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Sinemurian (Early Jurassic) stratigraphy at Last Creek, British Columbia and Five Card Draw, Nevada : paleontology and environmental implications

University of British Columbia

Over 400 ammonite specimens were collected from the Last Creek Formation in Last Creek, British Columbia and the Sunrise Formation in Five Card Draw, Nevada. A detailed taxonomic study from the Involutum Zone to the lower part of the Harbledownense Zone of the Sinemurian is presented. At least 38 species are identified and described, distributed amongst 15 genera and 5 families. Three species are introduced as new: Tipperoceras n. sp. A, Tmaegoceras obesus n. sp., Arnioceras n. sp. A. Also, Echioceratidae gen. et sp. indet. is tentatively introduced as a new species and genus. The following zones of the current Sinemurian zonation for western North America (Taylor et al., 2001) can be recognized, based on the stratigraphical distribution of the taxa identified, namely the Involutum, Leslei, Carinatum and Harbledownense zones in ascending order. The Jamesi Zone placed between the last two by Taylor et al. (2001) cannot be recognized in the study areas in either Canada or the United States and therefore it is removed from the current zonation scheme. A revised zonation and definition of the zones, with precise correlation to the primary standard northwest European zonation, are provided. The transgressive and regressive events in both Last Creek and Five Card Draw are calibrated with the revised Sinemurian zonation. Their relationships with eustatic changes as well as ammonite biodiversity and faunal turnover are also investigated. The Early Sinemurian transgression proposed by Hallam (1981, 1988) is well represented in both the Last Creek and Five Card Draw sections, and co-occurs with ammonite diversity maxima and a possible global positive CIE (carbon isotope excursion). The mid-Late Sinemurian regression and Late Sinemurian transgression are represented by facies and palebathymetric changes in Five Card Draw. The contrast in paleobiodiversity and faunal turnover as well as in carbon and osmium isotope profiles of the two study areas suggest significant differences in depositional environment, relative connection to the open ocean, and differences in terrestrial sediment input.

Text

2014-08-19

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/50066

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/