--> Abstract: Deformation of Allochthonous Salt and Evolution of Related Structural Systems, Eastern Louisiana Gulf Coast, by D. C. Schuster; #90987 (1993).

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SCHUSTER, DAVID C., Shell Offshore Inc., New Orleans, LA

ABSTRACT: Deformation of Allochthonous Salt and Evolution of Related Structural Systems, Eastern Louisiana Gulf Coast

Salt tectonics may involve not only vertical diapirism, but also horizontal silling (or lateral flow) into wings and tablets. Various combinations of these two modes of salt deformation, concurrent with sediment loading and evacuation of salt, have produced many complex (multi staged) structures in the northern Gulf of Mexico margin. Good examples of allochthonous salt bodies and structures resulting from evacuation of allochthonous salt are found in the coastal/offshore region of southeast Louisiana, a prolific oil and gas province. Many salt domes in the coastal/offshore region and tabular salt masses under the upper slope were fed by north-dipping feeders, now usually collapsed. Therefore, these salt bodies are detached from their Jurassic source. The large growth faults in the nor heastern Gulf Coast are due to salt evacuation, and many flatten into intra-Tertiary horizons which were formerly occupied by allochthonous salt masses. These former salt masses, like the salt tablets offshore today, were sourced along north-dipping feeders. Two end member structural systems involving evacuation of (former) tabular salt masses are recognized: "roho" systems and "stepped counter-regional" systems.

Roho systems are characterized by large, listric, down-to-the-basin growth faults which sole into intra-Tertiary horizons marked by high amplitude reflections continuous with residual salt masses. Salt from the former tablets has been loaded and squeezed laterally and downdip. An isolated roho system southeast of the Mississippi delta contrasts with larger, amalgamated roho systems under the central and western Louisiana shelf.

Stepped counter-regional systems off southeast Louisiana are comprised of counter-regional faults which sole into intra-Tertiary horizons (evacuated salt) before stepping down again further north--probably to the Jurassic salt horizon. Large salt withdrawal basins north of the counter-regional faults include south-dipping strata which terminate onto the evacuated horizons. These systems have a multi-staged evolution, including (1) initial formation of a south-leaning diapir, (2) progressive development into a semitabular allochthonous salt body, and (3) subsequent loading and evacuation of the allochthonous salt into secondary domes. The salt in these domes (e.g., WD 133, WD 137, and possibly the Bay Marchand trend) was sourced from the Jurassic salt horizon 10 to 15 miles north of th domes. Recognition of these more complex, deep-seated salt geometries should befactored into our analyses of hydrocarbon charge and migration, in light of the strong correlation between oil and salt in the Gulf Coast.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.