From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Baylor University Seminar, February 18, 2005

Understanding Growth-Faulted, Intraslope Subbasins by Applying Sequence-Stratigraphic Principles: Examples from the South Texas Oligocene Frio Formation

L. Frank Brown, Jr.

Abstract:

Analysis of Oligocene Frio Formation intraslope growth-faulted subbasins in the Corpus Christi, Texas, area indicates that deposition during relative lowstands of sea level was the main cause growth faulting. Lowstand depocenters on the low-gradient upper continental slope comprising basin-floor and slope-fan systems, and prograding lowstand delta systems exerted sufficient gravity stress to cause major sections of outer shelf and upper slope strata to fail and move basinward. The faults sole out deep in the basin. Footwall block rotation mobilizes deep-water sediments and forces distal mud bodies basinward and upward to form mud (shale) ridges that constitute the basinward flank of intraslope subbasins.

Sedimentation associated with third-order relative falls of sea level generated a major regional syndepositional growth-fault system. Subbasins on the downthrown side of each arcuate fault segment that comprises a regional fault system are filled during the lowstands of sea level. Consequently, genetically similar but diachronous lowstand depositional systems filled each subsequent growth-faulted subbasin trend. The subbasin stratigraphy becomes younger basinward because the subbasin development and fill process extended the Frio shelf edge stepwise into the Oligocene Gulf of Mexico basin, coinciding with relative third-order sea-level cycles.

The subbasins, prolific petroleum targets for decades, are now the focus of prospecting for deep gas. Lowstand sandstones are principal reservoirs, and synsedimentary tectonics produced anticlinal fault traps and associated stratigraphic pinchout traps on the flanks of the structures. Understanding the origin of the faulted subbasins and their chronostratigraphic relationships and depositional processes provides a perspective that can improve deep gas exploration.