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Uppermost Hettangian to lowermost Pliensbachian (Lower Jurassic) biostratigraphy and ammonoid fauna of the Queen Charlotte Islands, British Columbia

University of British Columbia

Extensive fossil collections from the upper part of the Sandilands and the lowermost Ghost Creek Formations allowed a detailed taxonomic study of the latest Hettangian to earliest Phensbachian ammonoid fauna of the Queen Charlotte Islands. 61 taxa, allocated to 27 genera, are described. Sunrisites senihlevis, Plesechioceras yakounense, Tetraspidoceras pacificum and Tetraspidoceras recognitum are introduced as new species. The vertical distribution of ammonites, documented in measured sections, serves as a basis to distinguish six successive assemblage zones: the Canadensis Zone, "Coromceras" Zone, Arnouldi Zone, Varians Zone, Harbledownense Zone, and Recognitum Zone. Of these only the Canadensis Zone was established earlier, the remaining five are defined here for the first time. This zonation permits high-resolution correlation of the sections. The total thickness of uppermost Hettangian to lowermost Pliensbachian strata in sections on Kunga Island is estimated at 385 m. A comparison of selected zones in different sections shows a subtle thickness increase to the south. The contact of the Sandilands Formation and the overlying Ghost Creek Formation is diachronous, younging gradually to the south. The faunal succession in the Queen Charlotte Islands agrees well with that of the Taseko Lakes area and Nevada, promising regional applicability of the proposed zones. Intercontinental correlation with the northwest European standard zonation is possible at diffrent levels. The Hettangian/Sinemurian boundary is contained within the Canadensis Zone. It is best approximated by the first appearance of Badouxia columbiae and Metophioceras spp. The position of the Recognitum Zone at the Sinemurian/Pliensbachian boundary remains problematic; most evidence on hand points to its Pliensbachian affinities. The ammonite fauna consists of taxa with pandemic, Tethyan, Athabascan, East Pacific, and Pacific distribution. Provincialism was not prominent but existed during the Sinemurian time. The high proportion of Tethyan forms is in accord with the theory suggesting a more southerly original paleolatitude for Wrangellia. The distribution of Tethyan forms can be explained by the early opening of the Hispanic Corridor, proven to be in existence by the Pliensbachian. Alternatively, the pantropic distribution model cannot be ruled out, although the faunal record from the eastern Tethys is inadequate to prove it. Thestrong representation of Athabascan and East Pacific elements renders long-distance longitudinal tectonic dislocation of Wrangellia unlikely. The paleoecology and taphonomy of ammonites, associated macrofauna, and trace fossils is used for paleoenvironmental reconstruction. The general lack of bioturbation, predominance of thin-shelled, presumably pseudo-planktonic bivalves, and the intact preservation of fish and a crinoid specimen indicate prevailing oxygen deficient bottom conditions. Trace fossils provide evidence for periodic improvements of bottom oxygenation. Different modes of ammonite preservation are controlled by shell morphology as well as the varying sedimentation rate and diagenetic regime. The latter reflects changes in redox conditions in the upper sediment layers.

Text

2010-12-02

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http://hdl.handle.net/2429/30266

Geological Sciences

University of British Columbia

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