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Abstract
Gulf Coast Association of Geological Societies Transactions
Volume 54 (2004)
EXTENDED ABSTRACT: 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.
Fiduk, Joseph C.,1 Anderson,
Lynn E.,1 and Rowan, Mark G.2
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 around the northern
Gulf of Mexico continental margin but fails to explain why Upper Wilcox deltaic
sediments are greatly expanded but also relatively confined compared to other
Paleogene deltaic centers.
Reprocessed 2-D seismic data across the Wilcox
Depotrough show 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.
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