Jurassic Faults of the Southwest Alabama and Offshore Areas

MINK, ROBERT M., BERRY H. TEW, BENNETT L. BEARDEN, and ERNEST A. MANCINI, Geological Survey of Alabama, Tuscaloosa, AL

Four fault groups affecting Jurassic strata occur in the southwest and offshore Alabama areas. They include the regional basement rift trend, the regional peripheral fault trend, the Mobile graben fault system, and the Lower Mobile Bay fault system. The regional basement rift and regional peripheral fault trends are distinct and rim the inner margin of the eastern Gulf Coastal Plain. The regional basement rift trend is genetically related to the breakup of Pangea and the opening of the Gulf of Mexico in the Late Triassic-Early Jurassic. This fault trend is thought to have formed contemporaneously with deposition of Late Triassic-Early Jurassic Eagle Mills Formation and to displace pre-Mesozoic rocks. The relative thickness of Jurassic Louann Salt near the basin margin was controlled s mewhat by paleotopography associated with the regional basement rift trend. The regional peripheral fault trend consists of a group of en echelon extensional faults that are parallel or subparallel to regional strike of Gulf Coastal Plain strata and correspond to the approximate updip limit of thick Louann Salt. Most of the salt-soled faults in the system are down to the basin, listric, and display growth. Nondiapiric salt features are associated with the trend and maximum structural development is exhibited in the Haynesville-Smackover section. The Mobile graben fault system consists of a series of north-south-trending fractures that generally mark the eastern boundary of the Mississippi interior salt basin. Salt piercement features are associated with the Mobile graben fault system and generally occur on the upthrown blocks of faults. Structural development along the system may have been initiated in the Jurassic; however, major salt tectonism is clearly evidenced in Tertiary strata. The Lower Mobile Bay fault system consists of a group of east-west-trending extensional faults in offshore Alabama occurring south of the Wiggins arch complex. These salt-soled faults are generally down to the basin, listric, and exhibit growth in the Haynesville-Smackover section. Salt structures associated with the Lower Mobile Bay fault system are pillow-stage features that principally affect pre-Cotton Valley strata. After deposition of the Haynesville section, effective salt movement was minimal.

No hydrocarbon accumulations have been documented in the pre-Jurassic strata of southwest and offshore Alabama. Productive hydrocarbon reservoirs occur in Jurassic strata along the trends of the fault groups, suggesting a significant relationship between structural development in the Jurassic and hydrocarbon accumulation. Hydrocarbon traps are generally structural or contain a major structural component and include salt anticlines, faulted salt anticlines, and extensional fault traps. All of the major hydrocarbon accumulations are associated with movement of the Louann Salt along the regional peripheral fault trend, the Mobile graben fault system, or the Lower Mobile Bay fault system.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)