ABSTRACT

Middle Frio, here defined as material deposited from the time of Anomalina cocoaensis until the time of Nonion struma, manifests seven environments of deposition in the subsurface of Chambers County, Texas: continental, brackish, saline bay, shallow inner shelf, deep inner shelf, middle shelf, and outer shelf. From absence or frequency of various foraminiferal species, environments can be delineated by comparison of the Frio assemblages with distribution patterns of living Foraminifera.

In Chambers County, eight persistent sand bodies can be correlated using seven index foraminiferal and distinctive electric log characters. Isopach maps, based on approximately 400 electric log sections, suggest depositional environments of the sand.

The county is cut by a system of middle Frio growth faults. Faunal analyses of sedimentary units indicate topographic relief on the faults during deposition, with turbidity currents at certain times moving shallow-water sediments and their contained faunas to deeper water on the downthrown sides.

Environments indicated on paleobathymetric maps, constructed from faunal analyses of thirteen wells, correspond closely to those suggested by geometry of the sandstone bodies.