Abstract: Growth-Fault Evolution in Offshore Texas

Barry E. Bradshaw, Joel S. Watkins

Growth-fault systems in offshore southeast Texas suggest the presence of two major growth-fault provinces. Shale-withdrawal growth-fault systems occur beneath the present inner-shelf region where deposition of mid-Jurassic salt was restricted. Salt-withdrawal growth-fault systems occur further offshore beneath the modern outer shelf and slope region where up to 3000 m of mother salt was deposited in a deep salt basin. Observations from these provinces suggest a four-stage model of growth-fault development in offshore Texas. A primary withdrawal stage occurs in viscous substrates of both salt and overpressured shale. Primary withdrawal faults are generated and accommodated when differential loading induced by progradational wedges or gravity spreading displaces salt and shale by pure s ear deformation away from the zone of maximum overburden loading into massifs, reactive diapirs, and shortening structures. Secondary withdrawal stages apply only to salt substrates and occur after the primary withdrawal faults have bottomed out. Secondary withdrawal of salt from massifs and pillows generates additional withdrawal faults and collapse structures, and is accommodated by the growth of diapirs and increased shortening. An allochthonous salt sheet emplacement stage involves the accommodation for continued secondary withdrawal through emplacement of extensive allochthonous salt sheets from both overthrusted nappes and/or diapir close up. Supra-allochthonous growth-fault systems may then be generated in a new phase of primary salt withdrawal. Complete four-stage growth-fault sy tems are referred to as megacycle structures.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994