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Geologic Framework and Hydrogeologic Properties of the Seco Creek Watershed, Texas in 1991, the U.S

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Geologic Framework and Hydrogeologic Properties of the Seco Creek Watershed, Texas in 1991, the U.S In cooperation with the U.S. Department of Agriculture and the Texas State Soil and Water Conservation Board Geologic Framework and Hydrogeologic Properties of the Seco Creek Watershed, Texas In 1991, the U.S. Geological Survey Seco Creek watershed are disrupted by Formation the top of the Edwards aquifer, (USGS), in cooperation with the U.S. faults of the Balcones fault zone (fig. 1). the formation is considered part of the Department of Agriculture (USDA) and Extensional normal faults in the Bal­ aquifer. The Georgetown Formation, the Texas State Soil and Water Conserva­ cones fault zone typically have strikes which is 0 10 about 82 ft thick (table 1), tion Board, began a study to evaluate the trending slightly northeastward and dip a consists of whitish, fossiliferous, dense effects of agricultural best-management few feet per mile to the southeast. Bal­ limestone near the bottom that grades practices on surface- and ground-water cones faults are en echelon (in steplike upward into a buff to yellow argillaceous quantity and quality in the Seco Creek arrangement), high-angle (nearly verti­ limestone near the top. watershed. The USGS study is part of a cal), normal, and generally downthrown Groups and formations above the larger study known as the Seco Creek to the southeast. Normal faults have jux­ Georgetown Formation consist of the Water-Quality Demonstration Project, taposed stratigraphically younger rocks Upper Cretaceous Del Rio Clay, Buda which is intended to demonstrate to urban against stratigraphically older rocks. Limestone, Eagle Ford Group, Austin and rural land and water users the poten­ South of the Balcones fault zone, the Group, Anacacho Limestone, and Escon- tial to reduce transport of agricultural regional dip steepens toward the south- dido Formation; the Tertiary Midway chemicals and sediment, improve ground- southeast (15 to 20 ft/mi) (Holt, 1956, Group, Wilcox Group, and Uvalde water and downstream surface-water p. 14). Gravel; and the Quaternary Leona Forma­ quality, and improve the quality and The Lower Cretaceous upper member tion and alluvium (table 1). The Del Rio abundance of vegetative cover. The Seco of the Glen Rose Limestone is greater Clay is about 45 to 90 ft thick and consists Creek watershed is approximately 50 than 500 feet (ft) thick in the Seco Creek of blue-green to yellow-brown clay. The miles (mi) west of San Antonio (fig. 1) watershed and consists of alternating Buda Limestone consists of about 60 to and has a drainage area of approximately beds of dense limestone and fossiliferous 90 ft of dense, massive micritic limestone 255 square miles (mi2). Agricultural impermeable marl (table 1). Overlying (Maclay and Small, 1984, p. 12). The activities account for more than 99 per­ the Glen Rose Limestone in the Balcones Eagle Ford Group, about 55 to 70 ft thick, cent of the land use in the watershed fault zone are the Devils River, Fort consists of shale, sandy siltstone, and (Steffens and Wright, 1996). The purpose Terretl, and Segovia Formations (Lozo flaggy limestone (Holt, 1956, p. 29). The of this fact sheet is to describe the geo­ and Smith, 1964). logic framework and hydrogeologic prop­ The Devils River Formation is strati­ erties of the Seco Creek watershed. graphically equivalent to the Fort Terrett Geologic Framework and Segovia Formations. The Devils Rocks exposed in the Seco Creek River Formation is about 380 to 620 ft watershed are of sedimentary origin. thick (table 1) and is mostly hard, miliolid The formations in the watershed (fig. 1; (a small foraminifera) grainstone to mud- table 1) primarily are Lower and Upper stone, with abundant chert nodules and Cretaceous marine limestone and marl, beds. Nodular argillaceous limestone is with some Tertiary and Quaternary sand­ found in the lower Devils River Forma­ stone and shale; marine invertebrate fos­ tion, collapsed breccia in the middle, and sils are common as well. Alluvial-plain rudist mounds at the top. The Fort Terrett sediments (clay, sand, gravel, cobbles Formation, the thickness of which ranges of limestone, dolomite, and chert) are from 0 to about 250 ft, consists of por- extensive and cover much of the bedrock celaneous limestone, collapsed breccia, throughout the study area. Generally, and chert (Rose, 1972). The 0- to 80-ft- the age of the exposed bedrock decreases thick Segovia Formation overlies the Fort from north to south. The oldest rocks Terrett Formation and consists of a marly exposed are the Lower Cretaceous rocks limestone in the lower part, dolomite and in the north, which are topographically collapsed breccia in the middle, and higher than the Upper Cretaceous, Ter­ miliolid and shell-fragmented limestone tiary, and Quaternary rocks in the south. and chert at the top (Rose, 1972, p. 35). Strata in the north have a regional dip of The Georgetown Formation is not approximately 8 to 9 feet per mile (ft/mi) known to yield water in the study area. (Reeves and Lee, 1962, p. 17) toward the However, because well drillers histori­ Seco Creek watershed, Medina County, southeast. However, regional dips in the cally have considered the Georgetown Texas (Bat Cave). (Photograph byJ.R. Gilhousen, U.S. Geological Survey) USGS Fact Sheet FS-104-98 U.S. Department of the Interior U.S. Geological Survey January 1999 EXPLANATION | | ALLUVIUM | | LEONA FORMATION ^ UVALDE GRAVEL | | WILCOX GROUP ^ MIDWAY GROUP ESCONDIDO FORMATION | | ANACACHO LIMESTONE [ | AUSTIN GROUP [ I EAGLE FORD GROUP BUDA LIMESTONE Ks Kdvr SEGOVIA FORMATION (Ks), FORT TERRETT FORMATION (Kft), AND DEVILS RIVER FORMATION (Kdvr), UNDIVIDED UPPER MEMBER OF THE GLEN ROSE LIMESTONE INFERRED FAULT 5 FAULT U, upthrown side; D, downthrown side Base modified from University of Texas, Bureau of Economic Geology, 1974, Universal Transverse Mercator Projection Zone 14 Figure 1. Location and surficial geology of the Seco Creek watershed, Texas. 230- to 290-ft-thick Austin Group overlies The Tertiary Midway Group, Wilcox Quaternary deposits (cobble, gravel, the Eagle Ford Group and consists of mas­ Group, and Uvalde Gravel overlie the sand, silt, and clay) in the Seco Creek sive chalky to marly, fossiliferous lime­ Escondido Formation. Glauconitic sand­ watershed include the Leona Formation stone. The Anacacho Limestone is about stone and shale predominantly compose and alluvium. These deposits are found 290 to 530 ft thick and consists of fossilif­ the 20- to 150-ft-thick Midway Group along stream valleys and flood plains, with thicknesses of as much as about 70 ft for erous limestone, chalk, marl, and clay. The (table 1). The Wilcox Group is about 200 the Leona Formation and about 40 ft for Escondido Formation, about 550 to 770 ft to 400 ft thick and consists of thin-bedded the alluvium (table 1). thick, conformably overlies the Anacacho sandstone, siltstone, and shale. The Uvalde Limestone and consists of flaggy to Gravel, which overlies the Wilcox Group, Hydrogeologic Properties argillaceous, fine-grained sandstone, silt- is 0 to about 30 ft thick and consists prima­ The Edwards aquifer and the Leona stone, and shale, with lenses of sandy marl rily of caliche-cemented, cherty gravel, Formation are the primary aquifers in the and limestone. which caps hilltops and stream divides. Seco Creek watershed, and the Trinity Table 1 . Summary of the lithologic and hydrologic properties of the geologic units of the Seco Creek watershed, Texas [Colors keyed to those of figure I; groups and formations modified from Welder and Reeves (1962), Lozo and Smith (1964), Rose (1972), Humphreys (1984), Miller (1984); lithology modified from Dunham (1962); porosity type modified from Choquette and Pray (1970). AQ, aquifer; gal/min, gallons per minute; CU, confining unit] Hydro- Hydro- Thick­ Sys­ Group or Field Relative porosity/ tem geologic formation logic ness Lithology permeability unit function (feet) identification and well yields Alluvium AQ 0^0 Gravel, sand, silt, and clay Gravel, sand, silt, and clay; confined High porosity/high permeability; where to stream valleys water-yielding Quaternary saturated Leona Formation AQ 0-70 Cobble, gravel, sand, silt, and clay Broad, flood-plain deposits of High porosity/high permeability; beneath terraces along larger cobbles, gravel, sand, silt, and clay water-yielding (2 to 86 gal/min) streams Uvalde Gravel AQ 0-30 Coarse, cherty gravel and caliche Alluvial deposits of coarse gravel Low to high porosity and permeability; where on hilltops and divides composed of chert and limestone lack of saturation probably limits saturated cemented by caliche water-yielding capacity in watershed £> a Wilcox Group CU 200-400 Thin-bedded, iron-rich sandstone, Fine-grained sandstone, shale, and Low porosity/low permeability siltstone, and shale; contains siltstone; possibly some lignite & lignite and calcareous nodules Midway Group . CU 20-150 Shale, siltstone, fine-grained Glauconitic sandstone and shale; Low porosity/low permeability; not sandstone and limestone locally fossiliferous known to yield water in the watershed Escondido Formation CU 550-770 Flaggy to argillaceous, fine­ Brown, fine-grained sandstone; Low porosity/low permeability; water- grained sandstone, siltstone, and locally fossiliferous; may contain yielding (3 to 12 gal/min) in places Upper shale; sandy marl and limestone, pyroclastics and asphaltic sand confin­ with conglomerates of fossil ing unit shells Anacacho Limestone CU 290-530 Fossiliferous limestone, chalk, White to gray argillaceous lime­ Low porosity/low permeability; not marl, and clay stone; light-gray chalk; light-
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