Cephalopods from the Cretaceous/Tertiary Boundary Interval on the Atlantic Coastal Plain, with a Description of the Highest Ammonite Zones in North America. Part 1. Maryland and North Carolina

Abstract

The sedimentary deposits on the Atlantic Coastal Plain in New Jersey, Delaware, Maryland, North Carolina, South Carolina, and Georgia span the Cretaceous/Tertiary boundary. We investigate the ammonites of the Severn Formation on the western and eastern shore of Chesapeake Bay, Maryland, and the Peedee Formation, North Carolina. We describe three ammonite assemblages from the Severn Formation and their associated dinoflagellates, defining three successive ammonite zones in the upper Maastrichtian. The lowest ammonite zone is the Discoscaphites conradi Assemblage Zone. It occurs near the top of the Severn Formation in Prince Georges County, Maryland, just below the Paleocene Brightseat Formation. The ammonite fauna consists of Sphenodiscus pleurisepta (Conrad, 1857), Sphenodiscus lobatus (Tuomey, 1856), Discoscaphites conradi (Morton, 1834), Discoscaphites gulosus (Morton, 1834), Jeletzkytes nebrascensis (Owen, 1852), Glyptoxoceras rugatum (Forbes, 1846), Baculites vertebralis Lamarck, 1801, and Eubaculites latecarinatus (Brunnschweiler, 1966). Dinoflagellates from a sample of matrix include Isabelidinium aff. I. cooksoniae (Alberti, 1959) Lentin & Williams, 1977, which correlates with calcareous nannofossil Zone CC25b, indicating the lower part of the upper Maastrichtian (68.2–67.4 MaBP). The D. conradi Zone is also present in parts of the Corsicana Formation, Texas, the Prairie Bluff Chalk, Alabama and Mississippi, the Peedee Formation, North Carolina, and the Navesink and New Egypt Formations, New Jersey. The next higher zone is the Discoscaphites minardi Assemblage Zone, which occurs in the Severn Formation approximately 6 m below the base of the Hornerstown Formation at Lloyd Creek, Kent County, Maryland. The ammonite assemblage is dominated by Discoscaphites minardi, n.sp., B. vertebralis, and S. pleurisepta, with rare specimens of Sphenodiscus sp., Discoscaphites iris (Conrad, 1858), and E. latecarinatus. A sample of dinoflagellates from the same bed as the ammonites includes Deflandrea galatea (Lejeune-Carpentier, 1942) Lentin & Williams, 1973 and Thalassiphora pelagica (Eisenack, 1954) Eisenack & Gocht, 1960, which correlate with the Neophrolithus frequens calcareous nannofossil Zone between Subzones CC26a and CC26b, indicating the middle part of the upper Maastrichtian (66.4–66.0 MaBP). The D. minardi Zone is also present in the New Egypt Formation, New Jersey. The highest zone is the D. iris Assemblage Zone, which occurs near the top of the Severn Formation at its type locality at Round Bay, Anne Arundel County, Maryland. The ammonite assemblage is dominated by D. iris and E. carinatus, although elsewhere this zone also includes Pachydiscus (Neodesmoceras) mokotibensis Collignon, 1952, Pachydiscus (Pachydiscus) jacquoti jacquoti Seunes, 1890, S. lobatus, S. pleurisepta, Discoscaphites sphaeroidalis Kennedy and Cobban, 2000, and E. latecarinatus. Dinoflagellates from a sample of matrix surrounding one of the ammonites include Palynodinium grallator Gocht, 1970 and T. pelagica indicative of the P. grallator Zone, Tpe subzone, which correlates with the upper part of calcareous nannofossil Zone CC26b, indicating the upper part of the upper Maastrichtian (65.6–65.0 MaBP). The D. iris Zone is also present in the upper part of the Corsicana Formation, Texas, the Owl Creek Formation, Mississippi, Tennessee, and Missouri, and the New Egypt and Tinton Formations, New Jersey.