WIDESPREAD DETRIMENTAL OCEANIC CONDITIONS ALONG THE WESTERN MARGIN OF NORTH AMERICA DURING THE EARLY TRIASSIC: ANOXIA WITHOUT BORDERS

Analysis of Lower Triassic facies from the western United States and Canada reveal the occurrence of widespread, harsh environmental conditions within Panthalassic waters that impinged on the western margin of North America. Data collected from the Union Wash Formation of east-central California, the Virgin Limestone of western Nevada, the Dinwoody and Thaynes Formations of the Phosphoria Basin (Wyoming, Idaho, Nevada, and Montana), and the Sulphur Mountain Formation (western Alberta) suggest the widespread occurrence of anoxic marine waters during the Early Triassic. In general, the Lower Triassic facies examined lack, or exhibit minimal bioturbation, and lack, or contain only a limited fauna, comprised of species that specialize in living under stressed conditions (e.g., the inarticulate brachiopod Lingula, the bivalves Claraia and Unionites). Anoxic conditions presumably extended to the north, as anoxic facies are reported from rocks found in northern Canada. Beyond the De Indio Formation of northeastern Baja California, which has been offset 800 km to the south, the author knows of no other Lower Triassic localities south of the U.S.-Mexico border. Evidence from the Marble Creek Formation of the Cache Creek Terrane, which was presumably deposited under open-ocean conditions in the Panthalassic Ocean, reveal that Panthallassic anoxia was not confined to the eastern border of the mega-ocean, but was more widespread. In addition, the Union Wash Formation contains large, synsedimentary seafloor cements believed to have formed near the boundary between a deep, anoxic, alkaline water mass, and oxygenated surface waters. Mixing of the two water masses would have led to carbon dioxide degassing of the deep water mass and supersaturation of the remaining water with respect to calcium carbonate. The combination of anoxia and elevated carbon dioxide levels along the western margin of North America are of paleobiologic significance, because they may have played a role in extending the biotic recovery from the Permian-Triassic mass extinction by delaying the initiation of the recovery.