Ostracoda of the Latest Cretaceous and Earliest Tertiary of the Gulf Coastal Plain: Biostratigraphy, Paleoenvironments, and Systematics.

Examination of the Ostracoda of the latest Cretaceous and earliest Tertiary of the Gulf Coastal Plain indicates that this microscopic crustacean is a practical, valuable tool for biostratigraphy and paleoenvironmental interpretation in this major geologic and geographic province. The newly established Veenia parallelopora Zone, an ostracode interval zone for the late Maastrichtian, allows correlation of the uppermost Cretaceous units across the Gulf Coastal Province. Veenia parallelopora, Brachycythere foraminosa, and Veenia adkinsi are characteristic species of this zone. The uppermost, latest Maastrichtian, part of this zone can be recognized by the presence of these three species plus Antibythocypris johnsoni, Brachycythere ovata (transitional form), Eucytherura reticulata, Opimocythere aff. O. hazeli, and Pectocythere hughesi. Many of the Upper Cretaceous ostracode taxa found in the Gulf Coastal Province seem to occupy the same or similar habitats as their present-day representatives. High percentages of species of one group of genera can be interpreted to indicate proximity to the coast. Whereas another distinctive group indicates deposits laid down in middle to outer shelf environments. The study of the Cretaceous-Tertiary boundary sections at Braggs, Shell Creek and Lynn Creek reveals that many long-range, widely-distributed ostracode species suddenly disappeared at the end of the Cretaceous. However, no general decline in the species diversity is observed in the later part of the Cretaceous. The Cretaceous-Tertiary boundary can be recognized by the disappearance of many Cretaceous ostracodes species. The boundary at Braggs, where the Cretaceous part of the section is the most complete, is at the contact of the Clayton Formation with the Prairie Bluff. At Shell Creek and Lynn Creek the uppermost part of the Prairie Bluff is missing. Here a sand unit, the Clayton sand, is present between the Prairie Bluff and the marls of the Pine Barren Member of the Clayton. The lower part of this sand contains pseudomorphs of microspherules, interpreted to represent extraterrestrial impact ejecta. At least the lower part of the sand containing the microspherule pseudomorphs correlates with the Cretaceous-Tertiary boundary clay in the stratotype at El Kef, Tunisia