One of the most challenging plays of the recent years has been exploration below extensive allochthonous salt sheets in the lower shelf and upper slope of the northern Gulf of Mexico. Different published models explain the variety of suprasalt structures associated with roho, stepped-counter regional, salt stock, salt tongue, and salt nappe systems. In contrast, almost nothing has been published on subsalt deformation structures.

Recent 2-D and 3-D seismic surveys having long offset and long-records have illuminated the subsalt frontier region. A variety of subsalt structures can now be recognized. In addition, deep detachment surfaces appear to link the subsalt structures to the original Jurassic Louann source layer between 8 and 10 seconds.

Southward, the main suprasalt structures are as follows. (1) Down-to-the-basin, deeply detached growth faults form extensional roho systems that dominate the northern half of the North Additions. (2) Counter-regional systems formed by large-scale seaward expulsion of allochthonous salt in the southern half of the North Additions. (3) Minibasins segment extensive salt canopies in the South Additions.

Deeper still, three end members of subsalt structures can be recognized: (1) expulsion rollover anticlines forming part of subsalt counter-regional systems; (2) subsalt rim synclines surrounding evacuated salt stocks; and (3) tilted blocks rotated. along down-to-the basin growth faults representing a subsalt roho system. None of these structures are unique to the subsalt region, but other structures are expected to emerge with more data and analysis. Current seismic profiles hint at the presence below allochthonous salt of ultra-deep raft blocks and turtle structures, thrust duplexes, strike-slip systems, and block-faulted autochthonous salt.

Links between the subsalt and suprasalt structures are still unclear. However, the structural grain of the sub-salt sequences in the South Additions appears to be controlled by interaction between (1) allochthonous salt sheets associated with counter-regional systems and (2) salt stocks or more elongated salt tongues. These relationships can be better understood by mapping the deep detachment surfaces (both faults and salt welds). These surfaces have a wide variety of directions and provide three-dimensional linkage between the different salt structures.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas