Limited Recovery from the Permian-Triassic Mass Extinction despite the Presence of Complex Ichnofabrics in the Lower Member of the Union Wash Formation, California

Signs of biological recovery typically appeared quickly after most of the mass extinctions in Earth's history. Delayed recovery following the Permian-Triassic mass extinction is thought to be due to unusual oceanic conditions, including anoxia and hypercapnic stresses, which persisted from the Permian-Triassic boundary until the end of the Early Triassic. In areas where oceanic conditions returned to normal, however, organisms recovered at normal Phanerozoic rates. The thin-bedded limestones and silty limestones of the Dienerian-Smithian Lower Member and Middle Member of the Union Wash Formation near Darwin, California are characteristic of shallow shelf and middle to outer shelf environments, respectively, and were examined in order to better establish the relationship between biotic recovery and environmental conditions. The Lower Member of the Union Wash Formation shows extensive biologic activity in the form of abundant Thalassinoides trace fossils. The upper portion of the Lower Member and the Middle Member of the Union wash Formation show a return to stressful oceanic conditions, reflected in the rock record by the presence of finely laminated facies, seafloor precipitates, and a lack of metazoan fossils. The bioturbated beds of the Lower Member of the Union Wash Formation were examined to determine the extent of the biologic recovery during the Early Triassic, using the ichnofabric index of the tops and sides of bedding planes and burrow diameter as recovery proxies. Despite the presence of complex ichnofabrics, the small burrow diameters and monotypic trace fossil type of the Lower Member of the Union Wash Formation indicate that only limited recovery occurred in the shallow shelf environment, and was ultimately squelched when anoxic waters flooded the area. The results of this study suggest that more complex factors than the presence or absence of harsh environmental conditions may have been responsible for determining the timing of biotic recovery.