The Biostratigraphic Basis for Gulf Coast Miocene Sequence Stratigraphy

SHAFFER, BERNARD L., JORY A. PACHT, and BRUCE E. BOWEN, Calibre Consulting Services, Inc., Houston, TX

A chronostratigraphic framework was developed as a prelude to a regional sequence stratigraphic and associated systems tracts analysis of the offshore Louisiana and Texas Miocene. The integrated data base for this study included over 10,000 line mi of seismic data, 230 wireline logs, and detailed biostratigraphic analyses on 35 wells. The objective of the study was to map systems tracts and facies that may help delineate prospective areas.

Because they are a fundamental stratigraphic unit within a sequence, this framework emphasizes condensed sections and their calibration by microfossil highest occurrence datums. Condensed sections have been delineated utilizing planktonic foraminiferal and calcareous nannofossil abundance and species diversity patterns, as well as detailed paleo-water depth determinations based on benthic foraminiferal biofacies.

Twelve Miocene condensed sections are recognized as maximum flooding events associated with seismically resolvable and regionally mappable stratigraphic sequences. Each can be calibrated to planktonic foraminiferal and nannofossil datums. Their relation to the classic Gulf Coast benthic foraminiferal zonation is evaluated. A new upper Miocene sequence is documented and dated by nannofossils. Otherwise, the scheme presented here is in general agreement with the Exxon Cycle Chart, the applicability of which is enhanced by calibration to Gulf Coast planktonic highest occurrence datums.

Seven other condensed sections occur between the lowstand slope fan and the lowstand prograding complex. These apparently develop in favorable paleoenvironmental settings, usually at the distal end of the prograding wedge, in response to diminished sedimentation rates during the time lag between slope fan deposition and prograding wedge deposition. Stacked and coalesced condensed sections develop in deeper water environments that are removed from lowstand terrigenous clastic input.

Pitfalls in the use of condensed sections may arise in shallow paleo-shelf settings where datums may be unreliable, and in deeper environments where multiple abundance/diversity peaks may occur. However, an event stratigraphic scheme is generally the most satisfactory method for overcoming correlation problems inherent in the Gulf Coast unstable prograding shelf margin.

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