Extensional Rafting: A New Idea for Structural Control of Shelf Margin and Slope Facies Deposition in Upper Wilcox Strata of South Texas, Northwestern Onshore Gulf of Mexico

Joseph C. Fiduk¹, Lynn E. Anderson¹, and Mark G. Rowan^2
1 CGG Americas, Inc., Houston, Texas
² Rowan Consulting, Inc., Boulder, Colorado

The prevailing paradigm for Upper Wilcox deposition in South Texas is that margin sedimentation was accommodated by growth faulting and slope failure. This same general mechanism is invoked around the Gulf of Mexico but fails to explain why Upper Wilcox deltaic sediments are greatly expanded but also relatively confined compared to other Paleogene deltaic centers.

Reprocessed 2-dimensional seismic data across the Wilcox Depotrough shows that it is located between partially extended upper slope strata and a completely detached raft block, which extends from Starr County northward to Live Oak County, Texas. Seismic data show little evidence of significant salt evacuation and/or extrusion. Forward modeling suggests that extensional rafting with ramp-flat fault trajectories on several detachment levels can provide all the accommodation. The raft system may be segmented along strike by switching vergence of the master fault cutting the Cretaceous, thereby providing possible local escape points for Wilcox sediments to more distal locations.

Extensional rafting is known and incorporated into exploration strategies along the South Atlantic margins of West Africa and Brazil, but not in South Texas. Back-filled incisions across raft structures, turbidites draped over or between rafts, and basin floor fans downdip of rafts are known exploration targets elsewhere but not recognized in South Texas. Additionally, because updip extension requires downdip contraction, contractional structures of this age could exist basinward of the raft. Employing a raft model explains why Upper Wilcox shelf margin deposition was confined and opens new exploration possibilities in this mature producing trend.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004