Subsidence as Factor in Development of Listric-Faulted Rollover Anticlines, Gulf of Mexico Basin

John B. Padgham

Syndepositional subsidence of deltaic clastics into finer grained sediments has long been recognized in the Gulf of Mexico basin. This subsidence influences the size and shape of typical listric-faulted rollover anticlines.

Rollovers that form through loading, subsidence, and fault slip involve the compaction of faulted beds. This reduces the fault dip angle and changes the shape of the faulted limb. However, compaction cannot account for severe reductions in this angle. Based on conceptual models, photographs of a laboratory experiment, and current seismic interpretations, listric faults are depressed and flattened as new sedimentation presses underlying abnormally pressured shales, deforming them through flexure flow into ridges and synclines.

Examples include a rollover formed through downthrown loading, and thick Pleistocene rollover limbs extended through massive loading. Through subsidence, fault dip azimuth was reversed in one case; in another, loading uniformly depressed tilted upthrown beds, forming a conveyor that supplied a growing downslope diapir. Synclinal and anticlinal curvature in many rollover limbs apparently results from compaction. Similar curvature was developed in computer models.

A conceptual model of a rollover formed as a slump is compared to rollovers offshore Texas. It is suggested that shallow subsidence along the Brazos Ridge fault produced a broad, low-relief anticline, and that subsidence, not deep-seated uplift, flexed crestal beds to produce tension faults.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.