Cephalopods from the Badlands National Park area, South Dakota: Reassessment of the position of the Cretaceous/Paleogene boundary

摘要

Terry et al. (2001) proposed that the Fox Hills Formation in the area of Badlands National Park, southwestern South Dakota, USA, contains the Cretaceous/Paleogene (K/Pg) boundary, marked by a thick layer of contorted bedding called the Disturbed or Disrupted Zone (DZ). Examination of the ammonites from just below this layer yields Hoploscaphites nicolletii (. Morton, 1842), H. spedeni (. Landman and Waage, 1993), Discoscaphites gulosus (. Morton, 1834), D. conradi (. Morton, 1834), and Sphenodiscus lobatus (. Tuomey, 1856). The abundance of Discoscaphites and the presence of a coarsely ornamented specimen of H. spedeni suggest that this assemblage corresponds to the upper part of the H. nicolletii Zone in the type area of the Fox Hills Formation. No ammonites are present above the DZ, but previous analyses of the dinoflagellates from just below, within, and above the DZ by Palamarczuk et al. (2004) are consistent with the ammonite results. Together, these fossils indicate that the interval just below and above the DZ represents the upper part of the lower upper Maastrichtian (≈ middle upper Maastichtian). Belemnites are present in the strata just below the DZ and occur as guards either isolated in the matrix or associated with fragmentary ammonites in concretions composed of soft sandy marl. The belemnites are assigned to Belemnitella bulbosa Meek and Hayden, 1857a, and B. badlandsensis n. sp., which is characterized by an unusually large fissure angle. The distribution of ammonites and lithofacies at this time reveals that the western shoreline of the Western Interior Seaway trended northeast-southwest across western South Dakota and adjacent parts of North Dakota. Based on an examination of the oxygen isotopes of the belemnites and scaphites in this area, the seawater temperature was nearly constant along the coast, approximately 17-20 °C. A comparison of the contact between the Pierre Shale and Fox Hills Formation at various localities in the Western Interior Basin indicates that this contact rises in the section toward the east, reflecting the final retreat of the Seaway during the late Maastrichtian.