M. A. Jordan
Driving on Interstate Highway 35 between Austin and San Antonio, one observes that the land to the east is for the most part relatively flat, whereas the land to the west is more rugged and higher, Austin, San Marcos, San Antonio and other towns along this part of the Interstate Highway 35 are all located at or near this obvious change in topography, which commonly is called an escarpment. Generally speaking, an escarpment is the relatively steep face that separates two regions of markedly different elevation. Along most of the Balcones Escarpment, the land to the west averages about 300 feet higher than the land to the east.
Most escarpments, including the Balcones, are the result of differential erosion, which occurs when there are variations in the ability of exposed rock to resist erosion. The old saying "the rain falls alike on the just and the unjust" can be applied to resistant and non-resistant rocks. Erosional agents, such as rain, attack with essentially equal vigor all over the land along the Balcones Escarpment. But the rocks on the west side are more resistant to erosion, and have been worn down less than the rocks on the east side, with the passage of time and countless rainstorms. So, "differential erosion" describes the result of erosional processes acting on rocks with different properties -- properties which influence the rocks' susceptibility to erosion. Figure 36 is a profile of a hypothetical escarpment, showing locations of resistant and non-resistant rocks.
The face of the escarpment usually looks more scarred by erosion than any other part of the area, because erosive power of water is greater on the steep slope of the face. Gullies and canyons occupied by fast-flowing streams may extend into the high-country side, but the streams normally become more serene as they flow into the low country. The face of an escarpment may be the site of waterfalls if it is a "young" escarpment, or if the contrast in resistance to erosion of the rocks is especially great. The Colorado River exhibits some of these features as it flows across the Balcones Escarpment and through Austin. In the hill country west of town the river flows through a relatively deep, steep-walled valley which has been dammed to make Lake Austin, Lake Travis, and other lakes further upstream. However, for most of its path through Austin the Colorado River flows across a relatively flat surface. The change in the river occurs just as it flows past the Balcones Escarpment, the result of differences in the ability of rocks to resist erosion. Now we shall examine the reasons for the differences in the rocks.
In the Austin area the rocks along the Balcones Escarpment have been assigned, according to their composition, to several units. The resistant rocks to the west belong mostly to either the Glen Rose Limestone or to the Edwards Limestone. Less resistant rocks dominate the area to the east. There one finds rocks of the Austin Group, made up mostly of several soft limestone formations. We will not bother with the names of the individual units of the Austin Group in this discussion, but it is worth noting that for many years, the entire unit has been known informally as the "Austin Chalk" (a thorough examination of the Austin Chalk is given in Chapter 2). Other units exposed east of the Balcones Escarpment include the Del Rio Clay, and the Eagle Ford and Georgetown Formations. All of these are relatively "soft" units. The Buda Limestone, a relatively resistant unit, is also found on the east side of the escarpment, but it is only about 40 feet thick and has a minor influence on the overall topography. For that matter, the non-resistant Walnut Clay occurs with the Edwards Limestone to the west, but is also too thin to have much influence on the topography. In summary, we have at the surface in the Austin area several different types of rock, each with different abilities to resist erosion relative to the other rock units. Figure 37 briefly outlines the erosive properties and reviews what we know about thickness, lithology, sequence of deposition and relative ages of these units.
Relative Resistance to Erosion of the Rock Units Exposed in the Austin Area (M.A. Jordan)
Knowing that the lower units (Glen Rose, Walnut, and Edwards) appear at the ground surface west of the escarpment, and that the upper units are exposed at the surface on the east, we see that the normal stratal sequence has been disrupted. In effect, the earth's crust has broken and the rocks have moved so that rocks of different age or position in the rock column are brought next to each other. This happened by faulting; faults are fractures along which movement occurs or has occurred. Figure 38 shows an idealized fault and how rock units are displaced along it. The Balcones faulting resulted in upward movement of the rocks on the west side of the fault zone.
A Fault - Before and After (M.A. Jordan)
Figure 38 also suggests that faults themselves can produce clifflike topography. In fact "fault scarps" can often be found associated with recent faults, but erosion usually attacks them vigorously and they disappear rapidly. Normally it takes only a few years or decades--"an instant" from a geologist's point of view--to remove or make practically unrecognizable such evidence of faulting. Much of the time erosion can wear down the rocks faster than faults can lift them up. Unless a contrast in resistance to erosion is produced, bringing a "hard" rock to a "soft" one at the surface, a fault scarp will not last long. In the Austin area, hundreds of feet of overlying rock have been removed by erosion since the time of Balcones faulting. The Balcones Escarpment of today is essentially a product of differential erosion, although it probably was expressed as a fault scarp at various times during the actual faulting.
Between the time of faulting and the present, as erosion progressed, there may have been times when there was no escarpment at all, or even times when the topography was reversed. After faulting ceased, the topography depended on which rock units were exposed to erosion. Figure 39 shows how the topography near a fault might change through time. With so much erosion in the Austin area, it is coincidence that the topography along the Balcones Escarpment "agrees" with the fault movement.
Evolution of Topography Along a Fault (M.A. Jordan)
Classification of Fault Types (M.A. Jordan)
With normal faults, the movement is such that the fault plane dips, usually steeply, toward the side of the fault where the rocks have dropped. It is as though the rocks slid down the inclined fault surface, propelled by gravity. Reverse faults are so named because the movement they show is opposite to that of normal faults. With reverse faults, the rocks appear to have been pushed up the fault plane, against the force of gravity. In strike-slip faults the movement is horizontal, similar to the way in which two ships might grind together if they try to pass too closely by each other.
In the Balcones Fault Zone the movement was predominantly along a group of normal faults. The eastern side of the fault zone is the "downthrown side", or it is equally correct to say that the west side is "upthrown". In places along the fault zone where the rock exposures are good, we can measure the amount of movement that occurred. The maximum figure we get is a little over 700 feet. The few reverse faults in the Balcones Fault Zone are the result of minor localized "adjustments" during the faulting. The greatest known movement on a reverse fault in this area is only a few feet.
Balcones faulting occurred in the Miocene Epoch (27 to 12 million years ago), not too far back in geologic time. We can date the age of the faulting because of the presence, a few miles east of Austin, of Miocene-age conglomerates, containing clay, sand and pebbles derived from the initial fault scarp. It probably took several millions of years for all the fault movement to take place. We can be sure of this because, around the world, presently-active faults are not observed to move more than about 30 feet at a time. The movement episode may last only a few seconds--and usually results in an earthquake--but this is followed by a period of relative quiescence which may last for hundreds of years or more. During the quiescent period subterranean forces build up slowly to the level necessary for another episode of fault movement. Erosion acts at an accelerated pace on fault scarps, explaining the origin of the conglomerates by which Balcones faulting was dated. The scarp that existed during the faulting was probably never very high, however.
All the evidence indicates that Balcones faulting has ceased. The Balcones Fault Zone does not threaten us with earthquakes. In fact, no earthquakes have been recorded as originating from here since the instruments have been available with which we might observe them. Austin is located in an "aseismic zone" on the Seismic-Risk map of the United States, which means that there is little or no reason to expect damage from earthquakes.
The main problem associated with faults in the Austin area is that builders must be very careful in their appraisals of construction sites. Faults observed at the surface are a warning that the bedrock in an area may be inhomogeneous. Detailed study of a construction site with faults is needed, because construction plans may have to allow for the variations in the bedrock. If hard rocks turn up unexpectedly where the builder thought there were soft rocks, he may lose money because the cost estimate in his excavation contract was too low. On the other hand, unexpectedly soft rocks can increase foundation costs, because additional precautions against damage by settling will be necessary. The Leaning Tower of Pisa is a famous example of a building with a settling foundation: it is scenic, and a great tourist attraction, but nobody wants a leaning apartment building or a sunken office complex!
The Balcones Fault Zone actually consists of many individual faults. No single fault extends for the whole length of the fault zone. The amount of movement that can be measured on each individual fault varies, depending upon where along the trace of the fault measurements are made. Near the middle part of a fault's length is where we usually find the maximum movement. Going toward either end of the fault trace we typically observe that the amount of movement dwindles down to nothing, and the fault "dies out" into unbroken rocks. However, if we observe adjacent faults, we can see that the movement on them may increase as the movement on the other fault decreases, so that the amount of movement measured across the entire fault zone remains relatively constant. Only as we approach the ends of the entire fault zone do we find that the number of faults, and the total movement on them begins to decrease. Figure 41 shows how two or more faults can share the total of movement across a fault zone, and how one fault can decrease in magnitude as an adjacent fault increases in magnitude. Figure 42 shows the "dying out" of a neighborless fault. Such phenomena are common all along the Balcones Fault Zone. Faults can also split up or join together, as in Figure 43, producing what are commonly known as fault slivers, drag-blocks, or fault slices. Figure 44 shows a drag-block formed by the splitting and rejoining of a fault. A good example of this in the Austin area will be described later.
The Balcones Fault Zone is part of a large fault system. Near the town of Luling, several miles south of Austin, is another fault zone trending roughly parallel to the Balcones Zone, called the Luling Fault Zone. The Luling Fault Zone is made up largely of normal faults which dip toward Austin, and the rocks are downthrown to the northwest. Thus the rocks between Luling and Austin are like a strip which has dropped downward between the two fault zones. This type of structure is called a graben, or a block downdropped between two normal-fault zones (Fig. 45).
Sharing of Displacement of a Fault (M.A. Jordan)
"Dying-out" of a Fault (M.A. Jordan)
Joining (or Splitting) of a Fault (M.A. Jordan)
In turn, the Balcones-Luling Graben System is one of many grabens which have been discovered throughout the margin of the Gulf of Mexico. In most of these faults, the faults trend roughly parallel to the present shoreline of the Gulf. Several of the grabens nearer the Gulf have been partially or completely buried by sedimentation, and were recognized mainly in the course of drilling for oil, or other subsurface exploration.
A Drag-Block (M.A. Jordan)
A Graben (M.A. Jordan)
The Cretaceous rocks of the Austin area formed predominantly as deposits on the floor of shallow seas and associated lagoons. In effect, about 100 million years ago, the Gulf of Mexico covered a much greater area than it now does. Beneath these relatively young deposits lies an assemblage of several kinds of older rocks, some of which were formed as long ago as 1.2 billion years. As it turns out, Austin and much of the Balcones Fault Zone lie almost directly above a "seam" in the pre-Cretaceous rocks, along which some profound changes have been discovered.
To the west of the fault zone in the Austin area, at depths of 100 to 650 meters (300 to 2000 feet), are Precambrian granites and strongly metamorphosed rocks, and Paleozoic rocks, mostly hard limestones.
Somewhere near the fault zone this subsurface assemblage gives way to the deeply buried remains of the Ouachita Mountain belt, produced by late Paleozoic mountain building. In Arkansas and southern Oklahoma, deeply-eroded remnants of the Ouachita Mountains are still exposed. From the study there we know that they contain a large proportion of shale and mudrocks, and similar rocks have been encountered in drill holes just east of Austin. For more information on the earlier history of Austin see Chapter One.
If we could backtrack in time to the late Paleozoic Era, we would see high mountains standing on the present site of Austin. If we could then move forward in time, watching the landscape as we move, we would witness the erosion of the mountains to a relatively flat surface. This would be accompanied by a general subsidence of the area, which allowed the Gulf of Mexico to flood areas formerly occupied by mountains and cover them with Cretaceous deposits. Later on, renewed uplifting and tilting would begin. The area to the west would rise, while we might note sinking to the east. The shoreline of the Gulf of Mexico would recede, mainly because of the filling in by deposition of material eroded from the high areas to the west.
Apparently rocks of the Ouachitas, being softer and flowing more readily under load than the western "basement" rocks, allowed the overlying Cretaceous rocks to settle east of the Balcones Fault Zone while they were relatively firmly supported on the west. The Cretaceous rocks "broke" by faulting at or near the place where Ouachita rocks comprise the "harder" basement rocks of central Texas, as shown in Figure 46. Note that Figure 46 shows eastward dip of the Cretaceous rocks. The actual dip of these rocks is significant, but too slight to show well in a small diagram, so it has been exaggerated.
Cretaceous rocks have moved deeper into the basin with time. Literally, they are "slipping away" from rocks further northwest, being stretched as they move along on the soft flowing rocks beneath them. This stretching in itself could be a significant part of the cause of the faulting we observe. Figure 47 shows now stretching (extension) is associated with the formation of a graben. The stretching that caused the Balcones Fault Zone could continue only as long as the rocks beneath were able to flow. Apparently the flowage has stopped, because the Balcones Fault Zone is no longer active.
Figure 48 shows a structural cross-section through the Balcones Escarpment in Austin. This section shows the relative movements of blocks faulted. The legend for the stratigraphic units is presented in Figure 49. A close-up cross-section of Barton Springs is given in Figure 56.
Cross-sectional Sketch of Subsurface Relations in Austin Area (M.A. Jordan)
Extensional Origin of Graben (M.A. Jordan)
Cross-section Through Balcones Fault Zone (M.A. Jordan)