WHITBREAD, TONY, and TODD NICHOLSON , Fina Oil Chemical Company, Houston, Texas

The Oligocene Vicksburg succession of South Texas was deposited as a coarse clastic system on the fringe of the uplifted Sierra Madre. Deposition within the Vicksburg is considered to have taken place almost entirely in the delta top and delta front environments and delta growth was by a complex series of syndepositional growth faults. For much of the Vicksburg the delta was detached from the underlying Eocene and crept seaward `en-masse' with sediments supplied by the rivers infilling and subsequent troughs.

An acceleration in tectonic control in Middle Vicksburg caused uplift, erosion and a change in structural style. The NW-SE ridges began forming in conjunction with significant counter regional growth faults. The ridges became the focus of current gas accumulations. The counter regional growth faults are not oriented the same as the synthetic growth faults and it is suggested that this change in tectonics relates to wrench fault movement in the basement.

The key to the successful exploitation of the Vicksburg is being able to unravel the extensive low angle discontinuous faults sets that dominate the structure in the majority of rotated fault blocks. These faults form during the growth and rotation of the individual slump structures. Their formation occurs when the curvature on the listric fault plane is reduced as the slump rotates to a horizontal position. When rotation is complete all the curvature from the base of the slump is transposed to the upper surface and the consequent low angle faults dissect the crest of the slumps.

AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas