A SURVEY AND ASSESSMENT OF LACUSTRINE AUTHIGENIC MINERALS FROM THE MORRISON FORMATION, BIG HORN BASIN, WYOMING

A survey of authigenic minerals found in Morrison Formation lacustrine deposits in the Big Horn Basin, Wyoming, revealed barite nodules, pyrite, anhydrite, and dioctahedral clays. Synthesis of mineral associations and patterns make it possible to differentiate subtle changes in hydrogeological and environmental lake characteristics. Microscopic textures, mineral-matrix relationships, and stable isotopic signatures record changes in lake water composition, hydrological fluctuations, and Eh-pH conditions.

Microbial activity associated with decaying organic material and subsequent anoxic conditions facilitated sulfate reduction and changes in clay composition. Barite nodules with granular textures formed next to dinosaur bones as organic matter decayed in Ca/Mg clays of shallow, near-shore water. Formation of barite nodules in continental deposits suggests unusual alkaline-saline water chemistry.

Iron-rich smectites were found with fine-grained ash material in transgressive deposits. Gypsum formed in the inner lake basin during regressive events. Reducing and low Eh-pH conditions of wetland areas surrounding the lake allowed framboidal pyrite formation and preservation of abundant organic material.

Geochemical patterns recorded in authigenic minerals detected in otherwise homogenous mudstones support the presence of a closed, alkaline-saline lake with a dynamic history of fluctuating lake levels and corresponding changes in water chemistry.