DIAGENETIC HISTORY OF TERRESTRIAL FOSSIL BONE FROM THE PALEOGENE WHITE RIVER GROUP, BADLANDS NATIONAL PARK, SOUTH DAKOTA

The late Eocene-Oligocene White River Group of SW South Dakota and NW Nebraska represents ~5 million years (30-35 Ma) of fluvial, eolian, and minor lacustrine deposition during the transition from global greenhouse to icehouse conditions. Twenty bones from various depositional environments throughout ~85 m of strata were analyzed in thin-section to identify patterns of authigenic mineral precipitation in bone cavities as a function of stratigraphic and geographic location, lithology, and paleoenvironments. Samples from channel sandstone of the Late Eocene Crazy Johnson Member of the Chadron Formation are unusually well preserved with mostly empty trabecular chambers and minor deposits of sucrosic calcite. Trabecular chambers in fossils from a single mudstone unit at the top of the Chadron Formation contain moderate to thick peripheral Fe oxide masses within blocky or sucrosic calcite, and chalcedony occurring pre- and post-calcite mineralization. Fossils from fluvial deposits of the Early Oligocene Scenic Member of the Brule Formation contain abundant, thick Fe oxide deposits, and in some cases, especially dense Fe oxide and carbonate flecks in bone material and Fe oxide encrusting at least part of bone surface. Bones from the eolian dominated Middle Oligocene Poleslide Member of the Brule Formation manifest blocky calcite in trabecular chambers preceded by a sucrosic calcite rind containing sparse Fe oxide masses relative to other fossils whereas others contain locally dense sucrosic calcite that obscures or replaces bone material. Sucrosic calcite and Fe oxides are associated with well-drained and hydromorphic environments, respectively; therefore, the decrease of Fe oxides and increase of sucrosic calcite rinds up section likely corresponds with the change from fluvial to eolian dominated environments. The occurrence of chalcedony is mostly restricted to the Upper Chadron and Lower Brule Formations in the SW region of Badlands National Park in association with prominent post-lithification chalcedony vein fills. These data will be used in conjunction with lanthanide and trace element composition to determine the influence of depositional and diagenetic environments on fossilization.