Sedimentology of Inorganic Calcite Precipitates from the Union Wash Formation, East-Central California, U.S.A.: Implications for the Biotic Recovery from the End-Permian Mass Extinction

Adam D. Woods
CSU Fullerton, Fullerton, CA

Inorganic calcium carbonate precipitates, while common in the Proterozoic, are uncommon throughout the Phanerozoic with one notable exception: the Lower Triassic. The Smithian-Spathian Union Wash Formation of east-central California contains a variety of inorganic synsedimentary calcium carbonate precipitates that were apparently growing directly on the seafloor. Inorganic calcium carbonate precipitates have been documented from two localities representing deposition in environments ranging from outer shelf (Darwin, CA) to basin (Union Wash, CA). Precipitates at the Union Wash locality occur as layers of acicular to bladed calcite crystals that grew with their c-axes oriented perpendicular to the seafloor and over large areas, similar to blades of grass growing within a lawn. These cements are interbedded with mudstone, and range in thickness from less than 1mm to nearly 10mm. The precipitates at the Darwin Hills locality grew as hemispheres and fans up to 10cm across. Growth is complex; in some cases precipitates appear to have grown downwards from other crystal fans or hemispheres. Some hemispheres have been overturned or disturbed, probably due to gravity flows or currents. Growth also appears to have been cyclic in the area, as cements often occur within dark gray, 3-5cm thick mudstone layers that are interbedded with light gray, precipitate-barren mudstone layers of about the same thickness. The occurrence of inorganic calcium carbonates are likely due to the establishment of unusual oceanic chemistry in the region, specifically the mixing between anoxic, alkaline deep waters and oxygenated surface waters, leading to carbon dioxide degassing and calcite supersaturation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005