GEOCHEMICAL CONTROLS ON FOSSIL PRESERVATION IN THE FLORISSANT FORMATION, FLORISSANT FOSSIL BEDS NATIONAL MONUMENT, COLORADO

The Late Eocene freshwater Florissant Formation can be described as a lagerstatten locality due to the reported excellent and unique preservation, and high diversity, of its flora and fauna. The fossils are preserved within the formation’s lacustrine and fluvial shales and mudstones and in pumice and ash conglomerates originated from the Thirtynine Mile volcanic field. Preservation in the shales of the paleontological material has been attributed to a sedimentation cycle in which clay and ash alternate with diatomaceous algal mats. Recently, other studies on the taphonomy of the carbonized botanical and entomological impressions have demonstrated that excellent preservation of specimens is rarely observed. The preservation quality instead is ranked as fair or poor in most cases, leaving doubt as to how well the shales and other lithologies truly preserve the fossils. In addition to the botanical and entomological specimens, recent detailed, stratigraphically-controlled field studies have shown that gastropods and bivalves occurred throughout the lake’s duration and are preserved within its different lithologies. However, original shell-calcite is only recorded in fossils from a single horizon of conglomerate, and in other stratigraphic horizons they are only observed as casts and molds or the shells have been entirely replaced by silica. In an effort to elucidate the mechanisms of preservation recorded within these lithologies, analyses of the biogenic calcites of these invertebrates have been completed. Electron microprobe and cathodoluminescence microscopy of thin sections from both the calcitic shells and their surrounding lithologies were used to describe their mineralogies. For comparison, bulk sample mineralogy was obtained through X-Ray diffraction analysis. These analyses have shown that the geochemical drivers of preservation within the paleolacustrine system influenced a chemical gradient that drove carbonate dissolution during the lake’s existence. This system controlled the amount of carbon (calcite shells or impressions) that was preserved in the lake’s associated deposits. Unraveling the geochemical drivers of preservation within the Florissant lake system furthers our understanding of depositional environments and their potential to preserve paleontological material.