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- known to yield significant quantities yellow to blue marl; and sandy of water (0 to 6 gal/min) yellow clay; asphaltic limestone UpperCretaceous Austin Group CU 230-290 Massive chalky to marly, White, chalky limestone, with Low porosity /low to moderate fossiliferous, flaggy limestone locally abundant Gryphaea aucella permeability; water-yielding (0 to 2 gal/min)

Eagle Ford Group CU 55-70 Black shale, sandy siltstone, and Weathers to yellow clay and brown Low porosity/low permeability flaggy limestone flagstone; petroliferous

Ifiuda Limestone | CU 60-90 Buff, light-gray, dense, massive, Light-yellow to buff, porcelaneous Low porosity/low permeability; not micritic limestone limestone; nodular, angular, or con- known to yield water in the study area coidal fractures b^ -^^^^^^H Del Rio Clay CU 45-90 Blue-green to yellow-brown clay Fossiliferous clay; Ilymatogyra Low porosity/low permeability; least arietina permeable part of the upper confining unit of the Edwards aquifer

Georgetown Formation CU 0-82 Whitish, dense limestone that Dense to argillaceous detrital lime­ Low porosity/low permeability; con­ grades upward into a buff to stone; fossiliferous; Waconella sidered part of Edwards aquifer but yellow argillaceous limestone wacoensis acts predominantly as a confining unit

Spgnvia ^^H AQ Light-gray, Limestone and High porosity/high Formation2 ^H where miliolid, dolomite, permeability; seeps satu­ shell- miliolid, and springs at base rated fragmented collapsed along marl contact; 0-80 limestone; breccia, lack of saturation Devils Hard, miliolid, dolomite; Massive, and marly might limit water- taceous Edwards River 380- shell- chert fossiliferous limestone; High porosity/ yielding capacity in aquifer Formation, 620 fragmented limestone; chert high perme­ watershed undivided Q grainstone Caprinids', ability; major CreLower Fort Terrett AQ to mudstone; chert nodules aquifer (7 to High porosity/ Formation where chert; and beds; 950 gal/min) high permeability; satu­ argillaceous Porcelaneous collapsed springs and seeps rated limestone limestone, breccias; Limestone at base; lack of 0-250 collapsed miliolids and dolomite; saturation might breccia, and chert limit water- chert yielding capacity in watershed

Upper Trinity Upper member AQ Greater Gray to yellow, dense to thinly Stair-step topography2; alternating Low porosity/relatively impermeable Trinity Group of the Glen than 500 bedded limestone and marl limestone and marl in upper part and permeable in lower aquifer Rose Lime­ part; springs and seeps ; water-yielding stone (2 to 18 gaVmin)

Balcones fault zone (southern two-thirds of watershed). North of Balcones fault zone (northern one-third of watershed).

k U.S. GOVERNMENT PRINTING OFFICE: 1999 773-457 / 33005 Region No. 8 aquifer (Glen Rose Limestone) is second­ In the Balcones fault zone (southern Holt, C.L.R., Jr., 1956, Geology and ground- ary. Many formations in the study area two-thirds of the watershed), the Trinity water resources of Medina County, contain ground water of limited quantity or aquifer underlies the Edwards aquifer and Texas: U.S. Geological Survey Water- Supply Paper 1422, 213 p. quality, which makes their use as aquifers is considered the lower confining unit of Humphreys, C.H., 1984, Stratigraphy of the limited. Ground water is unconfined in the the highly productive Edwards because of Lower Cretaceous (Albian) Salmon Peak northern part of the Seco Creek watershed its low permeability relative to that of the Formation of the Maverick Basin, south (fig. 1) where the Glen Rose Limestone Edwards. The characteristics of the upper Texas, in Stratigraphy and structure of and the Devils River Formation are member of the Glen Rose Limestone the Maverick Basin and Devils River exposed at land surface. In the southern described above might not apply in the trend, Lower Cretaceous, southwest Texas: San Antonio Geological Society, part, ground water in the Leona Formation Balcones fault zone. p. 34-59. is both unconfined and confined and in the Alluvium in the study area is not used Lozo, F.E., Jr., and Smith, C.I., 1964, Devils River Formation is confined. locally as an aquifer, although water Revision of Comanche Cretaceous potentially could be withdrawn in limited stratigraphic nomenclature, southern Currently (1998), the Edwards aquifer Edwards Plateau, southwest Texas: Gulf provides public water supply to more than quantities in the high-porosity and high- permeability stream-valley deposits. The Coast Association of Geological Societ­ 1 million people and sustains several ies Transactions, v. 14, p. 285-306. endangered species in south-central Texas. potential yield of this unit would be limited Maclay, R.W., and Small, T.A., 1984, Large-capacity municipal, military, and by its saturated thickness and by climatic Carbonate geology and hydrology of agricultural wells withdraw water from the conditions. the Edwards aquifer in the San Antonio Edwards aquifer. Permeable zones are The Uvalde Gravel above the upper area, Texas: U.S. Geological Survey confining unit, although classified as Open-File Report 83-537, 72 p. associated with the collapsed breccias and Miller, B.C., 1984, Physical stratigraphy and rudist mounds in the limestone (Maclay an aquifer (table 1), does not yield signifi­ facies analysis, Lower Cretaceous, and Small, 1984, p. 12). The Edwards cant quantities of water to wells in the Maverick Basin and Devils River trend, aquifer has undergone extensive diagene- study area, probably because of lack of Uvalde and Real Counties, Texas, in sis, resulting in primary depositional tex­ saturation. Stratigraphy and structure of the Maver­ tures being masked or destroyed by The Upper Cretaceous Del Rio Clay ick Basin and Devils River trend, Lower Cretaceous, southwest Texas: San recrystallization. Recrystallization, princi­ probably is the least permeable part of Antonio Geological Society, p. 3-33. pally by dedolomitization due to extensive the upper confining unit of the Edwards Reeves, R.D., and Lee, F.C., 1962, Ground- leaching in the freshwater zone, has aquifer. Above the Del Rio Clay, other water geology of Bandera County, resulted in increased porosity (Maclay and units also contribute to the confinement Texas: Texas Water Commission Small, 1984). Dissolution along bedding of the Edwards aquifer. The Buda Lime­ Bulletin 6210, 74 p. planes also has been observed in the out­ stone and Eagle Ford Group generally do Rose, PR., 1972, Edwards Group, surface not yield water. The Austin Group yields and subsurface, central Texas: Austin, crop; rainfall rapidly infiltrates into the University of Texas, Bureau of Eco­ very porous and permeable aquifer. small quantities (about 2 gal/min) of bitter- nomic Geology Report of Investigations tasting water, and the overlying Anacacho Wells completed in the Quaternary 74, 198 p. Limestone yields no significant quantities Steffens, T., and Wright, P., 1996, Seco Creek Leona Formation typically yield about (about 0 to 6 gal/min) of water. The Escon- Water-Quality Demonstration Project, 7 gallons per minute (gal/min) of water. dido Formation can yield moderate quanti­ annual project report fiscal year 1996: Holt (1956, p. 51) reported that the Leona ties (about 3 to 12 gal/min) of moderately Project Administrators, McFarland, M. Formation generally does not contain (Texas Agricultural Extension Service), to highly mineralized water (about 357 to Colwick, A. (Natural Resources Conser­ water where the underlying formation is 1,320 milligrams per liter dissolved solids) permeable. However, in areas of imperme­ vation Service), and Orange, S. (Farm but generally has low porosity and perme­ Service Agency), 338 p. able bedrock (southern part of the water­ ability, which precludes its use as an aqui­ University of Texas, Bureau of Economic shed), observed well yields (about 2 to 86 fer. Moderately to highly mineralized Geology, 1974, Geologic atlas of Texas, gal/min) are adequate for domestic and water typically is not suitable for human San Antonio sheet: Austin, scale stock use in the Seco Creek watershed. 1:250,000. consumption but can be suitable for stock Welder, F.A., and Reeves, R.D., 1962, Geol­ North of the Balcones fault zone (north­ use. The Tertiary Midway Group and Wil- ogy and ground-water resources of ern one-third of the watershed), the Trinity cox Group are not known to yield signifi­ Uvalde County, Texas: Texas Water aquifer is largely exposed at land surface. cant quantities of water within the study Commission Bulletin 6212, 263 p. The upper member of the Glen Rose Lime­ area, although the Wilcox Group forms the D.S. Brown, J.G. Mosier1, and G.M. Nalley stone composes the uppermost part of the lower part of the productive Carrizo- 1 Formerly U.S. Geological Survey. Trinity aquifer. This member is relatively Wilcox aquifer outside the study area. impermeable in its upper part and perme­ References Cited able in its lower part. Springs and seeps Any use of trade, product, or firm names is Choquette, P.W., and Pray, L.C., 1970, commonly are associated with the contacts for descriptive purposes only and does not Geologic nomenclature and classifica­ imply endorsement by the U.S. Government. of marl, limestone, and evaporite beds in tion of porosity in sedimentary carbon­ the upper member of the Glen Rose Lime­ ates: American Association of Petroleum For more information, please contact: stone. Springs issuing from the Glen Rose Geologists Bulletin, v. 54, no. 2, Subdistrict Chief Limestone provide perennial base flow in p. 207-250. U.S. Geological Survey the northern part of the Seco Creek water­ Dunham, R.J., 1962, Classification of carbon­ 435 Isom Road, Suite 234 shed. Wells completed in the Glen Rose ate rocks according to depositional tex­ San Antonio, TX 78216 ture, in Classification of carbonate Limestone can yield sufficient quantities rocks Symposium: American Associa­ Phone: (210) 321-5200 of water (about 2 to 18 gal/min) for domes­ tion of Petroleum Geologists Memoir 1, Email: [email protected] tic and stock use. p. 108-121. World Wide Web: http://tx.usgs.gov