ROBERTS, HARRY¹, RICHARD FILLON², and BARRY KOHL^3
1Louisiana State University, Baton Rouge, LA
²Texaco Exploration, New Orleans, LA
³Tulane University, New Orleans, LA

Abstract: Warm Water Recorded in Lowstand Shelf Edge Deltas: Paleoceanographic Model for the Northern Gulf of Mexico

Glacioeustatic fluctuations forced fluvial-marine stratigraphy during the Cenozoic, prograding the shelf edge during falling-to-lowstands, and aggrading the shelf surface during rising-to-highstands. Southeastern rivers dramatically prograded the northern rim of the Gulf creating a broad, shallow continental shelf along the Louisiana-Texas coast. The latest Pleistocene Mobile River delta just east of the Mississippi River's present delta is a studied case of lowstand deposition at the edge of this broad shelf. Within this delta, however, and within lowstand Mississippi delta systems, paleontological data offer paradoxical paleoecological interpretations. During glacial maxima, when global climatic conditions were registering thermal minima, shelf edge deltas of the Gulf harbored a benthic foraminiferal fauna related to calcareous bank and turtle grass habitats indicative of warmer conditions than are found today in the Mississippi delta. Our model analysis suggests that this apparent contradiction was related to withdrawal of the sea from the continental shelf during lowstands. A flooded shelf has limited heat storage capacity, and acts as a site of cold water generation, which chills highstand coastal environments and the shelf edge. During lowstands when the shelf was emergent, coastal depocenters were close to deep water, which has an unlimited heat storage capacity. Warm water therefore bathed the lowstand shelf edge shoreline creating a marine environment warmer than today's. Periodic lowstand incursions of warm tropical water are suggested also by the rare but persistent presence in prodeltaic sediments of the planktonic foraminifer Globorotalia menardii, which is characteristic of the Gulf Loop Current and its eddies.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas