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Late Triassic-Early Jurassic Calcareous Nannofossils of the Queen Charlotte Islands, British Columbia

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Late Triassic-Early Jurassic Calcareous Nannofossils of the Queen Charlotte Islands, British Columbia Late Triassic-Early Jurassic Calcareous Nannofossils of the Queen Charlotte Islands, British Columbia PAUL R. BOWN Dqinrfiiteiifof Geological Sciences, Micropalaeontology Unit, University College London, Gower Street, London WClE 6BT, U.K. ABSTRACT 5diments of the Kunga and Maude groups (Early Norian-Aalenian) from the Queen Charlotte Islands, British Columbia,have yielded the first records of early Mesozoic calcareousnannofossils in North America. These occurrences are the only record of Triassic nannofossils away from the recent discoveries in the Tethyan area (Austria,Indonesia, and the northwestern Australian margin). The nannofossil assemblages from the Queen Charlotte Islands are generally poor to moderately preserved, with species diversities typical for this time interval (2-10 species).The assemblages are comparable to those from northwest Europe, for the Lower Jurassic, and to those from Austria, Indonesia, and the Australian margin, for the Upper Triassic. They show compatible stratigraphicalranges, allowing the applicationof existingbiostratigraphical zonations. Palaeobiogeographical interpretations are inconclusive and fail to positively confirm a low latitude (Tethyan) position for the Queen Charlotte Islands, as indicated by macrofossil data. However, Triassic nannofossil assemblages have yet to be described from high palaeolatitude locations. In addition, certain features of the Lower Jurassicassemblages may indicate Tethyan affinitiesbut also distinguishthese assemblages from those which have been previously documented. These features include the absence of Schizosphnerelln piinctulata, whichoccursabundantlyinthecircum-Mediterraneanregionandwas previously thought to be “cosmopolitan“ in distribution; and the absence of Mitrolithus jansae which characterizes “Tethyan” assemblages in the circum-Mediterranean area. The Queen Charlotte Islands results are com- pared with new nannofossil data from Argentina, Tmor and North America. I. Micropalaeontol., 11 (2):177- 188, December 1992. 133 iiz 131 INTRODUCTION Calcareous nannofossils are relatively well studied for much of the Late Cretaceous to Recent interval and provide valuable Point biostratigraphicaland palaeoenvironmentalinformation.From their first appearance in the Late Triassic and through their Island initial evolutionary diversification during the Jurassic, they Point are less well known and data are geographically restricted, ISLAND primarily to the northwest European region. Although Branch 59 advances have been made in biostratigraphical resolution for Rd. this time interval, our knowledge of their palaeobiogeography is still rudimentary. At present it is possible to recognise STRAIT significant differences between northern European (Boreal) and circum-Mediterranean/proto-Atlantic (Tethyan) assemblages, but data from the remainder of Tethys and the 5 Pacific region are extremely rare (Bown, 1987). The discovery of calcareous nannofossils in the Upper Triassic and Lower Jurassic of British Columbia is therefore of interest in terms of testing biostratigraphical schemes established in the European area, and also for determining palaeobiogeographical patterns in a region of the early Mesozoic ocean unstudied by nannofossil workers. GEOLOGY OF THE QUEEN CHARLOTTE ISLANDS The Queen Charlotte Islands are part of the Insular Belt of the 0 20 50 Canadian Cordillera, situated at the western edge of the North American continent and separated from the Pacific Ocean floor by the Queen Charlotte Transform Fault. To the east lies Fig.1. Queen Charlotte Islands location map. 177 Bown 'UFNS INEMUMAN EITANGIAN u52 -uz< NORlAN M NORIAN PERIL - L CARNIAN CARNlAN Kl Fig. 2. Composite stratigraphy and nannofossil distribution of lower Mesozoic sequences, Queen Charlotte Islands. Stratigraphy from Carter et 01 (1989)and Tipper et 01. (1991). the shallow shelf region of Hecate Strait; to the south, Queen Formation (thinly-bedded and banded, hard, dark siliceous Charlotte Sound and Vancouver Island; and to the north, siltstone with interbedded tuff, fine to medium grained Dixon Entrance and the Alexander Archipelago of southeast sandstone, andshale). TheTriassic/ Jurassicboundary interval Alaska (Fig.1). Theislandsarecomposed of a varied assemblage falls within the Sandilands Formation. of Mesozoic and Cenozoic plutonic, volcanic, metamorphic, Conformably overlying the Kunga Group is the Maude and sedimentary rocks (Sutherland Brown et al., 1983). Group, of Early to early Mid Jurassic age (Sinemurian- Basement is formed by the Upper Triassic Karmutsen Aalenian). This unit is divided into five formations: the Ghost Formation which is predominantly composed of volcanic Creek Formation (dark grey shale and silty shale, with pale rocks. There is, however, recent evidence of older, Palaeozoic limestone interbeds and lenses); Rennell Junction Formation rocks (Carboniferous-Permian), below the Karmutsen (fine grained sandstone, argillaceous siltstone, grey shale, and Formation (Hesthammer et al., 1991).Overlying the basement limestone interbeds and lenses);Fannin Formation (sandstone, is a relatively complete early Mesozoic sedimentary sequence minor siltstone, and rare tuffaceous shales); Whiteaves which ranges from Carnian (Upper Triassic) to Aalenian Formation (glauconitic, greenish-grey silty shale with minor (Middle Jurassic) and is for the most part only moderately sandstone interbeds, limestone nodules, and bentonitic ash tectonised and of low metamorphic grade (Fig.2). All units are beds); and Phantom Creek Formation (brown, fine to coarse relatively well dated by integrated ammonite, conodont, and grained sandstones). The Mid to Late Jurassic succession radiolarian studies (e.g. Carter et al.; 1989, Tipper et al., 1991). which includes the Yakoun and Moresby groups is dominated The lowest unit is the Kunga Group which is Late Triassic to by pyroclastic lithologies, along with conglomerate, shale, Early Jurassic (Carnian-Sinemurian) in age and divided into and siltstone. three formations: the Sadler Limestone Formation (thickly- This early Mesozoic sequence is at the western edge of an bedded, dark limestone); the Peril Formation (thinly-bedded allochthonous tectonostratigraphic terrane known as dark limestone with subordinate dark shales and siltstone; Wrangellia. The lithological units can be related to those common Halobia and Monotis bivalves), and the Sandilands within the Wrangellia Terrane on Vancouver Island and in 178 Triassic-Jurassic Calcareous Nannofossils KENNECOTT POINT - SECTION KP-M FREDERICK ISLAND - SECTION E C.R.. ..... A,. z KP-8 c.. 5 ..... [I KP-7 KP-6 A. R.. P KP-5 c.. W 10444 ..... 10446 A. R.. ' W KP-4 A. F. 'i oa KP-3 A..R. 5 10445 A.F.. KP-2 C. R,. KP-1 AF.. W FREDERICK ISLAND - SECTION C 4n I 10436 10435 10434 10433 10432 FI-1 m FI-8 KENNECOTT POINT - SECTION X FI-7 10431 . FF FI-6 ..... 5 FI-5 ..... a 10438 ...R. 10437 ..... FI-3 ccc.. FI-4 ..... I KEY R = Rare (0-2 counts) v) F = Few (3-5 counts) D C = Common (6-100 counts) z Q. KP-10 A = Abundant (1 01 -1 000 counts) nd KP-11 VA = Very abundant (>lo01 counts) W z ? = Questionably present av) . = Not present 3 Fig.3. Stratigraphical distribution of nannofossils from Upper Triassic sections of the Queen Charlotte Islands. 179 Bown MAUDE ISLAND MORESBY ISLAND - WHITEAVES BAY ............ ............ U. MI-22 jj ............ MI-20 ............ 3 MI-19 ............ Lu- ............ z t5 MI-18 Z $ MI-17 ............ Z $ 3 MI-16 ............ s7MI-15 ............ 0I- MI-14 . F. FFF.. .... MI-13 RF. F. F. R.. ?R 3v) MI-12 .R.R...F.... 5 MI-11 .R.R........ UJ LU MI-10 .R.......... a MIL8 RR. F?F?. , . , . MI-7 ............ 2 MI-6 . F. FRF. R.. ?. 3 $ MI-5 RC. C. C. RR., . d MI-4 RF. FR.. ..... MI-3 . C. CRFFR., . , MI-2 RF?C. R. R?R., MI-1 FCRCFFRF.. .. GRAHAM lSLA ND - GRAHAM ISLAND - YAKOUN RIVER GHOST CREEK ROAD 2*kz GCR-6 ...... z:g$ GCR-5 ...... -ufiG%GCR4 F R F C R R ‘v) GCR-3 F . C R. R $iEg GCR-2 A F C C R R ~Cn$jGCR-1 A R F F . KEY R = Rare (0-2counts) F = Few (3-5 counts) C = Common (6100 counts) A = Abundant (101-1000counts) VA = Very abundant (>I001 counts) ? = Questionably present . = Not present Fig. 4. Stratigraphical distribution of nannofossils from Jurassic sections of the Queen Charlotte Islands. 180 Triassic-JurassicCalcareous Nannofossils mainland BritishColumbia. The palaeogeographical setting is thought to be that of a back-arc basin adjacent to the North In American craton. The sediments were deposited in a marine basin of fluctuating water depth which was periodically euxinic, and received variable amounts of volcanic detritus. Vulcanism was sporadic through the Late Triassic and Early Jurassic but became more persistent in the Mid Jurassic (Cameron and Tipper, 1985). Macropalaeontological, stratigraphical, and palaeomagnetic evidence suggest that the n Wrangellia terrane has undergone post-Pliensbachian latitudinal displacement of around 2400 km, moving north v) from a more “equatorial” position in the northern hemisphere, KP-45 . CRRR eastern Pacific (Smith and Tipper, 1989). KP-M .... KP43 F,, RF KP-39 ,FR.. NANNOFOSSIL RESULTS 10455 , R.. KP-38 F. R.. 1. Calcareous nannofossils from the Queen Charlotte Islands KP-37 ?.... A total of 200 samples were collected from the Kunga and KP-36 Maude groups (Upper Triassic to Middle Jurassic) during the KP-35 AF. .. KP-34 F.. 1990 field season. 75 of these, from throughout the KP-33 CF. .. stratigraphical interval, yielded calcareous nannofossils. The 10454 A.. KP-32 ..... nannofossil assemblages are generally poorly preserved
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