MARTIN, RONALD E., Department of Geology, University of Delaware, Newark, DE; and RUTH R. FLETCHER, Department of Geology, University of Hawaii, Honolulu, HI

ABSTRACT: Biostratigraphic Expression of Pliocene Sequence Boundaries, Gulf of Mexico

During the last two decades, the oxygen isotope curve has been used extensively as a proxy for Neogene ice volume and sea level change. Unlike the oxygen isotope curve, however, the relation of the Globorotalia menardii-based Ericson and Wollin zonation to paleoclimate and sea level change has remained obscure. Utilizing the warm-water Globorotalia menardii complex and cool (temperate) -water G. inflata, we have previously recognized sequence boundaries in the Pleistocene of the tropical Atlantic and American Mediterranean (Gulf of Mexico, Caribbean Sea) in both cores and an industrial well at (approx.) 0.09 Ma, 0.2 Ma, 0.4 Ma, 0.525-0.620 Ma, 0.7-0.9, 1.0, 1.2, 1.4-1.5, and 1.8-1.9 Ma. These boundaries have been recognized by other workers using integrated data bases of seismic secti ns, well-logs, biostratigraphic, and paleoecologic information.

We have extended our approach to the Pliocene of the Gulf of Mexico (ODP Core 625B) and Caribbean (DSDP Site 502). Relative abundance of keeled globorotalids (analogous to the G. menardii complex) tracks the oxygen isotope curve, and in both cores odd-and even-numbered abundance peaks and valleys (analogous to the oxygen isotope curve) are coeval (constrained by biostratigraphic markers). Based on relative abundance of keeled Globorotalia, we recognize sequence boundaries at 2.4, 2.7, 3.1, 4.4-4.6, and possibly 5.4 Ma, which correspond to those of other workers. Based on abundance peaks, we also recognize potential sequence boundaries at 1.9-2.0, 2.9, 3.4, 3.6, 3.9, 5.0, and (one or more) at 5.1-5.4. Potential sequence boundaries correspond to cooling "events" (approx.) 0.5% of the ox gen isotope curve, and therefore suggest the occurrence of further reservoir sands. In some (perhaps all) cases, these latter abundance fluctuations instead reflect changes in water column stratification (depth of thermocline, upwelling) more than water temperature or ice volume (sea level). Comparison of seismic, well-log, and oxygen isotope data sets with Globorotalia curves would allow us to "filter" the eustatic (climatic) sea level component from "background" (water column) changes, and enhance the predictive ability of ecostratigraphic (abundance) curves.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.