Abstract

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 elsewhere around the Gulf of Mexico but fails to explain why Upper Wilcox deltaic sediments are greatly expanded vertically but also relatively confined spatially compared to other Paleogene deltaic centers.

An examination of 2D seismic data in South Texas has identified what is now interpreted to be a large, rafted block of Eocene, Paleocene, and Cretaceous strata, analogous to rafts identified in the Kwanza Basin of Angola. Preliminarily named the “Wilcox raft” because of its association with the Wilcox depotrough, it has been identified in the subsurface extending from Starr County on the Texas - Mexican border, northward over 200 kilometers into Live Oak County, Texas. The actual extent of rafted material may extend farther to the north and/or south. The raft’s detachment surface is interpreted to be at the base of the Jurassic Louann salt.

The Wilcox raft contains one primary block more than 150 kilometers long and 15 to greater than 30 kilometers wide. The primary raft block may be segmented, and the entire rafted unit may include a number of smaller branching arms, ramps, and offset fault blocks. Various portions of the raft have downdip displacements from 5 to greater than 30 kilometers. The raft is bound on the west by expanded Upper Wilcox (early Eocene) strata and on the east by expanded Queen City (middle Eocene) strata. Other incompletely detached blocks lie to the west of the raft across the Wilcox depotrough. Raft geometries suggest that at least one additional rafted block lies farther basinward of the Wilcox raft, possibly beneath expanded Vicksburg (early Oligocene) strata. 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.

A proposal for rafting in this area of South Texas is not entirely new. Earlier modeling and restorations across the Wilcox depotrough have incorporated rafts. However, these models were predicated on large-scale salt withdrawal and incorporated more than three kilometers (>10,000 feet) of autochthonous salt occupying the area of the Wilcox depotrough. We believe that a much thinner autochthonous salt layer existed beneath South Texas. In other areas of the northern Gulf of Mexico where thick autochthonous salt existed, salt stocks are abundant. Although a few salt structures do exist in South Texas, there are very few compared with other Gulf coast interior salt basins. Forward modeling suggests that large sedimentary structures in the Wilcox depotrough, which can be misinterpreted as turtle structures, are related strictly to deposition during raft extension and not salt withdrawal. The geometries can be produced purely by extension on multiple detachments (Louann and lower Paleocene Midway shales) linked by ramps that dip both basinward and landward.

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 Wilcox age could exist basinward of any rafts. Employing a raft model explains why Upper Wilcox shelf margin deposition was confined and opens new exploration possibilities in this mature producing trend.