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 understand- ing Paleogene floodplain faunas, little work has addressed the deposi- tional history of these carbonates or evaluated potential taphonomic biases affecting the assemblages they produce. Recent studies of Big- horn 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. Ped- ogenic overprinting obscured primary textures and produced diage- netic grainstones, packstones, and wackestones. Biological and chemi- cal pedogenic processes formed most carbonate allochems, including micrite grains and calcified plant cells. Microbial fermentation oc- curred 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 phre- atic calcite cements during late-stage diagenesis. Fossil assemblages contained within the limestones likely represent fauna derived from rarely sampled floodplain microenvironments. Hy- draulic transport of bones was minimal. Accumulation of fossil mate- rial occurred episodically, and bones were rapidly encased in micrite. The uncommon body size distribution characteristic of limestone fau- nas is attributed, in part, to buffering of chemical and physical pro- cesses normally effecting bone decomposition during surface exposure and pedogenesis.