Mark G. Rowan¹, Art Leibold², Agostino Maccagni², Giuseppe Valenti²

(1) Rowan Structural Consulting, Boulder, CO
(2) Agip Petroleum Co, Houston, TX

ABSTRACT: Allochthonous Salt, Contractional Folds, and Turtle Structures of the Louisiana Deepwater Province

The Louisiana deepwater in Mississippi Canyon, Atwater Valley, Green Canyon, and Walker Ridge contains both salt-tongue and salt-stock canopies. The distribution of the two controls subsalt structural styles as well as the transport and deposition of Miocene reservoir sands. Salt-stock canopies are generally considered more prospective because subsalt contractional anticlines and turtle structures are more prevalent.

The turtle-structure trend is largely landward of the foldbelt trend, although there is some overlap. Turtles occur between canopy feeders, have various orientations, and may not have four-way closure. The best turtles are those that contain thick source rock intervals and that formed after deposition of the main reservoir intervals, so that sands are thickest in the anticlinal cores. Timing of turtle formation is variable and depends on the original or inflated thickness of the Louann salt and the suprasalt sedimentation rate.

The foldbelt trend narrows to the southwest, except for two prongs of folds extending farther landward that are related to the basement rift-basin geometry. In Atwater Valley and Green Canyon, structures include symmetrical folds, thrusted asymmetric folds, squeezed diapirs, and flap folds, in which the inclined weld of a squeezed diapir is used as a high-angle reverse fault. Most folds had a four-stage history: Upper Jurassic to Cretaceous small-wavelength folding; Paleogene and lower Neogene relative quiescence; upper Miocene shortening; and Plio-Pleistocene burial. Walker Ridge has only low-relief folds that accommodated significantly less shortening. The majority of gravity-driven contraction in this area was taken up farther landward and/or by shortening above allochthonous canopies.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado