SHALLOW – WATER FACIES OF THE UNION WASH FORMATION, EAST-CENTRAL CALIFORNIA: WINDOWS INTO RECOVERY FROM THE PERMIAN – TRIASSIC MASS EXTINCTION

Views of recovery from the Permian-Triassic mass extinction have changed over time from a long, sluggish rebound to one that was strongly controlled by the presence or absence of extreme environmental conditions. Shallow-water facies of the Union Wash Formation of east-central California allow analysis of the interaction between harsh environmental conditions and Early Triassic recovery as these sediments were deposited in outer shelf to slope environments that were highly susceptible to the impingement of deep, anoxic waters. Early recovery is present in the Lower Member of the Union Wash Formation at Darwin Hills, CA in the form of extensive networks of Thalassinoides burrows that are indicative of a moderate level of recovery. Highly burrowed facies (ii = 4 – 5) are overlain by ~600 m of finely laminated sediments of the Middle Member and indicate cessation of the nascent recovery as anoxic and alkaline waters flooded the region. Recovery resumed following the deposition of seafloor cements (Woods et al., 1999; 2007) in the lower portion of the Upper Member based on the presence of sponges, echinoderm ossicles, microgastropods and peloids. The sponge-bearing unit is separated from bioturbated (ii = 4 – 5; primarily Chondrites) calcisiltite rich in bivalves (Eumorphotis and Leptochondria) by laminated mudstone, > 1 m thick. This second fossil-rich interval is overlain by an unfossiliferous green shale, 18 m thick that is capped by another precipitate-bearing limestone. A third fossil-rich unit immediately overlies the precipitate-bearing limestone and contains bivalves, microgastropods, and terebratulid brachiopods. This interval is overlain by unfossiliferous calcisiltite and micritic limestone that may represent a final bout of environmental stress or a preservational bias. While each individual occurrence of fossil-rich sedimentary rocks may be confused for repeated, failed recoveries, they more likely represent the slow steady recovery of fauna in adjacent shallower environments that migrated into the region when environmental conditions improved (e.g., Schubert and Bottjer, 1992). The results of this study therefore indicate that recovery is nuanced and dynamic, and analysis of the relationship between environmental stress and recovery must take broader regional trends into consideration.