Chapteir 5 Tectonic Implications"
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Ministry of Employment and In vestment CHAPTEIR 5 TECTONIC IMPLICATIONS"- REGIONALCORRELATION AND which extends continuously along the southwestside ofthe SIGNIFICANCE OF MAIN Yalakom fault for 80 kilometres to Chilko Lake (Figure 39). At the southeast end of thisbelt, within theTaseko . Bridge LITHOTECTONIC ASSEMBLAGES River map area, this clastic succession is inferred to have BRIDGE RIVER TERRANE been deposited abovethe Bridge River Complex($e? Chap- ter 2). The Tyaughton basin beltis also bounded onits north- The Bridge River Terrane is represented the West side by sparse exposures ofthe BridgeRiver Complex BridgeRiver Complex, which includes Mississippianto (Riddell eT 1993a,b), indicating that the Bridge River Middle Jurassic chert, pillowed and massive greenstone, Complex probably underlies, in the subsurface, th3 entire limestone, clastic rocks and blueschist. The Taseko - Bridge belt of clastic sedimentary Theexposures at th north- River map area encompasses the northwestern end Of the west end of the belt, east of Chilko Lake, are bounded to the mainoutcrop beltoftheBridgeRiverComplex~A1ougstrike north by fault-bounded panels OfCadwaIlader and p/Iethow to thenorthwest is the main exposure beltJura-Creta-of terranes, and are the exposures of tl,e ceous clastic sedimentary rocksof the Tyaughton basin, Bridge River complex. Figure 39.Map showing the distributionof major tectonostratigraphic elementsof the southeastern Coast Beltin the Pemhrton, Taseko Lakes and Mount Waddington map sheets. BRC=narrow lens of Bridge River Complex east of Chilko Lake. Map is basec on the compilations of Schiarizza et ai. (1994) and Monger and Joumeay (1994). The Bridge River Terranealso includes local exposures cludes Permian limestone containing Tethyan fusilin :ds of clastic sedimentary rocks assignedto the Gun Lake and (Brandon et 41.. 1988). Downton Lake units in the southern part of the Taseko - The Bridge River Complexin the Taseko - Bridge River Bridge River map area. These undated rocks rest strati- area is thought to have accumulatedas an accretion--suhduc- graphically above the Bridge RiverComplex, and are cor- tion complex on the basisits ofwide age range, the appanmt related with theCayoosh assemblage, a thick successionof lack of an internal stratigraphy, commonly observed out- Jura-Cretaceousclastic sedimentary rocksthat conformably crop-scale tectonic disruption, and presence of Middle to overlies the Bridge River Complex farther to the south Late Triassic blneschist. Accretionary tectonics, presnm- (Journeay and Northcote, 1992; Mahoney and Journeay, ably related to subduction, apparently continued until at 1993; Journeay and Mahoney, 1994).Parts of the Cayoosh least latest Middle Jurassic time, as cherts of this age ue assemblage, including the Gun Lake unit, may conelate known to be imbricated within the complex (Cordey 2nd with the basal unit of the Relay Mountain Group, whichis Schiarizza, 1993) whereas clastic rocks which overlie the also inferred to have been deposited directly above the complex (Gun Lake unit and Relay Mountain Group) (lis- Bridge River Complex.Parts of the Caywsh assemblage are play a coherent stratigraphy. The continuoussedimentation probably older, however, as basal turbidites of the assem- recorded across thecontact between more coherent sectims blage locally overlie, withapparent conformity, limestone- of Bridge River Complexand the overlying Jura-Cretaceous bearing portions of the BridgeRiver Complex which have Cayoosh assemblage farther to the south indicates that the yielded Upper Triassic (Norian) conodonts (Journeay and early to mid Mesozoic phase of accretionary tectonics iid Mahoney, 1994). This suggests that the onset of sustained not effect all parts of the complex or completely close the clastic sedimentation was markedly diachronons within the Bridge River ocean basin. However,by late Middle Jurassic Bridge River basin. time the basin had narrowedto the extent that youngersedi- The Bridge River Complex and overlying Cayoosh as- mentation was dominated by clastic deposits, perhaps $e- semblage outcropin a belt that extends for about 150 kilo- rived from flanking arc terranes (Mahoney and Journeay, metres southeastward from the Bridge Riverarea, where it 1993; Journeay and Mahoney. 1994).A later pulse of sib- is truncated bytheFraserRiverfault.Rockscorre1ativewith duction-related deformation within Bridge River Termne the Bridge River Complex the on east side of thefault com- may be recorded by Early Cretaceous blueschists of the prise Permian to Jurassic chert, greenstone and pelite of the Shuksan Terrane of the North Cascade Mountains, which Hozameen Group, which outcrops withinthe eastern Cas- have been correlated with the Settler schist and Caymsh cade foldbelt of southern British Columbia and adjacent assemblage of the southeastern CoastBelt (Monger. 1991b, Washington state (Haugerud, 1985; Monger, 1989). The Monger and Journeay, 1994). Thisepisode may have led to metachert and metabasite-bearing Napeequa unit (Mad final closure of the Bridge River basin, and culminated in River Terrane) and Twisp valley schists of the Cascade the mid to early Late Cretaceous contractional deformation Metamorphic Core may also correlate with the Bridge River that characterizes the sontheastern Coast - north Cascade Complex (McGroder,1991; Miller et al., 1993h). Otherpo- orogen. Mid-Cretaceous clastic sedimentary rocks deFos- tentially correlative assemblages include the Cogburn ited in the Tyaughton - Methow basin during this conuac- Group within the metamorphic culmination of the south- tional deformation containdetritus derived from the Bri,ige easternCoastBelteastifHarrisonLake,andtheElbowLake River Complex and provide the first record of uplift ,md Formation withinthenorthwest Cascadesystemtothe south erosion of Bridge River Terrane (Gamer,1989, 1992). (Monger and Journeay, 1994). Structurally higher levels Some workers (e& Rusmore etal., 1988; Rusmore .md within this part of the orogen may correlate with the Woodsworth, 1991a) have correlated the Bridge River Cayoosh assemblage;these include the Settler schist.of the Complex with parts of the Cache Creek Terrane, which out- southeastern Coast Belt, (Monger and Journeay, 1994), the crops between Quesneland Stikine terranes in the Intermon- Chiwaukum schist of Nason Terrane within the Cascade taneBelt.Althoughthetwoassemblagesarebroadlysimi1ar Metamorphic Core (McGroder, 1991) and the Danington in structuralstyle and lithologic components, including'hi- phyllite and Shuksan greenschist-blueschist of the North- assic bluescbists, there are also important differences ,hat west Cascades Thrust System(Monger, 1991h). argue against their correlation.The first is a significant(lis- The Bridge River Complex may also correfare with parity in the youngest known radiolarian cherts, whichare chert, basalt and limestone of the Deadman Bay Terrane late Middle Jurassic inthe Bridge River Complex and Late (including Deadman Bay volcanics and Orcas chert, Bran- Triassic in southernCache Creek Terrane; Early or Micldle donetaZ.,1988),whichisacomponentoftheSanJuanthrnst Jurassic radiolarians extracted from the westem CaEhe system (see Figure 42). This correlation is based on a very Creek are in tuffaceous argillite, andtherefore record a dif- close correspondence in the known ages of chert,as those ferent type of sedimentation (Cordey et al., 1987; Cordey in the Deadman BayTerrane are mainly Permianto Lower and Schiarizza, 1993). A second important different,: is Jurassic, hut also include a single Mississippian locality based on thestructural relationships exposed in theTaszko (Whetten et al., 1978), as well as comparahle geochemistry - Bridge River map area (Chapter 3). which suggest !hat of hasalts, which resemble tholeiitic and alkalic basalts of Bridge River Terrane originated west ofCadwallader 7%- modem Ocean islands (Potter, 1983, 1986, Brandon et al., me. This does notsupport the proposedreconstructions of 1988; Macdonald, 1990a.b). The Deadman Bay Terrane Rusmore and Woodsworth (1991a) whichsuggest that the also includes Upper Triassic limestone, comparableto most Cache Creek - Bridge River basin lay a of a system of limestone bodies inthe Bridge River Complex,hut also in- arcs represented by Stikine and Cadwallader terranes. Fi- _- 156 Geological Survey Bnlnch Ministry ofEmpioymenr andhesfment nally, relationships in the southeastern Coast Beltindicate Cadwallader Terrane is inferred to have originaledeast that the Bridge River ocean basin remained open wellinto of Bridge River Terrane because it typically occur!; struc- the latter part of the Mesozoic (Joumeay and Mahoney, turally above the Bridge River Complexacross son:.hwest- 1994). and that its final collapse, coupled with uplift and directed thrust faults (Chapter 3). This relative erosion of its contents, did notoccur until mid-Cretaceous paleogeographic positioning is consistent with the ?resent time (Garver, 1989,1992).This contrasts markedly with the distribution of terranes along the northeastern marginof the history of Cache Creek Terrane, which was uplifted and southeastern Coast Belt in the Camelsfoot Range and its eroded followingits amalgamation withadjacent terranes in offset counterpart east of Chilko Lake, comprisinj:, from Early Jurassic time (Monger et al., 1982). northeast to southwest, Methow, Cadwal1ade.r and Bridge The above differences indicate that the Bridge River River (Figure 39). It is also consistent with the inlariable and Cache Creek terranes had distinctlydifferent