Abstract: Deltaic Depositional Style in Salt-Withdrawal Basin, Frio Formation, Brazoria County, Texas

D. G. Bebout, R. G. Loucks

The Frio formation along the Texas Gulf Coast consists of a gulfward dipping and thickening wedge of sandstone and shale. A high sand depocenter consisting of deltaic, strand-plain, and barrier-bar sandstones is present near the center of the wedge. Updip of this depocenter, thin fluvial-plain sandstones occur within a predominantly shale section; downdip, in the shelf and prodelta environments, sandstone bodies are also thin and in a thick shale section. However, in Brazoria County southwest of the Chocolate Bayou oil field, extensive progradation occurred during deposition of the lower part of the formation (Anomalina bilateralis zone) and large quantities of sand were transported far gulfward of the normal trend of main sand deposition. Deltaic sands up to 1,100 ft (33 m) thick accumulated in a large salt-withdrawal basin which is bounded on the updip side by enormous growth faults that developed contemporaneously with deposition and withdrawal of the salt. These sandstones are now at a depth of 12,000 to 17,000 ft (3,600 to 5,100 m).

This lower Frio unit is composed of seven facies sequences, each with prodelta shale at the base, delta-front shale and sandstone in the middle, and delta-plain sandstone and shale at the top. The most rapid filling of the basin occurred during the early depositional events as evidenced by rapid thickening of the facies sequences into the center of the basin. Paleodepositional maps of the depositional events on the downdip side of the major growth-fault system outline an area 24 mi (38 km) wide and 30 mi (48 km) long composed of overlapping lobate to slightly elongate deltas. In general, as a result of rapid subsidence, the deltaic lobes are stacked vertically but in the relatively more stable areas at the edge of the basin there is evidence of progradation of the younger delta systems slightly beyond the older deltas. The deltaic sedimentation was confined to within the salt-withdrawal basin

Sandstone porosity is lowest in parts of the basin where subsidence was most rapid and highest in the relatively stable areas.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma