Abstract

Floodplain limestones are increasingly important sources of fossil vertebrates from the Clarks Fork basin, the northern extension of Wyoming's Bighorn basin. Despite their importance for understanding Paleogene floodplain faunas, little work has addressed the depositional history of these carbonates or evaluated potential taphonomic biases affecting the assemblages they produce. Recent studies of Bighorn basin paleosols have increased our understanding of parameters affecting the paleo-floodplain, and large collections of limestones now exist; this has allowed detailed investigation of limestone petrography and geochemistry and the proposal of a new model for their formation.

Floodplain limestones record a complex depositional and diagenetic history. Precipitation of micritic low-Mg calcite and accumulation of bone occurred in low-energy ponded water at the earth's surface. Pedogenic overprinting obscured primary textures and produced diagenetic grainstones, packstones, and wackestones. Biological and chemical pedogenic processes formed most carbonate allochems, including micrite grains and calcified plant cells. Microbial fermentation occurred at shallow burial depths, and methane production, migration, and oxidation affected the carbon isotope composition of diagenetic carbonate and primary micrite. Pore space was fully occluded by phreatic calcite cements during late-stage diagenesis.

Fossil assemblages contained within the limestones likely represent fauna derived from rarely sampled floodplain microenvironments. Hydraulic transport of bones was minimal. Accumulation of fossil material occurred episodically, and bones were rapidly encased in micrite. The uncommon body size distribution characteristic of limestone faunas is attributed, in part, to buffering of chemical and physical processes normally effecting bone decomposition during surface exposure and pedogenesis.

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