Schematic mode1 of tectonic doIwiitization ...... -231

Figure 7.1 . Map of weIIs penetratïng the LRduc Formation, east-central Alberta .... -234 Figure 7.2 . Woodbend Group reef complexes. east-central Alberta ...... 236 Figure 7-3 . Cross-section across cenîral Albcaa of the Woodbend Group ...... 239 Figure 7.4 . Location map of avaiiable seismic pmfb ...... 242 Figure 7-5- Synthetic tie dom to the Precambrian basernent (PCP Killarn) ...... 246 Figure 7-6. Total-field magnetic data and Leduc buildups. easî-central Alberta ...... 247 Figure 7-7 . TOM-fieldmagnetic and magnetic HGV data, east-central Alberta ...... 250 Figure 7-8 . Magnetic HGV data (plotted 'uphill'). east-central AlberCa ...... 251 Figure 7-9 . Interpreted magnetic linearnents, east-centrai Albena ...... 253 Figure 7-10 . Bouguer gravity data and Leduc bddups. east-central Alberta ...... 255 figure 7-1 1. Bouguer gravity and Bouguer gravity HGV &ta, east-central Alberta .. -257 Figure 7-12 . Bouguer gravity HGV &ta (plotted iiphill') .eastcentral Alberta ..... 259 Figure 7-1 3. Interprmci Bouguer gravity lineaments. east-central AIberta ...... -261 Figure 7-14 . initiation of Leduc Formation reefs on depositional highs ...... 268 Figure 7-15 . Migrateci seismic data kom CAT iine 5 ...... 271 Figure 7-16. Migrated seismic data fkom line RB ...... 272 Figure 7-17 . Cornparison of migrateci seisrnic data from iines 5 and RB ...... 273 Figure 7-18 . Scaled migrated version of line 5 ...... 276 Figure 7-19 . Interpretation of a scaled rnigraîed line 5 ...... -276 Figure 7-20 . Scaled migrated version of line RB ...... 276 Figure 7-21 . hterpretation of a scaled migrateci line RB ...... 276 Figure 7-22 . Line 5 after flattening on the SWS marker ...... 281 Figure 7-23 . Interprcted line 5 after flattening on the SWS marker ...... -281 Figure 7-24 . Line RB after flattening on the SWS marker ...... 281 Figure 7-25 . Interpreted heRB after flattening on the SWS marker ...... 281 Figure 7-26 . Line 5 after flattening in the Deadwood Formation ...... 287 Figure 7-27 . hterpreted line 5 after flatiening in the Deadwood Formation ...... 287 Figure 7-28 . Line RB after flattening in the WwoodFormation ...... 287 Figure 7-29 . Interpreted line RB after flattening in the Deadwood Formation ...... 287

xiv Figure 7.30 . Mïgrated seismic data from CAT line 7 ...... -293 Figure 7.31 . Interpreted migrated seismic data fiom CAT line 7 ...... 293 Figure 7.32 . ScaIed migrated version of Iine 7 ...... -296 Figure 7.33 . Iatcrprctatim of a scaied migrated linc 7 ...... -296 Figure 7.34 . Linc 7 after flattening on the SWS der...... -2% Figure 7.35 . Interpreted line 7 afkflattening on the SWS marker ...... -2% Figure 7.36 . Line 7 after flattening in the Elk Point Group ...... -296 Figure 7.37 . hterpreted liue 7 ahrfiattening in the Elk Point Gmup ...... -296 Figure 7.38 . Migrated seismic data fkom CAT lines 9 and 10 (line 9/10) ...... -305 Figure 7.39 . Scaîed migrateci version of line 9/10 ...... 307 Figure 7-40 . Interpretation of a scaied migrated line 9/10 ...... -307 Figure 7.41 . Line 9/10 after flattahg on the SWS marker ...... 310 Figure 7-42 . Interpreted line 9/10after flattening on the SWS marker ...... 310 Figure 7.43 . Line 9/10 after flattening in the Elk Point Group ...... 310 Figure 7-44. hterpreted line 9/10 after flattening in the Elk Point Group ...... 310 Figure 7-45. Compresseci display of basement structure and Leduc Formation reefs .. 3 17

Figure 8-1 . Modes of basement-Devonian carbonate intetaction. central Alberta .... -326 Figure û-2 . Location map of wells penetrating the base of fish Scdes Zone ...... 331 CHAPTER 1

Introduction

" Borehole myopia, isotropism, and symmetresis are occuparronaZ geobgical diseases of petroleum geologists anà geophysicists, diseases which inhibit Mectfve analysis of &a as well as development of sound exploration philosophy. Undermnding of the Mture of the basentent and iîs relation to and injiuence on sedintentluy basiras is the first srep kkto geological Wth."

P.T. Flawn (1965, p.883)

As the introductory quote by Flawn (1965) suggests, the crystalline basement beneath a sedimentary basin cm have a significant influence on the subsequent stnictd and stratigraphie development of the sedirnentary cover. However, for one reason or another, such an ancestry is not always recognized. In the case of the Western Canada Sedbentary Basin (WCSB),most earlier publications have underestimateci basement control on basin developrnent; but more recentiy, the relationship between basement faults and Phanerozoic geology is receiving considerable attention. Nevertheless, when basement controis on local basin stnictrve are acknowledged, this is usually done in faicly genera.1 tem. For example, Greggs & Greggs (1989) have carried out a thorough compilation of earlier work that hypothesized basement faulting as a control on sedimentation in the WCSB, and synthesized these concepts into a mode1 of fdt-block tectonism in the WCSB. The present smdy attempts to develop, apply and evaluate several techniques, mainly geaphysical ones in tandem with geological ones, in order both to provide some conclusions for, or against, the hypothesis of extensive tectonic control on the deposition of Devonian carbonate facies on a number of different scales in a central Alberta region of the WCSB.

1.1 Background The subsurface of central Alberta contains one of the best known Paleozoic reef provinces in the world. In this extensive area, Middle and Upper Devonian strata generally consist of 2 shallow-water reeiàl and platforml arbmatcs, evaporites, siliciclastics, and widesptcad deeper-water basin-f5ll shde units (Figuxe 1-1). Through drillhole and seisniic data acquired over nearly five decades, the subsurface distribution of these Devonian cabnate facies has been mapped fairly extensively withm central AIberta and contains many linear arrangements (Figure 1-2). Numerous depositional and diagenetic models have been published for these carbonate buildups. On the other han& the mechanisms behind the initiation and development of such features remains unclear. There are still fundamental problems that are unresolved and new insigbts are necdtd to assess the validity of current hypotheses. This saidy endeavors to shed some Iight on the question of why Middle and Upper Devonian recfs and platforms grew where they did in central Alberta.

In other sedimentary basins, a close correlation has frequently been observed between basement structure and carhme depositional patterns, For example, in the Canning Basin of NW Austraiia, the carbonate buildups associateci with the socalled Dsvonian Great Baker Reef are of sirnilar age and morphology to those found in central Alberta and it is generally accepted that they bave ken at least partialiy influenceci by basement tectonics (Begg, 1987; Playford et al., 1989). A similar relationship between Devonian carbonate basin NI and basement has been posarlated to exist in central Alberta ever since the initial discovery of such reefs in the late 1940s. However, a fundamental difference between these two areas is the obvious effects of structurai control within the sediments of the Canning Basin. This inchdes numerous srnail- and large-scale faults, locally abundant mineralization and abundant nepûtnian dykes (cement-filleci extension fractures). Over the past Hty years, apart ibn studies of carbonates in the Peace River Arch area of north- central Alberta, evidence supporthg a structural cantroi on carbonates in central Alberta has been largely inconclusive. Previous work on the topic of basement-cover interaction in central AIberta is surnmarized in Chapter 5 through 7.

Of all the drillholes that intersect these Devonian carbonates, only a small fraction actuaUy continue downwards and intersect derlying units. Our understanding of the nature of the foundation upon which Devonian reefs and platforms initiateci is lirnited and, at best, extremely localized Table of hrmations

-- Slave Point .Fort Vermillion Givetian

l'hi Devwian succession m central Alberta, modifieci from AGAT Laboratones (1988); BHL Gp = Beaverhiii Lake Group. Stage boundaries are approxhate cmiy. Swan Hilis Fm

-- Swan WsFm upper platform 7 -Swan Hills Fm lower pladom 1 O 100 km v

Figure 1-2. Regional map of Upper Devonian reef complexes and platforms in central Alberta (modified fiom Mountjoy, 1980). Note the distinctive NNE alignrnent of carbonates, particularly in east-central Alberta. The disaibution of these, and other, carbonate buiidups is discussed in Chapters 5 through 7. 5 1.2 Dissertation objecîives Simply put, the main focus of this research is to find out what evidence is availabIe to support a comection between reefd morphology and the crystalline basement in the subsurface of central Alberta. Specincdy, several different geophysical datasets are examineci using recentIy developed techniques for evidence of basement structure or basement &&ce morphology that can be correlated with the distributional trends of Devonian carbonate buildups. Pre- and post-Devonian sedimentary successions in areas containing carbonate buildups are also evduated for direct or indirect evidence of reactivated kztonic acitivity-

An important subject to be addressed in ibis study is the deformation of the Phanerozoic cover in areas containing Devonian carbonates, From a geological perspective, this dissertation sets out to describe any evidence for tectonic heredity in the Devonian to Carnbrian succession of cenaal Alberta. The term tectonic heredity refers essentially to the ancestral or evolutionary relationship that fauIts and folds developed in cover rocks bear to structures in the cystalline basement or at the basement surface. A major objective is to place some constraints on the question of whether any such parentage may even be postulated to exist in areas where few structures have been identifiai pviously.

This dissertation dso attempts to provide at Ieast partial answers to more particular questions, such as: 1s it possible to delineate a dominant basement structural fabric? What is the configuration of the basement surface ldybeneath Middle and Upper Devonian carbonate buildups? Where was the basement origidly high in these areas, and where has it been elevated by later tectonism? What interactions, passive or active, have actualIy taken place between the Precambnan crystalline basernent and the Devonian sedimentary cover that has accumdated upon it? And, if Devonian reef and platfonn development appears to be controlled by fault trends, did vertical throw or differential compaction and erosion create the relief that promoted reef growth? In fact, did these reefs always develop in shallower water over topographie highs?

By virtue of recently acquired or donated crustal-scaie seismic data and the cuxrent availability of potcntial-field data across the central AIberta study area, the possibility of more fuliy addressing these questions is continualiy increasing. Because of the significant economic importance of hvonian hydrocarbon reservoirs in centrai Alberta, greater understanding of their origins and morphologies is clearly significant to Merreef 6 exploration, The implications of establishing basement umtrol on carbanate scdimentation are varied. If a duiect relationship between retf morphoIogy and movement along basement zones of weakness can bt shown to exist, it rnay be possi'ble to prtdict other potential phys by exiraplation of pvenbasement trends or lineamcnts to adjacent areas- Reactivatim of faults may have crtated conduits not only for the verticai migration of hydrocarbons to suitable resemoirs but also, in other places, as a potcntial dolomitization mechanism. In areas such as central Alberîa, with a long history of production hmDevanian carbonates, new targets can only be found by more refined geology. Flawn (1%5, p. 884) wrote that "thisbegins with the beginning, it ~rarrswith the henrent't

13 Methodology Several types of geophysical and geological datasets are available for this study. The geophysical data include hundreds of kilometers of seismic profdes, together with regional magnetic and Bouguer gravity data. Interpretations of these data are geologically constrained by extensive drillhole data found in many areas of this highly active hydrocarbon exploration region. In îbis dissertation, these multiple datasets are iategrated to provide bth a local and regional approach to understandhg the effects that structures in the crystahe basement cm have on Devonian carbonates. Integrated analysis is needed to overcome the problem of non-uniqueness in standard geaphysical interpretations. As Beaumont & Tankard (1987, p. vii) state: #no single piece of evidence is diagnostic of a process ... it is only through a synthesis of what rnay initially appear to be disparate data thrprogress in our understmiding is mader'.

Three principal units and geographical areas of Devonian carbonate deposition are investigated in this study: Keg River Formation (Ek Point Group) carbonates of north- central Albe- Swan Hiiis Formation (BeavdLake Group) carbonates of west-central Alberta, and Leduc Formation carbonates (Woodbend Group) of east-central Alberta (Figure 1-3). These areas were selected primarily on the basis of seismic data availability and the amount of geophysical and geologicai investigation undertaken by earlier studies. The regional distribution of these thrce Devonian fonnatims in cenual Alberta is fairly well hown fiom a large number of published sources. In addition, access to extensive proprietary datasets was also made avaifable to this study by Unocal Canada Ltd and Dinard Resources (Calgary). Possible basement interaction with these carbonates is investigated on two contrasting scales for each ara: locaI basement conml on individual Figure 1-3. Location map of seismic daîasets utilited in this siudy, together with an outline of the three distinct areas of carbonate sedimentation studied within central Alberta: Keg River Formation carbonates of north-cenîrai Alberta; Swan Hills Formation carbonates of west- central Alberta, and Leduc Formation carbonates of east-central Alberta. 8 reefs or reef compIexes using seismic profiIes and weU-log data; and the effect of larger tectonic elements on reef complexes and platfoms through regiond aeromagnetic and - gravity data. By combining these two scales of data, an ampt is made to relate local basement fabric identifiai on seismic data to the regional structural heworkinferred fkom potential-field data.

Two basic types of seismic data are available to this study crustal-de profiles recorded with extended iistwing times, and standard oil industry profles nodyrecorded to image only the Phancrozoic sedimentary cover (Table 1-1). In addition, shailow records fkom some seismic lines may be extrapolateci to obtain deep mflections by the process of 'self- tnincating' extended correiation (Okaya & Jarchow, 1989). This technique requires both a Vïbroseis source and data that have originally been recorded in an uncodateci fonnat. It presmes the bandwidth in the original record times but continues with an ever-decreasing bandwidth for the extra correlation time. Importantly, a proficïent source is stiU needed to ensure good signal penetration to longer Iistening times. This rnethod was tested on heRB in east-central Alberta and is described m Chapter 7.

Crustal-scale seismic data allow basement structure to be examined at depth, thereby allowing locaiized shallow faulting and folding to be placed in a large-scale structural framework, Prior to the early 1990s, very few seismic nflection profiles were recorded with sufficiently long listening times to properIy image basement structure in central Alberta. In 1992, as part of LITHOPROBE'Sobjectives to gain a better understanding of the deep crystalline basement, a 520-km transect was recordai across east-central Alberta. To supplement this regional aansect, basement interpretation is assisted by several industry- donated datasetS. Although originally recordeci with extended listening tirnes, these seismic data were initially processed to image only the sediientary cover. Processing and repr&essing of these various seisrnic datasets was undertaken using Landmark Graphics Corporation's Insight software on a UNE workstation at the LITHOPROBE Seismic Processing Facility (LSPF) at The University of Calgary. Acquisition and processing parameters associated with individual seismic iines are described in detail in Chapters 5 through 7.

Coherency filtering of seismic &ta tends to accentuate strong events but mask more subtle reflectors. This makes it gdfor crustal-sale interpretation but not so appropriate for Area Line Company Record length o=c.) Mrth-central Albertir Panny River Panny 1 Sigma Explorations Ltd. Panny River Panny2 Sigma Explorations Ltd. north-central Alberta 3-D survey International Colin Energy Ltd, West-central Alberta Carson Creek North CCk 1 Husky Oil Ltd, Carson Creek North CCk2 Talisman Energy Ltd, Ante Creek Ante 1 Unocal Canada Ltd, EdsonErith Edson l Mobil Oil Canada Ltd. East-central Albertq east-central Alberta LITHOPROBE (central Alberia transect [CATI: Homeglen-Rimbey Compton Resources Ltd.

Table 1-1. List of seismic datasets used in study. Figure 1-3 shows the location of each of these lines, 10 more detailed studies of the shallow basement. In this saidy, coherency nltering was only applied to datasets m order to outline the overall regidstnichire of cenfral Alberta.

A fundamental problem often associated with these deepcrnstai seismic datasets is contamination by long- and short-period muitipIe energy fiom the overlying sedimentary cover, as outlined by Vasudevan et al. (1993). Subsequent processing, perfomed by various contractors and at the LSPF, has generaIiy failed to remove these features completely. Subhurizmtal rmtltipies appear to be strongest i,mmdhly k1ow the basement event, thereby severely biadering the tracking of low-angle rcflectors to the basement-cover interface. In an attempt to resolve any low-amplitude dipping events that may be hidden amongst these multiple reflections, the shallow part of the data has been strongly scaled. This scaling also assists interpretation of the sedimentary Section.

In order to remove possible interpretational arnbiguities associated with tectonic overprinting and heterogeneous velocity anomalies, seismic sections are nonnally Qattened on several horizons above and below Devonian carbonates. For example, the authenticity of paleotopographic highs identifiai on the basement surface bedow Devonian carbonate buildups is ohsubstantiated afka seismic section has been flattened on the first reliably picked horizon imrnediately underlying a particular buildup. This is to account for lateral velocity variations frequently associated witb on-buildup and off-buildup rocks. Furthemore, the occurrence of localized thifining of pre-Devonian units over such a basement structural high indicates that the topographie high probably existed at the tirne of unit deposition.

In order to improve our understanding of some of the interpretational problems often associated with seismicalIy imaging features underlying carbonate buildups, physical sei&c modeIlkg was undertaken fen an area of north-&ai Aiberta. Acquisition of a 3- D survey over a reef and fauk-block mode1 was possible using the CREWES (Consortium for Research in Elastic Wave Exploration Seismology) physical modeIlhg faciliaes at The University of Calgary and is meroutlined in Chapter 5.

To facilitate the picking of Phanerozoic events on seismic data, a signifiant number of deep and shallow exploratory wells were utilized in this study. Synthetic seismograms were constnicted using digitizcd sonic logs and formation density logs, courtesy of Digitech Information Services Ltd., accessed at The University of Calgary. Correspondhg 11 formation-top information was avaiIabIe from two database sources, Digitech Information Se,Yices Ltd. and Geobase (CD Pubco Inc.), enabling a comparison of log picks and, hence, reliability. The LogM software of Geaphysicai Mim Cornputer Applications Ltd (GMA),a well log-based seismic modeIling system for personal cornputers, was then used to produce synthetic seismograms fiom a variety of dHcrent wavelets.

Signincantly, only a very limitecl numbei- of wells actually penetrate the Devonian in the central Alberta shidy area. For exampIe, Iess than 150 weiis reach the Middle Devonian Eik Point Group and only 32 wells extend to Precambnan basement depths in east-central Alberta, giving a very sparse sampling of the basement sutface of approximately one well per 2000 square kiiometres (roughly 20 townships). However, a number of these drillholes are found feasonably close to seismic profiles, enabling fairiy conndent ties to the basement surface. Recently, as part of the Industrial Partnefs Program of the Geological Survey of Canada (GSC), members of the Luwer Paleozoic Project (as outîined by Nowlan, 1994) have investigated cores and cuttings from these east-central Alberta wells that penetrate pre- Middle Devonian unis and even intersect the Prccambrian in some cases. This dissertation has directly benefited fiom the geological approach of the Lower Paleozoic Project by incorporating evidence of localized tectonic activity fiom some of these examinexi web. In north-central Alberta, because the basement lies at a shallower depth and the fact that several exploration targets lie directly above the basement, the number of wells reaching basement depths increases considerably. In fact, an estimated 85% of the reported 4000 basement intersections throughout Alberta come from this area (Ross, 1990). hterpretation of basement structure relies on a synthesis of available seismic profiles and well data with regional magnetic and gravity data over relevant areas of central Alberta. Public-domain aeromagnetic coverage in centrai Alberta has ken improved subsmtially through surveys conducted recently by several consortia consisting of the GSC and selected oil and mining companies. Compiled by the GSC, the national aeromagnetic database covering central Alberta consists of both data digitized from contour maps collected before the advent of digital recording @re-1980) and more recent digitally acquired data. Importantly, these various surveys have been levelled to a common dam through either upward or downward continuation (as outlined by Dods et ai., 1985). One should be mindful of the fact that merges of adjacent surveys are sometimes evident in the compiled regional magnetic maps and these features may be ampIified in derivative maps. Areas of north-central Alberta between 55" and 58" N are covered by digitized data 12 recorded in the early f 950s. To the south, digitally flown aeromagnetic surveys cover several blocks of central Alberta. Ont of these surveys (latitude 52" to 54" N, longitude 110" to 118" W) has just recently been released as GSC Open File 3235. Data hman adjacent survey are due to be released in 1997. Typicai fiight-lioe spacmg for these surveys is 1500 m. Based on the available coverage, the magnetic data fm central Alberta used in this dissertation was griddeci at a 2-lmi inte~alfrom the 1991 data base. These data were plotted using RTICAI) (Real Timc Imaging and Cornputer Aided Drafting), a PC-based software package developed by GEOPAK Systems, at the LSPF. Map sheets at a scale of 1:250,000 and digital &ta for seIected areas may be obtained .fkom the Gcophysid Data Center (GDC) of the GSC, in Ottawa The nationai gravity database is similarly available through the GSC. In the case of centrai Albexta, coverage by pvity data is complete- Gravity data were recorded, and therefore resampled, more sparsely, with a coxresponding Ioss of resolution, Bouguer gravity values were computed and griddecl at a 5-km interval from the 1991 data base and again plotted using RTICAD software. Map sheets at a scale of 1: 1 ,ûûû,ûûû and digital data are avaiIabie from the GDC.

Generally, for reasons outlincd in Chapter 2, most magnetic anomalies found in central Alberta are believed to be sourced in the crystalline basement. On the other hand, gravity anomalies are believed to reflect laterd density conmm both in the basement and in the sedimentary cover, though the most prominent anomaiies are still thought to orighate in the basement. Potential-field interpretations are not unique, but can be constrained by availabIe seismic profiles and the large number of drillholes located in cenûal Alberta. By vircue of the information they cm reveal on lithological and stnictwal grain in the basement, magnetic and gravity maps, in conjunction with stratigraphie information, constitute a powerful tool in the recognition of geologic inheritance in sedirnentary basins.

One.technique of transfomiing and displayhg potemial-field data that appears to highIight source-body edges is the horizontal gradient of the field. The harizontal gradient is a two- component vector quantity containina both magnitude and directional information. Conventionally, only gradient magnitude has been displaycd on particular rnaps. However, in this dissertation, an algorithm for generating and plotting magnitude and direction by vector-arrow length and azimuth is utilized: the horizonml-gradient vector (HGV) method. This type of mapping of gravity or pseudogravity data (the latter derived from magnetic data; see Baranov, 1957) should produce patterns that conespond to source-body edges by highlighting the loci of extrema of the horizontal gradient. Smctly, this is valid only for 13 vertical-sided bodies and for the gravity or pseudogravity field, and not for total-field magnetic data, Nevertheles, synthctic 3-Dmodehg descri in Chaptcr 2 mdicates that the HGV method cm still produce patterns from aeromagnetic &ta that assist m source- body edge detectim. In thîs dissertation, it is used in conjuncticm with total-field magnetic or Bouguer gravity data. Coupled with geological idonnation, the HGV technique off'a tool for mapping major lithologid or sûuctural contacts in the basement.

1.4 Dissertation ontiine In this d.isseri&on, chapters can generally be categorized hto two main sections: those that provide a detailed overview of the geophysical methods and geological background necessary for an integrated anaiysis of basement-carbonate interaction; and those chapters that deai specifically with each of the three carbonate units and atws sel& for study-

Chapter 2 provides an outline of the horizontal-gradient vector (HGV) method and descrii the >D synthetic modelling of geometric bodies likely to camprise the basement of central Alberta- A review of probable sources for regional magnetic and gravity anomalies observed in central Alberta is also provided. From correlations with established geoIogical feanires in the exposed Canadian Shield of the Wopmay Orogen, it is shown why overall patterns of regional magnetic HGV Lineaments are beIieved IargeIy to reflect Iithological and sîructurai contacts in the basement

In Chapter 3, a framework for the regional basement structure of central Alberta is presented. This combines a synthesis of earlier findings with recent seismic and potential- field HGV interpretations undertaken as part of this dissertatioa

In order to evaluate the influences that basement structures may have had on carbonates depositd about 370 million years ago, it is first necessary to appreciate the factors controllhg contemporary carbonates. Chapter 4 examines the various processes having a direct and indirect impact on modern reef development

Chapters 5,6 and 7 detail examples of proposeci basement coneol on carbonates of the Keg River, Swan Hills and Leduc Formations, respectively. In each of these chapters, the regional geologicd background and stratigraphy of a particular formation is summarized, together wirh an outline of the hydrocxbon potential and available data. Local and regional 14 features interpreted in the shallow basement and rn the basement surface are highlighted and compartd to pre-Devonian stratigraphie variations and Devonian carbonate buildups. Both direct and indirect examples of geoIogical hertdity between the basement and sedimentaq cover are identifiai m several key areas of oentraI Alberta.

Finally, Chapter 8 sunimarizes the major hdings of îhis study. Modeis that schematically illusirate the major types of interaction between Recambrian crystalline basement and Middle and Upper Devonian abnates in central Alberta are presented CHAPTER 2

The potential-field horizontabgradient vector (HGV) method

" Whor SMwe say thofthe double quest whcrc new knowledge jhgs wi& the doors offiesh discoveries, d wkre science [ays hethhgs imisibk kneath the earth? "

Eve & Keys (1929. p. ix)

Through the examination of magnetic and gravity maps, useful insights can be gained on the rcgional structure of sedimentary basins. Though such maps lack the vertical and lateral resolution of seismic data, they can help locate lithdogical and structural boundaries in the subsurface. Within central Alberta, one cment challenge is to elucidate in more detail the distribution of such feams, both in the crystalline basement and the sedimeatary cover. Through a better understanding of re@onal basin structure, we rnay be able to more My descn'be the largtr mechanisms of structural control on Devanian carbonates.

Anomaly-enhancement techniques based on taking residuals, derivatives, shadowgrams, or downward continuation can ali k used to sbarpen subtle magnetic and gravity variations. Several of these processing methads were considered to help delineate structural and lithological trends from potential-field data in central Alberta, In this dissertation, apart from available deep seismic reflection profiles, the horizontal gradient of magnetic and anomalies is cOI1Sidered one of the most appropriate indicatm of regional basement structure. A vector version of the horizontal gradient is used to generate a series of potential-field maps of varying scales where distinctive lineaments an readily apparent This chapter considers why horizontal gradients, and the vector version in particular, are suitable for preferentially detecting edges of bodies. 16 2.1 Utility of magnetic and gravity data in centrai Alberta Magnetic anomalies are reldto lithology ttirough total rock mapetization, remarient plus induced (Reynolds et al., 199û). Magnetization is controiied Iargely by the distriiution in rocks of the mineral magnetite. Sedimentary rocks usuaiiy contain less mapetite, and are therefore Iess magnetic, than igneous or metamorpbic rocks. Magnetic susceph'bi1ity is a signilïcant parameter in magnetics and is expressed as the ratio of the intensity of magnetization acqukd by a substance to the strength of the magnetizing force acting on the body. Average magnetic susceptiiilities for basernent rocks cm range from 4,400 (SI units) for metamorphic rocks up to 32,000 m the case of basic igneous rocks, often resulting in signincant W wntrasts (Dobrin & Savit, 1988). In cornparison, average magnetic susceptiiiIity values for sedimentary rwks may range hm25 (in the case of dolomite) to about 150 (m the case of shale) (Dobrin & Savit, 1988). T'us, variations m magnetic-field intensity over many sedimentary basins result fiom 1ithoIogical changes m the crystaiiine basement, or from intrabasinal volcanic layes and plutons.

In central Alberta, igneous rocks are rarely found in the Phanerozoic sedimentary cover, and most magnetic anomalies are sourced in the crystaiiine basement. This conclusion is supported by the similarity of wave-number spectra of aerornagnetic data over the Western Canada Sedimentary Basin and the Canadian Shield, after appropriate upward continuation is applied to zcount for different &pths to source (Teskey et ai., 1989). Small anomalies due to magnetite-rich placers, secondax-y rnineraiization, etc., may nevertheless occur in centrai Aiberta For exampie, local magnetic anomalies may be caused by Cretaceous Crowsnest Formation volcanics, which have a Iimited distribution in southwestern Alberta.

Gridded aerornagnetic data from the central Alberta study area, show in Figure 2-1, was extracted from the &tabase of the Geological Survey of Canada (GSC). Most of this area is covekd by surveys digitaiiy recorded in 1990, 2992 and 1992. Where not recorded digitally, the data have been digitized from existing aeromagnetic surveys carried out in 1951-1953 using an average flight altitude of 305 rn above ground level. Records were created by locating the intercept of a contour interval (normaUy every 10 nT) with flight iines. Any discrepancy in altitude between surveys was accounted for by upward or downward continuarion (Dock et al., 1985). The data have hem reduced using t4e Definitive Geomagnetic Reference Field (DGRF) (LangeI, 1992) for the year of the partidar survey and adjustments made to remove residual errors at survey boundaries. The DGRF provides an improved regional field representation over the International Figure 2-1. Total-field aeromagnetic map of central Alberta study area, scale i:5,000,ûûû.Data hm the Geological Survey of Canada; grid spacing 2 km. 18 Gmmagnetic Refereace Reld (IGRF) (PiUcington & Roest, 1996). The digitized and digitdly-recordeci magnetic prome data were gridded at the GeophysicaI Data Center (GDC)of the GSC at a cell size of 2 km by 2 km with no high-cut or anti-aliasing filtering applied prior to gridding (W. Miles, personal commuaication, 1997). Due to the 2-Ian sampling of the data in the griddbg process, anomaIies with wavelengths shorter than 4 km may therefore be aliased. However, this is not thought to be a significant ptoblem m aeromagnetic data fiom centrai Aiberta because most signifïcant magnetic anomalies are believed to be sourced within the basement. The average depth to basement regionally varies across the centrai Alberta study mahm about 1-4 in the NE to greater than 5.5 lan in the SW (Burwash et al., 1994). A common deof tfiumb is that we can expect anomalies caused by sources near the basement Surface to have wavelengths equivdent to about twice the distance fiom the sensor to the source body (after Ebner et al., 1995). Therefore, the possibility of wavelengths of less ttian 4 km king aiiased by a 2-km sampling interval is Iargely restricted to the NE part of the study area. However, aliasing of relatively short wavelength anondies could be a factor where source bodies are not buried under a considerable thickness of mostly non-magnetic sediments. This basic iimitation may apply to 2-km gridded aeromagnetic data fiom the Northern inteniides of the Wopmay Orogen, Northwest Territories, discussed m Section 2.5. Further discussion on the basic compilation of aeromagnetic data by the GSC is found in Dds et ai. (1985). The dataset was plotted using RTICAD (Red Time Imaging and Computer Aided Drafting), a PC- based software package developed by GEOPAK Systems, at the L~IHOPROBESeismic Processing Faciiity (LSPF).

The amount of magnetite in basement mks may Vary patiy: Dobrin (1976, p. 534) cautioned that "themagnetic reliefobserved over sedimentary basin areas is almost always controlled by the lithology of the basement rather thmr by its topography ': The presence of many strong aeromagnetic anomalies in the relatively flat Canadian Shield confirms the criticai importance of iithology variations. By cornparison, the local effects due to topographie or paleotopographic relief on the surface of the Shield are minor. Although magnetic contours can successfully map out trends related to laterai changes in rock type, additional information is necessary to determine reliably the structure of the basement surface.

Gravity anomaly maps reflect laterd density variations in the subsurface. Density contrasts occur in both the crystalline basement and the sedimentary cover but the most prominent anomalies, still likely reflect basement structure. Rocks from the Canadian Shield of 19 northeastern Alberta can be used to estimate realistic ranges of values expected for basement rocks of central Alberta. High-grade metasedimentary rocks, rnafic-rkh granitoids and granïtic gneisses have relatively high average deosities between about 2650 and 3000 kg/m3 while more felsic granitoids typically have relatively low average densities of 2600 to 2700 kg/m3 but there is substantial overlap in many cases (Sprenke et al., 1986). By cornparison, likely density con- as derived hmborehale measurements in the sedimentary section seldom exceed 250 kg/m3 (Dobrin & Savit, 1988). Spredce et al. (1986) relateci gravity anomaly patterns fÎom northeastem Alberta to Merences in near- dacelithology, subsurface rock density, sa~ctualfabric and metamorphic facies. Where geological contacts are not sharp, a gradational density contrast results in a relatively smooth gravity anody profile (Gendzwill, 1970). A gradual change in metamorphic grade analso pduce a gradual change in density.

The Bouguer gravity anomaly map of the central Alberta study area (Figure 2-2) was consmicted from gravity measurements spaced approximately 8 km apart. One basic limitation of a 8-km station spacing is that ifmdywavelengths shorter than 16 km exia in a particular area, they are likely to be aliad. As outlined by Goodacre et d. (1987), ail gravity measurements have been nduced to the Intemational Gravity Standardization Net 1971 (IGSN71) dam(MoreIli, 1974). Theoretical gravity values have been calculated fiom the Geodetic Reference System 1967 (GRS67) gravity formula (International Association of Geodesy, 1971). The density used for the Bouguer correction is 2670 kg/m3.Terrain corrections have been applied to the data, with sea level used as a datum. The largest source of uncertainty in the Bouguer anomaly caiculation is due to the uncertainty in the station heights. These are detennined by barometric levelling have a relative accuracy of about &5 m. The final accuracy of readings is about k2 mGaI (1 mGal-10" m/s2). The data was interpolated onto a 5-km grid at the Geophysical Data cent& (GDC) of the GSC.

Correlation between gravity and magnetic anomalies may ohbe complicated The prirnary reason is that magnetic and gravity maps reflet different rock properties, and basement structures may affect variations of these properties differently. For example, muieralization in fracture zones by fluids may give such zones a strong magnetic signature, far out of proportion to the amount of displacement on these faults. A second factor is that the magnetic field is dipolar, which complicates that anomaiy pattern fuaher. A third conmbuting factor is the unequai grid spacing of avaüable data; the magnetic and gravity 0-w 1ii-w I 1 1 1O'W 116'W 1 14'W

Figure 2-2. Bouguer gravity rnap of central Alberta study area, scale 1:5,000,000. Data from the Geological Survey of Canada; grid spacing 5 km. 2 1 data were gridded at 2-km and 5-km intervals, rcspectively, resuiting in dissimilar resolution.

On their own, magnetic and gravity interpretations are not unique, but hard constraints are available in the study area fiom thousands of drillholes, together with regional and local seismic profiles.

23 The horizontal gradient of potentiai-field data Figure 2-3 illustrates that even in the case of a relatively shallow (c 2 km) source with steeply dipping contacts, the total-field anomaly has smoothcd or smeared boundaries. The correspondhg vertical-gradient anomaly has a maximum over the center of the causative body and a namw surrounding ring.

Mapping the horizontal gradient of magnetic and gravity data is believed to enhance the images of anomaly-causing geomemes such as steep body edges (Hoad & Teskey, 1989). For gravity and pseudogravity anomalies (transformed total-field magnetic &ta; see Baranov, 1957), which are both intrinsically vertical-component data, a maximum horizontal gradient marks a vertical contact for long, linear faaires but may be subject to error for corners or small objects. Because western Canada lies at a high magnetic latitude (inclination > 70°), and total-field data is similar to vertical-component data, this relationship can also be proposed for regional magnetic data. However, the maximim horizontal gradient of magnetic data rarely coincides exactly with a vertical contact but the error involved is considered negligile in the case of the 2-km sampled magnetic data used in this dissertation. Reduction to the pole (RTP),to eliminate the effects of nonvertical inclination of the anomaly-inducing geomagnetic field in advance of horizontal-gradient calculations can not contribute much at such latitudes (Lyatsky et al., 1992). Furthemore, the relatively small lateral anomaly shifts (< 1 km) that would be involved at such magnetic inclinations may weil be accompanied by signal degradation that wipes out any net benefit- For exarnple, Jain (1988) undertook a modelluig study involving several RTP operators and concluded that anornaly distortion is typically introduced over bodies that have non- vertical sides. As applied, this extra processing step may be avoided at least initially in order to determine whether body edges appear to be present and mappable in a particular area. Then, if exact location of such edges is desireci, these extra steps could be appiied on JI - 6AT and& i J 1-1 I \ 1 +totai field

Fi- 2-3. North-south oriented profiIes of total-field, vertical-gradient and horizontal-gradient data over an infinite prism at the magnetic pole (after Hood & Teskey, 1989). This illustrates the advantages of using horizontal-ment data for the detection of steep body edges such as faults.

(within 5 x 5 window)

Figure 24 Calculation of a horizontal-gradient vector by centerhg the local window at each node of the data grid (after Lyatsky et al., 1992). Nodes are centred on grid-line intersections. 23 a case-by-case bis. Overall, such source-body edge detectioa is a usefiil tool h regidy mappmg basement grain, Mt patterns and mtmsive igaeous bodies.

Horizontal-gradient techniques are amoag the most mtuitive derivative methods in tenns of conceptuali7irtion, and among the simplest in tenns of mathematics. Based on the horizontal gradient, automateci dgorithms for detectmg edges in digital images have been devised in geophysical and remote-sensing applications. Previous techniques of horizontal-gradient computation can be classifieci as meof three principai methods: finite differences (CordeIl, 1979; CordeIl & Grauch, 1985), wavenumberdomain digital filter design (Dole 6 Jordan, 1978), and least-squares polynomial fitting (Sharpton et al,, 1987; Thurston, 1991). These - computational methods, reviewed by Thunton (1991), demonstrate that horizontal-gradient maps are an effective means of detecthg trends.

23 The horizontal-gradient vector method Earlier techniques generally collsidered the magnitude of the horizontal gradient, a scalar, and maps were generated simply by contowing or colour-coding gradient magnitudes. The directional aspect of the horizontal gradient was ignore& In the search for structural features in cemtral Alberta, the horizontal-gradient method used in this dissertation involves vector computations that take into account gradient directionality. ConceptuaUy, the horizontal gradient is a two-component vector quantity, having magnitude and direction, that rnay be understood in terms of an analogy with topographic relief (Lyatsky et al., 1992). The direction of a ment vector is that of the steepest increase of the quantity in question, for example, elevation, and is perpendicular to contclurs of, for example, elevation. The gradient magnitude is -test where the contour spacing is least, and vice versa.

A detailed discussion of HGV computatim rnethods was provided by Lyatsky et al. (1992) and Thurston and Brown (1994). The latter papa preseats a more computationally eficient variant which was utilized in this dissertation with a number of subsequent alterations. A horizontal gradient is calcuiated by least-squares fitting of a polynomial to a window of grid points centred on the node for which calculations are performed (Figure 2-4). At the maaI point of the window, the componeats of the HGV are determined by computhg first derivatives in two orthogonal directions (x and y). A thicd-order polynomial and a 5 by 5 window appeared to be the optimal choice for al1 the available data. Secondader surfaces 24 contain no infiedons and are thus capable of degrading the quaiity of the fi^ Orders higher than tfiree, although aiiowing for a better mathematical fit to the data, also quite often introduce greater error because of the increased importance of any spurious data points. The third order is Iow enough for the best-fit surface not to be greatly affecteci by such noise.

The HGV algorithm is essentîaiiy a hi&-fiequency cutoff, or smoothing, operator. In addition, with mmmagnetic and gravity anomalies found in centrai Alberta sourced in the basement, bigh-hquency anomalies are not prevalent because of muting by the sedimentary cover, The reduction of frequency content brought about by HGV dculation generaliy improves the coherency of basement-sourced regional anomalies. Thurston (1991) and Thurston & Brown (1992) established that cut-off mencies are inversely proportid to the window size and proportional to the polynomial der. In other words, fiequency content decreases with an increasing window size and a lower-order polynomial fit.

At any point on a map, an fIGV can be displayed as an arrow whose orientation represents the direction of the maximum change or gradient and whose length is proportional to the magnitude of the gradient, In this scheme, anows originate at grid nodes and can be generated either to point away hmlocal maxima in the potential field (i.e. dom,or negative gradient) or toward the local maxima (Le. uphill, or positive gradient). Vector maps are plotted using the public-domain UNIX-based GMT-SYSTEM mapping software hmLamont-Doherty Geologic Observatory. In HGV maps, the vector arrows are longest in zones of maximum horiontal gradients.

PIorting the potential-field HGV data in a single colour, nodyblack, has proven to be a usehl tool for highlighting lineaments trends. To illustrate this, total-field magnetic and magnetic HGV maps of the entire central Alberta study area are shown in Figures 2-1 and 2-5, respectively. In addition, Bouguer gravity data ftom this same region (Figure 2-2) cmbe compared wiîh the Bouguer gravity HGV equivalent (Figure 2-6). Vector arrows in both these HGV maps are ploued away from local maxima, or downbill, and are seen to clearIy accentuate subtie potentiai-field anomalies, giving the interpreter a feel for the remre (Lyatsky et al., 1992) of the magnetic or gravity field. However, lineament trends in regional maps such as Figure 2-5 are of- produceci by darker shading due to a greater Figure 2-5. Magnetic horizontal-gradient vector (HGV)map of the central Alberta study area; scale 1:5,00,000. Vector arrows are plotted pointing away from local maxima or downhiii. Figure 26. Bouguer gravity horizontal-gradient vector (HGV)map of the central Alberta study area; scale 1:5,000,000. Vector arrows are pIotted pointing away from local maxima or downhili. 27 densiiy of mtersectmg imowheads. It is the tail-ends of such arrows that wïiI approximately map out the edges of vertical-sided bodies.

On a regional scale, potential-field HGV data nodydisplay a wide range of arrow Iengths. GeneraIly, areas containing large and srnd horizontal gradients ca~otbe adequately displayed an one HGV map. In most cases, it is thezefore necessary to generate a suite of magnetic or gravity HGV maps for a particulaf area, containulg a range of scaling parameters. In the HGV aigorithm used in this study, it is possible to select a value by which to sale the vecux amw length. A specific scaling factor may prefere~ltiallyhighlight arrows of a particular length. Through sigdicant testing of merent scaIing factors, it is possible to determine an optimum scaIing factor for an arta For example, Figure 2-7 shows several magnetic HGV maps of east-central Alberta plotted with mows poinMg downhill using a wide variety of scales. Overall, Figure 2-7B cm be considered to be plotted using the optimum scaling factor for vector mwsin this ana Similarly, Figure 2- 8 contains a Scaled range of Bouguer gravity maps for eaa-central Alber& In this case, the best scaIing factor seems to be that used for Figure 24B. Linearnents observed on these various magnetic or gravity HGV maps are compiled to produce a.overall htefpretation. Elsewhere in this dissertation, all displayed magnetic and gravity HGV data are plotted usirtg only the optimum scaling factor. However, interpretation of such HGV maps used the full suite of scales, as displayed in Figures 2-7 and 2-8.

In addition to scaled magnetic and gravity HGV maps, different polaritïes of HGV data can also be generated, in a manner somewhat analogous to normal and reverse polarity in seismic data. As mentioaed previously, vector arrows are computed either to point away from, or toward, local maxima in the potential field (domand uphill, respectively). For example, Figure 2-9 contaias the uphül versions of the optimally scaled Figures 2-7B and 2-8B. On cornparkg these plots of opposing &ow directions, it is evident that although the same edges or boundaries are pnsent in both versions, they are not always straightforward to inteqret. This is ohdue to the dominant effect of arrowhead shading on HGV map intcrpretation, particdarly in the case of the higher-resolution magnetic data. Visually, it is easier to pick lineaments from HGV data pIotted using both polarities. For the three areas of centrai Alberta investigated in this study, outIined in Chapter 1, both a downhill and an uphill version of the magnetic and gravity HGV data are presented (Chapters 5 through 7). Figure 2-7. Magnetic horizontal-gradient vector (HGV) maps of east-central Alberta iuustrating the effect of scaling on overall vector arrow display; scale 1 :3,750,000: (A) scaling parameter 0.2, (B) scaling parameter 0.4, (C) scaling parameter 0.6, @) scaling parameter 0.8. Vextor arrows are plotted pointing in the negative-gradient direction or downhiI1. chiginal data frorn the Gedogical Survey of Canada, grid spacing 2 km. (J3) was plotted ushg the optimum scaling parameter.

Fie28. Bouguer gravity horizontal-gradient vector @GV) maps of east-central Alberta illustrating the effect of scaling on overd vector arrow display; de1:3,750,00: (A) scaling parameter 0.2, (B) scaling parameter 0.3, (C) scaling parameter 0.4, @) scahg parameter 0.5. Vector arrows are plotted pointing in the negative-gradient direction or downhiU. Original data from the Geological Survey of Canada, grid spacing 5 Zan, (B) was piotted ushg the optimum scaling parameter.

Figure 2-9. Potential-field horizontal-gradient vector (HGV)data hmeast-central Alberta, plotîed with optimum scaling parameters determineci hmFigwes 2-7 and 2-9 but with vector arrows pointing in the positive-gradient direction or uphill; scale 1:3,750,000. (A) magnetic HGV map using a scaling factor of 0.4, (B) Bouguer gravity HGV map using a sca1i.gfactor of 0.3,

36 A full set of magnetic and gravity HGV maps are normally cornpkd, compareci to the original total-field magnetic or Bouguer gravity data, anci interpreted to produce final maps of magnetic and gravity HGV Iinearnents in an area. An interpretation of the potential-field HGV data fiom east-central Alberta shown in this chapter (Figures 2-7 to 2-9) is fomd m Chapter 7.

2.4 Synthetic 3-D modehg of potential-fieid HGV data In order to quantitatively conmin or guide the intexpretation of gravity and magnetic HGV maps, it is useful to carry out rnodening saidies mcorporating some simple geologic bodies likely to comprise part of the basement of central Alberta. Importantly, modelling studies help to vewwhether any aaifacts are introduced durhg the caldation of HGV data. As mentioned previously, the calculation of HGV &ta assumes that vertical body edges produce potential-field gradients that are steepest directly over these boudaries. This assumption is ody strictly valid for .vertical-compnent data sucb as gravity and pseudogravity. However, because rnagnetic data hmcentral AIberta has a steep inchation (> 70°), it is proposesi that body edges can be similarly detected in magnetic data. This supposition is verined through the examination of seved synthetic rnagnetic HGV data examples.

Three main model geometries were considered: a prism, a dome, and a dipping layer. Models were created using Sieds MIMIC geologic modeliing software package and then loaded into LCPs S3MOD, an interactive 3-D potentid-field modeihg program. Mode1 dimensions were taken from an examination of typicd potential-field HGV lineament patterns found in east-central Alberta (Edwards et ai., 1996). Some models have been simplifieci to represent topography on the basernent surface. Such a basic two-layer model eliminates any possible ambiguity in tems of relating the potentiaI-field anomaly to a particular level.

Magnetic and Bouguer gravity HGV maps were computed for the various models. A complete suite of synthetic magnetic fIGV maps and a representative Bouguer gravity HGV example are presented in this dissertation. In the case of Bouguer gravity modelling, an average density of 2500 and 2700 kg/m3 was used for Phanerozoic sediments and crystalline basement rocks respectiveiy (after Dobrin, 1976). For magnetic modelling, constant magnetic susceptibility values of 500 and 25,000 (SI units) were used for 37 sediments and basement rocks respectively (after Dobrin, 1976)- An inducing field of maguitu& 59500 UT,an inclination of 765" and a declination of 21 O were calculated hm the Intematid Geomagnetic Refierence Field (IGRF) 1990 model for eastcentral Albcrta. The rnodelling assumes ali magnetization is induced with no component of remence. Synthetic Bouguer gravity and magnetic data were computed using a grid spacing of 5 km.

2.4.1 Prism model The pnsm model represents a basernent block that may have been created by orthogonal- trending faults, heterogeneous basement rock litbology, or a combination of rhese two geologicai scenarios. The aim of such syathetic modelling is to show that gravity and magnetic HGV maps can accurately and easily detect the edges of such blocks, regardless of their exact ongin.

The basic model consists of a 100 km by 100 km prism centred in a survey area of 1000 km by 1000 km and raiseci 200 m above a reference plane located at a depth of 1.8 km (Figure 2-1 OA). Using a grid spacing of 5 km, the entire survey area contains 20 1 by 20 1 grid nodes. The calculated Bouguer gravity data and comsponding Bouguer gravity HGV maps across the prism mode1 are show in Figures 2-IOB, 2-10C and 2-10D. For consistency, a scaling factor of 0.1 was used in the generation of aU synthetic HGV data. F'rism boundaries are easily defhed by a zone of Ionger vector arrows that represent greater gradient magnitudes. Arrowhead directionality where amws either point to, or away from, a boundary depends upon the relative changes in magnitude across that particula.contact. When vector mows are plotted away fkom local maxima, the origins of the arrows clearly map out the edges of the prism. In the case of vector arrows orienteci toward local ma. arrow tail-ends still outline the same body edges. At the corner points of the prism, where two orthogonal-mnding edges are juxtaposed some minor vaIiatims in arrow directionality are apparent.

The same basic vecmr patterns can also be recognized m magnetic HGV data over a prism model. However, in contrast to the simple Bouguer gravity HGV anomaly pattern associated with the prim model, corresponding magnetic HGV patterns are slightly more complicated. In the calculated total-field magnetic map (Figure 2-1 lB), a prism boundary is associated with a pair of high and low anomaly values. Although this results in a number of small magnitude HGV mwhead reversais adjacent to the prism edge, the overd extent of the prism is again delineated by zones of longer HGV arrows (Figure 2-1 lC, 2-1 ID). T& prim model: (A) mode1 geome~r,(B) Bouguer gravity data; (C) Bouguer gravity HGV data with vector mows plotted away fiom local maxima, or dowuhiil, with an outhe of the prism; @) Bouguer gravity HGV data with vector arrows plotted toward local maxima, or uphill, with au outhe of the prism.

Figure 2-11. The prism model: (A) mûdtl geomeûy; (B) total-field magnetic data; (C) magnetic HGV data with vector arrows plotted away from local maxima, or downhill, with an outline of the prism; (D) magnetic HGV data with vector atrows plotted toward local maxima, or uphill, with an outline of the prism.

In order to determine whether consistent HGV lincament patterns emerge regardless of data-grid orientation, total-field magnetic data over a series of rotated prism modeIs was cornputeci. Figures 2-12 and 2-13 show a prisru orientcd at 45" and 30" relative to north, respectively. The laterai extent of these rotated prisms is again delineated by a zone of longer magnetic HGV amw lengths with no apparent artifacîs introduced.

To help assess the sensitivity of the HGV &od in a rclatively simple geological scenario, several rotated prïsm models were generated with varying vertical relief. Figure 2-14 shows one such prism, rotated 30" clockwise, with a height of only 10 m. Total-field magnetic anomalies over the prism bowidanes in this model have significantly reduced amplitudes compared to anomalies associateci with the 200 m prism model (Figure 2-13). On comparing the comqmding HGV maps shown in Figures 2-13B and 2-1 4B (using a consistent scaling factor of 0.1), conttasting HGV mwlengths across these boundaries are also evident. Vector arrow lengths in Figure 2-14B do not appear to Vary considerably across the model. However, prism boundaries are subtly rnafked by a reguiar pattern of HGV arrowhead reversais. It would appear that a 10 m vertical displacement is close to the resolution Mt of the HGV metiiod, at lest for this particular model. In general, this type of sensitivity analysis is heavily dependent on the type of geological model, the susceptiiility contrast between the layers, and the level of non-mode1 "noise' in the data

A more complicated HGV pattern arises when a mode1 comists of more than one prism. Multiple prisms coliectively define a basement block fianiework, a geological scenario discussed in Chapter 3. Four juxtaposed two-layer prisms were modelled at depths of between 1.4 and 2.0 km, and a reference plane at 1.8 km. These prisms were oriented at 30" relative to north, thereby avoiding the dgnment of any body edges with the grid orientation. In order for this four-bhck rnodeito have w inanecting edges or cross-over points, it is necessary to separate individuai prisms by a small gap (Figure 2-15A). Such a gap results in a nurnber of edge effects in both the total-field magnetic and magnetic HGV data (Figures 2-15B and 2-15C). Several small variations in HGV arrowhead orientation are evident adjacent to prism boundaries. These variations in smali magnitude vectors can result when a 5 by 5 window centred adjacent to a particular prism edge contains a grid node that lies over an edge. Overall, prisrn boundaries are defined by zones of longer vector arrows. Figm 2-u* Prism model rotated 45' clockwise: (A) total-field magnetic data; (B) magnetic HGV data with vector amours pointhg away hmlocal maxima, or downhili, and an outiine of part of the vrism peorneay. The model dimensions are the same as the basic prism model shown b ~ig&2-la Fi2-13. Prism model rotated 30" clockwise: (A) total-field magnetic data; (B) magnetic HGV data with vector arrows pointing away fiom local maxima, or downhill, and an outhe of part of the pnsm geometry. The model dimensions are the same as the basic pnsm mode1 show in Figure 2-1OA. Figure 214. Rism modcl, with reduced vertical relief, rotatd 3W cloc%se: (A) total-field magnetic data; (B) magnetic HGV data with vcetor arrows pointuig away aOm local maxima, or downhill, and an outiïne of paR of the prism geomeay. The mcdel is the same as the basic prism body shown in Fi- 2-10A but with a height of only 10 m. Figure 2-15. Multiple prism mode1 rotated 30" clockwise: (A) mode1 geometry; (B) total-field magnetic data; (C) magnetic HGV data with vector arrows pointing away from local maxima, or downhill, and an outline of part of the mode1 geometry.

49 2.4.2 Dame model Another common geometric shape is the dome which may represent a pluton or intrusive body within the basement of centrai Alberîa. AdditionaUy, a dome-IÏke body could be produced fkom the erosion of a regular prism at the basement surface. In contrast to the near-vertical sides and level top of the prism, the flanks of the dome model slope gradually away from its apex. Two types of dome-like body are considered= a relatively flat dome or plate; and a standard dome.

The fiat dome or plate has relief of 200 rn above the reference plane and the radius of its circular projection, as seen in mep view, is 50 km 2-16A). Significantly, this body slopes less than 1' away fkom its apex. Figure 246B shows the calculated magnetic data across the model. The base of the dome cm easïiy be Iocated on magnetic HGV data by a series of longer vector arrows, both in the case of arrows plotted pointmg away hmlocal maxima (F@re 2-16C) and toward local maxima @gure 2-16D).'Ibe relatively flat nature of this plate-like modcl is reflected in the radiahg pattern of small magnitude vectors.

To test how the HGV algonthm responds to a feature with a great depth extent, a dome (or standard dome) model with a height of 10 km and a radius at its base of 50 km was generated (Figure 2-17). The fia& of this modd slope gradudy away fkom its apex with an average inclination up to 15'. The extent of this dome is clearly defineci on magnetic HGV data, particularly by a well-defined diverging pattern of HGV arrows when arrows are pointing away fiom local maxima (Figure 2-17C). In contrast to the flat dome or plate model, a rnarked variation in HGV mow length is obsewed within the radiating pattern of vectors, indicating that the gradient is changing considerably fiom the apex to the boundary .

2.4.3 Dipping-layer model The third major type of model consists of a plane dipping at approxhately 27' between two levels. This inchation corresponds to the apparent dip of some intra-basernent reflections identifid on seismic profiles acquired across areas of central Alberta, discussed in Chapter 3. In order to get such a Iow-angle dip, it is necessary to gnatiy exaggerate the amount of vertical relief on the model. As illustrateci in Figure 2-l8A, the two-layer mode1 is centred around a 10-km-wide zone that extends fkom a depth of 1.5 to 6.5 km. This low- angle step is orientecl 30' relative to north, again avoiding an alignment with the grid Figure 2-16. The fiat dome (or plate) model: (A) model geometry; (B) total-field magnetic data; (C) magnetic HGV data with vector mows pointing away hmlocal maxima, or downhiU, and an outline of part of the dome base; @) Magnetic HGV data with vector arrows poinhng toward local maxima, or uphill, and an outline of part of the dome base.

Fii217. The dome model: (A) model geometry; (33) tod-field magnetic data; (C) magnetic HGV data with vector arrows pointing away from local maxima, or downhill, and an outline of part of the dome base; @) Magnetic HGV data with vector arrows pointing toward local maxima, or uphill, and an outiine of part of the dome base.

Figure 2-18. The dipping Iayer model: (A) model geometry; (B) total-field magnetic data; (C) magnetic HGV data with vector mows pointing away from local maxima, or downhill, and an outline of part of the model geornetq-

56 orientation. A zone of longer vector arrows in the rnagnetic HGV map 2-18C) successfuily locates this mchd zone.

Modemg of simple geometric bodies serves to illustrate the aptitude of the HGV method for edge detection. Significantly, HGV lineament patterns emerge regardas of data-grid orientation and contain no apparent m.The edges of bodies are marked by I112UCimum horizontal gradients or the longer arrows in both gravity and rnagnetic HGV data. Basement features may be distinguished on the basis of consistent orientations and lengths of vector arrows, ohbounded by sharp breaks in the arrow patteni- Importantly, the effect of arrowhead shading is to produce some prominent lineaments that indicate the shape of bodies but not always the exact location. To accurately delineate body edges it is necessary to locate the tail end of long HGV arrows.

2.5 Geological corroboration of HGV interpretation: an example hmthe Wopmay Orogen Even though it fias been derncmstmed that HGV maps can preferentially locate edges, there stiU remainn an uncertainty in terms of the actual origin of such edges. In order to test the validity of the HGV method for delineating lithology and structure in the subsurface of central Alberta, it was first necessary to understand magnetic HGV patterns in an a. where the geology has benfairly well established, in an area without sedimentary cover. However, one basic limitation is that smce magnetic bodies could be buried at a shallow depth or even exposed at the surface in such an area, short-wavelength anomalies may be present and become aliased by a relatively coarse sampling mterval.

The test area selected covers the Northem Intemides of the Wopmay Orogen, District of Mackenzie, Noahwest Temtories. Important prequisites for selecting a suitable trial area include available potential-field data and a well-established regional geological model. The Northem Internides of the Wopmay ûrogen is covered by regional aeromagnetic data (2- km grid) from the Geological Survey of Canada and has been extensively mapped ibn fieldwork by numerous workers, summarized by Hoffman (1984). Figure 2-19 is a simplified geological rnap of the noahwestern Wopmay Orogen. A brief description of regional geological characteristics of this mcturaiiy complex area is incorporated into this chapter, extremely simpIified, to provide an elementary basis for conelating different HGV linernent patterns. Figure 219. The Northern Internides of the Wopmay Orogen, District of Mackenzie, Northwest Temtones: (A) Geological map simplifieci fkom Hoffman (1984); (B) map legend. Western channel diabase

Homby Bay Group

Great Bear Intnisive Suites diarite (Gq syenogranite, monzogranite (G3) granodiorite, momagmite (G2) m011206iorite (G1)

Early - Proterozoic

Hepbum intrusive suite Coronation Supergroup The map area extends fkom longitude 116" to 128" W, Iatitude 66" to 67' N and includes two tectonic zones of the Wopmay Omgen: the Great Bear Magmatic Zone, and the edge of the Hepbum Metamorphic Plutonic Belt (Hoffman, 1984). Four periods of coiiisional foreshortening, marked by compressional deformation stages Dl to D4, have been identified in the Wopmay ûrogcn. Great Bear magmatism (1.8û-1.84 Ga) developed after the first collision (Dl), probably after D2. The Great Bear Magrnatic Zone generaily consists of calc-aikaline volcanic rocks and continental sediments intruded by epizonal granitoids. Postdating the magmatisrn in the study area is a system of dominantly northeast- trendhg transcmt faults and oblique east-vergent thnists, comprismg the last signincant period of compressional deformation @4)- This tectonic zone therefore provides an excellent test area for assessing the ability of the HGV method to detect both structural and lithological boundaries.

Figure 2-20 contains an outline of the geologicai map (shown in Figure 2-19) superimposed on the total-field magnetic data. A number of rock units appear to have distinctive magnetic signatures. For example, G2 grauodiorite and monzogranite rocks (Great Bear Intrusive Suites) are associated with relatively high total-field aeromagnetic anomalies. In contrast, Archean basement and Hepbum intrusive rocks are largely marked by anomalously low magnetic field values. The curvilinear boundary between these geological units is clearIy identified on total-field magnetic data (Figure 2-20) and is commonly refend to as the Wopmay Line or fault zone (Hoffman, 1984). However, there are numerous cases where a particular rock unit does not appear to have a consistent magnetic character. The G3 syenogranites and monzogranites provide a good example of this.

For a given area, a suite of HGV maps are nonnally produced that incorporate a range of different scaling factors. Figure 2-21 displays an example of a magnetic HGV map fiom the Wopmay Orogen, with vector arrows pointhg away from local maxima or downhiil, superimposed on the total-field magnetic anomaly map. Identifying the main lineament trends is more straightforward using such a combined display. As illustrated by synthetic modeiliug, HGV lineament trends are hancl-picked on the basis of consistent vector arrow length and orientation to make line drawings or sri~kmaps.On comparing an interpretation of interpreted magnetic HGV lineaments with an outline of the geoIogical map (Figure 2- 22), a number of correlations are evident Firstiy, many northeast-trending lineaments Figure 2-20. Outline of the geological map of the Northem Intcmides of the Wopmay Orogen, shown in Figure 2-18, with total-field magnetic data (wr-Wopmay Line). The magnetic data is taken from the Geological Survey of Canada database, grid spacing 2 km. Figure 2-21. Magnetic HGV map of the study ma, with vector armws plotted pointing away from local maxima, or downhill, with total-field magnetic data. The magnetic data is taken from the Geological Swey of Canada database; grid spacing 2 ian. Figure 2-22. Interpreted magnetic lineaments (in black) with an outline of the geological map of the Northern Internides of the Woprnay Orogen (in grey) (modified from Hofian, 1984). Lineament interpretation is based on magnetic HGV and total-field magnetic anomaly patterns. 'A' represents a series of lineaments that closely correlate with a mapped major fault that separates clastic and ignirnôrite rocks. 'B' outlines a series of faults not detected on regional total-field or HGV magnetic data. A regional bundary between two tectonic zones of the Wopmay Orogen is clearly defined magnetically, marked as 'C'. Several magnetic anomalies, such as 'D', appear to be unrelated to mappcd gcological features. 63 closely coincide with mapped transcurrent faults. For example, 'At in Figure 2-22 highiights one such major stnictural feaatre that separates Hornby Bay Group clastic rocks hmignimbrites of the McTavish Supergroup. ûverall, it is evident that such faults are ody imaged on mgnetic HGV data where they sepatate rock units of conhasthg magnetic character and lithology. Where transcumnt faults nm through a rock unit with a fely unifonn lithology, dey are not resolved on HGV data. This is illustrated by 'B' on Figure 2-22. Significautly, it would appear that regional potential-field HGV lineaments best defiue major faults where such structures mark the contact between different rock types. Secmdly, non-mcatral geological contacts can also be identifiecl using regionai magnetic HGV data in this area. Some northwest- or north-northwest-trcnding cmearelements are aligned with regional variations in rock composition. For instance, the regionally extensive series of heaments marked by 'Cf in Figure 2-22 outlines the boundary between rocks of the ûrcat Bear Magmatic Zone and the Hepburn Metamorphic Plutonic Belt or Archean basement rocks. Thirdly, 'D'in Figure 2-22 illustrates tbat some magnetic HGV lineaments appear to be completely unrelateci to any mapped feature. In such cases, it is plausible that some magnetic anomalies may be associateci with unmapped rock units such as, for example, zoning in plutons or the mineralizatim of fracture zones by fluids.

Overd, it is apparent that iineaments identifiecl on regional magnetic HGV maps generaily refiect trends from a variety of sources. Major tectonic zone boudaries, lithological and faulted contacts are all definecl, Importantly, for the objectives of this dissertation, the capability of HGV data of delineating at least some significant basement feattues on a regional scale has been demonstrated .

2.6 Snmmary Regional magnetic and Bouguer gravity data provide an areal view of basin structure, limited, of course, by resolution constraints and the problem of non-uniqueness. Many large-scde lithological or faulted contacts between basement units of contrasting rock composition can be detected using such regional data.

To date, horizontal-gradient techniques have not been in widespread use, although it has been demonstrated that they could be applied to the mapping of basement geology. The horizontal-gradient vector (HGV) method is used in this dissertation because it is one of the 64 more useful potential-fiel8 processing techniques for the identification of basernent grain. Basement contacts cm be markcd by zones of maximum horizontal gradients represented by longer HGV anows. The HGV method is essentially a pattern-recognition tool but it is important to realize that HGV anowhead shading, particularly in the case of regional magnetic HGV maps, caa produce Iineament trends that weU define the shape of a particular basement feature but not necessady the exact IOcaficm of a partïculat boundary. It is the ongins of the long HGV anows that map out such boundaries.

Although magnetic and Bouguer gravity HGV maps help to define the lithological and structural heworkof the basemen&additional drillhole or seismic information is needed to indicate if a particular feature has relief on the basement SuIf' and, if so, whether it is a relative high or low structure. This is especially relevant for assessing how basement stnicaires have affectai overlymg carbonate sedunentation.

In rhis dissertation, the central Alberta study area depicted in Figures 2-1,2-2,2-5 and 2- 6 is sub-divided into three principal areas: north-cenîral, west-central, and east-central Alberta. The interpretation of potential-field HGV data in these regions, and how the structurai and MhoIogical trends they identify relate to Devonian carbonate moxphology are discussed in Chapters 5 through 7. CHAPTER3

Regional basement strnctute of central Alberta

" ïïwse who cure about husins nuut be concemed with the basin's beginning - to regard the basement as the stopping point insteud of the stmting point is fatal "

P.T. Flawn (1%5, p.883)

As used in this dissertation, the term basement refers to those crystalline rocks which underlie or form the foundations of Phanerozoic sedimentary sequences. The wide diversity, intriate historical, geometricai and rnineralogical relationships of such igneous and metamorphic rocks, characteristic of the basement of most regions, gave rise to the term basement cornplex prucha et al., 1965). This expression "serwed nineteenth century geologists as a catch-al1for the uninteresting &-nt upon which the Earth's structural architecture and stratigraphie record reste@' (Cordell & Grauch, 1985, p. 181). However, with the generai acceptance that basement structures can influence overlying structure, facies and fluid migrations within the sedimentary cover, basement tectonics has subsequently received considerable attention, particularly of late. This concept that younger structures that control Phanerozoic sedimentation could show an inheritance from older Protemzoic basement stmctures is, by no means, new. However, early belief was mostly intuitive due to only indirect evidence, strong on merely a priori grounds (Hoppin & Palmquist, 1965). Clearly, improved knowledge of basement structure would provide valuable insight in determinhg whether any such relationships may have existed in a particular area.

The crystalline basement of centrai Alberta has not received anything like the geological attention that the overlying sedimentary section has experienced due to a general lack of hydrocarbon exploration interest, and the often considerable cost and risk involved with drilling to pater depths. Nevertheless, over the years these hidden rocks have attracted interest from a variety of sources. Rccent discussions concedg the structure, petrology and age of basement in central Alberta include those of Ross & Stephenson (1989). Ross et 66 al. (1991), Burwash et al. (1993), Villeneuve et al. (1993), and Ross et al. (1995). Currently, ongoing multidisciplinary research by UTHOPROBE in the form of Aiberta Basement Transccts (surnmanzed by Ross, 1993a; 1994; 1993, particularly using crustal- scale seismic profiles to map deep subsurface structures, has helped dramatically to improve our howledge of the basement in central Alberta. The main purpose of this chapter is to review briefly recent advances on basement tectonics in this area and compare some of these eariier findings with anomaly patterns yieIded by regional magnetic and gravity HGV maps.

3.1 A heterogeneous basement: regional domains of central Alberta Hoffinan (1988), using field mapping and U-Pb geochronology, ouîlined a heterogeneous tectonic framework of domainal structures for the Canadian Shield. Importantly, he recognized that the Shield was assembled in a series of accretionary and collisional events akin to modern plate tectonics. These events established a series of major cnistai shear zones and magmatic belts or arcs in the western Shield. GeneraUy, Roterozoic magrnatic belts act as a glue (G. Ross, personal communication, 1995) to weld older Archean cmstal provinces or blocks together. Sigaificatltly, these magmatic sutures and shear zones are associated with distinctive aeromagnetic signatures, as outlined by Ross et al. (1991), and others, allowing discrete cmstal domains to be merdisthguished for the western Shield.

In Alberta, early work involving the examination of drillcores that penetrate the crystalline basement was mostly concemed with locally determinhg rock petrology (e.g. Bwash & Culbert, 1976) and geochronology using K-Ar analysis (e.g. Burwash et al., 1962). However, it is the interpretation of potential-field data ihat allows a regionai delineation of basernent domain boundkes, As mentioned previously, regional magnetic data over central Alberta are interpreted as king predominantIy sourced in the basement. Villeneuve et al. (1993) point out that such a depth to source would amuut for the long wavelengths and magnitudes of anomalies commonly observai, and is consistent with the slight loss of the higher frequency components with increasing thickness of sedirnentary cover (Wright et al., 1994, Figure 3.2), or depth to basement (Bmash et al., 1994, Figure 5.1).

Potential-field studies generally using only sparse, low-resolution data available at the time, such as those of Garland & Burwash (1959) and Coles et al. (1970, inferred that anomaly trends marking regional domains continued from the Shield into the Western Canada 67 Sedimentary Basin (WCSB). Using recently acquired aeromagnetic data (shown in Figure 2-3) and through the development of new analytical procedures for isotope analyses of recovered basement Qiucom, numerous authors (e-g. Ross & Stephenson, 1989; Ross et al., 1991; Bunvash et al., 1993; and referaces therem) have establistied that several major tectonic units in the westcm part of the Shield extend benexth the WCSB. Phanerozoic sediments in central Alberta are floortd by Precambrian crysdhe basemcnt that form part of the subsurface western extension of the Canadian Sel&The adjacent Shield therefore constitutes exposed hasement and provides an excellent opportunity to investigate these rocks at mthand. FUrthermore, the expression of feams mapped using potential-field data in the Shield may provide an analogue for constrainmg the interpretation of subsurface anomalies m central AIberta. The conclusion that most magnetic anomalies found in central Alberta originate in the basement is supporteci by the coincidence of magnetic anomaly wavelengths in centrai Alberta with magnetic data fkom the adjacent Shield that has been upward continued to account for near-surface high frequency anomalies associateci with such a shailower basement source (Teskey et al., 1989).

Ross et ai. (1991) have defined a mosaic of Archean and Early Proterozoic tectonic domains for the basement of Alberta and northeastern British Columbia, Figure 3-1 summarizes the tectonic franiework and age map constructeci for the basement of central Alberta. Cnistal domain boundaries were derived primarily from aeromagnetic signatures and precise U-Pb geochronology of basement core samples. However, of the reported 4000 basement drillhole intersections in luberta, only about 400 have recovered core fiom the basement and almost 85% of these originate in the Peace River Arch area (Villeneuve et al., 1993). Some domains in central Alberta (e.g. the Wabamun) contain only one sample of crystalhe basement These domains, together with the location, name and lithology of analyzed basement drillcore or drill cuttings, are fuily demi by Ross et al. (1991) and Vieneuve et al. (1993).

Ross et al. (1991) and ViIleneuve et al. (1993) mapped basement domain boundaries largely by picking or folIowing the zero contour of their plotted aeromagnetic data (Figure 3-2). Simply by changïng their display parameters through redefining the number of colour divisions used in the colour bar, a different crossover point could be recognized. It is possibIe that such a modification in the coIour disuibution or mlevelof the aeromagnetic rnap would result in the delineation of sligbtly conflicting domain boundaries. Potential- field horizontal-gradient vector (HGV) maps are plotteci independent of zcro-level and FieM. Regional basement domain map of the crystalline basement of the central Alberta study area (modifiai fiom Ross et al., 1991). On the basis of regional potential-field data and locaiized U-Pb geochronology, interpreted domains are: the Lacombe (L) domain; Rimbey (R), Talston O and Ksituan (K) magmatic arcs; Wabamun 0,Buffalo Head (B) and Honah (H) accreted temes; Thorsby (Th), Chinchaga (C) and Kiskatinaw (K) magnetic lows; and the Archean domains of Slave (S), Rae (R), and Heame (subdivided into the Lovema Block ml, Eyehill High [He] and Matzhiwin High). Rodent shear zones identified are the Snowbird Tectonic Zone (STZ) and Great Slave Lake (GSL). Cmstal seisrnic lines 1 through 10 correspond to L~OPROBE'Scentral Alberta transect (CAT). Iudustry-donated crustal-sale seismic profiles mclude lines CCkl and Antel. Lacombe domain 1x1 Accreted terrans (2.4-2.0 Ga) Magmatic arcs (20-1.8 Ga) Archean (2.62-8 Ga)

Magnetic lows (24-2.0 Ga) Figure S2. Total-field aeromagnetic map with regional basement domains (in white) of the centrai Alberta study area (modified from Ross et al., 1991), scaie 1:5,000,000. Magnetic data from the Geologicai Survey of Cm,grki spacmg 2 km. Intexpreted domains are: the Lacombe (L) domain; Rimbey (R), Talston (T) and Ksituan (K) magmatic arcs; Wabamun (W), Buffalo Head (B) and Hottah (H) accreted temes; Tborsby (Th), Chinchaga (C)and Kiskatinaw (K) magnetic lows; and the Archean domains of Slave (S), Rae CR), and Heame (subdivided into the Loverna Block IIQ], Eyehill High [He] and Matzhiwin High). Prominent shear zones identified are the Snowbird Tectonic Zone (STZ) and Great Slave Lake (GSL). Crustai seismic lines outlined in Figure 3-1 are highlighted in grey.

72 subsequent interpretations are therefore not restricted by its choice. On comparing the domain map of Ross et ai. (1991) with the rnagnetic HGV map (Figure 3-3), it appcars that the original choice of domaius becomts more jusiïfied. Overall, îhere is a good agreement between domains and contrasting regionai fabncs highlighted by magnetic HGV lineament patterns. The picking of domain boundaries may be justified on a regional scale but introduce potentially signincant geographic variations when used at the local scale.

One of the most prominent basement features in central Alberta is the NE-trendhg Snowbird Tectonic Zone (Sm. This crusta1 smcture is 2800 km long, and was identified by geologic field rnapping and in gravity and rnagnetic maps across the ShieId (Stiarpton et al., 1987; Cornmittee for the Magnetic Anomaly Map of North America, 1987). The subsurface extension of the STZ in central AIberta is also weli estabiished- From gravity data, Burwash & Culbert (1976) referred to it as the Edmonton-Kasba low. Earlier, Burwash et ai. (1964) had inferreci a structurai grain oriented NE-SW in the basement. On a rnap sale of 1:5,000,000, the S'IZ is clearly visible on total-field magnetic and magnetic horizontal-gradient vector (HGV)data (Figures 3-2 and 3-3, respectively), where it appears to extend into the eastern Cordillera The potential-field expression of the Sm is outlined in detail in Chapter 7. Many ideas exist regatding the exact nature of the Sm,but most studies generaüy consider it as a major crustai discontinuity. More specifically,recent field mapping by Hanmer et al. (1994) in the Shield of northem Saskatchewan showed the SïZ is represented by several dipslip and strike-slip shear zones of late Archean age, at least locally. However, an Early Proterozoic age has been postulateci by Ross et aI. (199 1) for the STZ in the subsurfaoe. It is possible that the STZ varies considerably in characrer dong-strike; thus, contrasting deveIopment stages and assigneci ages for formations may be inferred between the exposed and buried STZ.

The Thorsby domain threads its way into a narrow (about 30 km) magnetic low between the Rimbey and Wabamun domains and appears to merge with the STZ to the noaheast. Recovered drillcore fkom this domain consists of two plutonic rocks and a pegmatite. Intedy,the Thorsby Low is characterized by a distinctive striated pattern of Wtrending magnetic HGV lineamcnts, some of whkh appear to localiy crosstut the Thorsby-Rimbey domain boundary. Watanabe [1%5) recognized similar Iinear magnetic anomalies in areas of the Canadian Shield of northeastern Alberta where shearing has produced regional mylonitic zones. It seems likely that this domain represents a ductile southem extension of the S'El as proposeà by Villeneuve et al. (1993). Figure 3-3. Magnetic horizontal-gradient vector (HGV)map with regional basement domains (in rd) of the central Alberta study area (modifieci fkom Ross et al., 1991). scale 1:5,000,000. Vector arrows are plotmi pointing away from local maxima or downhill. Interpreted domains are: the Lacombe (L) domain; Rimbey (R), Talston (T) and Khan (K) magmatic arcs; Wabamun 0,Buffalo Head (B) and Hottah (H) accreted terranes; Thorsby (Th), Chinchaga (C) and Kiskatinaw (K)magnetic lows; and the Archean domains of Slave (S), Rae (R), and Heame (subdivided into the Loverna BIock BI, Eyehill High [He] and Matzhiwin High). Prominent shear zones identified are the Snowbird Tectonic Zone (STZ) and Great Slave Lake (GSL). Cnistal seismic lines outlined in .Figure 3-1 are highlighted in green. The location of Edmonton in shown by the srnd red square.

75 The Wabamun High is apparently stnicturauy enclosed between the STZ and an inferrecl splay of the S?Z to the north- Fmtotal-field magnetic and magnetic HGV data 3-2 and 3-3, respectiveIy), th dornain is clearly dominated by a series of pronou~~ced subcircular to arcuate positive anomalies, up to 20 km across and unevenly distributed. Similar magnetic anomaly patterns have been descrii fiom northeastem Alberta by Sprenke et al. (1986) and directly correlated with granitoid masses. It is piausibk that the distinctive magnetic HGV anomaly patrenrs within the Wabamun domain represent similar pluton-like bodies. However, chis broad domain contains only one basernent drilIcore, a relatively undeformeci tonalite.

The proposed ductile southcm extension of the STZ almg the Thmby Low also marks the noizhem boundary of the Rimbey High, a magnetically prominent curvilinear domain that extends northeastwatds to outcrop in Saskatchewan where it joins the Vigin River sbear zone, interpreted as a splay of the STZ (MacDonald, 1987). The domain cau be subdivided into: a southwestern part characteized by a striatory magnetic fabric, sùniiar to that described for the Thmby Low and possiibly reflecting ductile deformation associated with the STZ; and a northeastern part consisting of subcircular to arcuate positive magnetic anomalies, possibly corresponding to pluton-like intrusions similar to those of the Wabamun domain. Most basement intersections consist of biotite granite (syenogranite to monzogranite). Due to its magnetically positive nature, arcuate shape and granitic Iithology, this domain has been interpreted by Ross et al. (1991) as a possible magmatic belt.

To the southeast of these Sn-Muenced domains lies the Lacombe domain, poorly dated to be Early Roterozoic, and consisting of low-grade supracrusta1 rocks. The southeasteni boundary of this dornain fomthe niagnetically prominent, narrow (10-15 km) Red Deer trend (Figures 3-2,3-3). This NE-SW aligmemt of positive anomalies is particularly well imaged on the magnetic HGV map in Figure >3. Away fkom this distinctive zone, the interna1 fabric of the Lacombe domain is mainly dominated by NE-trading magnetic lineaments and, to a lesser extent, some W-tcending lineaments. Randomly distributed within this pattern ate several subcircuiar magnetic highs, probably of magmatic origia

Eastwards, the study area is underlain by reworked Archean-age cmst of the Loverna Block and Eyehill High, both considered to be part of the Hearne province by Hoffman (1988). In the Lovema Block, both N\H and NE magnetic trends are well developed, together with localized subcircdar to arcuate positive anomalies. AU recovered basernent 76 sampIes core these isolated anOmSLLies and mainly con& of biotitc-magnetite granites, suggesting isolated plutonic activity. Tbe curvilinear N-frenduig boundary between this domaui and hi@-grade metapIutOnic and gneissic rocks of the Eyehill High is weli defÏned on total-field mgnetic and magnetic HGV data (Figures 3-2 and 33, respectively). In con- the southan part of the Heame proMicc contaias the Matzhiwin Hïgh subdomaïn, tentatively marked by a broad positive total-field magnetic anomaly but appears hdistinguishable on the magnttic HGV map. NW- and NE-trendhg magnetic HGV lineaments cleariy cut across this proposed boundary.

The southem extension of domains in north£entral Alberta is termuiated by the inferreci trend of the STZ or a splay of the STZ. The Talston Arc wraps armd the Archean Rae Province and is sharpIy juxtaposai hgthe Sm.The Taiston domain, for the most part, reflects a striated NW-or NNW-irending interna1 HGV fabric consisting of magnetically positive and negative rocks, intersecteci by considerably weaker MWlineaments to the West- The stronger heament trends, marked by relatively long HGV arrows, may be extrapolateci into the exposed Shield of northeastem Albexta where they parallel several regional shear zones recognized by Bostock et al. (1987)- This magmatic belt has a compIex lithology consisting of highly deformed and intnided gneissic and granitic rocks. The Talston domain rnay have a structural eastern boundary, apparently represented by a ductile shear zone known as the Ailan Fault Zone (Sprenke et aL, 1986).

The Buffalo Head texrane is West of, and considerably older than, the Talston Arc and is a composite of largely metaplutonic and metavolcanic rocks (Ross et al., 1991). Magnetically, the N and NE part of the domain is characterized by a series of positive anomalies that &fine a roughly orthogonal arrangement of NE- and NW-trending HGV lineaments. To the SW, this fabric passes into a broad region of negative magnetic anomalies where similar HGV trends are recognized, albeit to a lesser extent. Several discrete subdomains have been recognized on the bais of this variation in intemal magnetic fabric and are srunmarized in Villeneuve et al. (1993).

The western boundary of the Bufiàlo Head terrane is associateci with the margin of a broad W-SE-aligned positive total-field magnetic anomaïy (Figure 3-2) but is more clearly delineated by a pronou~cedheament in the magnetic HGV map 3-3). To the West, lies the Chinchaga hw,a domain rnainly consisting of metaplutonic and metasedimentary gneisses. Ross et al. (1991) suggest that this sharp domain contact may represent a structural contact or relationship üetween these regional b10cks. Cornparcd to adjacent divisions, the Chinchaga domain is distinctive in that it is magnetidy quiet over a relatively Iarge area, rnakmg any features that do have magnetic signahires dyapparent. Overall, this domain contains a number of weak NW-and NE-trendkg curvilinear HGV anomalies.

With characteristics similar in nature to those of the proposed Rimbey magmatic bel&the irregular, cuspate Ksituan domain to the West of the Chinchaga Law is considered to be of magmatic rather than stmcairal origin. htemally, this proniment magnetic high is dissected by NW- and NE-trending magnetic HGV lineaments that mark rnagnetically-positive elongate blocks separateci by nanow Iows, which Vieneuve et ai. (1993) suggest reflect a penetrative deformation. The Ksituan High is tnincated to the NW by a namow NE- trending magnetic low, tentatively named the Kiskatinaw domain by Ross et al. (1991), and UIfemd to be a fault zone, possibly associated with the Great Slave Lake Shear Zone (also known as the Hay River Fault).

Overall, using total-field magnetic and magnetic HGV data, regional basement domains can be recognized through the actual delineation of boundaries and the identification of contrasting intra-domainal fabrics. Significaatly, proposed stnictwal contacts between domains (e.g. Buffalo Head terrane and Chinchaga Low) are well defined on the magnetic HGV rnap. Several distinctive types of magnetic patterns are commody observed in central Alberta, not generally localized to a particular domain- regional NE- or NNE-trendkg linearnents, thought to refiect deformation associated with the STZ; localized curvilinear anomalies trendinp approximately NE-SW or NW-SE,sometimes definhg a block-like frarnework of structural or lithological origin; and randody-distributcd subcircular to arcuate anomalies of varying dimensions, probably related to magmatism- These characteristic anomaly geometries are very similar to magnetic patterns observed by Sprenke et al. (1986) over Canadian Shield rocks in NE Aiberta.

Although both Ross et al. (1991) and Villeneuve et al. (1993) initially cornmented upon characteristic intemal magnetic anomalies of particular domains, they did not recognize that basement fabrics may sometimes cut across same domain boundaries (e.g. the Chinchaga- Ksituan contact). Significantly, the observed crosscutting of largely NW-trendhg magnetic HGV lineaments with interpreted domain boundaries is not readily apparent on published total-field or shaded-relief magnetic maps overhin with basement domains of central 78 Alberta. Such NW-SE beaments represent structural or lithological trends that are obviously younger than the assembly of domains that they emdacross.

Bouguer gravity data also provide a usefd means to regidycharacterize some basement domains, aIbeit to a Iess obvious extent. The Bouguer gravity anornaly map of ce& Alberta, shown in Figure 3-4, is domlliated by a lmg-wavelength gravity Iow associated with crustal thickening in the Corciillem Orogen and the presence of high heat flow in the Omineca Belt of the Cordillera (Sweeney et al., 1991). The Bouguer gravity HGV map effectively suppresses this long-waveiength contribution and accentuates the more subtle gravity anomalies, thcreby allowing a significantly improved comparison with the domain framework (Figure 3-5). However, the nurnber of examples of Bouguer gravity HGV patterns that can be closely tied to the magnetically cons- domains of central Alberta appears to be very lùnited. Villeneuve et al. (1993) pointed out that there are exampIes of major domainai boudaries in the Canadian Shield, such as the Great Slave Lake Shear Zone, that are not coincident with any marked horitontal-gradient magnitude anomalies of Sharpton et al. (1 987). AU toId, apart fiom local conespondence, most domains in central Alberta appear to cut amss regional Bouguer gravity and Bouguer gravity HGV anomaly patterns. The major exception to this rule is the series of promulent NE-to NNE-trending Bouguer gravity HGV iïneaments that are aligned with the Snowbird Tectonic Zone and continue unbroken through the Thorsby Low into the Corciillem (Figure 3-5). Adjacent to this zone, a Bouguer gravity HGV lineament, also thought to be related to this deformational zone, parallels the Rimbey-Lacombe domain contact. To the southwest, the area becornes dominated by NW-SE trends. A more detailed interpretation of gravity HGV lineaments in no&-, west- and east-central Alberta is included in Chapters 5, 6 and 7, respectively.

3.2 Crustd-scale seismic data and basement structure Previously, the number of seismic reflection profiles that were recorded deep enough to successfully image intrabasement structure in central Alberta was relatively small, compared with the vast number of conventional seismic surveys acquired by the oil industry every year. A bibliography of deep seismic studies in the Western Canada Sedimentary Basin (WCSB) published prior to 1990 is found in Ross & Stephenson (1989). As part of LITHOPROBE'Sinvestigation of the deep crystalline basement beneath the Figure 34. Bouguer gravity rnap with regional basement domains (in white) of the cenaal Alberta snidy area (modifieci fÎom Ross et al., 1991), scale 1:5,000,000. Bouguer gravity data kom the Geological Survey of Canada-, grid spacing 2 km. hterpreted domains are: the Lacombe (L) domain; Rimbey (R), Talston (T) and Ksituan (K) magmatic arcs; Wabamun (W), Buffalo Head (B) and Hottah (H)accreted temanes; Thorsby (Th), Chinchaga (C)and Kiskatinaw (K) magnetic lows; and the Archean domains of Slave (S), Rae (R), and Heame (subdivided into the Loverna Block ml, Eyehill High [He]and Matzhiwin High). Prominent shear zones identified are the Snowbird Tectonic Zone (STZ)and Great Slave Lake (GSL). Crustal seismic lines outlined in Figure 3-1 are highlighted in black.

Figure %S. Bouguer gravity horizontal-gradient vector (HGV)map with regional basement domains (in red) of the central Alberta study area (modified kom Ross et al., 1991), scale 1:5,000,000. Vector arrows are plotted pointing away ftom local maxima or downhill. hterpreted domains are: the Lacombe (L) domain, Rimbey CR), Talston (T) anal Ksituan (K) magmatic arcs; Wabamun (W), Buffalo Head (B) and Hottah (H) accreted ternes; Thorsby (Th), Chinchaga (C)and Kiskatinaw (K) magnetic lows; and the Archean domains of Slave (S), Rae (R), and Hearne (subdividcd into the Loverna Block [Hl], Eyehill High [He] and Matzhiwin High). Prominent shear zones identified are the Snowbird Tectonic Zone (STZ) and Great Slave Lake (GSL). Cmstal seismic lines outiined in Hgure 3-1 are highIighted in green. The location of Edmonton in shown by the pink box.

83 WCSB, a regional seismic reflection transect was ncorded with extended Iistening tums across central Alberta 3-1). hterpretation of this central Aberta transect (CAT) data is supplemented by several industrydonated 2-D datasets. Cornpared to conventÎd seismic data, a reWe mtcrprctation of d-deseismic data is often extremeiy difficult to m&e due to sideswipe, multiple energy and increased noise commonly found at extended listening times. Brown (1986), Wamer (1987) and Klemperer & Hobbs (1991) outlined how these factors can generally degrade the quality of deep cnistal data. Side~wipe~,or out-of-the-plane reflections, are nodydiffidt to detect on 2-D seismic data except where an intersecting seismic profile is available. In addition, the shallow basement of centrai Alberta is often seismically nodective or wntaminated by multiple energy originating fkom the overlykg sedimentary cover, thereby further masking any subtle signaaires associated with basement faulting. For most deep seismic sections, a low signal-to-noise (Sm ratio coupied with poorly knomvelocities can signincantly eect the migration process. The effect of ambient noise on the migration of stacked seismic data has been weU addressed by Yi(1987). Where noise dominates the deeper part of a stacked section, the migrated section frequenty contains a smearing of amplitudes and 'srniles' (or synformal events), often obscuring weak reflection patterns. When migrating crustal-scde seismic data, the wavefkonts are very large and the coxresponding 'smiles' produced are very long (Wamer, 1987). This means that noise bmts which may be lucalized on the stacked section could affect a large area on the migrated section, thereby making any subsequent interpretation questionable. The possible presence of such migration artifacts in CAT and indu~ay-donatedseismic data presented in this dissertation is Merdiscussed in Chapters 6 and 7.

A 520-km-long profile across cenaand east-central Alberta (lines 1 to 10 on Figure 34), recorded in 1992, is shown in Figure 3-6. Acquisition and processing parameters for these lines are found in Table 7-1. Importantly, this Il-second dataset crosses a number of basement domain boundaries. allowing camparisons to be made on the cnistaI strucaues of adjacent domains. Seismic reflection data offer a direct radier than inferred look at the domainal orgdtion of the basement and thereby help to mnove the uncertainty or non- uniqueness associated with potential-field interpretations. Whereas potential-field data, and HGV maps in particular, facilitate the areal recognition of domain boundaries and often- distinctive fabrics within domains for the shallow basement, seismic data provide a geometrical dip conml for some of these prominent anomalies and fabrcs. Figure M. Compressed cnistal-scale seismic display of the UTE3OPROBE central Alberta transect mes 1 to 10 in Figure 3-5) fkom Ross et al. (1995): (A) migrated coherency-filtered seisrnic reflection data; (B) tme-scaIe line drawing interpretation of (A). Transect Iine segments are numbered dong the top of (B), together with their corresponding orientation. The locations of basement dornaÏns and subdivisions interpreted by Ross et al. (1 991) are also indicated. A corresponds to an excellent example of a crustal-scale fold associated with the East Alberta Orogen. This broad feature has been interpreted by Ross et al. (1995) to represent a hanging-wall antiformal structure associated with adjacent thrusting. B is a major reverse fault that appears to continue throughout the cmst and actually offsets the Moho. This feature is correlatecl with the Snowbird Tectonic Zone. C refers to a single mid-crustal thrust slice or wedge. The same fauIt is crossed by line 1 mnning N-S and line 2 oriented W-E.

86 A prei.imimq crustd-sale interpretation of this dataset is reportai by Ross et ai. (1995). The upper crust of seismic lines 7 to 10 is characterized by pronounced, low-angle basement reflections with an apparent unifom dip tu the east or southeast that appear to soIe into a highly reflective Iower crust. Within thîs shaliow-dippmg WC,distinctive roll- over structures in the form of hanging-wall anticlines are also sem (e-g. Figure 3-45). Oved, the charactenstics of these particular Iarge-scale structures indicate that the mclined basement geometries are, most likely, tbst faults. At depth, these low-angle refiectors appear to sole or htten onto a major midmstal decollement or detachment dace.They fonn part of a crustal-scale thmst imbrication system or belt, termed the East Alberta Orogen by Ross et ai. (1995), where a signifiant amount of ctustal shortenhg is evidentt Such crusral-sale low-angle reflections appear, in fact, to coincide regionaliy with the potentid-fielddefineci Lacombe and Hearne domains (Luverna and Eyehill blocks).

in trying to relate some of these proposed mtra-basement thnist faults to discrete anomalies recognized on potential-field data, a number of possïbilities have to be considered. Firsùy, fiom the cornparison of regional magnetic anomaly patterns to mapped stnrctures in the Northem Internides of the Wopmay Orogen (discussed in Chapter 2), it appears that regional magnetic lineaments can mark tectonic zone boundaries and major lithological or stmctural contacts. Generally, for regional magnetic data to be able to detect large-scale fauIts, such structures need to create a contact between magneticallycontrasting rock types. It is iikely that many faults interpreted cm seismic data do not satisfy such a requirement and therefore will probably not be detected using regional magnetic data. Secondiy, not al1 of these interpreted crustal-scale thstfaults appear to ptopagate upwards into the shdlow basement in this area. In such cases, these structures will likely not have a mark& nagnetic signature.

NevertheIess, it is stiii possible to tentatively correlate some magnetic HGV lineament patterns with major hstsassociated with the East Alberta Orogen, at least on this regionai scale of investigation. For example, several prominent NE-trending magnetic lineaments within the Lacombe domain (Figure 3-3) seem to approximately coincide with the two principle thnist faults interpreted on line 7 @gure 3-6). In this area, the inferred faults can be traced close to the basement surface and may even locally breach this sedimentary- crystalline boundary. This proposed stnicûuai-conîact lineament trend is often complicated by other features present in the shallow basement. These examples are discussed in more detail in Chapter 7. Significantly, the synthetic modelling of a similarly-inclinecl basement 87 structure, outlined m Chapta 2, resu1ted in a HGV hearrimt une successfully locating the low-angle contact. However, it is important to nalue that anowhead shading in regional magnetic HGV maps can produce Iineament trends tbat weU define the shape of a particuIar basement feature but not always the exact location. It is the taiI-ends of long vector arrows that ofkm mark major bgsement boundanes.

The fomiation of the East Alberta Orogen is related by Ross et al. (1995) to the cornplex collisional assemblage of domains to the northwest Fnnn this reco~lstnrctiionand the NE- trending nature of domain boudaries, I have Se& that some NESW elements on the magnetic HGV map could relate to the strike of these low-angle thrust faults. This conclusion is supported by cleavage measurements fiom oriented basement dnllcore. For example, an orientation of N40° E was measund by Villeneuve et al. (1993) in strongly deformed schistose rock ncovered fkom well 1&I%3&î 1W5 in the eastern part of the Lacombe domain. This orientation is approximately paraUeI to the proposed strike of the thrust faults fkom magnetic HGV data. Bouguer gravity HGV lineaments within this domain @gure 3-5) also follow this NESW and.

Cornparrd to the Lacombe and Hearne domains, the basernent of lines 5 to 3 on Figure 3-6 is represented by a less well-defined fabric. Seismic events with dips similar to those previously encountered to the east are observed on Iines 3 and 4, albeit with a marked change in vergence. This reversal apparently coincides, at least partiaIly, with the Rimbey domain. Thus, the Rimbey magnetic high may delineate the western edge of the East Alberta ûmgen.

Interrupting this inclined basement fabric within the Thorsby domain is a major revene fault that extends throughout the crust, and appears to offset the Moho in the seismic section shown in Figure 3-6. Ross et al. (1995) posailate that the STZ or a strand of the STZ cm be comlated, at least locally, with this major structural discontinuity seen on seismic data. The location of this structure coincides with a regionally-extensive Bouguer gravity HGV lineament inferred to -sent part of an extension of the Snowbird Tectonic Zone (Sm.This relationship is expected since such a marked offset of the Moho would create a significant density contrast. Overall, the STZ is one of the most regionally signincant events affecting the basenient of cenaal Alberta. 88 To the northwest of the STZ, orthogonally-oriented seismic Iines 1 and 2 lie within the Wabamun domain and appcar to image a single dinist slice that offsets a pronounced series of subhorizontal reflectors, ailowing both stnke and dip to be deiineated (Figure 3-6). Using simple geomeûy, Burwash et al. (1995) demomtmted that this sûucture likely trends NW-SE and dips with a low angle to the southwest, probably resulting fiom NE-directed compression. In Figure 3-3, any hear magnetic trends Iikely to be related to this structure are masked by the presence of circular to arcuate magnetic anomalies thought to be associated with granitoid plutonism. In any case, the inteIpretation that this Mt cannot be followed into the shallow basement means it is unlikely to be observed on magnetic mdydata

Similar intra-basement geometries with shallow dips up to 20" were interpreted by Kaaasewicb et al. (1974) and Ganley & Cumming (1974) fkom a seismic profile near Edmonton. However, these reflectors could have a structural or lithological origin.

About 100 km northwest of line 1, seismic section CCkl was acquired by Husky Oi1 Ltd across the Chinchaga-Buffalo Head domain bouadary (Figure 3-2). Acquisition and proçessing parameters for this line are summarized in Table CI. On this NE-SW-orientecl Iine, intra-basement seismic events with an apparent dip of 20" to 30" SW are recognized fkom the basement-sediment interface down to around 4 to 5 seconds where they become masked by a midcmstal zone of high reflectivity (Figures 3-7). These low-angle events crosscut subhorizontal reflections at about 3 seconds, Bunivash et al. (1995) have suggested that these subhorizontal markers are WeIy amphibolite horizons. This layering may be regional in character, possibly correlating with subhorizontal structures conspicuous on lines 1 and 2.

The fact that no marked cut-off relationships can be reliably identifieci on the low-angle reflectors in seismic section CCkl, together with the la& of any large-de hanging-wall anticlinal structures at depth, indicates that these incliaed reflectors more likely represent lithological rather than structural boundaries in the shallow basement Although Villeneuve et al. (1993) regarded the sharp magnetic contact between the Buffalo Head Terrane and the Chinchaga Low (Figures 3-2 and 3-3) to be a Uely structural boundary, there is no seismic evidence from iiae CCkl to coxroborate this conclusion. The pronounced positive magnetic anomaly that deheates this boundary is more likely related to a sfiallow-dippi.ng layer containing basement rocks of relatively high susceptibility. On the magnetic HGV Figure 3-7. Migrated coherency-fiitered seismic data fiom Line CCkl. The vertical exaggeration is approximately 0.75. The basement-sediment interface is marked by a dashed iine. The arrows indicate low-angle refiectors in the shallow basement that are interpreted as layered basement rocks with an overall apparent dip to the SW. Some of these events appear to crosscut subhorizontd layering as about 3 seconds. Additional seismic processing appIied to this line is mmmizdm Table 61. TIME (seconds) 9 1 map (Figure 3-3), this seismic ihe is also situated adjacent to a pair of subcircular anomalies believed to have a magmatic origin. ûveraii, from the available seismic and magnetic anomaly data, it wauld appear diat the area around line CCkl is largely dominated by iithological rather than stnichrral bascment features. However, in the case of Bouguer gravity HGV data (Figure 3-5), a series of NW-and NE-trending lineaments define a regional block-like pattern around this seismic section that may reflect a possiiIe large-sale stnicaual inauence on basement lithology.

To the West of iine CCkl, Unocal Canada Exploration Ltd recently acquireü a regiond seismic profile, line Antel, recordeci to 8 seconds (Figure 3-5). Table 62outlines the acquisition and processing parameters for this seismic line. As shown in Figure 3-8, a series of low-angle refiectors can aIso be distinguished in the shallow basement of this NE- oriented line. Taking into consideration the quite difterent scales of Figures 3-7 and 34, the shallow basement fabric of line Autel is similarly inclineci to the SW. It is proposai that these dipping events also probably correspond to layered basement rocks. This seismic section extends across a iarge part of the Chinchaga magnetic low damain (Figrue S2)and intersects several NW-trending curvilinear anomalies on the magnetic HGV map (Fqpre 3- 3). The line also paraliels a NE-SW-alignai magnetic HGV iineament. Most of these anomalies are believed to be directly related to lateral variations in basement litho10gy. In contrast, line Antel appears to be bounded by a pair of NW-trending Bouguer gravity HGV anomalies that appear to fom part of larger block-like fhmework within west-central Alberta (Figure 3-5). The relatimhip of interpreted basement features in the Chinchaga domain with overlying Phaneromic sedimentary cover rocks is discussed in Chapter 6.

33 Basement structure and the sedimentary cover of central Aïberta From the regional distribution of &a-basement feahires described in this chapter, it is clear that the basement of central Alberta is far from homogeneous. In the past, the influence of basement structure and lithology on sedirnentary basin evolution in central Alberta was generally underestimateci due to a general lack of knowledge and a misconception that the basement was regionally and locally feaîureless. However, the advent of cnistal seismic profiles and publicdomain regional magnetic data, coupled with recent potential-field HGV analysis, as in this dissertation, provides a means to detect regional lithological and strucîurai complexities in the upper crust With this increased understanding of the different types of basement features that may exist within centrai Alberta, the tirne has corne to re- Fi3-8. Migrated coherency-fïitered seismic data hmLine Antel. The vertical exaggeration is approximately 3. The basement-sediment interface is marked by a dashed line. The arrows indicate a series of low-angle refiectors chat can be followed in the shallow basement. SimiIar to heCCkl (Figure >7), these SW-dipping events are thought to represent lithologicd contacts. Additional seismic processing applied to this line is summarized in Table 6-3. 9 9 4 d \O TIME (seconds) 94 evaluate the influences that the basement cornplex can have on the sdhenîary cover, both riegidy and ldy.

Table 3-1 details different types of surface and subsuxface trends identifieci m central and southern Alberta. The many suailarities between these different types of lineament maps and regionai trends observed in magnetic and Bougucr gravity anomaly maps presented in this chapter indicate that some basement features appear to have influenceci part, or au, of the Wanerozoic sedimentary cover in some areas. These alignmcnts serve as an indication of the various direct and indirect genetic influences that the basement may have on the geornetry of featues developed in overlying sediments. For instance, estabiishg a direct basement rdationship with sedimentation may involve recopking where bernent faults have ken locally reactivated and propagate upsection into Phanerozoic cover rocks. A more subtle basement-sediment mteraction nlay involve structural or üthological contacts in the basement infiuencing the morphology of the basement surface which, in tum, could indirectly affect overlyuig sedimentation patterns. The styie and extent of tectonc heredity exhibited between the basement and the seûimentary cover in central Alberta has recently received considerable attention (e.g. Eaton et al., 1995; Edwards et ai., 1996).

Chapters 5, 6 and 7 of this dissertation detail investigations into Wtand indirect basement control on Devonian carbonate morphology on botb a Id reefal scale and regional platfdscale in central Alberta. Area Lineament trend typ Lineament Source or fature orientations

cenws. Alberta N60% & N30"W set N~PE& ~65wset S. mna field observations of Mis ciastic dykes centrai Alberta spatiai filtering of weii data, Robinson et al. stratigraphie undulafions (1969) west-cenfd Alberta N& & NSOWset NS& W-E set Alberta ~54%& ~36% set N-S & W-E set cenuailS. AIberta regional bedrock joint ~5545%.~39-25"W Babcock (1 974) systems N5"E & N95"E set cenWS. AIberta

centrai Amena ~45%& ~45%set Eiaman & Iurgens N30"IK & ~60%set (1974) Alberta oil and gas field ~65%' Jones (1980) orientations west-cenuai Alberta, geolhermal gradient andes S. Alberta gravi@ and seismic data. stratigraphie correlations cendAlberta infemd maximum stress trajectory from weilbore breakouts S. Alberta photolineamcnts ~65-70% Moiiard (1986) (1957 database) local oil pool trend Ns- Alberta NE&NW Posavec (1991) Alberta NE&W Misra et al. (1991) cenuai Alberta regional ae~miagnetic NI* Burwash et al. trends (1994) S. Alberta seismic refiection data ~15% Wright et al. basement black faulting (1994)

Table M. Bibliography of surface and subsurface lineament trends identified in central and southem Alberta, c-4

Controls of modem reef development

" Ccvbome buildups are Iike Shakespeare, the play go on - only the actors chge"

RN. Ginsburg (In: James & Kendall, 1992, p. 268)

Over the last few decades, there has been an signiflcant amont of research into aii aspects of modem carbonate buildups and their foundations. Dramatic advances in geotectonics, oceanography, and marine geophysics have provided a current basis for structural studies of contemporary reefs and platforms. As Wilson (1975, p. 379) stated, "geologic thought directed toward interpretation of carbonate buildups evolves as contimously as do the organisms which generate such masses". One of the principal reasons for studying modem carbonate environments is îhat they can be used as analogues for interpretation of ancient carbonate environments, drawing attention to the hypothesis, the present is the key to the pst.

In order to assess the possible impact tectonism might have had on Devonian carbonate morphology in central Alberta, it is first necessary to understand the factors influencing carbonate sedimentation at present This chapter consists of an overview of modem shallow-water cartxinate accumulation that is considered pertinent to the reefs and platforms found in the geologic record.

4.1 The carbonate factory Most, though not all, present-day carbonate sedimentation results from chernical or biochernical processes occurring in warm, clear, shallow waters which are well agitated and free of clastic sediment (Wilson, 1975). The carbonate factory is the shallow, illuminated seafloor where carbonate particles are born or accumulate in situ, either 97 crystallizing as skeletons or precipitating directly out of seawater. Bathurst (1971) and Milliman (1974) provide excellent overvîews regardhg the complex chernid processes of precipitation of calcium carbonate minerals fiom sea water. In water that is essentially saturated with regard to calcium carbonate, any process îhat removes carbon dioxide will encourage precipitation. Removal of catbon dioxide by photosynthesis is considered by Wilson (1975) to be of prime importance. In warm-water setthgs, carbonate production is highest and fairly constant to depths of about 15 rn, marking the depth limit of abundant carbonate-secreting phototrophic organisms, such as calcareous algae. Thus, the rate of carbonate production is strongly depth-&pendent. Typicaliy, shaliow tropical waters are supersaturateci with respect to both aragonite and calcite, whilst deeper waters are either saturated or undersaturateci. At some depth in the water column, texmed the compemation depth, the rate of carbonate dissolution equds the rate of carbonate deposition (qttokowicz, 1970). Below this %th carbonate sediments do not normally accumulate.

Clear water or lack of terriginous influx is also important for carbonate formation. Contarninants such as suspendeci silt and clay reduce light peneuation, thereby inhibiting photosynthesis, and are detrimental to benthonic invertebrates capable of contn'buting significant amounts of calcium carbonate. Strong evaporation and water agitation are additionai processes important, but not essentiai, for carbonate deposition.

A variety of interrelateci processes operate on the carbonate factory to produce the carbonate deposits seen in the geologic record. The importance of climatic and hydrologic factors in causing and mo-g buildups has long been msed. As WeUs (1957) States, "organic- reef communities build reefs whose shape and simtion are guided by hydrologic, mereorologic, and pre-existent geonwrphic con troistr.

4.2 The modern carbonate growth window: the requisite environment The most widespread and volurnetrically important reefs in modern oceans are constructeci by herrnatypic corals and calcareous algae. in a typical coral reef, although coralline algae are usually more cornmon, corais are the critical organimn in that their skeletons fom the basis of the reef framework (Milliman, 1974). The limitations of coral growth thereby determine the distribution of coral mfs.The depositional window, shown schematidy in Figure 4-1, is controlled by a combination of envirwment.1 factors including: water depth and light, temperature, salinity, aubidity and wave action. Figure 4-2 shows an attempt to tem erature G 1 is+ts.c ' /-

Figure 4-1. T'hg modem carbonate depositional window, schematically showing factors for optimum reef growth (after James & Bourque, 1992).

Figure 4-2. Replicating the optimum growth conditions for a small-scale coral reef at the Smithsonian Museum of Natural History, Washington, D.C. 99 mode1 optimum growth conditions in a man-made environment to mate a small-scale coral reef ecosystem.

Light intensity decreases exponentiaiiy with water depth. Because hermatypic corals rely upon light-dependent symbiotic zooxanthellae for rapid calcification, reef corals are generally limiteci to those shallow depths at which zooxantheiiae can photosynthesize. Such symbionts enabIe hemtypic cor& to produce calcium carbonate several cimes faster than ahermatypic (non-reef-building), azooxanthallate corals. In low-latitude carbonate enWonments, the lower limits of hermatypic coral and caicareous green algai growth are 80-100 n Coxal growth fonn is in part an adaption to varying light conditions.

The ideal water temperature range for optimum hermatypic coral growth is about 25°C to 29°C. but corals can grow in waters between 18°C and 36°C (James & Bourque, 1992). Outside this range the coials begh to lose their ability to capture fd.Some species survive temperatures lower than 18'C but growth is restricted to individual colonies rather than anastomosing reefs (Milliman, 1974). Because waters are wmer on the western sides of ocean basins, most coral reefs are confinai to these areas. Whilst most studies have concentrated on the adverse effocts of lower temperature, coral bleaching descrï'bed by Glynn (1984) has drawn attention to the adverse effects of raised temperature.

Corals grow best in s;ùinities between 2.5% and 35% (James & Bourque, 1992). Raising salinities through severe evaporation, or lowering by expom to torrentid rains or river outflow, may affect the entire reef development. In the geological record, inferreci salinity variations have triggered shifts between coral and algal-dominated communities on numerous occasions in restricted seas (Hubbard, 1988).

In high-nutrient settings, the carbonate producers are outpaced by soft-bodied cornpetitors (Schlager, 1981). Corals thnve in nutrient-impoverished areas largely because they retai. and recycle nutrients very efficiently. Even rhough waters may be low in nutrients, in areas of high wave action adequate amounts are supplieci by high water flux. In addition, crashing waves prevent silt accumulation which could suffocate the corals. Active water movement aise maintains high oxygen levels and discourages coral predation. 100 The derscale over which these environmental factors operate inakes them suitable for physical and biological study. Larger, prïmariIy geologicd controls, exert direct and indirect controls on the location and morphology of modern carbonate buÏldups.

43 Generai controis oa carbonate buiidnps Accordïng to Jones & Desrochers (1992), the most important universal, extrinsic controls affecting reefs and platforms are tectonic subsidence, eustatic changes in sea level, plate movernent and clastic sediment input. Specific to a parcicular setting, the inrrinsic contmls of antecedent topography, biotic evolution and carbonate growth potenrial are all important factors governing sedimentaticm. Such factors may be cddered as the buiMing blocks of modern coral reefs, as outlined in Figure 43. Similady, HubW (1988) merseparates these gend controls on the basis of the scales over which they operate (Figure 4-4). Macroscopic-scale physical factors such as tectonics, sea-Ievel history and temperature, together with antecedent tapography set the stage for reef development and are responsïble for initially detennining the gross morphology of the carbonate buiIdup. Operating more dong gradients, mesoscopic-scale factors (e.g. lighî, wave energy, nutrients, sediment Ievel and predation) are more respomile for mouIding or fine-nming the surficial character and interior depositional fabric of a buildup.

43.1 Antecedent topography EarIy reef mdies in modem settings (e.g. Darwin, 1842) were generally tied to lands that rose above sea level and little was known about underlying submarine topography. In the past forty yem, an increased understanding of the roie of local bathymetry in reef disüibution around the world has developed. Specific to a particular tectonic setting, antecedent or pre-existing topography is now regarded to be an intrïnsic control, or sometimes even a blueprint (Jones & Desrochers, 1992) for carbonate sedimentation. In carbonate systems, organism growth tends to perpetuatc and accentuate depositional topography by selectively colonizing high areas. Many modem-&y reefs, shoals and islands are located on topographic highs where phototropic organisms are in an improved photosynthesis position, closer to sea level, and increased agitation was suitable for ooid shoal formation. In contrast, sheltered, quiet-water areas associateil with topographic lows preferentially accumulate muds. Clearly, the geometry of the underlying surface can exert a profound influence on the nature and distribution of carbonate buildups, at Ieast in their early stages. Antecedent topographic underpinnings may take many fonns and have PHYSICAL FACTORS BlOLOGlCAL FACTûRS

Figure 4-3. ~Gldin~bbcks of a reef: major fectors influencing and mntrolling reef development (afk Longman, 198 1).

.Macroscopic MesaCopic Microscopie inter- between with %lidI.crrn mo v. lmv. ES! Iwstemreef antecedent topography tectonism

sea level light temperature salinity wave energy nutrients turbidity

ti& currents predation

dominant Iimited mcontmiled by contro~ lm anm1 other factors Figure 4-4. SU-mary of factors favouring reef development, and the scaies over which they operate (modified from Hubbard, 1988). 102 multiple origins. Pre-enisting protrusions remlting fkorn old reefs and shods, structural features, karsts, erosion, and silicicIastÎc/volcanic deposits can al1 influence later depositional patterns of carôo~tes.

Many present-&y reefs have developed on older, dead reefs nom the preceeding stage of deposition, so reefs are cornmonly stacked. This lends credence to the adage that "reefs beget reefsl' (Hine, 1983, p. 3-13). ExampIes have been described from patch nefs in Belize Weyet al., 1973, Bennuda (Garrett & Hïne, 1979), and other areas.

Carbonate buildups may dso nucleate on topographic highs that are stxucturaUy induced such as horsts, or on f0otwal.l upliffs associated with tilted half-graben block topographies. Conversely, adjacent graben structures may serve to trap clastics. ShaUow-water carbonates are presently king deposited upon raised fault-blocks in the Gulf of Suez (Purser et al., 1987). Here, reefs developed dong the guIfward side of the block, with Iagoond sediments behind.

Purdy (1974) has suggested that antecedent highs developed during Pleistocene sea-level lowstands where prolonged subaerial exposure created karstic surfaces on many carbonate platforms. In Bermuda, karstification has increased topographic relief and, with later sea- level rises, Holocene reef growth occdon these karstic highs (Gama & Hine, 1979).

In the case of erosional topography, Goreau & Land (1974) proposed that terraced Pleistocene surfaces, most likely formed by shoreline erosion, have influenced Holocene reef development nez Jamaica. The history of reef development at St. Croix in the CaribW is closely tied to similar dope breaks encountered by rising sea level (Adey et al., 1977). The relief provïded by these features faciiitated sediment removal during the early stages of reef development New reef growth has also taken place on depositionally induced relief; for example, the foundations of many afsin Belize are early Pleistocene river deposits (Choi & Ginsburg, 1982). Volcii1loc1astic deposits are another example (Carnoin et al., 1988).

Overall, a pre-existing seafloor substrate or hard bottom high of whatever origin and composition enables reef commW1ities to nucleate directly on the seafloor. As pnviously discussed, many factors control carbonate buildups and facies, and with such complex controls one factor alone may not be dominant, Nevertheless, linear reef trends do often 103 exist because of the sensitivity of carbonate production to any kmd of antecedent hi@, whether mcairal or geomorphic. However, as pointed out by James & Macmtyre (1985), antecedent positive topography is not always a requirement. Many modem reefs have developed where no obvious underlying topographie highs are discemible. For exarnple, some of Bemuda's Holocene patch reefs may have originated fiom randomiy distniutcd coral heads acting as pioncer cornmunities (Logan, 1988). Furthemore, reef rnounds of Florida have actudy developed in areas of negative relief where muds preferentially accumulated and provided suitable substrates for colonization by seagrasses which helped mate the mounds (Vos, 1988).

4.3.2 Tectonism The controls and effects that tectonic activity can have on the nucleation, develapment and evolution of a modem carbonate buildup may be signifiant and in some places rnay actually dominate sedimentation processes. The tcctonic fiamework of a region operates to control carbonate deposition on several levels. Fdy,on a large scale, reef development is markedly affected by tectonically induced subsidence and the overall tcansgressive or regressive nature of a particular basin. Secondly, within that basin, localized tectonic activity rnay provide favourable sites for reef development

A basic control of a particular tectonic setting is the rate, style and continuity of subsidence, without which sedimentation is usuaily negligible. Subsidence rates affect the morphology of reefs, and indeed, entire reef tracts. Subsidence, together with eustasy, control water depths, which in turn control rates of carbonate accumulation as a function of water depth. Calcium carbonate accumulation is very rapid when conditions are optimum; as much as 50% of all calcium carbonate is formed in waters less than 10 m deep (Hine, 1983). However, the thickness of carbonate sediment produced during a given period is actually controlled by the accumulation space, not the rate of sediment production. Subsidence is the primarily control on the accommodation space, whereas eustatic sea-level controls how that accommodation space is fdled (Vail et al., 1990).

Tectonics may locally control the distribution and nature of the foundation upon which a buildup nucleates. The role of structurally controlled bathymeaic highs on reef initiation is discussed in the precediag section. From area to area, the structural influences responsible for initiation of a reef or platorm can be highly changeable and may recur repeatedIy during later evolution. This is reflected by the gross clifferences in morphology exhibiteci by reefs 104 fiom tectonically contrasting areas. Mullais (1983) provides a regional review on the diverse tectonic cmtrols responsiile for the contemporary development of several well- studied carbonate platforms. In the Bahamas, the most recent stnictural mode1 invokes wrench-fadt tectonics as a possible influence on platfimn configuration. Along the Belize continental margin, carbonate buildups are strongly cmtrolïed by basement ridges formed either by transfom faulting or rotational rifting. In contrast, around NE Australia, extensional tectonics, together with possible aulacogens, are thoaght to have guidai the Queensland carbonate me.

The stnicanal ftamework may also affect the amount of temgenous sediment input to, and water circulation patterns within, a carbonate system. Regional tectonics determine the nature of the adjacent hinterland and therefore, in part, the rate of temgenous cIastic sediment supply to a basin or shelf (James & Kendall, 1992). Fine-grained clastic and carbonate sediments are often suspended by stmms and organism activity. Importantly, regional and 104structures may resîrict the movement of such suspendeci sediments. This is detrimental to coral growth because it decreases lîght petration and covers or clogs the polyps (James & Bourque, 1992). Furthemore, poor water circulation results in departures from oceanic water composition (James & Kendall, 1992) and local salinity changes, again detrimental to reef grnwth. To illusnate such indirect tectonic effects on reef development, Wilson (1975) uses the complex morphology of the Arabian side of the Persian Gulf. Here, structures control the regularity of coasthes and embayments, thereby exerting control on the degree of water restriction. Structural orientation normal to prevailing wind and wave direction encourages rapid organic growth.

43.3 Eustatic sea-level changes As previously discussed, carbonate buildups are particularly susceptible to changes in sea level, whether of epeirogenic or eustatic ongin. Eustatic changes in carbonate systems determine the health or even the existence of the carbonute fuctory. Both worldwide glaciation (glacioeustasy) and megatectonic shifting of plates (tectonoeustasy) may induce sea-levei changes, a process operaihg either with, or in opposition to, local tectonic subsidence (WiIson, 1975). Global changes in sea level, when superimposeci on regional differences in tectonic movement, can result in very different relative sea-level histories fiom one reef system to anoiher. 105 Eustatic sea-level fluctuations are important over a variety of des, Cycles of sea-level change are recognized on five orders of magnitude, hma --order cycle of htmdreds of millions of years down to a fifth-ordtr cycle of tcns of thousands of years. James Bi Kendall (1992) discuss the effects of long-term and short-term fluctuations in relative sea level on carbonate sedmientation. Even srnail changes in sea level wiU have a rnarked influence on the depositionai regime of shallow carbonates. Modeni reefs are commody thin, young veneers over steep antecedent topographie daces which were shaped during the recent, high-fiequency glacÏoeustatic sea-Ievel fluctuations (Longman, 1981).

In the case of rising sea level, the relative rates of sea-level increase and reef growîh are important. Since the growth potentiaI of reefal organisms is greater than most tectoaic or subsidence rates (Schlager, 198 l), particularly rapid eustatic sea-level rise relative to reef growth, or a reducticm in reef gmvth itself, is necessaq for a reef to be drowned (the give- up reefs of James & Macintyre, 1985). If sea-level rise is not rapid relative to growth, the reef rnay transgress in either a retreating (onlapping) or, more commoniy, backstepping manner. Backstepping reefs transgress in stages, moving each time to a saallower. higher position. Where the rates of sea-level rise and reef growth are similar, continuous upward accretion result in the keep-up reefi of James & Macintyre (1985). In Holocene reefs, a reduced growth rate or lag period is typically recorded after an initial transgression, in which reef growth has caught up with sea-level (the catch-up reefS of James & Macintyre, 1985). If the sea-Ievel rise is les than the growth rate, the reef will quickly grow up to sea- level and expand lateraily.

By Wtue of growing in shallow water, reefs are prone to exposure by even minor sea-level drops. If the faii is rapid and the reef is exposed for a period of time, coral growth will cease. During less rapid falls, some recfs may prograde seawards giving an offlap relationship of successive reef complexes (Tucker & WnghS 1990).

Excellent discussions on the often complex interactions of subsidence, eustasy and carbonate sedimentation are found in Kendall & Schlager (1981), Tucker & Wright (1990), and Goldhammer et al. (1990). 106 4.4 Artificial reefs Over a relatively localized area and limited timt intaval, artificiaI reefs provide a fÏrst-band opportunity to study optimum ccmditim for small-scde reef initiation and development, includhg possïïle tectonic umîrols such as the vertical relief, depth and substrate necessary for preferred colonization by hermatypic corals. The main purpose of artificial reef construction is to enhance, supplement, or rnimic naairal reef habitats by providing relief on flat, featureless ocean bottom (Seaman & Sprague, 1991). Consmction increases habitat diversity, vertical reIief, and hard botrom, creating new fishing and diving opportunities. In ment years, artificial patch reefs have kncreated in the Fiorida Keys (Kruer & Causey, 1992), Japan (Grove et al., 1991), Hawaii (Brock & Grace, 1987) and elsewhere using large concrete slabs, boulders and rubble (fkom bridges), prefabricated units, large steel vessels (mtentionally sunk) and even mbber tires.

According to Bohnsack (1990). important structural characteristics for artificiai habitats include profile, shadow, vertical relief, surface area and substrate. Generally, vertical relief of prefabricated units rarely needs to be more than 5 m (Grove et al,, 1!BI), and is often considerably less. In terms of subsmte, results rcported by Bauer (1987) indicate metals appear to be the most successful materials, but recniitment of organisms on concrete closely resembles that on naairal surfaces such as dead coral. In the Florida Keys, Kmer & Causey (1992) report that shallow (7 m), mid-depth (14 m) and deep (24 rn) units with varying profiles have aii been successfully used to encourage coral remitment and attract different fish populations.

Overall, &ficial reefs mimic their naturai counterparts and it appears that the creation of relief by an appropriate substrate or hard bottom at a suitable depth is of primary importance for colonization by hermatypic corals.

4.5 Discussion Reef start-up or initiation can take place when the seafloor cornes into the growth window through the rise or fa11 of sea level, tectonism, or underlying mound development. Additionally, reef tum-on can occur when factors adverse to reef pwth are tumed off. The nature of the original community depends on where in the growth window accretion begins and the nature of the underlying substrate. Reef growth during the Holocene transgression generally began on Pleistocene limestone or other hard substrates. At present, there appears to be no consistent paûmi ta initial reef growth. Overall, modern reefi reflect a dynamic process of biogenic activity and sedimentatim that ail takes place in the original evironment of deposrion. Importanîiy, such in situ production of carbonates can directly mirror underlying structure.

As Klovan (1974) suggested, recent reefs can serve as models of development for those in the geological record but shdd not be used as explicit analogues. The development of any two carbonate regions separated in space and time is not Iikely to correspond exactly but similar controls are likely to have affecteci ment and ancient reefs. Essentially, modem carbonate buildups are dependent upon the relationship between consiructive and destnictive processes supwimposed upon an antecedent topographie wfiw and projected through time to account for tectonic setting, subsidence, eustatic sea-Ievel fluctuations, climaWhydrology changes, and evolution (Hine, 1983).

As Mullins (1983) pointed out, contemporary carbonate reef and platfirnargins appea. to maintai. and emphasizc undedying structures much more than siliciclastic margins. This can be related to in siru carbonate production exceeding any dative, geologically induced rise of sea level (SchIager, 1981). It is this high growîh potentiai that enables carbonates to keep pace with tectonic subsidence and preserve the pssmorphology of îheir underlying structural controls. The locations of well-studied modem carbonate reefs, platfonns and embayrnents al1 may have th& origins tied to precarbunate basement tectonics. However, the factors governing the location of one reef may have little or no devance to that of another. Carbonate bddups as diverse as the Great Bamier Reef, the Bahamas, and the Belize continental matgin al1 have unique tectonic histories dut have shaped the underlying rock structure and affected carbonate accumulation differently. This incrtasing awareness of the importance of sûuctural features in the localization of modem carbonate buildups has led to a re-evaluation of the factors influencing ancient reefs and platfom in the geological past. Baument Muences on the Keg River Formation of north-central Alberta

The objective of this chpter is to outlint a relationship between Precambrian crystalline basement structure and Devonian reef development in an area of central Alberta where some form of tectonic control or influence on the dism'butim of carbonates has been previously established. In this way, it rnay be possrile to better understand the interaction that can take place between the basement and Devonian carbonates in areas where basement srructure is les obviuus or poorly und-, such as that underlyiag the Swan Hills Formation of west-central Alberta or the Leduc Formation of centrai Alberta, discussed in Chapters 6 and 7 respectively,

Although discovered only relatively recently compared to Swan Hills and Leduc Formation buildups, the Keg River Formation (Middle Devonian) carbonates of north- centra1 Alberta have been the subject of numerous studies. The area covered in this chapter extends from latitude 55" N to 58" N (township 69 to 103) and longitude 1 12" W to 119" W (range 14W4 to 7W6). In this active hydrocarbn exploratiw aea, the Pace River Arch is the principal regional tectonic element and is lmown to be associated with often considerable block faulting of the basement surface (O1ConneIl,1994). Known Keg River Formation carbonate buildups Lie dong a NW-SE linear trend near the eastem margin of the Arch (Figure 5-1). In order to assess the impact of basement structures on the initiation and morphology of such carbonates, an integrated approach incorporating 2-D and 3-D seismic, driiShole and regional poteatial-field data was undertaken. In this dissertation, this methodology is similarly applied in areas containing Swan Hills Formation and Leduc Formation carbonates, An extrapolation of the tectonodepositional framework Iocally established for Keg River carbonates may therefore help to substantiate or refute the notion of a similar cause-and-effect relationship where the effects of basement structure may be even more subtk in order to facilitate fiuther hydrocarbon exploration of Keg River Formation carbonates in adjacent areas of north-central Alberta, the importance of acquinng detailed knowledge Figure 5-1. Location map of established Keg River Fonnatim (Middle Devonian) carbonate buildups in the eastern Peace River Arch area of northtentral Alberta (modified after Podmski et al., 1988). The regionai boundaries of the Arch are taken fiom O'Conneii et ai. (1990). The ' approximate location of the 3-D seismic survey across the Panny Fieid is also rnarked Keg River Formation reef plays carbonate buildups

III Arch boundaries (aficr O'Connel1 cl al,, 1990) 111 of basement stmctum is emphasized in this chapter. The interpretational benefits of using a 3-D seismic survey over 2-D seismic profiles across localid basement structures are also discussed. The 3-D seismic portion of this chapter has been pubiished by Edwards (1 996). In addition, a discussion cm the scaled physical seismic modelbg of a Keg River Formation reef overlying steep basement faults is expanded upon, to better constrain some of the problems sometimes associated with seismicaily imaging features directly underlying carbonate buiidups.

5.1 Geologicai setting and shtigraphy The Peace River Arch is the predorninant regional structural feature in central Alberta. The generalized Devonian stratigraphie nomenclature for the eastern part of the Peace River Arch area is summafized in Figure 5-2.

The basement surface below the Arch is regionally uplifted about 800 to 1000 m above the level of the adjacent basement over an atea of about 400 km est-west by 140 km north-south (Cant, 1988), and is cored by mostly granitic and gneissic rocks (Burwash et al., 1964). The approximate configuration of the ENE-trending Arch in northcentrd Alberta is indicated in Figure 5-3. Superimposeci on a regional dip to the SW, a number of topographie highs and lows are seen locally on this surface wiîhin the Arch.

Overlying or flanking Precambrian basement rock is a series of siliciclastic sands, conglomerates and carbonate interbeds, coIlectively known as Granite Wash or basal clastics. Many of the larger Precambrian boulders sometimes also show evidence of encrustation with stromatoporoids (Campbell, 1987). According to Angus et al. (1989), the age of these coarse-grained beds is poorly constrained due to a lack of fauna and by its diachronous nature. Cant (1988) proposed that the distribution of Granite Wash deposits was strongly influenceci by Precambrian pdeotopography at that tirne. At Red Earth and Utikuma Fields, both situated in a southeastem. area of the Peace River Arch, Granite Wash sediments locaiiy infrlled structural lows and thin abruptly over structurally high areas of the basement surface (Trotter, 1989). Cyclicity has been observed within these units and is compatible with recurrent movement on basement faults. These sediments are commoniy derived fiom the erosion of basement surface. Subsequent fluvial reworking of these sediments in a subaerial fan-delta envisornent has been proposed by Sabry (1989) for Granite Wash of the Red Earth Field. Signrficantly, these Stage Table of Formations

Beaverhill Lake Gp 1

Givetian Muskeg Fm

Figure 5-2. s&tigraphic chart for the Devonian of the eastm Peace River Arch area (modified from Campbell, 1987; Cmt, 1988). The Precambrian basement and diachnous basal clastic units are shaded. Figure 5-3. Structure contour map of the Precambnan basement sdxein the north-central Alberta study area nom drillhole data (modifiecl fkom Trotter, 1989); scale 1:2,500,000. The regional boundaries of the Peace River Arch are taken fkom OIConneilet al. (1990). The eastem part of the Arch is offset hmthe West by an aeromagnetic low inferreci to represent a shear zone in the basement (as outIined by O'Connell et al., 1990). The approximate location of the 3-D seismic weyis &O highlighted.

115 nits may depositionally pinchout agaiast or drape over basement highs, creating poten@ reservoir targets.

The Keg River Formation is dated as late EifeIian to early Givetian and is stratigraphically situateci in the Elk Point Group. It generally consists of open-marine carbonates with a variable degree of argillaceous content and is divided generally into upper and Iower members (Campbell, 1987). The Upper Keg River Member is the main reef-forming unit and varies in thickness fkom less than 20 m in interreef areas to 100 m or so in full buildups.

Based on contemporary global wind patterns, the equatond location of north-central Alberta during deposition of the Upper Keg River Member suggests the presence of northeasterly prevaiLing winds and currents (Campbell, 1987). In terms of the paleogeography during this the(Figure W), the Senex area was part of a frîaging shelf surroundhg the Elk Point Basin. Shoreward of the shelf edge, low-relief patch reefs developed under high-energy conditions, normally rooted on or above prominent basement structures (Figure 5-5). According to Dunham et al. (1983), the top of the underlyhg surface was probably emergent in some cases because the reefs grew on the windward side of a high, correspondhg to the right-hand side of raised basement structures shown in Figure 5-5.

Bioclastic gravel-stromatoporoid sequences appear to form the buk of the deposits associated with these reefal facies. The reservoir is a clean, recrystallized dolomite with good-to-excellent vuggy porosity and solution-enhrged fractures. The Keg River Formation is productive where sîructurally closed across Frecambrian highs. Typically, it is draped across these structures as a result of differential compaction of lower Paleozoic sediments. These clastic sediments can also be productive. Recognition of closed Precambrian stmcture is thus vital to successful Keg River Formation exploration.

In 1969, the initial discovery of Keg River Formation reefs in tbe eastern Peace River Arch area was made at the Senex Field (Podniski et A., 1988). However, the presence of a low-energy carbonate mudstone of poor reservoir quaiity at this locality hindered further exploration until the discovery in 1983 of a higher-energy carbonate trend at Amocors 2/341-9-5 test well at the Panny Field. In the five years following, an estimated 55 million bbl(8.7 x 106 m3) of recoverable oil was discovered and anywhere R22 W4 middle shelf shelf margin inner shelf . . (A) (4 Figure 5-4. Keg River Formation paleogeography: (A) Location map of the eastem Peace River Arch (PRA) area, showing the wtline of the Elk Point Basin in which patch reefs were deposited; (B) Keg River Formation reservoirs in the eastem Peace River Y Arch (after Campbell, 1987). The main producing reefs are situated on a middle carbonate shelf fringing the Arch, Ci OI cry stalline basement Granite Wash high energy carbonate buildups

Figure 5-5. Schematic cross section, eastern Peace River Arch area, illustrating the direct relationship between fault-bounded topographic highs on the Precambrian basement surface and the developmeni of patch reefs of the Upper Keg River

Member (rnodified after Campbell, 1987). Ci Y 4 ll8 between twenty and thirty operatmg companies have been actively pursuing the Keg Riveraenex play, Apart from initial discoveries at the Senex, Panny, Trout and Kidney Fields w~gure SI), this play is characterimi by a large number of oil pools with an anomalously low average pou1 size of less than 106 m3. These oils are thought to be sourced from the Upper Devonian Duvemay Formation (Kirkby 6 Tinker, 1991) but fluid migration paths are complicated by fie presence of structure. Recovery factors range fcom 15% to 50%, owing to the reefal depositional facies and tectonic-diagenetic enhancement of porosity and penneabiIity, In assessing the remaining potentiai in tbis play, Podruski et al. (1988) estimateci a median expectation value of 33 x 106m3 OP.

The Keg River Formation is merlain by îhe Muskeg Formation, an interlayered sequence of sdts and arihydrites. Evaporitic sediments of the Muskeg Formation thin over Keg River Formation reefs and thicken in interreef areas (Meijer &es, 1986). The basal anhydrite of this unit seals the Keg River Formation reservoir facies. Although the basal anhydrite layer has a slightly higher velocity and density than the underlying Keg River Formation, the contact between the two uni& cannot be mapped on seismic data. However, it is possible to seismicaiiy delimate the contact between the basal anhydnte and the overlying salt; this is referred to as the basal anhydrite/Keg River event (Mitchell, 1988), or near-Keg River event (Anderson et al., 1988a). Figure 5-6 shows a characteristic well tie to synthetic data illustrating these relationships.

5.2 Data avsilabiüty The seisrnic expression of Keg River Formation carbonate buildups in north-central AIberta has been previously examined fiom a 2-D perspective (e-g. Anderson et al., 1988a; Anderson et al., 1989a). In addition to 2-D seismic profiles available from these earlier publications, additional 2-D lines recorded across such buildups were also made available through Sigma Explorations Ltd. The acquisition and processing parameters for these lines are found in Table 5-1. An interpretation of some of these profiles is found in Edwards (1 992).

A 3-D seismic survey was aquired by Western Geophysical Ltd. in December 1991 and January 1992 across a Keg River-Senex prospect in the eastem Peace River Arch area of north-central Alberta. This dataset was kindly made available to this study through International Colin Energy Corp. Workstation access to the 3-D seismic data was

I ~cqnisitionparameters Source hydrapulse, 18 pop per source point, 40-m spacing Receivers - group mtervai: 20 m, record length: 15s Spread 1WMû * 6Cklûûû m 24-fold coverage

I Pmcesshg parameters I Demultiplexing 1 Amplitude recovery Deconvolution surface-consistent spiking operator length: 80 ms; 1% prewhitening Refraction statics Elevation staîics 65û-m dam, 1800-m/sreplacement velocity Trace editing fmt breaks, mute, trace kilis O

1 Residual statics 1 surface consistent; window: 2W12ûû ms StaCK Migration phase-shift with 100% stacking veIocities 1 Filter 1 16/20-80/90 Hz 1 Equalization 1 window: 2ûû-1200 ms

Table 5-1. Acquisition and processing parameters for seismic lines Pannyl and Panny2. AU processing was undertaken by Sigma Explorations Ltd. This dataset is a SigmaEnertec Hydrapulse joint venture. 121 provided by Exploration Innovations Inc. The exact location of the survey wittiin this active exploration area is not revealed for proprietary reasons but is shown on a regional de(Figure 5-1). An area of 16 h2(62 rd), measuring 5.8 km (3.6 mi) by 2.8 km (1.7 mi), was covered with a shotpoint spacing of 60 m (197 fi) and a line spacing of 300 m (984 fi) resulting in an average coverage of 2û-fold. The relevant acquisition and processing parameters are summarized in TabIe 5-2. A single-hole dynamite source with a 1-kg charge was use& Processmg foliowed a rdatively standard set of procedures through to one-pass 3-D migration. The primary purpose for shooting this survey was field development, considering this area already contains a number of esîabiished wells. Lt was anticipated that this dataset could contribute to enhancing the recovery fiom - producing pools by better 3-D delineation of Precambrian structure and irrpgular Keg River Formation patch reefs. Such seismic imaging may also add to reserves by nndmg suppIementary accumulations near existing pools. Any subsequent welI programs would obviously benefit fkom such a survey, helping to alleviate the uncertainty of drillhg in this area.

Considerable weli control exists in this active exploration area to allow the picking of horizons on these seismic data. Synthetic seismograms were constructed using GMA LOGM software from digitized sonic velocity logs and formation deusity logs, courtesy of Digitech Information Services Ltd., accessed at The University of Calgary. Corresponding formation-top information was available fkom two database sources, Digitech Information Services Ltd. and Geobase (CD Pubco Inc,). In this area, because the basement lies at a relatively shdlow depth and the fact that several exploration targets Iie directly above the Recambrian basement, quite a large number of drillholes acnidly penetrate down to basement or near-basement depths. This allows for a good regional delineation of structure on the basement surface, as shown in Figure 5-3.

The regional mapping of basement features may be assisted by potential-field data. A selected number of representative magnetic and gravity maps covering north-, West- and east-central Alberta are presented in Chapters 5 through 7 of this dissertation. For example, Figure 5-7 is a total-field magnetic map of nosthcentral Alberta, obtauled from the Geological Survey of Canada and gridded at a 2-km interval. As outlined in Chapters 2 and 3, rnapping the horizontal gradient vector (HGV) of poteutid-field data assists in source-body edge detection, a useïul ml in helphg to delineate regional lithological or faulted contacts in the basement. Figure 54combines magnetic HGV, with optimaliy- 1 Acquisition parameters dynamite; average charge: 1 kg, I shot point spacing 60 m; shot-line spacing: 300 n Receivers station spacing: 60 rn; Iine spacing: 250 m record Iength: 3 s no. of channels: varies (approximately 300) 20-fold coverage

I ~rocessiagparameters Demultiplexing Trace editing fmt breaks, mute, mekills Deconvolution surface consistent; 20&1100 ms design window 320-111soperator length, 0.1% prewhitening Refiaction statics Elevation statics 6Wm datum, 22Oû-mis replacement velocity Residual statics surface consistent NMO 1 3-D noise attentuation 1 30% noise add-back, 200 ms delta T I CDP trh statics I

one-pas 3-43 migration with 100% stacking 1 velocitics I Equalization 1 single gate, window: 7ûû-1300 ms

Table 5-2. Acquisition and processing parameters for the 3-D seismic suwey in the eastern Peace River Arch. Al1 processing was canied out by Geo-X Systems Ltd Figure 5-7. Total-field magnetic map of northcenrrai Alberta with an outhe of Keg River Formation carbonate buildups in the eastem Peace River Arch area (hm Figure 5-1); scale 1:2,500,000. The magnetic data is taken hmthe Geological Smey of Canada database; - @d spacing 2 Ean. The Peace River Arch regionai boundaries, the Keg River-Muskeg depositional edge and the approximate location of the 3-D seismic survey are also shown.

Figure 5-43, Optimally-ded magnetic HGV map of nc*£entral Alberta, with vector arrows plotted pointing away fkom locai maxima, or downhill, with totaI-field magnetic data; sale 1:2,500,000. The magnetic data is taken hmthe Geologicai Survey of Canada database; grid spacing 2 km- The appmximate Iocation of the 3-D seismic survey is marked- scaied vectors anows plotted pointing away fbm focal maxima (or dom),and total- field magnetic data. Optimaiîy scaled magnetic HGV data with vector arrows pointing toward local maxima or uphill is shown in Figure 5-9. From synthetic modelling described in Chapter 2, it is clear that magnetic and gravity HGV lineaments that represent body edges can be picked by mapping zones of longer vector arrows. Importantly, although vector arrowhead shading often produces a rnarked pattern of magnetic anornaLies, it is necessary to locate the tail-end of a particular arrow to help outline a boundary in the basement. Both total-field magnetic and magnetic HGV maps, with a full range of scaling parameters, are interpreted to produce a magnetic lineament map containing inferred basement body edges 5-10). Bouguer gravity maps are interpreted in a simiiar fashion (Figures 5-1 1 to 5-14). In this case, regional coverage by 5-km-gridded gravity data is complete across north-centd Albe-

5.3 Peace River Arch tecîonîsm. and basement surfâce topography The complex geological history of the Peace River Arch has recently been well documented (e-g- Cant, 1988; OIConneil et al., 1990; OIComeli, 1994; and references therein). On comparing the approximate location of the Arch with the tectonic basement domain map of Ross et al. (1989), shown in Figure 3-1, it becomes clear that no obvious correlation exists between the two. The Arch generally trends obliquely to the Buffalo Head, Chinchaga and Ksituan domains, as defined by regional magnetic data and geochronology. Figure 5-7 confllms that no clear relationship exists between magnetic anomaly patterns and the regional boundaries of the Arch, as defined by O'Connel1 (1994).General hypotheses regardhg the origin of the Arch are considered beyond the scope of this chapter, but detailed discussions can be found in Cam (1988), Stephenson et ai. (1989), OIConnell et al. (1990) and Ross (lm),This study is mainly concerned with the local tectonic expression of this cratonic uplift.

Because of a general lack of Lower Paleozoic units associateci with the Arch, there is an uncertainty regarding when this feature was initiaUy uplifted. Stratigraphic evidence for the tnrncation of Middle and Upper Cambrian sediments against the southern margin of the Arch was hterpreted by deMille (1958) to irnply that the basement may have warped up in post-Cambnan tirne. Later, Cant (1988) suggested that it ha& more likely, already been initiated by the Middle Cambrian, since the lithologies of these Cambrian units appear to become sandier toward the Arch. Outcrop evidence from Upper Roterozoic and Figure 5-9. ûptimally-scaIed magnetic horizontal-gradient var(HGV) map of noahcenaal Alberta with vector arrows plotted pointing toward local maxima or uphill-, de1:2,500,000. The approximate lacation of the 3-D seismic swey is marked.

Figure 5-10. Interpreted magnetic lineament rnap of north-central Alberta; scale 1:2,500,000. This map is compileri from downhiîi- and uphill-plottexi magnetic HGV data together with total-field magnetic data. Keg River Formation carbonate buildups, the Keg River-Muskeg depositionai edge and the regional boundaries of the Peace River Arch are highlighted (Çom Figure El).The approximate location of the 3-D seismic survey is also indicated

Figure 5-11. Bouguer gravity map of north-central Alberta with an outluie of Keg River Formation carbonate buildups in the eastem Peace River Arch area (from Figure 5-1); scale 1:2,5ûû,ûûû. The gravity data is taken nom the GeologicaI Survey of Canada database; grid spacing 5 km. The Peace River Arch regional boundaries, the Keg River-Muskeg depositimal edge and the approximate location of the 3-D seismic meyare dso shown.

Figure 5-12. OptimaIIy scaled Bouguer gravity HGV map of north-central Alberta with vector arrows plotted pointhg away from local maxima, or downhill, with Bouguer gravity data; sale 1:2,500,000. The gravity data is taken fkom the Geologicd Survey of Canada database; grid spacing 5 km. The approximaîe location of the 3-D seismic survey is marked.

Figure 5-13. OptunaUy scaled Bouguer gravity HGV map of noah-central Alberta with vector arrows plotted pointing toward local maxima, or uphill; scaie 1:2,500,000. The approxirnate location of the >D seismic mrvey is matked.

Figure 5-14. Interpreted gravity lineament map of north-central Alberta; scale 1:2,500,000. This map is compiled fiom downhill- and uphill-pointing gravity HGV data together with Bouguer gravity &ta. Keg River Formation carbonate buiidups, the Keg River-Muskeg depositional edge and rhe regional boundaries of the Peace River Arch are highlighted (taken from Figure 5-1). nie approximate location of the 3-D seismic smey is also mdicated.

140 Cambrian units in the Cordiiiera also seem to support such timing (O'Connell et al., 1990). Whatevcr the exact timing, regional Elk Point Group onlap indicates that it was an area of positive topograpbic expression by Keg River Formation times (Cant, 1988).

In the general Senex area of the casteni Peace River Arch (Figue SI),the Recambrian surface consists of anornalous structural highs with relief typicaUy ranging fiom 10 to 100 m (Figure 5-3). Two seismic Iines across the Panny Field, show in Figure 5-15, intersect one another across one of these basement highs. These 2-D sections are representative of this eastem part of the Arch and serve to illustrate the characteristic undulatory nature of the Precarnbrian basement event. ReIief on reflective horizons directly overlying this event is attributed to drape or differential compaction of the Pdeozoic section across anomalous Precambnan structures. Basement structures have been regionally mapped fiom available well data as having discrete trends and king areally closed. This pattern is consistent with the idea of a basement surface fi.actured by conjugate pairsets of faults, both parallel and perpendicular to the orientation of the Peace River Arch. Interpretation of recovered core data also supports at least a partly structural origin to basement paleotopography. For example, episodes of reactivated normal faulting have been documented by Campbell (1987) ftom the interfmgering of Granite Wash detritus and Elk Point sediments. Local relief on the basement surface was likely controiled by pre-Middle Devonian normal block faulting. However, it is probable that some fault blocks have been significantly srnoothed by ensuing erosional processes, often with a marked reduction in the amplitude of basement displacements

Local reactivation of basement faults at various times during the Paieozoic and Mesozoic has ken well estabiished since the 1950s (e-g. deMille, 1958; Sikabonyi Lk Rodgers, 1959; Williams, 1958, and others). The first documented episode of reactivated normal faulting affecting the Arch occurred during deposition of the Eik Point Group in the Middle Devonian (Campbell, 1987). There is furùier evidence, both geological (e.g. Cant, 1988) and geophysical (e.g. Anderson et al., 1988a), of renewed faulting since Devonian tirne, either on new surfaces or as reactivations of former faults. In addition to normal fault development, extrapolation of focal-mechanism evidence by Horner et ai. (1994) of adjacent, low level (Mc4) hydrocarbon-production-induced seismicity suggest that a dextral strike-slip component to the faulting is also present. More specifically, HaIbertsma (1994) inferred that structures observed regionally across the Arch may be grouped into NE-trending transverse faults and NW-trending extensional faults. Figure 5-15. Compressecl 2-D seismic data across the Panny Field: (A) mterpreted migrated data fiom line Pannyl; (B) interpreted migrated data from line Panny2. Note the characteristic undulatory nature of the Precambrian basement surface and draphg of overlying units, - hthof fairly low time-stnicturaI relief. WSW 1-3 3-1 1 ENE

shotpoint

-Muskeg #Basalanhy ,drite - Precambrian

Line Pamy2 143 Alternatively, it may bc speculated thaî some pie-Reg River Formation extensional fairlts have undergone subsequent mchreactivation mvolving lateral displacements.

On the basis of the morphology of features observeci on the Precambrian maure map (Figure 5-3), O'Connel1 et al, (1990) proposed a sepratio11 of the Arch into two principal blocks: a complex western province of large high-amplitude fault-bounded structures with relief up to 150 m; and an eastern province of smaller, broader basement stnictures, normally with less than 60 m of relief. These two blocks are apparently offset dong a northwest-trending line, possibly representing a shear zone in the basement. The Keg River Formation reefs discussed in this chapter lie in the eastem province.

As revealed by the geological comoboration of total-field magnetic and magnetic HGV data fiom the Northem Intemides of the Wopmay Orogen (Chapter 2), regional magnetic data often reflect major tectonic boundaries and large-scale lithological or stnictwal contacts. However, limited displacements of the basement surface in the Peace River Arch area, as suggested by drillhole and seismic data, are not likely to be detected using such regional magnetic data unless a particular fault corresponds to a major lithologïcal boundary. Nevertheless, the division of the Arch by O'Connel1 (1990) can be partly recognized on the total-field magnetic and magnetic HGV maps of north-central Alberta (Figures 5-7 to 5-10). At this regional deof investigation, Figure 5-10 indicates that patterns of subcircular and relatively short magnetic lineaments appea. more cornmon in the eastem block while longer curvilinear elements domhate to the West. The proposed basement shear zone separating these two regions is marked by a total-field magnetic low in Figure 5-7 (as reported by OtComell et al., 1990) and by a series of north- to northwest-trending magnetic Iineaments in Figure 5-10. This stmcnrre also roughly coincides with a zone of extensive Bouguer gravity lineaments (Figures 5-1 1 to 5-14). However, a west-east division of the Arch based on contrasting anomaly patterns is not possible in the case of regional Bouguer gravity data. The eastem part of the Arch is dominated by a north- to north-northeast-trending Bouguer gravity low. InterestingIy, part of this pronounced anomaly appears to be roughly coincide with the chah of Keg River Formation carbonate buildups. The area covered by the 3-D seismic survey interpreted in Section 5.4 lies near the norihem edge of this anomaly.

In surnmary, identification of basement faulting using 2-D seismic and regional potential-field data is difficult in this am,due to the limited-offset subvertical nature of 144 these localized displacements, Furthermore, vertical and latcral rccurrent movement on some faults is thought to have occrrrred, Detectim of subtle uudulations on the basement surface in the eastem Pace River Arch area bas significant implicatious in tmns of the proposeci development of overlying Keg River Formation patch reefs. The next section outlines the advantages of 3-D seismic data in imaging such basement structures and carbonate buildups.

5.4 Paleotectonism and Keg River Formation carbonate bnildups It has previously been established that pre-Keg River Formation block fauiting was important for establishing local relief on the basement surface. In turn, raised basement topography provided the loci for the development of patch reefs of the Upper Keg River Member. In this section, this tectonic-depositional relationship is investigated from a 3-D perspective.

A 3-D seismic survey over an area of the eastem Peace River Arch containhg known Keg River Formation carbonate buildups was intexpreted The 3-D grid for this survey is shown in figure 5-16. Two representative 2-D sections fkom this 3-D grid are presena an inline section oriented W-E across one of the larger Precambnan highs; and an arbitrary crossline section îhat parallels the NNE-SSW crest of a series of such positive structures (Figures 5-17 and 5-18, respectively). Synthetic and field data correlate weil at the Precambrian, near-Keg River (or basal anhydrite) and Watt Mountain events. These horizons are annotated pink, yellow and red, respectively. Sirnilar to the geometries observed in Figure 5-15, many units have an undulatory morphology that parallels the Precarnbrian basement event. This is attniuted to differential compaction of the overlying Paleozoic section. However, in the near-Keg River-Precambrian intemai, carbonates typically thin hmabout 60 m or so off-structure to about 30 m at on-structure locations.

The Recambrian surface itself has gently undulating relief with no definite offsets. These sections illusirate that all the major structural highs in this field are penemted by wells, frequently with successful results. These positive structures exhibit a maximum amplitude of 40 ms, corresponding to approximately 100 m. This value may be slightly overestimated since overlying lateral velocity variations, including those due to patch reef growth, aimve such paleohighs may manifest anornalous structure on the Precambrian surface. Figure 5-16. 3-D seisrnic survey gxid showing the relative positions of the seismic ïniïne and arbitary crossline shown in Figures 5-17 and 5-18, respectively. The Iocation of adjacent driIi holes are also marked. Figure 5-17. A representative inline section (#41), oriented West to east across one of the larger Precambrian highs and overlying Keg River Formation patch reefs. The Precambrian, near- Keg River Formation (basal anhydrite), and Watt Mountain Formation events are shown in pink, yellow and red, respectively. The depth of well penetration is indicated by green.

A significant reduction m ampiitude on the normdy stroug Precambrian event is clearly associateci with these sttuctural highs. The near-Keg River event shows similar amplitude changes moving laterally across such structures. Such on-structure amplitude dimming frequently indicates the presence of a Keg River Formation patch reef. As demonstrated by modeling (Mitchell, 1988; Anderson et al., 1988a), the Ioss of amplitude can be amibuted to interference effects resulting hma thinning of basal anhydrite (Muskeg Formation) over the Keg River buildup, and Granite Wash or basal clastic unit thinning (or even absence) over Precambrian structure. This provides a sharp conuast with the off- structure amplitude character. Representative velocities for these units are indicated in a - typical on-structure velocity log shown in Figure 54.

A the structure map of the Precambrian basement event (Figure 5-19) confirms that 3- D seisrnic is an invaluable tool when it cornes to accurately deheating paleostructure in terms of orientation, shape and size of basement highs in this area. These features, although subtle on both 2-D sections hmthe 3-D grid and 2-D field data, become well- defined on horizontal sections by a series of elevated structures aligned dong a crestline oriented NE-SW. A number of smaller peripheral basement highs lie adjacent to this main trend. This event represents ody 40 ms of time-stmctural relief,from 816 ms to 856 ms. Looking towards the NW,a 3-D perspective view of this horizon (Figure 5-20) further exaggerates this faint time-structural relief and shows clearly distinguishable closed Precambrian structurai hi@.

The Precambrian paleorelief is consistent with a conjugate fault pattern (Figure 5-21), where fault blocks were rounded and individual fault-scarp surfaces smoothed through ensuing erosional processes. The main set of paleohighs essentially constitute a NNE- to NE-trendhg horst structure, compartmentalized into a series of discrete blocks by NW- SE oriented normal faulting. However, even within this 16 ld area, the size, complexity and closure nature of these fault blocks Vary considerably. ImportantIy, these relatively small fault blocks are not Iikely to be seen on regional magnetic and gtavity maps, even if coverage was complete in this area.

The morphologies of the near-Keg River (basal anhydrite) and Watt Mountain time structure maps (i.e. the tops of the respective units) on Figures 5-22 and 5-23, respectively, closely pardlel that of the Precambrian basement reflector, confhhg the Figure 5-19. Precambrian time structure map; the colour bar represents time-structural relief in ms. Figure 5-20. 3-D perspective view of the Precarnbrian seismic event. The colour change from yellowlred to bluelpurple is the sarne as C in the Precarnbrian time structure map (Figure 5-19). O -interpreted fault

mm ma mmmm-Da disptayed seismic section Figure 5-21. hterpreted Precambrian fault structure map: tectonic interpretation of the Precambrian tirne structure map show in Figure 5-19. Wwm L000m Z00Wm Figure 5-22. Near-Keg River Formation (basal anhydrite) time structure map; the colour bar represents time-structurai relief in ms. am lH00m

Figure 5-23. Watt Mountain Formation tirne structure map; the colour bar represents time-structura1 relief in m. 154 draped nature of these events due to daferentid compaction. Consistent lateral tbickness variations above the Keg River Formation, rule out the possibility of reactivated tectonisrn as a possible source fm these similzuities.

The hot colours, red and yeilow, represent time-stmcûd highs whereas the cool colours, blue and purple, correspond to depressed paleotopography. In most cases, the areal extent of Keg River Formation patch-reef development confom to the hot colours shown on Figure 5-24. Comparison with the Precambrian time stnrcture map coniïms that the Keg River Formation carbonate buildups are directly rooted above paleohighs on the F'recambrian dace. This display indicates the morphology of these buildups as typically elongate and irregular. On comparing this time structure map with an inline section that intersects this structure (Figure 5-17), it becomes apparent that the aberrant tops and margins of these reefs are best interpreted &om a horizonta1 perspective. The areas designated by these colours also contain the majority of established oil wells for this field. Nevertheless, exploration potential exists through Merdevelopment of these established reefs, now that their skand shape is accurately hown, and via the number of neighbouring, smaller buildups that cm be disthguished. Certain satellite reefs are associated with unsuccessful drilling where the wells have apparently just missed such buildups. Some dry and abandoned wells are slightly obscured by darker colours (off- stmcture) on the structure maps.

Comparing the Precambrian-Near Keg River isochron map (Pc-nKR) (Figure 5-24) with the Precambrian structure map (Figure 5-19) indicates that the overall Keg River Formation sequence is commonly thinner above Precambrian hi@ where patch reefs have grown. Close investigation of the Precambrian surface around the largest paleohigh structure (Figure 5-25) reveals a marked reduction in amplitude undemeath full reef buildups. NW-oriented fault trends may be foUowed on either side of this main structure but not directly over the top. Such dimming is a response to the on-structure thinning of the Granite Wash unit. This location also corresponds with the pronounced thinning of the Pc-nKR isochron (Figure 5-24), and the full Keg River Formation patch reef development. Faultùig may be projected dong trend through the dimmed zone on the Precambrian surface, thereby indirectly inferring the presence of structure. Carbonate buildups growing laterally over fault scarps, whose raid sides initiated growth, seem to be a common feature of Keg River deposition in this area Figure 5-24. Precambrian to near-Keg River Formation isochron; the colour bar units are ms. Figure 5-25. hktpreted Precambrian time structure map for the northern part of the survey area, covering the largest patch reef-basement paleohigh expanse. The colour bar is shown in Figure 5-19. A lack of apparent basement structure directly beneath the full reef buildup is a result of an amplitude dirnrning of the Precambrian basernent event. 5.5 SDscaled physical seismic modeiiing of a reef-fauit block The usefuiness of 3-D data in gcneral, and horizontal displays in particular, for interpreting Precambrian basement tectonism in proximity to carbonate buildups is mertestecf usmg SDphysical seisrnic modeling. Acquisition of 3-D modelled seismic surveys is possible thtough the physical modelbg facilities at The University of Caigary. This physical modeling system is descrii by Cheadle et al. (1985). The main objective was to analyze the potential for directiy imaging high-angle faulting beneath carbonate buildups, nodycomplicated by on-structure amplitude dimming and velocity pull up of the basement surface.

Construction of the reef-fault block model incorporated geometry and velocity information determined from the eastem Peace River Arch 3-D dataset However, as indicated in Figure 5-26, the model is extremely sirnplified to include only the zone of interest, with no overburden, and to contain a combination of geologic type scenarios: an uplifted orthogonal fault block and basal clastic iafill, a patch reef partially overlying a fault block; and a patch reef situated above a simple basement platform. The success of physical seismic modeling is directly dependent on the choice of modeling materials and the scaling factors used for the simulation. The materials used to constnict the model are surnrnarized in Table 5-3. Scaling factors for velocity of 1:2, for depth and distance of 1:10,000, and for time of 15,000 were chosen for the model experiments such that the value of a model parameter multiplied by the appropriate scale factor resulted in the dimensions of the field prototype.

A 3-D zero-offset survey was carried out over an area of 25 cm by 20 cm (corresponding to field dimensions of 2.5 km by 2 km). 3-D zero-offset surveys essentidy have a 'stacked' trace at the centre of every bin. As indicated in Figure 5-27A, the survey consists of 100 inlines and 80 crosslines with a bin spacing of 0.25 cm (scaled to 25 m). The shooting direction was parallel to the short side of the reef. Post-stack processing techniques applied to the modeled 3-D dataset are generally comparable to those used previously for the 3-D field data.

A number of characteristic stacked and migrated displays from the modeled dataset are presented in this chapter. A representative inline section across the reef-fault block model is shown in Figure 5-27B. On this stacked section, a number of events can be picked tion a

I section a I

lt-""---.Isection b

Figure 5-26, A ichematic plan view of, and cross-sections across. the reef-fault block model. Modelling materials arc summarized in Table 5-2. seismic data physical3-û model E. Peace River Arch scaled parameters

-- Patch CanadianTi plastic 2840 1480 reef rcsinwithmp glas beads, VOL 1:I

bd Canadian Tire dastics plastic min

Table 5-3. Surnmary of dimensions and materials used for the constmction of the reef-fault block model. The original field velocities and dimensions are taken fiom the 3-D seismic survey in the eastern Peace River Arch area. The scaliug parameters used for the mode1 were: time 1 :5,000; deptWdistance 1 :10,000, and velocity 1 :2. The value of a model parameter multiplied by the sdefactor results m the dimensions of the field prototype. Figure 5-27. 3-D physical modeIling over a reef-fault block: (A) 3-D survey grid indicating the location of the arbitary crossline shown in 5-27B; (B) representative stacked section from 3-D survey. Note the relative absence of reflectors beneath the full carbonate buildup. 161 because the model geometry and velocity are aiready known. Seismic eveats associateci with the reef, pladorm and Granite Wash can all be readily identifid The base of the model is just below 1OOO ms. Significantly, pre-reef refiectors can be traccd away from and beneath the edges of the reef but are not well imaged where the reef attains its maximum thickness. Velocity pullup may also be inferred on the weak platform event directly below the reef. This stacked section also contains a number of diffractions originating hmboth the edges of the reef and the fadt block.

The extent of the elongate reef is weU imaged on the 820-ms timeslice of the migrateci dataset (Figure 5-28), where water surromding the reef gives rise, essentidy, to a zero amplitude. The presence of a weak anomaly to the SE indicates that the platform is not enhly flat but is slightly warped, Figure 5-29, an 860-111stimeslice below the reef, imaging the top of the fault block, exhiiits a relationship similar to that descnhxl for the inline section. This migrated timeslice provides an excellent example of the areal extent of the basal clastic block away fkom, and beneath, the reef margins. However, these amplitudes again decrease abruptly under the fuil reef. Because of the zero-offset nature of tbis dataset, a concentric reef flank reflection is also still observed on Figure 5-29. This anomaly has srnalier dimensions compared to observed difiction wavefronts simply due to refletions having steeper dips than diffractions.

From a 2-D perspective, numerical modeling by Anderson et al. (1988a) demonstrated that interference effects resulting in on-structure amplitude dimming of the basement @Iatform) surface rnay be caused by the thinning and velocities associated the overlying units. As hdicated earlier, the thiclcness of both Granite Wash sediments and Keg River Formation carbonates are frequently reduced over raised Precambrian topography. In some cases, carbonates may lie directly on Recambrian basement. This results in a reduced velocity contrast and a correspondhg loss of amplitude on the basement @latfom) event beneath catbnate buildups, as observed in the physical modeled dataset. Although the physical model is simplifieci to contain no post-reef units, this problem of imaging below carbonate buildups may also be partly due to a 'shadow-zone' effect where the basement surface becomes masked through transmission losses from the high- amplitude reef event. Whatever the precise origin of this amplitude dimming, timeslice investigation ailows the areal extent of the fault block containhg Granite Wash to k well imaged away fkom, and beneath, the reef rnargins. Importantly, it is again possible to III E

164 exîrapoIaîe these prefefzed trends to indirectly infer the pmence of fadting beneath reefs.

5.6 Future exploration of the Keg River-Senex play Using ody seismic data, it is difficult to deterniine whether undulations on the Precambrian basement event, apparently characteristic for this area of the eastem Pace River Arch, are dominantly produced by stmctural or geomorphic processes. However, the cyclicity of interkdded Granite Wash and Eik Point sediments recogaized fiom core data in this area is compatible with reactivated fad~gof the Precambrian basement . surface and subsequent erosion of that surface. Overali, it is evident that mmy Keg River Formation patch reefs in the eastern Peace River Ar& area were initiated through paleobathymetric differences exerted by Precambrian blwk faulting and many subsequently grew laterally to overlie these periodically active fault planes. Recognition of closed Precambria.structure is vital to successful exploration for the Keg RiverSenex play.

Keg River Formation patch reefs are normaliy iimited in size and ampiitude, growing up to a maximum thickness of approximately 50 m over uplifted blocks. Although 2-D seismic data can often successfuily locate sigaificant Recambrian paleostmcture and large carbonate buildups, smaller anomalies frequently Iack dennition and are not clearly resolved. Srnall satellite patch reefs may be located dong strike from larger buildups. These could easily be overlooked, particuIarly since this play is characterized by a large number of oil pools with an anomalously low average pool size.

3-D seismic would seem to provide an excellent tool for examining the shape, size and trend of Keg River Formation reef anomalies, together with the basement fault-bounded highs on which they grow. Using timeslices or the structure maps, extrapolation of preferred fault trends imaged away bmreefs indirectly infers the presence of faulting beneath, and within, buildups. These structures may represent potential fluid-migration conduits, or produce fiachire-related secondary porosity wirhîn buildups. A schematic diagram summariWag local reef-fault block interaction possible in this part of north- centrai Alberta is shown in Figure 5-30. The effcct of ensuing pre-reef erosion on this mode1 would be to significantly smooth rnany of these tilted fault blocks. Fault set (1) elevated part of tilted block locnlizes reef growth

reefal facies nshalylirnestone 1 marl pre-reef unit 1 basernent

Figure 5-30. Schematic block diagram showing the types of reef-fault block interaction representative of the eastern Peace River Amh area (modified after Brown & Brown, 1987). 166 Since a direct relatimhip between reef developrnent and movement on basement fdts has been established for this area, it may be possible to predict where reefs are likely to have grown by extrapolation of proven basement trends. Regional potential-field data is considered unlikely to be able to delineate the complex orthogonal faulting that initiated shoaling in this area. However, high-resolution aeromagnetic data wouid probabIy have more success in detecting such features. Seismic exploration for Keg River Formation reservoirs could then be localizcd dong productive trends delineated by such bigh- resolution data, Overall, the opportunity exists for the future discovery of Keg River Formation, and other, carbonate rcservoirs in the Peace River Arch area of north-cenaal Alberta using exploration philosophies oriented toward this basement-nef tectonic control model. Basement infiuences on the Swan ïïüis Formation of west-central Alberta

Bnefly stated, this chapter describes a geophysical investigation into the possible interaction between the Pr;ecambrian crystalline basement and the deposition of Swan HiUs Formation carbonates (Middle to Upper Devonian). The study area containhg these carbonates lies in west-central Alberta (F~gureGl), extending hmlatitude 53" N to 56" N (township 46 to 80) and longitude 114" W to 11Y W (range 1 WS to range 7 W6). From Figure 6-1, it is clear that the distribution of Swan Hüis carbonate buildups in this active hydrocarbon exploration area is fairly well resolved. However, compared to the tectonic control established for Keg River reef growth, discussed in Chapter 5, the mechanisms behind the nucleation and growth of Swan H'scarbonates remain relatively unclear and inconclusive to date.

Through the interpretation and integration of several different &tas&, th& chapter aims to better ascertain whether the Phanerozoic sedimentary sequence in west-central Alberta bas locally or regionaUy inherited any structures hmthe underlying Precambrian basement complex. In tu,the influence of any such pre-Devonian faults or folds on the presentday morphology of Swan Hills carbonates is assesseci using local deep and shallow seismic profiles, extensive drillhole data and regional potential-field data. Two morphologically distinct types of Swan HiUs buildups are considered. isolated atoll-like reefs, and an extensive carbonate platforrn or shelf.

Whilst structures underlying the Swan Hills reefs or plahmay directly or indirectly affect the loci and growtb of carbonates, the presence of basement faulting reactivated after deposition of Swan Hills carbonates is also thought to be of significance to this unit Any evidence substantiating the existence of such faulting may have important repercussions for discoverhg localized dolomite pockets within the otherwise tight limestone of the Swan EWls platfonn, or even the compartroentalization of certain reservoirs. The nature and importance of both tectonic heredity and dolomitization on Swan HiUs carbonates are stressed in this chapter. Figure 6-1. Map of Swan Hills Formatiw (Middle-Upper Devonim) paIeogeography ia west-central Alberta showing the distribution of tùe lower plamtogerher wirh the overïying isolated reef complexes and upper phtform; scde 1:2,500,000. Tbis configuration is modined hm several sources including Jansa & Fischbuch (1974), Walls (1988) and Unocal Canada Ltd. (Exploration staff, personai communication, 1996). Interpreted seismic profiles are also indicated, Swan HiUs Formation carbonam frhging the Peace River Arch are not discussed in this chapter. 6.1 Geologid se- and stntigraphy The Swan Hills Formation is stratipphidy situated m the Beaverhill Lake Group (late Givetian to early Frasnian stage). As shown m Figure 6-2, the Beaverhill Lake Group in west-centd Alberta comprises the Fort Vermilion, Swan Hüls and Waterways Forrnatioas. The subsurface of west-centrai AIberta is dominated by the Swan Hüls Forniaton, a unit occurring over at least 10,000 lm2.The mtigraphy of the Swan Hills Formation has been described in detail by Fong (1959), Fischbuch (1968) and Hemphill et al. (1970) and others, but early studies tended to use slightly diffemt nomenclature, as ootiined in Figure 6-2. In contcast to the work by Fong (1959), the base of the Swan Hills Formation is now regarded to lie at the top of Fort Vermilion Formation shales and anhydrites.

During a period of regional transgression, shallow-water carbonates of the Swan Hills Formation typically formed reefal complexes in an ernbayment south of the Peace River Arch. A regionally extensive basal carbonate platfom was initialiy deposited and served as a foundation for overlying reef-complex development. On the northem part of the basal platfonn, isoIated atoU-like reefs, isolated by marine channels, were deposited over existing topographie highs, while a backstepping, reef-rimmed shelf or upper plaaonn formed in the south (Leavitt, 1968). Overd, the Swan Hills Formation in this area ranges in thickness fiom about 150 rn for fidi carbonate buildups to less than 25 m in off-reef areas (Jansa & Fischbuch, 1974).

The Swan Hills Formation has been divided by several workers on the basis of composition, colour and geometry (Figure 6-2). Fong (1959) spiit the formation into a dark brown unit and a light brown unit, representing basal platform and upper shelf or reef development, respectively. Later, Fischbuch (1968) separateci the formation into nine informal units and designated these units as Divisions I to IX, moving fiom the bottom to the top. These divnons are interpreted by Fischbuch (1968) as representing stages of reef growth, each composed of several distinct facies. Each bounded division may npresent a single stage of reef growth, where boundaries represent a aear-termination of reef growth, possibly due to sea-level change and mbmarine erosion. In his scheme, Divisions I to III comspond to the Iowa platlm and Divisions IV to IX to the reef-rimrned shelf.

Basin filLing by westward-progradhg shale and argillaceous limestone clinofomis of the Waterways Formation occurred after drowning of the Swan Hills reefs (Wendte and Fong Fischbuch Hemphill et al. I Wabamun Group (1959) UPP~~ Watcrways Walerways Beavcrhill Formation Formation IE 1 Winterburn Group Lake 1

l

Muskeg Fm . Basal Po@, 'W. Bcaverhill Vermilion Vermilion Lake Formalion Formation Contact Rapids Fm I

(A)

Fieure 6-2. ~tkti~ra~h~of the Beaverhill Lake Group in the Swan Hills area of west-central Alberta: (A) generalized Devoninn stratigraphy highlighting carbonate buildups of the Swan Hills and Leduc Formations (modified from Hemphill et al., 1970; AGAT Laboratories, 1988); BHL=Beaverhill Lake Group; (B) proposed stratigraphic nomenclature of the Baaverhill Lake Group (after Ferry, 1989). 171 Stoakes, 1987)-The Waterways Formation reaches a maximum thickness of about 150 m (Fischbuch, 1968) but markedly thins over Swan Hiüs buildups to the extent that it is absent where Swan Hills carbonates are dirccüy overlain by carbonates of the Leduc Formation (Wdbend Group) @gure 62)-

The hydrocarbon potential of the Swan HilIs Formation in west-central Alberta was first reaIized in the Iate 1950s, with the discovery of the ViafIills reef cornplex in 1956 (Fong, 1959). At present, it is known that hydrocarbon reservoirs have developed within Swan Hills atolis or bioherms and a reef-fringed carbanate ban.often comprising both Stratigraphie and structural components. hmFigure 6-3, it is clear that both hestone - and dolomite reservoits are associated with these two general pIay types. fohski et al. (1 988) suggested that the overIying Duvemay Formation is the most lïkely source rock for hydrowbons trapped in Swan Hills carbonates.

In tbe case of lunestone resewoirs, regional dip influences the distrïïtion of hydrocarbons within stratigraphie traps such as carbonate banks (e.g. Anie Creek Field), shelf edges (cg. Sam Lake Field), and some isolated reefs (e.g. Virginia Hills Field)- Such porous carbonates are normally vertically and laterally seaied by the enveloping argillaceous sediments of the Waterways Formation (Podmski et ai., 1988).

Dolomite reservoirs are found within the extensive upper platfom or shelf of the Swan HilIs Formation and include the Hanlan, Erith (or Minehead), Rosevear and Kaybob South Fields. Reinson et al. (1993, Figure 43) outline other fields that may be grouped into this platformal play type. According to Walls (1988), the creation of porosity through dolomitization was generally confineci to narrow belts and often closely associated with either channels and embayments intemaily cutting the pIatform or the platfom margin itself. These porous zones are trapped either by the Waterways Formation or by updip tight platformal limestones.

Undiscovered pools may exist either in new reef complexes, downdip of subtle channels in discovered reef complexes, or in patch reefs formed on the platfixm (Podruski et al., 1988). There would also seem to be great potential for the presence of Iocalized doIomite pockets of porosity within the extensive Swan Hills platform. 20 40 km

limestone resemoirs

dolomite reservoh

Figure 6-3. Swan Hills Formation reservoixs in west-central Alberta (modifiecl fkom Walls, 1988). Published examples of proposed basement influences on carbonate deposition and diagenesis: (1) Martin (1%7), (2) Sawford (1%7), (3) Leavitt (1968), (4) Keith (1 WO), (5) Schultheis (1976), (6) Mauntjoy (1980), (7) Jones (1980), (8) Viau & Oldershaw (1984), (9) Viau (1983,(10) Waiis (1988), (11) Kaufman et al. (1991), and (12) Ross (1993b). 173 6.2 Data availability The interprebtim of the basemnt and sedimcntary cover in the west-central Alberta study area relies on a synthesis of available seismic reflection profiles, recorded with varying lisîening times, drillhole data and potentiai-field data from the Geological Survcy of Canada database. The location of the seismic profiles interpreted in west-central Alberta is show on a regional scale in Figure 61.

A deep seismic profile (be CCkl) was acquired by Husky Oil Ltd. in 1989, and recorded to t 6 seconds across the Carson Creek North Field of west-central Alberta, This field is an undolomitized, atoll-like feature associated with the Swan HiUs reef cornplex. AU processmg was undertaken at the OPR ROBE Seismic Rocessing Facility (LSPF) at The University of Calgary. A surnmary of the acquisition and processirrg parameters for he CCkl is given in Table 61. A second line (CCkZ), although only recordecl to 2.5 seconds, intersects line CCkl and provides some detailed well control for the stages of reef growih locdy established for the Carson Creek North buildup. This line was made available by Talisman Energy Ltd and Sigma Explorations Ltd. All presîack processing was carried out by Digitech Ltd in 1976, and later regrocesseci by Talisman Energy (Table 6-2). Figure 6 4 illustrates a synthetic well tie down to the Precambrian basement for the sedimentary cover in the Carson Creek North area.

More recentiy, Unocal Canada Ltd. acquired a large regional seismic profile (lhe Antel), recorded to 8 seconds. near an edge of the Swan HiUs upper platfonn or shelf. Table 6-3 outhes the parameters associated with acquisition and processing of this line.

In the Edson am.,about 100 km southwest of the Carson Creek North Field, a 3-second seismic profile @ne Edsonl) intersects a pronounced Phanerozoic structural feahrre hown as the Erith Graben (Mobil Oil Canada Ltd, exploration staff, personal communication, 1994). This profile, donated by Mobil Oil Canada Ltd. and initially processed by Geo-X Systems in 1978 (Table W),is used as a template line for studying the possible evolution of Phanerozoic faulting in west-central Alberta.

In this area, available coverage by reflection data is ody two-dimensional and local in nature, resmcted to sparse seismic profiles. However, regional images of basement features may be interpreted fiom regional potential-field surveys. Repeating the methodology introduced in Chapter 5, a series of total-field magnetic and magnetic HGV 1 Acquisition parameters Source dynamite; average charge: 4 kg; lOem interval Receivers groÜp interval: 20 m; record length: 16 s Spread symmetric split spread 23-10 * 10-2390 m 24-fold coverage

Demultiplexing Trace editing first breaks, mute, trace kih

Refraction statics 1-1ayer model, 9 15-m/s weathering velocity Elevation statics 945-m daturn, 280em/s replacement velocity

~otchfilter I Deconvolution minimum phase, 72-ms operator length 1% prewhitening I - AGC gain 1 500m Trace equalization 1 2to16s NMO Residual statics k32 ms maximum shift -- - - stack deconvolution gap: 32 ms; operator 256 ms 1% prewhitening F-X filter clip rejection filter (-0.004 to 0.004 seconds/trace) 1 F-X prediction- filter Migration 1 phase-shift; 100% stacking velocities Tirne-variant filtering 1 Time-variant scaling AGC gain * - 50-ms window I 1 * Flattening selected horizons

Table 6-1. Acquisition and processing parameters for seismic line CCk1. AI1 processing was undertaken at the LITHOPROBESeismic Processing Facility (LSPF). An asterïsk indicates optionai processes. Line CCkl Figure 6-4. Example of a synthetic tie down to the Precambrian basement, pmjected hmthe Mobil PR Carson 6-2M2-12W5 well to line Cal. Ttie synthetic seismogram was generated with an Omisby zero-phase wavelet (5110 to 50/70IL) with only primary reflections caiculated. Both synthetic and field data are displayed m normal polanty. Additional basement and near-basement correlations were also utilized in this study. 1 Acqiiisition parameters Source dynamite (charge unknown) Receivers group interval: 220 ft; record length: 3 s Spread 528iH.20* 220-5280 ft 24-fold coverage

1 Ampitude recovery 1 Deconvolution suface-consistent spiking 96-111soperator length, 5% prewhitening Refraction statics Elevation statics 3000-ft datum, 800eftfs replacement velocity Trace editing htbreaks, mute, trace kills Time-variant nIter - Notch filter 60Hk Amplitude scaling surface consistent 1 NMO 1 1 Residual statics 1 maximum shfi + 12 ms; window: 4ûû-3000ms

1 Migration 1 finite ciifference; 100% stacking velocities

Equalization window: 4W3000 ms * AGC gain window: 50 ms 1 * flattening 1 on selected horizons

Table 6-2. Acquisition and processing parameters for seismic line CCk2. Processing was carried out initially by Sigma Explorations Ltd and later by Talisman Energy Ltd. Items indicated with an asterisk were undertaken at the L~~HOPROBESeismic Pmcessing facility (LSPF). Acquisition parameters Source dynamite; average charge: 2 kg; 100-m interval Receivers group interval: 25 m; record length: 8 s Spread 4525-25 * M500 m 45-foId coverage kessing parameters Dern~Iti~lexina I

Deconvolution 1 surfaceconsistent minimum phase r Refkaction statics Elevation statics 1400-m danim. 3300-mls replacement velocity Trace editing fmt breaks, mute, trace kills Amplitude scaling swfiïce consistent mo Time-variant scaling window: WIûû ms T-Pmultiple attenuation window: 5~3000ms 1 Residud statics 1 window:400-3000ms stack deconvohtion 32-111sgap, 256-msoperator 1% prewhitening dip rejection Nter (-0.004 to 0.004 dtrace) F-X prediction filter 1 Migration 1 phase-shift; 100% stacking velocities

L The-variant scaling * AGC gain 1 5Oms 1 *Hattening 1 on selected horizons

Table 6-3. Acquisition and processing parameters for seismic heAntel. Pre-stack processiag was carried out by Geo-X Systems Ltd. and post-stack processing was undertaken at the LITHOPROBESeismic Rocessing Facility (LSPF). An asterisk indicates optional processes. Acquisition parameters

Source 1 dynamite; average charge: 8 x 1 kg; 1320-ft interval Receivers group interval: 330 ft; record length: 3 s Spread 1584&330 * 33ik15840 ft 14-fold coverage

Amplitude recovery 1 Deconvolution surface-consistent spiking 80-ms operator length, 0.1 % prewhitening Refraction statics Elevation statics 1000-m datum, 3050-mis replacement velocity Trace editing first breaks, mute, trace kills NMO ------Time-variant scaling Residual statics f 10 ms maximim shift; window: 600-2600 ms stack Migration Finite ciifference; 100% stacking velocities Filter 10114-SOI60 Hz EquaIization 1 window: 40-270û ms

Table 6-4. Acquisition and processing parameters for seismic line Edsonl. Al1 processing was carried out by Geo-X Systems Ltd- 179 maps of west-central Alberta, such as Figures 65to 67for example, are interpreted and compiled to produce a final magnetic heament map (Figure 6-8). Similarly, a single iineament map is produced fjrom regional Bouguer gravity and Bouguer gravity HGV data in this region 69to 612). Major lithological or structurai contacts in the basement may be idcntified Mmregionai meticand gravity maps. A cornparison of the distribution of Swan HiiIs Formation reefs and upper platfom with interpreted magnetic and gravity linearnents may indicate whether such regional basement features were a contributing factor in the deposition of these carbonates.

63 Basement structure and sarface topography As discussed in Chapter 3, the interprétation of seismic events at extended Listening times is often complicated by increased noise, multiple energy and possible sideswipe reflections. Furthmore, a low si@-to-noise ratio can lead to weak reflections becoming obscured by smeared noise bursts in the migrated seismic section. Nevertheless, the processing and interpretation of two NEAW Oneuted seismic profiles, lines CCkl and Autel, recordeci to relatively long iistening times across contrasting areas of Swan Hills Formation deposition have yielded a dipping basement fabnc. Line CCkl was acquired over the Carson Creek North reef complex and contains intra-basement seismic events, fiom 1.8 to 4 seconds, with an apparent dip of about 30" to the SW (Figures 6-13 and 6-14). Low-angle reflections with apparent dips around 10" are aiso evident between 2.5 and 4 seconds on Iine Antel (Figures 6-15 and 616). a regional profile situated near an edge of the Swan Mls Formation upper pIatfonn CoUectively, these inclined intra-basement seismic events are probably of a lithological rather than thst fault ongin. This interpretation is rnainly based on a general Iack of any large-scde hanging-wall structures or marked tnuications associated with these basement reflectors. For example, these proposed layered basement rocks appear to cross-cut subhorizonal layering at about 3 seconds on line CCkl with no signifiant offset.

Regional magnetic and gravity anornaly patterns in west-central Alberta, shown in Figures 65to 612,appear to display similar geometries to those observed in north-central Alberta (Chapter 5). Several distinctive types of anomdy patterns can again be identified: regionally-extensive iineaments; more Iocalized curviiinear elements, and randomly distributed subcircuIar to arcuate anomalies. Seismic hes Antel and CCkl are associated with contrasting regional magnetic signatures (Figures 65to 68). Figure 6-5. Totai-field magnetic rnap of west-central Alberta with an outline of Swan Hills Formation carbonate buildups (nom Figure 6-1); scaie 1:2,500,000. The magnetic daîa is taken from the Geological Survey of Canada database; grid spacing 2 km. Figure 6-6. ~~-&nall~-scalcdmagnetic HGV map of west-centrai Alberta, wiih vector arrows pld pointhg away fmm local maxima, or downhill, with tod-field magnetif data; sui* I:2,500,0ûû.The magnetic data is taken from the Geological Survey of Canada database; grid spacing 2 km. Figure 6-7. ûptimaily-scaied magnetic horizontal-gradient vector (HGV) map of westcentrai Alberta with vector arrows plotted pointhg toward local maxima or uphill; de1:2,500,000. Figure 6-8. ~ntkpretedmagnetic lineament map of west-central Alberta; de1:2,5ûû,ûûû. This map is compiled from do-- and uphill-plotted magnetic HGV data together with total-field magnetic data. Swan Hills Formatiun carbonate bddups (hFigure 61) are highlighted. Figure 6-9. Bouguer gravity map of west-centrai Alkria with an outiine of Swan Hills Formation carbonate buildups (from Figure 6-1); scale 1:2,500,000. The Bouguer gravity data is taken from the Geological Survey of Canada database; grid spacing 5 km. Figure 6-10. Optirnally-scaled Bouguer gravity HGV map of west-central Alberta, with vector arrows plotted pointing away from local maxima, or downhill, with Bouguer gravity data; scale 1:2,500,000. The Bouguer gravity data is taken hmthe Geological Survey of Canada dabbase; grid spacing 5 km. Figure 611. Optirnally-scaled Bouguer gravity horizontal-gradient vector (HGV) map of west-centrai Alberta with vector arrows plotted pointing toward local maxima or uphill; sale 1 :2,500,000. Figure 6-12. 16rpreted Bouguer gravity heament map of west-central Alberta; sale 1:2,500,000. This map is cornpihi fiom downhili- and uphill-plotted Bouguer gravity HGV data together with Bouguer gravity data. Swan Hills Foxmation carbonate buddups (hmFigure 6-1) are highlighted. Figure 6-13. Uninterpreted migracd seismic data from line CCkl. The vertical exaggeration near the basement surface is approximately 1.2, for an asmexi average veIot5ty of Mo0 m/s. Note the presence of several dipping events below two seconds that appear to cross-cut hi@- amplitude subhonZantal rdcctm. This lower part of the section is also contaminad by multiple energy.

Figure 6-14. Migrated seismic data from line CCkl with severai significant basement features highlîghted. Near the basement whce ( mdicated by a white marker at about 1.8 seconds), the vertical exaggeration is approximately 1.2, for an assumed average velocity of 5000 ds.

Fignre 6-15. Uninterpreted migrated seismic data hmIine Ante 1. The vertical exaggeration near the basernent surface is approxirnately 5.2, for an assumed average vdocity of 5000 ds.Note the regional dip and thickening to the SW of the sdhnentary cover on this hue (O to 2.25 or 2.5 seconds). A number of iow-angle events can be distinguished below 2.5 seconds, particularly m the SW,

Figure 6-16. Migrated seismic data fiom line Antel with the interpretation of a low-angle basernent fabric highlighted in white. Near the basement surface, the vertical exaggeration is approximately 5.2, for an assurned average velociîy of 5000 ds.The basement surface is indicakd by a white marker borizw- Some of the low-angle basement events intersect this surface. 4 c4 TIME (seconds) 9 9 4 m - TIME (seconds) 9 L? 9 X 2 - 4 3 m X - TIME (seconds) I I I 9 'f? 9 9 "7 O 2 m 4 rn 2 m rn d TïME (seconds) 193 Line Ante 1 is associated with relatively low magnetic anomaly values and is situateâ m the middle of the Chinchaga Low tectonic do& of Ross et al, (1991) (Figures 3-1 and 3-2)- From scaling between Figures 68and 616, two weak north- to northwest-trending magnetic heaments intersect the northeastern part of this seismic line at approximately shotpoints 650 and 1200. The magnetic Iineament at shotpoint 650 does not appear to correspond to any major basement feature idenaned cm the Antel seismic profile. On the other hand, tbe magnetic lineament at shotpoint 1200 lies in an area where several low- angle reflectors appear to appmach, and somctimes even breach, the basement &. One such seismic event is inttrpreted to reach the basement SUIface at shotpoint 1025, less than 5 km from the inferrd location of the lineament, However, no refiectors are seen to - directiy tie with the proposeci location of this lïneament. Figures 69to 6-12 meaI that several regional Bmguer gravity anomaiies are associated witb iïne Antel. A no*- noahwest-trendhg gravity iineament mecges with seismic Iine Antel near shotpoint 650, again adjacent to the interpreted locatim of dipping seismic events. A second linearnent @endsobliquely to the ke, and even nuis parallel to it between shotpoints 1625 and 1925. The relationship between this linment and the basement structure infexTed from seismic data is unclear. Overail, one possible interpretation is that the curviiinear north- to northwest-trending magnetic and gravity lineaments are related to the dipping intrabasement seismic fabric.

Seismic line CCkl intersects the propused boundary between the Chinchaga Low and the Buffalo Head domains (Figures 3-1 and 3-2). The northeastern part of this line coincides with a pronounced çubcircular magnetic high, flanked by a series of curvibear anomalies (Figures 65to 68). These anomaly patterns are interpreted to reflect lithological boudaries in the shallow basement, possibly originally related to magmatic intrusion. In this area, several dipping intrabasmcnt events are intespreted on Ihe CCKl between abut 2.5 and 4 seconds (Figure 614); some evcnts may extend upsection to mach the basement surface. It is plausibIe that these magnetic anomalies are related to inclineci basement layers containing rocks of laterdiy varying magnetic susceptibilitics- From Figures 69to 6-12, it is apparent that no signifiant gravity anomiaies are associated with îhis line.

Correlations between seismic and poteutid-field anomaiies are likely to be complicated because these geophysical techniques effectively respond ta contrasting rock parameters. Not every lithologicd boundary identified on seismic data is expected to bave a correspondhg magnetic or gravity signature, and vice-versa. Furthemore, discrepancies 194 betwem features mtapreted using these different geophysical techniques are to be expected due to the considerably greater resolving power of seismic &ta over potential-field data. At this local scale of hvestigaticm, some inaccuracies are Iikely to have been miroduced due to the regiond sampling of the magnetic and gravity data (2-km and 5-lan grid spacmg, respectively). Hi@-resdution aer~magll~c&ta is needed for a more conMent assessrnent at this locaIized de.

In cmîrast to the extensive hulting affecting the basement-sediment interface m the easteni Peace River Arch ara (outlined m Chapter 3, the area covered by available seismic data in west-central Alberta appears to be characterized by more subtle and less frequent irregularities on this Surface. As illustrated in Figures 6-17 to 622, the sedimentary- crystaIIine boundary on hes CCkl and Autel is represented by a gently undulating topography, apparentiy punctuated in places by discrete zones of unevenness. The presence of diffractions at the basement daceon stacked seismic sections often highlights such areas of localized basement relief (Figure 6-17). Following intrabasement low-angle seismic events upsection to this basement-sediment interface is often difficult due to contamination by multiple energy from the overlying sedimentary sequence. Smearing effects htrduced dltring the migration process can also conaibute to this problem. For example, noise bursts fiom 1.9 to 2.25 seconds, between shotpoints 101 and 251, on the stacked version of Iine CCkl (Figure 6-17) appear to result in synformal events or 'srniles' on the migrated version (Figure &18), thereby clearly hindering interpretatim in this area. The presence of migration artifacts in deep crustal data is Merdiscussed in Section 3.2. Nevertheless, some dipping seismic events in the shallow basement cm be traced to displacements at the basernent surface, without any sigaificant change in attitude. For example, Figure 622 shows an enlarged part of line Antel where local relief on the basement surface corresponds to the apparent intersection of a shailow dipping reflector with this surface. In some cases, where dipping reflectors in the shallow basement do not appear to coincide with displacements on this surface, it is possible that they may actually represent migration artifacts rather than real stnicîure.

It is conceivable that irregularities on the basement surface are solely the result of differential erosion acting on laterally varying basement lithology. The observation that dipping intrabasement events do not appear to extend beyond the basement surface into the sedimentary cover supports the theory that these features are more iikely to be lithological rather ùian structurai in natwe. If these events represented basement faulting, it is thought Figure 6-17. A scaled stacked version of line Cal with a display window of 1.25 to 2.25 seconds. Near the basement surface, the vertical exaggeration is approximately 3, for an assumed average velocity of 5000 m/s. Note the presence of several diffractions originating at the Precambrian basement Surface, some of which are highlighted by mws.

Figure 6-18. A scaled migrated version of lhe CCkl with a display window of 2.25 to 2.25 seconds. The vertical exaggeration is approximately 3 near the basement surface, for an assumed average velocity of 5000 m/s. Note the prescnce of signincant dtip1e energy and possible migration artifacts (himghted by arrows) in the shallow basement

Figure 6-19. An interpretation of line CCkl superimposai on the original data shown in Figure 6-18. The vertical exaggeration is approximatefy 3 near the basement surface, for an assumed average velocity of 5000 m/s. hterpreted seismic horizons are highlighted in white. The Swan Hills Formation carbonates and the Precambrian crystalline basement are both indicated by shading. Note the apparent presence of a dismete channe1 separahg the Carson Creek North and Carson Creek Fields.

oil well 2 km L Line CC k l migrated gas well Part of line flattened and displayed +dry & abandoned mouFigure 6- 18 in Figures 6-24 and 6-25

Figure 6-20. A scaled migrated version of line Antel with a display window of 1-25 to 2.75 seconds. For an assumed average velocity of 5000 m,snear the basement surface, the vertical exaggeration is approximately 7. The Precambrian basement contains a number of dipping events again overprinted by hi&-amplitude multiples. The annotation Pc indicates that this weil penetcates down to the Precambrian basement.

Figure 6-21. An interpretation of line Antel superUnposecl on the original data shown in Figure 620- The vertical exaggeration is approximately 7 near the basement surface, for an assumed average velocity of 5000 ds. Interpretcd seismic horizons are highlighted in white. Swan Hills Formation platform carbonates, Leduc Formation reefal carbonates and the Precarnbrian basement are aH indicated by shading. No Cambrian units are present dong this line and Swan HiIIs carbonates lie almost directly on the crystallùie basement.

Figure 6-22. (A) An enlargement of an unscaled line Antel illustratïng an inclineci basernent event intersecting the Precambrian basement horizon. Near the basement surface, the vertical exaggeration is approximately 2.25, for an assumed average velocity of 5000 mis. The trace of the low-amplitude shallow-dipping reflector amongst high-amplitude subhorizontal multiples is highlighted by arrows. (B) an expanded plot of the part of line Antel outlined by the white box in (A), with a vertical exaggeration of about 0.3. The Iow-angle reflector can be tracked to a displacement and adjacent relief on the basement &ce. TIME (seconds) 8 Y TIME (seconds) 3 Figure 622 203 they would show at Ieast some indications of minor reactivation. There are examples of local basernent i~~eguiaritiesthat do not secm to cohcide with any uitra-basement seismic events. For instance, around shotpoint 701 on the stacked seismic line CCkl figure 6- 17), diffractions are associated with local variations on the basernent surface that may be due to uneven tapography or roughness unrelateci to wrbasement structures.

On a larger scale, Laramide orogenesis (Late Cretaceous) significantly Muenced the regional character of the basement surface and sedimentary cover. In addition to impaaing a regional dip to the SW, areas of west-central Alberta undement Iow-amplitude folding. For example, relatively long-wavelength arching or warping of the basement surface and - overlying Phanerozoic cover is present at the NE end of Line CCkl (Figures 6-17 to 6 19). Isopach data rule out the possibility of a long-wavelength static associated with the Swan HilIs as a source for this anomaly. Flattening of this structure in an attempt to remove such tectonic overprinting is discussed in Section 6.4.

6.4 Tectonic heredity of the sedimentary cover

6.4.1 Fanlting in the sedimentary cover Although the Phanerozoic sedimentary cover in seismic lines CCkl and Autel, shown in Figures 6-1 7 through G21, contains no conclusive evidence of faulting, it is possible that relatively subtle displacements on near-basement units may have been overlooked or are simply beyond the resolution capabilities of available seismic data. Near-vertical normal faults have been intezpreted elsewhere in west-central Aibexîa, often resulting in the development of grabens or half-grabens which illustrate varying degrees of reactivation. For example, distinct normal faulting is seen to propagate upsectiun and offsets Mesozoic units on seismic data in the Edson area. A pronounced pair of no& faults fonning a relatively large graben structure hown as the Erith Graben (Mobil Oil Canada Ltd., exploration staff, personal communication, 1994) is interpreted on line EdsonI (Figure 6- 23). Bearing in mind the regional dip to the SW, a maximum displacement of 70 ms (about 160 m) can be estimateci for these high-angle structures, It is interpreted that these faults extend up at least to the Manuville Group of Lower Cretaceous age. It is inferred that the Erith Graben was created by significant movement on a preexisting zone of weakness in the basement, possibly reflecting the early stages of Laramide orogenesis. For regional potential-field data to be able to detect such a basement zone it has to fonn a major Figure 6-23. (A) Migrated seismic data hmline Edsonl. For an assumed average veIocity of 5000 mis near the basement surface, the vertical exaggeration is about 2. Note îhe presence of a distinct graben structure interpreted in white. It is not possible to diredy trace this structure within the Precambrian basement. (B) a compressed display of (A). TIME (seconds) 206 structurai contact between contrasting basement rock lithologies- The regional magnetic maps (Figures 6-5 to 68) show the middle of this seismic line is intersected by an east- West orientcd lineamtnt that appears to mark the tdge of some fom of in.tguIar basement block. It is possible that deformation of the sedimentary cover in this area is concentrated around the marghs of this block. Cmversely, fiom Figures 69to G12, it is apparent that no Bouguer gravity heaments intersect this line.

Fault activity iu the Edson and Hinton areas has previously been proposed by Lam and Jones (1985) thmugh detectim of a localized geothermal andyof relativeIy high average geothdgradients (approximately 36"C.k~).They suggest that the andyis caused by the movement dong fault planes of water that has been heated at depth This mechanism may constimte a potentially important dolomitization process within west-central AIkrta and is discussed in Section 6.7.

Using well-log data, Jones (1980) has suggested a number of other examples of Late Cretaceous faulting in the Swan Hills, Virginia Hills and Judy Creek Fields. Here, he proposed an apparent inversion of throw direction from Paleozoic to Mesozoic successions. In such examples, the Paleozoic movement on the main faults is downward to the SW, in the Mesozoic, it is usually to the NE. Clearly, overali fault throw on certain horizons would have been reduced, thus requiring a close examination of isopach data. Therefore, dthough no such normal-to-reverse fault inversion is evident in seismic data presented in this chapter, this relationship may exist elsewhere in west-central Aiberta and effectively mask the presence of potentiaily signii5caat Laramide-relateci faulting.

Srnall-de faulting during the Devonian has been suggested to exist in west-centd Alberta by several workers (e.g. Keith, 1970; Schultheis, 1976; Viau, 1987). More recently, Root (1993) proposed that these faults rnay have developeù in response to ïntraplate compressive stresses associated with the far-field stresses that created the Devonian-Mïssissippian thnist belt in western North America (cf: Dorobek et al., 1991). Ross (1992) also comments that in-plane compressive stresses during the Middle to Late Devonian may have Ied to reactivittion of basement weaknesses within central Alberta

6.4.2 Drape folding in the sedimentary cover When a displacement occurs on the basement surface, the overlying sedimentary beds norrnally adjust to the structural relief produced. Most sedimentary rocks cm adjust by 207 zones of passive accommodation in the initial stages of displacement with the remit that 'folchg' will precede any later possible faulting. This adjument of the Sedimentary beds to irreguiarities on the basement surface is termed 'drape folding' and wiIl commonly exhibit a monoclinal shape with beds flexed dong the fault and adjacent to it This structural style is weli documented in the Laramide defmticm of the Rocky Mountains in the western United States (e.g. hchaet al, 1965).

In west-centrai Alberta, Lower Paleozoic and Devonian layers may exhibit a type of drape stmcture known as a supratemous or compaction fold (after Bates & Jackson, 1992). This can be forxned by differential compaction of Spdiments over an uwven basement surface. Such roughness rnay have becn accentuated by differential erosion on Iatedy- vayhg basernent Iirhologies. Dinering degrees of compactibility and erosion will then emphasize any such relief in overlying Iayers. In the pas& several workers (e-g. Belyea, 1955; Greggs & Greggs, 1989; Martin, 1967, and others) have aiiuded to the possible existence of such drape features in central Alberta, but have generally offered Iittle corroborating evidence.

Tectonic overprinting by the Laramide ûrogeny (Late Cretaceous) in the Carson Creek North area may have masked any subtle features that may have been present in Devonian times; for example, the gentle folding and warping imparted by Laramide orogenesis descnbed in Section 6.3. In order to gain a better understanding of pre-laramide paleostnicture, it is necessary to carry out reconstructions such as the flattening of selected horizons. Of the various horizons picked and flattened, only one post-Swan Hills event and one pre-Swan Hills event are presented. Upon flattening, both of these sections (Figures 624 and 6-25) indicate the presence of pre-laramide paleotopographic structure on the basement surface with relief of 25 to 30 ms common (approximately 55 to 60 m). The fact that irregularities are stiU seen on reflectors in the seismic section flattened on a pre-reef event indicates that such feah~esare not entgely due to velocity pullup hmoverlying carbonates. These minor undulations on preSwan HiUs seismic events directly overiie similar features on the basement surface reflector. My mterpretaticm is that such feaaires are caused by passive drape folding. There is also an apparent thinning of the Cathedral Formation and, to a lesser extent, the Eldon Formation above a topographic high on the basement-sediment interface. Figure 6-24. (A) A scaled migrated version of part of line CCkl after fiattening on the ireton Formation. (B) An interpretation of part of line CCkl after flattening on the Ireton Formation and superimposed on the Onginai data shown in (A). Both seismic sections have a vertical exaggmtim of approximately 4.3, as-g an average velocity of Sûûû mis near the basement surface. Interpreted seismic horizons are highlighted in white and the extent of Swan Hills Formation carbonates and Recambrian basemcnt is indicated by shading, A iarge charnel or embayment cutting dom thugh the carbonates is clearly seen. Note the presence of localized topographie features on pre-Swan Hïiis Formaiion reflectors directly above displacements on the basement SUTface.

Figure 6-25. (A) A dedmigrated version of part of heCCkl after fiattenÏng on the Watt Mountain Formation. @) An interpretation of part of line CCkl after flattening on the Watt Mountain Formation and superimposed on the original data shown in (A). Both seismic sections have a vercical exaggeration of appmximately 4.3, assuming an average velocity of 5000 1x11s near the basement surface. Intexpreted seismic horizons are highiighted in white and the Precambrian bQsement is shaded. Note the presence of undulations on reflectors directly above offsets on the basement surface.

21 1. To the west, a number of Breguiarities arc also evident on the basement surface refiector of seismic line Antel. The regionai dip on this section is removeci by fla#enuig cm the Base of the Second White Speckled Shale marker 626 and 627). The Swan Hills Fonaation is separateci kmthe basement surface by dya vay thin sequence (about 10 to 20 m) of Middle Devonian Watt Mountain and GiLwood sandstone uni& but no Cambrian succession. Neighboring Precambrian-penetrating drill holes encountcr over 100 m of Cambrian sediments, confinning that this lack of Cambrian stratigraphy is only local in nature. This tends to suggest the presence of a broad basement high that was aiready uplifted by Cambrian times.

- These seismic lines incikate that the basement dacem this part of central Alberta is not smooth and featureless but is locally disrupted by zones of uneven paleotopography. My interpretation is that these lines suggest the existence of locd passive deformation on units directly overlying such basement irregularities. The significance of this type of indirect genetic relationship between the Precambrian basement and the distribution of Devanian carbonates in west-central Alberta is discussed in Section 6.6.

6.5 Basement control on Swan EIills Formation carbonates: previons work Several studies have suggested that basement smctures have played a substantial role in the sedimentological and diagenetic history of the Swan Hills Formation within this west- central Alberta area (Figure 63). Within the Swan Hills field aloae, Martin (1967), Jones (1980), Mountjoy (1980), Viau and Oldershaw (1984), and Viau (1987) have ail cornmented on indicators of structural deformation. Martin (1967) obseved that linear NW-trending marine channels, which separate Swan HiUs reefs, may have formed in response to basement disturbances. For example, the channel that isolates Judy Creek and West Judy Creek rnight have its origin in a deep-seated basement trend. Viau (1987) interpreted displacement of Middle Devonian-through-Mississippian log markers as indicating deep-seated faulting controllcd by basement structure and suggested that the timing and orientation of faulting at the Swan Hills Eeld controlled some depositional and diagenetic features. Structural indicators such as brecciation, fracturing mineralization and stylolites have been documented in cores described by Viau & OIdershaw (1984) for the Swan Hills area. Figure 6-26. A scaled migrated version of part of line Antel after flattening on the base of the Second White Speckled Shale marker horizon. Assuming an average velocity of 5000 mis near the basement surf..,the vercical exaggeratim is about 7.

Figure 6-27. An interpretation of part of üne Autel after flattening on the base of the Second White Speckled Shale marker and superimposed on the originai data shown in Figure 6-26. The vereical exaggeratim is about 7. Interpreted seismic horizons are highlighted in white. The extent of the Leduc Forrnaîion reef, Swan Hills platfonn and Precambrian basement are aU indicated by shading.

215 Walls (1988) has noted the NW-SE alignment of Swan HÎlls dolostone-hosted gas resewoirs that developed in the platform reef of westcenaal Alberta (Figure 6-3). These include the Enth, Hanlan, Rosevear and Kaybob South Fields, the latter two king directly aligned. A NW-SE oriented bascment structure could help to explain the pronound lineation of these dolomitized Swan Hills fields. In a study relating fault patterns to hydrocarbon occumnces, Jones (1980) ncognized several NW-ûending vertical faults passing through the Swan Hills Formation. Fadts were delineated by the displacement of log markers on strucdcross-sections. He noted that the Rosevear and Kaybob South fields lie adjacent to a N33' W Mthe. The Beaverhill Lake Group here is pradominantly a tight backreef limestone with production ody from the dolomites almg this trend. Fdt- controlled circulation of dolomitizing fluids at the Rosevear field has been suggested by Kaufman et al. (1991). Bank margins at Rosevear also appear to be datecl to basement faults (Walls, 1988).

A detailed core study by Keith (1970) suggested that tectonic control at Kaybob reef is exhibited at various leveis: (1) interior shelf-lagoon sequences initiated by small-scale pulses of subsideme; (2) larger-de subsidence variations that account for thickening and thinnmg of the geologicai rcef body as a whole; (3) an orthogonal pattern of sharp elagate folds îrending NWSE and NE-SW that are clearly expressecl at the base of the reef and are confined to the area of reefing, and (4) reefs in this region, together with their associated carbonate-shelf deposits, that fit into a well expressed orthogonal pattem controlled by larger-scale basement features. Normal faults with displacements of approximatdy 8 m were recognized at the Kaybob Field by Schultheis (1976), who concludeci that the initiation and areal extent of the lowest reef stage were controlled by local block faulting. Here, there is some evidence that shoaling on the carbonate platform was a reflection of irregularities on the &ace of the underlying Elk Point rocks.

From a depositional perspective, Jansa & Fischbuch (1974) have suggested that Swan Hills reef development in the Shirgeon-Mitsue area was initiated on widespread topographie highs that were probably related to the paleotopographic configuration of the underlying Gilwood delta cornplex. However, indirect contrd through some form of tectonic influence on Gilwood sedimentation cannot be mled out. Christie (1976) has postulated that NW-SEfaults have controlled sedirnentation of the Gilwood sand in the Mitsue Field are& NE of the Swan Hills Field. A case for feanites un the basement surface influenchg carbonate sedimentation within tbis area has been suggested by many workers but the= bas been litde, if any, direct seismic evidence for it. From the above examples, it is evident that the majority of studies concerning possible basement mvolvement on carbonate morphology are well- or core- based- Clearly, knowledgc of basement structure through seismic and potential-field investigation would provide valuable iasight regarding whether any such enigmatic relationships may have exisîed in westcentrai Alûem.

6.6 Paleotectonism and Swan HUls Formation carbonate bnildups As suggested for contemporary reefs in Chapter 4, the configuration of Swan Hills carbonates is a product not only of several phases of growth but also of other contemporaneous and pre-existing factors. Fischbuch (1968) emphasized that submarine currents, temperature, salinity and clarity of the water, subtle tectonic adjustments, and other processes not obviously reflected in the stratigrapbic record, have had considerable effécts on reef development in the Swan Hills area. Momtjoy (1980) suggests that some of the most impomt factors are: 1) relative sea-level fluctuations; 2) amount or location of mud deposition; 3) presence or absence of skeletai accumulations and hardgrounds; and 4) positive topographie elements in the basio, The main cantrol on reef initiation appears to be the development of organic shoals within the platform units at suitable bathymetric depths in the basin. The influence of Iocal paleotopography on carbonate sedimentation is therefore reflected primarily in its effect on paleobathymeuy.

In this chapter, the local and regional influences of basement structure on Swan Hills Formation buildups are examined in two specific areas where contrasting carbonate morphologies are recognizd isolatecl, localized reefal complexes such as Carson Creek North, and the extensive Swan Hills upper platform or shelf.

6.6.1 Isolated Swan Hills Formation buildups Although this study is mainly focusing on the possible interaction between basement structure and the initiation and development of the Carson Creek North reef, adjacent buiIdups are also examined through the interpretation of magnetic and gravity data. Through detailed examination of weil-log and core information by Leavitt (1%8), the aerial extent and shape of the Carson Creek North reef complex has already been quite accurately detennined. An interpretation of the gross topograpnic features of the reef platform just prior to the initiatiun of reef gmwth is prcsented in Figure 6-28, with the location of the two available seismic hes superposeci. As evidtnt from weli and seisrnic Iine CCkl 617 to 6-19), Carson Creek North reef is separated from Carson Creek Field on the Swan Hi& upper platfixm to the SW by a discrete channel

The general topographie trend of the Swan Hiiis platform pria to reef initiation at Carson Creek North appears to have been in a N to NE direction but there are many prominent irregularities in this trend, with the presence of highs ninning perpendicular to it. One such stnicaually hi& lobe with a NW-SE orientation appears to underiie the Carson Creek North reef cornplex. Upon cornparison of Leavitt's (1968) well-based paleotopographic map with the flattened versions of seismic line CCkl (Figures 624and 6-25), one cornes to the cmclusion that there is a direct coirelation betwea the structurai highs identifieci m buth srudies. Through flattening on a pre-reef seismic event, it is possiie to rninimize the effects of velocity pullup on reflectors beneath this carbonate buiIdup. Leavitt (1968) speculates that the irregularities on the reef platfom surface may ody be apparent, caused by correiation difficulties or post-Devonian folding. Leavitt did not correct for structures inflicted by Iater events such as Laram.de orogenesis, ccmsequentiy making his conclusions at that time somewhat questionable. However, through remova.1 of post-Devonian structures by flattening, this is no longer the case, and seismic evidence now corroborates the existence of such positive areas during reef initiation and subsequent stage growth.

It appears that the Ioci of the Carson Creek North reef was controlled in part by the basement-induced topography of the underlying reef platform. Furthemore, Figures 6-5 and 68indicate that this reef is centred on a subcircular positive magnetic anomaly, reflecting a large-scale lateral variation in basement rock properties. All told, it would appear that the effects of differential erosion acting w laterally-varying basement rocks is transferred into overlying pre-reef sediments through differential compaction or drape folding. This indirect basement control on carbonate deposition was alluded to in Section 6.4.2.

Figure 629 is a stratigraphie cross-section across part of Carson Creek North, the exact location of which is marked on Figure 6-28. The SW part of this section lies parallel to seismic line CCkl, and illustrates the lateral and vertical facies divisions commonly associated with isolated Swan Hills Formation buildups. The overalI growth of Carson Creek North reef and other Swan Hills Formation reefs is known to have been pulsatory or O 2h contour intemal 20 ft O 1 2 mi

DI seismic profile dry & abandoneci ...... well cross section

Figure 6-28. A stn~cturecontour map hmdrillhole data of the Swan Hi& Formation Iower plan- just prior to reef initiation at Carson Creek North (modined &ter Leavitt, 1968)- Regional dip has been removed. The location of available seismic lines and a stratigraphie cross- section, shown in Figure 6-29, is superimposed on an outline of uppez piatform and reef morphology. NE WNW NE

Carson Creek Carson Creek North reef '\

Figure 6-29. A stratigraphic cross-section across part of the Carson Creek North reef and the edge of the Swan Hills Formation upper platform (modified after Leavitt, 1968). The location of this section is indicated on Figure 6-28. Note the distinct lateral and vertical facies distribution associated with the buildup. C!rg 220 episodic (Fischbuch, 1968; Leavitt, 1968). Each reef can be dissected mto a number of layers or stages. Criteria for recognizjng stages are: (1) shale breaks; (2) backstepping of the reef stages resulting in reef 'terraces'; (3) overstepping of reef stages ont0 forereef and interreef sediments; and (4) disconformities in core samples (Schultheis, 1976). Each division represents a single stage of rcef growth that probably was terminateci by water shailowing and the original reef profile modifiecl by erosion. Subsequent reef growth took place on these erodcd but still elevated daces. Such divisions within various Swan Hilis buildups, as recognized by Fischbuch (1968) and others, suggests that a mode1 of sea-level control is valid. Continueci episodic inmases in the rate of sea-level rise produced such multiple stages of reef growth (Wendte and Stoakes, 1987). The initiation of each cycle corresponds to a markai increase in the rate of sea-level rise. During these tirnes, reef growth was generally outpaced by rising sea-level, resulting in a hardground surface. At times of lower sea-level rise or stüistaud, reef pwthkept pace with or exceeded the rate of sea-level rise, producing the characteristic upward-shoaling sequences. It may be postulated that relative rises in sea level were caused by the cumulative effect of subtle tectonic subsidence and eustatic sea-level rise, whilst relative drops were controlled by renewed uplift or by subsidence king outpaçed by a eustatic sea-level drop.

Seismic line CCk2 is used in tbis study as a template line for a detailed growth-stage interpretation of the Carson Creek North reef through weii estabLished core and well control (courtesy of Talisman Energy Ltd.). Moving from the isolated reef to the upper pla$orrn, seven stages of reef growth may be identifiai on this line (Figures 6-30 and 631). It is apparent that a number of offsets evident at the basement-sediment interface appear to coincide with successive stages of growth, implying some form of subtle spatial relationship even at this scale of carbonate deposition. A displacement of approximately 55 m on the basement surface corresponds with a stage4 to stage4 transition around shotpoint 147. Toward the southern part of this line (shotpoints 180 to 195), the reef backsteps and develops into a stage-7 buildup. It would appear that this developrnent stage can be correlated with a topographie high on the basement surface. Minor basement- controlled subsidence may be responsible for these observeci lateral facies and thickness variations within the Carson Creek North reef, with a reinitiation of reef growth having been at least partiy influenced by subtle basement-induced adjustments. Elsewhere, exarnples have been documentcd of renewed basement disturbances affecting some other Swan Hiils reef growth stages. A study of the evolution of the Swan HiUs buildup by Viau (1987) has indicated a sûucturai involvement on some contact surfaces between respective Figure 6-30. Migrated seismic data fiom line Cm.For an assumed average velocity of 5000 m/s near the basement surface, the vertical exaggeration is about 7. Considerable well control dong this line ailows a detailed division of the Swan Hills Formation into stages, as indicated at the top of the Section. Below the hcambrïan surface, the section is dominated by multiple energy.

Figure 6-31. An interpretation of line CCk2 superimposed on the original data shown in Figure 6-3 1. Seismic horizons are interpreted in white. The vertical exaggeration is approximately 7. The extent of Swan Hus Formation reevupper platfom and the Precambrïan basement are indicated by shading. Displacements on the basement surface are observed but it is not possible to follow these features within the basement off-reef foreslbpe stage 3 W~P &anne1 stase3 buiidup

stage 5 buiIdup

stage 7 buildup stage 7 buildup

TIME (seconds) off-reef farcslape stage 3 buildup FChannel stage 4 buiidup

stage 5 buildup

stage 7 buildup

stage 7 buildup

LI I œ N TIME (seconds) 224 stages. Keith (1970) has ais0 commented that small-scale pulses of subsidence have influenced interior reef facies in Kaybob d growth.

The development of marine channels may also closely mfluence the overall morphology of a Swan Hilis Formation reef. For instance, deep narrow channels separate the Carson Creek North reef from similar buildups to the north, West and south. Basement involvement with these channels has been alïuded to by Sawford (1%7), who demia NW-SE trendhg channel separating the Carson Creek North FieId from the Carson Creek Field. Sawford concluded that the channe1 was a primary feam rather than an exosional feature because of the nature of the sediments flanking the chamel, However, this channel is seismically evident near shotpoint 451 on line CCkI (Figures 617 to 619)and appears not to overlie any obvious basement structure. Oa the othtr band, the presence of a channel cutting through the upper part of the Carson Cr& North reef, identifieci from core data around shotpoint 75 on line Cm,seems to lie faïrly close to proposed disruption of the basement-sediment interface. Al1 told, in aying to relate a channel feature to a feature on the basement surface, significant dong-strike variations are conceivable. It is unlikely that a charme1 will correspond exactiy to any deepseated structure.

Away from the Carson Creek North area, the distribution of other isolated Swan fiills Formation buildups rarely coincides with mterpreteti regional magnetic or gravity lineament patterns. Many anomalies appear to cross-cut the edges of carbonate buildups with no apparent relationship.

In summary, on the basis of seismic, core and potential-field data, recognition of largely indirect basement control on carbonate sedimentation at Carson Creek North may be suggested on two possible scales or levek (1) paleotopography imparted by differential compaction and erosion over basement irreguianties which was important in controllhg reef complex initiation, and (2) resurgent minor basement activity in the fonn of subtle tectonic adjusmients, not detectable seismidy, th may have been signif?cant in localizïng or recommencing later individual teef-stage growth- However, the lack of correlation between regional magnetic and gravity lineament patterns and other isolated carbonate buildups suggests that the influence of large-scde basement lithology variations on carbonate depositicm may not be as important in adjacent areas of west-centrai Alberta. 225 6-62 The Swan Hills Formation apper platform Seismic line Antel lies entirely within the Swan Hilis Formation upper platform although the NE end of this heapproaches the northern pla$m edge near Ante Creek @gure 6 1). In this area, platform morphology is extremely undulatory in nature, apparently cut by a number of NE-trending embayments. Figures 6-26 and 627 show the uninterpreted and intexpreted versions of the NE part of this line respectively, after fiatterhg on the base of the Second White Speckled Shale marker to account for regiddip. Along this line, Swan Hills carbonates lie almost direcùy on a broad Precambrian basement high, separateci only by a thin veneer of Middle Devonian ciastics. Overlying these pbtformal carbonates to the SW is an edge of a large Leduc Formation reef.

As &scribecl in Section 6.3, dipping intrabasement reflectm on seismic line Antel are interpreted to represent lithological boudaries. Around shotpoint 1025, one such contact intersects the basement dace(Figure 6-27), AIthougb a broad structural high is evident on the basement surface adjacent to this intersection, this feature is thought to be at least partly the result of velocity puliup fkom an overlying Leduc Formation reef. The fact that this intersection is situated several kilometers northeast of the Leduc reef suggests that a direct relationship between this basement feature and the morphology of Leduc carbonate deposition is uniikely. However, some form of regid, indirect connection to the dipping basement fabric cannot be ruled out. To partly corroborate this hypothesis, a north- to northwest-trending magnetic linearnent W) Iocally corresponds to the northeastem margin of this Leduc buildup around shotpoint 1200. Aithough not directly imaged, a second intrabasement seismic event may be projected to inteet the basement surface just off to the NE of this Iine (Figure 627). This structure wouId then coincide approximately with the undulatory NW-trending margin of the Swan Hilis Formation upper plab.

On regionally comparing Swan Hilis upper pladorm morphology in this area with potential- field linearnent patterns, only a limited number of correlations are apparent. Figures 68 and 6-12 reveal most magnetic and gravity andypatterns largely intersect the platform margin obliquely rather than parailel it. However, one of these NW-trending Bouguer gravity lineaments closely corresponds to a large channel at Rosevear Field, in the eastem part of the upper plam(Figure 6-12). Direct evidence for NW-trending normal faulting associateci with this channel is known fkom seismic data (Suncor Inc., exploration staff, personal communication, 1994). One interpretation is that this lineament corresponds to a 226 structural contact m the basement that derimes a zone of weahess. This basement fault zone has then been reactivated mto the sedimentary cover.

In contnist to the relatively local-scale examination of Carson Creek North reef, inteqmtation of basement Muences on regional Swan Wsupper pladorm sedimentation is mody fiom a regional perspective. In summary, it appears that the development of large marine channels comiderabIy innuences the overall regional morphology of the platform. Some of these channels seem to be connecte& at least indirectly, to basement structures.

6.7 Tectonic dolomitization of the Swan Hiils Formation D~iomitizationis the pnxxss by which limestone is wholly or partiy converted to dolomite or dolomitic limestone through the replacement of calcium carbonate by magnesium carbonate, usualiy involvhg the action of magnesium-bearing water in the form of seawater or percoiating meteonc water (Bates Bt Jackson, 1992). Sisnificantly, this transformation can result in an increase in porosity of up to 13% (AIlaby & Allaby, 1990). According to Wilson (1975), two principal types of dolomite are generally disthguished: a stratigraphically controlled, early diagenetic type, and a hydrothermal type precipitated dong faults and veins. However, a clear distinction between these two kinds is often not possible.

From Figure 63, it is clear that dolomite reservoirs form an important constituent of the hydrocarbon potential of the Swan Hills Formation. At present, several important fields have been found, most associated with localized dolomite pockets of increased porosity within the extensive Swan Hills platform. A cornparison may be made with the Adsctt-type play (e.g. Helrnet Field) on the Slave Point platform of N.E. British Columbia. Hem, resemoirs again consist of stratifom lenses of dolomite within an otherwise tight ibnestone platform (Reinson et al., 1993). In both cases, dolomitization appears to folIow discrete hear trends, possibly refiecting deepseated controls.

Recently, fault- and hcture-conaolled dolornitization in the Devonian of centrai Alberta has ken locally documented. The best example appears to be associated with Upper Devonian Wabamun Group dolomites around the Peace River arch area of north-central Alberta (e.g. Stoakes, 1987; Churcher & Hamid Majid, 1989; Mountjoy & Halim- Dihardja, 199 1; Walbertsma, 1994). Hm, Mtmovcment emb1ed the upward migration of 227 hot, magnesium-rich dolomitizïng fluids mto Wabamun Iimestones. Such dolomites constitute part of the Tangent-type play, named aftcr the Wabamun-producing Tangent Field (Stoakes, 1987)- To the north, Qing & Mountjoy (1989) have also suggested that reactivated basement faults may have been responsible for the dolomitization of some Middle Devanian Keg River Formation carbonates.

In Swan Hills Formation carbonates of west-ceniral Alberta, resemoir configuration often corresponds to the distribution of original facies. However, tectonic activity and dolomitization may have created, modifieci or even dcstmyed original porosity in some instances (Fischbuch k Havard, 1977). Such secondary porosity is often not coincident - with the onginai facies. Viau & Oldershaw (1984) examineci cure dam hmthe Swan Hills Field and coxrelated a number of orthogonal mcturai offsets (5 to 13 m) orienteci NWSE and NE-SW with the distriiution of dolomite zones. Lead-zinc mheralization (galena and sphalerite), also inferred to be of hydrothermal origin, coincides with sorne dolomite zones. Viau & Oldershaw (1984) went on to indicate that two distinct pulses of upward-moving dolomitizing fluids were responsible: one episode probably occurred penecon- temporaneously with reef pwth, whilst the other episode occurred sometime after the termination of reef pwth. One interpretation is that this later pulse of dolomitization is related to Laramide fdreactivatim of basement zones of wealcness,

In the mostly Iimestone Swan Hills upper platform, WaUs (1988) bas outlined several fields associated with 1- pockets of dolomite. Fields producing gas fiom dolomite in the platform include Roseveu, Kaybob South, Erith (Edson) and Hanian (Figure 63). Reinson et al. (1993; Figure 43) delineates some neighboring derfields associated witb this play. To date, the rnapped distribution of dolomite, as outlined in Figure 6-32, has a strikingly linear NW-SE orientation suggesting some form of structural control on dolomitization.

At Rosevear Field, a NW-trending curvilinear Bouguer gravity lineament, interpreted to represent a structural boundary in the basement, cIosely coincides with a pronounced channel that cuts the Swan Hills upper platform complex in a NW-SE orientation. As highlighted in Figure 633, two discrete dolomite trends occur along the margins of this channel. Focusing of dolomitizing fluids along faulîs may account for this very localized linear distribution of dolomite and account for the large volumes of magnesium needed to be importecl (Kaufman et al., 1991). Corroùorating this proposed structural control, NW- Figure 6-32. Map of known dolomite reservoirs in west-central Alberta (ahWalls, 1988) with the distribution of Swan Hills Formation carbonates (hm Figure &1) and interpreted Bouguer gravity iineaments (hm Figure 6-12); sale 1:2,500,000. The Iocatiom of seismic profiles are also maricecl. limestone dolomite shale

Swan Hills i Formation

Figure 6-33. Localized doIomitizatim of the Swan Hilis Formation at the Rosevear FieId: (A) map outlining the occurrence of dolomite along the margins of a marine charme1 thai cuts the upper platfoxm in a NW-SE direction; (B) cross-section across the Rosevear Field indicating the proposed mode1 for dolomitization. Dolomitizing fluids moving updip m response to regional aItmg migrate hm channel-margin facies through a NW-trending Eault conduit (modifieci from Kaufinan et al., 1991). 230 trending normal faulting is known to be closdy associateci with both the channel and the dolomite zones fkom seismic data across this arui (Suncor Inc, exploration staff, pemd communication, 1994). Thenfore, Phmerozoic fauking, reactivated from a zone of wealcfless in the basement, may have influenced the formation of the chamel and played a later role in the movement of dolomititing fluids. Figure 6-34 illustrates a possible mechanism for the restricted dolomitization of Swan Hills Furmation carbonates mvolving localized fdtiag witbin west-centcal Albata

A seismic line across the Edson (.th)-Hanan dolomite trend is shown in Figure 623. As discussed in Section 6.4-1, this line intersects a pronounad W-trendhg graben structure, apparently reactivated during Laramide orogenesis. Such Phanerozoic normal faulting may have directly Iocalizcd doIomitkation almg tbis and. However, this structure is apparently not detected on interpreted regional magnetic and gravity data (Figures W and &12). In fact, magnetic heament patterns appear to define the edge of a large basement block that crossnits this trend obliquely.

In contrast to this direct tectonic control on dolomitization, Stoakes & Dixon (1992) have advocated that the distribution of dolomite in the carbonates of the Swan Hills platfom margin at Caroline Field, south of the study ana, is Iikely to have been controlled by the original depositional facies and to have resuIted fiom burial and dewaîering of adjacent shales. A similar scenario may be present at the Kaybob South Field Phanerozoic faulting is not known to be established dong this trend, at leas fkom availabb seismic data across this dolomite reservoir, and it does not coincide with any regional magnetic or gravity lineament trends. Nevertheless, the possibility still exists that dolomiîizing fluids are at Ieast partly focused or channeled by relatively small deep-seated faults or fractures in these areas.

Overall, an understanding of the origh and trends of dolomituation wili be of considerable importance to hture hydrocarbon exploration of Swan H.sFormation carbonates. The proposed role of feactivated basement structures in dolomitization has been stnssed in this chapter. Exploration for dolomite pockets within the extensive Swan Hills platfonn is likeIy to be meradvanceci by structural mapping fiom high-molution aeromagnetic surveys. In recent years, several such sweys have been fiown across west-central Alberta but are cunently proprietary in nature. Figure 6-34. Schematic mode1 of possible tectonic dolomitization within the Swan HUS Formation and Leduc Formation carbonates of west-central Alberta. (A) Basement-related normal fault reactivation resuits in an open fracntrt system and breccia formation witùh the Phanemzoic cover. (B) Local dissolution of bxeccia and waii-rock units by percolating fluids, with some limited dolomite rim cementation. (C) Dolomitization occurs in the breccia and wall-rock units and spreads to adjacent porous limestoncs (modifiecl after the Tangent-style dolomitization described by Stoakes (1987), G.M. Ross (personal communication, 19%)).

Basement infiuences on the Leduc Formation of east-central Alberta

Since the initial discovery in February 1947 of Leduc Formation (Middle Devonian) reefs in central Alberta, the stratigraphy, paleogeography, and geologicai bistory of this unit has received considerable attention. This chapter concentrates on carbonates of the Leduc Formation found between latitudes 51" N and 54" N (township 24 to 57) and fiom longitudes 110" W to 116" W (range 1W4 to 14W5). Significantly, Leduc Formation carbonates in this study area accommodate approximately one quarter of recoverable conventional crude oil reserves fond in Alberta (Burrowes & Krause, 1987). In view of this, and the great abundance of drillhole data acquired over nearly five decades, the distribution of these carbonates in the subsurface of central Alberta is now firmly established. By comparing Figures 7-1 and 7-2, one can see that by simply locating concentrations of weils drilled mto this unit, it is possible to map the occurrence of large Leduc Formation reefs and reefal complexes, at least on a regional scale.

Although the distribution of Leduc Formation carbonate buildups is well known, the mechanisms behind the initiation and development of many such carbonates remains relatively inconclusive. Of all the drillholes that intersect the top of the Leduc Formation (Figure 7-l), only a small fraction actually continue downwards through the Leduc Formation and inte- underlying units. Our understanding of the nature of the foundation upon which Leduc Formation reefs initiateci is limited and, at best, extremely localized. Because of the economic importance of Devonian hydrocarbon reservoirs, greater understanding of their origins and morphologies is clearly significant to further reef exploration, even in this relatively mature part of the basiu, and elsewhere.

Underlying basement controls have long been invoked to help explain prominent Leduc Formation reefal trends in the subsurface of east-central Alberta. However, apart fkom studies of Leduc carbonates associateci with the Peace River Arch of no&-central Alberta (e.g. Dk, 1990), supportive evidence has been largely inconclusive- The objective of this chapter is to investigate the shallow basernent beneath Leduc Formation carbonate buildups Figure 7-1. Map of weils penetrating the Leduc Formation m east-cenmi Alberta, updated to February 1995 (hmDigitech Information Services Lui.); sale 1:2,500,000.

Figure 7-2. Map of Woodbend Group (üpper Devonian) reef complexes, eastcentral Alberta (modified after Switzer et al., 1994); de1:2,500,000. Ia the Laramide disarrbed belt, reef positions were restored palinspastically (Andrews, 1987). The highlighted box contains several seismic profiles, outhed in Figure 7-4. As indicated, a number of previous snidies have detailed direct or indirect evidence of faulting associated with these reef complexes: (1) Haites (1960); (2) Knight & Hannon (1960); (3) Andrichuk (1961); (4) Jones (1980); (5) Ross Bi Stephenson (1989); (6) Arestad et al. (1995); and (7) bonet al. (1995). Voodbend rdcomplexe8 Leduc Pm.reef complcxes Cooking Lake Fm, ncarbonate plalfom oil and gas pools

MLE Mcadow Lake Escarpment D8 edge of thc Larnrnidc disiurbcd bolt O 50 ml 238 and relate possiïle basement tectonism or Iithology to carbonate morphology. Tlme principal areas of Leduc Formation carbomk deposition are investigatad m this cha- the Rimbey-Leduc-Meadowbrookreef chain, the Bashaw complex and tbe Southern Alberta shelf (Figure 7-2). Furthemore, for cach area, possible basement involvement with carbonate sedimentation is analyzed on two different scales: local basement control on individual reefs or reef complexes using seismic profiles recordeci with extended listerhg times and well-log data; and the eff&ct of larger basement elements on reef complexes and platfom through regionai magnetic and Bouguer gravity data

7.1 GeologicaI setting and stratigraphy The Leduc Formation is dated as Frasnian in age and is stratigraphicaily situateci in the Woodbend Group. The central Alberta study aracontains the stratigraphie type area of the Woodbend Group, summarized in Figure 7-3. Here, the Woodbend Group contains a thick sequence of shallow-water platformal and reefd carbonates of the Cooking Lake and Leduc Formations, and basin-nIling fine-grained argiliaceous and calcareous sediments of the Majeau Lake Member, and of the Duvemay and bonFormations (Stoakes & Wendte, 1987)- As in earlier chaptem. expressions such as the 'Leduc event' or 'horizon' refer to a seismic reflection from the top of a particular unit unless otherwise specined.

Briefly stated, the deposition of the Woodbend Group can be related to one major cycle that involves a lower transgressive phase, domhatecl by platfom and reefal carbonates, and an upper regressive phase, containing prograding sbale and limestone units (Stoakes, 1992). In central Alberta, the transgressive portion corresponds to Leduc Formation linear reef chahs, isolated r&s or reef complexes, and an extensive shelf complex, all developed on a regional platformai facies, the Cooking Lake platfonn (Figure 7-2). Although these buiidups display a wide variety of morphologies and sizes, a preference for a rough NNE aliment is evident on a regional scaie. Accordhg to Podniski et aI. (1 988), two stages of Leduc reef growth are recognized dong the Rimbey-Leduc-Meadowbrookreef chah A relatively thin lower biostromal development often ~e~edas a platform for thick upper Leduc buildups. mese widespread carbonates grew through an episodically rising sea level that accentuated the bathymeuic reiief between reef and basin. Later regressive basin Nling took place by progradation of fine-grainai limestones and shales of the Duvemay and ireton formations, gradually encroacbing upon and fjnaily terrninating reef growih (Stoakes NW SE Winterburn Op 1 iiniiœiriirFamennian-Frasnian boundarv

Anhydritiî carbonatelovaporitc

1,2,3 : Lower, Middle and Upper Leduc Fm rcspclivcly

Figure 7-3, Schematic cross-section across central Alberta illustrating the stratigraphic divisions of the Woodbend Group, flattened on the Famennian-Frasnian boundary (modified after Switzer et al., 1994). Although it does not cross the Bashaw reef cornplex, it does show a reef associated with the Rimbey-Leduc-Meadowbrook trend and the Southern Alberta Leduc shelf marign. t2rg 240 & Wendte, 1987). More rccently, the sequentid baSmwide devtlopment and depositional history of the Woodbend Group bas been detailed by Switzer et al. (1994).

Historically, high exploration activity has long been associated with these stacked Leduc reefs. The Rimbey-Leduc-Meadowbrook chah of reefs contains porous buildups from 250 rn to 300 m thick, sealed by Ireton Formation shales. The Duvemay Fomiation provides an excellent hydrocarbon source rock (Podruski et al., 1988). Dolomitization plays an important role in reefk of this trend but large adjacent limestone re.efS such as Redwater and Golden Spike are aIso important. At Bashaw, traps occur at updip terminations of Leduc reef complexes, pinnacle reefs, patch reefs, and in channcItd rim feaaires (Podniski et al., 1988). To the east, sûatigraphic closure also occurs almg reenfrantsand embayments of the Leduc plat€= margin.

Although Leduc reefal plays are fairly mature, exploration is stilI continuing, focusing mainly on pinnade reefs and channels, and variations in reef thickness. Undiscovered buildups are still thought to occur in central Alberta, as highlighted by a recent presentation by Lemon & Taylor (1993): 'The Rumsey Leduc pinnade reefi where are the rest?". ûur understanding of why a reef initially grows in a particula. area is of fundamental importance to future exploration in central Alberta. If it is possible to recognize a relatiottship between Leduc Formation carbonate morphology and basement structure, it may be feasible to predict adjacent buiIdups simply by extrapolating deep-seated trends.

7.2 Data availability Previously, the number of seismic reflection profiles that were recorded wiîh sufficiently long Iistening times to successfully image basement structure in east-central Alberta was relatively few, compared to the vas number if conventional seismic surveys acquired by the oil indusüy every year. A bibliography of deep seismic studies in the Western Canada Sedimentary Basin published pnor to 1990 is found in Ross & Stephenson (1989). As part of LITHOPROBE'Sobjectives of better understanding feams of the deep crystalline basement, the 520-km-long central Alberta transect (CAT) was recorded in July 1992- Acquisition and processing parameters for the 60-fold CAT seismic data are summarized in Table 7-1. In this sîudy, only those seismic lines that directly cross interpreted Leduc carbonate buildups are show (Figure 7-4). I Acquisition parameten Source V~bseis,4 vibrators over 50 m, 100-m interval sweep 1&56 Hz, 8 sweeps per vibrator point Receivers group intend: 50 m (9 over 42 m) record length: 18 s Spread asymmetric split spread: 3 100-150 * 15CL9100 m 60-fold coverage Pnieessing parameters Dernul tiplex AGC 500-ms operator Minimum phase correction Deconvolution designature, O. 1% prewhitening Spectral balance 4 tirne windows, zero phase Refiaction statics 2 layer model; weathering velocity: 900 mis Elevation statics datum: 970 m; replacement velocity: 2800 ds Residual statics surf" consistent; maximum shift: f 30 ms window: 20-1200 ms NMO Trace editing 1 first breaks, mute, trace kills srack Noise attenuation FXdeconvolution, 30% noise addback Scaling muItigate Am,3 time windows Migration hite difference, 100% stacking velocities Filier 4 thewindows Scaling muitiple gate balance * Merge hes9 and 10 only * Data aügnrnent account for different datum elevations * AGC gain 50-ms window (&2200 ms) 1 * Flattening 1 selected horizons

Table 7-1. ~c~uisitioiand pcessing parameters for the 1992 LITHOPROBE Central Alberta Transect (CAT)survey. Acquisition was by Veritas Geophysical Ltd. and initial processing was carried out by Pulsonic Geophysical Ltd. An asterisk indicates optional processes undertaken at the LITHOPROBESeismic Frocessing Facility (LSPF). Figure 7-4. Map of available cnistd seismic profiles, with an outline of Woodbend Group reef complexes from Figure 7-2. Lmes 5, 7 and 9/10 are part of the LITHOPROBE Centrai Alberta Transect (CAT), whilst Iine RB is an extended correlation profile donated by Compton Resources Ltd, Both interpreted seismic-line segments that cross reef complexes and those Iines used for regional extrapolation and stratigraphie correlation to weii data are indicated. ûniy those wells that are directly used in the interpretation of iine segments are plotted. Note the Iocation of the one synthetic weii tie to the Recambrian included in this chapter (Figure 7-5).

244 In addition to these pronIes, a line dcmated by Compton Resources Ltd is included in this study: line Rb on Figure 74.Like line 5, this 24fold pronle also crosses the Homeglcn- Rimbey reef, providiag memuch-needtd hg-strike conml on interprttatim Table 7- 2 details the acquisition and pmcessing parameters for this dataset. This line was acquired with a Vibseis source with a sweep Inigth of 5 seconds and a record length of 8 seconds but was originaUy processeci to oniy 3 seconds. However, shaliow records hmthis line have been since exîrapolated to obtain deq reflectims by the process of 'seIf-tnmcating' extended correlation. As outlind by Okaya & Jarchow (1989), this method simply requires Vibroseis data to be originally recorded in an uncorrelateci format. It preserves the bandwidth m the orighai record rimes but continues with an everdeaeaskg bandwidth for the extra correlation tirne- However, a proficient source is di needed to ensure good signal penetration to depths beyond the basement surface. In this case, this prerequisite was not fully satisfied, resulting in poor data quaiity and a very low signal-to-noise ratio for deep crustal refl ections in the recorrelated &ta Nevertheless, interpretations are considered valid for the shalIow basement and are included in this study. Overall, bearing in mind the need for adequate acquisition parameters, it would appear that a great potential exists for Mer incorporation of standard oil industry seMches into futme studies of crustal structure in Alberta using this method.

To facilitate horizon picking on the seismic data, a significant number of deep and shallow exploratory weils were utiIized in this study. As indicated earlier, only a limitai number of welh penetrate the Leduc Formation in the study area Of these, less than 150 wells reach the Middle Devonia.Elk Point Group and only 32 wells extend to Precambrian basement depths, giving a very sparse sampling of the basernent surface of approxirnately one well per 2000 square kilometres. However, a number of these drillholes are found reasonably close to the CAT seismic data, enabling a fairly confident tie to the basement surface. Figure 7-5 illustrates a synthetic weU tie projected about 30 km to line 10, from the Second White Speckled Shale marker down to the Recambriau.

Similar to the integrated rnethodology used in northcentral and west-central Alberta (Chapters 5 and 6), seismic and well data in east-central Alberta are again complemented by potential-field data, providing a more regional perspective for assessing possible basement involvement with the sedimentary cover. The magnetic and Bouguer gravity data for this area are shown in figures 7-6 through 7-13. Acquisition parameters Source / Vibroseis; vibroseis point interval: 50 m

Receivers group interval: 25 m (9 over 25 m) record length: 8 s I Spread 1600-125 * 125-1600 m 30-fold coverage Pmcessing parameters Demultiplex 1 ~xtenddcorrelation seif-truncating, extrapolated to fidl 8 s Notch filter 60Hz,30Hz Trace editing first breaks, mute, trace kills Minimum phase correction Deconvolution spiking; operator length: 120 ms; O. 1% prewhïtening FK filter -2500 mis to 2500 m/s AGC gain 500-ms window Trace equalization 2to8s Refraction statics Elevation statics dam: 900 rn; replacement velocity: 2500 m/s Geometrical spreading I correction I

Residual statics surface consistent; maximum shift: + 24 ms Stack Deconvolution gap: 32 ms; operator: 256 ms; 1% prewhitening F-X filter dip rejection filter (-0.004 to 0.004 dtrace) F-X prediction fdter Mimation phase shift, 100% stacking velocities The-variant filtering 1 4 time windows 1 Tirne-variant scaling 4 time windows * AGC gain 50 ms * Flattening 1 selected horizons 1

Table 7-2. Acquisition and processing parameters for seismic line RB. Acquisition was by Western Geophysicai Co. Ltd. and al1 processing was carried out at the LITHOPROBESeismic Processing Facility (LSPF). An asterisk indicates optional processes. Base of FiScaies Zone- Viig Fm- Mannviie Gp- Niska Fm- fntm Fm- Leduc Fm-

Cooking Lake Fin- BeavertiiIi Lake Gp-

Elk Point Gp- Fmgan Fin- Deadwood Fm-

Earlic Fin -

basal randStone- unit Precambrian - lsscmcnt

Line 10 Figure 7-5. An example of a synthetic tie down to the Precambnan basement, projected about 30 km hmthe PCP Killam 15-34-43-10W4 well to line 10. The synthetic seismogram was generated with an Ormsby zero-phase wavelet (Y10 to Sol70 Hz) with only prirnary reflections calculated. Both synthetic amd field data are displayed in normal polar@. AdditionaI basement and near-basement com1ations were also utilized in this saidy. Figure 7-6. Total-field magnetic map of east-ce~~traiAiberta with an outline of MUCFormation carbonate buildups (hmFigure 7-2); sale 1:2,500,000. The magnetic data is taken from the Geological Survey of Canada database., grid spacing 2 km. The Laramide disturbed belt edge OB) and the proposed location of the Meadow Lake Escarpment (MLEJ are highlighted. The cities of Edmonton and Caigary are rnarked for reference. 2 'PL Figure 7-7. ûptimaliy-scaIed magnetic HGV map of east-central Alberta, with vector arrows plotted pointing away hm local maxima, or downhill, with total-field magnetic data; sale 1:2,500,000. The magnetic data is mken kmthe GeoIogicai Survey of Canada dambase; grid spacing 2 km. NANOTESLAS Figure 74. Optimally-scaled magnetic horizontal-gradient vvector (HGV) map of east-central Alberta with vector arrows plotted pointing toward local maxima or uphill, de1:2Jûû,ûûû.

Figure 7-9. Interpreted magnetic Lineament map of east-cenuai Alberta; scale 1 :2,500,000. This map is compiled fcom downhlll- and uphill-plou magnetic HGV data îogether with total-field magnetic data. Leduc Folznatim carbonate bddups (fiom Figure 7-2) and the Laraniide disturbed belt edge OB) are highlighted, togcther with the proposed IOCSttion of the Meadow Lake Escapment CMLE) and the Snowbird Tectwic Zone (Sm.Tbe cities of Edmonton (Ed) and Calgary (Cal) are marked for reference.

Figure 7-10. Bouguer gravity map of east-central Alberta with an outline of Leduc Formation carbonate buildups (fiom Figure 7-2); sale l:S,SOO,OOO, The Bouguer gravity data is taken fiom the Geological Survey of Canada daîabase; grid spacmg 5 km. The Laramide disturbed belt edge PB) and the proposed location of the Meadow Lake Escarpmezlt (MLEJ are highiighted. The cities of Edmonton and Calgary are rnarked for reference.

Figure 7-11. ûptimally-ded Bouguer gravity HGV map of east-cenaal AEbeaa, with vector arrows plotted pointing away hmlocal maxima, or downhill, with Bouguer gravity data; sale 1:2,500,000. The Bouguer gravity data is taken hmthe Geological Sumey of Canada database; grid spaciug 5 km.

Figure 7-12. Optimally-scaled Bouguer gravity horizontai-gradient vector (HGV) map of eastcenirai Alberta with vector mws ploaed pointing tod local maxima or upm de 1:2,500,000.

Figure 7-13. Interpreted Bouguer gravity lineament map of east-centrai Albeaa; sale 1:2300,000. This map is compiled from downhiil- and uphill-plotted Bouguer gavity HGV dam together with Bouguer gravity data. Leduc Formation carbonate buildups (hmFigure 7-2) and the Laramide disturbed belt edge PB)are higfilighted, together with the proposed location of the Meadow Lake Escarpment (MLE) and the Snowbird Tectonic Zone (Sm.The cities of Edmonton (Ed) and Calgary (Cal) are marked for reference.

73 Basement structure and sïirIace topography The crystalline basement of est-central AIberta has not received anywhere near the geological attenticm that the overlying sedimentary section has been given due to the lack of hydrocarbon exploration mterest and the often considerable cost and risk involved in drilling to greater depths. Despite the Scafcity of basément-penetratuig weiis, there has ben a considexab1e amount of conjecture on its regional and local nature.

As outlined in Chapter 3, one of the mmprominent basement features identined in central Alberta is the NE-trending Snowbird Tectonic Zone (STZ), a 280ekm long crustal discontinuity that cmbe traced across the Canadian Shield (Sharpton et al., 1987)- This regional structure has been correlated with several dip-slip and strike-slip shear zones of late Archean age by recent field rnapping in the Shield of northem Saskatchewan by Hanmer et al. (1994). The STZ can be observed on regional potential-field data fiom east- central Alberta but appears to have contrasting magnetic and Bouguer gravity signatures. Magnetic maps (Figures 74to 7-9) show the STZ is represented by several NIUE- or NE- üending parallel strands, each in tum consisting of a number of Iineaments that coiiectively mark a hadzone, with a width of about 40 to 50 km, throughout east-central Alberta. Bouguer gravity xnaps in Figures 7-10 to 7-13 indicate the STZ changes from a series of discrete, parallel Linear or cuvilinear strands to a single hear element in the SW. This sauthem STZ strand intersects the Cderandeformational front and continues into the Laramide-age (Upper Cretaceous) fold-and-thmst belt which forms the Foothills and the Front Ranges of the Canadian Cordillera. Unfoxtunately, no publicdomain magnetic data are available in that area

Away from the STZ, regional magnetic and Bouguer gravity maps reveal anomaly patterns that appear similar to arrangements des cri^ in West- and north-central Alberta. Again, localized subcircular to arcuate lineaments of varying sizes are thought to be related to plutonic intrusion while Iarger curvilinear elements may reflect major stmctural or lithological contacts in the basement.

In L~OPROBECAT seismic lines 7 to IO, the basement is characterized by pronounced law-angle reflections with an apparent uniform dip to the east or southeast that appear to sole into a highly reflective Iower crut (Figure 3-6). In contrast to the shailow-dipping basement fabric obsewed in seisrnic data from west-cenhai Alberta (Chapter 6), distinctive 264 rollover structures in the fonn ofhanging-wall anticlines have becn magnized by Ross et ai- (1995). The reIations of these particuiar iargc-scde structures mdicate that the inched basement geometrîes are, most likeiy, thrust fa&. At depth, these low-angle reflemrs appear to sole or fiaüen onto a major mid-staI decollement or detachment surface. They form part of a cmstal-scale thrust imbrication system or belt, termed the East Alberta Orogen by Ross et al. (1995), where a significant amount of crustd shortening is evident.

According to Sproule (1962), "initialfolding andfaulting alung tectonicalIy active belts, which are commonly rejuvenated in luter geological pen'oàs, and the intrusion and extrusion of igneous material, al1 tend to make ultimately for high structural and topographic relief at the surface of the Precanbrian, If we add to the situation larer differentialmetamorphism with related recrySEQllization,and segregarion of miner& and assemblages of minerais, we have a siîuaîion that promotes finrher differenficllphysicd and chernical erosion, also with a resultant tendency to accenîuate mpographic refiefot the Precambrian surfacee".Rom the limited coverage provided by available seismic profiles, the apparent effect of the East Alberta Orogen on present-&y basemcnt surface topography varies across east-central Aiberta and does not appear to confom exactly to Sproule's hypothesis. Overall, the basement reflector appears to be fairly subdued, aIthough locai basement faultïng with smaU vertical offsets and larger topographic structures have been recognized (e.g. Eaton et al., 1995; Hein & Breakey, 1!393). It may be that other subtie displacements on the basement surface are not imageci since rhey are beyond the resoiution capabilities of the seismic data. Eaton et al. (1995) calculateci the vertical resolutioa of the CAT dataset to be around 20 m near the basement-sediment interface. Therefore, basement topography of less than about 20 m relief will not be detected seismicaily. Away fhnthese Lines, several significant Precambrian basement highs have been identified from dnllhoie data in est-cenaal Alberta with relief of up to 200 m in some instances (Hein & Breakey, 1993; Slind et ai., 1994). The term tm~nadnockrbas hcquently been used to describe some of these Precambrian surface features. According to Allaby & Allaby (1990), a 'monadnockt is "an isolated hi11 or range of hills standing cunspicuousty above the general level of a peneplain (where erosion is least) and resvlting from the erosion of the szurounding terrain". It may be located on relatively resistaat rock. However, whether the origin of most basement topography in east-central Alberta is actually fault-induceci, lithological, erosional, or some combination has yet to be successfully shown and may vary from area to ara. 265 Recently, as part of the hdustnal Partners Program of the Geologicai Survey of Canada, members of the Lower PaIwmic Roject (as outlined by Nowlan, 1994) have investigated cores and cuttings )om weiis intersecting the Prccambrian in central Alberta. They identifiecl a number of local Precambrian basement highs, as well as direct and indirect evidence of tee- Ln successicms immediaidy overlying the Recambrian hmseveral wells in central Alberta (FL Hein, personal communication, 1995). This included: syndepositional faulting (such as very srnaIl-scale normal faulting) and folding; mineraikation; brecciatim; ~workedbasement regohth, and liquefaction or fluid-escape features. Establishing the presence of tcctonism in the shallow basemat and near-basement sediments, particularly considering the very localized sampling of the basement, bas potentially important repercussions for determuiing the nature of basement interaction with younger units.

7.4 Strucîural control on Leduc Formation carbonates: previoas work Over the past forty years, numerous snidies have suggested that structural movements originating in the Precambrian crystalline basement have controlled or affected deposition of Leduc Formation carùonates throughout east-central Alberta (Figure 7-2). As early as 1955, Belyea coxnrnented that "diferenfialsubsideme, probably controlled by lines of weakness in the basement cornplex, may have ken the important factor in detennining reef distribution" for the Leduc Formation. Solely on the basis of mapped reef morphology, many other workers have also suggested that the distinctive N'NE linear alignment of the Rimbey-Leduc-Meadowbrook reef chain, Bashaw-Malmo-New Norway-Duhamel group of reefs, and, in part, the Leduc shelf edge, may all reflect the influence of deeper seated- fault trends (e.g. Goodman, 1954; Illing, 1959; Haites, 1960; Martin, 1967; Jones, 1980; Mountjoy, 1980). Others have proposed the presence of more subtle low-angle tectonic hinge lines underlying Leduc buildups. For example, Downing & Cooke (1955) maintain that even a minor change in the dip of underlying unis may have been enough to create conditions favourable for reef growth during Woodbend Group time. Sunilarly, a deraileci study by Andrichuk (1961) in the Duhamel area concluded that 'yacies and thickness changes along a tecronic hinge line caused by possible fault movement in the Precambrian basement dere-ned the locus of hter (Duhamel)Leduc reefgrowth".Parailel to the axis of the Duhamel reef, he identified the underlying presence of a localized Cooking Lake carbonate shoal and a marked de.in basal Duvernay unit thickness. 266 These early studies were al1 mostly well- or core-based and proviciai vcry little direct evidence to corroborate their tectonic control theories. However, in this area of Alberta, once thought to accommodate iew structurai features of note, some workers have directIy or indirectly inferreci the presence of faultmg closely associateci with Leduc Formation carbonate buildups. For example, Haites (1%0) bas document& the existence of sphalerite in several Leduc cons ftom the Bannie Glen and WiLake fields of the Rimbey-Leduc- Meadowbrook reef chah. He also found that similar hydrothermal lead-zinc mineralization occurred in the Malmo, New Norway and Duhamel fields, but not in the Bashaw complex. In these areas it is inftrred that the mheralization process was fault-controlled and that fluids used existing fuit planes as mduits to access Leduc @3) carbonates. However, the - exact timing of these structures is often unclear since overlying Ireton and Nisku formations at Duhamel also cmÉain sphalerite. Hm, the proposed structure rnay simply be post-Leduc in age or may be the result of repeated reactivation. Such mheralization in fiacture urnes may give them a relativeIy strong magnetic signature, fa out of propomon to the amount of displacement. More recently, Arestad et al. (1995) have identifiai the existence of several NE-trending fault zones from a 3-D seismic survey across the nearby Joffre field, which produces hmthe NihFoxmation, These structures affect Nisku shelf carbonates and rnay compartmenîaiize the mewoir by influencmg fluid-flow directions.

merstructural iadicators include the presence of marked resemoir discontinuities between adjacent Leduc buildups. Knight & Hannon (1960) noted signifiant reservoir pressure discrepancies in the Erskine and Stettler fields that appeared to continue dong a NNE-SSW or NE-SW trend through the Ghost Fine Embayment and separate fields of the Southem Alberta Shelf complex. The presence of a fault dividing these Leduc fieids would help to explain such anomalies.

More recently, using total-field aeromagmtic data, Ross & Stephenson (1989) delineated a discrete positive anomaly trading NNE to NE across centrai Alberta, named the Rimbey arc or high. They speculated that this prominent aeromagnetic high appears to run subparallel to the Rimbey-Leduc-Meadowbrook chah of reefs. Bearing in mind that most significant magnetic anomaIies in central Alberta are believed to be sourced m the basement, this momaiy may correlate with a basement feature that constitutes some form of regional basement control on these teefs. However, as demonstrated by Eaton et al. (1995), this total-field aeromagnetic anomaly appeai.s largely to crosscut the reef trend, indicating that any such basement influence is largely indirect in nature. Near the Southem Albda SheLf Margin, more conciusive evidtnce for fâdting exists. The Drumheller fault is exposed on both banks of Michichi Creek (142%SOW4), and trends N53"E to separate die north and south pools of DrumheIlcr field (Haitcs, 1960). Hem, both vemcai and Iateral displacements are interpretcd at the Nisku Formation level. Fuahennore, seismic evidcnce for local post-LRduc half-graben faulting iu the nearby West Dnunheller area (T29-29, R22W4) also exists, agah with a roughIy NNE or NE trend (G. Thomas, personal cornmunicatian, 1994). These structures may rcflect recunent movement on zones of weakness that were estabfished prior to the Leduc, but this has yet to be substantiated-

In addition, although deep well conml is scarce, Belyea (1958) has postulateci that a major Lower Paleozoic crosionai escarpment may extend throughaut this east-centd Alberta ares This feature, known as the Meadow Lake Escarpment, is believed to be an ENE-trending erosional lineament that originated during the sub-Devonian hiatus in the Meadow Lake area of west-centrai Saskatchewan (van Hees, 1958). However, whether this feature is related to basement tectonics is still in question. van Hees (1958) bas mapped significant relief on pre-Devonian unis but no obvious Precarnbrian anomdy in the Meadow Lake area. According to Slind et al. (1994), up to 500 m of Cambrian and ûrdovician sediments were removed prior to deposition of Middle Devonian units. The Meadow Lake Escarpment has been projected both east and West by van Hees (1958), dong the northem margin of Ordovician carbonates. To the east, he has projected it to run parallel with a major Recambrian fault system displayhg vertical and horizontal rnovernents, the Kisseynew linment of the Flin Flon area @hison, 1951). To the West, several workers (e-g. Belyea, 1958; Switzer et al., 1994) speculate that this escarpment extends southwarcis to line up closely with the southem Alberta Leduc shelf margin @gure 7-2), the Beaverhill Lake Group southem bank edge and the southern Lower Elk Point margin, but they offer no supporthg evidence.

However, not ail studies have advocated a stnictural influence on Leduc Formation reef development. For example, Stoakes (1992) concluded that most Leduc reefs in central Alberta were situatecl on local highs on the Cooking Lake platfom that were purely depositional rather than stnictural in nature (Figure 7-14). More specifically, according to Stoakes & Wendte (1987) and Wendte (1994) the Redwater reef and, possibly, the Willington and Pinedale reefs were localized by depositional highs that mltcd from the i aas 269 development of deep-water rentrants or chameis in the uppermost cycle of Cooking Lake platform growih, thus allowing open weIl-circulated waters into the platform interior. As mentioned previously, very few wek and fewtr available cores amally pcnetrate the Cooking Lake Fonnation to intersect older uni&. TheLefore, it is pIausiïle that at least some other 'depositional' highs may genetidy refiect the configuration of older surfaces, possibly inherited fiom even deeper süuctures. The possibility of inherited topography is investigated in thîs study.

It is significant that at least some farm of antecedent topographie highs beneath buildups, albeit subtle, are recognized using core and well data. Most seismic studies across Leduc Formation buildups in cenîral Alberta have generally dismissed the presence of tirne- sîructural relief beneath reefs as velocity ariifkîs (e.g. Anderson et al,, 1989b; Anderson et al., 1989c; Eaton et al., 1995). Lateral velocity heterogeneity is exhibited in moving km relatively high-velocity carbonates of Laduc reefs to Iower-velocity Ireton shales off-reef, commonly manifesting itself as velocity pull-up beneath the carbonates. Assuming a constant velocity con- between such carbonates and shdes, the magnitude of velocity- generated relief is regarded by Anderson et ai. (1989b) to be mostly a function of reef thickness and, to a lesscr extent, magnitude of overlying stnicairal drape. Therefore, any proposed relief on pre-Leduc seismic events, whether interpreted as structural or depositional in origin, should be subject to extreme mtiny-In this study, the inclusion of any such questionable seismic structures is avoided by Battening on a pre-leduc horizon, thereby removing spurious Leduc velocity anomalies. In any case,pre-leduc relief may be so extrernely subtle as to be missed altogether or to simply be beyond the resolution capabilities of the available seismic data

As discussed by Anderson et al. (1988b), suspect tirne-structural relief on pre-leduc units may also be caused by salt dissolution. Re-salt seimiic reflections rnay be affected by velocity puhp or pushdown. Although the area of east-ceniral Alberta covered by seisrnic profiles used in this study likely contains iittle Wabamun Group sait, some EUc Point evaporites (Prairie Formation) are known to exist east of the Bashaw complex (Meijer Drees, 1986). In areas underlain by the Prairie Formation, care is therefore needed to avoid exroneously interpreting actual salt-dissolution features as deep-seated sûuctures.

Overall, it seems clear that the question of basernent interaction with Leduc Formation carbonates in central Alberta remains unanswered, fifty years after it was first proposed. 270 From the avaiiable Iiteratnre, strucftrral ofkt of Rt-Leduc m*ts immtdiate1y below reef trends in this area has never been documented. Although syndepositicmal tectonic controls on Leduc Formation buildups stem mlkly, subtle basement-related influences, direct or indirect, may have been overlooked, Recently, Eaton et al. (1995), also using CAT seismic data, offered some support for the hypothesis of subtle basement tectonics as a factor in carbonate sedimentaticm. Foilowhg on hmtheir work, this study aims to take a closer look at whether any connection with deep-seated trends may be postulated to exist for Leduc Formation carbonates, either on a local or regional sale. Detailed mapping and correlation of Leduc reef morphology, pre-Leduc stratigraphy (Upper Devonian to Precambrian), and the shallow Precambnan basement, will hopefully lead to a clearer undersianding of the dksûi'bution of reefs and phdorms fdin east-centd Alberta.

7.5 PaIeotectoaism and Leduc Formation carbonate bnildups In this snidy, the influence of basement structure on WCrecf development is mvestigated in three principal areas: the Pimbey-Leduc-Meadowbrwk feef chain, the Bashaw cornplex, and the Killam Barrier mf on the southem Alberta shelf. As mentioned previously, seismic and potential-field &ta are examinecl together in each of these areas to help determine any local or @onal influences the basement may have M

75.1 The RimbeylLeduc-Meadowbrmk reef chah These reefs form a NNE-trendkg chain in the western part of the study ara, roughly dong longitude 114" W (Figure 7-2). Over a distance of 150 km, from Wizard Lake to Sylvan Lake, this chah maintains a nearly constant ttend of about N20' E. It may be extended in the north to the Morïnville reef trend, and in the south to the Cheddarville reef complex. Seismic lines 5 and RB nui mssthe elongate Homeglen-Rimbey reef of the central part of the chah. These Iines are both oriented W-E, but line 5 lies about 28 km to the north of line RB (Figure 7-4). SeismicaUy, the shallow crystalline basement in this area is characterized by a low signal-to-noise ratio and significant multiple energy from the overIying sediments (Figures 7-15 to 7-17). Xnîerpretation of these lines is Mer complicated by the preseace of several possible migration artifacts. For exampIe, between 3 and 4 seconds on line 5 (l3gure 7-15), high-amplitude reflectors outline a synformal seismic event that is interpreted in this dissertation to be introduced during the migration process due to increased noise in the stacked section, as outlined in Section 3.2. AU told, it Figure 7-15. Migrateci seismic data hmCAT line 5. The vertical exaggeration near the basement surface is approximately 3.5, for an assumed average velocity of 5000 mis. The Precambrian basement surface is delineated by a dashed white line. Figure 7-16. Migrated seisrnic data fiom line RB. The vertical exaggeration near the boisement surEace is approximately 3.5, for an assumcd average velocity of 5000 mis. The Recambrian basement surface is deheated by a dashed white he. Figure 7-17. A cornparisrin of migrateci scismic data hmline 5 and me RB, plotted at the same scale. For an assurned average velocity of 5000 mls near the basement surface, the vertical exaggeration is about 3.5. The Precambrian basernent surface is delineated by a dashed line. Note that subtie differences are observed betwea the Middle Cambrian Earlie Foxmation and the Precambriaa event, even at this scale. A hi@-amplitude event on he5, termed the basement precursor (ôp) by Eaton et al. (1995), is not psent on line RB.

275 is not possibk to reliably distinguish any real seissnic events dircdy below the basement surface (at about 1.8 seconds) on line 5 or Iine Rb.

On directly comparing these two lines at the same sale in Figure 7-17, it appears that the reflectivity of the sedimenu immediately overlying that basement varies somewhat. An anomalously high-ampliaide event in the Middle Cambrian to Precambrian (Earlïe Foxmation) interval of line 5, temed the basement precursor (bp) by Eaton et al. (1995). is not present anywhere on line RB. It has been suggested that this anomaly may be caused by a mineralized, diageneticdy altered or fluid-enriched zone, possiily confined to the Rimbey basement do& Eaton et al. (1995) went on to speculate that it may have been a localizing mechanism for overlying Leduc reefs. However, the fact that this near-basement reflection is not present beneath the Homeglen--bey reef to the south wouid tend to suggest that this is not the case. Fuahermore, ment work constituting part of the Lower Paleozoic Roject nidicates that this seismic event on line 5 is simply the result of wavelet tuning or interference due to a thin locaîized wedge of Cathedral Formation îimestone that extends eastward into the Earlie or Stephen Formation clastics (J. Dietrich, personal communication, 1996). A close exmination of line 5 (E"~gures7-18 and 7-19) suggests the presence of a thin stratigraphie intemal. Such a local intemngering of Cathedra1 deposits within the Stephen/Earlie unit is also proposed by Slind et al. (1994, Figure 85) to exist in central Alberta.

An interpretation of the Homeglen-Rimbey reef of line 5 is shown in Figure 7-19. Here, the reef is approximately 5 Imi wide and reaches a maximum thickness of about 225 m, where it represents full buildup. To the east, this Upper Leduc reef is replaced by only a Lower Leduc partial buildup (basal reef or platform) that attahs a height of about 100 m, as suggested by the pronounced increase in Ireton Formation shale thickness and as supported by well data. Moving south, the ~ome~len&be~reef appears to thicken and Vary considerably in width. At iine RB, a larger fidl reefal buildup is developed to a thichess of 290 m or so and a lateral extent of over 9 km (Figures 7-20 and 7-21). Overall, the reefs of the Rimbey-Leduc-Meadowbrook chah seem to be characterizeà by such dong-strike variations in morphology.

On fiattening on the Second White Speckled Shale marker (Upper Cretaceous) in lines 5 and RB (Figures 7-22 to 7-25), it is apparent that a pronounced anomaly is present on horizons immediately kneath the Leduc reef and can be followed dom to the basement Figure 7-18. A scaled migrateci version of line 5 with a display window of 0.8 to 2.2 seconds. For an assumeci average velocity of 5000 m/s near the basement Sllfface, the vertical exaggeration is about 2.

Figure 7-19. An interpretation of line 5 superhposed on the onginal data shown in Figure 7-18. The vertical exaggeration is approximately 2. Intcrpreted seismic horizons are highlighted in white. The extent of the Leduc Homeglen reef and of the basement are indicated by shading.

Figure 7-20. A scaled migratecl version of heRB with a dispIay window of 0.8 to 2.2 seconds. The verticai exaggeration is approximately 4.

Figure 7-21. An interpretation of Iine RB superimposed on the original &ta shown in Figure 7-20. The vertical exaggeration is approximately 4. Interpreted seismic horizons are highlighted in white. The extent of the Leduc Homegien reef and basement are indicated by shading. TlME (SECONDS)

Figure 7-22. A scaled migrateci version of line 5 after Bamning on the Second White Speckled Shale marker (Upper Cretaceous). For an assumed average velocity of 5000 dsnear the basement surface, the vertical exaggeratim is about 2.

Figure 7-23. An interpretation of Iine 5 flattened on the Second White Speckled Shaie marker and superirnposed on the original data shown in Figure 7-22. The vertical exaggeration is approximately 2. Interpreted seismic horizons are highlighted in white. The extent of the Leduc Homeglen reef and basement are indicated by shadïng.

Figure 7-24. A scaled migrateci version of line RB, after flattening on the Second White Speckled Shale marker. The vertical exaggeration is appmximately 4.

Figure 7-25. An interpretation of line RB flattened on the Second White Speckled Shaie marker and superirnposed on the original data shown in Figure 7-24. The vertical exaggeration is approximately 4. httrpreted seismic horizons are highlighted in white The extent of the Leduc HomegIen reef and basement are indicated by shading- II.

0- TIME (SECONDS)

286 Ievel. For example, the top of the basement IocaIly appears to be about 25 to 30 ms higher beneath the full buildup than elsewhere. Although some pre-Leduc reflectors Iack continuity on these lines, they appear nonetheless to lie the-StnicturaUy parailel to one another, with no sigaincant isochron variations on Middle Devonian and Lower Paleozoic uni& evident underneath the reef. Mermore, fiatmhg on a pre-Leduc event, m this case an intra-Deadwood reflector (Upper Cambnan) removes al1 trace of this anomalous structure. As demonstrate-in Figures 7-26 to 7-29, the basement Surface almg hes5 and RB is fairly flat after this flattening, indicating that this anomalous relief was mostly velocity pull-up resulting hmIateral velocity differences in the Woodbend Group. The presence of a break in dope toward the West of line RB is probably a flattening artifact related to poor data quality.

From a regional perspective, thtre appears to be a only lÏmited correlation between carbonate morphologies obsenred dong this chah and the magnetic anomaly patterns shown in Figures 7-6 to 7-9. For example, the southern part of the extensive Homeglen- Rimbey reef coincides with a lineament that defines tbe edge of a pronounced positive magnetic anomaly. However, the remainder of this reef chah is crosscut by a senes of NNE-trending magnetic lineaments that arie believed to be reWto deformation associated with the Snowbird Tecmnic Zone (STZ). On this basis of these magnetic maps, it appears that no large-scale relationship exists between these two regional features. In contrast, Figures 7-10 to 7-13 suggest that there is a closer relationship between the Bouguer gravity-defined SIIZ and this reef chah In Figure 7-10, a broad Bouguer gravity positive anornaly extends NESW across east-central Alberta and continues across the Cordilleran deformational front. Significantly, this anomaly is believed to be related to part of the S'IZ and coincides with a significant part of the chah, hmthe Leduc to Cheddarville reef complexes. This is more clearly seen in the interpreted Bouguer gravity lineament map of Figure 7-13. Furthemore, the infexred southwestem extension of the STS foiiows a trend that coarsely coincides with the palinspastically restored Bentz-Bearberry reef complex. Overall, it may be that we are looking at subtle dong-strike variations in the factors influencing Lower Leduc reef initiation almg this trend Although published seismic evidence is lacking, it seems as though large parts of the mbey-Leduc-Meadowbrook reef chah in east-central Alberta may be influenced, at least indirectly, by variations of basement structure and lithology associated wirh the STZ. Figure 7-26. A scaled migraîed version of line 5 after aaûenmg ion an mtra-Deadwood Formation (Upper Cambrian) reflector. For an assumed average velocity of 5000 mis near the basement surface, the vertical exaggeration is about 2. Only the Ear1ie Formation and Precambnan events are annotateci.

Figure 7-27. Au interpretatiion of line 5 flatteneci on an intra-Deadwd reflector and superimposed on the original data shown in Figure 7-26, The vertical exaggeration is approximately 2. The Earlie Formation is highlighted in white and the Recambrian basetnent is shaded.

Figure 7-28. A scaled rnigrated version of line RB after flattening on an intra-Deadwood Formation (Upper Cambrian) reflector. The verticai exaggeration is appmximately 4. Only the Earlie Formation and Precambrian events are a~otated

Figure 7-29. An intexpretation of line RB after flattening on anperirnposed on the original data shown in Figure 7-28. The vertical exaggeration is approximately 4. The Earlie Formation is highlighted in white and the Precambrian basement is shaded.

292 7.5.2 The Bashaw reef cornplex The Bashaw cornplex is isolated fkom the Leduc-Rimbey and Cheddarville-Bentz complexes to the West by the NNE-brending Bmt Timber embayment (Figure 7-2). OveraIl, the Bashaw complex has a NE to NNE orientation with a fairly straight western edge, and is segmenteci by a number of channeis mto several srnalier subcomplexes. To the north lies the string of Duhamer, New Norway and Malrno reefs, with a clear NNE trend As outiined by Figure 7-4, seismic line 7 cuts W-E across the northern part of the Bashaw reef complex. Although partly masked by multip1es, the Mowbasement is characterized seismically by two shallowdipping reflectors, as illustrated by Figures 7-30 and 7-31. These west-verging features represent a continuation of deeper, crustal-scale thrust stxuctures identifid by Ross et al. (1995). Upsection, the tip of an infened thnist may be traced to the basement-sediment interface where it apparently induces a marked break m dope or hinge line (Figures 7-32 and 7-33). My interpretation is that there is a structurai component to basement topography beneath the Bashaw carbonate buildup. In addition, thrust faulting of the shdow basement may have separateci contrasting lithologies that are susceptiïle to âifferential erosion, thercby accemating any smcnually-induced relief on the basement surface.

An interpretation of the Bashaw complex is shown in Figure 7-33, where it extends laterally for approximately 22 km and reaches a maximum thickness of 240 m. On flattening this heon the Second White Speckled Shale marker to remove a regional dip to the west, it is clear that a marked tirne-structurai hi@ is present on reflectors below the carbonate buiidup (F"1gures 7-34 and 7-35). The basement suriace has tirne-structural relief of the order of 30 rns, relative to regionaI levels observecl to the east. To the west, this apparent relief is increased due to the presence of the low-angle basement break in slope. Overall, data quality on line 7 is good, suggesting that any anomalous topography produced due to flattening on an incorrectly picked horizon is minimal in this case.

In contrast to the parallelism exhibited by pre-Leduc reflectors beneath the Homeglen- Rimbey reef, distinct isochron variations can be recognized on ihe 7. Seismicaily, the Beaverhill Lake sequence consists of several low-angle clinoforni reflections, the break in slope of which roughly line up with the western rnargin of the Leduc buildup and the basement high. Underlying units display more subtle regional thïchess variations. For example, the thickness of the Upper Cambrian Deadwd Formation decreases gradually to the West, and no localized on-structure thianing is evident. Middle Cambrian events (Eadie Figure 7-30, Uninterpreted rnigrated seismic data Çom part of CAT line 7. For an assumed average velocity of 5000 dsnear the basement dace,the vertical exaggeraticm is about 3.5. Note the presence of a series of low-mgle events in a shallow basement otherwise thought to be dominated by muitipk eaergy.

Figure 7-31, Migrated seismic data fiom line 7 with several signincant basernent features highlighted. The vertical exaggeration is approxirnately 3.5. The Precambnan basement surface is delineated by a dashed white line. TIME (seconds) TiME (seconds) Fignre 7-32. A scaled migrated version of line 7 with a display window of 0.8 to 1.8 seconds. For an assumeci average velocity of 5000 m/s ncar the basement surîace, the vertical exaggeration is about 8.5.

Fignre 7-33. An interpmtation of line 7 supemsed on the original data shown in Figure 7-32, The vertical exaggeration is approximately 8.5. Interpreted seismic horizons are highlighted in white. The extent of the Bashaw Leduc reef complex and basement are indicated by shading.

Fignre 7-34. A scaled migrateci version of Line 7 aftcr flattening on the Second White Speckled Shale rnarker. The vertical exaggeration is approximately 8.5.

Figure 7-35. An interpretatian of line 7 after flattening on the Second White Speckled Shale mark and superimposed on the original data shown in Figure 7-34. The vertical exaggeration is approximately 8.5. Interpreted seismic horizons are highlighted in white. The extent of the Bashaw Leduc reef complex and basement are indicated by shading.

Figure 7-36. A scaled migrated version of line 7 after flattening on an Elk Point (Middle Devonian) reflector. The vertical exaggeration is approximately 8.5. Only the Deadwood, Earlie and Precambrian events are annotated-

Figure 7-37. An interpretatian of line 7 after ilattening on an Elk-Point reflector and superimposed on the original data shown in Figure 7-36. The vertical exaggeration is approximately 8.5. The Deadwood and Earlie Formations are highlighted in white and the Precambrian basernent is shaded.

TIME (SECONDS)

TiME (SECONDS) 303 Formation and basal sandstone units) seem to drape passively over an undufathg basement surface. By flattening on pre-leduc refiectors (e-g. the top of the Elk Pomt Group, in Figures 7-36 and 7-37), andousbasement structure due to Woodbend Group velocity heterogeneity is removeci. However, a broad, Iow-amplitude basement high with reIief of about 50 m (20 ms) is still prtsent. Although this stnicturt has a gradual eastem fiank, it is abruptly bounded to the west by the proposed intersection of the deep-seated thrust where the basement is offset by 40 m (15 ms). From Figure 7-37, one can see that it this basement break in slope that indirectly imparts the dominant stmcturai control on Middle and Upper Cambrian stratigraphy. It appears to have acted as a local hinge line to produce signifiant topographie irrtguiarities on Lower Paleozoic surfaces. Subtle hwerPaleozoic facies changes may also be associated witb this feature. For instance, Cambrian carbonates appear more abundant west of this low-angle hinge line (J- Dietrich, personal communication, 1996). In contras&only very local accommodation of Middle Cambrian unit5 takes place above the fault near shotpoint 1093. However, more laterally extensive drape is evident on Cambrian seismic events over the eastem part of the basement high, albeit more subtle. The cumulative effect is to yield a zone of local, low-amplitude Lower Paleozoic relief that can be extrapolated upward to coincide with both BeaverhilI Lake clinoform geometries and the western margin of the Bashaw complex, implying some form of indirect relationship. Andrichuk (1961) suggested that movement dong a similar basement fault-induced hinge line Muenced Leduc reef growth at the Duhamel Field, to the north of the Bashaw complex.

Basement thrusting of the East Alberta Orogen is likely to have formed during the complex collisional assembly of basement domains, possibly obliquely, to the northwest (Ross et al., 1995). From this reconstniction and the NE-trending nature of domain boundaries, it is Üiferred that these thrusts may strike NE-SW. The thmst that extends to the basement surface on line 7, the~bycceating a stratiPp-hically important hinge he,possibly ands in a similar direction. This supposition is supported by the orientation of cleavage measured by Villeneuve et al, (1993) from basement drillcore recovered from 1M5-38-21W4, about 40 km to the south of line 7. Here, a strongly deformed schistose rock possesses a cleavage trend of N4W E, similar to the strike of some regional magnetic and gravity lineaments (F@res 7-9 and 7-13). However, no large-de correlation is evident between Bashaw carbonate morphology and magnetic anomaly patterns. Interpreted magnetic lineaments largely crosscut, and subcircular anomalies lie adjacent to, the northern part of the reef complex. No magnetic lineaments are interpreted ta iatersect seismic line 7. 304 Regionai Bouguer gravity anomalies, on the other hand, seem to display a closer relationship to the overall margins of the Bashaw complex (F~gure7-13). Several linearnents appear to bound large segments of this fragmcnted reef complex. It is piausible that some of these NE- or NNE-trending lineaments are reflecting lithology variations that are directly reIated to basernent thrusting. Line 7 mtersects an arcuate gravity linearnent oriented obliquely to this proposed structural trend; its relationship to the proposed tbrusting is unclear. To the east, the Stettler reef is simkly flanked by NE-trending lineaments. CoiIectively, such coincidences could indicate a subtle form of regional interaction between ttie basement and Devonian carbanate scdimentation patteims.

75.3 The southern Alberta shelf margin The Leduc carbonate shelfends to the West at an abrupt, linear, NE-trendhg edge, known as the KilIam Barrier. It appears reefal in naaire and is extensively ddomitized (Switzer et al., 1994). The sheif edge crosses the CAT seismic data near the intersection of lines 9 and 10 (Figure 7-4). The line segments adjacent to the edge are merged and aIigned to account for different dam elevations. Figure 7-38 illusmates that identifying ml seismic events in the shaiiow crystaiijne basement of the southem Alberta shelf margin area is extremely problematic. This is due to a low signal-to-noise ratio, several possible migration artifacts, and relatively high-amplitude multiple events originating fiom the sedimentary cover. Hypothetically, although clear seismic evidence is lacking, it is pIausible that some of the thrust faults identifieci by Ross et al. (1995) for the deeper cmst of Iine 10 may actually propagate upsection to the shallow basement and even intersect the basement-sediment interface. If this is the case, then these structures may have been instrumental in the formation of a large basement topographie high that aüains relief of about 125 m (50 ms) and extends for a distance of 25 lan or more dong this Iine (Figures 7-39 and 7-40). Alternativdy, non-structural processes rnay represent the dominant agent in the creation of this basement arch. For example, such relief c&d largely be the result of erosion acting on laterally-varying basement rock composition. Whatever its exact origin, this arching of the basement surface clearly affects overlying Cambrian and Devonian cover rocks.

Figures 7-39 and 7-40 illustrate some of the Cambrian and Devonian isochnrn variations and reflector truncations associated with this structure. To account for pst-Devonian regional tectonics and to accurately reconstruct reflector geornetries, this seismic section is flattened on the Second White Speckled Shale rnarker (F@res 7-41 and 7-42). ïntra-Basal sandstone unit Mectors onlap across both the West and east fiank of the basement hi@. Figure 7-38. Migratecl seismic data fiom part of merged CAT profiles 9 and 10 (line 9/10). For an assumeci average velocity of 5000 m/s near the basement surface, the verticai exaggeration is approximately 3.5. The Precambrian basement surface is deIineated by a dashed white line.

Figure 7-39. A scaled migrateci version of line 9/ 10 with a display window of 0.6 to 1.6 seconds. For an assumed average velocity near the basement surface of 5000 mis, the vertical exaggeration is approximately 85.

Figure 740. An interpretation of line 9/10 superimposeci on the origmal data show in Figure 7-40. The vertical exaggeration is approximately 8.5. Interpreted seismic horizons are highlighted in white. The Southeni Alberta Leduc shelf edge, pinchout of Basal Sandstone unit reflectors, and Precambriau basement are ail indicated by shading.

Figure 7-41 A scaled migrated version of line 9/10 aftcr Battcning on the Second White Speckled ShaIe marker. For an assurneci average velocity of 5000 mis near the basement surface, the vertid exaggeration is approximaîely 85.

Figure 7-42. An interpretation of line 9/10 after flaüening on the Second White Speclcled Shale marker and superimposed on the original data shown in Figure 7-42. The verticai exaggeration is approximately 8.5. Interpreted seismic horizons are highlighted in white. The Southern Alberta Leduc shelf edge, pinchout of Basal Sandstone unit refiectors, and Recambrian basement are all indicated by shding.

Figure 7-43. A scaIed migrateci version of line 9/10, after flattening on au Elk Point (Middle Devonian) reflector. The vertical exaggeration is approximaîely 8.5. The Deadwood, Earlie, Basal Sandstone and Aecamùrian events are annotateci.

Figure 7-44. An interpretation of an Eik-Point-flatteneci luie 9/10, superimposed on the original data shown in Figure 7-44. The vertical exaggeration is approxirnately 85. The pinchout of Basal Sandstone unit reflectors and the Precambrian basement are indicated by sbading.

3 15 The shading m Figure 743 highlights pmchouts at different Ieveh within this unit. There is also niarked thinning of the Earlie Formation over this structure. AIthough only minor onstructure tfiinning is evident m the overlying Deadwood Formation dong thk line, mtra- Deadwood reflectors show some marked isochron changes, possibly representing low- angle ciinoform gwmetries. This conclusion is substantiatecl by au examination of Figures 743 and 7-44, where the section is fiattened on an Eîk Point Group reaector. Although the top of the DeadwOOd unit is fairly fiat, intra-Deadwood, Earlie and Basai sandstane unit reflectors appear to regiondy thin and drape over basement topography. These observations indicate that the hi@ was already uplifted by the the of Middle Cambrian sedunentation and continucd to influence sedimentation, albeit subtly, through to the onset of Devanian sednnentation.

The Leduc Formation on seismic Iine 9/10 comprises thme distinct reef stages: the Lower, Middle and UprLeduc- A full buildup comprising dl three stages is found in the eastem part of this line, where it reaches a maximum thickness of about 240 m. Moving westwards, the Lower Leduc shelf passes IateralIy into about 60 to 70 m of Duvernay Formation shales. Middle and Upper Leduc carbomtes grew on this older buildup and proceeded to overstep the Lower Leduc shelf margin by as much as 15 km. Between shotpoints 134 and 516 on line 10, Upper and Middle Leduc carbonates ovefie Duvernay shales. This overstepping of the MiddleAJpper Leduc relative to the Lower Leduc of the southem Alberta shelf is also suggested by Switzer et aL (1994, Figure 12.7).

The effect of this Recambrian stnictural high is also seen further upsection in the Devonian sequence, Here, it appears that the western Bank of the high became progressively more stratigraphically important than the eastem side. Within the Elk Point Gmup, there is clear evidence of reflectiw downlap, probably Muenced by a combination of hwer Paleozoic unit rhioning and drape over the West side and fmt of the basement hi@. In mm, drape over this Middle Devonian downlap may have accentuated any topographie relief enough tû provide favourable conditions for the initiation of Lower Leduc carbonate sedimentarion to the east. This Lower Leduc shelf buildup would then have controiled the development of later growth stages.

Some workers (e.g. Belyea, 1958; Switzer et al., 1994) have postulated that the pre- Devonian Meadow Lake Escarpment (MLE), described in Section 7.4, may have partly influenceci the morphology of the southem Alberta Leduc shelf edge. In Figures 74to 7- 316 9, regional rnagnetic anomalies appear to show no comspcmdcnce to the proposed NNW- SSE onented W.A Bouguer gravity lineament is hterprtted to partly coincide with this structure but not to extend along mdto mtersect seismic line 9/10 7-10 to 7-13). Significantly, no evidence of a pronounceci escarpment affectmg the Lower Paleozoic is present on LITHOPROBECAT seismic dam This suggests that the MLE either dies out or changes orientation in this part of east-central Alberta. Overali, the inferred trend of this escarpment largely crosscuts the edge of the Leduc shelf in the nortbeastem part of the study area, apparentiy exhibithg litîie regional influence. The boundary of an arcuate positive magnetic anomaly mughiy coincides with the shelf margin where it deviates away from the MLE trend 7-6 to 7-9), implying an indirect relationship could be present. As in adjacent areas, these types of anomalies are ttiought to be related to plutonic intrusion.

To the south of the proposed location of the MLE, Bouguer gravity and rnagnetic anomalies show few correlations with the edge of the Leduc sheIf. For instance, NNE- to NE- trending magnetic anomalies obliquely cut the shelf margin and seismic line 9/10. My interpretation is that some of these Iinear anomalies are reflecthg large-sale variations in basement Iithology induced by thrust faulting. If this is the case, it is possible that non- structural changes in basement lithology may constitute a more important dong-st&e conml than basement tectonism on this rnargÏn.

7.6 Discussion. Basement structure and lithology appears to have had an indirect effect on carbonate depositional environments during the Middle to Upper Devonian in the Bashaw and Southern Alberta Leduc shelf areas. This cbapter has detailed the spatial relationship between a number of local and regional irregularïties on the basement surface and Cambrian to Devonian sedimentation patterns. Cornpressed displays of seismic data often help to distinguish such subtle localized basement anomalies, as iilustrated in Figure 7-45. Considering the highly localized sampling of the nm-basement by ciriilholes and the low- relief nature of basement topograpby, many more potentidy significant structurally- or lithologically-induced topographie features may have kenoverlooked on industty seismic data and warrant re-examination. Recent aaificial reef studies ourlined in Chapter 4 (e.g. Grove et al., 1991) suggest that the amount of fief necessary for reef nucleation is rarcly more than 5 m, and is often considerably less. Relief of the order of a few metres will not IbLeduc Fm sedimentary cowr 1 Horlmnri flatlened on Second White Speckld Shak marker 1

Lhe 7 Lhe W0

Figure 7-45. A compressed-scale display of the basement surface and Cambrian to Devonian sedimentary cover across east-central Alberta, as evident from seismic data. The vertical exaggeration is approximately 30. 318 be imaged on the CAT seismic &ta, whcrc the nsolution at the basement level is limited to 20 m or so (Eaton et al., 1995). In addition, the low-amplitude nature of Cooking Lake reflectors directly beneath Mucreefs can obscure local topography that is large enough to be imaged. It is feasible that low-relief topographie highs, possibly inherited from the basement, may also be prcsent beneath reefs of the Rimbey-Leduc-Meadowbrook trend.

Post-Leduc reactivation of basement faults is also thought to have occurred in east-central Aiberta. Both direct and indirect evidence of post-Leduc faultmg in the study area has been discussed in Section 7.4, The fact that such stmcaires are not evident on the seismic profiles interpreted in this study codd be taken to infer that subtle fault displacements could be present but are simply beyond the resolution capabilities of the data. Furthemore, if these Phanerozoic faults are subvertical in nature, as indicated in west-central Alberta (Chapter 6), th& detection using the reflection seismic technique becomes problematic. Importantly, such stnictures, if present, may act to compartmentalize a carbonate reservoir or play a role in the dolomitization of some Leduc Formation carbonates. To date, such basement-carbomte interaction remaias questionable, at lest for the Leduc Formation reefs of east-central Alberta. CHAPTER8

Snmmsry and conclusions

" rfa topographie high is postJated on the Precmnbrian. it is iogicai t~ mume that this wouid r&ct its presence in rhe sirata ùzid down arounù anà mer it, and ako Ur vQnOw stages ir would create mi ideaiphjônn on which reef deveiopment could take pke. "

R.L. Rutherford (1954, p.209)

There are many unanswered questions regarding the influence of the Precambrian crystalline basement on Phanerozoic stratigraphy in Alberta This study has focused on the distribution of Middle and Upper Dcvonian carbonates in central Alberta and their possible relationsbip to deepseated basement features. Clearïy, with the configuration of carbonate buildups having been previously mapped fairly extensively through hydrocarbon exploration, it is now lmowledge of underIying basement structure that is one of the key factors in detennining whether any such association may have existed. Through the inkgration of available seisrnic, potential-field and well &ta, this poorly understood aspect of carbonate sedimentation in central Alberta is now assessed from a geophysical perspective.

8.1 Limitations of an integrated geophysical methodology In trying to relate feanues identified using conmsting geophysical techniques that effectively respond to different rock parameters, correlations identified throughout tbis dissertation are often seen to be complicated. For instance, the acousric basment identified using seismic data can be tied to the structural basement thraugh deep weii information (sonic, density), but may be very different to the magnetic basement deterrnined fkom aeromagnetic data m central Alberta. Regionai magnetic and Bouguer gravity data in central Alberta are known mostly to reflect the mcture of the crystailine basement 320 The seismic rnethod should detect topographie relief on the basement surface where the amplitude of a particular structure is greater than the resolution of the data; for example, the Central Alberra Transect (CAT) seismic data wiiinot image displacements of less than 2û m at the basement level. However, the magnetic signature of a rclatively large offset may be minor compared to the critical importance of major Iithology variations in the basement. Therefore, unless large-scale basement structure or topography corrtsponds to changes in basement lithology, which is possibly the case for some anomalies, it is uniikely it will be detected using a regional total-field magnetic sumey. On the other hand, a basement Mt may lateraiiy separate very different basement lithologies resulting in a large magnetic anomaly. This structural contact may have very linle displacement at the top of the basement, resulting in little or no seismic image. Lack of smiilarity of seismic and potentiaI- field data is also likely to occur due to the marked ciifference in resolution of these geophysicd techniques. Seismic profiles have considerably pater resolvuig power than magnetic or gravity data. Nevertheless, if we bear m mind that discrepancies will exist, this combination of a seismic and potential-field approach to investigating pre-Devonian smcture can only improve our understanding of local and regional tectonic controls on the distribution of cadmmes m central Aiberta

83 Structural elements of the crystalline basement in central Alberta Interpretation of seismic profiles recorded with extended listening times have yielded a well-defined low-angle basement fabric compnsing part of a crustaI-sale thrust imbrication system in east-central Alberta termed the East Alberta Orogen by Ross et al. (1995). Large- scale roll-over structures carrieci on the hanging wail of these E- or SE-dippmg hstscan be identifie& In seismic data from west-central Alberta, inclined intrabasement events have also ken recogoized, albeit with an inferred reversal in the vergence and orientation compared to reflectors observed to the east. Based on the lack of any marked üuncatiom or large-sale hanging-wail structures associatecl with these seismic events, they are inferreci to be of Lithological rather than structurai origin. However, some dipping seismic events may also lx interptieted as migration artifacts, inWuceci as a result of a Iow signd-to-noise ratio in the stacked data. Overall, where dipping events can be correlated with displacements at the basement surface, such features likely represent real strucnues.

In addition to these two conirasting basernent fabrics, a major crustal-scale discontinuity, known as the Snowbird Tectonic Zone (STZ), extends NNE-SSW or NE-SW throughout 321 the study area. Using regional magnetic and Bouguer gravity data, tbis shear zone has been tracked hmthe Canadian ShieId to the su- of central Alberîa. This geologic check helps to confum the interpretation of some potential-field anomaiies as faults. However; defoxmation associated wiîh the S?Z is delineated by cmtrasting magnetic and Bouguer gravity signaaires, thercby illusirathg the need for the kind of joint interpretation undertaken m this study.

The structure aad lithology of the shallow bascment in centrai Alberta are believed to be primary conmls governing beisemcnt surface paieotopography, bath lowlly and regidy. Relief on the basement surface was also like1y to have ken affecteci by ensuing geomorphological processes. For instance, Iaterally varying basement rocks at the basement surface are like1y to be subject to cliffimential erosion accentuating or smoothing promsioas on this surface, The cumulative effect of these various agents was to produce srnail- and large-scale irregularities on the basement-sediment interface in many areas of central Alberta. Areas of raised basement topography, normaliy recognized fiom seismic data and sparse drillhole penetratiom of the near-basement and basement, often play an important role in the deposition of overlying Phanerozoic sedimcnts.

8.3 Tectonic inheritance of the Phanerozoic -ver in central Alberta The evolution of a sedimentary basin is usualIy affected by a multitude of geological processes, some of which are controlled by pre-existing smctures in the basement. Basement faults which bound ngid crustd biocks rnay accommodate aItemating pulses of uplift, subsideme, tilting, or strke-slip motion. Reactivation of old basement fadts into the ovedying sedimentary cover provides a direct example of tectonic inheritance. Such faults may propagate up-section through the cover by episodic reactivation induced by changes in the regionai stress field, On the basis of the kcniralûamework genedy observed in sedimentary cover rocks, the basin architecture of central Alberta can be regiondly suwivided into the Peace River Arch area of noahcentrai Alberta, and the interior plains to the SE of thstrucaual feature.

In north-central Aiberta, the basement surface is regionaily uplifted and locally dominated by Phanerozoic tectonism associated with the ENE-trending Peace River Arch. In the eastem part of the Arch, the basement diceconsists of a regular pattern of anomalous stnicturai highs locally mapped hmseismic and drillhole data as king closeci, a pattern 322 consistent with tht notion of a surface fr;zctrind by cmjugate Mt pairSc&. Thm is both geophysical and geological evidence that NE- and NW-uending nodfaults were periodicalIy rcactivw Mi,during, and after Middle Dcvonian sedimentalion, In this dissertation, it bas been shown that rhe preferred technique for mapping local structures on the basement surface is 3-D scismic.

Away fmm the Peace River &ch, faulting m the Phanerozoic cover of centrai Aiberta has not been well documented in the pubiished literature. This is epitomized in the foiiowing quote by Haites (IW, p. 58): "'ulruig in [cetla Alberta] has long been thought to be the remit ofslumping, &aping, glacial &n, or sait dissolution. As n matter of fm,iz semas thar any inte'pretatiun but a tecîonic one has ken preférred to explainjzuZ&*. Phanemoic faulting may have been locally active during txo main phases of reactivation: Middle Cambrian and Late Cretaceous tirnes. Whereas subtIe Middle Cambnan reactivation of basement faults is inferred in West- and eastatrai Alberta, later Cremxous movement appears to be largely restncted to areas of west-centrat Alberta, at least fiom the available seismic data. These basement faults have ldywted into the overlying sedimentary cover with a near-vaticai attitude. This later reactivationis probably in reqxmse to kgh in- plane stresses imposed by the Laramide Orogeny to the West. Tectmic overp~tingin the fom of gentie foIding and warping is also imparmi by Laramide orogenesis in this western region.

In east-central Alberta, basement thnist faults have kniaterpreted to dybreach the basement surface, amntlyinducing very local variations in the thclmess of the Middle Cambrian Basal Sandstone unit. However, on a Iarger de,the cumulative effect of some faults is to produce significant Iow-relief changes in dip at the basement level. In theory, these structures couid be carrying basement rocks of contrasting lithologies, likely to be vulnerable to varying degrees of erosion. As a result of differentiai compaction, Phanerozoic sediments may subsequently drape over such features, thereby providing an indirect example of tectonic mheritance behveen the basement and sedimentary cover. The influence of some of these mctures on the morphology of Middle and Upper Devonian carbonates is summarized in the wgsection. 323 8.4 Modes of basement-Devonian carbonate interaction in centrai Aïberta As the introductory quote by Rutherford (1954) implies, ever since the initiai discovery of reefs in the subdace of centrai Alberta, underlying basement controls have long been invoked to help explain prominent reefd trends. However; apart hmstudies of carbonates assoçiated with the Peact River Atch, supportive evidence has been largely inconclusive. In this study, the regionai variation observed in the morphology of the basement surface and deformation of the Pbaneromic cover is simkly reflected m their respective infiuences on overlying carbonate deposition. Pre-, syn- and pst-depositional basement structures have been interpreted to be closely associated with MiddIe and Upper Devonian carbonates in selected areas of central Alberta. The timing of these features relative to Devonian sedimentaiion is fundamental to understanding how such smctures may have affecteci reef initiation, development and diagenesis.

8.4.1 Pre-depositional structures As outlined in Chapter 4, the importance of paleobathymetry on reef initiation is well established from studies of modem carbonates. Even minor changes in the relief of a depositionai surface cmhave a pronounced effect on a carbonate environment. of raised topography could have given rise to potentially high-energy reservoir rock such as oolite shoals, whereas depresseci areas could have led to deeper-water facies such as shaley limestone.

Typicaily, the impact of predepositional basement structure on the nucleation of carbonates can vary significantiy fkom area to area In central Alberîa, both direct and indirect styles of basement control on the initiation of Middle and Upper Devoaian carbonates are discemeci. These two categories are somewhat similar in nature to the generic classification of local basement interaction on the sedimentary cover of central Alberta, describeci by Eaton et al. (1995), in that they ldcely reprrseat end manbas of a series of possïbilities.

Raised topography generated by reactivated basement tectonism can sometimes directly provide the loci for reef gr6wth. For example, it is well established that Middle Devonian patch-reef dolomites of the Keg River Formation typically developed in higher-energy conditions on, or above, prominent fault-bounded basement structures in the eastem Peace River Arch area of north-central Alberta. From 3-D seismic, a patch-reef trend consisting of irregular, low-relief buildups, together with smaller neighbouring bodies, is clearly identifid to be rooted on raised structure. On-structure amplitude nimming of the normally 324 strong near-Keg River event marks the location of buildups. 3-D mapping aiIows faults imaged adjacent to Recambrian on-strucaire dimmed zones to be traced beneath reefs. Such structures likely constitute important fiuid-migration conduits that may have controlied later dolomitization and the hydmarbon potential of Keg River Formaiion reefs, Such a direct tectono-depositional relationshïp is schematidy illustrateci iu Figure 8-lk

Relief on the basement surface may influence the paleotopography of Cambrian and Devonian units which, in tum, can sometimes indirectly localize reef nucleation. In west- central Alberta, in conrmst to the eastem Peace River Arch area, the present snidy indicates that pre-Devonian drape or differential compaction over irregularities on the basement - surface created the subtle topographic highs that initiated and promoted Swan Hills Formation reef growth. An example of such an indirect tectcmo-depositimal relatiOI1Sbip is shown in Figure 8-1B. On a larger scale, the developrnent of some charnels or embayments that strongly affect the ovaall mmphology of the Swan Hills Formation upper platform are kuown to be tied to areas where recumnt movement on basement Eaults has occurred.

The basement daceis inferred to impart an indirect influence cm the morphology of some Leduc Formation carbonates in eastcentral Alberta. Here, the cumulative effect of some large-scale thmst faults, possibly separating contrasMg basement lithologies, is to produce subùe low-relief changes in dip at ttie basement Ievel, Beneath the southem Alberta Leduc shelf margin, for instauce, basement arching IocaUy affects the thickness of Cambrian stratigraphy. In mm, Upper Cambrian topography created by on-structure thinning of underlying units is accentuated thniugh simple drape or differential compaction. This process of inheriting underlying topography is transferred ùIto the Devonian systern where on-structure initiation or thickening of carbonates is discemed. To the west, topographic inheritance in the form of drape of Cambrian &rata over a low-angle basement hinge line appears to have influenceci the western edge of the Bashaw reef complex. Conversely, the enigmatic Rimbey-Leduc-Meadowhk xeef chah does not scem to correhte with any real basement structure identifid on seismic data. However, the Homeglen reef, as imagecl by available intersecting seismic profiles, may not necessanly be representative of the encire chah Furthemore, the possibility of seismically unresolvable undulations existing on p~- Leduc reflectors dong this trend cannot be ruled out. On a Iarger scde of investigation, a broad Bouguer gravity anomaly, bclieved to be related to part of the STZ, coincides with a Figure 8-1. Modes of basement-Devonia. carbonate interaction in central Alberta (ckarbonate buildup). (A) Direct basement fault controi on reef initiation, e.g. some kgRiver Formation patch reefs, eastem Peace River Arch. (B) Indirect basement control on reef initiation through inhexïted topography, e.g. some Swan Hiils Formation reefal bddups of west-centrai Aiberta and some Leduc Formation buiIdups of est-central Alberta- Raised topography on the basernent suffice is schematidy shown to be influenceci by laterally varying basement lithologies which are bounded by low-angle stnictural or lithological contacts.(C) Basement fault control on local dolomitization, e.g. Swan Hills Formation upper plaiform of west-central Alberta,

327 significant part of this chai.My interpretation is that some fonn of indirect regional relationship probably existai between these two kaûms.

8.4.2 Syn-depositional structures At some localities in central Alberta, detailed interpretation of drillcore by other wotkers, coupled with seismic &ta interpreted in this study, bas yielded direct or indirect evidence of faulting that was apparently active at the same the that Middle and Upper Devonian carbonates were being deposited- There is geological evidence for tectonism occurrhg synchronously with the deposition of Keg River Formation carbonates in noah-central Alberta, Cyclicity has been previously observed fiom drillcore recovered hmwithin these carbonate buildups. For example, Campbell (1987) identified Granite Wash detritus, eroded from uplifted fault bloch, to be interbedded with Keg River carbonates. This contempomy depositional relationship is compatile with recurrtnt movement on basement faults. Sirnilarly, at Carson Creek North reef, west-central Alberta, my interpretation is that minor tectonic adjusments have indirectly contributeci to the Iocalizing or reinitiating of Swan Hilis Formation reef-stage growth. However, subtle displacements presumed to be associateci with such murgent basement activity are UnIikely to be resolved on the avadable seismic data. In this case, interpreting syn-depositional structures within Swan Hills Formation carbonates largely depends upon carrelating inferred structural offsets of the basement surface with lateral variations in reef stage pwth. The reactivation of small-scale faulting may also have influenced interior reefal facies in adjacent carbonate buildups in west-central Alberta.

8.4.3 Post-depositional structures Movement on basement faults occming after deposition of Middle and Upper Devonian carbonates provided a potential mechanismmfor dolomitizïng fluids to locally access carbonates. Porosity enhancement through fault- and fracturecontrolled dolomitization is thought mainly to affect carbonates found in north- and west-central Alberta. For example, in the eastern Peace River Arch area of norih£enttal Alberta, basement faults reactivated after the termination of reef growth may have been responsible for the hydrothennal dolomitization of Keg River Formation carbonates. Similarly, in west-central Alberta, discrete linear trends of dolomite occur in the otherwise tight limestone Swan Hills Formation upper platform. Hem, Late Cretaceous reactivation of deep-seated faults is thought to be at least partly responsible for such regular trends (Figure 8-1C). Post- 328 depositional stnictures may ahact as a seal to compmmentalize a carbonate teservoir. Therefore, establishing the presence of basement faulting reactivated after deposition of a &nate buildup is potentially signincant m terms of field development

Although several examples have been included in this dissertation where reactivated basernent faulting and the morphology of the basernent surface were believed to play a role in the depositional history of carbonate reefs and platforms, it does not mean that the basement was necessarily involved with every carbonate buildup in central Alberta Analysis needs to be undertaken on a case-by-case basis in order to better cons& the relative importance of basement stnicture on the initiation, development and diagenesis of a particular reef.

8.5 Implications for future hydrocarbon exploration By exposing stmcturai grains in basement terranes, seismic-reflection profihg has assistecl in the recognition of geological inbentance in central Alberta-a relationship of potentiaiiy substantiai economic importance. Generally, this aspect of basin evolution is underestimated, in part because many swp faults are often thought to accommodate only limited offsets and are poorly resolved with seismic reflection techniques. Furthemore, many subtle basement strucnires infdfrom seismic data are not detected using regional potential-field data. In this study, both direct and indirect relationships between reef morphology and basement structure and lithology have been interpreted for several hydrocarbon-producing areas of central Alberta. It may be possible to predict other carbonate plays in this fairly mature part of the Western Canada Sedimentary Basin by the regional extrapolation of interpreted basement trends to adjacent areas. In addition to influencing the loci and gros morphology of carbonate buildups, mctivation of basement faults may have created conduits, in places, for the vertical migration of unsaturateci waters or hydrocarbons to suitable Devonian carbonate reservoirs, or the upward migration of deeper fluids leading to dolomitization or base-metal accumulations (Nesbitt & Muehlenbachs, 1993). Furthemore, the hydrocarbon-bearing potential of a reservoir might be improved through fracture-related secondary porosity. An understanding of the origin and trends of dolomitization, in particular, is considemi by the present author to be of paramount importance to future hydrocarbon expIoration of Devonia. carbonates, especially in areas Iike west-central Alberta. It is c0nceiva.eülat some hard-mdetect traps m other Paieozoic and Mesozoic SUCCeSSions are aIso systematidy mtroIied by deeper structures in the basement, thereby offering a means for the prediction and detection of such often elusive targets. The economic importance of recognizing the different modes of tectonic inheritance that rnay exist in a particular area is fuaher highlighted by examining the regional alignment of selected oil and gas fields in parts of central Alberta For instance, reservoir trends in east-central Alberta are indirectly indicated in Figure 8-2, which shows ail welis penetrating at Ieast the mid- Cretaceous Base of Fish ScaIes Zone (cf. Misra et al., 1991). Because most targets were most likely defined independently of one anothcr, the obsewed linear regional arrangements of some wells re!flects a genuine alignment of resemoirs. Significantly, many of these trends contain fields produchg not dyfrom carbonates but aiso hmcMcs that were considered purely stratigraphie in nature. It is possible that ma-of these reservoirs roughly coincide with mctural or lithological boundaries in the basement. Overall, there would seem to be considerable potential for the discovery of additional resewoirs using integrated exploration philosophies oriented toward the basement-control models outlined in this dissertation,

8.6 Future work and recommendations More work is needed to mess the degree of local subtle tectonic heredity present in the Phanerozoic cover of this extensive study area. This requires the interpretation of additional seismic profles recorded with sufficiently long listening times (about 8 seconds) to image the basement The process of extended correlation, describexi in Chapter 7, provides a way of incorporating some conventionai oil indu- seismic data into such future studies. This method only requires data to be acquhd with an efficient ViBoseis source and recorded in an uncorrelated format Potentially, this makes a very large database avaiiable for delineating the local structure of the shailow basement and the recognition of additional subtle basementaver mlatimships.

The methodology employed in this study may be similarly applied in other areas of the Western Canada Sedimentary Basin where sufficient data is available. For instance, two other LITHOPROBEtransects, PME(Peace River Arch Industry Seismic Experiment) and SALT (Southem Alberta Lithospheric Transect), have been acquired and processed since 1994. These data may be evaluated for corresponding modes of interaction between Figure 8-2. Location map of weiis penetrating the base of Fish Sdes Zone (mid-Cretaceous) in east- central Alberta, updated to February 1995. On viewing this rnap obliquely, Iike a seismic section, and slowly rotating it, numemus weii alignments becorne apparent. Such prefd arrangements reflect alignrnents of expIoration targets, perhaps dong reactivated basement faults.

332 the basement and the scdimentary covg. In the ar#is cuvercd by these mmcts, a matchhg procedure codd be equally applied to impmve our understanding of the factors iduencing Mesozoic clastic lithosbratigraphy, m addition to Dcvonian carbonate morphology.

One of the major problems encountcred in the intqmtatim of crustal-scale seismic profila in this study is the inability to completely remove multiples direcdy below the basement surface. Considering the structural significance of tracking dippmg refiectors to offsets at this interface, the removal of these featuns is important for any future wodc mvolving these particular seismic lines. By comparing these data with other LITROPROBE transats in Alberta that have recently been acqumd and processed ushg conaasting parameters, it may be possible to qualify whether the presence of such multipies varies si@cantly hmarea to area and if they can actually be removed hmugh the application of irnprovd acquisition and processing parameters.

Recently, high-resolution aeromagnetic surveys have been conductcd over areas of Alkm and MC British Columbia, but rrmain mostly proprietary at present Some of the few published examples include tfiose of Ebner et al. (1995) and Berger et al. (1996). In conaast to the regional potential-field data utïlized in this study, such surveys provide significantly improved capabilities for mapping structures, both in the basement and sedimentary cover. Enhanced reso1ution is achieved through advanced data acquisition Oower altitudes, tighter line spacing and broad-band fkequency retention) and data processing techniques (e.g. microlevelling) (Berger et al., 1996). More densely sampled surveys allow gridding at a smaller grid-ceII size. This means subtle anomalies associated with intrasedimentary faults and fxactures may now be mapped, pnsumably because they have been mineralized in some way. Applying the horizontal-gradient vector (HGV) technique outlined in this study to such hi@-resolution &ta may help to delineate an in&edirnentary magnetic fabric. Ail told, there would seem to be a gnat potential for the application of such sweys to betîer constrain interpretation of the local effects of tectonic heredity on carbonate deposition in central Albena. References

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