Growth Faults, Salt, and Deep Structure of Northern Gulf of Mexico

Dan M. Worrall, Sigmund Snelson

The major Cenozoic growth fault systems of offshore Texas and Louisiana are of two main styles: type I--short, arcuate fault systems associated with minibasins and salt domes, most typically found offshore Louisiana; type II--laterally extensive, linear, listric fault systems, most common in coastal Texas. Both styles are the results of updip extension and related downdip, gravity-driven, lateral spreading of strata and salt.

The short, arcuate type I fault systems typically occur when rapid, local sedimentary loading has displaced thick salt. Downdip spreading of this thick salt and its clastic cover resulted in the Sigsbee salt nappe offshore Louisiana, earlier hypothesized by others. Seismic data now show that this nappe has overthrust abyssal strata a distance of at least 60 mi (80 km). The basal nappe surface dips north and is last seen truncating lower Tertiary strata beneath the middle slope, where extensive allochthonous salt can exceed 25,000 ft (7.6 km) in thickness.

Palinspastic reconstructions of major, linear type II fault systems offshore Texas suggest that at least some of the extension in these systems is also compensated by downdip deformation of salt. Mostly Paleogene salt-cored(?) folds (Perdido foldbelt) at the base of the lower Texas slope are too old and exhibit far too little horizontal shortening to compensate for the tens of kilometers of Neogene extension under the Texas shelf. Downdip compensation for these Neogene faults occurred in the salt-rich middle and lower slope. It is likely that Paleogene fault systems farther updip also were compensated by downdip mobile salt, probably in conjunction with stratal shortening by Perdido folds.

The contrast in the nature of faulting between the two types of fault systems may well be a function of depositional style. Short, arcuate fault systems associated with minibasins and trapped salt (type I) are more likely the result of irregular point loads provided by shifting deltas. Long, linear, listric fault systems (type II) are generally associated with strike-oriented barrier island/interdeltaic deposition.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.