106°

102°36°

32°

28°

98°94°

Texas

Austin

San Antonio

Rio Grande

Pecos River

Colorado River

Gul

fof M

exico

Travis

KinneyUvalde

Bandera

KendallRealEdwards

Kerr

Atascosa

BexarGuadalupe

Comal

HaysBlanco

Gillespe

Medina

Colorado River

A

B

Edwards and Trinity aquifers catchment areaOutcrop and recharge zone of the Edwards aquiferEdwards aquifer artesian zone

EXPLANATION

0

0 100 200 KILOMETERS

100 200 MILES

Catchment area? ?

EDWARDS PLATEAU HILL COUNTRY GULF COASTAL PLAIN

A B

Trinity Group

Rechargezone

Artesianzone

Balcones fault zone

UpperConfining

units

Midway Group

Surficialdeposits

Wilcox and Claiborne Groups

Edwards GroupTrinity Group

Fresh-/saline-water interface

Undifferentiated Paleozoic rocks

15,000 20,000

 and fault displacement 

U.S. Department of the InteriorU.S. Geological Survey

Fact Sheet 2006–3145June 2007

This five-year USGS project, funded by the National Coopera-tive Geologic Mapping Program (NCGMP), is using multi-

disciplinary approaches to reveal the surface and subsurface geologic architecture of two important Texas aquifers: (1) the Edwards aquifer that extends from south of Austin to west of San Antonio and (2) the southern part of the Trinity aquifer in the Texas Hill Country west and south of Austin (fig. 1).

The Edwards aquifer is one of the most productive carbonate aquifers in the United States. It also has been designated a sole source aquifer by the U.S. Environmental Protection Agency and is the primary source of water for San Antonio, the nation’s seventh largest city. The Trinity aquifer forms the catchment area for the Edwards aquifer and it intercepts some surface flow above the Edwards recharge zone. The Trinity may also contribute to the Edwards’ water budget by subsurface flow across formation boundaries at considerable depths. Dissolution, karst development, and faulting and fracturing in both aquifers directly control aquifer geometry by compartmentalizing the aquifer and creating unique ground-water flow paths.

The Edwards aquifer and the southern extent of the Trinity aquifer are characterized by three areas (or zones): (1) catchment area (exposed Trin-ity aquifer rocks), (2) recharge zone, and (3) artesian or confined zone. The stratigraphic and structural framework of the Edwards and Trinity aquifers are conceptualized along cross-section A–B (fig. 2).

Precipitation falls on Lower Cretaceous Trinity Group rocks in the catchment area (also called the contributing area) and travels down gradient as surface water until crossing the Edwards recharge zone. There, the water enters the aquifer through fractures and faults and eventually reaches the artesian zone (fig. 2).

Geohydrologic Framework of the Edwards and Trinity Aquifers, South-Central Texas

Figure 1.  Distribution of the Edwards aquifer and catchment  area (Trinity aquifer).

Figure 2.  Structural schematic cross section of the Edwards aquifer and catchment area (Trinity aquifer).

The primary geologic structures are high-angle normal faults that are arranged in a down-to-the-southeast en echelon pattern (in step-like arrange-ment). Depending on the fault architecture and the rock types involved, the faults can restrict or redirect flow or may enhance flow by creating linear zones of increased permeability.

The project’s principal areas of research include: (1) Geologic Map-ping, (2) Geophysical Surveys, (3) Geochronology, (4) Three-dimen-sional Modeling, and (5) Noble Gas Geochemistry. Individual study areas (fig. 3) and published products can also be viewed at http://esp.cr.usgs.gov/info/edwards/index.html

 George B. Ozuna 3

 Water Science Center DeZavalla Road, Suite 290 Antonio, TX 78249 

v

Printed on recycled paper

Well 001

Northwest Southeast

0

530

374

265

187

132

93.4

66

46.6

32.9

23.3

16.5

11.6

8.22

5.81

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2 GEOPHYSICAL SURVEYS

Bracketville

Hondo

KINNEY COUNTY

EDWARDS COUNTY

UVALDE COUNTY

DrippingSprings

SanMarcos

Castroville

LytleNatalia

DevineMEDINA COUNTY

Kyle

Buda

New Braunfels

San Antonio

BEXARCOUNTY

COMALCOUNTY

HAYSCOUNTY

Uvalde

Ground-waterdivide

1Geologic map compilationof the Edwards recharge zoneSIM 2005–2873

1Infiltration potential map ofComal CountyOFR FY 07

2HEM Survey ofNorthern Bexar CountyOFR 2005–1158

1Hydrostratigraphic map ofNorthern Bexar County OFR FY 07

1Geologic map compilation ofUpper Seco CreekOFR 2004–1430

43-D subsurface model ofUpper Seco CreekSIR FY 07

2HEM Survey ofUpper Seco CreekOFR 2003–0226

2Aeromagnetic survey ofMedina and Uvalde CountiesOFR 2002–0049

Occurrence ofShallow Igneous Rocksin Uvalde andMedina CountiesSIR FY 07

3Argon dating ofmafic intrusive rocks inUvalde and Kinney CountiesOFR 2004–1031

43-D subsurfacegeologic model of theEdwards recharge zoneSIR 2004–5226

2AMT Surveys offresh-/saline-water interfaceOFR 2002–0118

5Noble gas analyses of thefresh-/saline-water interfaceSEE REFERENCES

1Geologic map of theEdwards recharge zone inKinney CountySIM FY 07

1Geologic map ofNorth Medina and NE Uvalde CountiesOFR 2006–1372

Rio Grande

MedinaLake

GuadalupeRiver CanyonLake

BraunigLake

CalaverasLake

Ground-waterdivide

29˚30’

30˚

100˚

99˚

98˚

1 GEOLOGIC MAPPING2 GEOPHYSICAL SURVEYS3 GEOCHRONOLOGY4 THREE-DIMENSIONAL MODELING5 NOBLE GAS GEOCHEMISTRY

PROJECT RESEARCH AREAS

Mapped recharge zone

Unmapped recharge zone

Balcones fault zone

Normal faults

County boundaries

Streams and rivers

Fresh-/saline-water interface

Mafic intrusive rocks

EXPLANATION

0 105 20 MILES

OFR USGS Open-File Report

SIR USGS Scientific Investigations Report

SIM USGS Scientific Investigations Map

1

2

3

4

5

1

2

3

4

5

UVALDE COUNTY MEDINA COUNTY

BEXAR COUNTY

COMAL COUNTY

HAYSCOUNTY

TRAVIS COUNTY

Balcon

es fa

ult

zon

e

Uvaldesalient

Edwards recharge zone

SAN MARCOS PLATFORM

MAVERICKBASIN

DEVILS RIVER TREND

100˚ 99˚

98˚

29˚30’

1 GEOLOGIC MAPPING Geologic Mapping

The complex geology of the recharge zone, as defined by the Texas Commission on Environmental Quality (TCEQ), includes lithologic units assignable to the Lower Cretaceous Edwards Group, which is underlain by the Glen Rose Limestone (lower confining unit) and overlain by the Upper Cretaceous Del Rio Clay, Buda Limestone, and Eagle Ford and Austin Groups (upper confining units).

The geology of the Edwards aquifer in the northeastern part of the recharge area is characterized by the Kainer, Person, and Georgetown Formations, which are subdivided into eight informal hydrostrati-graphic units. In the area west of San Antonio in Medina, Uvalde, and Kinney Counties, significant facies changes (fig. 4) exist across the Devils River trend reefal facies (Devils River and Georgetown Forma-tions) and into the deeper water Maverick Basin facies (West Nueces, McKnight, and Salmon Peak Formations).

Past and present 1:24,000-scale geologic mapping in the Edwards aquifer area by the USGS Texas Water Science Center was con-ducted county-by-county but was never compiled at a regional scale. Compilation efforts for this project began in late 2002 and resulted in the publication of U.S. Geological Survey Scientific Investigations

Figure 3.  Project research tasks and published products through FY 07.

Figure 4.  Distribution of depositional facies across the Edwards aquifer.

Map 2873 (http://pubs.usgs.gov/sim/2005/2873/). This compilation is the first effort of its kind to digitally synthesize the geology of the Edwards recharge zone.

Geologic mapping of four-quadrangles along the upper Seco Creek area, Medina and Uvalde Counties (http://pubs.usgs.gov/of/2004/1430/), represents a digital compilation of work by the Texas Bureau of Economic Geology and contains new geologic inter-pretations for the southern two quadrangles. This map, as well as geologic mapping to the east in Medina County (http://pubs.usgs.gov/of/2006/1372), has been used to provide the digital geologic frame-work used in a helicopter electromagnetic (HEM) geophysical survey of the area as well as for ongoing fracture and three-dimensional (3-D) EarthVision™ (EV) modeling.

One of the primary goals of the project is to produce infiltration-potential assessment maps for critical parts of the Edwards recharge area. An infiltration-potential assessment model for Comal County, which includes a 10-meter Digital Elevation Model (DEM) grid and numerical values for hydrostratigraphic units, faults and fractures, karst features, soils, vegetation, fracture density, and basin accumulation indices, is ongoing.

Geophysical Surveys

The presence of volcanic intrusive rocks in south-central Texas has been known since the earliest surveys. A large exposure of volcanic rocks, sometimes called the Uvalde igneous field, is centered in Uvalde County and extends west into Kinney County, south into Zavala County, and north and east as far as the city of Austin. The rocks comprising the Edwards aquifer and the lower and upper confining units are essentially nonmagnetic, with magnetic susceptibilities on the order of 0-5 × 10-5 (SI units). In contrast, the igneous rocks have susceptibilities of 200–3,000 × 10-5 SI. New occurrences of intrusive bodies at both surface and subsurface levels were unveiled by the 2001 aeromagnetic survey (http://pubs.usgs.gov/of/2002/ofr-02-0049/) which reveals how the intrusives may control the unique morphology of the Edwards’ major flow paths (fig. 5).

Another geophysical method used in studying the subsurface geohydrology of the Edwards and Trinity aquifers is audio-magnetotelluric sounding or AMT (http://pubs.usgs.gov/of/2002/of02-118/). AMT is an electrical technique that uses either natural signals or a controlled transmitter to measure earth conductiv-ity as a function of depth.

The AMT sounding locations were specifically selected to resolve a number of key geological issues, including a volcanic plug along the Frio River near the town of Knippa in Uvalde County. Additional AMT studies conducted in and around selected transect wells near Kyle, Texas, mapped the fresh-/saline-water interface (fig. 3).

A helicopter electromagnetic and magnetic (HEM) survey (http://pubs.usgs.gov/of/2003/ofr-03-226/) was completed in 2002 for a 209-square-kilometer (81-square-mile) area in the Seco Creek drainage area, Medina and Uvalde Counties. The surface geology can be viewed at http://pubs.usgs.gov/of/2004/1430/. The survey area was centered on Woodard Cave (Valdina Farms sinkhole), a signifi-cant karst feature in northwestern Medina County. The primary objective of the survey was to image the subsurface electrical resistivity of select geologic features that control the area’s ground-water resources.

Well 001

Northwest Southeast

0

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374

265

187

132

93.4

66

46.6

32.9

23.3

16.5

11.6

8.22

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4.1

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Well 002 Well 003 Well 005 Well 004

ResistivityOhm-Meters

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2 GEOPHYSICAL SURVEYS

29˚30’

99˚07’30”

100 kHz Apparent ResistivityOhm-Meters

1259

839

654

532

435

361

288

209

123

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87

77

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Seco

Creek

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2 GEOPHYSICAL SURVEYS

30˚38’10”

29˚30’20”

29˚22’30”

29˚14’40”

100˚00’20” 99˚44’40” 99˚29’00” 99˚13’20” 98˚57’40”

Deep source

Sabinalanomaly

Geologicstructures

Pipelines

Broad anomalytypical of deep sources

0

0 5 10 KILOMETERS

5 10 MILES

2 GEOPHYSICAL SURVEYS

 of depositional  the Edwards aquifer.

Figure 6.  Upper Seco Creek HEM (helicopter electromagnetic and magnetic) survey. The highest fre-quency used is 100 kHz, and it defines the geoelectric signatures of near-surface strata.  See text for descriptions of the rock units in this area.  Seefigure3forlocationofthismaparea(OFR2003-0226).  The white circle indicates the location of Woodard Cave.

Figure 5.  The total magnetic field (reduced to pole) of the geomagnetic survey area, Uvalde igneous field, Uvalde and Medina Counties. Magnetic intensities increase from blue (lowest) through shades of green, yellow, red, and purple (highest). Seefigure3forlocationofthismaparea(OFR2002-0049).

-5 (SI units). In contrast, the -5 SI. New occurrences

/) which

-

http://pubs.usgs.

/. The -

The HEM data were pro-cessed to produce apparent resistivities for each of the six electromagnetic and magnetic coil pairs and frequencies. The higher frequencies have the least depth of penetration. A map of the 100 kHz apparent resistivity shows that the catchment area and recharge and confined zones all have numerous linear features that likely represent faults and fractures. The maximum depth of penetration for this band is 3-5 meters. From this frequency, the warmer colors (reds, orange and purples, fig. 6) denote more resistive rocks (limestone and dolostone) and the cooler colors (blues) denote conductive rocks (shale and mudstone). The reds and purples are indicative of the Devils River Formation and Buda Limestone. The dark blues denote the Del Rio Clay and the Eagle Ford Shale. The inter-mediate colors (greens and light blue) represent the Glen Rose Limestone and the Austin Chalk.

A similar HEM geophysi-cal survey of northern Bexar County (Camps Bullis and Stanley) was flown in Decem-ber of 2003 (http://pubs.usgs.gov/of/2005/1158). This survey reflects the complexity of the Cretaceous rocks comprising the Trinity aquifer and, in particular, the Glen Rose Limestone. The Edwards recharge zone at the southern end of the survey is characterized by resistive lime-stones versus the more conductive siltstones and mudstones of the Trinity aquifer.

106°

102°36°

32°

28°

98°94°

Texas

Austin

San Antonio

Rio Grande

Pecos River

Colorado River

Gul

fof M

exico

Travis

KinneyUvalde

Bandera

KendallRealEdwards

Kerr

Atascosa

BexarGuadalupe

Comal

HaysBlanco

Gillespe

Medina

Colorado River

A

B

Edwards and Trinity aquifers catchment areaOutcrop and recharge zone of the Edwards aquiferEdwards aquifer artesian zone

EXPLANATION

0

0 100 200 KILOMETERS

100 200 MILES

Catchment area? ?

EDWARDS PLATEAU HILL COUNTRY GULF COASTAL PLAIN

A B

Trinity Group

Rechargezone

Artesianzone

Balcones fault zone

UpperConfining

units

Midway Group

Surficialdeposits

Wilcox and Claiborne Groups

Edwards GroupTrinity Group

Fresh-/saline-water interface

Undifferentiated Paleozoic rocks

1

2

3

4

5

1

2

3

4

5600

-400

5,000

Kyle well no. 1

Edwards Aquifer

Edwards Aquifer

24°C

25°C

27°C

26°C

Kyle well no. 2 Kyle well no. 3 Kyle well no. 4

0

-200

Conductancemillisiemens/meter

0

05001,0001,5002,0002,5003,0003,500

400

10,000HORIZONTAL DISTANCE (FEET)

ELEV

ATIO

N IN

FEE

T (0

IS A

PPRO

XIM

ATE

MEA

N S

EA L

EVEL

)

15,000 20,000

200

Del Rio Clay

Glen Rose Limestone

Del Rio Clay

Glen Rose Limestone

5 NOBLE GAS GEOCHEMISTRY

1

2

3

4

5

3 GEOCHRONOLOGY

3255000UTM COORDINATES

32600003265000

472000

464000UTM COORDINATES

4 THREE-DIMENSIONAL MODELING

Figure 9.  Diagram showing conductance (millisiemens/meter) and fault displacement near Kyle, Texas.

U.S. Department of the InteriorU.S. Geological Survey

Fact Sheet 2006–3145June 2007

This five-year USGS project, funded by the National Coopera-tive Geologic Mapping Program (NCGMP), is using multi-

disciplinary approaches to reveal the surface and subsurface geologic architecture of two important Texas aquifers: (1) the Edwards aquifer that extends from south of Austin to west of San Antonio and (2) the southern part of the Trinity aquifer in the Texas Hill Country west and south of Austin (fig. 1).

The Edwards aquifer is one of the most productive carbonate aquifers in the United States. It also has been designated a sole source aquifer by the U.S. Environmental Protection Agency and is the primary source of water for San Antonio, the nation’s seventh largest city. The Trinity aquifer forms the catchment area for the Edwards aquifer and it intercepts some surface flow above the Edwards recharge zone. The Trinity may also contribute to the Edwards’ water budget by subsurface flow across formation boundaries at considerable depths. Dissolution, karst development, and faulting and fracturing in both aquifers directly control aquifer geometry by compartmentalizing the aquifer and creating unique ground-water flow paths.

The Edwards aquifer and the southern extent of the Trinity aquifer are characterized by three areas (or zones): (1) catchment area (exposed Trin-ity aquifer rocks), (2) recharge zone, and (3) artesian or confined zone. The stratigraphic and structural framework of the Edwards and Trinity aquifers are conceptualized along cross-section A–B (fig. 2).

Precipitation falls on Lower Cretaceous Trinity Group rocks in the catchment area (also called the contributing area) and travels down gradient as surface water until crossing the Edwards recharge zone. There, the water enters the aquifer through fractures and faults and eventually reaches the artesian zone (fig. 2).

Geohydrologic Framework of the Edwards and Trinity Aquifers, South-Central Texas

Figure 1.  Distribution of the Edwards aquifer and catchment  area (Trinity aquifer).

Figure 2.  Structural schematic cross section of the Edwards aquifer and catchment area (Trinity aquifer).

Three-Dimensional Modeling

Three-dimensional (3-D) geologic modeling of aquifers can help to quantitatively evaluate the connectedness of hydrostratigraphic units across fault and fracture zones and to estimate the distribution of geologic units and structures in the subsurface. Geologic 3-D framework modeling is also useful for visualizing features within fault zones and the interactions of en echelon fault strands and flexed, relay ramps. All of these parameters are complex variables that reflect original depositional conditions and subsequent alteration and dislocation. The following 3-D models have helped project staff to evaluate some of the geologic processes controlling the Edwards and Trinity aquifers.

An interactive 3-D EarthVision™ (EV) model of the northern Bexar County area (http://pubs.usgs.gov/sir/2004/5226/) reveals the subsurface geology of the Edwards and Trinity aquifers where water wells are 200-1,000 feet or more in depth. This model is based on mapped geologic relationships that reflect the: (1) Balcones fault-zone structures, (2) detailed interpretations of 40 principal wells, and (3) geometry of the Edwards Group hydrostratigraphic units.

A similar 3-D EV model of the north Seco Creek area (Medina and Uvalde Counties) is ongoing. This model (fig. 8) is being built using a variety of digital datasets, including: (1) the current geologic map (http://pubs.usgs.gov/of/2004/1430/), (2) detailed lithologic descriptions and interpretations from 40 drill holes, and (3) helicopter electromagnetic geophysical data.

Noble Gas Geochemistry

Noble gases have proven to be excellent tracers for ground-water studies, and they can help to recog-nize ground-water origins and water/rock processes. The very rapid ground-water movement through the Edwards aquifer is controlled by two major flow paths: the western Medina flow path and the eastern flow path. Within both flow paths, the fresh-water zone of the aquifer is bounded to the south and southeast by the transitional downdip saline zone. This interface (fig 3), locally referred to as the “bad-water line,” is arbitrarily defined where aquifer waters exceed 1,000 mg/L of total dissolved solids (TDS).

Fluid logging and discrete isotope and noble gas sampling were conducted in four transect wells across the fresh-/saline-water interface in Uvalde, Medina, Bexar, Guadalupe, and Hays Counties (Lambert and others, 2003). The fluid logs obtained from the moni-toring transect wells show a distinct interface in the eastern part of the study area, such as near Kyle (fig. 9). In the western transect wells, the interface appears to be more gradational. The dissolved gas samples also indicate that the saline-zone water is much older than previously thought.

The helium isotopic (3He and 4He) data from the Kyle transect wells tell an interesting story. Measured isotopic compositions of the excess

helium in the saline waters of the Edwards aquifer have a composition of 0.22 ± 0.02 R/RA (R/RA is the sample

3He to 4He ratio normalized to the present-day atmos-pheric ratio of helium). This homogeneous isotopic composition suggests a uniform source reservoir of excess helium associated with the aquifer. Measured helium compositions from Kyle 3 and 4 wells are similar to those of the other saline samples of the Edwards aquifer (0.23 ± 0.01 R/RA), but excess helium values from Kyle 1 and 2 wells (freshwater and fresh-/saline-water transition zone) are on the order of 0.14 R/RA (Hunt and others, 2003). This differ-ence in isotopic composition suggests that the waters have a different origin and may be attributed to sub-surface discharge of groundwater from the underly-ing Trinity aquifer. Major ion and stable isotope data also corroborate the existence of a sub- surface communi- cation between the aquifers as well as dramatic salinity changes in the Kyle 2 well follow- ing a major recharge event in the spring of 2003.

Figure 8.  Three-dimensional EarthVision™ model of the North Seco Creek area.  Note the multiple faults (shown in red) and the electromagnetic geophysical profiles (shown in blue) used to construct the model.  

AcknowledgmentsMany thanks to W.R. Page, D.V. Smith, and

T.W. Judkins who improved this Fact Sheet with thorough reviews.

ReferencesLambert, R.B., Hunt, A.G., Landis, G.P., and Waugh, J.R., 2003, Fluid logging

and discrete sampling of the freshwater/saline-water interface of the Edwards aquifer, south-central Texas [abs.]: Implications for groundwater flow and origin of salinity (Part 1—Fluid logging): Geological Society of America, Abstracts with Programs, v. 35, no. 6, p. 280.

Hunt, A.G., Lambert, R.B., Waugh, J.R., and Landis, G.P., 2003, Fluid logging and discrete sampling of the freshwater/saline-water interface of the Edwards aquifer, south-central Texas [abs.]: Implications for groundwater flow and origin of salinity (Part 2—Geochemistry): Geological Society of America, Abstracts with Programs, v. 35, no. 6, p. 280.

The primary geologic structures are high-angle normal faults that are arranged in a down-to-the-southeast en echelon pattern (in step-like arrange-ment). Depending on the fault architecture and the rock types involved, the faults can restrict or redirect flow or may enhance flow by creating linear zones of increased permeability.

The project’s principal areas of research include: (1) Geologic Map-ping, (2) Geophysical Surveys, (3) Geochronology, (4) Three-dimen-sional Modeling, and (5) Noble Gas Geochemistry. Individual study areas (fig. 3) and published products can also be viewed at http://esp.cr.usgs.gov/info/edwards/index.html

By Charles D. Blome,1 Jason R. Faith,2 and George B. Ozuna 3

1U.S. Geological Survey, Denver, CO2U.S. Geological Survey, Stillwater, OK3U.S. Geological Survey, San Antonio, TX

Contacts:

Charles D. Blome     George B. Ozuna  U.S. Geological Survey  USGS Water Science Center  MS 980, Box 25046, DFC  5563 DeZavalla Road, Suite 290  Denver, CO 80225     San Antonio, TX 78249  cblome@usgs.gov

This fact sheet is available online at http://pubs.usgs.gov

Edited by F. Craig BrunsteinPublication design and digital layout by Carol A. Quesenberry

Geochronology

The Uvalde igneous field consists of fine- to coarse-grained ultramafic and hypabyssal rocks that exist as dikes, plugs, and shal-low intrusions. The five rock types identified in this field include alkali basalt, melilite-olivine nephelinite, olivine nephelinite, nepheline basanite, and phonolite.

A 2001 aeromagnetic survey (http://pubs.usgs.gov/of/2002/ofr-02-0049/) detected over 200 shallow, igneous intrusive bodies; fewer than 20 had previously been mapped. The apparent random distribution of these igneous bodies raised questions as to whether they represent a single intrusive episode or multiple episodes.

A preliminary 40Ar/39Ar geochronologic study (http://pubs.usgs.gov/of/2004/1031/) of various mineral separates and groundmass concentrates from a variety of rock types exposed in Uvalde County revealed two distinct age groups, one at approximately 82 to 80 Ma (million years ago) and the other at 74 to 72 Ma. Previous K/Ar ages had suggested that igneous activity spanned a range in age from 90 to 60 Ma.

Figure 7.  Columnar joints in 80 to 82 million-year-old melilite-olivine nephelin-ite exposed at the Knippa Traprock Quarry east of Uvalde. USGSphotograph.

Printed on recycled paper