THE RELATIONSHIP BETWEEN THIRD- AND FOURTH-ORDER SEQUENCES AND GROWTH FAULT DEVELOPMENT—FRIO FORMATION, SOUTH TEXAS

Subregional 3-D seismic volumes and wireline logs permitted definition of second- to fifth-order (~10 my–10 ky) Frio and Anahuac (Oligocene) sequences, systems tracts, and associated syntectonics within and across growth-faulted subbasins. Third- and most fourth-order sequences were correlated within several subregional wireline-log and seismic networks. Third-order cycles correlate across subbasins, whereas higher frequency cycles solely correlate within subbasins. Typically, growth faults develop because of sediment loading during third-order relative sea-level lowstands. However, in some instances fourth-order lowstands also generate syndepositional faulting. Postdepositional faults often nucleate above preexisting structural weakness related to older growth faults. Lithostratigraphic Frio and Anahuac strata comprise six chronostratigraphic, third-order depositional sequences and many fourth- and fifth-order sequences. Except for incised-valley fills, lowstand tracts comprise off-shelf systems deposited within active, growth-faulted, intraslope subbasins. Maximum Anahuac flooding provided a regional, dated marker to which latest published ages of sequence surfaces were calibrated. Maximum flooding surfaces and type 1 unconformities are essentially isochronous, but sand-rich lithofacies are mostly diachronous. Seaward, lowstand sedimentary wedges and superposed shelves become younger. Entrenched rivers supplied sediments via ephemeral deltas for gravity transport to basin floors and slope fans. Eventually, overloaded lowstand depocenters initiated gravity faulting, mobilized mud, and, hence, produced younger faulted, shale-withdrawal subbasins. Diminished faulting permitted lowstand deltas to extend shelf edges basinward until the deltaic ramps were anchored at the basinward margin of buried subjacent shale ridges. These shale buttresses stabilized the upper continental slope and shelf edge. During a later cycle, highstand shorelines prograded basinward over the shallow, lowstand ramps. On-shelf regression eventually stalled by increasing accommodation space near the continental shelf edge, establishing another depocenter and intraslope subbasin. These cycles not only span third-order sequences but also occur at higher frequency scales displaying all or part of the third-order facies successions.