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**Geologic Map Compilation of the Upper Seco Creek Area, Medina and
Uvalde Counties, south-central Texas**

By Charles D. Blome^1^, Jason R. Faith^2^, Edward W. Collins^3^, Diana
E. Pedraza^2^, and Kyle E. Murray^4^

Any use of trade, firm, or product names is for descriptive purposes
only and does not

Imply endorsement by the U.S. Government

open-file REPORT 2004-1430

^1^u.s. Geological Survey, Denver, CO 80225

^2^U.S. Geological Survey, San Antonio, TX 78249

^3^Bureau of Economic Geology, University Station, Austin, TX 78713-8924

^4^University of Texas at San Antonio, San Antonio, TX 78249-1130

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

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**Contents**

Page

Introduction..............................................................................3

Geologic
Setting\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....4

Tectonic and Hydrologic
Setting......................................................6

Quaternary and Upper Tertiary
Stratigraphy.................................\...\...\...7

Upper Cretaceous
Stratigraphy...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....8

Lower Cretaceous
Stratigraphy........................................................13

References...........................................................................\...\....17

**Geologic Map Compilation of the Upper Seco Creek Area, Medina and
Uvalde Counties, south-central Texas**

By Charles D. Blome, Jason R. Faith, Edward W. Collins, Diana E.
Pedraza, and Kyle E. Murray

**INTRODUCTION**

The Edwards aquifer of south-central Texas lies within and adjacent to
the Balcones fault zone and is considered one of the most complex and
productive carbonate aquifers in the United States. Many agencies have
worked together toward gaining a better understanding of the geologic
and hydrogeologic characteristics controlling the productivity and
sustainability of this prolific ground-water resource. The Edwards
aquifer in the upper Seco Creek area lies within the Devils River
Formation, a predominant unit within the Devils River reef-trend facies
of the Edwards Group (see map inset showing facies distribution). The
Devils River Formation contains approximately 600 feet of highly porous
and permeable limestone deposited on a carbonate platform along the
presently defined Balcones fault zone and Edwards Plateau. The
stratigraphy of the area ranges from the Lower Cretaceous Glen Rose
Limestone (oldest) to the Quaternary alluvium (youngest). Edwards Group
lithostratigraphic units in the mapped area contain the Devils River and
Fort Terrett Formations, the latter being equivalent to the lower part
of the Devils River Formation.

The network of overlapping Balcones faults has a significant impact on
the hydrogeology of the Edwards aquifer throughout south-central Texas
by controlling regional ground-water flowpaths in a general southwest to
northeast direction (Maclay and Small, 1976). The general trend of the
flowpaths parallels the major faults in the area. The fault control on
the major ground-water flowpaths highlights the need for more accurate
digital mapping and compilations of geologic data for use in
hydrogeologic assessments of the Edwards aquifer region.

The impetus for compiling the geology of four 7.5 degree quadrangles
(Comanche Waterhole, Flatrock Crossing, Sabinal NE, and Texas Mountain)
at a scale of 1:50,000 was to provide a digital geologic map database
for a helicopter electromagnetic (HEM) survey flown in 2002 (Smith and
others, 2003) as well as ongoing 3-D EarthVision modeling of the
Edwards/Trinity aquifer boundary in the north Seco Creek area. The
majority of the geology contained within these four quadrangles is from
the mapping of Collins (1997, 1998, 1999a, and 1999b). Earlier mapping
in this area included the efforts of Holt (1959) and Welder and Reeves
(1962), and parts of their geology are included in the Collins' reports.
More recent mapping by Small and Clark (2000) and Clark (2003) reveal a
number of additional faults in the upper Seco Creek area not recognized
by Collins. However, this map compilation represents the original
mapping by Collins (1997, 1998, 1999a, and 1999b) and subsequent mapping
by Blome and Faith in FY-03 and FY-04. Also, new structural data will be
included in upcoming 3-D geophysical and structural models as needed but
only after each fault is field checked.

# Geologic Setting

The lithostratigraphic units of the upper Seco Creek area are provided
on the map explanation and are described in detail below. The oldest
lithology in the study area is the Lower Cretaceous Glen Rose Limestone,
which represents the upper part of the Trinity Group. The Devils River
Formation is commonly divided into informal upper (about 250 feet thick)
and lower (about 350 feet thick) units (Collins, 2000). The upper and
lower parts of the Devils River Formation are also equivalent to the
Segovia and Fort Terrett Formations, respectively, in the Edwards
Plateau area to the north. According to Collins' mapping of the Flatrock
Crossing and Texas Mountain 7.5 quadrangles, lithologic changes between
the Devils River and Fort Terrett deposits are gradational and are
related to minor facies variations. The Georgetown Formation, comprised
of limestone and marl, typically overlies the Edwards Group lithologies
in much of the region (Small and Clark, 2000) but is absent in the study
area.

The Upper Cretaceous lithologic units consist of several formations,
including (from oldest to youngest), the Del Rio Clay, Buda Formation,
Eagle Ford Formation, Austin Group, Anacacho Limestone, Escondido
Formation, and isolated, mafic intrusive rocks. The Del Rio Clay
directly overlies the Edwards Group due to the absence of the Georgetown
Formation in the upper Seco Creek area.

Although the precise age of the Uvalde gravel is not known, it is
considered late Tertiary and Quaternary in age. The Leona Formation
consists of fine silt to coarse gravel and, along with the Uvalde
Gravel, terrace deposits, and unconsolidated alluvium, comprise the
Quaternary stratigraphy. The gravel and alluvial deposits occur
abundantly and conceal a large portion of the underlying geology and
structural features in the study area.

# Tectonic and Hydrologic Setting

Uplift of the Edwards Plateau and subsidence of the Gulf of Mexico
during the Miocene created the Balcones fault zone in south-central
Texas. The network of overlapping *en echelon* normal faults trend, on
average, to the northeast, is downthrown to the southeast, and is linked
by scattered cross faults trending southeast (see map insert showing
facies distribution). Relay ramps often form between overlapping faults
with varying lateral displacement gradients to accommodate the
deformation occurring in their bounding hanging wall and footwall
blocks. Continued displacement of the bounding faults and the subsequent
increasing extension parallel to them produce cross faults that link the
major faults and breach the relay ramp. On the surface, Seco Creek and
other drainages in the area flow south, nearly perpendicular to the
major faults, with the exception of the area just south of the Woodard
Cave Sinkhole where Seco Creek intersects a series of cross faults, and
its path is abruptly redirected to the southeast. These cross faults
breach a southwest-dipping, large-scale relay ramp observed in the
Comanche Waterhole and Sabinal NE quadrangles.

The Edwards aquifer in Medina and Uvalde Counties is contained in the
Edwards Group rocks and overlying Georgetown Formation. Faults in a
carbonate aquifer commonly affect the ground-water flowpaths by: 1)
acting as a pathway for flow through increased porosity and permeability
parallel to the faults, or 2) acting as a flow barrier and preventing
hydrologic communication across the fault. In either situation, when the
displacement of hydrostratigraphic units is minor, faults may not
significantly affect the flow of ground water. Regional ground-water
flowpaths in the Edwards aquifer, including those in the four-quadrangle
study area, tend to follow regional fault trends regardless of whether
the water is flowing within or adjacent to the fault planes. []{.mark}

**DESCRIPTION OF MAP UNITS**

**QUATERNARY AND UPPER TERTIARY STRATIGRAPHY**

**Qal Alluvium (Quaternary)---**The alluvium is comprised of
unconsolidated gravel, sand, silt, and clay along streams and rivers
that are inundated regularly. The gravel contains mostly limestone and
chert. Low terrace deposits found along minor drainages include some
local outcrops that cannot be subdivided

**Qu Undivided terrace, slope-wash deposits, and fan deposits
(Quaternary)---**This undivided unit contains gravel to mud

**Qt Terrace deposits (Quaternary)---**Terraces in the mapped area
contain unconsolidated gravel, sand, silt, and clay along streams and
rivers. They occur mostly above flood level. Deposits of adjacent
terraces exposed along different elevations have been mapped separately

**Qle Leona Formation (Pleistocene)---**The Leona Formation ranges in
thickness from a few feet to 80 feet and is composed of lenticular beds
of sand, gravel, silt, and clay. The pebbles and cobbles in the Leona
predominantly contain limestone with some chert. Coarser gravels are
present near the base of the formation with silt increasing upwards. The
Leona is locally a prolific ground-water source and has rare cavern
development and variable (low to high) porosity due to the poor sorting
in the gravels. Locally, silt and clay cement significantly reduces the
permeability. In general, the formation is thickest near the stream
channels or older abandoned meander channels

**QTu Uvalde Gravel, older alluvium (Quaternary and upper
Tertiary)---**The Uvalde Gravel contains mostly gravel and sand with
some silt and clay. Well-rounded, pebble- to cobble-sized gravel are
common, few boulders. The unit contains mostly chert and limestone,
commonly cemented by caliche. The Uvalde deposits typically cap
topographically high areas. The precise age of the unit is unknown but
has been estimated to be approximately Quaternary to late Tertiary in
age. Thickness of the unit ranges from several feet of gravel lag to
more than 10 feet

**UPPER Cretaceous STRATIGRAPHY**

**Ki Mafic intrusive rocks---**The igneous exposures generally are not
well exposed. Welder and Reeves (1962) first mapped and located many of
the intrusive exposures. However, some small outcrops noted by Holt
(1959) in Medina County are not shown.

The igneous rocks consist of fine- to course-grained ultramafic and
hyabyssal rocks that occur either as surface exposures or in the
subsurface as dikes, plugs, and shallow intrusions. The ultramafic field
is centered in Ulvalde County and extends to the west into Kinney
County. The igneous rocks are represented by five rocks types, which
include alkali basalt, melilite-olivine nephelinite, olivine
nephelinite, nepheline basinite, and phonolite (Miggins and others,
2004).

A high-resolution aeromagnetic survey flown in 2001 (Smith and others,
2002) revealed over 200 shallow (in the subsurface) igneous intrusive
bodies whereas fewer than 30 had been mapped (Fisher, 1983). A
^40^Ar/^39^Ar geochronologic study (Miggins and others, 2004) shows that
there have been at least two distinct phases of magmatic activity in
Uvalde County. The first phase of intrusive activity occurred
approximately 82-80 m.y. ago whereas the younger intrusive rocks
(phonolites) were emplaced 74-72 m.y. ago

**Kes Escondido Formation---**The Escondido Formation contains mudstone,
siltstone, sandstone, and silty limestone. The map unit also includes a
thin (as much as \~30 feet) lower marl and mudstone unit called the
Corsicana Marl. Outcrops are not common. Thickness of the Escondido
ranges between 550 and 900 feet

**Kan Anacacho Limestone---**Unit contains limestone and marl.
Grain-rich limestone is common. The Anacacho is light gray to white in
outcrop view, thin- to thick-bedded, glauconitic, and contains fossil
fragments. Thickness of the unit ranges from 240 to 500 feet

**Kan+Kau Anacacho Limestone and Austin Group, undivided---**In areas of
poor exposure, the Anacacho Limestone and Austin Group cannot be
differentiated

**Kau Austin Group---**The Austin Group, commonly called the Austin
Chalk, contains massive to slightly nodular, gray to white, chalky to
marly, fossiliferous mudstone ranging from 135 to 200 feet thick. It is
identified in the field as white, chalky limestone (microgranular
calcite) containing the fossil oyster *Gryphaea aucella*, abundant
*Inoceramus* prisms, minor foraminifera and ostracode tests as well as
echinoid debris. The chalky mudstone forms ledges and alternates with
marl and locally with bentonitic seams. Glauconitic pyrite nodules
partly weathered to limonite are also found. Thick caliche is common on
the surfaces of most outcrops.

The Austin Group is generally referred to as an aquifer-confining unit.
However, there is local occurrence of ground water associated with
fractures. The Austin Group has scarce cavern development and low
porosity and permeability. The unit forms thick black soil with juniper
and live oak in low-relief areas

**Kef Eagle Ford Formation---**This unit is typified by brown, flaggy
and sandy shale, siltstone, and argillaceous limestone, 15 to 30 feet
thick. The upper part of the unit contains limestone and shale and is
dark gray in color. Limestone is generally light, yellowish brown,
flaggy, and occurs in beds as much as 4 feet thick. The lower part of
the unit contains silty, medium-brown, laminated siltstone and weathers
easily, forming flat to gently rolling topography. The unit also is
known as lignite by local drillers.

No cavern development is evident and the primary porosity has been lost,
thereby reducing permeability. Outcrops are rare in the mapped area and
are generally covered with dark-brown soil. Strata at slope break of the
Eagle Ford/Buda contact are commonly fossiliferous with oysters,
ostracodes, foraminifers, fish bones, teeth, and *Inoceramus* prisms
present

**Kbu Buda Formation (Upper Cretaceous)---**The unit contains poorly
bedded to nodular, hard to chalky, mudstone, 40 to 56 feet thick. The
mudstone commonly contains calcite-filled veins and is glauconitic and
locally fossiliferous with abundant shell fragments. Coloration is light
gray to pale orange; weathers dark gray to brown. Unit is thinner bedded
and argillaceous near its upper contact. The upper contact with the
Eagle Ford Formation is disconformable, sharp, and conspicuous, and
outcrops as resistant caps on hills. Lower part of formation contains
soft, chalky limestone.

The Buda Formation weathers to form thin, red-brown soil containing
rounded cobbles of limestone, and is less glauconitic and less
iron-oxide-stained than the underlying Georgetown Formation (present
outside the map area; Collins, 1997, 1998, 1999a, 1999b). The Buda also
contains more fossil gastropods than the Austin Group in addition to
burrows filled with chalky marl. The Buda fossils include abundant
pelecypods, foraminifers, ostracodes, serpulids, echinoid spines, and
bryozoans. Locally, solitary corals and green algae are also found. The
unit has minor karst features with low porosity and regionally is
considered a confining unit for both the Edwards and Trinity aquifer
systems

**Kdr Del Rio Clay---**This unit is 15 to 50 feet thick and contains
gypsiferous and calcareous, poorly indurated, dark gray to olive brown
clay with abundant pelecypods. Minor, thin lenticular beds of highly
calcareous siltstone also may occur. The Del Rio becomes less calcareous
and more gypsiferous up section and weathers light gray to yellowish
gray. The unit is slope forming or under hanging where slumped below the
overlying Buda Formation. Upper and lower contacts with the Buda
Formation and upper Devils River Formation, respectively, are
gradational.

The unit contains highly expansive soil, has no significant porosity or
permeability or recognized cavern development and is the primary upper
confining unit of the Edwards aquifer system. Unweathered Del Rio Clay
is composed of kaolinite, illite, and lesser amounts of montmorillonite
(Collins, 2000). Water tanks for livestock are commonly found on
outcrops. Marine megafossils include abundant *Ilymatogyra arietina*
(formerly *Exogyra* *arietina*) in addition to other pelecypods (Young,
1967)

**Lower Cretaceous STRATIGRAPHY**

The lithostratigraphy of the Edwards aquifer varies from northeast to
southwest due to the three fluctuating depositional environments, the
San Marcos Platform, the Devils River Trend, and the Maverick Basin (see
map inset). The Edwards Group of the San Marcos Platform, formed in
shallow marine waters, is divided into seven informal hydrostratigraphic
members (from youngest to oldest): cyclic and marine, leached and
collapsed, regional dense, grainstone, kirschberg evaporite, dolomitic,
and basal nodular (Maclay and Small, 1976). The overlying Georgetown
Formation represents an eighth hydrostratigraphic member. The San Marcos
Platform facies grades abruptly into the Devils River Trend facies in
northern Medina County. The central portion of the Edwards aquifer,
represented by reefal facies of the Devils River Trend (see map inset),
is comprised of the upper and lower Devils River Formation (historically
interpreted as one unit by the U.S. Geological Survey) and overlying
Georgetown Formation. However, the lateral transition from the Devils
River Formation to the deeper-water West Nueces, McKnight, and Salmon
Peak Formations (Maverick Basin facies) is highly gradational and
variable along their assumed boundaries (Clark, 2003)

**Kdvru upper Devils River Formation---**The upper Devils River
Formation contains limestone and dolomitic limestone deposited in
shallow, subtidal to tidal-flat depositional environments. Some rudistid
mounds are also found. The unit is generally 200-250 feet thick in the
mapped area. The upper Devils River is comprised of miliolid,
shell-fragment wackestones and grainstones containing rudists as well as
abundant caprinids, monopleurids, and requieniids. The unit also
displays extensive cavern development and is classified as having
fabric-selective porosity, which grades downward into more
nonfabric-selective porosity near the base

**Kdvrl lower Devils River Formation---**The lower Devils River is
typified by 350 to 400 feet of limestone and dolomitic limestone formed
in shallow subtidal to tidal-flat depositional environments. The unit
contains some leached evaporitic rocks and breccias. The lower 20 to 50
feet of the unit contains nodular limestone, and the basal part (100 to
200 feet) of the Devils River Formation contains sparry limestone and
nodular, burrowed mudstone to wackestone containing gastropods,
*miliolids,* and the pelecypod *Exogyra texana*. Solution-enlarged
fractures are present in the lowermost part of the formation.

The Texas Bureau of Economic Geology geologic maps and this study show
the Devils River Formation as subdivided into upper and lower units
whereas USGS mapping to date depicted the Devils River Formation as a
single undifferentiated unit. A helicopter electromagnetic survey flown
in the Seco Creek area in Medina and Uvalde Counties (Smith and others,
2003) clearly shows the Devils River as two separate units based on
contrasts in resistivity.

In his mapping of Uvalde County, Clark (2003) demonstrated that the
Edwards basal nodular (informal) member could be traced from the San
Marcos Platform into the Maverick Basin. Recent fieldwork by a number of
authors (Clark, Blome, and Faith) also shows that at least the informal
basal nodular and dolomitic members (Kainer Formation) can be traced
laterally from the San Marcos Platform into the Devils River Trend and
even into the deeper-water Maverick Basin (see map inset). This lateral
continuity of the lowermost units was first observed by Rose (1972) and
later revisited by Miller (1983, 1984). Nevertheless, local aquifer
researchers have not, to date, accepted the informal hydrostratigraphic
subdivisions through the Edwards Group. Hydrostratigraphic mapping of
the lower part of the Devils River Formation in Medina and Uvalde
Counties and mapping of Maverick Basin lithologies in Kinney County are
planned for FY 2005

**Kft Fort Terrett Formation---**This unit contains limestone, dolomitic
limestone, and marl that originally formed in shallow subtidal to
tidal-flat depositional environments. The upper part of the unit
contains leached evaporitic rocks and breccias, and the lower 20 to 40
feet of the unit contains subtidal limestone

**Kgru upper Glen Rose Limestone---**The formation contains alternating
beds of light gray to yellowish-brown limestone, dolomitic limestone,
and argillaceous limestone and marl. Carbonate textural fabrics include
wackestone, packstone, and grainstone. The upper Glen Rose Limestone is
about 400 feet thick in the study area, contains minor evaporite layers,
and originally formed in shallow subtidal to tidal-flat depositional
environments; upward-shoaling cycles are common. Some intervals of
disturbed bedding and collapse breccia are also present and possibly
were created by evaporite dissolution. The upper Glen Rose unconformably
underlies the Devils River Formation in the Devils River Trend (Miller,
1984) and forms the lower confining unit of the Edwards aquifer system.

Field identification of the Glen Rose Limestone is commonly associated
with stair-step topography that forms through differential erosion of
the alternating limestone and marl beds. The Glen Rose is locally
burrowed and produces honeycomb porosity in some areas. Fossil evidence
includes sparse casts of marine faunas, specifically molluscan
steinkerns (*Protocardia texana* and others), rudistids, oysters,
echinoids, gastropods (*Tylostoma* sp., *Turitella* sp.), the
foraminifera *Orbitolina minuta*, as well as local dinosaur tracks.

Surface cavern development associated with faults and fractures and some
water production at evaporite beds have been noted but are considered a
rare occurrence. The unit is classified hydrologically as having mostly
nonfabric-selective porosity and generally low permeability (Small and
Clark, 2000; Clark, 2003)

**REFERENCES**

Clark, A.K., 2003, Geologic framework and hydrogeologic characteristics
of the Edwards aquifer, Uvalde County, Texas: U.S. Geological Survey
Water-Resources Investigations Report 03-4010.

Collins, E.W., 1997, Geologic map of the Texas Mountain quadrangle,
Texas: Bureau of Economic Geology Open-File Map, Scale 1:24,000.

Collins, E.W., 1998, Geologic map of the Sabinal NE quadrangle, Texas:
Bureau of Economic Geology Open-File Map, Scale 1:24,000.

Collins, E.W., 1999a, Geologic map of the Comanche Waterhole quadrangle,
Texas: Bureau of Economic Geology Open-File Map, Scale 1:24,000.

Collins, E.W., 1999b, Geologic map of the Flatrock Crossing quadrangle,
Texas: Bureau of Economic Geology Open-File Map, Scale 1:24,000.

Collins, E.W., 2000, Geologic map of the New Braunfels, Texas, 30X60
minute quadrangle: Geologic framework of an urban-growth corridor along
the Edwards aquifer, south-central Texas: Bureau of Economic Geology,
Miscellaneous Map No. 39, 1:250,000-scale map, with text, p. 1-28.

Fisher, W.L., 1983, Geologic Atlas of Texas, San Antonio Sheet:
University of Texas, Bureau of Economic Geology, 1:250,000-scale sheet
with text.

Holt, C.L.R., Jr., 1959, Geology and ground-water resources of Medina
County, Texas: U.S. Geological Survey Water-Supply Paper 1422, 213 p.

Lozo, F.E., Jr., and Smith, C.I., 1964, Revision of Comanche Cretaceous
Stratigraphic nomenclature, southern Edwards Plateau, southwest Texas:
Gulf Coast Association of Geological Societies Transactions, v. 14, p.
285-306.

Maclay, R.W., and Small, T.A., 1976, Progress report on geology of the
Edwards aquifer, San Antonio area, Texas, and preliminary interpretation
of borehole geophysical and laboratory data on carbonate rocks: U.S.
Geological Survey Open-File Report 76-627, 65 p.

Miggins, D.P., Blome, C.D., and Smith, D.V., 2004, Preliminary
^40^Ar/^39^Ar geochronology of igneous intrusions from Uvalde County,
Texas: Defining a more precise eruption history for the southern
Balcones Volcanic Province: U.S. Geological Survey Open-File Report
2004-1031, 31 p.

Miller, B.C., 1983, Physical stratigraphy and facies analysis, Lower
Cretaceous Maverick basin and Devils River trend, Uvalde and Real
Counties, Texas: University of Texas at Arlington, M.A. thesis, 217 p.

Miller, B.C., 1984, Physical stratigraphy and facies analysis, Lower
Cretaceous, Maverick basin and Devils River trend, Uvalde and Real
Counties, Texas, *in* Smith, C.L., ed., Stratigraphy and structure of
the Maverick basin and Devils River trend, Lower Cretaceous, southwest
Texas\--A field guide and related papers: South Texas Geological
Society, p. 3-33.

Rose, P.R., 1972, Edwards Group, surface and subsurface, Central Texas:
University of Texas at Austin, Bureau of Economic Geology Report of
Investigations, no. 74, 198p.

Small, T.A., and Clark, A.K., 2000, Geologic framework and hydrogeologic
characteristics of the Edwards aquifer outcrop, Medina County, Texas:
U.S. Geological Survey Water-Resources Investigations Report 00-4195, 10
p.

Smith, D. V., Smith, B. D., and Hill, P. L., 2002, Aeromagnetic survey
of Medina and Uvalde Counties, Texas: U.S. Geological Survey Open-file
Report 02-49, 10 p.

Smith, B.D., Smith, D.V., Hill, P.L., and Labson, V.F., 2003, Helicopter
electromagnetic and magnetic survey of the Seco Creek area, Medina and
Uvalde Counties, Texas: U.S. Geological Survey Open-File Report 03-226,
47 p.

Welder, F.A., and Reeves, R.D., 1962, Geology and ground-water resources
of Uvalde County: Texas Water Commission Bulletin 61212, 263 p.

Young, Keith, 1967, Commanche Series (Cretaceous), South-Central Texas,
*in* Hendricks, Leo, ed., Commanchean (Lower Cretaceous) stratigraphy
and paleontology of Texas: Society of Economic Paleontologists and
Mineralogists, Permian Basin Section, Publication no. 67-8, p. 9-29.