PALEOSOLS AND VERTEBRATE TAPHONOMY OF THE OLIGOCENE POLESLIDE MEMBER, BRULE FORMATION, BADLANDS NATIONAL PARK, SOUTH DAKOTA

The Badlands of South Dakota are extremely fossiliferous. Unfortunately, the abundance of vertebrate fossils and the easy access to outcrops attracts fossil poachers. In an effort to document and preserve vertebrate fossil resources, we have been working in conjunction with Badlands National Park over the past several years to determine the sedimentologic and paleopedologic relationships of vertebrate fossil accumulations. Nine sites were excavated last summer within the late Oligocene Poleslide Member of the Brule Formation. Fossils ranged from isolated postcranial bone elements and skulls to articulated masses. In most cases, fossils were preserved within a massive, light gray, volcaniclastic siltstone that lacked any macroscopic evidence of association with a defined ancient land surface, although many were preserved within calcite cemented nodules. The most abundant evidence of former landscapes and associated paleosol profiles is in the form of clay filled root traces, usually no larger than 2-3 mm in diameter, and rare clay lined fractures (argillans). Macroscopic evidence suggests that the siltstone represents an aggradational eolian landscape in which bones were quickly buried. The lack of defined land surfaces suggests that aggradation was rapid. Several well defined land surfaces throughout the eolianite are marked by dense concentrations of calcified and clay filled root traces, dung balls and associated beetle burrows, sweat bee pupae, and occasional vertebrate remains. These ancient land surfaces represent periods of extended stability during which eolian influx was reduced. They can be traced throughout the study area and serve as key marker beds for stratigraphic correlations.