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Hydrologic evolution of the Edwards Aquifer recharge zone (Balcones fault zone) as recorded in the DNA of eyeless Cicurina cave spiders, south-central Texas
Abstract
The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en echelon fault zone where Cretaceous carbonates have been modified by karst processes influenced by structural and stratigraphic controls. While the modern confined Edwards Aquifer flows through cavernous voids at the base of the escarpment, air-filled caves perched in the escarpment are relicts of paleoaquifer hydrology. The structural geology of the Balcones Escarpment and the phylogeography of its endemic cave spiders provide mutually informative frameworks from which to establish relative dates for the activation of discrete groundwater recharge areas. The mitochondrial genetic variability of troglobitic spiders is correlated with the structural elements of the Balcones fault zone in and around the San Antonio relay ramp. Older (basal) genetic lineages occur in structurally high, mature karst terrains, while the younger (derived) lineages occur in structurally low, emergent karst terrains. Based on mitochondrial deoxyribonucleic acid (mtDNA) data, Cicurina diversity is interpreted as the product of the progressive availability of vadose zone habitat as discrete recharge areas have developed.
... If KFRs are evolutionary units, we expect patterns of population relatedness and connectivity to be congruent with KFR limits, that is, two populations within the same KFR would be more closely related and have higher connectivity (gene flow) than populations from different KFRs. However, prior studies explored the genetic variation of C. madla and closest relatives in Bexar County using mitochondrial data and some results were contrary to this expectation (Paquin and Hedin 2004;White et al. 2009;Hedin et al. 2018). For instance, some caves in the Government Canyon KFR seem to be more closely related to caves in the Helotes KFR. ...
... This age agrees with the diversification of flightless ground beetles (Rhadine) in Bexar County (Gómez et al. 2016). Cicurina population divergence dates also agree with the age of the fragmentation of subterranean environment, which indicates that the population subdivision was possibly caused by natural changes in the habitat (White et al. 2009). However, as we discuss below, human activities might have had a recent influence on population connectivity and reduction in genetic diversity. ...
... Populations of C. madla appear to have been declining in the last hundred thousand years (Fig. 5), as has also been reported for cave spiders from Europe (Pavlek et al. 2022). It has been suggested that cave habitats in the Edwards Aquifer have suffered fragmentation and reduction since the beginning of the Pleistocene (~ 3 Mya; White et al. 2009), which may explain the reduction in C. madla populations. However, the rate in which population effective size declines seems to be faster in the period when there is evidence of portion of a species range (KFR) should be "protected" in perpetuity. ...
Karst systems have great ecological, cultural and economic importance, but are constantly threatened by human development. Northwestern Bexar County, Texas (USA), is an example of a region where urbanization threatens an essential karst landscape. Many studies have provided substantial contributions to the understanding and conservation of biodiversity in this area, including the delineation of Karst Fauna Regions (KFRs). However, no study has evaluated fundamentally important measures of genetic diversity, changes in population sizes, and gene flow for listed regional species. Here we used population genomic data derived from the sequence capture of ultraconserved elements (UCEs) to describe patterns of structure and connectivity among cave populations of Cicurina madla, and to estimate demographic and phylogeographic processes underlying those patterns. In addition, we evaluated if population genetic processes could have been influenced by human activities. Although there is weak phylogenomic differentiation between populations, the distribution of genetic diversity and patterns of gene flow within and between areas suggest important differences in population dynamics among KFRs. Our genomic data show that populations of C. madla in Bexar County currently carry comparatively low levels of heterozygosity and nucleotide diversity, and that these might have been influenced by human activities. Ultimately, we present genomic evidence for declining cave spider population sizes perhaps coincident with increases in an expanding human population and discuss implications of our findings for conservation.
... Although those two karst rivers were separated by the dams, they had diverse habitats such as riffle and pool mesohabitat and therefore the impact of Jiuyuxi and Shiniuzui dams was limited for the fish community structure. In addition, the similar results were found in some other karstic landsform rivers (White et al., 2009;Pedro et al., 2018). Even in karst landscapes, where stream continuity is naturally interrupted by obstacles such as subterranean stretches, the river continuum is still maintained (Vannote et al., 1980;White et al., 2009;Pedro et al., 2018). ...
... In addition, the similar results were found in some other karstic landsform rivers (White et al., 2009;Pedro et al., 2018). Even in karst landscapes, where stream continuity is naturally interrupted by obstacles such as subterranean stretches, the river continuum is still maintained (Vannote et al., 1980;White et al., 2009;Pedro et al., 2018). This suggested that karst geomorphic rivers may be somewhat resilient to disturbance (Kollaus et al., 2015). ...
... This suggested that karst geomorphic rivers may be somewhat resilient to disturbance (Kollaus et al., 2015). Nevertheless, despite fish community dynamic systems being resilient, karst river systems are not immune to anthropogenic disturbances such as overfishing (White et al., 2009). Therefore, in future studies, we need to be more focused on the influence of anthropogenic disturbances on the longitudinal structure of fish community in karst rivers. ...
... The Texas component of the uplifting is known as the Llano Uplift and is thought to have raised the strata of Central Texas several thousand meters [14,15]. This uplifting provided forces contributing to the Balcones faulting during the mid-Cenozoic [14][15][16]. The faulting resulted in a vertical displacement of strata by as much as 260 m [17]. ...
... One of the Cretaceous formations (Edwards Limestone) in the footwall block still exposed along the eastern and southern slopes of the Balcones Escarpment is riddled with caverns, the youngest of which are below the fault trace and support permanent subterranean streams of the Edwards Aquifer [14], with estimates of total water capacity ranging upwards to over 200 km 3 [17]. At progressively higher strata of Edwards in the now vadose portion of the formation above the trace there are progressively older caverns which formerly carried aquifer water but now harbor terrestrial endemics, all this indicating a long Cenozoic history of coastward hydrological evolution in the Edwards Plateau [16]. It is in the perennial rheocrenic springs that have been issuing from this series of caverns for millions of years that we focus our attention. ...
... Krejca [65] used evolutionary patterns of stygobionts to aid in interpreting the hydrogeologic history of the central Texas karst aquifers and adjacent aquifers of northern Mexico. White et al. [16] studied a troglobiontic spider clade from vadose caverns of the Edwards karst and established links among phylogeography, geomorphology, and hydrogeology. They also developed a robustly supportable interpretation of the Pleistocene evolutionary history of the clade indicating that the spiders had apparently been epigean during successive glacial maxima and then retreated into caves that had been left at progressively lower elevations in the vadose zone during the three interglacial warming periods of the Pleistocene as the aquifer evolved coastward [66]. ...
The contemporary distribution of crenobiontic endemics in central Texas is enigmatic because only some springs are occupied by crenobionts despite other unoccupied springs having seemingly suitable habitats. In the absence of complete paleo-records, a cohesive and widely accepted explanation for this biogeography has eluded researchers for many years. We suggest that data on contemporary species with obligate coevolution, such as parasites with multiple obligate hosts in their life cycles, can help to fill intervening gaps in the paleo-record because the contemporary distribution of such a parasite indicates that its hosts cohabitated without interruption since the arrival of the parasite. To test this conjecture, we studied one such parasite endemic to a select few central Texas springs, Huffmanela huffmani. By studying the distribution of the intermediate host, geologic and paleo-climatic records, performing lab experiments with live animals, and examining archived museum specimens of the definitive hosts from the 1950s, we were able to test multiple predictions about how the distribution of H. huffmani became what it is today. Our results corroborate a narrative suggesting that several severe droughts since the Wisconsin glaciations are responsible for having sculpted the present-day distribution of central Texas crenobionts.
... Although those two karst rivers were separated by the dams, they had diverse habitats such as riffle and pool mesohabitat and therefore the impact of Jiuyuxi and Shiniuzui dams was limited for the fish community structure. In addition, the similar results were found in some other karstic landsform rivers (White et al., 2009;Pedro et al., 2018). Even in karst landscapes, where stream continuity is naturally interrupted by obstacles such as subterranean stretches, the river continuum is still maintained (Vannote et al., 1980;White et al., 2009;Pedro et al., 2018). ...
... In addition, the similar results were found in some other karstic landsform rivers (White et al., 2009;Pedro et al., 2018). Even in karst landscapes, where stream continuity is naturally interrupted by obstacles such as subterranean stretches, the river continuum is still maintained (Vannote et al., 1980;White et al., 2009;Pedro et al., 2018). This suggested that karst geomorphic rivers may be somewhat resilient to disturbance (Kollaus et al., 2015). ...
... This suggested that karst geomorphic rivers may be somewhat resilient to disturbance (Kollaus et al., 2015). Nevertheless, despite fish community dynamic systems being resilient, karst river systems are not immune to anthropogenic disturbances such as overfishing (White et al., 2009). Therefore, in future studies, we need to be more focused on the influence of anthropogenic disturbances on the longitudinal structure of fish community in karst rivers. ...
The Datong and Xiaotong rivers are karst tributaries in the upper Yangtze River drainage and represent considerable habitat diversity that contributes to high fish diversity and rich fishery resources in the two rivers. During the few decades, fish resources have abruptly declined due to overfishing, water pollution and habitat degradation. In order to further strengthen conservation and restoration of fish resources and provide scientific support for fishery management in these rivers, the temporal and spatial dynamics of fish assemblage in the Datong and Xiaotong rivers were investigated in the present study. A total of 45 species were captured at 30 sample sites during four surveys conducted from March 2016 to January 2017. Species diversity and richness both increased from upstream to downstream in the two rivers. The fish assemblages significantly differed between river reaches, while did not differ in significantly between 4 months. The spatial variations in the abundance of Rhynchocypris oxycephalus , Zacco platypus , Triplophysa bleekeri , Sinogastromyzon szechuanensis , Platysmacheilus nudiventris , Sinibrama taeniatus , Saurogobio gymnocheilus , Pseudorasbora parva , Pseudobagrus truncates and Opsariichthys bidens were considered to contribute most to the spatial pattern of fish assemblages. The lack of a temporal pattern of the fish assemblage in the rivers might be attributed to ecological habits of short-distance migration and diversified mesohabitats with riffles and pools for fish spawning, feeding or overwintering within a very short river range. Our results suggest the spatio-temporal dynamics of fish assemblages is highly adapted to the habitat characteristics in the two karst rivers. We recommend that fish conservation of these tributaries be focused on the mesohabitats, particularly the many pools and riffles.
... Recognition of parapatry and understanding underlying mechanisms of maintaining parapatry provide a framework for understanding zoogeographical history and high rates of endemism in the Edwards Plateau region. Karstification of the limestone began at least 6 MYA (White et al. 2009). With karstification, aquifers formed in the Cretaceous limestone and continued to erode downward in a south and southwest direction (Deike 1990, White et al. 2009). ...
... Karstification of the limestone began at least 6 MYA (White et al. 2009). With karstification, aquifers formed in the Cretaceous limestone and continued to erode downward in a south and southwest direction (Deike 1990, White et al. 2009). Erosion by streams and rivers often intercept and down-cut confining layers of the aquifer, forming artesian springs (Abbott andWoodruff 1979, Grimshaw andWoodruff 1986) and also accelerates formation of the vast aquifer (Deike 1990). ...
... However, the aquifer continues to erode downward, as well as rivers and streams continue to erode downward, and once high flowing spring complexes evolve into low flowing springs, and eventually dewater (Abbott and Woodruff 1986), exposing cavernous limestone and caves. Newly formed cavernous limestone and caves (i.e., hydrologic evolution) parallel allopatric speciation events in cave spiders within the Edward Plateau (White et al. 2009). ...
Base flow is the portion of stream flow attributed to groundwater, and few studies quantify the pure effects of base flow reductions on stream fish communities. Spring complexes within the karst terrains of the Edwards Plateau Region of central Texas offer a unique opportunity to test hypothesized relationships between base flow and stream fish communities. Spring complexes are numerous within the Edwards Plateau, providing multiple independent observations, stable hydrographs dominated by base flow conditions, similar groundwater sources, and support endemic fishes that are associated with the spring complexes (i.e., spring-associated fishes). Primary objectives of this study were to assess spring-associated fish richness, relative abundances, and densities across a gradient of base flow magnitudes with predictions that metrics of spring-associated fish communities would linearly decrease with reductions in base flow. To control potential confounding variables, additional objectives were to test for the presence and strength of parapatry that is hypothesized to exist between spring-associated fishes and riverine-associated fishes (i.e., fishes with distributions not typically associated with spring complexes). Patterns in richness, relative abundances, and densities indicated parapatric distribution between spring-associated and riverine-associated fishes. Strength of parapatry depended upon base flow magnitude. Correspondingly, differences in spring-associated fish richness, relative abundances, and densities along a base flow gradient were detected, but only densities were linearly related to base flow. Richness and relative abundances of spring-associated fishes were non-linearly related to base flow, suggesting that spring complexes have a level of buffering capacity against base flow reductions. The relationship between spring-associated fish communities and base flow gradient was used to support the reported parapatry between spring-associated fishes and riverine-associated fishes with in the area and to highlight the conservation value of spring complexes to regional fauna. Predictive models generated in this study can be used to evaluate spring-associated fish community integrity within the Edwards Plateau Region and to predict future changes in Edward Plateau spring complexes related to increases in groundwater extraction.
... Several authors examining biological and geological discrepancies (Paquin and Hedin 2004, White 2006, White et al. 2009, Ledford et al. 2012) questioned the karst fauna region concept as a valid biological model and recovery strategy for all listed karst invertebrates in Bexar, Travis, and Williamson counties. USFWS (2011) also acknowledged such discrepancies in the Bexar County Karst Invertebrates Recovery Plan. ...
... The range of the species does not coincide with the existing karst fauna region concept for reasons not considered by GVA (1992) who did not publish considered alternatives to his hypothesis. Ranges of the troglobitic Batrisodes are consistent with modern interpretations of the hydrologic evolution of the Edwards Aquifer in tandem with troglobitic biogeography as discussed by White et al. (2009). Down-cutting of the Edwards Limestone outcrop by the South Fork of San Gabriel River did not prevent southward radiation of B. texanus and cannot explain the barrier preventing further dispersal into the rest of the Georgetown Karst Fauna Region. ...
Current distribution records for Batrisodes texanus Chandler have become more refined since the species was federally listed as endangered in the United States almost 30 years ago. Distribution and range records are compiled from a variety of sources, including dedicated surveys of karst biota in various preserves in Williamson County, Texas, and karst feature investigations triggered by development requirements established for compliance with the Endangered Species Act. The species range is determined by factors not considered in current delineations generated in 1992 of karst fauna regions that serve as a model for de facto recovery units used by the U.S. Fish and Wildlife Service. Current recovery unit delineations might inhibit official downlisting of species despite significant existing and planned conservation efforts. This untenable situation probably is the direct result of improper boundary placement between recovery units; the authors recommend modifying a single recovery unit boundary to better reflect the distribution of the species as determined by in situ field surveys. The authors recommend modifying the recovery unit system to include the entire range of the species within a single recovery unit.
... The Edwards Plateau is a large (3200 km 2 ; Longley 1981) karst-terrain ecoregion of central Texas and with its voluminous subterranean aquifers and numerous surface water discharges (e.g., artesian springs) provide perennial aquatic resources. Karstification of the Edwards Plateau began at least 6 MYA within faulted and downdipping Cretaceous limestone along the margin of the Gulf of Mexico sedimentary basin (White et al. 2009). As aquifers evolve downward within Cretaceous-age limestone (Deike 1990, White et al. 2009), surface streams and rivers continuously erode and intercept confining layers of the aquifer, forming and later bypassing artesian springs Abbott 1979, Grimshaw and. ...
... Karstification of the Edwards Plateau began at least 6 MYA within faulted and downdipping Cretaceous limestone along the margin of the Gulf of Mexico sedimentary basin (White et al. 2009). As aquifers evolve downward within Cretaceous-age limestone (Deike 1990, White et al. 2009), surface streams and rivers continuously erode and intercept confining layers of the aquifer, forming and later bypassing artesian springs Abbott 1979, Grimshaw and. Consequently, artesian springs in arid and semi-arid regions of the Edwards Plateau provide perennial flows that are moderately decoupled from local precipitation and recharge events but not completely. ...
Aquifers and spring complexes within the arid and semi-arid karst Edwards Plateau region of Texas are evolutionary refugia supporting a large number of endemic flora and fauna. Spring fishes, many of which are endemic, are associated with the spring complexes, whereas river fishes generally avoid spring complexes. Purposes of this study were to experimentally test affinities of spring fishes and river fishes for spring complexes and rivers among six independent spring-river systems of the Edwards Plateau and to quantitate relationship among spring fishes, river fishes and spring flow magnitude within spring complexes. We found that spring fishes did not exclusively use spring complexes but were more abundant in spring complexes than rivers and that species richness, relative abundance, and density of spring fishes were directly related to spring flow magnitude. Patterns in affinities for river fishes were less distinct. Quantitation of fish–habitat relationships will assist efforts in conserving aquatic resources within the Edwards Plateau and provides predictive models to assess biological integrity of other spring complexes. In addition, consistency in distributional patterns of spring and river fishes provides a framework for testing underlying processes of species segregation and diversification, establishes boundaries of aquatic evolutionary refugia, and expands the value of evolutionary refugia from historical refugia to contemporary and future refugia for Edwards Plateau aquatic biota.
Cave trechines beetles (Coleoptera: Carabidae: Trechini) are members of cave communities globally and important models for understanding the colonization of caves, adaptation to cave life, and the diversification of cave-adapted lineages. In eastern North America, cave trechines are the most species rich group of terrestrial troglobionts, comprised of over 150 taxa in six genera with no extant surface members. Previous studies have hypothesized the climate change during the Pleistocene was a major driver of cave colonization and diversification in this and other temperate terrestrial cave fauna. However, our time calibrated molecular phylogeny resulting from the analysis of 16,794 base pairs (bp) from 68 UCE loci for 45 species of the clade supports an alternative hypothesis whereby cave colonization of the surface ancestor of eastern North American cave trechines likely began in the middle Miocene in the Appalachians Ridge and Valley (APP) and dispersed into the Interior Low Plateau (ILP) in an east to west manner around 11.5 Mya. The APP served as a cradle for diversification and also as a bridge linking the southern Appalachians and Interior Low Plateau enabling the dispersal and subsequent diversification of these cave beetles. Major clades in our time calibrated phylogeny attained their present day geographic distributions by the early Miocene followed by multiple additional episodes of cave colonization and diversification occurring throughout the Pliocene and Pleistocene. The genera Neaphanops, Darlingtonea, Nelsonites, and Ameroduvalius were nested within specious genus Pseudanopthalmus supporting the hypothesis that these genera are derived Pseudanophtlamus. Moreover, while several morphologically derived species groups of Pseudanopthalmus were recovered as monophyletic, others were not warranting future taxonomic and systematic research. The molecular systematics and biogeography of these unique cave beetles offer a model for other comparative evolutionary and ecological studies of troglobionts to further our understanding of factors driving speciation and biogeographic patterns.
During the Pleistocene in the northern part of Europe and Asia, the presence of ice sheets not only limited the range of species but also influenced landscape and thus the contemporary habitat system that determines the pattern of biodiversity. The aim of the research was to find out whether and how a lowland landscape, which formed as a result of subsequent Pleistocene glaciations (five) that in Eurasia covered various and generally successively smaller areas, affected the genetic differentiation of a species. The research was carried out in eastern Poland on the root vole Microtus oeconomus (Arvicolinae, Rodentia), a model boreal and hygrophilous species. Samples were collected from 549 vole individuals at 33 locations. Based on the analysis of 12 microsatellite loci and the 908 bp of cytochrome b sequences (mitochondrial DNA), the genetic structure of M. oeconomus in the landscape zones of the Polish Lowlands was determined. The results show that the latitudinal variability of the relief in eastern Poland (resulting from different ranges of Pleistocene ice sheets) and the related specific configuration of hydrogenic habitats are reflected in the genetic differentiation of the root vole. Therefore, it may be concluded that the history of landscape development affects the genetic structure of hydrophilic species.
This catalogue lists 1,084 species of spiders (three identified to genus only) in 311 genera from 53 families currently recorded from Texas and is based on the “Bibliography of Texas Spiders” published by Bea Vogel in 1970. The online list of species can be found at http://pecanspiders.tamu.edu/spidersoftexas.htm. Many taxonomic revisions have since been published, particularly in the families Araneidae, Gnaphosidae and Leptonetidae. Many genera in other families have been revised. The Anyphaenidae, Ctenidae, Hahniidae, Nesticidae, Sicariidae and Tetragnathidae were also revised. Several families have been added and others split up. Several genera of Corinnidae were transferred to Phrurolithidae and Trachelidae. Two genera from Miturgidae were transferred to Eutichuridae. Zoridae was synonymized under Miturgidae. A single species formerly in Amaurobiidae is now in the Family Amphinectidae. Some trapdoor spiders in the family Ctenizidae have been transferred to Euctenizidae.
Gertsch and Mulaik started a list of Texas spiders in 1940. In a letter from Willis J. Gertsch dated October 20, 1982, he stated “Years ago a first listing of the Texas fauna was published by me based largely on Stanley Mulaik material, but it had to be abandoned because of other tasks.” This paper is a compendium of the spiders of Texas with distribution, habitat, collecting method and other data available from revisions and collections. This includes many records and unpublished data (including data from three unpublished studies). One of these studies included 16,000 adult spiders belonging to 177 species in 29 families. All specimens in that study were measured and results are in the appendix. Hidalgo County has 340 species recorded with Brazos County at 323 and Travis County at 314 species. These reflect the amount of collecting in the area.
The placement of the genus Cicurina Menge in the Dictynidae is discussed and accepted. The subgenera of Cicurina are enumer- ated and Cicurata Chamberlin and Ivie is formally synonymized un- der Cicurella Chamberlin and Ivie. The synonymy of Cicurella un- der Tetrilus Simon is rejected; both subgenera are accepted as valid. The female genitalia of C. (Tetrilus) japonica (Simon) is reillustrated. Cicurina elliotti Gertsch is reillustrated and synonymized under C. buwata Chamberlin and Ivie (both from caves in Travis and Williamson Counties, Texas). Cicurina gatita Gertsch is synonymized under Cicurina pampa Chamberlin and Ivie. A taxonomic key and many new cave records to females of cicurinas known from Bexar County are provided. Females of the 12 species of Cicurina known from Bexar County (one species known only from epigean habitat) are redescribed and reillustrated. In addition, five new troglobitic species are described that are endemic to caves in the county. Varia- tion and species definitions are evaluated with doubt being cast on the validity of C. davisi Exline. The possible evolution of cicurinas in Bexar County is briefly outlined. Notes on biology, endangerment, and captive culturing of cicurinas are provided.
There is reasonable doubt that a collection of vertebrate fossils listed by Dumble and quoted by later authors came from the Oakville formation in southwest Texas. Vertebrate fossils low in the Fleming formation have been collected between Burkeville and George West, Texas, extending the Fleming formation vertebrate faunas into the upper Oakville of Renick and the emended Lagarto of Plummer. It is recommended that both be included in the Fleming formation, and the Oakville sandstone be restricted to beds from the top of the Catahoula to the top of the Moulton sandstone member, Oakville formation of Renick. The vertebrate fauna found at Cold Spring is appreciably younger than that found at Navasota. The former is in the Fleming formation and the latter in the Oakville formation. Vertebrate remains of the same age as those found at Cold Spring within the Fleming formation are also found at the type section of the emended Lagarto formation (Plummer, 1933). Traceable biostratigraphic units herein called faunas, are used to correlate and identify Miocene formations along the Texas Coastal Plain. Arikareean, Hemingfordian, Barstovian, and Pliocene biostratigraphic units are recognized.
Obligatory cavernicoles, or troglobites, have traditionally been of special interest to evolutionary biologists for several reasons. The existence of animal life in caves and other subterranean spaces at first attracted attention because of its novelty; intensive biological exploration of caves began little more than a century ago. Although the discovery and description of the cave faunas of the world is far from complete, especially in the Western Hemisphere, so much descriptive information has been compiled that we can safely assert that, at least in unglaciated, temperate parts of the world, the occurrence of numerous species of troglobites in any major limestone region is a common and highly probable phenomenon.
The Edwards Aquifer, the major source of water for many communities in central Texas, is threatened by population growth and development over its recharge zone. The location of the recharge and confined zones and the flow paths of the aquifer are controlled by the structure of and deformation processes within the Balcones fault system, a major system of predominantly down-to-the-southeast normal faults. We investigate the geologic structure of the Edwards Aquifer to assess the large-scale aquifer architecture, analyze fault offset and stratigraphic juxtaposition relationships, evaluate fault-zone deformation and dissolution and fault-system architecture, and investigate fault-block deformation and scaling or small-scale (intrablock) normal faults. Characterization of fault displacement shows a pattern of aquifer thinning that is likely to influence fault-block communication and flow paths. Flow-path constriction may be exacerbated by increased fault-segment connectivity associated with large fault displacements. Also, increased fault-zone deformation associated with larger-displacement faults is likely to further influence hydrologic properties. Overall, faulting is expected to produce strong permeability anisotropy such that maximum permeability is subhorizontal and parallel to fault-bedding intersections. At all scales, aquifer permeability is either unchanged or enhanced parallel to faults and in many cases decreased perpendicular to faults.
The Edwards Limestone (Albian) is a mosaic of shallow water, back-reef, carbonate lithofacies averaging about 150 m thick, that have been dolomitized, chertified, and calcitized.Intermittent subaerial exposure during and shortly after deposition of the Edwards resulted in solution-enlargement of some primary voids. Slow upwarping of the northwestern margin of the subsiding ancestral Gulf of Mexico basin elevated the Edwards Group above sea level late in the Cretaceous. Down-to-the-coast, en echelon, normal faulting along the Balcones system during the Early Miocene accentuated the topographic position of the Edwards above sea level. Rejuvenated Gulf-ward flowing streams cut into the upthrown fault block and exposed the uppermost Edwards Limestone in the bottoms of deep canyons, which created the discharge sites that initiated a continuously circulating groundwater system within the Edwards Limestone in the Miocene. The early porosity systems have increased in volume through the self-ramifying cavern solution process that occurs in carbonate rocks.The down-dip boundary of the aquifer, or “bad-water line”, represents an early bypass boundary of the aquifer that has become deeply ingrained with time. Cavern development is largely sub-horizontal and reflects the control of bedding partings and beds of differing lithology. The Edwards Limestone aquifer has only a few widely separated natural discharge sites which, when viewed in scale, make the confined aquifer appear like a regional master conduit. This cavernous system is known as the Edwards underground reservoir and it supplies the water for most of south-central Texas.