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Hydrogeologic Aspects of the Knippa Gap Area in Eastern Uvalde and Western Medina Counties, Texas

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Hydrogeologic Aspects of the Knippa Gap Area in Eastern Uvalde and Western Medina Counties, Texas Prepared in cooperation with the U.S. Army Corps of Engineers Hydrogeologic Aspects of the Knippa Gap Area in Eastern Uvalde and Western Medina Counties, Texas The Knippa Gap are complex, and this complexity is associated with the Balcones Fault Zone, a prominent structural feature in The Edwards aquifer is the primary source of south-central Texas, and other related geologic structures. water for a large area of south-central Texas (fig. 1). The existence of a geologic structure known as the The geologic structure, stratigraphy, and hydrogeologic “Knippa Gap” first was postulated by Maclay and Land characteristics of rocks that make up the Edwards aquifer (1988) as a subsurface structural low or “gap” located and its confining units control the amount and direction in an area beneath Knippa and Sabinal, Tex. (fig. 1), that of groundwater flowing through the aquifer. Groundwater restricts regional groundwater movement from the west flow pathways in the western part of the Edwards aquifer to the east. The existence of the gap was postulated on the R iv er 99°45' 99°00' 98°15' S a b i D Leakey n W a r e y l s R C t F i EDWARDS REAL i BANDERA b N r v o i e l COMAL u o o COUNTY COUNTY r COUNTY e c C COUNTY e R r s i e v MEDINA COUNTY e R e Utopia k GONZALES iv r COUNTY er 29°30' Laguna San Antonio KINNEY COUNTY UVALDE COUNTY UVALDE Sabinal D'Hanis BEXAR H COUNTY o Brackettville S n WILSON Knippa e d Uvalde c o COUNTY o S an A C n C t Knippa Gap area r on Leona Springs 1 e r i L e e ATASCOSA COUNTY o eo k e Leona Springs 2 n k a R i N R v u iv e e e r c r e s F MAVERICK COUNTY R ZAVALA COUNTY r LA SALLE COUNTY i io v R 29°00' e r iv er Base modified from U.S. Geological Survey 1:250,000-scale digital data Universal Transverse Mercator projection, zone 14 KARNES COUNTY S North American Datum of 1983 a 0 10 20 30 40 MILES n M ig u e l C 0 10 20 30 40 KILOMETERS re ek FRIO COUNTY Trinity EXPLANATION McMULLEN COUNTY aquifer LIVE OAK COUNTY Edwards aquifer (George and others, 2011) Outcrop T E X A S San Antonio segment of the Subcrop BALCONES FAULT Edwards aquifer er ZONE Study boundary FrioUVALDE Riv COUNTY Approximate location of the Uvalde Salient BEXARer RiCOUNTYv DIMMIT COUNTY Spring and name es ec Leona Springs 1 u Study area MEDINAN COUNTY Gulf of Mexico Figure 1. Location of the Knippa Gap area in south-central Texas. U.S. Department of the Interior Fact Sheet 2014–3045 U.S. Geological Survey Printed on recycled paper June 2014 Outcrop of eroded and dissected extinct volcano located adjacent to the Frio River and east of the city of Uvalde in Uvalde County, Texas. basis of a series of numerical simulations of groundwater flow made by Maclay and Land (1988) in an attempt to synthesize the geologic, structural, and hydrogeologic characteristics of the aquifer. Although the presence of the Knippa Gap near the boundary of the Devils River Trend and Maverick Basin Cave in fractured Austin Group rock located depositional provinces (fig. 2) in eastern Uvalde and approximately 9.3 miles northeast of city of Uvalde in western Medina Counties, Tex., had been supported by Uvalde County, Texas. EXPLANATION Depositional province Devils River Trend Maverick Basin San Marcos Platform Central Boundary of Balcones Fault Zone Texas Stuart City Reef Tectonic Hinge Line Platform Balcones escarpment Llano Uplift Ouachita Structural Belt San MarMarcos Devils River Trend Arch cos N San Marcos Maverick Platform Basin Ancestral Gulf of Mexico Present-day 0 50 100 150 200 MILES Gulf of Mexico 0 50 100 150 200 KILOMETERS Figure 2. Regional depositional provinces, boundary of the Balcones Fault Zone, and the Balcones escarpment in south-central Texas. numerical simulations, efforts to map it in detail had not and drainage patterns. In eastern Uvalde and western been previously attempted. In cooperation with the U.S. Medina Counties, sedimentary rocks from the Lower Army Corps of Engineers, the U.S. Geological Survey Cretaceous to the Pleistocene along with igneous rocks developed the first detailed surficial geologic map of the from the Upper Cretaceous are exposed at land surface Knippa Gap area with data and information obtained from and in roadside outcrops (Clark and others, 2013). These previous investigations and field observations (Clark and rocks include limestone, chalk, gravel, sand, silt, shale, others, 2013, plate 1). A simplified version of the detailed clay, and basalt. Igneous rocks also were mapped to show geologic map depicting the hydrologic units, faulting, the spatial distribution of the igneous bodies and their and structural dips of the Knippa Gap area is provided in relation to the geologic structure of the area. Borehole figure 3. geophysical data were used to determine formational Surficial geologic units and surficial expression contacts in the subsurface and regional dips in each of of fractures, faults, and lineaments were identified and the geologic units. This information was used to interpret mapped with the aid of well log data, aerial imagery, rock structural dip directions and the approximate location of and soil exposures, and observed changes in vegetation the structural trough in the Knippa Gap (fig. 4). 99°24' EXPLANATION Hydrologic unit 99°31'45" 29°28'45" Upper confining unit to 99°16'15" Edwards aquifer Edwards aquifer Study boundary Fault—Dashed where 99°39'30" B inferred A A' Line of section Inferred structural dip A' 29°21' 99°47'15" Structural trough of the Knippa Gap area Approximate width of B' 29°13'15" the Knippa Gap structural trough ' A COUNTY UVALDE MEDINA MEDINA COUNTY Base modified from U.S. Geological Survey 1:24,000-scale digital data 0 2.5 5 7.5 10 MILES Universal Transverse Mercator projection, zone 14 North American Datum of 1983 0 2.5 5 7.5 10 KILOMETERS Figure 3. Hydrologic units, faulting, and structural dips of the Knippa Gap area in south-central Texas. Full version of geologic plate is available in Clark and others (2013). SOUTHWEST NORTHEAST A A' FEET BEND IN SECTION BEND IN BEND IN BEND IN BEND IN BEND IN BEND IN SECTION SECTION SECTION 1,200 SECTION SECTION SECTION ' 1,100 B – 1,000 B SECTION BEND IN 900 SECTION 800 Approximate width of 700 the Knippa Gap 600 structural trough 500 EXPLANATION 400 Hydrologic unit Upper confining 300 unit to Edwards 200 aquifer Edwards aquifer 100 Trinity aquifer NAVD 88 Land surface -100 Fault—Dashed -200 where inferred. Arrow shows -300 relative movement -400 -500 -600 -700 0 3,000 6,000 FEET -800 VERTICAL SCALE GREATLY EXAGGERATED -900 NAVD 88, North American Vertical Datum of 1988 Figure 4. Hydrologic section A-A’ showing approximate width of the structural trough in the Knippa Gap area. Location of trace A-A’ shown on figure 3. Sources of Groundwater in the Knippa Gap that generally are associated with solutionally enlarged Area bedding planes and fractures. Although most of the permeability probably results from fractures, the aquifer The Edwards aquifer, a karst aquifer with large also contains bedding plane, fracture, breccia, vug, porosity and permeability that includes geologic units channel, and cave porosity. The vertical fractures near the from the Devils River Trend and the Maverick Basin top of the Edwards aquifer in the Devils River Formation depositional provinces, is the primary source of water provide the most effective paths for recharge, and where in the Knippa Gap area (fig. 3). The Edwards aquifer subaerially exposed, these fractures greatly increase the in the Devils River Trend is one of the most porous rate of recharge when precipitation occurs in the area and permeable units in the area, with permeable zones (fig. 3). Channel porosity developed along a bedding plane in the Austin Group located near the intermittent Leona Springs 2 on the Leona River in Uvalde County, Texas. NORTHWEST SOUTHEAST B B' BEND IN SECTION BEND IN SECTION BEND IN SECTION FEET 1,200 1,150 ' A BEND IN SECTION 1,100 – BEND IN SECTION 1,050 A 1,000 SECTION 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 Sandstone ledges in the Escondido Formation located 150 100 approximately 7 miles south-southeast of the community of 50 NAVD 88 D’Hanis in Medina County, Texas. -50 -100 -150 -200 -250 -300 Additional minor sources of water in the Knippa Gap -350 -400 area include the Leona aquifer, a minor aquifer associated -450 -500 with alluvium of the Pleistocene Leona Formation that -550 -600 crops out along stream channels; the Uvalde Gravel; the -650 -700 Indio Formation; the igneous basalts and serpentines; -750 -800 and other Edwards aquifer confining units (Del Rio -850 -900 Clay, Buda Limestone, Eagle Ford Group, Austin Group, -950 -1,000 Anacacho Limestone, and Escondido Formation) (Clark -1,050 -1,100 and others, 2013). NAVD 88, North American Vertical Datum of 1988 0 3,000 6,000 FEET EXPLANATION VERTICAL SCALE GREATLY EXAGGERATED Complex Structure and Groundwater Flow Hydrologic unit Upper confining Paths unit to Edwards aquifer The Balcones Fault Zone is a structurally Edwards aquifer complicated, Miocene-aged fault complex that trends Trinity aquifer Land surface approximately west to east across Uvalde and Medina Fault—Dashed where inferred. Counties and is composed of high-angle, en echelon, Arrow shows relative vertical or nearly vertical normal faults that are movement downthrown to the southeast (fig.
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