Counterregional-Style Deformation in the Deep Shelf of the Northern Gulf of Mexico
Mark G. Rowan1 and Kerry F. Inman2
1Rowan Consulting, Inc., 850 8th St., Boulder, Colorado 80302; [email protected]
2Exploration Consultant, 1114 Barkdull, Houston, Texas 77006
The stepped counterregional style of allochthonous salt is well developed and documented in the northern Gulf of Mexico. In this model, a tabular salt tongue or canopy is evacuated into isolated diapirs connected by counterregional (or landward-dipping) growth faults, with the withdrawal basin having the geometry of a basinward-dipping, monoclinal growth wedge. We suggest that a similar style dominates the deeper, supra-Louann level in large parts of the northern Gulf of Mexico, most notably significant portions of the deep shelf province.
In our model, a basinward-dipping and -thickening minibasin subsides into Louann salt that is displaced into a basinward-leaning primary diapir. Salt withdrawal is the dominant process, although extension, contraction, and strike-slip movement can locally be important. The diapir eventually forms a counterregional weld as the salt moves into an allochthonous level. Subsidence of the minibasin relative to outlying areas is greatest at the diapir and decreases away from the diapir along counterregional faults until it is taken up only by folding beyond the fault tips. Depending on the orientation of a two-dimensional cut through such a system, the geometry can range from an asymmetric fault-bounded growth wedge to a faulted fold to a symmetric growth syncline. Two or more such systems with variable orientations in proximity to each other results in a wide variety of structural styles: repeated counterregional faults, growth synclines, faulted synclines, turtle structures, fault-bounded turtles, and horst blocks. Although four-way closures are certainly possible, structural traps tend to be three-way closures against welded feeders or counterregional faults.
AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana