AN OLIGOCENE SPRINGFED CARBONATE LAKE IN THE MIDDLE OF A VOLCANICLASTIC EOLIANITE, BADLANDS NATIONAL PARK, SOUTH DAKOTA

The Oligocene Poleslide Member of the Brule Formation is composed primarily of reworked eolian volcaniclastics and occasional fluvial channel and overbank deposits. Well developed paleosols within the study section define periods of landscape stability during which eolian input was reduced. One such period of landscape stability is marked by a distinctive lacustrine carbonate that contains charophytes, aquatic snails, stromatolitic mounds, and the remains of fish. The lacustrine unit is areally restricted over several hundred square meters and laterally grades into terrestrial sediments marked by fossil roots. Periods of lake expansion are recognized by the overlap of lacustrine carbonate over rooted and fossil bearing terrestrial sediments. The lake was likely springfed, based on the paleoclimatic interpretation of associated paleosols which suggest that this region was arid at this time. Similar carbonate lakes lower in the section are associated with reduced periods of sedimentation and shifting siliciclastic depocenters, whereas some are related to spring/tufa activity controlled by regional structure. This particular lacustrine system may have been a focal point for mammalian activity as the surrounding area is extremely rich in vertebrate fossils of the Whitneyan North American land mammal age.