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TURNER, JAMES R., Fina Oil and Chemical Company, Tyler, TX

ABSTRACT: A Model for the Evolution of Salt Dome Overhang, East Texas Basin

Interpreting the geometry and attitude of the salt stock and beds beneath a salt overhang is difficult. Often the geophysical data sets are ambiguous or incomplete and interpretative thinking has to be guided by geologic models. However, some geologic models are based on an apparent geometry of the salt on seismic lines and result in a volume problem between the salt and sediment. The model shown here preserves bed lengths and sediment volumes in cross sectional drawings and block diagrams during the pillow, diapiric, and postdiapiric stages of growth. The volume of the diapir is accommodated by bed thinning and drawbridge movement instead of by sediment removal by erosion.

The growth of the overhang is interpreted as a second episode of salt movement after the diapir stage has concluded. Competent beds above the diapir form a barrier to the renewed growth of the stock. The upward force of the salt lifts the overlying beds instead of breaching them. The salt then spreads laterally, splaying the beds apart and swelling the overhang salt mass in a balloon-like fashion. The overhang growth continues until the basal salt chamber is evacuated. As the basal salt is taken into the overhang, the flank sediments surrounding the stock may cave in, squeezing off the salt stock and filling the void created by the evacuated salt.

The application of the model to seismic line interpretation results in a unique geometry with a small salt stock, a vent closing fault, and an umbrella-like overhang.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.