Depositional architecture of growth-fault related wave-dominated shelf edge deltas of the Oligocene Frio Formation in Corpus Christi Bay, Texas

摘要

Growth faults within the Frio Formation define six subbasins on the South Texas shelf and add to the complexity of the sediment dispersal along the shelf margin. These growth faults influenced sediment pathways, controlled sediment partition and provided increased accommodation for deltaic depocenters. Rollover and thickening of sediments occur on the downthrown side of growth faults with offsets of at least 150 m and up to 750 m. Growth strata are indicated by intervals that thicken landward (by several tens of meters) from anticline crests. Individual 4th-order (100—500 ky) regressive cycles expand about five to seven times across growth faults; expansion ratios as great as 10 are observed. The shelf edge is associated with the largest displacement, expansion ratios and thickness of prograding deltaic sequences. Sedimentary structures and trace-fossil associations identified in cores from Nueces, Encinal and Red Fish Bay subbasins indicate deposition on a wave-dominated deltaic shoreline. The cores exhibit highly bioturbated muddy sandstones alternating with decimeter-thick clean sandstones with hummocky cross-stratification. Alternation of fair-weather wave deposits with sandy storm beds in two successive fault zones suggests that all subbasins developed under similar conditions during growth-fault development High sediment supply conditions favored accumulation of thicker sediments on the down-thrown sides of the faults. Wave dominance of deltas on the outer shelves in the growth compartments suggest that building of the upper slope rather lowers slope margin, and decreases the likelihood of the presence of sandy deep water fans. Correlation of about 700 well logs integrated with seismic data provided the areal distribution and relative timing of sediment filling of the growth-faulted compartments to unravel the evolution of higher-order (fourth and fifth) sequences affected by these growth faults. Core data were used to recognize depositional environments, to calibrate lithology to well logs and to characterize seismic amplitude anomalies. Within this shoreface succession the clean sandy storm beds will make attractive hydrocarbon reservoirs especially on the crest of rollover anticlines in downthrown compartments. These sandstone bodies pinch out seaward into open marine shelf mudstones and are sealed by overlying shelf mudstones deposited during transgressive episodes.