LATERAL CHANGES IN PALEOSOLS, SEDIMENTOLOGY, AND VERTEBRATE TAPHONOMY WITHIN THE OLIGOCENE SCENIC MEMBER OF THE BRULE FORMATION, BADLANDS NATIONAL PARK, SD

The Oligocene Scenic Member of the Brule Formation is composed of ca. 50 m of fossiliferous fluvial and eolian deposits that change laterally from southwest to northeast across the North Unit of Badlands National Park. Lateral changes are manifested as differences in facies architecture, paleosols, and vertebrate taphonomy. Throughout the study area the base of the Scenic Member is marked by an unconformable contact with hummocky, greenish gray mudstones of the underlying Eocene Chadron Formation. At the extreme southwest part of this transect, at the north end of Sheep Mountain Table, fluvial deposits are composed of thin overbank mudstones superimposed by weakly developed paleosols and numerous channel sandstones with large width to depth ratios. Vertebrate fossils occur as isolated elements, with the exception of numerous complete tortoise shells. Just to the northeast of Scenic, overbank facies become dominant, but occasional large channels are found. Channels toward the base of the unit are finer grained and lack a coarse bedload, with the exception of bone fragments. Paleosols range from very weak to weakly developed depending on the proximity to channels. Fossils are very well preserved, commonly as articulated to associated remains within defined zones. Farther to the northeast within Conata Basin, overbank mudstones are dominant, and paleosols become better developed. Fossils occur as isolated elements and occasional articulated remains, but also as discrete high density bonebed accumulations within former oxbows. The lateral change in fluvial facies and associated paleosols is likely due to asymmetric basin development with the northeast end of the study area experiencing less frequent depositional events due to its proximity to the Sage Arch and Sage Fault, whereas the southwest end was more frequently flooded. In addition to lateral changes, vertical changes in paleosols morphology are noted in the Dillon Pass area as paleotopography on the underlying Chadron Formation was infilled, resulting in a change from paleosols affected by hydromorphy to well drained paleosols higher in the section.