Abstract

Growth faults are ubiquitous features within the Oligocene Frio Formation and separate several sub-basins on the South Texas shelf. These growth faults add to the complexity of the sediment dispersal along the shelf margin because they influence pathways, control sediment partition and provide accommodation for deltaic depocenters. Growth faults also affect the development of the stratigraphic sequences and associated surfaces (e.g., sequence boundaries). The stratigraphic and sedimentological character of the Frio Formation is examined in detail in cores and interpreted from well logs. Sedimentary structures and trace-fossil associations indicate a transition of depositional environments from lower shoreface to offshore and slope fans. The same stratigraphic interval (28.5–30.2 million years) has been examined in cores located in different sub-basins. The cores exhibit highly bioturbated (Thalassinoides, Asterosoma, Teichichnus, Paleophycus, and Planolites) muddy sandstone alternating with decimeter-thick clean sandstone beds with hummocky cross stratification and common Ophiomorpha trace fossils. These beds are interpreted as shelf (below fair-weather wave) deposits composed of low-energy, muddy sandstones alternating with sandy storm beds. Other cores display decimeter-thick, parallel-laminated or massive sandstone with sharp bases alternating with decimeter-thick mudstones; both lithologies lack bioturbation. These sediments are interpreted as gravity flow (turbidite) deposits on the slope.

Correlation of core-calibrated logs with about 700 well logs provides the amount (volumetric) and relative timing of the filling of the growth-faulted compartments to unravel the evolution and the formation of higher-order (fourth and fifth) sequences affected by these growth faults.