PALEOZOIC STIKINE ASSEMBLAGE IN THE ISKUT IRIVER P,ND CHUTINE RIVER REGIONS, NORTHWESTERN BRITISII-I COLU MBIA (104B/ll, 12; 104F/9, 16) By W.C. McClelland University of California, Santa Barbara
-0" -0" ." KEYWORDS: Regionalgeology, Stikine assemblage, Alaska suggest that metamorph c rocks west of and within Yukon-Tanana Temne, correlations. the Coast Belt are correlative with the Yukc n-Tanana Nisling terranes (e.g., Gehrels er a/., 1950, in xess; Garzau, INTRODUCTI'ON 1991; Rubin andSaleehy, 1991; Samson el a/.. l')91; McClelland er al., in press; Fitlure 1-17-1). Although ths: Paleozoicrocks included in theStikine assemblage juvenile Sm-Nd isotopic signatt re of the Stil ine Terrars: is (Monger, 1977) an: well exposedeast of theCoast Belt distinguished from the evolved signature ct aracteristi': of between the Taku and Iskut rivers (Figure 1-17-1). Recent the Yukon-Tanana Terrane (San~,soner a/., I )91), 1Md:lel- studies have provided insight into the age and stratigraphy landand Mattinson (1991) s.1ggested thktthe Stikin,z of these Lower to Middle Devonian, Carboniferous and assemblagemay be partly corrdative with nid-Pileozoi: Permian strata in theForrest Kerr - NewmontLake and rocks in the Yukon-Tanana Tenme. ScudRiver regions; (e.g., Anderson, 1989;Brown a/., ef Fieldwork during 1991 foctis8i:d on pre-Pel mian rock:; of 1991). These studies provide a framework for interpretation theStikine Terrane to estaidi:;h andcornlare theage, and correlation of poorly known, Permian and older rocks character and geologic relation5,hips of the :tikinian base- described by Kerr (1948).Geological Survey of Canada ment with the Yukon-Tanana Terrane in southeastem (1957) and Souther (1959, 1972) in the Chutine River area Alaska. Permian and older rock!. of the Stiki le assembhg: and south the Iskut River (Figure 1-17-1). of were examined inthe Chutin:: Riverand Forrest Kerr The nature of the contact between Paleozoic rocks of the regionsand south of the lskul River to p~widea strat- Stikineassemblage and metamorphic rocks in the Coast igraphic and structural framework for ;geoct ronologic ;Inti Belt to the west is uncertain. Recent studies in southeastern isotopic studies. Thefollowing aniclc: sun marires i'iel Kerr (1948) and the Geological Survey of1 :anada (1057) outlined the regional distribution of metarnor lhic rocks that underlie limestone of known or suspected F mnian age: in the Stikine and Iskut region. Schistose to gn ,issic :qi'Iitc!, metavolcanic rocks, quartzite a1-1dlimestone were reprlte,j and examined in thisstudy south of the :~kutRivcr a.t localities shown on Figure I-! 7 -2. BRUNT CREEK Brunt Mountain (Figure 1-1'7-:2) is underlai I by a massiv: section of clinopyroxene-p(,rphyritir tufl, flows, 1.01- caniclastic rocks and argillite #of probable La e Triassic ag: (Kerr,1948; Geological Surve:, of Canada, 1957). 'r'lese volcanicrocks overlie a section of interayered b1,Ic.r argillite, siliceous tuff, fine-grained volcanicl; stic rocks ;mj discontinuous layers of light grey weath(:ring. white ma4k The marble layers may either bt: Permian in age, based on along-strike projection of limestone pro )able Penrlian Figure 1-17-1. Simplified tectonstratigraphic map show- 0' ing the location <:f the study area and distribution of the ageexposed at the mouth of lilrunt Creekind along the Paleozoic Stikine assemblage (modified after Wheeler and CraigRiver; or Triassic,bawd on comp Irison of this McFeely, 1987 and Brown el al., 1991). sequence with similar rocks in the Telegra )h Creek. area Grulo,yira/ Fieldnvrk 1991, Paper 1YY2-I 207 Figure 1-17-2. Location map of Iskut-Craig rivers study area showing the distribution of the Stikine assemblage (modified after Kerr. 1948 and Wheeler and McFeely, 1987) and general location of sections examined during this project. (Souther, 1972; D.A. Brown,personal communication, hornblende-biotite quartz diorite of unknown age. To the 1991). In Brunt Creek, the marble-bearing section is under- west, the quartzose turbidites grade upwards intoa thick lain by phylliticargillite and fiqe-grainedvolcaniclastic sequence of light green tuffaceous clastic rocks dominated rocks with subordinate brown-weatheringmarble and mafic by centimetre-scale bedsof fine-grained sandstone, siltstone pillowed flows, fragmental rocks and tuff of uncertain but and mudstone. These rocks are in turn overlain by black possibleCarboniferous age. In Brunt Creek andnorth of argillite interlayered with dark brown marble and biotite- BruntMountain, hornblende clinopyroxene gabbroand amphibole schist derived from mafic tuffs and flows. The diorite that are inferred to be Late Triassic in age (Alldrick argilliteand volcanic section is cappedby light grey ef a!., 1990)and appear compositionally similar to the weathering, white marble that is apparently laterally contin- uppermost volcanic sequence of probable Late Triassic age, uous with limestone of probable Permian age exposed along intrude all of the above units. the Inhini River (Figure 1-17-2). Quartz-rich clastic rocks at the base of this section are CRAIG RIVER SIMMACREEK similar to continent-derived sedimentsof the Yukon-Tanana - Terrane in southeastern Alaska (e.#., Gehrels et u/., 1990). Theridge between the CraigRiver and Simma Creek This correlation and the apparent depositional relationship (Figure 1-17-2) is underlain by a thick sequence of gamet- between the quartzose clastic rocks and Permian rocks of biotite-white mica-feldspar-quartz schist derived from fine- theStikine Terrane suggest that the Paleozoic Stikine grainedquartzose, turbiditic strataand quartzite. Eastern assemblage either depositionally overlies orlaterally grades exposures of the clastic sequence are intruded by foliated into the Yukon-Tanana Terrane. 208 British Columbia Geological Survey Brunch DICK CREEK- INHINIRIVER PEIWIAN LIMES rONE East of the Inhini River, limestone of probable Permian age is faulted against a thick sequence of pyroxene crystal- lithic tuff and volcanlclastic rocks probably correlative with VOI..CANIC: RO( :KS theUpper Triassic Stuhini Group (Figure 1-17-2). Never- theless, the contact is probably a faulted depositional con- PALEOZOI ;CLAS tact. Rocksconformably underlying the limestone in the unnamed creek south of Fizzle Mountain include tuffaceous siltstone and sandstone. siliceous siltstone, mafic tuff and minor brown-weathel,ing limestone. North-dipping,massive Permian limestone along thenorth side of DickCreek is underlain by probable Carboniferous interlayered green to browntuffaceous siltstone, mafic flows, lapilli tuff and breccia. Thin limestone lenses within the volcanic section contain abundant crinoid fragments and rugose corals. Vol- caniclastic rocks at the base of the unit grade downwards intolight grey siliceous argillite. The upper part of the argillite contains a re'latively thin (IO m) coarsely crystalline white marble. Exposures south of Dick Creek are dominated by a thick section of light green, tuffaceous to quartzose turbiditic rocks that are similarto the clastic rocks overlying the sequence of quartzose turbidites and quartzite south of Simma Creek. Centimetre-scale beds of fine-grained sand- stone, siltstone and mudstone that make up the section may be Carboniferous or older as they appear to underlie the Figure 1-17-3. Generalized gedogic map of I le Chutine volcanic section exposed north of Dick Creek. River area (modified fromSouthe-, 1959) !:howil g distribu- tion of units discussed in text. C'L Chutine Lake Paleo.coic Stikine assemblage (modified aftcr Wheeler an, McFeely, 1987 and Brown et a/., 1991). MOUNT GEOFFRION- Mouwr FAWCETT Mount Geoffrion and Mount Whipple (Figure 1-17-2) are underlain by a thick iequence of probable Triassic mafic to West of Triumph Creek, the n1etaclaslic se'tion is o\cr- intermediate, pyroxene, amphibole and plagioclase-bearing lain by a thick section of tuffaceous argillit :, felsic tuff, tuff,debris flows, ,volcaniclastic rocksand subordinate limestone andmafic to intem1e:iiate flows, uff and \[)I- argillite that depositlonally overlies massive light grey to caniclastic rocks. Souther (1959) inferred a TI assic age lor whitelimestone of' knownPermian age (D.A. Brew, the volcanic portion of the sectian. A Permiar or older age unpublished data). West of Mount Geoffrion, the limestone is more likely as these rocks are overlair, by I ermian lime- overlies centimetre-sm-ale beds of fine-grained volcaniclastic stone to the southeast (D.A. Brc wn, personal communica- rocks, tuff andargillite. Thelower sequence contains at tion,1991). least two undated, massive limestone layers 5 to 20 metres thick. Probable Permian limestone along the ridge north of MountFawcett is underlainby mafic volcanic rocks, FORREST KERR REG~ION(I04B/ LO, 15) argillite and fine-gralned tuffaceous clastic rocks. This sec- tion is similar to that below limestone of probable Permian TheStikine assemblage expo:ied we:it of Forrest Kcrr age south of Simma Creek. Creek (Figure 1-17-1) includes complex8:ly d# formed fire- grained clastic strata, siliceous ilrgillite. lim, stone, mid'ic and felsic tuff, and mafic volcanic rocks (An( .erson. 19?9; CHUTINE RIVIER REGION (104F/9, 16) Read et a/., 1989; Logan et c/,,1990a, b Ilrown et 5'/., 1991). Massive limestone in the :section has yi :Ided Middle Souther (1959) assigned rocks east of the Coast Belt in Devonian fossils (Anderson, l!W; Read er a! , 1989). It is the Chutine River region to a metamorphic sequence that interlayered with argillite,fine-grained tuffa :eous cla:;tic includes marble, quartzite and orthogneiss, a sequence of rocks, mafic volcanic rocks, intelmediats: frai mental ro:ks quartzose clastic rocks, mafic volcanic rocks and limestone. andintermediate to felsic tuff. his west-dillping sectim These rocks were examined at Chutine Lake and west of gradesstructurally down (to the east) into t linlybedded Triumph Creek, respectively (Figure 1-17-3). Penetratively siliceous argillite and fine-grained clasti,: roc1 s, tuf1ace.1~~ deformed metamorp'hic rocks at Chutine Lake are derived greywacke. maroon debris flows intemxdiatf to felsic luff fromfine-grained turbiditic clastic strata and siliceous and fine-grained tuffaceous cl;stic rocks. Th : age of [his argilliteintruded by granodioriticdikes. Theages of the lower clastic sequence is uncertaln, howEver, :he section is metaclastic rocks and the granodiorite are unknown. These similar to Permian or older clar;tic rocks th; t are REFERENCES McClelland, W.C. and Mattinson, J.M. (1991): U-Pb (Zir- Alldrick, D.J., Britton, J.M., MacLean, M.E., Hancock, con) Constraints on the Age of the Yukon-Tanana Ter- K.D., Fletcher, B.A. and Hiebert, S.N. (1990): Geology rane in the Coast Mountains, Central Southeast Alaska and Mineral Deposits of the Snippaker Area; B.C. 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( 11337): Tectoni : Assemblage: of theUpper Paleozoic and Lower Mesozoic Alava Map of the Canadian Cordi lera; G~ologr.a/ Survr,~ojr Sequence:A Revised View of thePolygenetic Taku Canada, Open File 1565. Geological Fieldwor-k 1991, Paper 19Y2-I 211 NOTES