Chapteir 2 Lithologic Units

CHAPTEIR 2 LITHOLOGIC UNITS INTRODUCTION mainly as turbidites. The Hurley Formation is stratigraphi- cally underlainby mafic volcanicrocks that are alsc' partof The Taseko - Bridge River map area is underlain by the Cadwallader Group, andis locally overlainby a succes- several distinct, partially coeval, Late Paleozoic to Meso- sion of Lower to Middle Jurassic shales, siliceous argillites zoic tectonostratigraphic assemblages (Figures4 and 5) that are juxtaposed across a complex system offaults of mainly and siltstones assigned to the Junction Creek urlit. The Tyaugbton Group and Last Creek formation arc: facies Cretaceous andTertiary age. These Paleozoic and Mesozoic rocks are intruded by Cretaceous and Tertiary stocks and equivalentsoftheHur1eyFormationandJunctionCn:eknnit dikes of mainlyfelsic to intermediate composition, andare that are in fault contact with the Cadwallader Group north- locally overlain by Paleogene volcanic and sedimentary west of GoldBridge. The Tyaughton Group comprises mid- rocks and by Miocene to Pliocene plateau lavas. They are dle to upper Norian nonmarine and shallow-marine intruded by a large mass of Late Cretaceous granodiorite, conglomerate, sandstone and minor limestone, w;de the referred to as the Dickson-McClure batholith, along the overlying Last Creek formationis a transgressive sfquence southwestern marginof the map area.The pre-Tertiary tec- comprising upper Hettangian to Sinemurian conglomerate tonostratigraphic assemblages within the map area are and sandstone grading upward into upper Sinemmian to briefly described in the following paragraphs. middle Bajocian shale. The volcanic rocks of the Cadwal- lader Group have traceelement compositions simi1.uto is- Bridge River Terrane: The Bridge River Terrane is rep- land arc tholeiites, and the clastic rocks of the Hurley resented mainly by the Bridge River Complex, an assem- Formation, Tyaugbton Group and Last Creek folmation blage of cherr, argillite, greenstone, gabbro, blueschist, serpentinite, limestone and clastic sedimentary rocks with contain clastsof limestone, basalt, andesite, dacite,lhyolite no coherent stratigraphy (Potter, 1983, 1986). Datedcherts and granitoids, suggestingthat Cadwallader Terran,: repre- and limestones within the complex range from Mississip- sents part of a LateTriassic volcanic arc and fringini: clastic pian to late Middle Jurassicin age (Cordey and Schiarizza, apron. Rusmore ef al. (1988). Umhwfer (1990) and Rus- 1993; F. Cordey, Appendix 2; M.J. Orchard, Appendix I), more and Woodsworth (1991a)point out general similiui- and blueschist-faciesmetamorphismoccurredin theMiddle ties between CadwalladerTerrane and theTriassic - :iurassic to Late Triassic (Archibald ef al., 1990, 1991a,b).The wide rocks of Stikine terrane, and suggestthat they may be cor- age range,structural complexity and presence of blueschist- relative or related portions of thesame arc system. facies metamorphic rocks suggest that this assemblage ac- TyaughtonBaQn: TheTyaughton basin (Jeletzkyznd Tip- cumulated as an accretion-subduction complex. The upper per, 1968) is a belt of Jura-Cretaceous clastic sedimentary part ofBridge River Terrane comprises a thick coherent suc- rocks that extends from the Taseko - Bridge Ri~erarea cession of clastic metasedimentary rocks referredto as the northwestward to beyond Cbilko Lake. The lower part of Cayoosh assemblage that conformably overliesthe Bridge the basinis represented mainly bythe upper Middle Jurassic River Complex to the south of the Taseko - Bridge River to Lower Cretaceous Relay Mountain Group,whict. is well area (Journeay and Northcote, 1992; Mahoney and exposed in the centralpart of the Taseko - Bridge Riber area, Journeay, 1993; Journeay and Mahoney, 1994). The southeast of Big Creek. Volcaniclastic strata assignedto the Cayoosh assemblage is locally represented in the sonthem Tosh Creek unit are locally exposed in the northwestern part part of the Taseko - Bridge River area by an undated suc- of the map area and mayrepresent a Lower Cretaceous vol- cession of siltstones and sandstones that were in part in- canic facies within the upper part of the group. The basal cluded in the Noel Formation ofCaimes (1937, 1943). and contact of the Relay Mountain Group is not expo!:ed, but are here assigned to the informal Gun Lake and Downton indirect evidence suggests that it was deposited on the Lake units.A narrow fault-bounded lensof Jura-Cretaceous Bridge River Complex, and thereforeis a facies equivalent conglomerate, here referred to as the TrnaxCreek conglom- of the Cayoosh assemblage. However, Jura-Cretaceous silt- erate, is spatially associated with the Gun Lake unitmay and stones and fine-grained sandstones that are r:xposl!d very have been derived from higher inthe section. locally in the CamelsfootRange (Grouse Creek unit of Fig- Cadwallader Terrane: Cadwallader Terrane includes the ure 4) are in apparent stratigraphic contact with Cadwallader Upper Triassic Cadwalladerand Tyaughton groups together Terrane, whereas 1OOkilometresto thenorthwestaseparate with Lowerto Middle Jurassic rocksof the Last Creek for- belt of Jura-Cretaceous rocks assigned theto :Relay Mouu- mation and Junction Creek unit (Rusmore, 1987, Umhoefer,tain Group is in stratigraphic contact with Middle i'urassic 1990). The most extensively exposed componentis the Hur- rocks of Methow Terrane (Tipper, 1969a: Schiarizza et al., ley Formation of the CadwalladerGroup, which consistsof 1995). These relationships suggest that the Relay Mountain upper Carnian to upper Norian sandstone, siltstone, con- Group and correlative rocks constitute an overlap assem- glomerate and minor micritic limestone that were depositedblage linking Bridge River, Cadwallader and Methowter- ___ Bulletin IO0 I1 Figure 4. Major tectonostratigraphic assemblages of the Taseko- Bridge River area. Abbreviations, fromleft to right, are:nKPC=Povd Creek formation; IuKTC, IKTC=Taylor Creek Group; IKWS=Tosh Creek succession; muJRM1, 3-2, IKRM3=Relay Mounlain Group; luKSQ= Silverquick formation; JKT= TN~XCreek conglomerate; IKGk Gun Lake unit; DL= Downton Lake unit: MJBR= Bridge Rivcr Complex; hnJLC=Last Creek formation; u'K=Tyaughton Group; JKG=Gmnse Creek unit; ImlJC=Junction Creek unit; uTCH=Hurlcy Formation; nTCV=volcanic unit of the Cadwallader Group; PBEkBralorne-East Liza Complex; PSMSbnlaps serpentinite m6lange unit; PSH=Shnlaps harzbnrgite unit; IKJMyl, IKJMy2, IKJMcl, IK.JMcZ=Jackass Mountain Group; ImJys, mlyv, mIcs=Jurassic rocks correlated with the Dewdney Creek Formation; KDC=Dash- Chum succession. ranes. The upper part of the Tyaughton basin consists of of ultramafic cumulates, layered to isotropic gabbros, am- synorogenic clastic sedimentaryrocks of the Taylor Creek phibolitic dike fragments, rodingite, and volcanicand sedi- Group and overlying Silverquick conglomerate, which werementary rocks. Late Paleozoic radiometric dates from deposited during mid-Cretaceous contractional deforma- plutonic and metamorphic knockers withinthe serpentinite tion. These strata were deposited above the Bridge River mdangeunit (Archibald etal., 1991a; R. Friedman,Apptn- Complex in the Bridge River area, and above the Relay dix 8) are interpreted to be the age of ocean-floor p1utoni:;m Mountain Group farther to the northwest. They include de- and metamorphism associated with constructionof Shulrips tritus derived from the Relay Mountain Group, Bridge Riveroceanic crust (Calonetal., 1990). Large-scale structuralin- Complex, Cadwallader Terrane and associated ophiolitic version of the original ophiolite stratigraphy is inferred to rocks, as well as detritus derived from a volcanic terrainto have occurredin mid-Cretaceous time during its thrust-e n- the west, anda newly uplifted metamorphic- plutonic belt placement above Cadwallader Terrane, whichlies beneflth to the east (Garver, 1992, Garver and Brandon, 1994).The the Shnlaps complex across a southwest-vergent thrustS:IS- Tyaughton basin depositsare capped by Upper Cretaceous tem that is exposed in the southwest comer of the Shulpps nonmarine volcanic and volcaniclastic rocksof the Powell Range. The Bralome-East Liza Complex consists of green- Creek formation.These rocks are extensively exposedin the stone, diorite, tonalite,gabbro and serpentinite that.are im- northwestern part of thearea, where they overliethe Taylor bricated with Cadwallader Terrane throughout the sonthern Creek Group acrossan angular unconformity. Local expo- part of the Taseko-Bridge River area.It includes rocks tflat sures farther east, however,are in gradational contact with were assigned to the Bralome and President intrusions by the underlying Silverquick conglomerate. Cairnes (1937, 1943) as well as some rocks that Caimes Ophiolitic Complexes: Ophiolitic rocks in the Taseko - included in the Pioneer Formation of the Cadwallader Bridge River area are assigned to the Shulaps Ultramafic Group. These rocksare lithologically similarto the plutonic Complex and the Bralome-East Liza Complex.The Shnlaps and volcanicknockers found within the Shnlaps serpentinite Complex covers mostof the northern Shnlaps Range andis m€lange unit, and have yielded similar Late Paleozoic IB- subdivided intotwo major structural divisions (Calonet al., diometric dates (hitch et al., 1991a). They may n:presf:nt 1990): an upper unitof harzburgite anddunite with a mantle slices of Shulaps oceanic crust that were imbrical:ed w th tectonite fabric, and a structurally underlying serpentinite Cadwallader Terrane during obductionthe of Shulaps Coin- melange unit comprising

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