Paleogene Stage and Planktonic Foraminiferal Zone Boundaries and Unconformity-Bounded Depositional Sequence Contacts in Southwestern Alabama

MANCINI, ERNEST A., Geological Survey of Alabama and University of Alabama, Tuscaloosa, AL, and BERRY H. TEW, Geological Survey of Alabama, Tuscaloosa, AL

In the southwestern Alabama area, Paleogene Stage boundaries, which are based on planktonic foraminiferal ranges, can correspond to unconformity-bounded depositional sequence contacts. The Maestrichtian-Danian (Cretaceous-Paleocene) Stage boundary is coincident with a type 1 unconformity at the Selma-Midway Group contact, and the Selandian-Ypresian (Paleocene-Eocene) Stage boundary corresponds to a type 1 unconformity in the Wilcox Group. The Danian-Selandian (lower-upper Paleocene), Lutetian-Bartonian (middle Eocene), Bartonian-Priabonian (middle-upper Eocene), and Rupelian-Chattian (lower-upper Oligocene) Stage boundaries correspond to type 2 unconformities in the Midway, Claiborne, and Jackson groups and Chickasawhay Limestone, respectively. The Priabonian-Rupelian (Eocene-Oligocen ) and Ypresian-Lutetian (lower-middle Eocene) Stage boundaries, however, do not coincide with sequence-bounding unconformities and occur at the Jackson-Vicksburg Group contact, which is the surface of maximum sediment starvation in condensed section deposits of a type 2 depositional sequence and within transgressive deposits of a type 1 depositional sequence in the Claiborne Group, respectively.

Planktonic foraminiferal zone boundaries evident in the Paleogene strata of the eastern Gulf Coastal Plain generally correspond to marine transgressions (rises in relative sea level). These biostratigraphic zonal boundaries are observed to occur at depositional sequence bounding surfaces (unconformities), transgressive surfaces within depositional sequences, or within transgressive or condensed section deposits of depositional sequences. The stratigraphic occurrence of the biostratigraphic boundary is influenced by the paleogeographic setting and paleoenvironmental conditions of a given site of deposition. Differential amounts and rates of sedimentation associated with paleobathymetry and/or distance from the shoreline at various depositional sites and differential subsidence within t e depositional basin resulting largely from local tectonics are integral controls on the stratigraphic occurrence of biostratigraphic datums. Other factors affecting the stratigraphic occurrence of planktonic foraminiferal zonal boundaries are the presence of unconformities and associated paleontological hiatuses resulting from nondeposition or erosion, the degree of postdepositional erosion and removal of sediments, and the presence or absence of lowstand deposits associated with type 1 unconformities or shelf margin deposits associated with type 2 unconformities.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)