Rusk County lies between the Sabine Uplift on the east and the East Texas Basin on the west. These two prominent structural features resulted from faulting that began in the Triassic Period (200 to 250 million years ago).
The axis of the East Texas Basin trends north to south generally along the western boundary of Smith County. The Sabine Uplift, which centers in Panola County, Texas, and northwestern Louisiana, forms the eastern boundary of the basin.
The development of the two complementary structural features (basin and uplift) contributed to the setting for some of the largest petroleum reservoirs in the world: the East Texas Oil Field and the Carthage Gas Field.
The Mt. Enterprise Fault System trends east-west across the southern part of Rusk County. Movement along the fault system has been variable. Some areas show a maximum vertical displacement of over 200 feet, with beds of the Queen City Sand downdropped against the Carrizo Sand.
Strata in northern and central Rusk County show a general dip away from the Sabine Uplift. In the southern part of the county, the gradient increases and becomes more variable in close proximity to the Mt. Enterprise Fault System.
The geology of Rusk County, as it relates to fresh groundwater, is comprised of alternating sequences of continental, deltaic, and marine sediments that are predominantly of Eocene (33.9 to 56 million years ago) to Paleocene (56 to 66 million years ago) age.
Continental and deltaic units, composed of quartz sand with varying amounts of silt and clay, contain the fresh groundwater in the area and form the major conduits for its movement.
Marine portions of the section, consisting largely of clay or shale with lesser silt and glauconitic sandstone, form the intervening aquitards that restrict the movement of groundwater.
The deepest fresh water aquifer in Rusk County is the Carrizo-Wilcox aquifer, composed of the Wilcox Group and the immediately overlying Carrizo Sand. Excellent aquifer characteristics have made the Carrizo-Wilcox the most productive aquifer in East Texas. Recharge through its outcrop areas in Rusk County contributes significantly to the availability of Carrizo-Wilcox groundwater throughout much of the region.
The Wilcox is underlain by the Midway Group, a predominantly marine and lagoonal shale. No significant fresh groundwater is known to exist in the Midway or deeper strata; therefore, the top of the Midway marks the base of fresh groundwater in Rusk County.
The Midway Group is overlain successively by the Wilcox Group, Carrizo Sand, Reklaw Formation, Queen City Sand, Weches Formation, and Sparta Sand. The Reklaw and Weches have extremely poor water-bearing qualities and are insignificant as aquifers in Rusk County.
Sparta and Queen City sediments are preserved on downdropped blocks of the Mt. Enterprise Fault System and on higher elevations in northern Rusk County. However, the limited areal extent of the Sparta and Queen City restricts their use as a water supply to low-yield, shallow wells.
To explore Texas geology, please visit this USGS interactive map.
Explore the hydrogeology of the Northern Carrizo-Wilcox Aquifer with the TWDB Interactive Map.
View Carrizo-Wilcox Aquifer data here.
*Information on the geologic units and their water-bearing properties within Rusk County is available in the District’s Management Plan, located in the District Documents section of the District’s website.
