U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5131

Three-Dimensional Geologic Model of Complex Fault 

Structures in the Upper Seco Creek Area, Medina and 

Uvalde Counties, South-Central Texas

Three-Dimensional Geologic Model of 

Complex Fault Structures in the Upper Seco 

Creek Area, Medina and Uvalde Counties, 

South-Central Texas

By Michael P. Pantea, James C. Cole, Bruce D. Smith, Jason R. Faith, 

Charles D. Blome, and David V. Smith

Scientific Investigations Report 2008–5131

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Pantea, M.P., Cole, J.C., Smith, B.D., Faith, J.R., Blome, C.D., and Smith, D.V., 2008, Three-dimensional geologic 

model of complex fault structures in the upper Seco Creek area, Medina and Uvalde Counties, South-Central Texas: 

U.S. Geological Survey Scientific Investigations Report 2008–5131, DVD-ROM, 9 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Geologic Setting.............................................................................................................................................1

Modeled and Mapped Stratigraphy and Lithologic Descriptions .........................................................2

Model Construction and Methodology ......................................................................................................4

Structural Uncertainties in Southeastern Part of Model ........................................................................6

Modeled Structure.........................................................................................................................................7

Conclusions.....................................................................................................................................................8

References Cited............................................................................................................................................8

Disc Contents.........................................................................................................................................9

Figures

1. Index map showing the location of the model defined by the quadrangles used 

and part of the Balcones fault zone within the upper Seco Creek area of 

Medina and Uvalde Counties, south-central Texas................................................................2

2. Correlation chart showing relationships of the geologic and hydrogeologic 

units used in the model. The Anacacho Limestone and Austin Group (Chalk) 

are combined in the model..........................................................................................................3

3. Surface of the Glen Rose Limestone showing the inferred north-trending 

fault bounded by the discontinuous Seco Valley Road fault on the north and 

the continuous Parker Creek fault on the south......................................................................7

Abstract

This multimedia report shows and describes digital 

three-dimensional faulted geologic surfaces and volumes of 

the lithologic units of the Edwards aquifer in the upper Seco 

Creek area of Medina and Uvalde Counties in south-central 

Texas. This geologic framework model was produced using 

(1) geologic maps and interpretations of depositional environments and paleogeography; (2) lithologic descriptions, 

interpretations, and geophysical logs from 31 drill holes; (3) 

rock core and detailed lithologic descriptions from one drill 

hole; (4) helicopter electromagnetic geophysical data; and (5) 

known major and minor faults in the study area. These faults 

were used because of their individual and collective effects 

on the continuity of the aquifer-forming units in the Edwards 

Group. Data and information were compared and validated 

with each other and reflect the complex relationships of 

structures in the Seco Creek area of the Balcones fault zone. 

This geologic framework model can be used as a tool 

to visually explore and study geologic structures within the 

Seco Creek area of the Balcones fault zone and to show the 

connectivity of hydrologic units of high and low permeability 

between and across faults. The software can be used to display 

other data and information, such as drill-hole data, on this 

geologic framework model in three-dimensional space. 

Introduction 

The Edwards aquifer is one of the most permeable and 

productive carbonate aquifers in the United States and is 

designated a sole source aquifer by the U.S. Environmental 

Protection Agency. The Edwards aquifer is important to the 

livelihood of south-central Texas residents and serves the 

domestic, industrial, agricultural, and recreational needs of 

nearly 2 million people. It is the major source of water for 

the city of San Antonio, Texas, the seventh-largest city in the 

United States. 

This three-dimensional model shows integrated geologic 

data of the Edwards aquifer hydrostratigraphic units and structural features within a study area composed of four 7.5-minute 

Three-Dimensional Geologic Model of Complex Fault 

Structures in the Upper Seco Creek Area, Medina and 

Uvalde Counties, South-Central Texas

By Michael P. Pantea, James C. Cole, Bruce D. Smith, Jason R. Faith, Charles D. Blome, and David V. Smith

quadrangles: Comanche Waterhole, Flatrock Crossing, Sabinal NE, and Texas Mountain (fig. 1). The model evolved using 

Dynamic Graphics EarthVisionTM software and shows faulted 

hydrogeologic surfaces representing data from drill holes and 

rock core, mapped geologic surfaces, faults and other geologic 

structures; geologic contacts mapped from aerial photography; 

and helicopter electromagnetic survey (HEM) geophysical 

data. The HEM survey of the study area was conducted in the 

spring of 2002 (Smith and others, 2003) to identify and map 

subsurface geologic features. Drill-hole data and mapped surface contacts were used to confirm the HEM-defined surfaces.

Goals of this study were to provide scientific information 

to a nontechnical audience and integrate different geoscience 

data types to accurately represent the local faulted geologic 

units that form the Edwards aquifer. Integrating different 

geoscience data types made the model internally consistent 

because drill-hole data helped to confirm HEM and surface 

geologic data, surface geologic data helped to confirm HEM 

and drill-hole data, and HEM data helped to confirm drillhole and surface geologic data. The resultant model is a more 

accurate interpretation due to the diversity, integration, and 

cross-validation of the data types. 

We developed this visual interactive tool for the nontechnical and technical audience to show and help evaluate the 

shape and form of the lithologies that define the Edwards aquifer within the study area. The model shows some properties 

of the geologic framework that may affect the behavior of the 

aquifer: (1) the complex relationships between fault-bounded 

lithologic units, (2) possible downgradient flow pathways 

along or across fault boundaries, and (3) possible downgradient flow pathways along the tilted and fractured ramplike 

structures created by unequal displacement of faults within 

this part of the Balcones fault zone. 

Geologic Setting

The Edwards aquifer lies within the Balcones fault zone. 

Stratigraphic units that compose the aquifer host numerous 

subparallel, northeast-trending, near-vertical normal faults 

that are downthrown to the southeast. Locally, major faults 
2 Three-Dimensional Geologic Model of Complex Fault Structures in the Upper Seco Creek, South-Central Texas

may be connected by southeast-trending cross-faults shown 

by Clark (2003), Small and Clark (2000), and other workers. 

Locally and regionally, faults within the Balcones fault zone of 

south-central Texas have significant effect on the hydrogeology of the Edwards aquifer by (1) acting as areas of enhanced 

waterflow parallel to the faults where porosity and permeability are increased and (or) (2) acting as barriers preventing or 

restricting downgradient waterflow. 

This three-dimensional (3–D) model is made up of, and 

shows, geologic units that confine the Edwards aquifer (fig. 2). 

The lowermost confining unit shown in the model is the upper 

member of the Glen Rose Limestone. Hydrogeologic units of 

the Edwards aquifer include the Devils River Formation of 

the Devils River trend and the overlying thin, discontinuous 

Georgetown Formation (Clark, 2003), not separately shown in 

the model. The uppermost confining unit shown in the model 

is the Del Rio Clay. 

Exposed stratigraphy in the Seco Creek area extends from 

the Lower Cretaceous Glen Rose Limestone to the Quaternary 

alluvium and terrace gravels (not shown in the model). The 

Edwards Group (Devils River Formation) are reefal-facies lithologies, very porous and permeable, dolomitized, rudist-bearing limestone containing shell fragments that formed along the 

western, northern, and eastern margins of the Maverick Basin 

(fig. 1) as described by Lozo and Smith (1964). 

The Lower Cretaceous units shown in the model are 

the Glen Rose Limestone and Devils River Formation of the 

Edwards Group. The Glen Rose Limestone is shown as a 

single unit in the model; however, it is informally subdivided 

into upper and lower members by Stricklin and Amsbury 

(1974). Upper Cretaceous stratigraphy modeled from youngest 

to oldest are the Anacacho Limestone and Austin Group (undivided), the Eagle Ford Formation, the Buda Formation, and 

the Del Rio Clay. A younger Upper Cretaceous mafic intrusive 

unit in the southwest corner of the model penetrates all modeled units. Details about the geologic setting and stratigraphic 

relationships are beyond the scope of this model and are 

described by others such as Lozo and Smith (1964), Collins 

(1997, 1998, 1999a, 1999b), and Blome and others (2004).

Modeled and Mapped Stratigraphy and 

Lithologic Descriptions

The following brief descriptions of rock units modeled 

were compiled from different sources that include Collins 

(1997); Small and Clark (2000); Clark (2003); and Blome and 

others (2004).

Figure 1. Index map showing the location of the model defined by the quadrangles used and part of the Balcones 

fault zone within the upper Seco Creek area of Medina and Uvalde Counties, south-central Texas.
Modeled and Mapped Stratigraphy and Lithologic Descriptions 3

Figure 2. Correlation chart showing relationships of the geologic and hydrogeologic units used in 

the model. The Anacacho Limestone and Austin Group (Chalk) are combined in the model.
4 Three-Dimensional Geologic Model of Complex Fault Structures in the Upper Seco Creek, South-Central Texas

The upper member of the Glen Rose Limestone is greater 

than 500 ft (152 m) thick and is the lower confining unit of the 

Edwards aquifer. The Glen Rose consists of alternating beds of 

light-gray to yellowish-brown limestone, dolomitic and argillaceous limestone, and marl. Locally, the unit includes packstone to grainstone and collapsed sedimentary breccia. Local 

fossil assemblage includes oysters, gastropods, foraminifers, 

and dinosaur tracks. This unit extends to the base of the model 

and was not defined by thickness.

The Devils River Formation (undivided in the model) 

varies from 500 to 600 ft (152–183 m) in thickness and is 

informally divided into upper and lower units. Within the 

study area, the upper unit thickness varies from 200 to 250 ft 

(61–76 m), and the lower unit thickness varies from 300 to 

350 ft (91–107 m). The lower unit unconformably overlies the 

upper Glen Rose Limestone and is composed of limestone and 

dolomitic limestone with local breccia. The lower one-half of 

the unit may contain sparry limestone and burrowed mudstone 

that locally hosts gastropods, miliolids, and pelecypods. The 

upper part of this unit is composed of limestone and dolomitic limestone with shell fragments supported by grainstone. 

Locally, the upper part of the Devils River Formation may 

have interfingered beds of the Georgetown Formation (Small 

and Clark, 2000).

The Del Rio Clay is about 60 ft (18 m) thick and is the 

upper confining unit of the Edwards aquifer. The Del Rio Clay 

shares an abrupt unconformable contact with the underlying 

Devils River Formation. The unit is gypsiferous, calcareous, 

poorly indurated clay with beds of pelecypods. Locally, the 

Del Rio Clay contains thin, lenticular, calcareous siltstone 

beds. 

The Buda Formation is variable in thickness, averaging 

70 ft (21 m) between the top Del Rio Clay and the bottom of 

the Eagle Ford Formation. Where exposed, the unit is lightgray to pale-orange, poorly bedded to nodular, hard to chalky 

mudstone containing abundant pelecypods, foraminifers, and 

ostracodes; solitary corals and algae are common. 

The Eagle Ford Formation is about 40 ft (12 m) thick 

and consists of flaggy, sandy shale, siltstone, and argillaceous limestone. The base of the group commonly contains 

oysters, ostracodes, and fossil fish. The lower part of the unit 

commonly is laminated siltstone that grades upward to silty 

limestone and shale. 

The youngest rock units modeled are the Upper Cretaceous Anacacho Limestone and the Austin Group locally 

known as the Austin Chalk. These units are not differentiated 

in the model due to poor surface exposures and subsurface 

contact information. The Austin Group varies from 135 to 200 

ft (41–61 m) in thickness and is composed of gray to white, 

massive to slightly nodular, chalky and marly, fossiliferous 

mudstone. The unit often contains grain-rich, thin- to thickbedded glauconitic limestone and marl with fossil fragments. 

The chalky limestone contains fossil oysters, abundant Inoceramus prisms, foraminifers, and echinoid debris. Bentonite 

seams and glauconitic pyrite nodules partly weathered to 

limonite are common. The overlying Anacacho Limestone is 

greater than 470 ft (143 m) thick and is composed of white to 

gray mudstone and packstone with thick interbeds of bentonite 

clay. 

A mafic igneous intrusion of fine- to coarse-grained alkali 

basalt is modeled in the far southwest corner of the study area. 

The intrusive rocks are identified and described by Blome and 

others (2004) and Miggins and others (2004). 

Model Construction and Methodology

Data for the hydrogeologic surfaces within the model 

were identified and defined from three primary sources: 

(1) Thirty-one drill holes with stratigraphic picks based on 

geophysical logs, core, and (or) cuttings. 

(2) HEM data were used to help define locations and 

displacement of faults and interpret and define the 

tops of two electrically conductive stratigraphic units: 

the Del Rio Clay and the Eagle Ford Formation. 

Elevations of top surfaces of these units were 

validated with geophysical logs from selected drillhole and lithologic descriptions from drill-hole core. 

HEM data also were used to validate the spatial 

location for parts of the Devils River Formation 

by showing what appear to be two geophysical 

conductivity signatures within the formation. 

Inadequate subsurface and surface information and 

inconsistent drilling reports prevent verification of the 

HEM data within the Devils River Formation for this 

model. 

(3) Contacts defined from surface geologic mapping 

and local field checking of fault locations and 

stratigraphic contacts at the 1: 24,000-scale. 

Data points for all modeled surfaces are labeled in the 

data files with one of the following: (1) a drill-hole name 

or a line number such as TD6938601 or 10050; (2) an “m” 

designation such as m50. These data are defined as stratigraphic contact data or calculated data from known contact or 

drill-hole information; (3) an “md” designation such as md48. 

These data were needed to help with the computations for the 

modeled surfaces, especially for displacements along faults. 

We kept these data points to a minimum. 

The primary reference surface for the model is the top 

of the Del Rio Clay. This surface was selected because it was 

easily identified in drill-hole data and HEM surveys. The 

HEM surveys provided a lot of data covering a large area in 

the central part of the model. The HEM surface was validated 

using drill-hole geophysical logs. We also used the top of 

the Glen Rose Limestone as a secondary reference surface to 

build the model on and to provide a foundation for modeled 

structure. A critical data source was drill hole YP–69–37–402 

in the west-central part of the model, locally known as the 

Sabinal drill-hole (Small and Maclay, 1982). This drill hole 

was continuously cored and serves as the local stratigraphic 

reference for all surfaces. Average thickness of modeled units 
Model Construction and Methodology 5

based on the “Sabinal” core and field mapping are: Del Rio 

Clay, 60 ft (18 m); Buda Formation, 70 ft (21 m); Eagle Ford 

Formation, 40 ft (12 m); and the Devils River Formation at 

500 ft (152 m) in the western part of the model, thickening 

eastward to 600 ft (183 m). 

EarthVisionTM modeling software was used to create 

three-dimensional mathematically defined surfaces from 

scattered x, y, and z data points where x and y are a Universal 

Transverse Mercator (UTM), NAD 27, Zone 14 location in 

meters, and z is an elevation in feet. In the model, x, y, and z 

values define top surfaces of lithologic units. Volumes of units 

are defined and shown as the space between two surfaces and 

surfaces and fault planes or model extents. EarthVisionTM software follows basic geologic rules to define depositional, channel fill, or unconformable contacts and surfaces. These rules 

can be modified by adding data points to a surface, altering 

gridding parameters or detail, or using smoothing algorithms 

in any or all of the x, y, or z dimensions. This allows considerable discretion to define or refine a surface. Modeled surfaces 

were derived using “minimum surface-tension” gridding 

algorithms, gridding modifications and detail modifications, 

and user-defined data points. Details of the algorithms and 

how data points are used by the software are beyond the scope 

of this report but are available from Dynamic Graphics, Inc., at 

http: //www.dgi.com. 

EarthVisionTM software was used to create, compile, 

and display the model because of its ability to use and view 

different data types. In addition, it can accurately define and 

show faulted geologic surfaces while maintaining structural 

complexity and integrity in three dimensions. The model has 

25 faults and fault segments. Faults were located using the 

geologic map of Blome and others (2004) and HEM interpretations (Smith and others, 2003). Location and displacement 

of faults were cross validated by using field observations, 

HEM data interpretations, and drill-hole information. Faults 

were defined as normal and nearly vertical on the basis of our 

field mapping and the recognized extensional movements of 

faults within the Balcones fault zone (Barker and others, 1994; 

Maclay, 1995; and Collins, 2000).

The geologic units modeled occur west to east in the 

study area. Erosion removed parts of younger units in the 

northern one-half and along the eastern margin of the study 

area. For example, the Del Rio Clay crops out and underlies 

units in the central and southern one-half of the study area but 

does not exist in the northern or eastern part of the study area. 

The model prototype was built using subsets of all available 

data adopted by consensus of the authors. Accurate modeling 

of structure was critical to develop the model so 10 faults were 

chosen to prototype the modeling process. The model evolved 

using an iterative methodology to integrate different data sets 

and define relationships between stratigraphic surfaces and 

faults. Some data sets were often limited to different parts 

of the study area. For example, drill-hole data are scattered throughout the southern one-half of the model, greater 

accuracy of surface mapping occurs in the northern one-half 

of the model area, and HEM data for two clay-bearing units 

are greater in the central part of the study area. Data sets 

and information were cross-checked against each other. For 

example, the Del Rio Clay surface elevations built from HEM 

data were validated by the Del Rio Clay elevations shown on 

drill-hole logs and geologic maps. As more data were added 

to the model, interpretations of stratigraphy and structure were 

refined, and the model evolved, increasing in complexity. The 

resultant model is the accumulation of interpretations through 

consensus of the authors as constrained by the data.

Topography for the model was created using U.S. Geological Survey 10-meter Digital Elevation Model (DEM) data. 

Detail of the surface is a result of the volume of data present, the size and number of grid cells used in the model, and 

resolution used to display the whole model. We designed the 

model to show topographic and structural detail. However, we 

also balanced this with what can be interactively displayed on 

a typical IntelTM single processor computer system running 

Windows XPTM as defined in the readme1.txt file on this disc.

Where possible, surfaces were defined using data points 

defined by surface contacts, drill-hole stratigraphic identification, or HEM data. Where data were missing, data points 

were mathematically derived from known data points based 

on local thickness of modeled units or average thickness based 

on reference well YP–69–37–402. For example, if only local 

data for a surface was the contact defined on the geologic 

map, we used the x, y, and z values for the mapped surface 

contact. Local overlying and underlying surfaces were than 

derived by maintaining the x and y location of the mapped 

contact and calculating the z elevation value based on local 

thickness where possible, or average thicknesses based on 

YP-69-37-402. This held true for elevations of units defined 

by drill-hole data or HEM data. In the subsurface, some x 

and y coordinates were moved slightly to maintain geometry 

of faults. This was necessary because we had no downhole 

positioning coordinates for any subsurface stratigraphic units. 

Because these data points were derived from reference data, 

they were assigned an “m” label in the data files. Where 

needed, data points were estimated and added to a surface data 

file to refine known relationships such as fault displacements 

or to help the mathematical generation of a surface. These data 

points have an “md” label in the data sets. Local stratigraphic 

surface and thickness variations of units were expected and 

modeled. Such variations may reflect faulting, dissolution, 

and collapse features or inherent uncertainties of defined picks 

from geophysical logs or other data. The greatest local variations occur in surfaces defined by HEM data due to the high 

spatial density of data and system noise and sensitivity of the 

geophysics modeling process. For the model, HEM data were 

filtered to limit noise and spikes caused by local anomalies. 

The authors believe that thickness variations shown in the 

model reflect reasonable interpretations based on the objective 

criteria in surface maps, drill-hole log interpretations, lithologic descriptions, observations of drill-hole core, and HEM 

interpretations. 
6 Three-Dimensional Geologic Model of Complex Fault Structures in the Upper Seco Creek, South-Central Texas

The lowermost unit modeled is the Glen Rose Limestone, 

the lower confining unit of the Edwards aquifer. The top of 

the Glen Rose is defined by surface mapping in the northern 

one-half of the study area and was calculated from drill-hole 

geophysical logs or defined by reference data that defined the 

surface of the Del Rio Clay. The top of the upper Glen Rose 

Limestone was used as secondary reference surface because it 

provided the base to build the model on and was constrained 

by reference data. As the bottommost unit in the model, the 

top surface of the Glen Rose Limestone influences positions of 

overlying units in three-dimensional space, such as the Devils 

River Formation. Top of the Glen Rose was also used to 

propagate and validate fault structures defined and shown on 

geologic maps and HEM interpretations. This was necessary 

because the software defines fault displacement from modeled 

surfaces and propagates the displacement until other data are 

available or some model extent or boundary is reached.

The Del Rio Clay is well defined by HEM geophysical conductivity, drill-hole, geophysical, and lithologic data. 

The Eagle Ford Formation HEM data were used to calculate 

top of the Del Rio Clay in the southern one-third of the study 

area because the HEM survey did not penetrate the stratigraphic unit due the volume of clay. An approximate top for 

the Del Rio Clay was extrapolated from the top of the Upper 

Glen Rose Limestone in the northern one-third of the study 

area. This extrapolated surface helped validate geologic map 

contacts on hill and mesa tops and as an aid to model other 

geologic units and structures. The top of the Del Rio Clay 

was used as the primary reference surface because of the 

abundance of data, certainty of picks, and interpretations 

from known reference points. This surface was used to help 

define overlying and underlying units where data were sparse. 

Because of the abundance and accuracy of HEM data for this 

surface and the overlying Eagle Ford Formation, these surfaces 

show more local variation in the model. 

The Buda Formation in the model was defined with a 

thickness of 70 feet, or as the volume between the top of the 

Del Rio Clay and the bottom of the Eagle Ford Limestone, 

using the top of the Del Rio Clay as its reference horizon. 

HEM data could not be used to build the Buda Formation 

surface because HEM signals cannot penetrate thick, electrically conductive clay-bearing units such as the overlying Eagle 

Ford Formation. However, HEM soundings did show the Buda 

Formation where exposed. 

The combined Austin-Anacacho unit is the upper surface 

shown in the southern one-half of the model. Topography for 

the model was defined using USGS 10-meter topographic 

DEM data. EarthVisionTM software was used to create a 

detailed topographic grid. This grid was used to truncate modeled units to build the topographic surface. The topographic 

surface shows the approximate location of exposed geologic 

and hydrogeologic units and is an aid to locate these units on 

topographic maps. 

All data were evaluated for quality and accuracy by using 

two- and three-dimensional visual analysis and modeling 

and by field checking surface outcrops. All data types were 

iteratively cross-checked against each other. Due to the 

iterative and visual nature of the model, data inconsistencies 

were identified early in the modeling process and resolved. 

The results of the ongoing visual evaluations are a better 

understanding of the geology and structure in the study area as 

well as increased model accuracy. 

Structural Uncertainties in 

Southeastern Part of Model

The existence of faults parallel to the Seco Creek drainage south of the intersection of Cedar Creek and Seco Creek 

has been questioned for decades. Historically, the U.S. Geological Survey (USGS) started mapping the upper Seco Creek 

area of Medina and Uvalde Counties in the early 1930s (Sayre, 

1936, pl. 1). Their published results and recent mapping by the 

USGS and the Texas Bureau of Economic Geology (TBEG) 

show that the local bedrock geology is obscured by undifferentiated Quaternary terrace deposits and Pleistocene Leona 

Formation gravels; in addition, erosion and a lack of surface or 

subsurface drill-hole evidence neither prove nor disprove the 

existence of local faults. Because of years of speculation, the 

authors believe a brief history about the existence of a local 

fault or faults is warranted. 

Arguments for faulting: 

•	 Sayre (1936, pl. 1) inferred a possible cross-fault that 

cuts across Seco Creek. This fault, located near the 

Medina and Uvalde County boundary in the area of 

Little Seco and Seco Creeks, was inferred to offset 

the younger Eagle Ford and Austin Groups and Anacacho Formation against the older Buda Formation. 

•	 Small and Clark (2000) show undifferentiated Devils 

River Formation rocks faulted along the Seco Creek 

drainage and against the much younger gravels of 

the Leona Formation in the same area. 

•	 1: 75,000-scale mapping by Clark and others (2006) 

along Seco Creek shows an inferred cross-fault 

informally named the “Seco Creek Valley fault” 

bounded by the Woodard Cave fault to the north and 

the Parker Creek fault to the south. 

•	 Clark and Journey (2006) suggest that a fault forms 

at least a partial barrier to waterflow because an 

offset of several hundred feet of the Edwards aquifer 

and juxtaposed confining units might direct part or 

most of the waterflow southward. 

•	 Geophysical soundings used for this model show 

that reflections from the the Del Rio Clay end in the 

Seco Creek drainage—a lateral discontinuity that 

may suggest a local fault. 

Arguments against faulting: 
Modeled Structure 7

•	 C.L.R. Holt, Jr. (1959, pl.1), in cooperation with 

the Texas Board of Water Engineers, does not show 

any cross-faults along Seco Creek. Also shown are 

the Leona Formation gravels overlying the other 

lithologic units. 

•	 Collins (1999a, b) mapped the geology along Seco 

Creek in the Flatrock Crossing 7.5-minute quadrangle and did not include a cross-fault in the Seco 

Creek drainage south of its intersection with Cedar 

Creek. 

•	 Recent mapping by Blome and others (2004) shows 

no faults along the Seco Creek drainage.

•	 The west and southwest dip of the Eagle Ford 

Formation under the Leona gravels and Quaternary 

gravels, the Buda Formation, and thinner parts of the 

Austin Group as shown in the HEM data and drillhole data and by surface geologic mapping would 

position the Eagle Ford Formation at the surface 

along Seco Creek. This, coupled with erosion, may 

explain why the Eagle Ford Formation and Del Rio 

Clay abruptly end east of Seco Creek locally. 

•	 HEM data show local faults that are not shown on 

surface geologic maps, under alluvium deposits in 

parts of the study area (Smith and others, 2003). The 

data also infer that the Eagle Ford Formation and Del 

Rio Clay dip east and southeast in cross section. In 

cross section, this dip shows the Del Rio Clay cropping out along Seco Creek in the area of the controversial fault. Erosion, perhaps caused by events that 

deposited the Leona Formation gravels and local 

terraces, may account for the abrupt termination of 

the Eagle Ford Formation and the Del Rio Clay. 

Modeled Structure

We developed both a model without the controversial 

fault and one with the northwest-trending cross-fault. Both 

models show the Del Rio Clay ending in the Seco Creek drainage. This model includes the cross fault. The fault location 

inferred from study of the HEM data is south of the intersection of Cedar Creek with Seco Creek, in the Seco Creek drainage along a line where data for the Del Rio Clay end (fig. 3). 

Fault placement is west of the current location of Seco Creek 

and west of where others inferred a fault (Sayre, 1936, pl. 1; 

Small and Clark, 2000; Clark and others, 2006; and Clark and 

Journey, 2006). HEM data further constrain a possible fault 

location between the Seco Valley Road fault to the north and 

the Parker Creek fault to the south. 

Figure 3. Surface of the Glen Rose Limestone showing the inferred north-trending fault bounded by the 

discontinuous Seco Valley Road fault on the north and the continuous Parker Creek fault on the south. 

View is toward the northeast.
8 Three-Dimensional Geologic Model of Complex Fault Structures in the Upper Seco Creek, South-Central Texas

To model the fault, we created data points east of where 

the HEM data for the Del Rio Clay ended. We modeled a 

maximum displacement of about 50 feet, or the approximate 

thickness of the Del Rio Clay; this displacement is similar to 

other local fault displacements. In addition, more than 50 feet 

of displacement caused problems with known stratigraphic 

surfaces and the spatial relationships between faults and stratigraphic surfaces that are known to exist by geologic maps, 

drill-hole data, and HEM data to the north, south, east, and 

west. Though the fault provides an abrupt termination of the 

Del Rio Clay surface, models without the fault showed the Del 

Rio Clay terminating in the same area.

Conclusions

Evolving this model both visually and interactively 

simplified the integration of the data. The model is internally 

consistent because drill-hole geophysical and lithologic data 

were used to confirm HEM and surface geologic data; surface 

geologic data were used to confirm HEM and drill-hole data, 

and HEM data were used to confirm drill-hole and surface 

geologic data. The resultant model is a more accurate interpretation due to the diversity of the data used and the integration and cross validation of the different types of data. The 

model shows the complex relationships between fault-bounded 

lithologic units, possible downgradient flow pathways along 

or across fault boundaries, and tilted and fractured ramplike 

structures that resulted from faulting in this part of the Balcones fault zone. The interactive nature of this model makes it 

an excellent tool to study these relationships and the relationships of other data sets in the modeled area.

HEM data for the Eagle Ford Formation and the Del Rio 

Clay in the center part of the model help identify and accurately locate faults shown by Blome and others (2004). An 

area of importance is the west-central part of the model, south 

of the Woodard Cave fault. In this area, fewer faults were seen 

in the data at shallow depth, and faults that were retained are 

based on interpretations of drill-hole geophysics and HEM 

surveys. Faults modeled were traced across flight lines until 

they faded out. Several named faults in the central part of the 

model were relocated based on HEM data. Locally, location adjustments of some faults exceeded one-half mile. Of 

note was the Seco Valley Road fault. Although inferred to be 

continuous by published maps, the HEM data showed a break 

in the continuity of the fault in the central part of the model. 

Even with these modifications, overall structural trends are 

consistent with mapping done by Blome and others (2004), by 

Collins (1998, 1999a, 1999b), and in other earlier works. 

Major structural trends and outcrop of this model are 

mostly consistent with the mapping done by Blome and others 

(2004). Differences in the details are seen for the following 

reasons: some faults were removed or their locations changed, 

faults were added because of the interpretation of HEM data 

that showed fault locations at shallow depth, the modeled 

DEM surface is less detailed than the original DEM, and there 

are limits to the detail the software can model. Also, we chose 

to show less surface detail to keep the model robust on commonly available hardware. 

References Cited

Barker, R.A., Bush, P.W., and Baker, E.T., Jr., 1994, Geologic 

history and hydrogeologic setting of the Edwards-Trinity 

aquifer system, west-central Texas: U.S. Geological Survey 

Water Resources Investigations Report 94–4039, 51 p.

Blome, C.D., Faith, J.R., Collins, E.W., Pedraza, D.E., and 

Murray, K.E., 2004, Geologic map compilation of the upper 

Seco Creek area, Medina and Uvalde Counties, southcentral Texas: U.S. Geological Survey Open-File Report 

2004–1430, 21 p., 1 pl., scale 1: 50,000.

Clark, A.K., 2003, Geologic framework and hydrologic 

characteristics of the Edwards aquifer, Uvalde County, 

Texas: U.S. Geological Survey Water-Resources Investigations Report 2003–4010, 17 p, 1 pl., scale 1: 75,000.

Clark, A.K., Faith, J.R., Blome, C.D., and Pedraza, D.E., 

2006, Geologic map of the Edwards aquifer in 

northern Medina and northeastern Uvalde Counties, southcentral Texas: U.S. Geological Survey Open-File Report 

2006–1372, 22 p., 1 pl., scale 1: 75,000.

Clark, A.K., and Journey, C.A., 2006, Flow paths in the 

Edwards aquifer, northern Medina and northeastern Uvalde 

Counties, Texas, based on hydrologic identification and 

geochemical characterization and simulation: U.S. 

Geological Survey Scientific Investigations Report 

2006–5200, 48 p.

Collins, E.W., 1997, Geologic map of the Texas Mountain 

quadrangle, Texas: University of Texas at Austin, Bureau of 

Economic Geology Open-File Map, scale 1: 24,000.

Collins, E.W., 1998, Geologic map of the Sabinal NE quadrangle, Texas: University of Texas at Austin, Bureau of 

Economic Geology Open-File Map, scale 1: 24,000.

Collins, E.W., 1999a, Geologic map of the Comanche 

Waterhole quadrangle, Texas: University of Texas at 

Austin, Bureau of Economic Geology Open-File Map, 

scale 1: 24,000.

Collins, E.W., 1999b, Geologic map of the Flatrock Crossing 

quadrangle, Texas: University of Texas at Austin, Bureau of 

Economic Geology Open-File Map, scale 1: 24,000.
References Cited 9

Collins, E.W., 2000, Geologic map of the New Braunfels, Pantea, M.P., and Cole, J.C., 2004, Three-dimensional 

Texas, 30 x 60 minute quadrangle-Geologic framework of geologic framework modeling of faulted hydrostratigraphic 

an urban-growth corridor along the Edwards aquifer, south- units within the Edwards aquifer, northern Bexar County, 

central Texas: University of Texas at Austin, Bureau of Texas: U.S. Geological Survey Scientific Investigations 

Economic Geology Miscellaneous Map No. 39, 28 p., 1 pl., Report 2004–5226, CD–ROM.

scale 1: 100,000. Sayre, A.N., 1936, Geology and ground-water resources of 

Holt, C.L.R., Jr., 1959, Geology and ground-water resources Uvalde and Medina Counties, Texas: U.S. Geological 

of Medina County, Texas: U.S. Geological Survey water- Survey Water-Supply Paper 678, 146 p.

supply paper 1422, prepared in cooperation with the Texas Small, T.A., and Clark, A.K., 2000, Geologic framework and Board of Water Engineers, 213 p., 6 pl. hydrologic characteristics of the Edwards aquifer outcrop, 

Lozo, F.E., Jr., and Smith, C.I., 1964, Revision of Comanche Medina County, Texas: U.S. Geological Survey WaterCretaceous stratigraphic nomenclature, southern Edwards Resources Investigations Report 2000–4195, 10 p, 1 pl.

Plateau, southwest Texas: Gulf Coast Association of Small, T.A., and Maclay, R.W., 1982, Test-hole data for the Geological Societies Transactions, v. 14, p. 285–306. Edwards aquifer in the San Antonio area, Texas: Texas 

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aquifer in the San Antonio area, Texas: U.S. Geological Smith, B.D., Smith, D.V., Hill, P.L., and Labson, V.F., 2003, Survey Water Resources Investigations Report 95–4186, Helicopter electromagnetic survey of the Seco Creek area, 64 p. Medina and Uvalde Counties, Texas: U.S. Geological 

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Disc Contents

In addition to this discussion, this publication contains 

a complete faulted 3–D surface and volume model with 

supporting digital data sets of stratigraphic units and HEM 

information in the study area. The data sets include drill-hole 

from core and geophysical logs, surface geology and elevation, 

and HEM data. The model may be explored and .jpg or .tiff 

images saved of user-defined views. Both a basic “getting 

started” help file by Pantea and Cole (2004) and detailed help 

files are provided as aids to understanding the 3–D viewer. 

We included viewers for Windows XPTM, Linux, and SUNTM 

platforms. However, the SUNTM platform requires additional 

drivers available from SUNTM. Once in the viewer program, 

all functions operate the same. The USGS has licensed the 

rights from Dynamic Graphics, Inc., to provide an encrypted 

model and the viewers to use the enclosed data sets and 

interpreted model. The license allows the USGS the service 

and rights to provide unlimited distribution. We designed this 

product to function from the DVD–ROM media but 

recommend the necessary files be copied to a local hard drive 

for better performance. No additional installation programs 

are needed to view the model and data sets using the 3–D 

viewer. More information about the viewing software and

EarthVisionTM may be obtained from Dynamic Grapics, Inc., 

at http: //www.dgi.com. 

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Team Chief Scientist, USGS Earth Surface Processes

Box 25046, Mail Stop 980

Denver, CO 80225

(303)236-5344

Or visit the Earth Surface Processes Team Web site at:

http://esp.cr.usgs.gov/
Pantea and others—Three-Dimensional Geologic Model of Complex Fault Structures, South-Central Texas—Scientific Investigations Report 2008–5131