THE MOENKOPI FORMATION (SOUTHWESTERN UNITED STATES) AS A MODEL SYSTEM FOR DETERMINING LAND-SEA LINKS DURING EARLY TRIASSIC ENVIRONMENTAL AND BIOTIC CRISES

Limestones of the marine Lower Triassic (Spathian) Virgin Limestone Member of the Moenkopi Formation, as well as their offshore and basinal equivalents in the Union Wash Formation, outcropping in southern Nevada and eastern California, contain sedimentary features indicative of stressful environmental conditions like those interpreted for a number of Tethyan Permian-Triassic boundary sections, as well as other stratigraphic intervals occurring throughout the Lower Triassic. Such sedimentary features include deposition of microbialites, including stromatolitic and thrombolitic reef mounds, as well as large seafloor calcium carbonate precipitates (e.g., Woods et al., 1999; Pruss et al., 2006). Deposition of these features is interpreted to have occurred as a response to incursion of deep euxinic and CO2-rich ocean waters from Panthalassia onto the shelf margin along the western edge of Pangea (e.g., Marenco et al., 2003). Such features indicate a largely toxic global ocean which was responsible for much of the end-Permian mass extinction as well as the prolonged Early Triassic recovery from this event.

These carbonates of the Virgin and the Union Wash have been of great utility towards understanding the biotic and sedimentary signal of Early Triassic environmental crises in marine environments along this Pangean continental margin. Associated Moenkopi sedimentary units may provide a significant opportunity to examine the effects of these Early Triassic stressed environments upon terrestrial settings as well. The Virgin Limestone interfingers extensively with the Moenkopi Lower Red Member and correlatives towards the east. The Lower Red Member represents primarily siliciclastics deposited in coastal and fluvial environments. These outcrop relationships can serve as a framework in which to examine environmental conditions on land when adjacent Early Triassic marine systems were undergoing significant stress. These sections, like the variety of marine and non-marine Permian-Lower Triassic stratigraphic sections which exist in China, as well as non-marine Permian-Lower Triassic sections of the Karoo in South Africa (e.g., Ward et al., 2000), present outstanding opportunities to develop an understanding of land-sea process links during end-Permian and Early Triassic environmental crises.