Anatomy of an anachronistic carbonate platform: Lower Triassic carbonates of the southwestern United States

Abstract

Examination of Lower Triassic carbonate rocks from around the world has revealed the resurgence of atypical carbonates that were last common during the Proterozoic (‘anachronistic facies’). Anachronistic facies are attributed to a combination of post-extinction paleoecologic factors and widespread, harsh environmental conditions, particularly the presence of anoxic, alkaline waters in the global ocean. The marine rocks of the Moenkopi Formation (Virgin Limestone) of southern Nevada and laterally equivalent deep-water limestones of the Union Wash Formation of east-central California provide a means to reconstruct depositional environments across a Lower Triassic carbonate ramp and place the atypical facies and fabrics that occur within those rocks into a larger ‘Anachronistic Platform Model.’ Examination of shallow subtidal and intertidal facies reveal widespread ooid shoals in both the Union Wash Formation and Virgin Limestone, as well as the presence of small, fringing stromatolite bioherms in shallow subtidal environments of the Union Wash Formation. Inner to middle ramp environments of the Virgin Limestone contain aggregates of cabbage-head stromatolites that formed 1–2 m-high patch-reef bioherms, in addition to flat-pebble conglomerates, mud-chip facies and wrinkle structures. Deep-water facies from the Union Wash Formation contain synsedimentary calcium carbonate precipitates that form laterally extensive crusts in basinal environments, or interlocking mosaics of hemispheres and fans in outer ramp to slope environments. The results of this study reveal that carbonate sedimentation during the Early Triassic was controlled by a combination of biotic factors, environmental stresses, unusual oceanic chemistry and normal carbonate sedimentary processes that resulted in the development of carbonate platforms that had affinities to both Phanerozoic and Proterozoic depositional systems.