Low-temperature geothermal ground water in the Hosston/Cotton Valley hydrogeologic unit, Falls County area, Texas
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In the Falls County study area in east-central Texas, the Cotton Valley/Schuler Member and the overlying Hosston Sand (Late Jurassic to Early Cretaceous?) act as a single hydrogeologic unit (aquifer) that contains low-temperature geothermal ground water. The hydrogeologic unit consists of coarse- to very-fine-grained sandstone which becomes more fine-grained to the east, or basinward. Minor amounts of red and black shale are also present, and chert conglomerates become less common basinward. The unit was probably deposited as a bedload-dominated fluvial system in the western part of the study area, as shallow shelf sands or other sand-rich marginal-marine sediments in the central part of the study area, and possibly as sand-rich submarine-fan systems in the eastern part. Both the depositional history and the Balcones/Ouachita structural hinge control the hydrology of the aquifer. Where Balcones faulting interrupts the continuity of the aquifer, ground-water movement may be channelled along the faults. Transmissivity and hydraulic conductivity are enhanced along the Balcones Fault Zone as well. Where faulting is absent or less evident, transmissivity and hydraulic conductivity are controlled by net-sand thickness (depositional axes) and ground-water movement is downdip or radially toward a cone of depression centered in McLennan County. This cone of depression, which has resulted from years of ground-water withdrawal, influences ground-water movement throughout most of the meteoric part of the aquifer. In the eastern half of the study area, the ground water in the Hosston/Cotton Valley is saline and the pressure heads are much higher than in the western part of the aquifer, suggesting there is potential for updip movement of the saline ground water. Along the Mexia Fault Zone, which is approximately parallel to and east of the Balcones Fault Zone, pressure heads are somewhat lower than surrounding heads, suggesting that the Fault Zone is the locus of groundwater discharge from the Hosston/Cotton Valley, and possibly from aquifers beneath the Hosston/Cotton Valley. The Balcones/Ouachita hinge also appears to control the geothermal regime in the study area. In the west, the aquifer overlies Ouachita rocks and is relatively thin. East of the Balcones Fault Zone, where the Ouachita rocks begin to dip steeply into the basin, the aquifer thickens rapidly. These two regions correspond to areas of conductive and forced-convective heat flow, respectively. Geothermal gradients are high (40°C/km) along the Balcones Fault Zone, presumably due to conductive heat flow from the basement rocks, and are probably higher than normal along the Mexia Fault Zone to the east, where the faults may be acting as loci for upwelling ground water.