PEERING INTO THE "MAD MAX" WORLD: AN ANALYSIS OF TEMPORAL AND SPATIAL SHIFTS IN ENVIRONMENTAL CONDITIONS ALONG THE WESTERN MARGIN OF NORTH AMERICA DURING THE EARLY TRIASSIC

Studies of Lower Triassic sedimentary rocks from a variety of locations around the globe reveal that the Early Triassic was one of the most remarkable periods in Earth history. Sedimentary facies deposited during this time demonstrate widespread and enduring anoxia in much of the world ocean and the prevalence of inorganic seafloor cements and microbialites further hint at unusual marine conditions. In an attempt to better quantify Early Triassic environmental conditions and associated temporal and spatial variations, several marine sequences from the western margin of North America were examined, including the Sulphur Mountain Formation of western Alberta, the Dinwoody and Thaynes Formations of the Phosphoria Basin, and the Union Wash Formation and Virgin Limestone of eastern California and southwestern Nevada. This data was combined with previously-published paleoxygenation data from other localities in the region. Results from the Phosphoria Basin (Wignall & Hallam 1992), and the northern (Wignall & Twitchett 2002) and southern Canadian Rockies indicate anoxic deep waters grading laterally into suboxic shallow waters during the Griesbachian. During the Dienerian shallow water successions were deposited under conditions of improving oxygenation in the Phosphoria Basin, with deeper waters remaining anoxic. Deep-marine facies continued to be deposited under anoxic conditions during the Smithian-Spathian interval, with anoxic waters possibly flooding into shallower environments. In addition, the growth of inorganic seafloor cements around the Smithian-Spathian boundary in the Union Wash Formation, and concomitant (?) growth of stromatolitic mounds in the Virgin Limestone suggest a sustained episode of carbon dioxide degassing in this region, probably related to the presence of alkaline, anoxic waters offshore. Anoxic conditions in deeper-water environments apparently persisted into the early Middle Triassic before finally dissipating. While more detailed paleoenvironmental work remains to be done on Lower Triassic sedimentary sequences, evidence continues to be discovered that implies Early Triassic environmental conditions were uncharacteristic for the Phanerozoic, and may have played a strong role in determining the shape and timing of the recovery from the Permian-Triassic mass extinction.