Dr. Grimshaw’s interest in the MCQ and SMNQ goes back to the 1970’s, when he studied the area for his Ph.D. dissertation. When he returned to UT after completing his M.A. in Geology and a tour of duty in the U.S. Army, the environmental movement was rapidly accelerating tin the U.S. and worldwide. The 1970s are often referred to as the “decade of the environment” because many of the major new laws for environmental protection were enacted, including the National Environmental Policy Act (enacted in 1969 and implemented on January 1, 1970) and the clean air act, clean water act, solid waste act and hazardous waste act.
Dr. Grimshaw chose a career in environmental geology, which he began with Dr. Keith Young as his supervisor. During this timeframe Dr. Young published his book “Geology: The Paradox of Earth and Man”. Dr. Grimshaw’s dissertation utilized the San Marcos area as a case study in the rapidly-growing Austin-to-San-Antonio growth corridor. The dissertation has two major components – detailed geologic mapping followed by an environmental analysis that focused on land capability to support various human uses of the land. The area covered is the southern 1/3 of the Mountain City Quadrangle, all of the San Marcos North Quadrangle, and the upper 2/3 of the San Marcos South Quadrangle.
The original 1976 dissertation was a single volume with a pocket for 10 folded plates. In 2013 Dr. Grimshaw upgraded the document by utilizing word processing, replacing the photos with color versions, and dividing it into two volumes. Both volumes were also converted to PDF format for easy access. Volume 1 includes the text, photos and diagrams. The 10 plates in the pocket of the original dissertation are in Volume 2. Also in 2013, Dr. Mark Helper, faculty member of UT’s Department of Geological Sciences, worked with Dr. Grimshaw to digitize the dissertation geologic map (Plates 9 and 10). This map, “Geologic Map of the San Marcos North Quadrangle and Adjacent Portions of the Mountain City and San Marcos South Quadrangles, Hays, Caldwell and Guadalupe Counties, Texas“, accompanies Volumes 1 and 2 of the 2013 version of the dissertation,
Several components of the dissertation are extracted from Volume 1 for presentation on this webpage, including the original frontispiece, table of contents, abstract, summary and conclusions, references cited, and appendix.
Frontispiece: The Balcones Escarpment between Austin and San Antonio
The city of San Marcos can be clearly seen just south of the center of the case study area (outlined in white). Note the offset in the scarp in the northern half of the area. This Landsat (ERTS) Band 5 image was made on July 21, 1973.
Volume 1. Table of Contents
Volume 1 Table of Contents
Preface to Digital Version 7
Original Preface 9
Frontispiece. The Balcones Escarpment between Austin and San Antonio 11
Abstract 13
List of Tables 19
List of Illustrations 21
Plates (In Volume 2) 25
1 Chapter 1. Introduction 27
2 Chapter 2. A Methodology for Environmental Geologic Investigation of Urban and Urbanizing Areas 29
2.1 Components of Geologic Environments and Urban Systems 29
2.1.1 Components of the Geologic Environment 29
2.1.1.1 Substrate 30
2.1.1.2 Processes 31
2.1.1.3 Landform 31
2.1.2 The Urban System 31
2.1.2.1 Urban Situs 31
2.1.2.2 Urban Input 33
2.1.2.3 Urban Output 33
2.1.2.4 Transportation 34
2.1.3 Interaction of the Urban System with Its Geologic Environment 35
2.2 Generation of Data Sources 35
2.2.1 Definition of the Study Area 36
2.2.2 Natural Data Sources 37
2.2.2.1 Engineering Geology Map 37
2.2.2.2 Soils Map 37
2.2.2.3 Resources Map 38
2.2.2.4 Processes Map 38
2.2.2.5 Landform Map 38
2.2.2.6 Derivation of Natural Data Sources 39
2.2.3 Cultural Data Sources 40
2.2.3.1 Current Land Use Map 40
2.2.3.2 Land Use Control Map 40
2.2.3.3 Derivation of Cultural Data Sources 40
2.2.4 Summary and Discussion 41
2.3 Environmental Geologic Problems of Existing Urban Systems 41
2.3.1 Urban Situs 42
2.3.1.1 Impact of the Environment on the City 42
2.3.1.2 Impact of the City on the Environment 43
2.3.2 Urban Input 43
2.3.3 Urban Output 43
2.3.4 Transportation 43
2.3.5 Summary and Discussion 44
2.4 Environmental Geology in Physical Land Use Planning for Urban Growth 44
2.4.1 Selection of Land Use 45
2.4.2 Construction of the Blank Suitability Score Grid 45
2.4.3 Screening Procedure 46
2.4.4 Evaluation Procedure 46
2.4.4.1 Step 1. Conceptualize the Overall Objectives 47
2.4.4.2 Step 2. Construct the Demand Analysis Hierarchy 47
2.4.4.3 Step 3. Select the Physical Performance Measures 49
2.4.4.4 Step 4. Formulate the Suitability Score Functions 49
2.4.4.5 Step 5. Assign Weights to the Demand Analysis Hierarchy 50
2.4.4.6 Step 6. Adjust the Weights 51
2.4.4.7 Step 7. Prepare the Suitability Score Grids 52
2.4.4.8 Step 8. Calculate the Suitability Index Map 54
2.4.4.9 Automation of the Evaluation 54
2.4.5 Verification Procedure 56
2.4.6 Formulation of a Physical Land Use Plan 59
2.4.7 Discussion 60
2.4.8 Definitions 61
2.5 Summary of Methodology 62
3 Chapter 3. Description of the San Marcos Case Study Area and Delineation of
Natural and Cultural Data Sources 65
3.1 Description of the Study Area 65
3.1.1 Location and Boundaries 65
3.1.2 Physical Geography 66
3.1.2.1 Major Geographic Features 66
3.1.2.2 Climate 68
3.1.2.3 Soils 69
3.1.2.4 Vegetation 69
3.1.3 History 71
3.1.4 Population and Government 72
3.1.4.1 Population Statistics 73
3.1.4.2 Governmental Entities 73
3.1.5 Economy 75
3.1.5.1 Colleges and Universities 75
3.1.5.2 Federal Government 76
3.1.5.3 Manufacturing, Agriculture, and Amusements and Lodging 76
3.1.5.4 Elementary and Secondary Schools 77
3.1.5.5 Construction and Mining 77
3.1.5.6 Summary 77
3.1.6 Historical Influence of Geology on Land Use 77
3.1.7 Suitability of the San Marcos Area as a Case Study 78
3.2 Natural Data Sources 79
3.2.1 Engineering Geology Map 79
3.2.2 Soils Map 81
3.2.3 Resources Map 83
3.2.3.1 Water Resources 83
3.2.3.2 Aggregates Resources 84
3.2.3.3 Energy Resources 85
3.2.4 Processes Map 86
3.2.4.1 Fluvial Processes 86
3.2.4.2 Karst Processes 86
3.2.4.3 Mass Movement Processes 89
3.2.4.4 Shrink-Swell Processes 90
3.2.5 Landform Map 90
3.3 Cultural Data Sources 91
3.3.1 Current Land Use Map 91
3.3.2 Land Use Control Map 92
4 Chapter 4. Environmental Geology of Existing Urban Systems In the San Marcos Area 95
4.1 Urban Systems in the San Marcos Area 95
4.1.1 The City of San Marcos 95
4.1.2 The Town of Kyle 95
4.1.3 Outlying Housing Developments and Mobile Home Parks 98
4.2 Environmental Geologic Problems of Existing Urban Systems 99
4.2.1 Urban Situs 101
4.2.1.1 Impact of the Geologic Environment on Urbanization 101
4.2.1.2 Impact of Urbanization on the Geologic Environment 106
4.2.2 Urban Input 116
4.2.2.1 Aggregates Input 116
4.2.2.2 Water Input 124
4.2.2.3 Energy Input 124
4.2.3 Urban Output 134
4.2.3.1 Solid Wastes 134
4.2.3.2 Liquid Wastes 144
4.2.4 Transportation 151
4.2.4.1 Roads 151
4.2.4.2 Railroads 155
4.2.4.3 Airports 156
4.2.4.4 Pipelines 156
4.2.4.5 Power Lines 159
4.2.5 Summary 161
5 Chapter 5. Environmental Geology in Physical Land Use Planning for Urban Growth in the San Marcos Area 163
5.1 Urban Growth in the San Marcos Area 163
5.2 Land Suitability Analysis for a Sanitary Landfill 164
5.2.1 Objectives and Restrictions 164
5.2.2 Construction of the Blank Score Grid 165
5.2.3 Screening Procedure 165
5.2.4 Evaluation Procedure 167
5.2.4.1 Step 1. Conceptualization of the Overall Objectives 168
5.2.4.2 Step 2. Construction of the Demand Analysis Hierarchy 168
5.2.4.3 Step 3. Selection of the Physical Performance Measures 169
5.2.4.4 Step 4. Formulation of the Suitability Score Functions 171
5.2.4.5 Step 5. Assignment of Weights to the Demand Analysis Hierarchy 171
5.2.4.6 Step 6. Adjustment of the Weights 171
5.2.4.7 Step 7. Preparation of the Suitability Score Grids 174
5.2.4.8 Step 8. Calculation of the Suitability Index Map 177
5.2.4.9 Discussion of Results 177
5.2.5 Verification Procedure 183
5.3 Toward a Physical Land Use Plan for the San Marcos Area 183
6 Chapter 6. Summary and Conclusions 185
Appendix. Geology of the San Marcos Area, Texas 187
A.1 The Geologic Context: Geologic Elements of Central Texas 187
A.2 Stratigraphy 188
A.2.1 Glen Rose Formation 188
A.2.2 Walnut Formation 189
A.2.3 Edwards Group 189
A.2.4 Georgetown Formation 190
A.2.5 Del Rio Clay 190
A.2.6 Buda Formation 190
A.2.7 Eagle Ford Formation 190
A.2.8 Austin Group 191
A.2.9 Taylor Group 191
A.2.10 Corsicana Formation 191
A.3 Structural Geology 191
A.4 Geomorphology 194
A.4.1 Fluvial Processes 194
A.4.2 Karst Processes 196
A.4.3 Mass Movement 197
A.5 Summary 198
References Cited 199
Vita 207
Abstract
A major focus of environmental geologic work for the foreseeable future will be on the growing urban areas of the world. A systematic methodology for investigating the environmental geology of such areas can be developed by dividing the geologic environment into three major components (substrate, processes, and landform) and the urban system into four major components (situs, input, output, and transportation). These conceptual tools can then be used to develop a three-step procedure: 1) generation of the data source maps; 2) investigation of environmental geologic conflicts of existing urban systems; and 3) incorporation of geologic considerations in planning for future growth.
The data sources should be prepared in map form and should comprise two types of data, natural and cultural data. A conventional geologic map is the primary document used in the derivation of the five natural data source maps: 1) engineering geology; 2) soils; 3) resources; 4) processes; and 5) landform. The necessary cultural data source maps are a current land use and a land use control map.
In the investigation of environmental geologic problems of existing urbanization the urban system concept provides an excellent means of organizing the various facilities and activities of a city. If this scheme is used, the facilities can be systematically considered and their associated conflicts disclosed.
Environmental conflicts of future urbanization can be prevented by evaluating the ability of land to sustain the demands that will be imposed by projected urban facilities. The urbanization can then be fitted to the land by siting the facilities where the land can best meet their demands. The procedure for determining land suitability for an urban facility consists of three steps: 1) screening; 2) evaluation; and 3) verification. Areas that are totally unsuited for the facility are eliminated in the screening step. The remaining candidate areas are rated for their relative suitability in the evaluation step, which consists of an eight-step algorithm developed by adaptation of a formal decision-making technique. The results are confirmed in the verification step, and the final product, the suitability index map, should serve as the primary basis for determining where the facility will be located.
The San Marcos case study area comprises two 7-1/2 minute quadrangles in south-central Texas between Austin and San Antonio. The area is almost ideal for testing a methodology for environmental geologic investigation of growing urban areas. The methodology developed in this study was shown to be highly effective when applied to this area.
In its present state of development the methodology appears to be a conceptually sound and effective approach to optimizing the interaction between cities and their geologic environments. Future work should concentrate on adaptation of key sections for automation and on the application of the three-step procedure to a variety of different kinds of cities and geologic environments in order to refine and test it.
Summary and Conclusions
The procedure of this environmental geologic study has been to focus on growing urban areas and to delineate the components of urban systems and their geologic environments. These components were used as conceptual tools for development of a systematic three-part methodology for conducting environmental geologic investigation of growing urban areas. This methodology was then applied to a case study area around San Marcos in the Interstate 35 growth corridor of central Texas.
Recognition of the two conceptually distinct parts of the urban-environment interaction – the urban system and the geologic environment – makes the problem of environmental geologic analysis more manageable. The classification of the geologic environment into three broad categories (substrate, processes, and landform) appears to be conceptually sound and sufficiently comprehensive to be applicable to most of the inhabitable parts of the world. Also, the urban system organizational scheme, which uses the four categories (situs, input, output, and transportation) recognized here, apparently accounts for the urban facilities and activities having the most environmental geologic significance. The organization of the procedure of the methodology into three parts – delineation of the data sources followed by a curative and then a preventive procedure – appears to be a rational and rigorous approach to the analysis of the interaction between cities and their geologic environments.
In the derivation of the natural data source maps for the San Marcos case study area, the substrate-processes-landform scheme proved to be highly effective in organizing the geologic and geology-related phenomena having significance for urbanization. The curative part of the methodology has also apparently accounted for all the important environmental geologic conflicts in the area. The preventive part of the procedure was quite effective in determining land capability for the one projected urban land use, a sanitary landfill, for which the analysis was run. The application of this part of the methodology to the San Marcos area clearly demonstrated the advantages of this rigorous analysis. In the first place the procedure goes a step beyond the preparation of the data source maps and converts the information into an indication of land suitability for specific uses. Second, the procedure is based on an established decision-making technique, so the results (the Suitability Index Maps) are highly defensible. Finally, the results are easily understood by nontechnical people who are likely to be making land use decisions.
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Appendix. Geology of the San Marcos Area, Texas
The primary aim of this appendix is to present a geologic report to accompany the geologic maps of the San Marcos area (Plates 9 and 10). The procedure will be first to describe the regional geologic context and then to cover in turn the three major aspects of the geology of the area: stratigraphy, structural geology, and geomorphology. This report has been prepared in a very condensed form, and only the major points are covered in each topic. The geologic mapping for Plates 9 and 10 is original to this study, but most of the information in this text is derived from previous studies. These sources are too numerous to cite individually, but local studies by DeCook (1956, 1960,1963), Davis (1962), and Noyes (1957) have been relied upon heavily.
The geologic mapping was done on Agricultural Stabilization and Conservation Service (U.S. Department of Agriculture) aerial photographs that have a scale of 1:20,000. All mapping was accomplished in the field because frequent spot checking of outcrops was necessary to work out the complex fault patterns. The map information was transferred to topographic base maps by use of a mirror-type desk projector (Reed Research Model 635B). As noted in the text, U.S. Geological Survey 7-1/2 minute topographic quadrangles (scale 1:24,000) were used as base maps. The study area includes the southern one-third of the Mountain City quadrangle, all of the San Marcos North quadrangle, and the northern two-thirds of the San Marcos South quadrangle. Available subsurface data on file at the Texas Water Development Board were also examined. Most of these data are in the form of water well logs prepared by water well drillers having little training in geology and proved to be of only limited value. DeCook (1960, 1963) made the best possible interpretations of these data, so his studies are used extensively in this-report. Logs of wells drilled since these studies were published were also utilized to some extent.
A.1 The Geologic Context: Geologic Elements of Central Texas
The basement rocks of central Texas comprise two primary elements – the Texas Craton and the Oauchita foldbelt. The Texas Craton, a northwest elongated mass of Precambrian, mostly granitic rock, is the most fundamental basement element in Texas. It is bounded on the southeast by the strongly deformed and metamorphosed Paleozoic rocks of the Ouachita foldbelt. The study area lies over the Ouachita foldbelt just southeast of the boundary with the Texas Craton. Unconformably overlying this basement is a southeast dipping wedge of Cretaceous sedimentary rocks. This wedge, which thins to the northwest, is approximately 750 meters thick in the San Marcos area. The strata of this wedge are mostly carbonate rocks and fine-grained terrigenous rocks.
The Cretaceous wedge is intensely faulted along the Balcones fault zone, which extends in an arcuate band from Del Rio to north of Waco. The location of the fault zone closely follows the line of crustal weakness along the boundary between the Ouachita foldbelt and the Texas Craton. Displacement across the zone is downward to the southeast, and the zone is in a graben-like relation to the Luling-Mexia fault system, which is located further toward the Gulf Coast. The regional dip across this hinge-line fault zone increases from about 1.9 to 5.7 meters per kilometer northwest of the zone to about 9.5 to 18.9 meters per kilometer southeast of the zone. The displacement across the zone is maximum between Austin and San Antonio. Most of the faulting is believed to have occurred during the Miocene, and no well documented instances of fault movement have been recorded in historic times.
The topographic expression of the fault zone is well developed in the San Marcos area. The faulting of the soft, clayey Upper Cretaceous strata downward against the more resistant carbonate units of the Lower Cretaceous has resulted in the development of the east-facing Balcones Escarpment. The elevation across this fault-line scarp about doubles from about 150 to about 300 meters.
Uplifting of the west side of the Balcones fault zone has caused faster erosion there, and the result has been the exposure of generally older rocks there than east of the zone. However, the rate of erosion west of the scarp had been slowed considerably by the resistant Edwards Group carbonate rocks, thus giving rise to a relatively flat, elevated surface known as the Edwards Plateau.
A.2 Stratigraphy
Because of the emphasis of this study on the surface and near-surface bedrock, only rock units that crop out will be described. As shown on the geologic maps (Plates 9 and 10), the bedrock in the San Marcos area comprises mostly Cretaceous sedimentary rocks, and most of the Texas Cretaceous stratigraphic section is exposed in the area. Several gravel deposits of Quaternary age are also present in the area, but their discussion is deferred to the geomorphology section of this appendix.
Most of the Texas Cretaceous strata were deposited in shallow marine environments on a broad shelf that extended inland from about the middle of the present Gulf Coastal Plain. The dominant lithologies of the strata are limestones, dolomites, and marls. Most of the upper part of the section is composed of terrigenous clays. Highly uniform lithotopes over large areas of the shelf give rise to rock-stratigraphic units which may change materially in thickness and composition regionally but are relatively uniform in areas as large as a 15-minute quadrangle. Some of the rock-stratifigraphic units thin southward across the San Marcos area, owing to the influence of a subtle Cretaceous positive tectonic element, the San Marcos Platform, between New Braunfels and San Antonio. None of the units, however, show appreciable lithologic changes across the area. Each of the stratigraphic units cropping out in the area are described very generally in ascending order in the following paragraphs. For detailed descriptions and measured sections of these units, the reader is referred to DeCook, 1956, Noyes, 1957, DeCook, 1960, Davis, 1962, or DeCook, 1963.
A.2.1 Glen Rose Formation
The oldest rock-stratigraphic unit exposed in the San Marcos area is the Glen Rose Formation. It crops out in the northwest corner of area in the canyon of the Blanco River, where the upper 12 to 15 meters are exposed. The strata consist of finely crystalline dolomite interbedded with dolomitic limestone and dolomitic marl. Individual beds range from 0.6 meters to several meters in thickness (Davis, 1962). Because of its small area of occurrence, this formation does not have great significance for this study.
A.2.2 Walnut Formation
The next formation in ascending order is the Walnut, which was distinguished by Davis (1962) but was included by Rose (1972) in the overlying Kainer Formation. The Walnut crops out on the bluffs of the Blanco canyon in the northwest part of the area. Davis (1962) recognizes the Bull Creek and Bee Cave Members in ascending order. The Bull Creek is 12.0 meters thick and is mostly a massive limestone and dolomite. The Bee Cave is 2.5 meters thick and is a nodular marl. Like the Glen Rose, the Walnut Formation is not particularly significant to this study, and it is included with the overlying Kainer Formation on the geologic map.
A.2.3 Edwards Group
The Edwards Group is the next rock-stratigraphic unit. Although this unit has been elevated in rank to a group comprising the Kainer (below) and the Person (above) Formations (Rose, 1972), it will be described here as a group primarily because both formations serve as part of the important Edwards aquifer. The Edwards is a nearly pure carbonate unit with beds of hard limestone, dolomite, and all gradations between these two lithologies. Isopachous maps by Rose (1972) indicate a thickness of about 100 meters for the Kainer and 43 to 49 meters for the Person in the San Marcos area. The two formations are lithologically similar and are difficult to distinguish unless the distinctive marker bed (the Regional Dense Member) at the base of the Person crops out. For this reason, and because of the intense faulting, no section has yet been measured in the area. The Kainer and Person have been mapped separately on the geologic map where it was possible to distinguish between them. Elsewhere, the Edwards is mapped as undifferentiated.
The Edwards is one of the most important rock units in the area. It underlies most of the area west of the Balcones Escarpment and therefore determines the physical properties of the substrate in that area. More importantly, this unit is the aquifer which supplies the significant quantities of ground water in the area and provides the flow from San Marcos Springs. The outcrop area west of the escarpment, because it provides much of the recharge to this aquifer, should be subjected to rather tight land use restrictions. The Edwards is different both in lithology and in the chemical quality of its contained water on either side of the “bad-water line” shown on the Resources map (Plate 3). This line approximately demarks the downdip limit of freshwater circulation. West of the line the water is potable (although somewhat mineralized), and the rock has been greatly altered by the rapidly circulating water. East of the line the water is highly mineralized and is charged with hydrogen sulfide. The rock there has not been subjected to the solution, collapse, and recrystallization effects that are characteristic of the aquifer part of the formation, and it is more typical of a deeply buried petroliferous limestone. Two oilfields a few miles southeast of San Marcos have produced a combined total of about 270 million barrels of oil from the Edwards.
A.2.4 Georgetown Formation
The Edwards Group is overlain by the Georgetown Formation, which is from 9 to 12 meters thick in the San Marcos area. The contact between the Edwards and Georgetown is sharp and distinctive and is probably disconformable. DeCook (1956), in a description of a measured section around Sink Creek just north of San Marcos, indicates that this formation includes beds of shale, marl, argillaceous limestone, and limestone. The Georgetown crops out in several fault blocks in the intensely faulted zone along the Balcones Escarpment, but does not include large outcrop areas.
A.2.5 Del Rio Clay
The Del Rio is about 15 meters thick and is composed of a clay-shale almost uniformly through this thickness. In the unweathered state these clays are composed dominantly of kaolinite and illite with a small admixture of mixed layer illite-montmorillonite. In the weathered zone, however, the illite and mixed layer clay are converted into highly plastic montmorillonite. The contact of the Del Rio with the underlying Georgetown is usually obscured by mass movement of these clays. The formation is characterized by gypsum veinlets (at the outcrop) and by abundant specimens of the distinctive small oyster, Ilmatogyra arietina. A thin (about 0.3 meter) bed of Ilmatogyra lumachelle occurs at about the middle of the formation. Like the Georgetown, the Del Rio crops out in irregular polygonal fault blocks in the vicinity of the Balcones Escarpment. Despite the small total area of outcrop, this formation has considerable implications for this study, as noted in the text.
A.2.6 Buda Formation
The next formation in ascending order is the Buda, which is a relatively hard, nodular limestone in the lower part and a hard, resistant, thick-bedded limestone in the upper part. It is about 15 meters thick in the San Marcos area. The Buda, like the underlying Del Rio and Georgetown, occurs chiefly in fault blocks in the intensely faulted zone along the escarpment. The hard limestones of this formation often form a resistant cap on hills that are flanked by the less resistant Del Rio Clay. The contact between the Del Rio and Buda is usually obscured by slump failure of the Del Rio out from under the Buda.
A.2.7 Eagle Ford Formation
The Eagle Ford Formation, which is about 7.5 meters thick, overlies the Buda Formation. The Eagle Ford has three distinct parts – a lower bentonitic shale about 2.1 meters thick, a middle calcareous, flaggy sandstone or siltstone unit that is about 1.2 meters thick, and an upper shale having a thickness of about 4.2 meters. The lower contact between the lower shale and the upper Buda limestones is sharp and distinct. The contact with the overlying Austin Group can seldom be seen because of slumpage of the upper shale out from under the more competent beds of the Austin. The Eagle Ford, like the subjacent formations above the Edwards, occurs in fault blocks along the Balcones Escarpment, and it also occurs more prominently in the bluffs of the Blanco River in the eastern half of the San Marcos area. Despite the competent flaggy beds in the middle of the Eagle Ford, this formation is considered a clay unit for the purposes of this study.
A.2.8 Austin Group
The Austin Group has been elevated in recent years from formation to group status and has been subdivided into several formations. However, it has been mapped in this study as a single unit because the lithologic differences between the various formations are not in general great enough to be highly significant for environmental geologic purposes. DeCook (1963) reports a thickness of about 49 to 55 meters in the vicinity of San Marcos, but the upper contact is not exposed in the area. The lithology is chiefly an argillaceous or chalky limestone.
The Austin underlies a large area in the eastern half of the Kyle section of the study area on both sides of the Blanco River. It also crops out in a step fault block between the San Marcos Springs and Comal Springs faults southwest of San Marcos. Interestingly, the Austin also occurs in a fault block in the western part of the Kyle section not far from the highest elevation in the area. Because of its large area of outcrop in the northern half of the area, the Austin is very significant to this study.
A.2.9 Taylor Group
The Taylor Group, which is the next rock-stratigraphic unit in ascending order, has a thickness of about 90 meters in the San Marcos area. Like the Austin Group, the Taylor has in recent years been elevated to group status and has been divided into three formations (Young, 1965). These formations are the Sprinkle (lower), Pecan Gap (middle), and Bergstrom (upper). All three formations are composed of smectitic mudstone and are distinguished primarily on the basis of calcium carbonate content; the Pecan Gap is more calcareous and more resistant to erosion than the underlying Sprinkle or the overlying Bergstrom Formation.
The formations of the Taylor Group crop out extensively in the area east of the Balcones Escarpment. The Sprinkle was recognized in only one location southwest of San Marcos, but the Pecan Gap underlies almost all of the rest of the area east of the scarp. The Bergstrom is in contact with the Pecan Gap along an inferred northeast-oriented Balcones fault and occurs in the southeast corner of the area. Because of its wide areas of occurrence and poor engineering properties, the Taylor Group as a whole is very significant to this study.
A.2.10 Corsicana Formation
The youngest Cretaceous formation in the San Marcos area is the Corsicana Formation of the Navarro Group. Outcrops of this formation were observed in the far southeast corner of the area where the formation apparently overlies the Bergstrom in normal stratigraphic contact. A regional dip of about 19 meters per kilometer was assumed when this contact was drawn.
A.3 Structural Geology
The San Marcos area lies over the Balcones Escarpment in an intensely faulted part of the Balcones fault zone. This zone, as noted earlier, is a system of mostly normal faults having a net displacement downward to the east and southeast. At first glance, the part of the geologic map northwest of the escarpment (Plates 9 and 10) has the appearance of a shattered mirror or pane of glass, but with closer study a definite pattern emerges. In general, the faults of major displacement strike about N30°E. Near the northern margin of the area the Mustang Branch and Mountain City faults are step faults across which the Austin Group is 224 displaced downward to the southeast to about the same level as the Edwards Group. A third step fault, the Kyle fault, is located about 7 kilometers to the southeast near Kyle. It has displaced the Pecan Gap Formation down to about the level of the Austin Group. A similar relationship exists near the western margin of the San Marcos section of the area. Three step faults (an unnamed zone of faults, the San Marcos Springs fault, the Comal Springs fault) have displaced the Pecan Gap Formation downward to the southeast to an elevation lower than the Edwards Group. The two step fault zones in the northern and southern parts of the area are not aligned with each other, but are offset by about 4.6 kilometers. The complex faulting and outcrop pattern between these zones is the result of the adjustment of the intervening area to their en echelon relationship. The offset of the zones can be seen quite clearly in the Balcones Escarpment as shown on the satellite photo in the Frontispiece. In broad outline, the Edwards Group outcrops on the upthrown side of the southern set of step faults give way northeastward to progressively younger strata that are on the downthrown side of the Mustang Branch and Mountain City step faults (R. O. Kehle, personal communication). Figure A-1 shows in simplified diagrammatic form the ramp between the en echelon step faults. This ramp, instead of bending smoothly, as shown in the diagram, from the upthrown side of the southern fault zone to the downthrown side of the northern zone, has been intensely faulted and fractured, resulting in the mosaic of gravity fault blocks west of the Blanco River in the Kyle section. Unfortunately, a key part of the transition has been covered by alluvium of the Blanco River. This relatively straightforward picture is complicated somewhat by the intensely faulted graben-like downfaulted wedge in the west-central part of the Kyle section. Similar but somewhat smaller ramp-like structures can also be seen along the Balcones Escarpment near the eastern margin of the Kyle section. Three en echelon faults (the Kyle fault, the San Marcos Springs fault, and an unnamed fault between these two) separate two ramps which dip northeastward. Outcrops on these ramps change northeastward from the Austin Group to the Pecan Gap Formation, but the exact nature of the transition is obscured by alluvial cover.
Another interesting structural feature along the escarpment is the relatively undisturbed monolithic block upon which the northwestern half of San Marcos is built. This block is encircled by intense faulting but has not itself been appreciably disturbed except for a slight eastward tilt which may be a result of regional dip. The block stands quite high topographically on the divide between Sink Springs and Purgatory Creeks.
Southeast of the Balcones Escarpment the Balcones faulting does not appear to be as intense as northeast of the scarp. However, this apparent difference is due to the difficulty in recognizing and mapping faults in the clay terrane rather than to the absence of faults. Not only is it almost impossible to observe faults in the soft, uniform Upper Cretaceous clay units, but the structure is further obscured by the thick black soils that have developed on the clays. More than likely, intense faulting like that northwest of the scarp has also occurred southeastward to the Mexia.
If this block were eroded to a level surface, the strata exposed in the ramp would become progressively younger northeastward.
fault zone (Keith Young, personal communication). The major faults shown on the geologic map southeast of the escarpment are taken from previous work (chiefly DeCook, 1960, 1963 and Barnes, 1974), and their existence is inferred primarily from subsurface data rather than surface mapping.
The total vertical slip (throw) across the study area from the upper contact of the Glen Rose in the Blanco River canyon to the inferred contact between the Taylor and Navarro Groups in the southeast corner of the area is about 415 meters. An unknown part of this displacement is due to regional dip. The displacement on the major faults in the area is difficult to estimate because the blocks on either side of the faults are themselves usually broken up by cross faults. The individual blocks are often displaced by differing amounts. Also, the blocks are usually tilted and bent, which further increases the difficulty of estimating displacements. Despite these problems some very general estimates of displacements on the major faults or fault zones can be made. The minimum throw on the combined Mustang Branch and Mountain City faults equals the thickness of the stratigraphic interval between the Edwards and Austin Groups, and amounts to about 50 meters. The Hidden Valley fault, which is reported to have a throw of about 60 meters in Comal County (George, 1952), has a highly variable throw in the San Marcos area. The upper contact of the Glen Rose is apparently displaced only about 9 meters at one point in the Blanco River canyon, but it is displaced by more than 15 meters at another point in the canyon. DeCook (1963) states that this fault dies out to the northeast. The Morton Ranch fault has maximum displacement along a segment where the Austin Group is faulted down against the Person Formation, and the minimum displacement there is about 50 meters. Davis (1962) estimates the maximum displacement of this fault to be between 80 and 110 meters. DeCook (1956) estimates the throw of San Marcos Springs fault to be a minimum of about 90 meters. Comal Springs fault decreases in displacement northeastward across the area. The throw is estimated to be about 120 meters near the west edge of the San Marcos section and about 100 meters near San Marcos. Because of the en echelon relationship of some of these faults, their displacements sum to a value greater than the total vertical throw across the area.
An interesting structural feature southeast of the Balcones Escarpment is the inferred breached dome-like structure between Hunter Road and Interstate 35 southwest of San Marcos. A lone outcrop of the Sprinkle Formation observed near the center of this feature was found to be completely surrounded by outcrops of the Pecan Gap Formation. The interpreted structure, which assumes a reversal of dip near the Comal Springs fault of the type described by Cloos (1968), is advanced as the simplest explanation for these observations. An earth resistivity traverse was made parallel to McCarty Lane by an Engineering Geology class of the University of Texas at Austin to clarify the structural picture at this location, but the results were relatively inconclusive. The interpretation depicted proved to be compatible with the resistivity observations, but an alternative interpretation which proposes additional faults and a horst-like structure is equally likely. The reversal-of-dip explanation is used here to avoid depicting a minimum of two additional inferred faults, neither of which can be satisfactorily connected to known faults.
A.4 Geomorphology
The primary physiographic element in the San Marcos area is the Balcones Escarpment. As noted in the text, the elevation about doubles westward across the scarp within the limits of the area, and the landforms are different on either side. East of the scarp the topography on the soft Upper Cretaceous clays of the Blackland Prairie is gentle and rolling with low rounded hills. West of the scarp the rugged limestone terrane of the dissected eastern margin of the Edwards Plateau is known as the Texas hill country.
A.4.1 Fluvial Processes
The dominant geomorphic processes in the area at present are the fluvial processes. One of the more interesting aspects of the fluvial geomorphology of the area is the geologic history of the Blanco River. Woodruff (Baker and others, 1974) described evidence indicating that the Blanco formerly flowed eastward out of the Kyle section into what is now the drainage basin of Onion Creek. He proposed that the scarp (90°) bend in the course of the river in the north-central part of the Kyle section may be an elbow of capture. This elbow was created when the Blanco was diverted by a small stream that was eroding headward more or less normal to the Balcones Escarpment. A faintly visible trace of a short segment of the precapture course of the Blanco can be seen on aerial photos about 1.6 kilometers northeast of the capture elbow on the present divide between Onion Creek and the Blanco River. The increased gradient resulting from the capture may have been the primary cause of the incision of the Blanco to form the canyon where the river flows through the resistant Edwards Group limestones.
Evidence for a second capture of the Blanco can also be found south of Kyle, where terrace gravels cap the Austin Group uplands. This terrace stands about 12 to 15 meters above the present floodplain of the Blanco. Koenig (1940), who has studied this terrace, reports a maximum thickness of 12.8 meters for the gravels of this terrace. A thickness of no more than about 4.6 meters was observed during this study. The terrace is deeply dissected and retains none of its original landform, but it clearly indicates that the Blanco formerly flowed eastward south of Kyle into the present drainage basin of Plum Creek (Koenig, 1940). This former course of the Blanco is also clearly indicated east of the Kyle section on the Seguin sheet of the Geologic Atlas of Texas (Barnes, 1974). The alluvial gravels in this former valley are more resistant to erosion than the surrounding Upper Cretaceous clays and have resulted in an inversion of topography (Victor Baker, personal communication). The gravels now cap hills that are higher than the ‘surrounding clay terrane.
The present Blanco River and associated features also display several interesting characteristics. About 4.2 kilometers west of Kyle the narrow canyon of the river abruptly opens to a floodplain having a width of about 1.6 kilometers. The river has a meandering course within the limits of this floodplain, which remains relatively constant in width downstream to about Interstate 35. East of the Interstate the floodplain again increases in width, to about 5.8 kilometers. These abrupt changes in floodplain width are attributed to changes in bedrock resistance to erosion. The widening of the gorge to a floodplain west of Kyle occurs at almost the exact point that the river crosses a fault which marks the downstream limit of the hard Edwards Group limestones. Apparently the sidecutting action of the river is more effective in the less resistant post-Edwards strata downstream from this point. Interestingly, there is also a sharp nickpoint in the river at the same site. An old gravel terrace on the upland west of the river indicates that this location has marked a sharp change in the river’s behavior from a primarily downcutting to a sidecutting stream for some time in the geologic past. The second sudden broadening of the floodplain at the Interstate occurs at another change in bedrock resistance, where the nonresistant Pecan Gap clays are downfaulted against the more resistant Austin Group chalks and limestones.
The Blanco has incised slightly and has cut into or through the floodplain alluvium. As a result the flat surface along the river is no longer a floodplain in the normal sense of the term, but is rather an “infrequently flooded surface” (Victor Baker, personal communication). The river has cut into bedrock for most of its course downstream to Interstate 35, exposing in the alluvium a thickness of about 6 meters of gravel overlain by about 3 meters of sand, silt, and clay (see Figure 3-6 in text). Because of the exclusively carbonate rock source area, the gravel is composed chiefly of limestone clasts with a lesser amount of chert and dolomite. The succession of gravel overlain by fine-grained sediment is interpreted to be a normal channel gravel – overbank mud sequence. The deposition of this sequence at a particular location probably occurs during the passage of a meander loop which is eroding into the bedrock slightly. As a result, the preexisting alluvium at the location is entirely removed as the meander passes, and only one channel – floodplain sequence is preserved at the location. This process, which explains the presence of only one sequence in most places, is highly idealized and is not realized everywhere on the floodplain, resulting in several variations on the most commonly observed succession.
Downstream from Interstate 3S, the river probably has not cut through the alluvium along most of its length. Several abandoned channels can be identified in the floodplain almost from its beginning west of Kyle by tonal changes in large scale air photos. The present course of the river near the confluence into the San Marcos River has apparently been assumed relatively recently, as indicated by the steep slopes in the weak Pecan Gap clays along the cut bank there.
A minor but interesting fluvial geomorphic feature of the area is just south of San Marcos, where an example of stream piracy in action can be seen. In UTM 3304-601, the 580-foot contour line clearly indicates a connecting channel between Willow Springs and Purgatory Creeks. Longitudinal profiles of these creeks downstream from this channel show a gradient of about 3.8 meters per kilometer for Willow Springs Creek and a gradient of about 1.9 meters per kilometer for Purgatory Creek. The U.S. Army Corps of Engineers (1971, p. 20) reports that water from Purgatory Creek flows through the connecting channel into Willow Springs Creek when the Purgatory Creek discharge exceeds 120 cubic meters per second (4,200 cubic feet per second). It seems clear that the higher gradient Willow Springs Creek is in process of capturing the flow of Purgatory Creek at this natural channel. If these streams were left undisturbed by urbanization, erosion during 232 times of flood would almost certainly deepen the channel, and within a few hundred years all of the flow of Purgatory Creek would be diverted into Willow Springs Creek.
A.4.2 Karst Processes
Karst processes have been important on the Edwards Group carbonate rocks in the western part of the San Marcos area. The present climate in the area is not conducive to karst processes, however, and the karst features as a whole appear to be undergoing destruction by stream dissection. Many dolines nevertheless remain on the uplands some distance away from the major rivers where the dissection is most rapid. The relatively flat upland in the northwest corner of the Kyle section appears to be a fairly good remnant of the Edwards Plateau surface and has dolines in sufficient abundance to be termed a karst plain.
As noted in the text the most important karst process from an anthropocentric point of view is the recharge to the Edwards aquifer. Some of this recharge takes place by direct infiltration and through sinkholes in the uplands, but most recharge is believed to occur in the beds of intermittent streams (L. J. Turk, personal communication). An example of this stream recharge is depicted in Figure 3-7 in the text.
Abundant caves are a prominent karst feature associated with the Edwards Group. One cave, Wonder Cave, is operated commercially, and several others have been named, explored, and mapped. One of the more interesting caves is Dugger Cave (Tarbutton Cave), which is located near the Blanco River channel in UTM 3316-604. This cave has a vertical entrance, and the base flow of the Blanco is prevented from being diverted into the cave only by a lip of bedrock and cemented gravel alluvium about 0.9 meters high between the river level and the cave entrance. During flood stages, when the water overtops the lip, part of the river’s flow is diverted into the cave, thus becoming recharge to the Edwards aquifer. Interestingly, although small passages of the cave extend under the Blanco channel (Bill Russell, Texas Speleological Survey, personal communication), an appreciable part of the river’s base flow is apparently not lost to the cave. Dugger Cave is probably the furthest downstream point of recharge from the Blanco River to the Edwards aquifer.
Sink Springs is a vertical, water-filled cave in UTM 3308-603 near the channel of Sink Springs Creek (see Figure 4-26 in text). The water level in the cave is near the ground surface and is at the level of the potentiometric surface of the Edwards aquifer. The cave received its name because it takes water from Sink Springs Creek during flood flow (or did before a small dam was constructed between the cave and the creek) and apparently has been known to flow in historic time (Robert Knispel, local resident, personal communication). It is possible that the discharge of San Marcos Springs issued from Sink Springs before the Edwards was breached at the discharge point at Spring Lake, but little or no evidence can be advanced to support this idea. Two other noteworthy observations can be made concerning the location of San Marcos Springs. First, it may not be fortuitous that the springs are located at the margin of the undisturbed monolithic block under northwestern San Marcos described earlier. Second, the springs are situated at a large-displacement fault where the Pecan Gap Formation has been downfaulted into contact with the Person Formation. At other points along the scarp, this displacement is distributed over two or more step faults, giving rise to a fault block having the Austin Group at the surface between the Edwards and the Pecan. Gap. That is, outcrops of Austin occur between the Edwards and the Pecan Gap on either side of the springs along the Balcones Escarpment, but the Austin has been faulted out at the springs location.
In addition to these karst features in the Edwards Group, two sinks are present in the Austin Group chalks and limestones near the south-central part of the Kyle section. They are of interest primarily because sinks have not heretofore been reported in the Austin Group in central Texas (Keith Young, personal communication). These sinks, as well as karstic features in other post-Edwards strata (such as Academy Cave and the Devil’s Smokehouse, both of which are developed in the uppermost Buda Formation) have almost certainly resulted from roof collapse of large caverns in the Edwards. The dolines are thus created by a kind of natural stoping process from caverns in the Edwards upward through the stratigraphic section rather than by solution of post-Edwards limestones. Figure 3-8c in the text shows a cross-sectional view of a sinkhole in which this type of collapse has occurred.
A.4.3 Mass Movement
Mass movement of somewhat different types is active on either side of the Balcones Escarpment. East of the scarp, the weak Upper Cretaceous clay units are undergoing slumping and creep that give rise to the low, gently rolling hills. West of the scarp, a major type of mass movement is the calving off of large limestone blocks on cut banks of streams. Hills that are flanked by the Del Rio Clay and capped by the overlying Buda Formation often have large blocks of limestone from the Buda sliding down the hillside on the weak Del Rio. The best examples of such large sliding blocks are northwest of San Marcos in UTM 3308-599 and 600, where a hill is developed on a fault block of Del Rio Clay. This hill was apparently originally protected from erosion by a Buda cap, but this cap has become fragmented and broken up into large blocks. These blocks are sliding down the hill radially away from the hilltop and are now found at varying elevations on the hillside. Another notable example of the failure of the Del Rio out from under the Buda are two large slump blocks that are developed on the cut bank of the Blanco River at Five-mile Dam. The blocks are composed of Buda and Eagle Ford strata and have collapsed because of undercutting of the Del Rio Clay at the river level.
A.5 Summary
The bedrock units in the San Marcos area are Cretaceous sedimentary strata composed primarily of carbonates and fine-grained terrigenous rocks. The lithology and physical properties of the rocks are chiefly a function of the depositional conditions and source areas that were extant during deposition of the Cretaceous section, and the present occurrence and distribution of the rock units are controlled by the Balcones faulting in conjunction with the shape of the present erosional surface. Fluvial, karstic, and mass-movement processes are the dominant agents that have given rise to the existing geomorphic features, and they continue to shape the land surface today.
Dissertation Methodology at LBJ School (Eaton Course)
[In preparation]