THE TERRESTRIAL RESPONSE TO THE POST EOCENE-OLIGOCENE CLIMATIC TRANSITION, POLESLIDE MEMBER, BRULE FORMATION, BADLANDS NATIONAL PARK, SOUTH DAKOTA

The Early Oligocene Poleslide Member of the Brule Formation within Badlands National Park (BNP), South Dakota is composed of pedogenically modified siltstone deposits. Sediments consist of reworked volcaniclastics from the Basin and Range province and siliciclastics produced by the deroofing of the nearby Black Hills. Thirteen paleosol profiles, comprised of 6 different pedotypes, were described from 10 meters of section in the middle Poleslide Member, and represent deposition and soil formation from late C12r through early C11r. Paleosols were described based on soil structure, micromorphology, geochemistry, and clay mineralogy. Based on climofunction models, paleosol morphology, chemistry and clay mineralogies were closely controlled by changes in mean annual precipitation, which affected the depositional system and time constraints of pedogenesis.

Paleosols vary up section from seasonally aquic, humid open range profiles to semiarid soils with pedogenic carbonate in weakly developed profiles. Well developed profiles at the base of the study interval contain discrete horizons, clay films, and soil fabrics. Pedogenic development becomes limited up section by relatively high rates of loess deposition, with intermittent periods of landscape stability. Weakly developed profiles at the top of the study interval are characterized by dispersed root traces and pedogenic carbonate accumulation with no significant horizonation. This change in pedogenic development is associated with a biostratigraphic transition interpreted to be a change to a faunal assemblage better suited to arid environments.

Climate alterations during the preceding Eocene-Oligocene transition had differing effects regionally on the terrestrial system through time. Paleoclimate studies across the Eocene-Oligocene transition in Nebraska suggest a change in temperature and no change in precipitation, whereas these Early Oligocene soils in South Dakota were primarily affected by changes in precipitation. The extent, timing, and expression of climatic change induced by the Eocene-Oligocene transition across the Great Plains is not well constrained. Regional changes in paleosol development in conjunction with biostratigraphic and magnetostratigraphic data help to clarify the nature of this change.