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Paleoenvironmental interpretation and identification of the Norian-Rhaetian boundary in the Sinwa Formation (Mount Sinwa, British Columbia) using stable isotopes and conodonts

Pacific Centre for Isotopic and Geochemical Research

The Sinwa Formation is exposed at its type locality on Mt. Sinwa, located in the southern portion of the Whitehorse Trough in northwestern British Columbia. Composed of thick, fossiliferous Late Triassic carbonates, the formation is interpreted to have been deposited in the forearc basin between the Stikine Terrane (pre-accretion) and the North American Craton. This study marks the first detailed stratigraphic investigation of the Sinwa Formation and has significant implications for understanding Late Triassic paleoenvironmental changes in the region and the location of the Norian-Rhaetian boundary in North America. It also contributes to ongoing research refining Late-Norian to Rhaetian conodont biostratigraphy. This was achieved by recording lithologies, δ13C, and conodonts across a 231.3 m thick section. Carbonate facies progress up-section from rip-up clast rich mudstone, to coral fragment wackestone, to bivalve coral wackestone/packstone and intact coral boundstone, to a dark siliciclastic shale. This progression is interpreted as rising sea level caused by local tectonics. Petrographic thin sections show variation in depositional energy based on matrix texture. Conodont species recovered include Mockina englandi, M. carinata, M. cf. spiculata, M. bidentata, and M. mosheri. Being exclusively Rhaetian (Carter and Orchard, 2007), the appearance of M. mosheri at 231 m places the Norian-Rhaetian boundary below it. The δ13C record shows 3 negative excursions. Comparable to the western Tethys section investigated by Zaffani et al. (2017), the stratigraphically highest excursion starting at 230 m may coincides with the Norian-Rhaetian boundary. In considering evidence from both conodont biostratigraphy and carbon isotopes the boundary is tentatively constrained between 230-231 m from the base of the Mt. Sinwa section. In a lithological context, this places the Norian-Rhaetian boundary 0.3-1.3 m below the transition between coral boundstone (Facies 3) and shale (Facies 4).

Sinwa Formation (B.C.); Sinwa, Mount (B.C.)

Text

University of British Columbia. EOSC 449

Vancouver : University of British Columbia Library

10.14288/1.0379764

Science, Faculty of; Earth, Ocean and Atmospheric Sciences, Department of

Unreviewed

http://creativecommons.org/licenses/by-nc-sa/4.0/