ARMENTROUT, JOHN M., Mobil Exploration and Producing Technical Center, Dallas, TX

ABSTRACT: Graphical Display of the Biostratigraphic Signature in Sequence Stratigraphy--With Examples from the Gulf of Mexico Plio-Pleistocene

Graphical displays of biostratigraphic data should be formatted for immediate integration with exploration analyses. The primary datums and depositional systems tracts of sequence stratigraphy are especially useful and each can be recognized within biostratigraphic data sets. Displays in two-way time and stratigraphic depth must be standard report formats. Biostratigraphic reports should also include correlations with nearby wells and global cycle charts to facilitate geologic analyses. Alternate correlation schemes should also be illustrated to allow interpreters to test each possibility.

The primary horizons utilized in sequence stratigraphic analysis are the sequence boundary, and the maximum flooding surface which occurs within the condensed section in areas of low sediment accumulation rates.

The sequence boundary may be represented by a marked shift in biofacies assemblages from deeper below to shallower above. It may be marked also by an increase in reworked or displaced fossils, and by a decrease in both fossil abundance and diversity due to rapid accumulation of sediment in the shallow water, high-energy environments associated with the erosional unconformity.

The maximum flooding surface is often represented by significant increase in fossil abundance and by the deepest-water biofacies assemblage of the transgressive-regressive cycle. Despite their lateral variabilities, the seismic, lithologic and biostratigraphic signatures of the condensed section and of the associated maximum flooding surface are generally the most easily recognized and precisely dated regional correlative surfaces.

Fossil assemblages of the highstand systems tract reflect shallowing upward neritic conditions, with intervals in which faunal abundance decreases upwards. Fossil assemblages in transgressive systems tracts show deepening-upward biofacies and an increase in faunal abundance upward. Lowstand systems tracts are most significant seaward of the shelf-edge, and consist of depositional thicks with low fossil abundance. These lowstand systems tracts are separated by highly fossiliferous condensed sections containing the distal aspects of the transgressive and highstand systems tracts.

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