Growth-Faulted Compartments of the Oligocene Frio Formation in Proximity of the Shelf Edge in Corpus Christi Bay, Texas
Olariu, Mariana; Hammes, Ursula; Ambrose, William A.; Ogiesoba, Oscareni C.
Growth faults within the Frio Formation separate 6 sub-basins on the South Texas shelf and add to the complexity of the sediment dispersal along the shelf margin because they influence pathways, control sediment partition and provide locally increased accommodation for deltaic depocenters. Rollover and thickening of sediment occur on the downthrown side of growth faults with offsets of at least 500 ft, with more than 1000 ft being common. The shelf edge is associated with maximum displacement, maximum expansion rations and maximum thickness of prograding deltaic sequences. Sedimentary structures and trace-fossil associations identified in cores from Nueces and Encinal sub-basins indicate a transition of depositional environments from lower shoreface to offshore. The cores exhibit highly bioturbated (Thalassinoides, Asterosoma, Teichichnus, Paleophycus, and Planolites) muddy sandstones alternating with decimeter-thick clean sandstones with hummocky cross-stratification and Ophiomorpha trace fossils. Alternation of fair-weather wave deposits with sandy storm beds suggests deposition on a wave dominated shoreface-shelf environment. The presence of similar facies in two successive fault zones suggests that both sub-basins developed under similar conditions during growth-fault development and under high sediment supply conditions which favored accumulation of thicker sediments on the downthrown side of the fault. Locally decimeter-thick, parallel-laminated or massive sandstone with sharp bases are interbedded with decimeter-thick mudstones; both lithologies lack bioturbation. These sediments are interpreted as delta front turbidites deposited in the proximity of the shelf edge. Core data were used for recognizing depositional environments and for calibrating lithology to well logs and to seismic amplitude anomalies. Correlation of about 700 well logs integrated with seismic data provided the amount and relative timing of the filling of the growth-faulted compartments to unravel the evolution of higher-order (fourth and fifth) sequences affected by these growth faults. 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 sand bodies pinch out seaward into open marine shelf mudstones and are sealed by overlying shelf mudstones developed during the successive transgression.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013