Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Shale-Ridge and Fault Geometries in Growth-Faulted Subbasins along the Central and South Texas Gulf Coast: Oligocene Frio Formation
The Frio Formation of the southern and central Texas Gulf coast is dominated by growth-faulted subbasins that originated from mobile mud deformation during sea-level lowstands. Coarse clastic deltaic sedimentation shifted basinward and was deposited onto unconsolidated slope muds, which were mobilized into ridges. Each successive shale ridge composed the slopes of subsequent Frio subbasins. The mechanism of mobile mud deformation and associated growth faulting is poorly understood in these subbasins. Mobile mud deformation in basins such as the western Gulf of Mexico, offshore Niger, Indonesia, Trinidad, and southern Australia has been attributed to overpressure caused by a combination of burial compaction, diagenesis of clays, kerogen maturation, and compressive tectonics related to significant burial depth. However, mud movement in the Frio growth-faulted subbasins probably occurred fairly soon after lowstand deposition commenced and has been attributed to disequilibrium compaction. Rapid burial of the fine-grained basinal muddy sediments caused them to retain their pore water, which allowed them to deform easily into triangular shapes (ridges) as denser sediments accumulated above them. The morphology of the shale ridge is typically related to growth-fault geometry and ranges from a simple ramp shape to a more complicated ramp-flat/incline-ramp shape. The latter shape has been attributed to a second stage of shale movement related to overpressure caused by increased overburden. The resulting shale pillow created an anticlinal high in the sediments of the hanging wall and onlap, indicating timing of movement. Alternatively, the hanging-wall geometry was a direct consequence of ramp and flat geometry of the footwall.