MULTIPLE FAILED BIOTIC RECOVERIES ALONG THE OUTER SHELF EDGE OF WESTERN NORTH AMERICA DURING THE EARLY TRIASSIC

Biotic recovery from the Permian-Triassic mass extinction was complex, uneven and strongly controlled by the distribution of depositional environments and harsh environmental conditions. The Union Wash Formation at the Darwin Hills locality (east-central California) contains at least 2 and possibly 3 distinct Early Triassic recovery intervals that terminate abruptly with the onset of deleterious conditions. The first recovery interval is manifested as bioturbated (ii = 4-5) micritic limestone found in shallow marine facies of the Lower Member. The bioturbated beds consist of small diameter (2-3 mm) Thalassinoides burrows that occur periodically across a ~200m-thick interval that is overlain by laminated chert and micritic limestone. The second nascent recovery interval begins less than one meter above seafloor precipitate-bearing micritic limestone found at the base of the Upper Member and is marked by the occurrence of cm-scale siliceous sponges within isolated lenses. The lenses are ~2-3m across, 10-20cm thick and occur over a 3m-thick interval. The sponges are overlain by a thin (>1m) laminated and presumably anoxic micritic limestone, which is overlain by a 3-5m-thick bivalve-rich layer that contains extensively developed (ii = 4-5) and distinct trace fossils assignable to Chrondrites. The bivalves are poorly preserved, but recognizable genera include Eumorphotis and/or Leptochondria. The bivalve-rich layer is overlain by 18m of green shale and capped by a fossil-rich limestone, 30m-thick, which contains bivalves, microgastropods and ammonoids. This unit is brecciated throughout and contains transported and in situ seafloor precipitates in addition to vermicular limestone. The presence of multiple failed recoveries in outer shelf settings points to the importance of the amelioration of environmental conditions in sparking biotic recovery and highlights the vulnerability of nascent biotic recoveries to environmental perturbations in shelf-edge settings. As a result, a strong gradient in the timing and extent of recovery existed between nearshore and offshore environments. Faunas from nearshore environments appear to undergo an early and sustained recovery while contemporaneous shelf-edge faunas endured multiple phases of recovery and relapse as environmental conditions oscillated.