Evolution of Salt-Related Structures

R. S. Bishop

Several types of structures (piercements, turtles, and nonpiercements) are caused by salt movement. Reconstructions show that the emplacement process is basically the same for many geometrically dissimilar structures, but that the great differences of shape originated from different patterns of sediment loading, salt thickness, and basin evolution.

The reconstructions are generalizations derived from numerous real examples to show timing, evolution of dip, origin of thickness changes and overhangs, how the salt-sediment volume exchange occurs, and diagnostic criteria to interpret these events. Such reconstructions help to discriminate between turtles and nonpiercements, to interpret lithofacies, and to unravel the role of sedimentary events on the structural evolution. In addition, they illustrate the mechanism of diapirism, using criteria to help distinguish diapirism in an overburden having strength (the mechanism assumed here) from diapirism in a viscous overburden (the classical buoyancy theory).

In general, many piercements may start quite early (even before a density inversion exists) and move primarily by extrusion or may alternate between extrusion and intrusion beneath a thin overburden. The pattern of sedimentation largely determines the pattern of diapirism. In contrast, nonpiercements and turtle structures are passive features and may form whenever salt migrates away from them to an adjacent "escape hatch." For example, nonpiercements may not form by salt rising vertically, but rather by salt moving away horizontally to some point of escape. In other words, the dome remains static while the overburden collapses into the rim syncline.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.