Geology topics

Research about Medina County, Texas

Source-linked reports with geographic coverage including Medina County, Texas.

3 recordsLinked to original sources

USGS2024

Geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within northern Medina County, Texas

During 2023–24, the U.S. Geological Survey, in cooperation with the Edwards Aquifer Authority, revised a previous publication of the geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers that was completed during 2018–20 within northern Medina County, Texas. The purpose of this report is to present the updated geologic framework and hydrostratigraphy of the rocks containing the Edwards and Trinity aquifers in northern Medina County from field observations of the surficial expressions of the rocks. The report includes a detailed 1:24,000-scale hydrostratigraphic map with names and descriptions of the geologic and hydrostratigraphic units (HSUs) in the study area. This study includes updates to the geology of the Kainer Formation (or its stratigraphic equivalent, the Fort Terrett Formation) with the addition of a burrowed unit between the basal nodular and dolomitic members. The hydrostratigraphy was also updated with the addition of HSU IIA for the upper part of the Devils River Limestone and the Seco Pass HSU for the burrowed member of the Kainer (or Fort Terrett) Formation. The Cretaceous age rocks (listed in ascending order) in the study area are part of the Trinity Group, Edwards Group and stratigraphically equivalent Devils River Limestone, Washita Group, Eagle Ford Group, Austin Group, and Taylor Group, with isolated areas where Late Cretaceous age igneous rocks have intruded. The groups and formations are composed primarily of relatively thick layers of clays, shales, and limestone. The igneous rocks are coarse-grained ultramafic in composition. Hydrostratigraphically, the rocks exposed in the study area (listed in descending order from land surface) are igneous, the upper confining unit to the Edwards aquifer, the Edwards aquifer, the upper zone of the Trinity aquifer, and the upper part of the middle zone of the Trinity aquifer. The karstic carbonate Edwards and Trinity aquifers developed because of their original depositional history, primary and secondary porosity, diagenesis, fracturing, and faulting. These factors have resulted in development of modified porosity, permeability, and transmissivity within and between the aquifers.

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USGS2020

Geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within northern Medina County, Texas

The karstic Edwards and Trinity aquifers are classified as major sources of water in south-central Texas by the Texas Water Development Board. During 2018–20 the U.S. Geological Survey, in cooperation with the Edwards Aquifer Authority, mapped and described the geologic framework and hydrostratigraphy of the rocks composing the Edwards and Trinity aquifers in northern Medina County from field observations of the surficial expressions of the rocks. The thicknesses of the mapped lithostratigraphic members and hydrostratigraphic units were also estimated from field observations. The Cretaceous-age rocks (listed in ascending order) in the study area are part of the Trinity Group (lower and upper members of the Glen Rose Limestone), Edwards Group (Kainer Formation [and its stratigraphic equivalent, the Fort Terrett Formation] and Person Formation), Devils River Limestone, Washita Group (Georgetown Formation, Del Rio Clay, and Buda Limestone), Eagle Ford Group, Austin Group, Taylor Group, and Late Cretaceous igneous intrusive rocks. The groups and formations are composed primarily of relatively thick layers of clays, shales, and limestone. The igneous rocks are coarse-grained ultramafic in composition. The principal structural feature in northern Medina County is the Balcones fault zone, which is the result of late Oligocene and early Miocene extensional faulting and fracturing resulting from the eastern Edwards Plateau uplift. In the Balcones fault zone, most of the faults in the study area are high-angle to vertical, en echelon, normal faults that are predominately downthrown to the southeast. Hydrostratigraphically, the rocks exposed in the study area (listed in descending order from land surface as they appear in a stratigraphic column) are igneous, the upper confining unit to the Edwards aquifer, the Edwards aquifer, the upper zone of the Trinity aquifer, and the upper part of the middle zone of the Trinity aquifer. The karstic carbonate Edwards and Trinity aquifers developed as a result of their original depositional history, primary and secondary porosity, diagenesis, fracturing, and faulting. These factors have resulted in development of modified porosity, permeability, and transmissivity within and between the aquifers.

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USGS2000

Geologic framework and hydrogeologic characteristics of the Edwards Aquifer outcrop, Medina County, Texas

The hydrogeologic subdivisions of the Edwards aquifer outcrop in Medina County generally are porous and permeable. The most porous and permeable appear to be hydrogeologic subdivision VI, the Kirschberg evaporite member of the Kainer Formation; and hydrogeologic subdivision III, the leached and collapsed members, undivided, of the Person Formation. The most porous and permeable rocks of the Devils River Formation in Medina County appear to be in the top layer. The upper member of the Glen Rose Limestone, the lower confining unit, has much less porosity and permeability than that observed in the Edwards aquifer. The Edwards aquifer has relatively large porosity and permeability resulting, in part, from the development or redistribution of secondary porosity. Lithology, stratigraphy, diagenesis, and karstification account for the effective porosity and permeability in the Edwards aquifer outcrop. Karst features that can greatly enhance effective porosity and permeability in the Edwards aquifer outcrop include sinkholes, dolines, and caves. The Edwards aquifer rocks in Medina County change from the eight-member Edwards Group to the essentially indivisible Devils River Formation. The facies change occurs along a line extending northwestward from just south of Medina Lake.

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