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ABSTRACT: New Insights into the Post-Middle Jurassic Subsidence and Depositional History of Offshore Alabama, Northeastern Gulf of Mexico, based on Backstripping Analysis

LI, X., and R. T. BUFFLER

A backstripping analysis of the Shell Main Pass 154 well offshore Alabama provides new insights into the post-Middle Jurassic subsidence and depositional history of the northeastern Gulf of Mexico. The analysis was carried out using paleontological and lithology data provided by Shell tied to a sequence stratigraphic framework developed for the area. Paleobathymetry estimates were made by combining the well data with a seismic stratigraphic analysis and data from previous studies. The result is an anomalous tectonic subsidence curve that does not follow a normal passive margin evolution and differs from a previously published curve for the same well.

Very slow subsidence during the Jurassic-earliest Cretaceous rifting and seafloor spreading phases of Gulf evolution provided little accommodation, and the area remained high and only a thin sedimentary section was deposited. Post-seafloor spreading subsidence increased during the Early Cretaceous with a maximum rate occurring during the late Early Cretaceous. This latter event corresponds to a gulf-wide buildup of thick carbonate platforms with rimmed margins that surrounded the entire Gulf basin, but it can not be correlated with any regional tectonic event. Continued normal thermal subsidence during the Late Cretaceous combined with a rise in eustatic sea level and other environmental and oceanographic changes caused drowning and a stepback of the platform margins. An anomalous uplift of the area is reflected in the curve from Eocene to Early Miocene, which can not easily be explained but could be due to regional Laramide stresses or deep salt movement. Latest Cenozoic subsidence could be due to deep salt withdrawal.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana