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by Thomas W. Grimshaw and C. M. Woodruff, Jr.
ABSTRACT
Detailed geologic mapping in the Balcones
fault zone in the San Marcos area has revealed a structural style that may have
had a profound effect on the geomorphologic and hydrologic evolution of the
area. Two major en echelon step-fault zones are present in the area, and a
highly faulted ramp structure has formed in the zone between the en echelon
fault zones.
Differential erosion of rock units in the
ramp structure may have determined the course of a stream that captured the
Blanco River from an easterly flow direction into the Onion Creek basin to its
current southeasterly flow direction. Subsequently, the Blanco may have
"tapped" the Edwards Aquifer by down-cutting or side-cutting action
at or near the San Marcos Springs location. Thus, both the capture of the
Blanco and the current location of San Marcos Springs may have been indirectly
caused by the local structural setting between the two major en echelon fault zones
of the Balcones system.
Similar major ramp structures are apparent by
map inspection in at least three other locations in the Balcones fault zone,
one near Austin and two west of San Antonio. A fourth structure may also be
present near New Braunfels.
Figure 1. Principal features of the San Marcos case study area.
Figure 2. Simplified geologic map of San Marcos area, showing step fault zones and associated faulting in the area between.
Figure 3. Block diagram of en echelon step fault zones, showing intervening ramp structure.
Figure 4 (LARGE IMAGE: 472K). Detail from geologic map of San Marcos area, showing intensity of faulting in the ramp structure area (from Grimshaw, 1976).
INTRODUCTION
The Balcones fault zone is a tensional
structural system consisting of numerous normal faults, cross faults, grabens,
horsts, step faults, en echelon faults, and similar features in central and
south Texas. The fault zone extends from Waco southward to Austin and San
Antonio and then westward to Del Rio. Generally, the rocks exposed at the
surface west of the fault zone are Lower Cretaceous stiatigiaphic units
consisting of resistant limestones, dolomites, and marls; east of the zone, the
rocks exposed are Upper Cretaceous, nonresistant chalk and calcareous clay
units. The difference in resistance to erosion has resulted in a fault-line
scarp known as the Balcones Escarpment. Soils east of the scarp are deep and
well developed, and the predominant historical agricultural land use has been
for cropland. West of the scarp, the soils are thin and rocky, and ranchland is
the predominant agricultural land use.
The Balcones Escarpment and fault zone are
especially well developed in the area around San Marcos, Texas about 35 miles
south of Austin. The purpose of this paper is first to describe the structural
style in a case study area between two major en echelon step fault systems and
then to set forth hypotheses on geomorphologic and groundwater implications of
the zone of adjustment between these en echelon fault systems. The discussion
of structural style and resulting outcrop patterns is based upon detailed
geologic mapping for environmental geologic purposes in two 7-l/2 minute
topographic quadrangles in the rapidly growing San Marcos area (Grimshaw,
1976).
The principal features of the case study
area, besides the city of San Marcos, are Interstate 35, the town of Kyle, the
Blanco River, Purgatory Creek, Sink Creek, San Marcos Springs, and the San
Marcos River (Figure
1).
GEOLOGIC
OVERVIEW
A generalized geologic map of the San Marcos
area is shown in Figure 2. The
principal rock-stratigraphic units in the area. in descending stratigraphic
order. are shown below. All are Cretaceous in age.
|
Unit |
Approximate Thickness (ft) |
|
Taylor Group (clay) |
925 |
|
Austin Group (chalk) |
170 |
|
Eagle Ford Formation (clay) |
25 |
|
Buda Formation (limestone) |
50 |
|
Del Rio Clay |
50 |
|
Georgetown Formation (marl) |
33 |
|
Edwards Group (limestone) |
475 |
Major faults of the Balcones system traverse
the study area from northeast to southwest. The net displacement in the case
study area. as elsewhere in the Balcones fault zone, is downward to the
southeast. The faults of major displacement strike about N30 E. The Edwards
Limestone crops out over most of the area west and north of these faults. and
the Austin chalk and Taylor clay compose the subsurface east and south of the
faults. Outcrops of the thin intervening formations between the Edwards and
Austin occur in numerous fault blocks within the fault zone.
The regionally important Edwards Aquifer is
especially significant in the San Marcos area, both as an essential source of
copious fresh water and as a recreational resource associated with San Marcos
Springs. These springs are a major discharge point of the Edwards Aquifer;
discharge averages about 161 million gallons per day. The extensive outcrop
area of the Edwards limestone in the western and northern portions of the area
is an important part of the aquifer recharge zone.
The Balcones fault zone enters the
northeastern part of the case study area as two major step faults: Mustang
Branch fault and Mountain City fault (Figure 2).
These faults are referred to hereafter as the "northeastern step fault
zone." The Kyle fault, located farther to the southeast. is another step
fault in this succession. Similarly, the Balcones fault zone is represented in
the southwest part of the area by three major step faults -- Comal Springs
fault, San Marcos Springs fault, and Bat Cave fault. referred to hereafter as
the "southwestern step fault zone." The major northeastern and
southwestern step fault zones are thus in en echelon relationship across the
study area.
The fact that fault tracee are not at all
influenced by topography indicates that all faults are vertical or nearly so.
STRUCTURAL
INTERPRETATION
The cumulative displacement across the
northeastern step fault zone generally decreases to the southwest. Similarly,
cumulative displacement across the southwestern step fault zone decreases to
the northeast. Thus, the total displacement downward to the southeast remains
relatively constant across the study area, but is "transferred' from the
northeastern to the southwestern step fault zone as is typical for en echelon
fault zones in a tensional fault system like the Balcones.
The transfer of displacement between the step
fault zones is shown in block diagram form in Figure 3.
The area of adjustment between the step fault zones forms a ramp-like structure
that bends downward to the northeast from the upthrown side of the southwestern
step fault zone to the downthrown side of the northeastern step fault zone. If
this ramp were eroded to a level surface parallel to the bottom of the block in
the diagram, an outcrop pattern with older rock units exposed in the southwest
and successively younger units exposed to the northeast would be displayed.
Just such a pattern is exhibited in the case study area, where Edwards
limestone exposures in the southwest part of the area, on the upthrown side of
the southwestern fault zone, give way northeastward to Austin chalk and Taylor
clay bedrock on the downthrown side of the northeastern step fault zone.
The ramp structure is broken up into several
irregularly shaped grabens. horsts, and step fault blocks that are, in turn,
broken up into numerous small, irregular fault blocks as small as 100 yards (or
less) in dimension, as shown in Figure 4. The overall structural grain of the
faulting in the ramp is consistent with the regional Balcones fault zone, with
the larger-displacement faults having a northeastward orientation, but with the
smaller cross faults oriented in all directions.
The geometry of the larger grabens and horsts,
as well as the much smaller individual fault blocks within them. suggests that
the ramp was subjected to some torsional stresses, in addition to the dominant
tensional stresses, as the southeastern part of the area "dropped
away" from the northwestern part and the ramp area adjusted to the
transfer of displacement from the northeastern to the southwestern step fault
zone.
The resulting overall outcrop pattern, with
the Edwards giving way to the Austin and Pecan Gap northeastward along the
ramp, and with the complex, irregular faulting between the en echelon step
fault zones, is shown clearly in Figure 2.
The intervening units between the Edwards limestone and the Austin chalk are
exposed in the complexly faulted area extending generally southward from the
northeastern step fault zone to the southwestern step fault zone. The outcrop
pattern on a larger scale and in more detail is a mosiac of irregular fault
block outcrops having an appearance not unlike a shattered pane of glass (Figure 2). The complex faulting of the ramp into irregular
grabens and horsts at the smaller scale (Figure 3),
and the small irregular fault blocks at the larger scale (Figure 4) represent the adjustment of the ramp area to the
tensional and lesser torsional stresses during the Balcones faulting.
The intensity of faulting depicted in Figure 4 is most clearly displayed in the band of outcrops of
intervening stratgraphic units between the Edwards limestone and the Austin
chalk in the graben and horst area between the en echelon step fault zones. It
is likely that this intensity of faulting also exists in the areas of exposure
where the Edwards, Austin, and Taylor Groups are exposed, but the lithologic
homogeneity of these units does not allow the individual fault blocks to be
mapped in such detail. It is only where the succession of thin, lithologically
dissimilar units represented by the Georgetown (marl), Del Rio (clay), Buda
(limestone), and Eagle Ford (clay with thin siltstone layers) is affected by
the intense faulting that sufficient stratigraphic control allows the very
small individual fault blocks to be mapped.
Along the northeastern section of the
southwestern step fault zone, there is a major, high-standing fault block,
herein named the San Marcos horst, which is undisturbed by the intricate
faulting that characterizes most of rest of the area northwest of the step
faults. The cause of this large. undisturbed, monolithic fault block remains
problematical. The Balcones Escarpment is especially well developed along the
eastern margin of this horst; the Old Main Building of Southwest Texas state
University is built on this prominent scarp and is a striking local landmark
easily seen by travelers on nearby Interstate 35. Much of the western half of
San Marcos is built on this large fault block, and San Marcos Springs
discharges along its northeastern margin.
POTENTIAL
GEOMORPHOLOGIC IMPLICATIONS: A HYPOTHESIS
Woodruff (1977), in a discussion of
development of drainage patterns near the Balcones Escarpment, has shown that
the Blanco River formerly discharged into what is now the Onion Creek drainage
basin. The "elbow" in the course of the Blanco River in the northern
part of the case study area is near the point at which a headward-eroding
smaller stream captured the Blanco and diverted its flow from generally
eastward to a southeastward direction. The point designated "A" in
Figure 1 is the location of a distinctive erosional feature interpreted to be a
former channel of the pre-capture course of the Blanco. This feature is now
located on the drainage divide between the Blanco River and Onion Creek
drainage basins.
Not long after Balcones faulting occurred
(currently believed to be during the Miocene), the stratigraphic units exposed
in the vicinity of the en echelon faults and associated ramp structure in the
study area were Upper Cretaceous or younger. It may reasonably be expected that
as erosion occurred and the land surface lowered in the area, the more
resistant Austin chalk was exposed in the southwestern part of the ramp
structure while the less resistant Taylor clay was still present in the
northeastern part of the structure. Further, because of marked difference in
erodibility of these units, it may be expected that a small east-facing
escarpment would have formed along the ramp.
The hypothesis or question then arises:
"Could this small escarpment then have determined the course of the stream
that ultimately captured the Blanco River?" If the answer is in the
affirmative, then the physiographic development in the area was controlled by
the presence of the en echelon faulting and associated ramp, and the present
southeasterly course of the Blanco was ultimately determined by the presence of
the ramp.
POTENTIAL
HYDROLOGIC IMPLICATIONS: ANOTHER HYPOTHESIS
Woodruff and Abbott (1979) have hypothesized
that the current principal discharge points of the Edwards Aquifer, such as San
Marcos Springs and Comal Springs, have developed at or near the locations where
major streams and rivers traverse the Balcones Escarpment. In effect, these
locations represent the lowest elevation points where the Edwards limestone is
exposed. There, the actively downwardly eroding rivers have opened drains for
the aquifer.
After the Blanco River was captured into its
current southeastward course, its grade was increased and the currently visible
deeply incised canyon where the river crosses the Edwards limestone was formed.
This canyon opens to a wider valley containing alluvium at the point where the
river crosses a fault and begins to flow on outcrops of younger strata,
primarily the Austin chalk (Figure 2).
The valley widens again, this time to a much greater extent. at the point
farther downstream where the river begins to flow on the Taylor clay.
Although the river is still downcutting after
it passes out of the Edwards, it is also effectively sidecutting as evidenced
by the presence of alluvium in the valley in this stretch. The continued
downcutting is indicated by the presence of bedrock exposures in the river
channel. The thickness of the alluvium and the presence of a single sequence of
fining-upward grain-size pattern in the alluvium indicate that the river is
removing and redepositing its alluvial deposits as it shifts course in its
valley.
The question naturally arises: "Was the
Blanco River responsible for cutting a drain into the Edwards aquifer at the
current location of San Marcos Springs?" The current locations of the
river, the springs, and Sink Creek indicate that, if the Blanco did open this
discharge point of the aquifer by downcutting or sidecutting at the location of
the springs, it occurred many years ago when the land surface and the river
were at somewhat higher elevation than at present.
If the hypotheses outlined above and in the
previous section are correct, then the current location of San Marcos Springs
was determined by the presence of the Blanco River, whose location was in turn
determined by the northeastern and southwestern en echelon step faults and the
associated ramp structure. The role of the San Marcos horst in this sequence of
events awaits further study.
POSSIBLE
SIMILAR STRUCTURAL PATTERNS ELSEWHERE IN THE BALCONES FAULT ZONE
Inspection of maps of the Geologic Atlas of
Texas published by the University of Texas Bureau of Economic Geology (Barres,
1974a, 1974b, 1974c) suggests that similar structural patterns exist elsewhere
in the Balcones fault zone. For example, a similar ramp structure is indicated
in the Austin area, where outcrops of Edwards limestone in an en echelon step
fault setting give way northeastward to Austin chalk outcrops. A band of
outcrops of Georgetown, Del Rio, Buda, and Eagle Ford Formations extends
southward from the Colorado River to the town of Buda. This band separates
Edwards outcrops to the southwest from Austin chalk outcrops to the northeast,
just as in the San Marcos area.
A smaller scale but similar pattern may also
exist near New Braunfels, where outcrops of Edwards limestone give way
northeastward to Austin chalk outcrops in an apparent ramp structure, with a
transition zone of Del Rio, Buda, and Eagle Ford Formation outcrops between.
Southwest of New Braunfels, the en echelon
faulting in the Balcones fault zone reverses, with major displacement shifting
from southeastward fault zones to northwestward fault zones. Ramp structures in
this area are thus also reversed, with bending downward to the southwest rather
than to the northeast as in the San Marcos area. One such "reversed"
ramp structure, where Edwards outcrops give way southwestward to Austin chalk
outcrops, and with intervening exposures of Georgetown through Eagle Ford fault
blocks, is apparent immediately northwest of San Antonio. Another, larger one
appears to be present across most of Media County and far eastern Uvalde
County.
SUMMARY
Detailed geologic mapping of intensely
faulted Cretaceous strata in the Balcones fault zone in the San Marcos area has
revealed a structural style and bedrock geometry that have potential
implications for the geomorphologic and hydrologic development of the area. The
current location of the Blanco River and San Marcos Springs may have resulted
from a sequence of events whose course was determined by this structural style
and the relative resistance of the Cretaceous units to erosion. The presence of
the two en echelon step fault zones and associated ramp structure may have been
the ultimate cause of both the current, captured course of the Blanco River and
subsequently the location of San Marcos Springs.
Similar en echelon structural patterns with
ramps are indicated in at least three other locations in the Balcones fault
zone.
REFERENCES
Barnes, V. E., 1974a, Austin sheet: The University of Texas at Austin, Bureau of Economic Geology, Geologic Atlas of Texas, scale 1:250.000.
Barnes, V. E., 1974b, San Antonio sheet: The University of Texas at Austin, Bureau of Economic Geology, Geologic Atlas of Texas. scale 1:250,000.
Barnes, V. E., 1974c, Seguin sheet: The University of Texas at Austin, Bureau of Economic Geology, Geologic Atlas of Texas, scale 1:250.000.
Grimshaw, T. W., 1976, Environmental Geology of Urban and Urbanizing Areas: A Case Study From the San Marcos Area, Texas: The University of Texas at Austin. Ph.D. dissertation, 244 p.
Woodruff, C. M., Jr., 1977, Stream piracy near the Balcones fault zone, central Texas: Journal Of Geology, v. 85, p. 483-490.
Woodruff, C. M., Jr., and P. L. Abbott, 1979, Drainage-basin evolution and aquifer development in a karstic limestone terrain, south-central Texas, U.S.A.: Earth Surface Processes. v. 4, p. 319-334.
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Abbott, Patrick I.. and Woodruff, C. M., Jr., eds., 1986, The Balcones Escarpment, Central Texas: Geological Society of America, p. 71-76
