Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
Role of Lowstand Deposition in Mobilizing Shale Ridges that Established Successive Shelf Edges, Oligocene Frio Formation (32-23.6 Ma), Gulf of Mexico Basin, South Texas
L. Frank Brown, Jr., Robert G. Loucks, Ramón H. Treviño, and Ursula Hammes
Rivers entrenched at shelf edges during relative lowstands of sea level served as sediment point sources. The sediments accumulated in basin-floor fans, slope fans, and prograding deltaic-wedge systems. Increasing weight of the sediment load generated growth faulting that began during mid-slope-fan deposition. Initially, faulting accelerated accommodation rates in subbasins and increased load stress, which mobilized and forced deep, water-saturated, muddy facies basinward and upward in response to rotation of footwall blocks. Extensional faults developed in strata above shale ridges. Growth of uplifted ridges diminished fault movement enough to permit aggradational slope-fan deposition to in turn reduce water depths. Subsequently, deltas prograded over slope systems that were now sufficiently shallow for deltaic deposition. Minor uplift of shale ridges and normal fault movement were typically rejuvenated during later cycles. Individual arcuate (areally concave basinward) growth faults comprise segments of major strike-aligned regional fault and associated shale-ridge systems produced during temporally equivalent relative sea-level falls. Depositional systems that filled subbasins are genetically identical (although diachronous), with systems tracts becoming younger basinward. Shale ridges produced paleo-seafloor relief responsible for basinward thinning of slope fans and delta systems. Late lowstand deltaic progradation terminated on the crest of contemporaneous shale ridges. Consequently, basinward flanks of ridges progressively served to reestablish younger Frio shelf margins farther basinward, occurring synchronously with the end of each third-order relative lowstand of sea level. We suspect that sedimentary-induced tectonics may be a greatly underappreciated key factor in the origin and evolution of many types of sedimentary basins and depositional systems.