STROMATOLITES FROM THE LOWER TRIASSIC UNION WASH FORMATION OF EAST-CENTRAL CALIFORNIA: SEDIMENTOLOGY AND PALEOBIOLOGICAL SIGNIFICANCE

Recent studies have documented the complexity of biotic recovery from the Permian-Triassic mass extinction by revealing that recovery began quickly in some regions while the biota in other areas continued to suffer under harsh environmental conditions. 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 (Anisian?)) 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 including stromatolitic bioherms (Schubert and Bottjer 1992; Pruss and Bottjer 2004), oncoids (Woods, pers. obs. 2003), and oolites (Bottjer, pers. comm. 2005). 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 can be laterally extensive (some layers persist for at least 1 km) and typically exhibit planar to wavy laminations with some development of small domes (up to 10 cm in height). A shallow subtidal depositional environment is suggested by evidence of eroded or planed surfaces within the stromatolites, the occurrence of “coated grains and ooliths” within the interval (Stone et al. 1991), and the presence of bivalve-rich shell beds just below the interval. The stromatolites found here occur stratigraphically higher than inorganic calcium carbonate precipitates at either the Darwin Hills, CA locality (outer shelf to slope environment) or the Union Wash, CA locality (basinal environment). Therefore, these stromatolites may represent the last known occurrence of anachronistic facies and associated unusual paleoceanographic conditions in North America following the Permian-Triassic mass extinction.