The Environmental Importance of Biogenic Structure Distribution across An Early Triassic Mixed Carbonate-Siliciclastic Shelf: Implications for Assessing the Rate of Recovery Following the End-Permian Mass Extinction and the Nature of Post-Mass Extinction

The aftermath of the end-Permian mass extinction saw the expansion of harsh marine stresses such as anoxia and euxinia into shallow marine environments. These stresses can be reflected in the Lower Triassic rock record as unusual sedimentary features such as subtidal microbialites and wrinkle structures, both of which have been interpreted to form in the absence of pervasive bioturbation (Pruss et al., 2005). Stresses likely originated in the deep ocean, impinging upon the continental shelves during the Early Triassic, yet it is unclear how close to shore these stresses actually reached, and whether there was actually any refuge from this Early Triassic toxic ocean.

The Virgin Limestone Member of the Lower Triassic (Spathian) Moenkopi Formation of southern Nevada and southwestern Utah contains numerous indications that it was deposited during a period of marine stress, with features including subtidal microbialites and wrinkle structures (e.g., Pruss et al., 2005). Examination of sediments of the Virgin Limestone deposited in nearshore environments reveals a surprisingly different story, suggesting a sharp environmental contrast between offshore and onshore recovery patterns. Sediments deposited in lagoonal and shoreface environments have greater trace fossil diversities, higher ichnofabric indices, and in some instances much larger burrow diameters that those observed in offshore environments of the Virgin Limestone. Sediments deposited at the offshore transition, which may represent the divide between severe offshore stress and less severe onshore stress, appear to be the most variable, and proximal tempestites are observed to be either colonized by several ichnogenera or contain extensive wrinkle structures in the complete absence of bioturbation.

This study reveals the importance that environmental disparity can have upon the appearance of the recovery from the end-Permian mass extinction, and that the presence of harsh marine stresses in offshore environments may not necessarily indicate a shore-to-shore global toxic ocean.