ABSTRACT: Variation of Faulting on Coastal Salt Basin Diapiric Structures

Thomas G. Fails

Many of the major growth faults affecting diapiric structures (salt domes, clay, or "shale" domes) originate in the upper continental slope environment. They flank the diapiric ridges and massifs that subsequently segment into individual diapiric structures as continental shelf deposition commences. Upper slope sand depocenters are often associated with these faults and some of these growth faults remain active as continental shelf and alluvial plain deposition continues. Other faults are buffed and become inactive. The resulting fault patterns vary with respect to the type of diapiric structure on which they occur.

Diapiric structures can be conveniently classified by the diapirobjective section relationship (continental shelf sands and shales and sandy upper slope strata make up the objective section):
Penetrant (or shallow piercement) diapirs--pierce the entire objective section plus much or all of the overlying nonobjective alluvial strata. Many still active and unburied.
Semipenetrant (or intermediate piercement) diapirs--pierce part but not all of the objective section, arching the shallower overlying beds. Buried and inactive.
Nonpenetrant (or deep-seated) diapirs--buried in upper slope shales beneath the base of the arched objective section. Inactive.

Fault patterns on Coastal Salt Basin diapiric structures fall into three basic classes:
Single or multiple offset--one or more semiparallel faults downthrown in the same direction.
Compensated--two or more semiparallel faults downthrown in opposite directions, forming grabens or horsts.
Crossed offset--two or more faults in crossed orientation.

Over 200 diapiric structures in the Coastal Salt Basin (offshore and onshore south Louisiana and southeast Texas) were analyzed for fault type, orientation, and pattern. Significant variations and trends in fault characteristics occur by diapiric structure type and are illustrated graphically. While most diapiric structure fault patterns fall in one of the three basic classes, combinations of two patterns are relatively common on certain types of diapiric structures. And certain fault orientations and patterns occur repeatedly with each diapiric structure type. These data should be useful to salt dome workers interested in diapir/diapiric structure development and/or in origination of drilling prospects on both mature and undeveloped salt domes.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990