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Growth-Fault Evolution in Offshore Texas
Barry E. Bradshaw, Joel S. Watkins
Examination of growth-fault systems in offshore southeast Texas suggests two major growth-fault provinces. In one province, shale-withdrawal growth faults occur beneath the present inner-shelf region where an upstanding basement block restricted mid-Jurassic salt deposition. Salt withdrawal faults occur farther offshore beneath the outer-shelf and slope region where up to 9,800 ft (3,000 m) of mother salt was deposited in a deep salt basin. Our data indicate a four-stage sequence of growth-fault development. The sequence begins with a primary withdrawal stage of salt or overpressured shale. Primary withdrawal faults are generated in response to differential loading induced by progradational wedges or gravity spreading. Differential pressures displace salt and shale by pure shear deformation away from the zone of maximum overburden loading into massifs, reactive diapirs, and shortening structures. A secondary withdrawal stage occurs in the salt substrate province after the primary withdrawal fault has bottomed-out. No secondary withdrawal stage occurs in the shale province. Withdrawal of salt from massifs and pillows generates secondary withdrawal faults and collapse structures, and is accommodated by the growth of diapirs and increased shortening. An allochthonous salt sheet emplacement stage may subsequently occur to accommodate continued secondary withdrawal through emplacement of extensive salt sheets from 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 systems are referred to as megacycle structures.
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