A WIDESPREAD ZONE OF SOFT SEDIMENT DEFORMATION AND EJECTA IN THE FOX HILLS FORMATION OF SOUTHWEST SOUTH DAKOTA: AN IMPACTITE WITHOUT A CRATER

A regional zone of intense soft sediment deformation up to 5 m thick is preserved within the southernmost deposits of the Fox Hills Formation in and around Badlands National Park, South Dakota. The zone of disruption (DZ) is characterized by large scale slumping, clastic dikes, flame structures, foundering of coherent blocks into a massive liquefied substrate, and ejecta. Our previous age interpretations suggested that the DZ was probably a distal seismic manifestation of the Chicxulub impact and thus marked the K-Plg Boundary in this part of South Dakota. However, our new dinoflagellate data for these beds is more compatible with an age for the DZ of late middle to early late Maastrichtian. This interpretation is consistent with: 1) our macrofossil evidence which suggests that the South Dakota DZ is high in the Jeletzkytes nebrascensis ammonite zone; and 2) our Sr isotope data from belemnites 1.5 m below the DZ dated to 67.6 + 0.5 Ma. The question now becomes one of correlation. Is the South Dakota DZ the distal manifestation of the Chicxulub impact as we originally surmised? This seems unlikely even considering recent proposals which suggest that the Chicxulub crater is itself 300 ky older than the terminal K-Plg event. Does the DZ correlate with an impactite below the K-Plg boundary, within sediments lithologically similar to our sections in South Dakota, that has been recently recognized in the Vermejo Formation of the Raton Basin in New Mexico? Does the DZ correlate with some other yet unknown late Cretaceous impact or is it simply the manifestation of a regional geotectonic event? We are analyzing the biostratigraphy and magnetostratigraphy of the South Dakota DZ sections to help pinpoint the timing of the DZ, and provide answers to these questions.