SOILSCAPE ANALYSIS OF THE SCENIC-POLESLIDE MEMBER BOUNDARY, BADLANDS NATIONAL PARK, SOUTH DAKOTA

Thirteen paleosol trenches were excavated, described and sampled along the Scenic-Poleslide Member boundary of the Brule Formation, Badlands National Park, in an attempt to reconstruct a soilscape for the Mid-Oligocene terrestrial Northern Great Plains, North America. Our preliminary results suggest at least four distinct soil types, spanning the North Unit and a portion of the South Unit, were forming at approximately the same time along the boundary landscape. The individual paleosols measured range from very weakly developed to well developed soils, characterized by clay-illuviation, redoximorphic features, and some surface horizon truncation. Soils that formed at this level in the eastern portion of the North Unit are moderately developed soils characterized by clay-illuviation, subsequent Bt degradation, and drab mottles and root traces with manganese hypocoatings. Soils that formed in the Dillon Pass area vary from weakly developed to moderately developed soils with evidence of gleization, vermicular mottling, and over-thickened A horizons. Soils that formed in the Sheep Mtn. Table area are very weakly developed soils characterized by stacked A-C horizons and a possible cambic horizon. Our data suggest that the Scenic-Poleslide Member boundary soilscape experienced a complex progressive and regressive pedogenetic pathway, which varied as a function of pedofacies and location relative to the Sage Creek Arch. However, the majority of paleosols studied along this landscape do exhibit a common polygenetic theme, well-drained conditions promoting lessivage followed by poorly-drained conditions, waterlogging and Bt degradation. Whether this complex polygenetic soilscape resulted from changing base-level, climate, fluvial and hydrological dynamics, or intrinsic thresholds remains uncertain. Our results do confirm the need to conduct more laterally intensive soilscape analyses when attempting paleoenvironmental reconstructions using paleosols in the rock record.