ABSTRACT

Electric log patterns indicate that the Hosston Formation of central Texas consists of four principal facies: (A) massive sands greater than 80 ft thick; (B) distinct sands 10 to 80 ft thick interbedded with sharply defined shale breaks; (C) sands less than 10 ft thick intercalated with shale and silt beds; and (D) thin calcareous (dolomitic?) beds with interbedded silt. Facies A shows characteristics of fluvial sands; B of either deltaic or strand associations; C of interdistributary and overbank mud and silt deposits or lagoon-marsh deposits; and D of shallow intertidal to supratidal environments.

Cross sections show the fluvial and deltaic facies (A and B) to be concentrated on and north of the San Marcos platform while the intertidal and supratidal D facies occur downdip of these facies. Distinctive sequences of B with superposed D facies occur between these two belts. Facies C, typical of interdistributary floodbasin deposits and/or lagoon-marsh deposits is somewhat erratic in its distribution.

Warm waters (40°-50°C) are currently being produced from the A and B facies. The nature and distribution of the C and D facies isolate them from hydrologic communication with the recharge area and thus limit their use as a geothermal resource.