Along-strike variations and AUTHORS · 2017-11-17 · geology and tectonics, particularly in the Cana-dian Cordillera, has provided new insights on the structure and tectonic evolution

AUTHORS

Michael A. Cooley � Queen’s University,Geological Sciences and Geological Engineering,Miller Hall, Kingston, Ontario, Canada K7L 3N6;[email protected]

Michael A. Cooley is a Ph.D. graduate (2007)from the Department of Geological Sciences andGeological Engineering, Queen’s University,Kingston, Ontario, Canada. His structural geol-ogy background includes detailed mappingof complexly deformed rocks, from polydeformedmetamorphic rocks to nonmetamorphosedsedimentary rocks in thrust and fold belts. Heis currently a structural geology consultantfor the mineral exploration industry.

Raymond A. Price � Queen’s University,Geological Sciences and Geological Engineering,Miller Hall, Kingston, Ontario, Canada K7L 3N6;[email protected]

Along-strike variations andinternal details of chevron-style,flexural-slip thrust-propagationfolds within the southernLivingstone Range anticlinorium,a paleohydrocarbon reservoirin southern AlbertaFoothills, CanadaMichael A. Cooley, Raymond A. Price, John M. Dixon,and T. Kurtis Kyser

Raymond A. Price is a professor emeritus at theDepartment of Geological Sciences and Geo-logical Engineering, Queen’s University, Kingston,Ontario, Canada. His research in structuralgeology and tectonics, particularly in the Cana-dian Cordillera, has provided new insights onthe structure and tectonic evolution of the Cor-dilleran foreland thrust and fold belt in Canadaand for other thrust belts worldwide.

John M. Dixon � Queen’s University, Geo-logical Sciences and Geological Engineering,Miller Hall, Kingston, Ontario, Canada K7L 3N6;[email protected]

John M. Dixon has taught structural geologyat the Department of Geological Sciences andGeological Engineering, Queen’s University,Kingston, Ontario, Canada. His research on thrustand fold tectonics has generally focused onanalog (centrifuge) and numerical (finite ele-ment) modeling. Dixon is currently associate vicepresident Academic-International at Queen’sUniversity, but continues his research while onpermanent administrative leave.

T. Kurtis Kyser � Queen’s University, Geo-

ABSTRACT

The Livingstone Range anticlinorium marks a hanging-wallramp across which the Livingstone thrust cuts up eastward ap-proximately 1000m (∼3280 ft) between regional decollementsin the Devonian and the Jurassic strata. It is well exposed andprovides actualistic models for exploration of analogous sub-surface structures. More than 30 km (>19 mi) of detailed map-ping along strike reveals generally gradual changes, punctuatedby abrupt terminations or offsets of structures at northeast- toeast-trending tear faults, interpreted to be reactivated basementfaults. Some folds plunge out at tear faults, forming domal cul-minations between them.Hinge zones of chevron-style, flexural-slip thrust-propagation folds display a distinctive pattern of ramp-flat thrusting comprising stacked detachment thrusts, each ofwhich emerges from a different zone of interbed slip in thebacklimb and deflects the hinge zone eastward. Each succes-sively lower detachment thrust dies out in the hinge zone justbelow an overlying one. Displacements on the detachments,rotation of fold limbs, and interbed flexural slipwere integratedkinematically. Thrust-propagation folding involved a form of

logical Sciences and Geological Engineering,Miller Hall, Kingston, Ontario, Canada K7L 3N6;[email protected]

T. Kurtis Kyser is a professor of isotope geo-chemistry and the director of the Queen’s Facilityfor Isotope Research at the Department of

Copyright ©2011. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received September 5, 2007; provisional acceptance December 7, 2007; revised manuscriptreceived April 6, 2010; final acceptance January 27, 2011.DOI:10.1306/01271107097

AAPG Bulletin, v. 95, no. 11 (November 2011), pp. 1821– 1849 1821

Geological Sciences and Geological Engineering,Queen’s University, Kingston, Ontario, Canada.His research interests include isotope geochem-istry, origin and chemical evolution of the earth,evolution of fluids in basins, low-temperaturegeochemistry, geochronology, environmentalgeochemistry, and fluid-rock interactions.

ACKNOWLEDGEMENTS

This research was supported by Natural Sciencesand Engineering Research Council grants toRaymond Price, John Dixon, and Kurt Kyser. Weacknowledge the financial assistance of thesponsors of the Fold Fault Research Project. Field-work benefited greatly from the field assistanceof Greg Cameron, Scott Conley, and Kyle Larson.We are grateful for the constructive criticismsof Margot McMechan, Daniel Ciulavu, ChristopherK. Morley, and Alfred Lacazette. Digital topog-raphy was provided at nominal cost by AltaLISLtd., and access to well logs was provided byNexen.The AAPG Editor thanks the following reviewersfor their work on this paper: Daniel Ciulavu,Alfred Lacazette, and Christopher K. Morley.

DATASHARE 39

Unbroken electronic versions of Figures 5, 6,and 9 are accessible in PDF format on theAAPG Datashare website as Datashare 39 atwww.aapg.org/datashare/index.

1822 Chevron-Style Thrust-Propagation Folding

cataclastic flow: individual blocks of rock delimited by faults,joints, and sheared bedding surfaces underwent minor relativedisplacements with little or no internal deformation, despitelarge translations and rotations of the thrust sheet. Pressuresolution and vein formation were widespread but were minorcomponents of the deformation. Complex fracturing withinthe anticlines is dominated by conspicuous widely spaced(∼150m [∼490 ft]) transverse (east-northeast–striking) zonesof intense fracturing that transect hinge zones and limbs. Thesepermeable zones likely originated by reactivation of basementfaults, and their significance may not be predictable by cur-vature analysis.

INTRODUCTION

Thrust-propagation folds are common structures within fore-land thrust and fold belts. As thrusts propagate upsectionthrough stratified sedimentary rocks, folding occurs at the tiplines of thrust faults if the rate of fault propagation is less thanthe rate of displacement. Folding compensates for the short-ening deficit in thrust propagation. The geometry and kine-matics of thrust-propagation folds have been discussed bymany authors, including Dahlstrom (1969, 1970), Williamsand Chapman (1983), Jamison (1987), Suppe andMedwedeff(1990), Wickham (1995), and Allmendinger (1998). Previousmodels of thrust-propagation folding demonstrate how displace-ment along the fault diminishes to zero at the tip line (Figure 1).A few have discussed how fold shapes can be geometricallyrelated to such variables as ramp angle, fold interlimb angle, anddegree of thickening or thinning of the forelimb (Figure 1B);how they develop self-similarly as they grow (Figure 1C); andhow growth strata can be used to predict fold shape and toexplain how these folds develop (Figure 1D).

Thrust-propagation anticlines form important hydrocarbontraps, but like any reservoir, they are difficult to study exceptthrough the limited information provided by seismic imagingand drilling data. The steeply dipping limbs of this fold style arenot easily imaged on seismic reflection profiles, and as a result,it is difficult to locate the hydrocarbon-favorable hinge zones ofthese folds unless suitablemodels depicting true fold shapes areavailable. Structures can also change considerably along strike,and exploring for additional reservoir potential along strike isrisky unless geologic information is available to help predictthe possible changes. A further goal for studying any reservoiris to characterize the internal fault structures and fracture pat-terns, as these provide important pathways for hydrocarbon

, Livingstone Range, Southern Alberta

migration and trapping. However, even with bore-hole imaging and fracture predictionmodeling fromfold curvature analysis, a true or close approxima-tion of fracture patterns in a deeply buried reservoiris difficult to achieve. An invaluable method forunderstanding a particular reservoir style is to ex-amine analogous structures that are exposed at thesurface in outcrop.

An important, accessible, and verywell-exposedanalog to thrust-propagation fold-related hydro-carbon reservoirs lies in the southern part of the

Livingstone Range anticlinorium (LRA) in thefoothills of the southern Canadian foreland thrustand fold belt (Figure 2). The LRA is a long (65 km[40 mi]) narrow (<5 km [<3 mi] wide) north- tonorthwest-trending horizontally plunging structuralculmination of the Mississippian platformal carbon-ate rock that stands above the surrounding deformedJurassic and younger foreland basin strata of thefoothills belt. At the northern end, the anticlinoriumterminates along the eastern side of the broad domalPlateauMountain culmination; at the southern end,

Figure 1. A few thrust-propagation fold models. Panel Ais from Williams and Chapman(1983), defining displacementand distance of offset of markerbeds. Panel B is from Jamison(1987), showing how fold shapeis related to the fault rampangle, the fold interlimb angle,and the degree of thickeningor thinning of the forelimb, wheret represents the original thick-ness of a stratigraphic unit andtf is the thickness of the unitin the forelimb. Panel C is fromSuppe and Medwedeff (1990),showing a simple thrust-propagation fold growing self-similarly. Panel D is fromWickham(1995), showing a simple thrust-propagation fold that hasgrowth strata accumulating dur-ing fold development. In all ofthese models, the tip line of thethrust is still within the area ofthe model and the fold has notbeen detached from its footwall.

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it terminates abruptly as south-plunging conicalfolds just north of the Crowsnest Pass Highway(Figure 2). The LRA coincides with amajor hanging-wall ramp across which the Livingstone thrust cutsapproximately 1000 m (∼3280 ft) upsection froma regional decollement in lime mudstones of the

1824 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

upper part of the Devonian Palliser Formation toanother regional decollement within the marineshale of the Jurassic Fernie Formation. The match-ing footwall cutoff is located more than 30 km(>19 mi) to the west, under the Lewis thrust sheet(Price, 1981, 1994).

Figure 2. Regional geologicsetting of the study area (shownin Figure 3) at the southern endof the Livingstone Range, south-ern Alberta Foothills. (A) Regionalmap showing the geologic fea-tures described in the text.(B) Geologic map modified fromWheeler and McFeely (1991)and Price (2007, personal com-munication). Cross section XX′modified from Price (2007, per-sonal communication). Crosssection YY′ modified from unpub-lished cross section of P. MacKay(2003, personal communication).

nge, Southern Alberta

An important objective of this research wasto document the along-strike changes in geologicstructures along the southern approximately 30 km(∼19mi) of theLivingstoneRange (Figure 2), whichis themost deeply eroded and the best exposed partof the structure. The internal structure of the LRAvaries along strike as individual anticlines emerge,increase in size, and die out (Figure 3). These changesare generally gradual, but locally, they occur abruptlyacross east- or northeast-trending “tear faults.” De-tailedmapping of these transverse structures revealshow they may have originated and how they affectadjacent deformation during thrusting and folding.

A distinctive characteristic of the LRA is thepresence of steeply dipping east-verging blind thrustfaults that extend along the hinge zones of mostanticlines. These were identified in the central partof the LRA by Douglas (1950) and illustrated inDouglas et al. (1970); however, they do not appearon the 1:50,000-scale geologic map and structuresections of the southern part of the LRA by Norris(1993). Each blind thrust separates outward-facingplanar fold limbs that merge above the tip line ofthe thrust as a concentric parallel fold. The result-ing chevron-style, flexural-slip thrust-propagationfold differs significantly from the fault-propagationfold models outlined by Dahlstrom (1969, 1970),Williams and Chapman (1983), Jamison (1987),Suppe and Medwedeff (1990), Wickham (1995),and Allmendinger (1998). Most previous modelsdo not address the internal details of deformationthat occur within the fold cores; they do not showminor structures such as the blind thrust faultswithin the hinge zones like those observed in theLRA. This research does not compete with otherthrust-propagation fold models; it complimentsthembydocumenting in greater detail the geometryand kinematics of structureswithin the fold cores. Inaddition, the tip lines of the main underlying prop-agating thrusts in most previous models are stillwithin the field of view of the diagrams, and thefootwalls are still attached to the hanging-wall anti-clines (Figure 1). In the LRA, the hanging-wallstructures have been displaced from the footwallbymore than 30 km (>19mi) (Price, 1981, 1994),and this may have some relevance to the uniquechevron-style geometry of these folds.

Figure 3. Geologic map of the study area. The southern Living-stone Range anticlinorium (LRA) comprises generally north-trending segments that are separated by regularly spaced cross-strike discontinuities. Most are tear faults marked by abrupt changesin fold plunges and dextral offsets of folds, but the southern one(Rock Creek discontinuity) may be a blind tear fault or lateral ramp.The location of this map is shown in Figure 2. The cross sectionallocations depicted in this figure are for sections illustrated inFigure 6. Contoured equal-area plots of poles to bedding in equal-area projection from the lower hemisphere, plotted using thecomputer program GEOrient 9.1 (R. J. Holcombe). Jf = Fernie For-mation; Cm = Misty Formation; Ce = Etherington Formation; Cmh =undivided Mount Head Formation; Clvtv = Turner Valley Memberof the Livingstone Formation; Cb = undivided Banff Formation.

Cooley et al. 1825

An additional objective of this research was todocument the system of faults and fractures withinthe thrust-propagation fold cores in the southernpart of the LRA. Not only did these faults andfractures once provide important fluid pathwaysfor hydrocarbon migration and trapping (most frac-tured and faulted zones in the LRA contain hydro-carbon residues), making them important analogsfor subsurface reservoirs, but understanding theirkinematics can reveal how this style of fold forms.

Fieldwork for this project (Cooley, 2007) en-tailed three field seasons of geologic mapping at ascale of 1:10,000 and included examination andmeasurement of stratigraphic sections through theCarboniferous strata and extensive sampling of theorientations of bedding, fractures, faults, slickenlines,and kinematic indicators and veins. The geochem-istry of the vein minerals, which provides importantinformation about the nature and evolution of thefluids that were flowing through the fractures, is thesubject of a separate article within this journal issue(Cooley et al., 2011).

REGIONAL GEOLOGIC HISTORY

The foothills and rockymountains of southwesternAlberta form the eastern margin of the Cordilleranforeland thrust and fold belt. The thrust and foldbelt is a northeast-tapering accretionary wedge com-prising sedimentary strata that have been scrapedoff the underriding Laurentia craton and trans-ported eastward with the overriding Intermontanesuperterrane, which is a tectonic collage of oceanicvolcanic arc and sedimentary rocks that was ob-ducted over and accreted to the western margin ofLaurentia during the Late Jurassic to Paleoceneconvergence between Laurentia and subductionzones along its western margin (Monger and Price,1979; Price, 1981, 1994).

The structure of the accretionary wedge isdominated by east-verging listric thrust faults thatflatten with depth and merge into a basal decolle-ment situated just above the contact between thesedimentary cover and the underlying Paleopro-terozoic basement (Bally et al., 1966). Thrust-propagation and fault-bend folds developed during

1826 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

the thrusting, and older thrust faults have beenfolded along with underlying strata by displace-ments on younger, deeper thrust faults (Douglas,1950;Dahlstrom, 1970; Price, 1981). The displacedand deformed rocks include the Belt-Purcell strata(Figure 2A), which accumulated in a Mesoprote-rozoic intracontinental rift (Price and Sears, 2000;Sears and Price, 2003); the Neoproterozoic toJurassic Cordilleran miogeocline, which accumu-lated along the western rifted continental margin ofLaurentia; the platform cover on Laurentia, whichis the lateral equivalent of the miogeocline; and thesynorogenic foreland basin deposits that accumu-lated in front of the advancing accretionary wedgeand were partly incorporated in it. In the FrontRanges, imbricate sheets of cratonic platform car-bonate strata form conspicuous linear mountainranges. In the foothills, thinner imbricate slices offoreland basin siliciclastic strata form more sub-dued topography, but with isolated linear moun-tain ranges underlain by local culminations thatexpose Paleozoic platformal carbonate rocks.

STRATIGRAPHY OF THE SOUTHERNLIVINGSTONE RANGE ANTICLINORIUM

The southern LRA comprises Upper Devonian toMiddle Jurassic cratonic platform strata that areoverlain by Late Jurassic and younger foreland ba-sin deposits (Figure 4A). The uppermost approxi-mately 620 m (∼2035 ft) of the Carboniferousstrata, which was examined in detail for this study(Cooley, 2007), is described below.

The basal approximately 150 m (∼490 ft) ofthe Livingstone thrust sheet is not exposed at thesurface, but it has been intersected by many oilexploration wells, including two in the study area(Figure 3, Texaco 3-20-9-3W5 Livingstone Eastand Shell 10-33-7-3W5 Imperial Burmis). It alsohas been imaged on seismic reflection profiles inareas to the north andwest of the study area (Deline,2003). The lowest unit in the thrust sheet com-prises approximately 60 m (∼200 ft) of the UpperDevonian Palliser Formation lime mudstone. Thisupper part of the Palliser Formation overlies amajorregional decollement along the Livingstone thrust

nge, Southern Alberta

(Price, 1981). The Palliser Formation is overlainby approximately 10 m (∼33 ft) of the UpperDevonian–lower Mississippian black shale of theExshaw Formation, a regionally important hydro-carbon source rock (Obermajer et al., 1997) andpossible seal to hydrocarbon migration. The over-lying Banff Formation consists of approximately320 m (∼1050 ft) of the Lower Mississippian ar-gillaceous silty and cherty limestone, but only theupper approximately 230 m (∼755 ft) is exposedin the study area (Figure 4B). The lower approxi-mately 150 m (∼490 ft) of the exposed Banff For-mation consists of dark-gray, cherty, argillaceous,and silty limestone. This is overlain by two con-spicuous and laterally consistent marker units: alower unit that consists of approximately 45 m(∼150 ft) of coarse crinoidal grainstone (Figure 4B)and an overlying unit of approximately 60 m(∼200 ft) of lime mudstone or micrite with palechert horizons (Figure 4B). The overlying Living-stone Formation comprises approximately 230 m(∼755 ft) of predominantly coarse crinoidal grain-stone and packstone and minor interbedded limemudstone and dolomitic mudstone with rare thinshaly layers that are tentatively identified as theTurner Valley Member. The Mount Head For-mation, which overlies the Livingstone Formation,ranges in thickness from 130 to 190 m (427 to623 ft). It includes three members dominated byfine-grained silty dolomite and shale (Wileman,Salter, andMarston members) that are interlayeredwith three members that consist mainly of limemudstone (Baril, Loomis, and Carnarvon mem-bers). The Carnarvon Member is a distinctive con-spicuously bedded dark-gray limemudstonemarkerunit that forms the uppermost approximately 40m(∼130 ft) of the Mount Head Formation. Theoverlying approximately 83-m (∼272-ft) intervalof the upper Mississippian Etherington Formationcomprises interbedded fine-grained dolomitic mud-stone, shale, siltstone, and sandy limestone. TheLivingstone, Mount Head, and Etherington forma-tions comprise the Mississippian Rundle Group,which is overlain by approximately 40m (∼130 ft)of the Pennsylvanian quartz arenite of the MistyFormation of the Spray Lakes Group. The JurassicFernie Formation, which is poorly exposed along

Figure 4. Stratigraphy of the study area. (A) Schematic east-west section showing stratigraphic relationships of the CrowsnestPass area (modified from Norris, 1993). Potential hydrocarbonsource rocks are the Cretaceous units, most of which containorganic-rich shale and currently structurally underlie the LRA.(B) Composite stratigraphic section of rocks exposed in the studyarea. Potential reservoir rocks are coarse limestone layers withinthe Livingstone and Mount Head formations.

Cooley et al. 1827

the margins of the anticlinorium and within a fewsynclines, is predominantly shale with minor inter-bedded wacke. The Upper Jurassic “Passage beds”at the top of the Fernie Formation is the oldest unitin the foreland basin succession, representing thefirst occurrence of coarser grained westerly derivedsediments shed from the emerging Cordillera. TheFernie Formation is overlain by the coal-bearingJurassic–Cretaceous Kootenay Formation.

Most nearby petroleum gas fields produce fromdolomitized rocks of the Mississippian RundleGroup, from the Mount Head Formation and/orthe Livingstone Formation. The shale of the FernieFormation is generally regarded as the cap rock fornearby gas reservoirs, but extensive evaporite layersin the Mount Head Formation likely play a criticalrole as top seal or part of the top seal in these reser-voirs (G. S. Soule, 2009, personal communication).

STRUCTURAL GEOLOGY OF THE SOUTHERNLIVINGSTONE RANGE

Overview

The LRA is an array of linked, en echelon, thrust-propagation, chevron-style folds involving theDevonian–Carboniferous platformal carbonate rocksand overlying Mesozoic siliciclastic foreland basindeposits. One to three anticlines occur in individ-ual cross sections. The structure of theLRAchangesalong strike, as individual anticlines emerge, increasein size, and die out (Figure 3). The changes aregenerally gradual, but locally, they occur abruptlyacross cross-strike discontinuities that coincide withnortheast-trending tear faults that are kinematicallylinked to slip on the underlying Livingstone thrust.The four cross-strike discontinuities that occur inthe study area (Figure 3) separate the LRA intofive distinct segments, four of which were mappedin detail during this study.

TheGreenCreek–MorinCreek segment,whichis at the southern end of the study area and is easilyaccessible (Figure 5), provides excellent exposuresof the core of the anticlinorium along the deepcanyons occupied by these two creeks. The southend of this segment is delimited by the northeast-

1828 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

trending Rock Creek discontinuity, which is markedby abrupt changes in the plunge of the Centre Peakanticline, the Belleview anticline, and the Caudronanticline. The Centre Peak anticline has a relativelyhorizontal plunge betweenMorin Creek andGreenCreek, but the plunge changes abruptly to 41° south,where it terminates southward as a conical fold at theRockCreek discontinuity (Figure 5). North ofGreenCreek, the Belleview anticline dies out northward asa north-plunging conical chevron fold (Figure 6,section AA′); but south of Green Creek, it main-tains a relatively horizontal plunge and is offset bya 200-m (656-ft) dextral jog at the Rock Creekdiscontinuity. TheCaudron anticline has a relativelyhorizontal plunge at Morin Creek; a gently south-ward plunge atGreenCreek; and an abrupt increasein plunge to approximately 29° south, where theanticline terminates as a south-plunging conicalfold at the Rock Creek discontinuity (Figure 5).The north end of this segment is marked by thenortheast-trending, steeply dipping Morin Creektear fault, which dextrally offsets the Centre Peakanticline and Caudron anticline by approximately250m (∼820 ft) and approximately 130m (∼430 ft),respectively. A domal culmination in the CentrePeak anticline occurs midway between the RockCreek discontinuity and theMorin Creek tear fault.

The Centre Peak segment of the LRA, whichextends from the Morin Creek tear fault to theDaisy Creek tear fault, contains three anticlines.These are, from west to east, the Cross anticline,the Centre Peak anticline, and the Caudron anti-cline. The Cross anticline is narrow, straight, and6 to 7 km (3.7 to 4.3 mi) long, with a culminationat Caudron Creek (Figure 5). South of the culmi-nation, the anticline terminates abruptly againstthe Morin Creek tear fault; north of the culmina-tion, it plunges gently northward and dies out 2 km(1.2 mi) northwest of Centre Peak as a conicalfold within the Fernie Formation (Figure 5). Themorphology of the Centre Peak anticline changesabruptly across the Morin Creek tear fault. Southof the tear fault, the Centre Peak anticline has achevron style and the hinge of the anticline at thetop of the Livingstone Formation occurs at an eleva-tion of approximately 2250m (∼7382 ft) (Figure 6,cross section BB′). North of the Morin Creek tear

nge, Southern Alberta

fault, the closure at the top of the Livingstone For-mation is at approximately 1900-m (∼6235-ft) ele-vation, and the fold is open and concentric (Figure 6,cross section CC′). The Centre Peak anticline ex-tends northward as an open fold to the vicinity ofCentre Peak, where it changes into a tight chevronfoldwith planar limbs andwith an east-verging thrustfault cutting up through the forelimb (Figure 6,cross sections DD′ and EE′). The Caudron anti-cline has a similar structural style across the MorinCreek tear fault—a tight chevron fold with an east-verging thrust fault in the hinge zone. However,the structural style changes significantly along striketo the north. On the northern side of the MorinCreek tear fault, a west-verging back thrust emergesin the upper backlimb of the fold, forming the “roofthrust” of an eastward-tapering thrust wedge thathas been displaced toward the core of the Caudronanticline (Figure 6, cross sections CC′ and DD′).The “floor thrust” of this wedge, which carries thetop of the Banff Formation in its hanging wall inthe core of the Caudron anticline on the northernside of Morin Creek tear fault (Figure 5), is inter-preted to be the eastern part of a thrust splay fromthe underlying Livingstone thrust that cuts up-section at a low angle to bedding from the PalliserFormation to the Lower Livingstone Formation.North of Centre Peak, where only the deeper levelsof the LRA are exposed, the hinge zones of boththe Centre Peak anticline and the Caudron anti-cline are represented by simple west-dipping thrustfaults on either side of the Centre Peak syncline(Figures 5, 7, 8; section EE′ of Figure 6).

The Ernst Creek thrust, which underlies theCentre Peak anticline, is a major structure that hasbeen traced northward 20 km (12mi) fromCentrePeak towhere it dies out in the core of an anticline atthe northern end of the study area (Figures 5, 7–9).Northward from the northern side of Centre Peak(UTM 5512500N in Figure 8) to the Daisy Creektear fault, the structure of the LRA is mainly dom-inated by the west-dipping homoclinal panel in thehanging wall of the Ernst Creek thrust (Figure 8).

The steeply dipping, northeast-striking DaisyCreek tear fault (Figure 8) is marked by approxi-mately 160 m (∼525 ft) of dextral offset of theErnst Creek thrust. It appears to abut the under-

lying Livingstone thrust and is likely formed inconjunction with slip along the Livingstone thrust.However, upward, the Daisy Creek tear fault abutsa detachment zone within the upper part of theLivingstone Formation that forms the floor of aneastward-tapering thrust wedge that has delami-nated the Livingstone Formation (Figure 6, crosssection FF′ and profile section GG′). The backthrust forming the roof of the wedge juxtaposes theLivingstone Formation over the CarnarvonMemberof the Mount Head Formation and the EtheringtonFormation. A small klippe of Livingstone Forma-tion occurs above the Etherington Formation at theridge crest, approximately 200 m (∼656 ft) northof cross section FF′ (Figure 8). The back thrustdies out rapidly southward within the upper partof the Livingstone Formation in the core of a smallconspicuous box anticline (Figure 8).

In the segment of the LRA that extends north-ward from the Daisy Creek tear fault to the PocketCreek tear faults, the structure of the LRA is dom-inated by the ErnstCreek thrust (Figures 8, 9). Thewest-dipping, north-trending homoclinal panel ofBanff Formation and Rundle Group strata in thehanging wall of the thrust forms the backlimb ofthe anticlinorium. Thin thrust slices and attenuatedfolds of Misty, Etherington, Mount Head, and up-per Livingstone strata in the footwall of the ErnstCreek thrust form the overridden forelimb.

The Pocket Creek tear faults comprise threesteeply dipping, east-west–trending transverse faults(Figures 3, 9). They occur within a conspicuoustransverse, east-west–trending dextral monoclinalflexure that has been superimposed on the entireLRA. The flexure, which is approximately 1500m(∼4920 ft) wide and has an amplitude of approx-imately 800 m (∼2625 ft), is situated west of thenose of an underlying north-plunging anticline thatinvolves the Livingstone thrust as well as the Me-sozoic rocks in its footwall (Figure 9). This anti-cline is part of a group of folds that deformed theLivingstone thrust andwere related to displacementson thrusts that developed below the Livingstonethrust (figure 15 of Douglas, 1950). The two largesttear faults end downward against the Ernst Creekthrust and appear to have been linked kinematicallyto displacement on it (Figure 9). The other smaller

Cooley et al. 1829

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Figure 5. Geologic map of the southern part of the study area.

Cooleyetal.

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F ure 6. Cross sections through the Livingstone Range anticlinorium. The locations of cross sections are shown in Figure 2 and on maps Figures 5 (sections AA′–EE′), 8 (sections FF′,G ′), and 9 (sections HH′, II′, and JJ′).

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in

Figure 7. View southward at Centre Peakanticline on the northeastern side of Cen-tre Peak (approximate location of crosssection EE′ in Figure 6). Banff Formation isjuxtaposed over the upper part of theTurner Valley Member along a thrust thatcuts up through the forelimb of theanticline.

Figure 8. Geologic map ofthe Daisy Creek area. The legendfor this map is the same asthat of Figure 9. Jf = Fernie For-mation; Cm = Misty Formation;Ce = Etherington Formation;Cmhc = Carnarvon Member ofMount Head Formation; Cmh =undivided Mount Head Forma-tion; Clvtv = Turner ValleyMember of the Livingstone For-mation; Cbm = Banff Formationmicrite unit; Cbg = Banff For-mation grainstone unit; Cb =undivided Banff Formation.

1834 Chevron-Style Thrust-Propagation Folding, Livingstone Range, Southern Alberta

tear fault ends downward against a minor thrustthat repeats part of the Mount Head Formationwithin the Ernst Creek thrust slice. All three faultsdie out upward within the Etherington Formation.Overlying strata, including a minor east-vergingbacklimb thrust that repeats part of the Etheringtonand Misty formations, have been folded by dextraldisplacement on the southern tear fault.

In the segment of the LRA that extends fromthe Pocket Creek area northward to the northernedge of the study area, the structure of the LRA isdominated by theErnstCreek thrust and by the twothrust-propagation anticlines that occur on eitherside of it, namely, the West Gap anticline and theEastGap anticline (Figures 9, 10). The Ernst Creekthrust, which juxtaposes the Banff Formation overRundle Group strata along the west limb of theEast Gap anticline, is separated from the WestGap anticline by the Thunder Mountain syncline.Both anticlines have planar limbs, and both containeast-verging thrust faults along their hinge zones(Figures 9, 10; sections HH′ and II′ in Figure 6).North of TheGap, both the Ernst Creek thrust andthe overlying thrust that emerges northward fromthe hinge of the West Gap anticline die out in thehinge of a simple east-verging asymmetric thrust-propagation anticline (Figure 9) (Douglas, 1950).

Discussion/Interpretation of the Geology

Cross-Strike Discontinuities (Tear Faults or LateralThrust Ramps)Most tear faults in the LRA are interpreted to bepreexisting northeast-trending faults that becamereactivated during thrusting and folding. TheMorinCreek tear fault (Figure 5) and the Daisy Creek tearfault (Figure 8) are interpreted to have been activebefore or during deposition of the lower part of theMount Head Formation because both of these faultscut vertically upward only a short distance into thelower part of the Mount Head Formation. Abovethis stratigraphic level, the faults appear to branchinto thrust faults (including back thrusts) that cutsubparallel to bedding. The Rock Creek disconti-nuity is probably the same type of reactivated faultstructure, but at its present level of erosion, the onlyevidence for the underlying tear fault is the abrupt

change in plunges of folds in the overlying MountHead Formation and younger strata (Figure 5). Thesame fault structure that formed the Morin Creektear fault likely continues in the Livingstone thrustsheet to the southwest and forms the lateral rampthat marks the abrupt northward termination ofthe Turtle Mountain anticline in Bluff Mountain,as mapped by Norris (1993).

The Mississippian activity on these faults issupported by regional-scale lateral facies changesand thickness changes within the Mount Head For-mation that indicate that Paleoproterozoic base-ment structures influenceddeposition of theMountHead Formation (Brandley et al., 1996). The LRAis underlain by the Vulcan structure of southernAlberta (Figure 2A), which is marked by a con-spicuous east-northeast–trending regional negativebouguer gravity anomaly, and paired negative andpositive aeromagnetic anomalies that are interpretedto mark an east-northeast–trending fault structurebetween the Medicine Hat Block to the south andthe Loverna Block to the north (Eaton et al., 1999;Clowes et al., 2002). The east-northeast–trendingtear faults within the LRA could have originatedfrom the Late Mississippian activity on the under-lying Vulcan structure.

The Pocket Creek tear faults clearly differ fun-damentally from the other three cross-strike dis-continuities because they are east-west striking andformed within a conspicuous transverse, east-west–trending dextral monoclinal flexure that has beensuperimposed on the entire LRA by underlyingyounger thrust-related structures.

Tear faults are important structures to identifyand understand in the subsurface for hydrocarbonexploration. In theLRA,many folds abruptly plungeout at or very near to tear faults, forming domalanticlinal culminations that would have been idealhydrocarbon reservoirs when the LRA was stilldeeply buried. Understanding the basement tec-tonics that underlie a thrust and fold belt may pro-vide clues to predicting the orientation of subsurfacetear faults that have affected thrusting deformation.

Transverse Zones of Intense Fracturing and Minor FaultingA series of regularly spaced east-west– to east-northeast–striking, steeply dipping zones of intense

Cooley et al. 1835

18

Figure 9. Geologic map of the northern part of the study area.

Cooleyetal.

1837

fracturing and minor faulting transect the north-south–striking limbs and hinge zones of foldswithinthe LRA in the vicinity of Green Creek, MorinCreek, and Caudron Creek (Figures 5, 11). Thetransverse zones of intense fracturing, which havea regular spacing of approximately 150m (∼490 ft)and commonly contain one or more discrete, butdiscontinuous, fault surfaces, appear to be restrictedmainly to the lowest part of the Mount HeadFormation and older units. They commonly formconspicuous gullies in the steeper slopes and cliffs(Figure 11A, B). The fractured zones are generallywider andmore strongly deformed in the forelimbsof the folds than in the backlimbs. Fault offsets arecommonly 1 m or less (≤3.3 ft), and the sense ofdisplacement varies from one fault to the next.Some individual faults terminate abruptly againstbedding surfaces but are aligned with other faultsthat occur several meters away. Slickenlines andslickenfibers are rare but, where preserved, aregenerally parallel with the bedding. Solid black hy-drocarbon residues are common on fault surfacesand in fractures (Figure 11D). In a few transversefracture zones, multiple sets of crosscutting calciteand/or dolomite and/or hydrocarbon veins provide

1838 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

evidence of intermittent reactivation of the trans-verse fracture zones.

These transverse zones of intense fracturingand minor faulting are interpreted to have formedduring flexural-slip folding by reactivation of a setof preexisting steeply dipping, east-west–trendingmegajoints that had developed preferentiallywithinthe Livingstone Formation and underlying units.Propagation of interbed slip along individual bed-ding surfaces during flexural-slip folding evidentlywas interrupted by the preexisting transverse frac-tures. Some interbed slips may have been trans-formed into strike slip within the transverse frac-ture zones, some may have been transformed backinto interbed slips at another stratigraphic level onthe opposite side of the fracture zone, and somedisplacement may have been dissipated within thedense network of fractures within the transversezone. These intermittently reactivated fracturedzones may have been important conduits for fluidmigration before, during, and after thrusting andfolding; moreover, they provide a good example ofthe types of structures that may be important path-ways for hydrocarbonmigration and accumulation.Their occurrence within the lowest part of the

Figure 10. View south at thefolds and related thrust faultsalong the north slopes of ThunderMountain.

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Mount Head Formation and underlying units sug-gests that they originated as faults that were activebefore deposition of the overlying Mount HeadFormation,making themsmaller analogs to the largerMorin Creek tear fault and Daisy Creek tear fault.

DETAILED STRUCTURAL GEOLOGY OF THECENTRE PEAK ANTICLINE

Observations

The Centre Peak anticline is illustrative of thethrust-propagation folds in the LRA, and its inter-nal structure is well exposed in the transverse can-yons ofGreenCreek andMorinCreek (Figure 5). Inthe hinge zone of the anticline, at creek level on thesouthern side of Green Creek canyon (Figure 12), anear-vertical thrust fault separates the west-facingbacklimb from the east-facing forelimb. The thrusttruncates bedding in the upper part of the coarse

lime grainstone unit of the Banff Formation thatoutlines the anticlinal hinge, but it is subparallelwith bedding in cherty dolomitic micrite of the topof the Banff Formation in the steeply dipping tolocally overturned forelimb (Figure 12; and sectionAA′ of Figure 6). Displacement decreases updipalong this fault to a point approximately 150 m(∼490 ft) above the creek, where it dies out in thehinge zone of the anticline within lime grainstoneof the lower part of the Turner Valley Member(Figure 12; section AA′ of Figure 6). Above thislocation, a detachment thrust emerges from thebacklimb of the anticline along a bedding detach-ment in the upper part of the micrite unit of theBanff Formation. This detachment thrust cuts up-section eastward in its footwall, truncating the un-derlying Turner Valley Member strata in the fore-limb of the anticline. At the highest levels in thestructure, this detachment thrust becomes steeperand concave to the west (Figures 12, 13). The dipseparation between the footwall and hanging-wall

Figure 11. Transverse zones of intense fracturing and minor faulting in the LRA. (A) Vertical air photo of the cross anticline at CaudronCreek. The east-west–trending gullies mark the locations of transverse east-west zones of fracturing and faulting that cut Turner ValleyMember (Clvtv) limestone in the core of the anticline at regularly spaced (∼150-m [∼490-ft]) intervals, but note that they do not extendinto the overlying Mount Head Formation (Cmh). (B) View to the west across the core of the Centre Peak anticline south of Morin Creek.The black arrows mark the locations of transverse zones of fracturing and faulting that cut through the east limb. The dashed line marksthe surface trace of the thrust fault that lies along the hinge zone of the anticline. (C) View to the west at intensely fractured limestonewithin a transverse zone of fracturing and faulting. White arrow points along the main fault contact where photo (D) was taken. Thehandle of the rock hammer in the circle at lower center of photo is 33 cm (13 in.) long. (D) The white arrow points along a steeplydipping vein of black hydrocarbon residue along a transverse fault surface. View is to the west. Pencil is 14 cm (6 in.) long.

Cooley et al. 1839

cutoffs of the top of the Banff Formation indicatesthat thrust displacement is approximately 250 m(∼820 ft) at that stratigraphic level in the struc-ture (Figure 12; section AA′ of Figure 6). As it cutsupsection in its hanging wall through the lower partof the Turner Valley Member, it becomes sub-parallel to the steeply dipping strata in the upper-most Turner Valley Member in its footwall; thrustdisplacement decreases, and the fault terminatesin the core of the anticline below an unbroken foldhinge in the upper part of the Turner Valley Mem-ber (Figure 13A). A network of subvertical, bedding-parallel, and subhorizontal extension faults deformsthe rocks in the upper forelimb and in the core of thefold (near the top of the photograph in Figure 13A).Individual marker beds within this fault network

1840 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

are represented by discontinuous elongate blocksfloating within a matrix of highly faulted and brec-ciated host rock fragments (Figure 13A).

The same basic pattern of chevron-style foldingis displayed in theCentre Peak anticline on the steepslopes along the northern side of Green Creek, butwith some significant minor variations (Figure 14).In the core of the anticline at creek level, the near-vertical thrust fault observed on the southern side ofthe fault continues to the northern side (Figure 14B).Asymmetric minor folds in the nearly vertical east-facing strata of the micrite unit of the Banff For-mation and Turner Valley Member in the footwallare kinematically congruent with the sense of inter-bed shear for the flexural-slip folding (Figure 14B).This near-vertical thrust fault cuts upsection along

Figure 12. View south at the core of the Centre Peak anticline as exposed on the southern side of the Green Creek Canyon (projectednorthward into section AA′ in Figure 6). Two east-verging thrust faults (thick dashed lines) extend along the hinge zone. The lower near-verticalthrust fault dies out updip within the fold hinge within the Turner Valley Member of the Livingstone Formation (Clvtv). The upper thrust faultemerges from a bedding detachment approximately 20 m (∼66 ft) below the top of the uppermost micritic unit of the Banff Formation(Cbm) and deflects the hinge zone eastward. The footwall and hanging-wall cutoffs of the top of the Banff Formation along this thrust fault(X and X′, respectively) have been offset approximately 250 m (∼820 ft) along this fault. Cmh = Mount Head Formation; Clvtv-b = brownmicritic marker unit within Turner Valley Member of the Livingstone Formation; Cbg = grainstone unit near the top of the Banff Formation.

nge, Southern Alberta

its hanging wall, and displacement decreases up-ward to zero where the fault abuts an overlyingbedding-parallel detachment within the lower partof the micrite unit of the Banff Formation in thebacklimb of the fold (Figure 14A). This detach-ment thrust deflects the hinge zone of the CentrePeak anticline approximately 50 m (∼165 ft) east-ward (Figure 14A) to where it curves upward tobecome steeply dipping and dies out in the hingezone of the anticline just below a higher beddingdetachment zone in the lower part of the TurnerValley Member. This higher bedding detachmentthrust deflects the hinge zone of the Centre Peakanticline approximately 150m (∼490 ft) eastwardas it cuts upsection eastward through the steeplydipping beds in it footwall (Figure 14A). The con-

spicuous detachment that occurs approximately20 m (∼66 ft) below the top of the Banff Forma-tion on the southern side of Green Creek canyon(Figure 12) is still discernable on the northern side,but displacement on it appears to die out abruptlynorthward (Figure 14A).

Additional details in the pattern of chevron-style folding in the Centre Peak anticline are seenon the steep slopes along the southern side ofMorinCreek (Figures 5, 15). Near creek level in MorinCreek canyon, a near-vertical thrust fault juxtaposesthe lower part of the grainstone unit of the upperpart of the Banff Formation in the west-facing limbwith the upper part of the micrite unit of the top ofthe Banff Formation in the east-facing limb. Dis-placement on this near-vertical thrust decreases

Figure 13. View south at the upper part of the core of the Centre Peak anticline exposed in the Green Creek canyon. (AA′) A network ofsubvertical and bedding-parallel thrust faults deforms the upper part of the Turner Valley Member (Clvtv) within the upper forelimb.Extension faults (dotted lines) offset bedding in the core of the anticline. (BB′) The detachment thrust fault that emerges from thebacklimb approximately 20 m (∼66 ft) below the top of the Banff Formation (Cbm) dies out upward within the zone of intense faulting inthe upper part of the forelimb. Cmh = Mount Head Formation; Clvtv-b = brown micritic marker unit within Turner Valley Member of theLivingstone Formation.

Cooley et al. 1841

upward and it dies out in the hinge zone of the foldwithin the micrite unit of the Banff Formation,where it is overlapped by a bedding detachmentzone near the base of the Turner Valley Member(Figure 15). This detachment thrust deflects thehinge zone of the anticline approximately 200 m(∼656 ft) eastward and abruptly curves upward,cutting upsection in its hanging wall and becomingsubparallel to the bedding in its footwall. At theskyline in Figure 15, the thrust fault cuts into the

1842 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

lower part of overturned Mount Head Formationin its footwall.

The distinctive pattern of ramp-flat thrustingthat occurs along thehinge-zone thrust systemof theasymmetric chevron-style Centre Peak anticline con-sists of a series of stacked detachment thrusts, eachof which emerges from a different zone of interbedslip in the backlimbof the anticline anddeflects thehinge zone. Each detachment thrust consists of twocontrasting segments. The lower segment, which is

Figure 14. View north at thecore of the Centre Peak anticlineas exposed on the northern sideof the Green Creek canyon. Theplanar limbs of the anticlineare separated by three east-vergingthrust faults that extend alongthe hinge zone. One thrustemerges from a bedding-paralleldetachment in the lower partof the micrite unit (Cbm), anda second thrust fault emergesfrom a bedding-parallel detach-ment in the lower part of theTurner Valley Member (Clvtv).The structurally lowest faultsegment (lower photo B) juxta-poses a ramp that cuts throughthe grainstone unit (Cbg) andmicrite unit (Cbm) of the topof the Banff Formation in thehanging wall with overturnedand steeply dipping micrite unit(Cbm) and Turner Valley Mem-ber (Clvtv) in the footwall.

nge, Southern Alberta

parallel to bedding in the less steeply dippingbacklimb of the fold, juxtaposes a hanging-wall flatwith a footwall ramp. The upper segment, which issubparallel with the steeply dipping forelimb, jux-taposes a hanging-wall ramp with what roughlyapproximates an overturned footwall flat. Each suc-cessively higher detachment thrust dies out in thehinge zone at approximately the same stratigraphiclevel from which an overlying detachment thrustfault emerges from a bedding detachment zone inthe backlimb (section AA′ of Figure 6).

Displacements on the individual detachmentsthat occur near the top of the Banff Formationwithin the Centre Peak anticline clearly vary alongstrike. As displacement on one detachment diedout, displacement on an overlying or underlyingoverlapping detachment increased or decreased.The amount of horizontal shortening and degree ofinterbed shear that occurred within the limbs ofthe Centre Peak anticline appear to be fairly con-stant along strike, but displacement on individualdetachment horizons that accommodated the short-ening was not.

Interpretation: Evolution of the CentrePeak Anticline

The well-documented configuration of the hinge-zone thrust system of the Centre Peak anticlineplaces tight constraints on the interpretation of thekinematics and mechanics of this asymmetric chev-ron fold. In this fold, interbed slip was unevenlydistributed throughout the fold limbs, both verti-cally and along strike. Extra interbed slip occurredalong successively higher discrete bedding detach-ment thrusts that propagated into the hinge zone ofthe anticline, resulting in an axial surface with con-spicuous jogs. These bedding detachment thrustsare analogous to limb thrusts of Ramsay (1974),which tend to form at the bases of thicker, morecompetent strata within chevron fold limbs. Thehighest bedding detachment thrust, which formedlast and terminates in the core of a concentric, par-allel, flexural-slip fold, records the arrested growthof the anticline. It provides an actualistic model forthe geometry of the structure before the develop-ment of each of the underlying detachment thrusts

Figure 15. View south at the core of the Centre Peak anticline as exposed along the southern side of Morin Creek canyon. In the lowercore of the anticline, a steeply west-dipping thrust fault juxtaposes the west-facing grainstone unit (Cbg) of the backlimb over the upperpart of the micrite unit (Cbm) in the east-facing overturned forelimb. An overlying thrust fault that emerges from a bedding detachmentat the base of the Turner Valley Member (Clvtv) in the backlimb of the fold truncates most of the Turner Valley Member (Clvtv) along itsfootwall. Near the skyline, this detachment thrust curves upward and juxtaposes a hanging-wall ramp through most of the Turner ValleyMember (Clvtv) with a footwall flat that cuts subparallel to overturned bedding in the lowermost Mount Head Formation (Cmh).

Cooley et al. 1843

that deflect the hinge zone. This sequence of de-formation is modeled in Figure 16 as a series ofcross sections that represent stages in the develop-ment of the structural interpretation shown in crosssection AA′ in Figure 6. The reconstruction useskink band migration and conservation of beddingthickness.

1844 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

The initial stage in the evolution of the CentrePeak anticline involved the propagation of theLivingstone thrust up a ramp from the regional de-tachment in the upper part of the Palliser Formationand into the upper part of the Banff Formation(Figure 16A), forming a first-generation thrust-propagation fold (Al Saffar, 1993). A decollementbreakthrough (Suppe and Medwedeff, 1990) oc-curred when the Livingstone thrust was deflectedinto a bedding detachment in the micrite unit nearthe top of the Banff Formation (Figure 16B). TheLivingstone thrust then propagated upward fromthe detachment in the micrite unit of the top ofthe Banff Formation and into the overlying Living-stone Formation, above which the Caudron thrust-propagation anticline began to develop (Figure 16B).At this stage, displacement along the Livingstonethrust became diverted into the hinge zone of theCentre Peak anticline (Figure 16B), initiating asecond-generation thrust-propagation fold (Al Saffar,1993). A new thrust branched upward from theLivingstone thrust, juxtaposing the Palliser detach-ment in its hanging wall over the truncated steeplydipping Palliser Formation and Banff Formationstrata in its footwall. As the new thrust propagatedupward, simultaneous limb rotation and antitheticinterbed slip within fold limbs caused fault cutoffsto lengthen and shorten because of thrusting trans-formed into shortening by flexural-slip folding. Thesecondary fault-propagation folding caused the hingeof the initial anticline to migrate westward, length-ening the forelimb (Figure 16C). The new thrustdied out in the hinge zone of the anticline withinthe lower part of the Banff Formation. As the sec-ondary thrust fault stopped propagating, a newthrust fault emerged from a detachment in thelower part of the Banff Formation in the backlimbof the anticline; it propagated upward through thesteeply dipping grainstone unit of the upper partof the Banff Formation in the forelimb and dis-placed the anticlinal hinge eastward and upward.As it died out in the upper part of the micrite unitat the top of the Banff Formation (Figure 16D), athird thrust fault emerged from a detachment ho-rizon near the tip line of the underlying thrust fault.It also displaced the anticinal hinge eastward andupward as it propagated upsection through the

Figure 16. Proposed kinematic evolution of the Centre Peakanticline based on cross section AA′ in Figure 6. Light-gray linesare originally vertical material lines that track relative displacement.

nge, Southern Alberta

steeply dipping forelimb of the anticline into theupper part of the Turner Valley Member, where itterminated in the core of the anticline. At this stage,the evolution of the Centre Peak anticline was ar-rested, preserving the relationships between flexural-slip folding and thrust propagation. Subsequent tothe deformation sequence outlined in Figure 16,thrust-propagation folding would have resumedwithin the Caudron anticline when the Livingstonethrust continued to propagate upsection throughthe Paleozoic strata before flattening out within adecollement in the Jurassic Fernie Formation.

During each step in the evolution of the CentrePeak anticline, external rotation of the fold limbswas accommodatedby antithetic interbed slipwithinthem.As the backlimb underwent top-to-the-westrotation, a synchronous antithetic top-to-the-eastinterbed shear within the backlimb occurred. Thisismainly expressed by the conspicuous offsets alongthrusts that emerge from bedding detachmenthorizons. As the forelimb underwent top-to-the-east rotation, a synchronous top-to-the-west shearwithin the forelimb occurred. This is illustrated byminor folds within the forelimb in Green Creek(Figure 14B). The cumulative bedding-parallel shearrecorded by thrusts emerging from detachmenthorizons in the backlimb of the Centre Peak anti-cline is approximately 30° (Figure 16E). This top-to-the-east bedding-parallel shear was likely alsodistributed within the Livingstone thrust sheet inthe region to the west of the Livingstone Range.

The detachment thrust faults that occurwithinthe backlimb of theCentre Peak anticline and theirassociated conspicuous jogs in the hinge zone arefeatures that are not included in many models ofthrust-propagation folding (Dahlstrom, 1969, 1970;Williams and Chapman, 1983; Jamison, 1987;Suppe and Medwedeff, 1990; Wickham, 1995;Allmendinger, 1998) (Figure 16). Although somemodels of thrust-propagation folds that are basedon kink-band migration (Jamison, 1987; Suppeand Medwedeff, 1990) do involve interbed slip,these models show much less than the approxi-mately 30° observed in the Centre Peak anticline.The Centre Peak anticline evolved further thanthe stages of deformation shown by most modelsof thrust-propagation folding (Figure 1).

Strata within the Centre Peak anticline havebeen subjected to several deformation events dur-ing their translation from relatively undisturbedsubhorizontal beds to their present position withinthe chevron fold of the Centre Peak anticline. De-formation within the rock mass included displace-ments along and across networks of thrust faults,extension faults, and other shear fractures that cutacross bedding. These discrete displacement dis-continuities bound individual blocks of rock thatunderwentmajor translation and rotationwith onlyveryminor internal deformation. Generally little tono evidence of penetrative strain exists. The finestdetails of sedimentary and fossil structures are wellpreserved within the fracture-bound blocks. Thisstyle of deformation, which is analogous to defor-mation in a granular or blocky aggregate in whichindividual fragments are free to move relative toone another along their bounding surfaces (Price,1967), is an example of cataclastic flow (cf.Wojtal,1986; Ismat andMitra, 2005). Slickenlines provideevidence of the direction and sense of shear dis-placements along small extension faults, contractionfaults, shear joints, and bedding. Surface markingsand crystal druze on joints, faults, and beddingplanes provide evidence of dilation between blocks.Pressure solution and stylolitization on some frac-tures and precipitation of carbonate and quartz indilatant zones along other fractures were wide-spread but wereminor features of the deformation(Price, 1967). Cataclastic flow was most intenseand conspicuous in the hinge zone of the CentrePeak anticline, where beds are offset and stretchednear the tip lines of bedding detachment faults thatpropagated out of the backlimb (Figure 13). In thislocation, extension faults cut previously formedthrusts, implying that the local stress state withinthe fold hinge changed from an earlier episode ofcompression to extension during the later stages offolding.

Fracture Analysis of the Centre Peak Anticline

Data and ObservationsFracture orientations were measured in the Living-stone Formation at six localities in the backlimb ofthe Centre Peak anticline and at 10 localities in the

Cooley et al. 1845

Figure 17. Fracture orientation data from the Centre Peak anticline plotted in an equal-area stereographic projection from the lowerhemisphere, with the bedding plane horizontal. The forelimb (east limb) of the fold contains transverse zones of fracturing and minorfaulting and also flexural-slip–related back thrusts that involve highly variable fracture patterns adjacent to fault surfaces. Areas in theforelimb that are not adjacent to faults exhibit simpler fracture patterns (e.g., stations M687A and M687B). Stereonet images weregenerated using the program GEOrient 9.1 (R. J. Holcombe). Ce = Etherington Formation; Cmhc = Carnarvon Member of the MountHead Formation; Cmh = undivided Mount Head Formation; Clvtv = Turner Valley Member of the Livingstone Formation; Cbm = BanffFormation micrite unit; Cbg = Banff Formation grainstone unit.

1846 Chevron-Style Thrust-Propagation Folding, Livingstone Range, Southern Alberta

forelimb (Figure 17). The data from each localityhave been rotated to restore the local fold axis tohorizontal and then rotated about the local foldaxis to restore bedding to horizontal. Three of thelocalities in the backlimb at Green Creek are fromunpublishedmeasurements made in 1963 by R. A.Price (2004, personal communication). At mostlocalities, a few dominant fracture sets are obvious,but locally, substantial dispersion exists in fractureorientations. Fractures are generally more numer-ous and variable in the forelimb than in the back-limb. In both limbs, fracture orientation patternsare highly variable from one locality to another.However, north-south–trending bedding-normalfractures (b–c joints) are common. At one locality(Figure 17; locality M687) the dominant fracturesets apparent in one stratigraphic unit (localityM687A) differ conspicuously from those withinan adjacent stratigraphic unit (locality M687B).At most localities within the zone of transversefracturing (Figure 17; localitiesM131,M686,M688,M689, M693), a conspicuous steeply dipping, east-west–trending dominant fracture set exists. Theintensity of fracturing and variability in orientationdiminish abruptly away from the zones of trans-verse fracturing.

InterpretationThe generally more intense fracture developmentin the forelimb relative to the backlimb of theCentre Peak anticline can be attributed to minorback thrust faults and extension faults, which aremoreprevalent in the forelimb than in the backlimb.The contrasting fracture patterns between adjacentstratigraphic units at location M687 (Figure 17),which is in the forelimb but isolated from fracturezones, suggest that individual segments of thestratigraphic section separated by bedding detach-ment zones deformed independently. The along-strike variability in fracture development, evident inFigure 17, illustrates the importance of the trans-verse fracture zones, which exhibit the greatestdensity and complexity of fracture orientations,implying extensive strain and strain softening ofthe rock mass and a loss of cohesion along thesezones. These zones would have been the mostfavorable conduits for hydrocarbon migration, an

interpretation that is supported by the presence ofhydrocarbon residues in the fractures and faultswithin them (Figure 11D).

CONCLUSIONS

TheLRA is awell-exposed analog to similar buriedhanging-wall ramp anticlines, such as the TurnerValley anticline (MacKay, 1991), which is a pro-lific hydrocarbon-bearing structure that underliesthe foothills approximately 40 km (∼25 mi) south-west of Calgary (Figure 2A).

Along-strike changes in the structure of theLRA are generally gradual, but locally, they occurabruptly across cross-strike discontinuities. Somefolds continue across tear faults with some degreeof dextral offset, andmany abruptly plunge out nearthem, forming domal culminations that may oncehave been ideal hydrocarbon reservoir structures.Identifying tear faults in the subsurface in thrustand fold belts is, therefore, an important objectivefor petroleum exploration. Predicting the locationsand orientations of tear faults may be facilitated bystudying the underlying basement tectonics. In theLRA, the origin of the northeast-trending tear faultsare likely related to Mississippian activity on thenortheast-trending Vulcan structure, an importantsuture within the underlying Paleoproterozoic base-ment. Both the Morin Creek tear fault and DaisyCreek tear fault, which are northeast trending andterminate upward at detachments within the lowerpart of the Mount Head Formation, likely origi-nated as northeast-trending, steeply dipping struc-tures that were active before or during depositionof the Lower Mount Head Formation. The PocketCreek tear faults, which trend east-west and cutthroughMountHead Formation and younger strata,are interpreted to have formed much later whena dextral transverse fault-bend fold flexure wassuperimposed on the anticlinorium as the Living-stone thrust sheet was being transported eastwardon underlying thrusts.

The presence of tear faults in the LRA impliesthat similar structures occur in the subsurface. TheMorin Creek tear fault lines up to the southwestwith the northern end of Turtle Mountain, likely

Cooley et al. 1847

representing a continuation of the same structure.The lateral jog in the LRA at The Gap overlies alateral ramp in the subsurface at Racehorse Creek(Deline, 2003) and possibly other subsurface lat-eral ramps along strike to the southwest.

The distinctive pattern of ramp-flat thrustingthat forms the hinge-zone thrust system of theasymmetric chevron-style Centre Peak anticlineconsists of a series of stacked detachment thrusts,each of which emerges from a different zone ofinterbed slip in the backlimb of the anticline anddeflects the hinge zone. Each detachment thrustconsists of two contrasting segments. The lowersegment, which is parallel to bedding in the lesssteeply dipping backlimb of the fold, juxtaposes ahomoclinal west-facing structural panel above ahanging-wall flat with an east-facing footwall ramp.The upper segment, which is subparallel with thesteeply dipping forelimb, juxtaposes a hanging-wallramp with what roughly approximates an over-turned footwall flat. Each successively lower de-tachment thrust dies out upward in the hinge zoneat approximately the same stratigraphic level fromwhich an overlying thrust fault emerges from abedding detachment zone in the backlimb (sectionAA′ of Figure 6). The highest bedding detachmentthrust, which terminates in the core of a concen-tric, parallel, flexural-slip fold, records the arrestedgrowth of the anticline. It provides an actualisticmodel for the geometry of the structure before thedevelopment of each of the underlying detach-ment thrusts that deflect the hinge zone.

The initial stages of thrust-propagation foldingin the Centre Peak anticline may have resembledthemodels described byDahlstrom, (1969, 1970),Williams and Chapman (1983), Jamison (1987),Suppe and Medwedeff (1990), Wickham (1995),and Allmendinger (1998) (Figure 1). However,the folds in the LRA underwent additional defor-mation that caused the fold hinges to tighten to aninterlimb angle of approximately 60° and length-ened the fold limbs, which essentially doubled thesize of the anticline and resulted in a distinctivestyle of chevron folding.

The cumulative bedding-parallel shear recordedby thrusts emerging from detachment horizons inthe backlimb of the Centre Peak anticline is ap-

1848 Chevron-Style Thrust-Propagation Folding, Livingstone Ra

proximately 30° (Figure 16E). This provides anindication of the top-to-the-east bedding-parallelshear that was likely distributed throughout theMississippian strata within the Livingstone thrustsheet in the region to the west of the LivingstoneRange. This shearing occurred during the earlieststages of thrust sheet development, as the Living-stone thrust beganmoving up the ramp that formedthe LRA, and not during the more than 30 km(>19mi) of eastward transport that occurred later.Similar amounts of internal shearing have poten-tially occurred in other thrust sheets in this and inother thrust and fold belts.

Fracture patterns that occur along the CentrePeak anticline in the Green Creek–Morin Creekarea are dominated by northeast-trending transversezones of intense fracturing that cut through foldlimbs and that formed during reactivation of awidely spaced (∼150m[∼490 ft]) preexisting jointset. The transverse fracture zones exhibit more in-tense fracturingwithin the forelimbs of the anticlinesin the LRA, which would have made this the mostpermeable part of the structure during hydrocarbonmigration and trapping when the LRA was stilldeeply buried. These transverse structures do notextend up beyond the lower part of the overlyingMountHeadFormation, implying that they initiallyformed before deposition of the Mount Head For-mation. The fractured zones are, therefore, struc-turally similar to the larger tear faults in the LRA,in that they all probably formed during the sameepisode of Mississippian activity on large fault struc-tures in the underlying Paleoproterozoic basement.

REFERENCES CITED

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