--> ABSTRACT: Composite Origin of Allochthonous Salt, Offshore Louisiana, by Mark G. Rowan, Barry C. McBride, Paul Weimer; #91020 (1995).

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Composite Origin of Allochthonous Salt, Offshore Louisiana

Mark G. Rowan, Barry C. McBride, Paul Weimer

Salt geometries observed within the Sigsbee Nappe, the upper slope, and the shelf are commonly thought to represent immature, intermediate, and mature stages, respectively, in the evolution of allochthonous salt. However, the base of much of the Sigsbee Nappe and salt welds beneath the inner shelf both appear to have ramp-flat geometries, whereas welds beneath the upper slope of offshore Louisiana display an irregular topography characterized by elliptical depressions separated by saddles. This anomalous structural style cannot be explained simply by a decreasing salt budget; instead, it suggests distinct histories for different parts of the composite salt system.

During the early- to mid-Tertiary, rapid loading of salt in the area of the present coastline produced regionally extensive salt nappes, analogous to the Sigsbee. As these advanced basinward, they left behind stepped-counter-regional systems and isolated salt diapirs. At the same time, numerous diapirs grew in the current upper slope region, possibly sourced from the autochthonous Louann Salt. Because of slow sedimentation rates, the diapirs grew outward through time, often amalgamating to form salt canopies. Remobilized salt from these bowl-shaped diapirs formed salt bodies over the saddles between the depressions as well as the Sigsbee Nappe. Near the end of the Miocene, the Sigsbee-like nappe system to the north merged with the modified canopy system in the vicinity of the present uter shelf.

The composite origin of the allochthonous salt is probably due, at least in part, to the initial areal distribution and thickness variations of the autochthonous Louann Salt. For example, the pattern observed in offshore Louisiana could be explained by two thicker salt basins to the north and south that were separated by thinner salt beneath the present middle shelf. Thus, this model does not necessarily apply to other parts of the northern Gulf of Mexico.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995