STROMATOLITES OF THE LOWER TRIASSIC UNION WASH FORMATION AND THEIR SIGNIFICANCE FOR THE BIOTIC RECOVERY FOLLOWING THE PERMIAN-TRIASSIC MASS EXTINCTION

Recent studies have documented the complexity of biotic recovery from the Permian-Triassic mass extinction. Study of Lower Triassic rocks from the southwestern U.S. has produced a variety of sedimentologic and biotic evidence that point to persistent environmental stress following the extinction (specifically anoxic, alkaline waters in the deep ocean, and possible high carbon dioxide levels in the surface ocean). Examination of the deep-water facies that comprise the upper Lower Triassic (Smithian-Spathian) Union Wash Formation of east-central California documented a variety of abiotic calcium carbonate precipitates that were growing on the seafloor (Woods et al., 1999; Woods and Bottjer, 2000; Pruss et al., 2005), while study of the laterally equivalent, shallow-water Virgin Limestone (Moenkopi Formation) has detailed the widespread occurrence of unusual microbialites and abiotic carbonates during the period. Recent study of the Union Wash Formation at the Cerro Gordo, CA locality reveals the presence of stratiform stromatolites from an approximately 10m-thick interval. The stromatolites are laterally extensive (some layers persist for at least 1 km) and typically exhibit planar to wavy laminations (ranging from 0.5 to 2 mm in thickness) with some development of small domes (up to 10 cm in height). Other features of the stromatolites include disseminated fossil hash within the microbialite, rollup structures, and microfaulting. A shallow subtidal depositional environment is suggested by evidence of eroded or planed surfaces within the stromatolites, submarine channelization of the stromatolites, and the occurrence of coated grains and ooids within the interval (Stone et al. 1991). These stromatolites occur stratigraphically above inorganic calcium carbonate precipitates from the Union Wash Formation and stromatolite bioherms from the Virgin Limestone and may therefore represent the last known occurrence of anachronistic facies and associated unusual paleoceanographic conditions in North America following the Permian-Triassic mass extinction.