Structural Overprinting in the Northwestern Skeena Fold Belt(NTS 104B, H), Northwestern British Columbia1
by W. Loogman2, J-F. Gagnon2, J.W.F. Waldron3 and C.A. Evenchick3
KEYWORDS: Skeena Fold Belt, folds, faults, over print -ing
INTRODUCTION
The Skeena Fold Belt, as de fined by Evenchick(1991a), is lo cated in north west ern Brit ish Co lum bia, in the Stikine Terrane of the Intermontane Belt of the Ca na dianCor dil lera (Fig 1). The dis tri bu tion of the Skeena Fold Beltroughly mir rors the lo ca tion of the Me so zoic Bow ser Ba -sin, and the fold belt is pre dom i nantly de vel oped in theMid dle Ju ras sic – Early Cre ta ceous Bow ser Lake Group ofthe Bow ser Ba sin. The Early Ju ras sic Hazelton Group andthe Late Cre ta ceous Sustut Group are also af fected by fold -ing and thrust fault ing. Two re gional fold trends are pres entwithin the Skeena Fold Belt: north west-trending folds aredom i nant in the east ern two-thirds of the fold belt, andnorth east-trending folds are pres ent in the west ern third(Fig 1). Lo cally, these fold trends are found out side theirdom i nant do mains and over print each other (Evenchick,1991a; Evenchick, 2001). It is the aim of this study to eval -u ate over printed struc tures and their tim ing re la tion ships.Field work was con ducted in 2006 as a fol low-up to field -work car ried out in the sum mer of 2005 and re ported on byWaldron et al. (2006). This work ac com pa nies that re ported by Gagnon et al. (2007).
In an ef fort to aid in hy dro car bon ex plo ra tion withinthe Bow ser Ba sin, re cent stud ies of its pe tro leum po ten tialby the Geo log i cal Sur vey of Can ada and BC Min is try ofEn ergy, Mines and Pe tro leum Re sources (Osadetz et al.,2003; Evenchick et al., 2003) have fo cused on up dat ing thether mal ma tu rity mod els of Bustin and Mof fat (1989),which pre dicted un fa vour ably high heat flow to much ofthe Bow ser Ba sin, and on de vel op ing mod els for as sess ingthe ef fec tive ness of pe tro leum sys tems. In ad di tion to con -ven tional pe tro leum, there is po ten tial for coalbed meth aneand con ven tional coal. Min eral po ten tial ex ists in vol ca nicrocks on the flanks of the ba sin (Fig 1), as shown by thehigh-grade stratiform volcanogenic mas sive sul phide(VMS) Au-Ag de pos its at Eskay Creek (Roth et al., 1999).
Un der stand ing the struc tural his tory of the region willcontribute to exploration and evaluation of these resources.
REGIONAL STRATIGRAPHICFRAMEWORK
The mapped re gions of the Skeena Fold Belt ex poserocks of the Mid dle Ju ras sic – Early Cre ta ceous Bow serLake Group, in clud ing the Ritchie-Alger, Eaglenest andSkelhorne lithofacies as sem blages, de scribed in Evenchick and Thorkelson (2005). The lack of re gion ally cor re la tivebound aries in the Bow ser Lake Group makes the use of for -mal for ma tion/group des ig na tions dif fi cult. Syn chro nousde po si tion of shal low and deep ma rine fa cies oc curred; theBow ser Lake Group re flects an over all progradationaldepositional history.
The Ritchie-Alger as sem blage com prises sand stone,siltstone and shale, with rare chert-rich con glom er ate.Sheet-like in ter vals of fine to me dium-grained sand stone,sep a rated by shale in ter vals that are metres thick, have been in ter preted to re late to sub ma rine fan de po si t ion(Evenchick and Thorkelson, 2005). Ev i dence of turbiditeac cu mu la tion within the sub ma rine fan com plexes in cludes nor mal grad ing, groove casts, dewatering struc tures andBouma cy cles. The Ritchie-Alger as sem blage is es ti matedto reach thick nesses of 1800 m in the west ern third of theBow ser Basin (Evenchick and Thorkelson, 2005).
The Skelhorne lithofacies as sem blage com prises morethan 1000 m of siltstone, sand stone and con glom er ate(Evenchick and Thorkelson, 2005). Com mon me dium tothick-bed ded coars en ing-up ward cy cles, ma rine and plantfos sils, wave-gen er ated rip ples and bioturbation are in ter -preted to re cord a mod er ate-en ergy deltaic depositional en -vi ron ment (Evenchick and Thorkelson, 2005). TheSkelhorne as sem blage is most com mon in the west-cen tralpart of the Bowser Basin.
The Eaglenest lithofacies as sem blage com prises ahigh per cent age (up to 80%) of rusty- weath er ing con glom -er ate and sand stone, with mi nor siltstone and shale, andreaches thick nesses of 1000 m (Evenchick and Thorkelson, 2005). Con glom er ate clasts are well-rounded and well-sorted peb bles com posed pre dom i nantly of chert. Plant fos -sils, in clud ing sub stan tial si lici fied trees, are abun dant,whereas ma rine fos sils are rare. The abun dance of con -glom er ate, coars en ing-up ward cy cles and tree fos sils allsug gest a high-en ergy deltaic depositional en vi ron ment(Evenchick and Thorkelson, 2005).
EXISTING STRUCTURAL FRAMEWORK
The Skeena Fold Belt over lies the Stikine Terrane ofthe Intermontane Belt (Evenchick, 1991a). The Skeena
Geo log i cal Field work 2006, Pa per 2007-1 325
1 Geoscience BC con tri bu tion GBC0222 Department of Earth Sciences, University of Alberta,
Edmonton, AB T6G 2E33 Geological Survey of Canada, 605 Robson Street, Vancouver,
BC V6B 5J3
This publication is also available, free of charge, as colourdigital files in Adobe Acrobat® PDF format from the BCMinistry of Energy, Mines and Petroleum Resources website athttp://www.em.gov.bc.ca/Mining/Geolsurv/Publications/catalog/cat_fldwk.htm
Fold Belt was formed be tween the Albian and early Ter tiary (Evenchick, 1991a; Evenchick and Thorkelson, 2005). The fold belt has been in ter preted as a fold-and-thrust beltrooted in the Coast Moun tains (Evenchick, 1991b).Evenchick (2001) sug gested that oblique north west-south -east short en ing oc curred first, af fect ing the west ern Bow ser Ba sin, and was fol lowed by a long pe riod of north east-south west short en ing that gen er ated north west-trendingfolds, par al lel to the sur face strike of the Ca na dian Cor dil -lera. On the ba sis of work con ducted in the Groundhogcoal field, Mof fat and Bustin (1993) pro posed three pe ri odsof short en ing: ini tial north east-south west short en ing, thennorth west-south east short en ing and lastly north east-south -west short en ing. Bone (2002) ex am ined a basinal fold-in -ter fer ence pat tern iden ti fied by Evenchick (1991a), formed by in ter sec tion of two sim i lar-scale syn clines, con firm ingthe pres ence of type I fold in ter fer ence (Ramsay, 1967); in -ter fer ence of this type can produce domes favourable topetroleum accumulation.
2006 FIELD MAPPING
Dur ing the 2006 field sea son, six ar eas were mapped at1:25 000 scale from he li cop ter-po si tioned fly camps(Fig 1). Re sults from three of these ar eas are re ported onhere. The Iskut ridge area was se lected due to its prox im ityto the Eskay Creek area, an area known to have mul ti plegen er a tions of folds (Read et al., 1989; Lewis, 1992;Waldron et al., 2006). The Cartmel Lake area was se lectedas an area dom i nated by north west-trending folds with pos -
si ble over print ing of north east-trending folds, rec og nizedby Evenchick and Green (2004). The Sweeny Creek areawas cho sen for its con ver gence of fold trends, as shown byEvenchick (2004).
Iskut Ridge
A small ridge, east of the Iskut River and north ofPalmiere Creek at the west ern mar gin of the Bow ser Ba sin,is herein re ferred to as ‘Iskut ridge’ (Fig 1). All rock unitsob served in this area are as signed to the Ritchie-Algerlithofacies as sem blage (Evenchick et al., 2004). Struc turein the Iskut ridge area is dom i nated by north and north-north east-trending folds trace able through out much of themap area (Fig 2). North-trending folds are rounded andopen to tight. Mappable de flec tions in north-trending ax ialtraces oc cur near south east-trending mi nor folds. Mi norfolds are open to tight, and com monly plunge steeply(~60º) southeast.
Equal-area pro jec tions of poles to bed ding (Fig 3a)show a dif fuse gir dle dis tri bu tion with an eigenvector 1plung ing steeply to ward the south east. These data sug gestsmall-scale fold ing about a south east-plung ing axis, in con -trast to map-scale re la tion ships that in di cate north-trending folds as the dom i nant struc ture. Cleav age is ubiq ui touswithin fine units of the turbidite suc ces sion and spo radic incoarser sand stone. Pen cil lineations are wide spread, de -fined by in ter sec tion of steep bed ding and steep cleav age.Cleav age data shown in Fig ure 3b lie in two clus ters: adom i nant steeply dip ping cleav age that strikes north west,
326 Geoscience BC, Re port 2007-1
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CRETACEOUS
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MIDDLE JURASSIC TO MID-CRETACEOUS
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Upper Hazelton Group clastic rocks
Hazelton Group volcanic rocks
Mapping area
PALEOZOIC
57N
o56No128W
kilometres
500130W 128W
CartmelLakeCartmelLakeCartmelLake
IskutRidgeIskutRidgeIskutRidge
SweenyCreekSweenyCreekSweenyCreek
N
Fig ure 1. Sim pli fied re gional ge ol ogy of the north west ern Skeena Fold Belt (com piled from Evenchick, 1991a; Bone, 2002; Evenchick andThorkelson, 2005; Waldron et al., 2006), show ing fold ax ial traces and study ar eas re ferred to in the text; in set shows lo ca tion of fig urewithin the Ca na dian Cor dil lera.
and a lesser steep cleav age that strikes north east. In out -crop, cleav age ap pears to be a spaced pres sure-so lu tionfab ric. Slaty cleav age char ac ter ized by pen e tra tive min eralalign ment is rare. Ori ented sam ples were col lected and willbe used for fu ture petrographic anal y ses of cleav age. Be -cause of the scar city of min eral align ment, over printedcleav age does not dis play crenulation. The clus ters of
cleav age poles in Fig ure 3b are at trib uted to su per po si tionof two ep i sodes of short en ing. There is no gir dle dis tri bu -tion, such as might be ex pected if an ear lier cleav age werere folded by later short en ing. This is likely be cause ex ten -sion di rec tions dur ing both de for ma tion ep i sodes werenearly par al lel and ap prox i mately ver ti cal. Mapped de flec -tions of north-south ax ial traces by south east-trending folds (Fig 2) in di cate that the north to north-north east-strik ingfolds formed first and were sub se quently over printed byrel a tively mi nor south east-trending folds. This is con sis -tent with overall folding of bedding about a southeast-plunging axis (Fig 3a), suggesting that minor folds are partof a larger structure.
Equal-area plots of bed ding-cleav age in ter sec tionlineations in Fig ure 3c show a scat tered clus ter cen tred on anorth west-trending axis. The scat ter is at trib uted to the rel -a tive scar city of north east-strik ing cleav age in com par i sonto north west-strik ing cleavage.
Faults are rel a tively rare in this area com pared with therest of the west ern Skeena Fold Belt. Brit tle struc tures ob -served in clude en échelon veins at a num ber of lo ca tions.There are a num ber of ‘S’ and ‘Z’ asym met ric veins in ter -preted to re cord sinistral and dextral move ment, re spec -tively. In some cases, both asym me tries can be ob served ata sin gle lo cal ity (Fig 4, 5). The pres ence of en échelon veins sug gests a com po nent of brit tle strain, pos si bly re lated tothe nearby For rest Kerr fault (Read et al., 1989) and faultsin ter preted by Lo gan et al. (2000).
Cartmel Lake
An area ex pos ing rocks of the Eaglenest as sem blageap prox i mately 3 km south west of Cartmel Lake was se -lected for study of pre vi ously rec og nized over printed folds(Evenchick and Green, 2004). The to pog ra phy is dom i -nated by cliff-form ing con glom er ate beds in which a near-ver ti cal rough cleav age dom i nates the exposed surfaces.
The struc ture at Cartmel Lake is dom i nated by threema jor folds that trend south east (Fig 6). At map scale, agently dip ping sur face in the north ern sec tion of the maparea trun cates bed ding at a low an gle and is in ter preted as adécollement. A fault in the south ern sec tion of the map areatrun cates bed ding in a sim i lar fash ion and is in ter preted as acon tin u a tion of the same décollement (Fig 6). Bed ding cut -offs in di cate that this de tach ment climbs upsection to thesouth east. As so ci ated out crop-scale folds face and vergesouth east. A north east-trending map-scale fold at the south -ern ex trem ity of the area is pre sumed to be re lated to thesame de for ma tion that pro duced the south east-verg ing de -tach ment. Map-scale south east-trending folds can betraced through the de tach ment without offset, implying that it must have been folded.
Nu mer ous steep north west-strik ing faults, trace ableover less than a kilo metre, off set cliff-form ing con glom er -ate in a dextral sense, with off sets rang ing from metres totens of metres (Fig 6). Slickenline lineations in di catedextral strike-slip mo tion. Rare sinistral faults of sim i larsize and scale, but trace able over just a few metres, are pres -ent. These faults off set the mapped trace of the low-an gledécollement by sim i lar amounts. The sinistral faults may be con ju gate to dextral faults, as sug gested by their clock wiseori en ta tion rel a tive to the dextral faults and sim i lar off set ofstrata. The ori en ta tions of the con ju gate faults sug gest anap prox i mate north-south shortening direction.
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66229933000000
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15801600
16201640
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64
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44
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79
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8926
64
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74
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24
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68
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77
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69
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Legend
Cleavage
Bedding
Syncline
Anticline
Vertical beds
Contour elevations in metres.
Scale200 m
N
Axial trace
Fig ure 2. Struc ture of the Iskut Ridge area, show ing bed ding ori en -ta tions and mapped ax ial traces.
Spaced cleav age is well de vel oped in con glom er ate ofthe Cartmel Lake area, with some out crops show ing threedis tinct cleav age sets. The high per cent age (75–90%) ofcoarse-grained rock types within the Eaglenest lithofaciesas sem blage may ac count for the com mon oc cur rence ofcleav age within con glom er ate; li thol ogy is rel a tively ho -mo ge neous and there fore strain is not par ti tioned to finergrained rock types. The cleav age dis tri bu tion is un usualcom pared to other Bow ser Lake Group as sem blages thathave a bulk fine-grained com po si tion and only rarely de -velop cleav age in coarse rock types. Cleaved con glom er ateshows smooth weath er ing sur faces in stead of the ‘bumpy’weath er ing pat tern of peb ble con glom er ate else where in
the ba sin, as cleav age planes cut through peb bles. De spitede for ma tion of peb bles, sig nif i cant po ros ity is ob served tohave been re tained be tween the clasts. Equal-area plots ofcleav age poles in Fig ure 7b show a widely scat tered dis tri -bu tion, con sis tent with mul ti ple phases of de for ma tion.Equal-area plots of bed ding poles in Fig ure 7a show a dif -fuse gir dle nor mal to an east-south east-trending, shal lowlyplung ing eigenvector 1, con sis tent with mapping showingfolding of the low-angle detachment by northwest-trending folds.
Sweeny Creek
The area around the head wa ters of Sweeny Creek wasmapped, ex pand ing map ping of the Tumeka Creek studyarea re ported in Waldron et al. (2006). Rock units in thisarea be long to the Skelhorne as sem blage of the Bow serLake Group (Evenchick, 2004). Prior map ping in theSweeny Creek area re vealed north-north east and north-north west-trending folds con verg ing with out in ter sec tion,
328 Geoscience BC, Re port 2007-1
N
N = 73
2
1
N
N = 140
N
N = 68
CBA
E1
Fig ure 3. Lower-hemi sphere equal-area pro jec tion of struc tural data col lected in the Iskut ridge area: A) poles to bed ding; E1, eigenvector1; dashed line shows prin ci pal E1 gir dle plane; B) poles to cleav age, dot ted lines out line in ter preted dis tri bu tions of cleav age clus ters 1and 2; C) bed ding-cleav age in ter sec tion lineations.
Fig ure 4. Z-asym met ric en échelon quartz veins, in di cat ing dextraland re verse sense of shear; ar rows in di cate in ter preted sense ofshear; view to wards the west-north west.
Fig ure 5. S-asym met ric en échelon quartz veins, in di cat ingsinistral and re verse sense of shear; ar rows in di cate in ter pretedsense of shear; view to wards the west-south west.
within an area dom i nated by north west-trending folds(Evenchick, 2004). Folds mapped in the Sweeny Creekarea have vari able ori en ta tions, plung ing be tween north-north west and north-north east. Fig ure 8 shows ax ial tracesthat con verge with each other but do not cross cut. The mappat tern sug gests that these folds are of the same gen er a tion
and are part of a larger sys tem of con i cal folds. Map re la -tion ships also show that all north to north east folds ap pearto in crease in plunge to ward the north east. All bed ding inthe north ern part of the study area dips to the north east; re -gion ally, the struc tures in this area are part of the south westlimb of a gen tle north west-trending syncline, with a wave -
Geo log i cal Field work 2006, Pa per 2007-1 329
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474000
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71
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62
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8
12
77
17
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24
22
3216
21
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5526
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Bedding-form linesObserved
Inferred
Inferred decollement
Late steep fault
Antiform
Synform
Eaglenest Assemblage
Jurassic-Cretaceous Bowser Lake Group
Red and rust-weathered conglomerate and subordinate sandstone
Mainly grey sandstone, siltstone, with coal and fossil plant remains
Form lines mark thick conglomerate beds
1 km
N
2180
Contour elevations in metres
Fig ure 6. Struc ture of the Cartmel Lake area; note dis tri bu tion of early décollement and later folds.
330 Geoscience BC, Re port 2007-1
N
N = 76
N
N = 103
A B
E1
Fig ure 7. Lower-hemi sphere equal-area pro jec tion of struc tural data col lected in the CartmelLake area: A) poles to bed ding; E1, eigenvector 1; dashed line shows prin ci pal E1 gir dleplane; B) poles to cleav age.
Fig ure 8. Struc ture of the Sweeny Creek area, show ing ori en ta tions of struc tures and ax ial traces.
length on ap prox i mately the 5 km scale, ob served to thenorth of Sweeny Creek. The syncline is in ter preted to haveincreased the plunge of north-trending folds.
DISCUSSION
All three ar eas mapped show over print ing of struc -tures. At Iskut ridge, a ma jor north to north-north east-trending fold is re folded by smaller, south east-trendingstruc tures. The Cartmel Lake area is char ac ter ized by over -print ing of north west-trending folds on an ear lier de tach -ment; folds as so ci ated with the ear lier de tach ment shownorth east trends. The Cartmel Lake area may re cord the im -por tance of de tach ments in the de for ma tion his tory of thearea, as sug gested by Evenchick (1991b). Over print ing isless clearly dis played at Sweeny Creek, but pro gres sivedown-plunge steep en ing of north east-trending folds is con -sis tent with refolding by folds with north west trends. Thus,struc tures in all three ar eas are con sis tent with early, gen er -ally north west-south east short en ing, followed bynortheast-southwest shortening.
The de vel op ment of type I dome-and-ba sin in ter fer -ence pat terns is de pend ent upon over print ing of folds at ahigh an gle. Large domes pro spec tive for pe tro leum ac cu -mu la tion re sult from in-phase in ter fer ence of anticlines ofsim i lar am pli tude. Where struc tures of dif fer ent wave -lengths or am pli tudes in ter fere, the re sult ing pat terns aremore com plex. Fold am pli tudes, wave lengths and in ter sec -tion an gles are vari able in the Skeena Fold Belt. At Iskutridge, the early north to north-north east folds are map scale, and folds with south east trends are rel a tively mi nor struc -tures. If pres ent in the subsurface, such a struc tural pat terncould serve as an ar ray of small pe tro leum traps, as south -east-trending folds could trap pe tro leum lo cated in earlyfolds. At Sweeny Creek, there is a north west-trending kilo -metre-scale syncline and the north-trending folds havewave lengths on the hun dred metre scale. Down-plungesteep en ing of early folds by a north west-trending fold ofthis type could drive subsurface pe tro leum mi gra tion to thesouth, form ing a broad pool with sev eral pock ets withinearly folds. The Cartmel Lake area in cludes mul ti ple sets of spaced pres sure-so lu tion cleav age within con glom er atethat dis solves peb bles but does not oblit er ate po ros ity in the ma trix. At Cartmel Lake, early folds are likely as so ci atedwith décollements. Lo cat ing such structures in thesubsurface would provide insights on the location ofpotential petroleum traps.
ACKNOWLEDGMENTS
The au thors thank Geoscience BC for con tin u ing tosup port this re search. Ad di tional field costs were sup ported by Nat u ral Sci ences and En gi neer ing Re search Coun cil ofCan ada (NSERC) Dis cov ery Grant A8508 to J. Waldron.H. Tomes and C. Bloomberg pro vided ex cel lent as sis tancein the field. He li cop ter sup port was pro vided by Quan tum,Prism and Pa cific West ern He li cop ters. Re search atCartmel Lake was made pos si ble through a re search per mitfrom the Spatsizi Pla teau Wil der ness Pro vin cial Park.Sabina Sil ver Cor po ra tion and the Eskay Creek mine werevery hos pi ta ble in al low ing us to visit their sites. Help ful re -view by Philippe Erdmer and Rob Stevens improved themanuscript.
REFERENCES
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