TRACE FOSSILS FROM THE LOWER MEMBER OF THE UNION WASH FORMATION, CA: EVIDENCE OF AN EARLY RECOVERY FROM THE PERMIAN-TRIASSIC MASS EXTINCTION?

Earth's ecosystems experienced an extended recovery from the Permian-Triassic Extinction. Unusual oceanic conditions, such as widespread anoxia and hypercapnia, are reflected in the rock record by anachronistic facies, depositional features and fabrics that were more common during the Proterozoic than the Phanerozoic, and a low diversity of metazoan fossils. The thin-bedded limestones and silty limestones of the 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, during this time and will be examined in order to better establish the relationship between biotic recovery and environmental conditions during this critical period in Earth history. Initial study of the Lower Member of the Union Wash Formation reveals the occurrence of shallow water, highly bioturbated beds (ichnofabric index 4 to 5) that are comprised entirely of small Thalassinoides trace fossils (burrow width averages 2-3 mm), overlain by laminated, and probably anoxic cherts and shales. This sequence is indicative of an initial, low-level recovery of trace makers following the mass extinction that was abruptly terminated when deep, anoxic waters flooded the shallow shelf environment, and supports hypotheses that recovery from the mass extinction began quickly where harsh environmental conditions were absent. Future work will better establish the level of recovery represented by the bioturbated beds as well as the environmental conditions that terminated the nascent recovery.