ANCIENT PEDOGENIC OVERPRINTING OF THE K-T BOUNDARY INTERVAL WITHIN THE FOX HILLS FORMATION OF SOUTHWESTERN SOUTH DAKOTA

A zone of intense sediment disruption, which we interpret as a distal manifestation of the K-T Boundary impact event at Chicxulub, is preserved within four main outcrops of the Fox Hills Formation within the badlands region of southwest South Dakota. Within Badlands National Park, the most complete stratigraphic succession is as follows: Pierre Shale (black); Fox Hills Formation with approximately 4.5 meters of yellowish, slightly bedded siltstone with glauconite; a yellowish-pink zone of disruption that varies from 0.5 to 5 meters in thickness; approximately 6 meters of thinly bedded shale and crossbedded sandstone; 4.5 meters of bright yellow shale. The bright yellow colors are the result of a phase of ancient soil formation, herein referred to as the Yellow Mounds Paleosol. The relationship of the Yellow Mounds Paleosol to the deltaic facies between the four main outcrops of the zone of disruption was influenced by both syntectonic uplift of the Sage Arch, and also subsequent uplift and removal of Fox Hills strata prior to the deposition of the overlying Eocene-Oligocene White River Group. The resultant stratigraphic pattern is one in which the zone of disruption is at the top of the Yellow Mounds Paleosol in Badlands National Park, but 30 miles to the north at Creighton, South Dakota, the zone of disruption is overlain by approximately 30 meters of unmodified deltaic sediments. Only the uppermost several meters of the section at Creighton is overprinted by the Yellow Mounds Paleosol. From a facies perspective, the sections in Badlands National Park suggest that there was a rise in relative sea level following the formation of the K-T zone of disruption. This is supported by outcrops at Creighton which show thin beds of Pierre Shale lithology interbedded with crossbedded and ripple laminated deltaic sandstone and siltstone.