The top of the Midway Group of Paleocene age marks the base of the extent 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.
In Central Texas, the Wilcox Group of Paleocene to Eocene age is subdivided into the Hooper, Simsboro, and Calvert Bluff formations, corresponding to deltaic, fluvial, and fluvial-deltaic facies, respectively.
However, in East Texas, the Simsboro is no longer identifiable, and the Wilcox Group is divided into informal lower and upper units. The lower Wilcox represents the facies equivalent of the Hooper Formation, and the upper Wilcox includes both the Simsboro and the Calvert Bluff equivalent fluvial and fluvial-deltaic facies, respectively.
In East Texas and Rusk County, the Wilcox Group consists of beds of sand, silt, and clay, with locally economic amounts of lignite. These Wilcox Group sediments represent multi-facies, fluvial-deltaic systems where channels and associated sand facies form the framework for groundwater movement.
The sand bodies are elongated, sinuous, and laterally discontinuous with axes generally oriented north to south, consistent with the direction of sediment transport. The elongate sand bodies represent ancient fluvial systems and offer optimal locations for high-yield water wells.
In western Rusk County, the Wilcox reaches a maximum thickness of approximately 1,500 feet. The unit thins toward the uplift and is reduced to slightly over 600 feet thick in its outcrop in the eastern portion of the county.
The Carrizo Sand is a massive, relatively homogenous sand of Eocene age consisting of medium- to fine-grain quartz sand with minor occurrences of interbedded gray clay.
The Carrizo Sand is a clastic, near-shore deposit with beach, dune, barrier island, and lagoonal facies represented in outcrops throughout East Texas. In Rusk County, where not thinned or entirely removed by erosion, the formation can reach a thickness of over 125 feet.
The Reklaw Formation of Eocene age is a shallow marine shelf deposit that is primarily composed of glauconitic clay and silt. In some locales, the formation commonly contains minor amounts of sand in the basal portion of the formation, near its contact with the underlying Carrizo Sand.
In outcrop, the Reklaw forms a red clay soil that typically contains limonite seams and iron concretions. In Rusk County, the Reklaw Formation reaches a maximum thickness of approximately 130 feet and occurs primarily in the northern portion of the county and north of the Mt. Enterprise Fault System in the southern portion of the county.
The Queen City Sand of Eocene age was deposited by an extensive deltaic system and is primarily composed of sand, loosely cemented sandstone, and interbedded clay units with minor occurrences of lignite.
In East Texas, sand facies of the Queen City Sand are thickest near the center of the East Texas Basin and generally thin eastward along the strike of the formation, pinching out in the subsurface just west of the Texas-Louisiana border.
In Rusk County, the Queen City Sand occurs in outcrop and subcrop in the northwestern portion of the county and also in the Mt. Enterprise Fault System in the southern portion of the county. The formation ranges in thickness up to 130 feet.
The Weches Formation of Eocene age is a shallow marine shelf deposit that is primarily composed of glauconitic clay with only minor amounts of sand. The formation is green in unweathered sections, but weathers to red when exposed.
Relatively thin sections of the Weches Formation occur in the Mt. Enterprise Fault System in southern Rusk County, where it attains a maximum thickness of approximately 50 feet.
The Sparta Sand of Eocene age consists of fine sand and interbedded sandy clay and silt deposited in a deltaic environment similar to the Queen City Sand. In Rusk County, the Sparta Sand only exists as laterally discontinuous units within the Mt. Enterprise Fault System, where it attains maximum thicknesses of about 100 feet.
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*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.
