CHEMOSTRATIGRAPHY OF THE UNION WASH FORMATION: IMPLICATIONS FOR THE EARLY TRIASSIC RECOVERY FROM THE PERMIAN TRIASSIC MASS EXTINCTION

Previous studies have suggested that anomalous oceanic conditions may have persisted through Early Triassic time in the aftermath of the end Permian extinctions(s). Evidence for this includes low diversity marine faunas and anachronistic carbonate facies such as microbial buildups in subtidal normal marine settings and large seafloor calcium carbonate precipitates (more common to Archean and Paleoproterozoic time) in deeper paleoenvironments (Schubert and Bottjer 1992, Woods et al. 1999, Pruss and Bottjer 2001). Along the western margin of Pangea (western United States), microbial buildups occur in the Virgin Limestone Member of the Moenkopi Formation (southern Nevada) and seafloor precipitates are found in the Union Wash Formation (east-central California). These rock units record a shelf to basin transect deposited during Spathian (late Early Triassic) time.

We have initiated an intensive integrated chemostratigraphic (d¹³C_carbonate, d¹³C_organic, d³⁴S_{trace sulfate}, d³⁴S_sulfide) study of the Virgin Limestone Member and the Union Wash Formation in an effort to better understand the origin of these features. Preliminary d³⁴S results on trace sulfate from the limestones of the precipitate bearing unit in the Union Wash Formation reveal a dramatic positive excursion (from +5.3 to +25.1 per mil CDT) in sulfur isotopes concomitant with a decrease in sulfate abundance. These results are consistent with the hypothesis that bacterial sulfate reduction in an anoxic water column existed during this time (e.g., Woods et al., 1999; cf. Knoll et al., 1996). Periodic upwelling of this anoxic water mass may have contributed to the biotic stress and unusual carbonate facies seen in the Early Triassic. These results corroborate those of Holser et al. (e.g., 1986) who previously discovered an apparently global sulfur spike in Spathian marine evaporites with values reaching between +24 to +28 per mil (from +11per mil in the latest Permian), considered the largest known sulfur excursion in the Phanerozoic.