PALEOENVIRONMENTAL INTERPRETATION AND IDENTIFICATION OF THE NORIAN - RHAETIAN BOUNDARY IN THE WHITEHORSE TROUGH (STIKINE TERRANE, NORTHERN CANADIAN CORDILLERA)

The Sinwa and Aksala formations consist primarily of extensive Late Triassic carbonates deposited in the Whitehorse Trough, the forearc basin positioned between the pre-accretionary Stikine Terrane and the North American Craton. Despite excellent exposure of some of the best developed Late Triassic reef complexes in North America, these formations remain understudied. Here, we present the first detailed chemostratigraphic and biostratigraphic studies of these formations. Lithofacies progression and high-resolution sampling for stable isotope geochemistry is used to interpret paleoenvironmental shifts in the basin during the Late Triassic, with implications for regional and global trends, as well as tectonic reconstruction of the Northern Canadian Cordillera. Conodont collections allow for the refinement of a newly established Late Triassic conodont taxonomy, and the recognition of the Norian - Rhaetian boundary.

We present two stratigraphic sections logged on Mt. Sinwa, located in the Northwest corner of British Columbia, and two more sections in the Whitehorse region of Yukon: Lime Peak and Hill 4308. Mt. Sinwa shows a facies progression consistent with a gradual shift from a back-reef to a reef crest setting near the top of the succession. Integrated conodont biostratigraphy and stable isotope chemostratigraphy constrains the placement of the Norian - Rhaetian boundary in both Mt. Sinwa sections, with the first occurrence of the Rhaetian conodont species Mockina mosheri coinciding with a negative δ¹³C excursion. This excursion can be correlated with those identified in sections spanning the Norian - Rhaetian boundary in the Tethys region. Strong covariance between δ¹³C and δ¹⁸O in both Yukon sections show that the primary δ¹³C signal has been lost, unlike the British Columbia sections. This discrepancy in the Sinwa and Aksala Formations could point to a difference in depositional positioning within the Whitehorse Trough, and/or a difference in diagenetic history. Through the integration of lithostratigraphy, chemostratigraphy, and biostratigraphy, this study paints a full picture of paleoenvironmental progression off the West Coast of North American during the Late Triassic and improves stratigraphic correlation between North America and the Tethys during this time interval.