Stratigraphic Record of Initiation of Sedimentation in the Bowser Basin(NTS 104A, H), Northwestern British Columbia1

by J-F. Gagnon2, W. Loogman2, J.W.F. Waldron2, F. Cordey3 and C.A. Evenchick4

KEYWORDS: sedimentology, stra tig ra phy, hy dro car bon,Bow ser Lake Group, Hazelton Group, Spatsizi For ma tion

INTRODUCTION

The Bow ser Ba sin is a sed i men tary ba sin lo cated overthe Stikine Terrane in the Intermontane Belt of north-western Brit ish Co lum bia (Fig 1). In this area, the StikineTerrane is over lain by Early Ju ras sic sed i men tary rocks ofthe up per Hazelton Group (Troy Ridge fa cies of the Salmon River For ma tion in An der son [1993]; Spatsizi For ma tionin Thomson et al. [1986]) and then by a thick suc ces sion(ap prox i mately 6 km) of Mid dle Ju ras sic to Early Cre ta -ceous sed i men tary rocks mainly as signed to the Bow serLake Group (Evenchick and Thorkelson, 2005). The fill ofthe Bow ser Ba sin was de pos ited over the arc vol ca nic rocks and as so ci ated volcaniclastic rocks of the lower HazeltonGroup. A broad va ri ety of sed i men tary en vi ron ments char -ac ter ized the Bow ser Ba sin, rang ing from ba sin-floorturbidites at the base through mar ginal-ma rine clastic rocks to nonmarine redbeds in the uppermost parts of thesuccession.

Re cent ther mal ma tu rity in ves ti ga tions have shownthe ex is tence of an ef fec tive pe tro leum sys tem in the Bow -ser Ba sin (Evenchick et al., 2002; Osadetz et al., 2003;Stasiuk et al., 2005). Black shales of the Spatsizi For ma tion have re turned to tal or ganic car bon con tent up to 6%, andthese val ues might have been two to four times greater be -fore the ex pul sion of or ganic ma te rial (Ferri and Boddy,2005). This sug gests that un der the proper con di tions, or -ganic shales of the Spatsizi For ma tion would have acted asex cel lent source rocks. How ever, vari able ther mal mat u ra -tion lev els through out the ba sin and the lack of in for ma tionre gard ing the link age of source rocks of the up per HazeltonGroup and po ten tial res er voir rocks of the Bow ser LakeGroup create uncertainty in terms of exploration risks.

The aim of this study is to un der stand the con di tionsthat led to a change in the de po si tion style in the Mid dle Ju -ras sic from a vol ca nic arc-dom i nated en vi ron ment to a sub -

sid ing sed i men tary ba sin. For that pur pose, study ar easwere se lected based on known ex po sures that ex hibit thetran si tion from the Hazelton Group into the Bow ser LakeGroup in struc tur ally un dis turbed sec tions. Three ar eas arede scribed in this re port, Todagin Moun tain, Mount Willand Oweegee Dome (Fig 1). Sys tem atic mea sure ments ofde tailed strati graphic sec tions were con ducted in the fieldto doc u ment the onset of sedimentation in the BowserBasin.

STRATIGRAPHY OF THE LOWERHAZELTON GROUP

The low est strati graphic unit ob served in the vi cin ityof Todagin Moun tain con sists of a mix ture of green ish greyto ma roon vo l can i c l a s t ic rocks and l igh t g r eytrachyandesitic vol ca nic flows. This dom i nantly in ter me di -ate vol ca nic suite and as so ci ated plutons, whichuncomformably over lie the Tri as sic Stuhini Group, wasmapped as an de site brec cias and de rived epiclastic rocks by Ash et al. (1997). Five in di vid ual in tru sive bod ies from thissuite re turned U-Pb zir con dates from 205 to 198 Ma, sug -gest ing a Hettangian to Sinemurian age (Fried man andAsh, 1997). The lavas iden ti fied in this study are mostlyapha ni tic and con tain cal cite-filled amygdules. Small in tru -sive bod ies of leucocratic monzogranite are also pres ent toa lesser ex tent. Poorly sorted re worked tuff, con tain ingabun dant subrounded lithic frag ments, is interbedded withwelded lapilli tuff and re ces sive bands of very fine grainedcrys tal tuff. The pres ence of flat tened pum ice (fiamme) inme dium-grained lapilli tuff along with light beige euhedralfeld spar crys tals in a ma trix of fine-grained air-fall tuff sug -gests that ex plo sive vol ca nism was the pre dom i nant modeof de po si tion at that time. He ma tite is a com mon min eralamongst these pyroclastic rocks, suggesting that theymostly accumulated in an oxidizing subaerial environment.

In the Mount Will dis trict, the lower Hazelton Groupcon sists of a bi modal as sem blage termed the Cold Fishvolcanics by Thorkelson (1992). Ac cord ing to Thorkelson(1992), this unit can be di vided into a fel sic suite of high-sil ica rhy o lite and a mafic suite of ba salt, an de site anddacite. The up per 100 m of this suc ces sion were ex am inedap prox i mately 10 km south west of Gladys Lake. Ma roon to green, very poorly sorted epiclastic and indurated lapillituffs con form ably over lie a thick unit of ox i dized rhy o lite.The re ces sive volcaniclastic in ter val is capped by ap prox i -mately 30 m of densely welded ignimbrite char ac ter ized bylarge open vugs, fiamme, lava par ti cles and an gu lar vol ca -nic frag ments. This indurated fel sic ash flow con sti tutes the last ma jor pulse of ex plo sive vol ca nism in the re gion before the establishment of sedimentary conditions.

The vol ca nic arc rocks of the lower Hazelton Grouparound Oweegee Dome con sist of fine lay ers of fis sile dust

Geo log i cal Field work 2006, Pa per 2007-1 275

1Geoscience BC con tri bu tion GBC0232Department of Earth and Atmospheric Sciences, University of

Alberta, Edmonton, AB (jfgagnon@ualberta.ca)3Sciences de la Terre, Université Claude Bernard Lyon 1,

Villeurbanne, France4Geological Survey of Canada, Vancouver, BC

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

tuff interbedded with well-strat i fied ma roon and green vol -ca nic sand stone, peb ble to cob ble con glom er ate, de brisflows and mi nor andesitic vol ca nic flows. These units areover lain by a thick unit of fel sic por phy ritic rhy o lite thatwas dated at 199 ±2 Ma by Greig and Gehrels (1995).

STRATIGRAPHY OF THE UPPERHAZELTON GROUP

Rocks as signed to the up per part of the Hazelton Group are ex posed in a south west-trending sec tion west ofTodagin Moun tain (Evenchick and Green, 2004). The up -per Hazelton Group con sists of a bi modal vol ca nic suitewith mul ti ple fine-grained sed i men tary in ter ca la tions. Ade tailed strati graphic col umn rep re sent ing the sec tion ispre sented in Fig ure 2. Thick units of co lum nar ba salt con -sti tute most of the sec tion. Thin in ter vals of ma rine clasticsed i men tary rocks con tain nu mer ous ammonites of theFreboldi zone, which sug gest a Pliensbachian age(Evenchick et al., 2001; T. Poulton, pers comm, 2006). Thestrati graphic sec tion con tains an an gu lar dis cor dance at264 m (Fig 2) where a re ces sive unit of al tered ba salt is un -con form ably over lain by crosslaminated lime stone and ox -i dized ba salt de bris. A thin layer of dacitic lapilli tuff 50 cmabove this un con formity yielded a U-Pb zir con age of185.6 ±0.6 Ma (Palfy et al., 2000) which con firms thePliensbachian age ob tained from paleontological data. Thetop of the suc ces sion is char ac ter ized by a thick unit of ve -sic u lar ma rine pil low ba salt and hyaloclastite indicatingdeposition in a submarine environment (Fig 3).

Sed i men tary rocks as signed to the Spatsizi For ma tionare ex posed in the Mount Will dis trict (Evenchick andThorkelson, 2005), lo cated ap prox i mately 10 km north east

of Joan Lake, where Thomson et al. (1986) car ried out ear -lier strati graphic work. The low est strati graphic unit of theSpatsizi For ma tion at Mount Will is char ac ter ized bybrown ish to light grey arkosic sand stone de pos ited on topof welded ignimbrite. The high pro por tion of al teredvolcanogenic clasts in the sand stone in di cates re cy cling oflo cally de rived ex tru sive rocks of the un der ly ing Cold Fishvolcanics. Even though no an gu lar dis cor dance was ob -served, the con tact with the un der ly ing ignimbrite is ero -sional and prob a bly cor re sponds to an un con formity. Fineto me dium-grained subrounded lithic frag ments and feld -spar grains are mod er ately sorted and sed i men tary struc -tures such as rip ple crosslaminations and pla narcrossbedding are com mon. The basal sand stone unit con -tains mul ti ple cal car e ous in ter vals rang ing from 35 to55 cm thick with a broad va ri ety of ma rine fos sils(ammonites, cor als, bel em nites and bi valves; Fig 4). Largepieces of fos sil wood and plant frag ments found in thinlylam i nated siltstone and very fine grained sand stone in di -cate that a sig nif i cant quan tity of ter res trial de bris wastrans ported to the ba sin. The abun dance and high di ver sityof bioturbation sug gest that paleoenvironmental con di -tions, such as ox y gen con tent, tem per a ture and sa lin ity,were rel a tively sta ble. The pres ence of sym met ric wave-gen er ated rip ples is con sis tent with a depositional en vi ron -ment above the fair-weather wave base in an open, shal low-ma rine set ting. Upsection, there is a sig nif i cant in crease insilt and clay con tent, and the sand stone beds are usu allythin ner. Concretionary beds of lime stone are as so ci atedwith the coarser grained rock units whereas rounded cal car -e ous con cre tions are mostly found in shale. Trace fossilsand bivalves are omnipresent in those rocks suggesting thatthe depositional environment was still within the photiczone.

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Fig ure 2. De tailed strati graphic sec tion show ing interbedded vol ca nic rocks and ma rine sed i men tary rocks of the up per Hazelton Group,Todagin Moun tain area, BC.

The fossiliferous shal low-ma rine suc ces sion is over -lain by 140 m of interbedded black si li ceous mudstone andpale or ange tuffaceous siltstone (Fig 5). This unit was orig i -nally clas si fied as the Quock For ma tion of the SpatsiziGroup by Thomson et al. (1986) and later low ered to mem -ber sta tus within the Spatsizi For ma tion by Evenchick andThorkelson (2005). In the re gions sur round ing the Bow serBa sin, cor re la tive rocks are at trib uted to the in for mal nameof py jama beds be cause of the al ter na tion of pale or angeand black weath ered lay ers that give them a striped ap pear -ance (Fig 6). Ammonite fau nal as sem blages at the base ofthe suc ces sion in di cate an early Bajocian age (Thomson etal., 1986). In the mea sured sec tions, the si li ceous shales aregen er ally blocky and indurated. The si li ceous mudstonecon tains abun dant radiolarian microfossils, which sug gestde po si tion from sus pen sion in a low en ergy en vi ron ment.

Even though Thomson et al. (1986) con sid ered the vol ca -nic ash to be the pri mary source of sil ica in the rocks,ob ser va tions of the py jama beds in thin sec tions sug gestthat in tro duc tion of sil ica con tent to the sys tem viaremobilization of sil ica from radiolarian tests was sig nif i -cant. Re ces sive fine-grained tuff bands vary from 1 to 5 cmin thick ness and at test to dis tal vol ca nism. Chlorite andplagioclase are the most com mon min eral phases in thecrys tal tuffs. The rar ity of trace fos sils along with high or -ganic con tent can prob a bly be re lated to ox y gen-de fi cientcon di tions in a deeper wa ter en vi ron ment. How ever, theoc cur rence of thin to medium-bedded limestone in thesection signifies that sediments accumulated above thecarbonate compensation depth.

On the east ern edge of Oweegee Dome, a 270 m suc -ces sion with sed i men tary char ac ter is tics sim i lar to theQuock Mem ber was ob served above andesitic andvolcaniclastic rocks of the lower Hazelton Group. There,the lower and up per Hazelton Group are sep a rated by an an -gu lar un con formity (Greig, 1991; Waldron et al., 2006). Inthis area of the ba sin, the thinly interbedded si li ceousradiolarian mudstones and ash tuffs have been clas si fied asthe Troy Ridge fa cies of the Salmon River For ma tion (An -der son, 1993; Greig, 1991). The py jama beds, iden ti fiedaround Oweegee Dome, are thinly interbedded with oc ca -sional wavy par al lel lami na tions and con vo lute slumpstruc tures. Car bon ate con tent is com mon in the re ces sivecoarser tuffaceous bands whereas the mudstone is blocki erand com pletely ce mented by sil ica. Well-pre servedradiolarian sam ples in black si li ceous mudstone haveyielded an early Bajocian to early Bathonian age for the up -per part of the sec tion, which seems slightly older than theBathonian to Callovian age ob tained in the same area byEvenchick et al. (2001). The pre lim i nary re sults are pre -sented in Ta ble 1. Near the top of the sec tion, the tuff bands

278 Geoscience BC, Re port 2007-1

Fig ure 3. Up per part of the de tailed sec tion (335–414 m) mea sured west of Todagin Moun tain, BC, show ing thinly lam i natedsiltstone and shale over lain by ma rine pil low ba salt (MPB). See per son for scale.

Fig ure 4. Well-pre served ammonite fos sil of the Spatsizi For ma -tion, Mount Will area, BC.

Geo log i cal Field work 2006, Pa per 2007-1 279

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Fig ure 5. De tailed strati graphic sec tion show ing up per most vol ca nic unit of the lower Hazelton Group, ma rine sed i men tary rocks of theSpatsizi For ma tion and the con tact with the over ly ing Bow ser Lake Group, Mount Will area, BC. See Fig ure 2 for leg end.

be come thin ner and are less abun dant. The tran si tion intothe over ly ing turbidites of the Bowser Lake Group appearsconcordant and gradational.

STRATIGRAPHY OF THE BOWSERLAKE GROUP

The basal unit of the Bow ser Lake Group in the vi cin -ity of Mount Will con sists of thinly lam i nated very fine

grained sand stone with pale or ange weath er ing and darkgrey siltstone with shale in ter vals. It was clas si fied as theTodagin as sem blage of the Bow ser Lake Group byEvenchick and Thorkelson (2005) and is in ter preted to rep -re sent a prodelta slope set ting. Peb ble to cob ble chert-richcon glom er ate units are also found in the prodelta slopelithofacies as sem blage. The thick ness of these coarse-grained pack ages is highly vari able and can reach up to50 m. They are char ac ter ized by sharp ero sive bases thatshow scour ing into un der ly ing fine-grained rocks. Mea -sure ments on flutes and groove casts in di cate paleocurrentdi rec tions to wards the south east. Near Todagin Moun tain,Green (1991) in ter preted sim i lar con glom er ate bod ies aschannelized de pos its in in cised can yons and gul lies in aslope en vi ron ment. Overbank sand de pos its with troughcrossbedding and pla nar crosslaminations are also com -mon on the edge of the main chan nel axes. Centi metre-scale cal car e ous con cre tions are abun dant in the fine-grained rock units and are usu ally as so ci ated withdiagenetic py rite. Dif fer en tial com pac tion in di catesconcretionary growth early in the diagenetic pro cesseswhen pores were still filled with wa ter. Soft-sed i ment de -for ma tion fea tures such as load struc tures, sed i men tarydikes and syndepositional folds and faults are com mon fea -tures in the Todagin as sem blage. These are in ter preted torep re sent high fluid pres sure com bined with rapid sediment accumulation rates that created instability in the sedimentpile and resulted in slope failure.

Sed i men tary fa cies at the base of the Bow ser LakeGroup near Oweegee Dome are slightly dif fer ent fromthose ob served in the north ern part of the ba sin. Thick bedsof mas sive well-sorted sand stone interbedded with cal car e -ous fis sile shales are found above the py jama beds. The im -pres sive lat eral ex tent of the sandbodies along with gradedbeds and par tial Bouma se quences sug gest that these sed i -ments were de pos ited as deep water turbidites insubmarine-fan com plexes. Sharp ero sive bases, con vo lutelami na tions and flame struc tures are com mon. The ab senceof bioturbation fea tures and shelly fos sils is in ter preted torep re sent a hos tile en vi ron ment where sed i men ta tion in -flux was too high to sustain biogenic activity on the basinfloor.

These turbidite de pos its, as signed to the Ritchie-Algeras sem blage by Evenchick et al. (2006), rap idly grade upinto shal lower sed i men tary fa cies of the Muskaboo Creekas sem blage (Evenchick et al., 2006). An un in ter ruptedstrati graphic sec tion dis play ing mul ti ple se quences wasmea sured along an east-trending ridge east of Oweegee

280 Geoscience BC, Re port 2007-1

Fig ure 6. Thinly interbedded re sis tant si li ceous mudstone and re -ces sive tuffaceous siltstone (py jama beds) of the Quock Mem ber,Mount Will area, BC. See ham mer for scale.

TABLE 1. SUMMARY OF THE RADIOLARIAN FAUNA IDENTIFIED IN PYJAMA BEDS EAST OFOWEEGEE DOME. SAMPLE INDICATED AN EARLY BAJOCIAN TO EARLY BATHONIAN AGENEAR THE TOP OF THE UNIT (BASED ON ZONATIONS FROM CARTER ET AL. [1988] AND

BAUMGARTNER ET AL. [1995]).

Sample KA031A Sample KA032BEmiluvia chica s.l. Foreman Podobursa sp.Higumastra sp. Paronaella sp.Hsuum sp. Parvicingula sp. D CarterOrbiculiforma sp. Orbiculiforma sp.Napora sp.Parvicingula sp. cf. dhimenaensis s.l. BaumgartnerParvicingula sp. cf. schoolhousensis gr. Pessagno & WhalenPerispyridium sp.Tetratrabs izeensis Yeh

Dome (Fig 7, 8). Al ter nat ing pack ages of fine to me dium-grained sand stone and silty shale are in ter preted to rep re -sent fluc tu a tions of rel a tive sea-level, this be ing the pri -mary con trol on the mode of sed i men ta tion. Coars en ing-up ward cy cles in cal car e ous co quina beds found at thebases of the sandy in ter vals con tain mud rip-up clasts de -rived from each un der ly ing shale unit. The cyclicity ofthese de pos its sug gests that a shelf en vi ron ment was pe ri -od i cally sub jected to storm-in duced high en ergy con di tions that trans ported skel e tal ma te rial away from the place ofgrowth. The sand stone in ter vals vary from 35 to 100 m inthick ness and con tain abun dant trace fos sils, bi valves andrip ple crosslaminations. They are capped by max i mumflood ing sur faces above which are laminated shale andsiltstone, which accumulated from suspension duringrelative highstands.

DISCUSSION

Units of the lower Hazelton Group ob served in all thestudy ar eas seem to in di cate that these rocks mostly ac cu -mu lated in an ox i diz ing, subaerial en vi ron ment. The tran si -tion from the lower to the up per Hazelton is abrupt andmarked by a re gional ero sional sur face. Above this un con -formity, the de po si tion style of the Spatsizi For ma tion andequiv a lents is quite vari able on the ba sin scale, de pend ingon the paleotopography and the dis tance from ac tive vol ca -nic cen tres. In vol cano-sed i men tary suc ces sions, the ra tioof volcaniclastic rocks to ma rine sed i men tary rocks is afunc tion of prox im ity to the erup tive cen tre. For in stance,the Pliensbachian to Bajocian fine-grained sed i men taryrocks at Todagin Moun tain are interbedded with thick in ter -vals of ma rine pil low ba salt and hyaloclastite, whereas cal -car e ous fossiliferous sand stone and siltstone of the sameage in the Mount Will district show only minor volcanicinput.

Basal sed i men tary rocks of the Spatsizi For ma tion areheavily bioturbated, fos sil rich and con tain mul ti ple sed i -men tary struc tures such as sym met ri cal wave rip ples andpla nar crosslaminations, which at test to a shal low ma rineclastic-shelf en vi ron ment. Upsection, the per cent age offine-grained sed i men tary rocks in creases grad u ally andshelly fos sils are rare. This is in ter preted to rep re sent a pro -gres sively deep en ing en vi ron ment where clastic sed i mentsac cu mu lated via turbiditic in put. Lam i nated si li ceousmudstones and cal car e ous tuffaceous siltstones (py jamabeds) were found in ev ery mea sured strati graphic sec tion.De tailed map ping con ducted in this study re vealed strongsim i lar i ties be tween rocks of the Troy Ridge fa cies and thepy jama beds of the Quock Mem ber, as noted in pre vi ouswork (Greig, 1991; An der son, 1993; Ferri and Boddy,

2005). In the sec tions at Oweegee Dome and Mount Will,cal car e ous beds are pres ent through out the unit and the pro -por tion of rust-col oured vol ca nic ash grad u ally de creasesnear the top. Fur ther more, radiolarian sam ples ex tractedfrom black si li ceous mudstone sug gest a early Bajocian toearly Bathonian age for the up per part of the Oweegee sec -tion (Ta ble 1). These re sults are con sis tent with pre vi ousages ob tained from ammonites by Thomson et al. (1986)for the Quock Mem ber in the Joan Lake area. The tran si tion from the py jama beds of the up per Hazelton Group to theover ly ing clastic deep water sed i men tary rocks of the Bow -ser Lake Group is sub tle and cov ered in most sec tions. Theab sence of vein ma te rial in the float and con sis tent bed dingmeasurements on either side of the contact indicate that thelatter is likely to be concordant with no structural offset.

An un usu ally thin suc ces sion of deep water sed i men -tary rocks was ob served around Oweegee Dome. Sand-rich turbidites of the Ritchie-Alger as sem blage rap idly grade up into shal low-ma rine clastic-shelf de pos its of the Muskaboo Creek as sem blage (Evenchick et al., 2006). This is in ter -preted to be the re sult of de po si tion over an un even to pog -ra phy, where Oweegee Dome con sti tuted a struc tural highin the Bow ser Ba sin. The abun dance of soft-sed i ment de -for ma tion, ero sive bases and sed i men tary dikes in theTodagin and Ritchie-Alger as sem blages are at trib uted tohigh sed i men ta tion rates in a deep water en vi ron ment. Thismarks a dras tic change from the un der ly ing py jama bedunits where coarse clastic input was almost nonexistent.

The oc cur rence of or ganic radiolarian mudstonesabove struc tural highs such as Oweegee Dome sug geststhat these po ten tial source rocks have been de pos ited andpre served through out the en tire ba sin. Thick and lat er allyex ten sive sandbodies de pos ited on the edge of the Mid dleJu ras sic clastic shelf dur ing rel a tive lowstands might con -sti tute ex cel lent res er voir rocks for petroleum exploration.

Based on the ob ser va tions made in the mea sured strati -graphic sec tions, it ap pears that con tin u ous sed i men ta tiontook place in a tec toni cally ac tive, shal low ma rine ba sin onStikinia from the early Pliensbachian to early Bajocian. De -pend ing on the rel a tive po si tion of ac tive vol ca noes, thesed i ments in cor po rated more or less vol ca nic in put andwere de pos ited along with sub aque ous and subaerial bi -modal vol ca nic flows. Rapid lat eral fa cies change and thevari abil ity of sed i ment thick ness in the up per HazeltonGroup are at trib uted to ac tive nor mal fault ing dur ing anextensional back-arc stage that iso lated mul ti ple sub-basins. A sig nif i cant de crease of vol ca nism dur ing theAalenian and Bajocian is at trib uted to the ces sa tion ofsubduction of oce anic crust un der Stikinia and to the fi nalac cre tion of vol ca nic-arc rocks with the Cache CreekTerrane. Fol low ing the end of magmatism, ther mal re lax -

Geo log i cal Field work 2006, Pa per 2007-1 281

Fig ure 7. Pan orama view of the de tailed mea sured strati graphic sec tion west of Oweegee Dome, BC.

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Fig ure 8. De tailed strati graphic sec tion show ing al ter nat ing sand stone and shale in ter vals of the Muskaboo Creek as sem blage of the Bow -ser Lake Group, BC. See Fig ure 2 for leg end.

ation of the crust com bined with eustatic sea-level rise gen -er ated new ac com mo da tion space in the ba sin where con -densed sec tions of fine-grained deep water sed i ments werede pos ited. Obduction of the Cache Creek Terrane along theKing Salmon thrust fault (Evenchick and Thorkelson,2005) gen er ated a new source of chert-rich sed i ments thatwere shed in the basin via turbiditic input and fan-deltascorresponding to the deposition of the Bowser Lake Group.

ACKNOWLEDGMENTS

The au thors would like to thank Geoscience BC forsup port ing this re search through a grant to the Geo log i calSur vey of Can ada. Ad di tional field ex penses were sup -ported by Nat u ral Sci ences and En gi neer ing Re searchCoun cil of Can ada (NSERC) Dis cov ery Grant A8508 heldby J.W.F. Waldron. He li cop ter sup port was pro vided byPrism and Pa cific West ern He li cop ters. Heidi Tomes andCordell Bloomberg are thanked for their re mark able workin the field. Field work in the Gladys Eco log i cal Re servewas made pos si ble by BC Parks through a Re search & Ed u -ca tion Park Use Per mit. Sabina Sil ver and Eskay CreekMine were hos pi ta ble in al low ing us to use their site.Murray Gingras pro vided help ful comments on an earlierversion of the manuscript.

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