FOSSILS OF THE CRETACEOUS/TERTIARY TRANSITION, BADLANDS REGION OF SOUTH DAKOTA

A distinctive zone of disrupted, convoluted sediments with prominent flow rolls and localized clastic dikes, occurs over 300 km² in the Badlands area of South Dakota. The disrupted zone (DZ) is located in the Maastrichtian Fox Hills Formation about 20 meters above its transitional contact with the Pierre Shale, and ranges from 0.5 to 5 meters in thickness. The DZ occurs in an interval with uncommon, but significant fossil content. Scaphitid ammonites characteristic of the Jeletzkytes nebrascensis ammonite zone occur below and within the DZ, but not above it. In addition, the sections, including the DZ itself, contain nuculid and inoceramid bivalves, osteichthian and chondrichthian remains, crustacean crawling traces, leaf fragments, and disseminated, and sometimes carbonized, plant debris. Pollen typical of the late Maastrichtian Wodehouseia spinata palynostratigraphic zone also occur in the DZ sections. Based on fossil content, Sr age dates, sedimentary features, the presence of impact ejecta, and its position in the highest part of the Cretaceous marine section of South Dakota, we interpret the DZ as a distal manifestation of the end-Cretaceous Chicxulub Impact Event. Compared to somewhat earlier Fox Hills faunas in other parts of South Dakota, the Badlands K/T fauna is greatly impoverished taxonomically. This impoverishment precedes the end-K impact and is thus unrelated to it. It is more likely the result of localized high-stress conditions in the shallow, ephemeral tidal channel/beach-face environments within which the Badlands fauna was preserved.