Stephen F. Austin State University SFA ScholarWorks Faculty Publications Department of Geology 4-2011 Spring Hydrology of Colorado Bend State Park, Central Texas Kevin W. Stafford College of Sciences and Mathematics, Department of Geology, Stephen F. Austin State University, [email protected] Melinda G. Shaw-Faulkner College of Sciences and Mathematics, Department of Geology, Stephen F. Austin State University, [email protected] Jessica L. DeLeon College of Sciences and Mathematics, Department of Geology, Stephen F. Austin State University Follow this and additional works at: https://scholarworks.sfasu.edu/geology Part of the Geology Commons Tell us how this article helped you. Repository Citation Stafford, Kevin W.; Shaw-Faulkner, Melinda G.; and DeLeon, Jessica L., "Spring Hydrology of Colorado Bend State Park, Central Texas" (2011). Faculty Publications. 14. https://scholarworks.sfasu.edu/geology/14 This Conference Proceeding is brought to you for free and open access by the Department of Geology at SFA ScholarWorks. It has been accepted for inclusion in Faculty Publications by an authorized administrator of SFA ScholarWorks. For more information, please contact [email protected]. Spring Hydrology of Colorado Bend State Park, Central Texas By Kevin W. Stafford, Melinda G. Shaw, and Jessica L. DeLeon Department of Geology, Stephen F. Austin State University, 1901 N. Raguet, Nacogdoches, TX 75962 Abstract Karst development in Ellenburger carbonates near Colorado Bend State Park in central Texas exhibits complex polygenetic origins, with porosity development dominated by an early hypogene phase that has subsequently been overprinted to varying degrees by epigene processes. Quarterly physicochemical and continuous thermal monitoring analyses of eight springs in the study area indicate that modern groundwater flow paths are highly variable. Springs exhibit patterns that range from shallow, distributed recharge into diffuse-flow dominated systems, to focused recharge into well-connected conduit systems, to deep-circulation systems that equilibrate with bedrock. All springs, except Sulphur Spring, exhibit physicochemical characteristics indicative of proximal epigenic groundwater flow through Ellenburger carbonates, while Sulphur Spring shows elevated temperature and dissolved-ion concentrations indicative of longer groundwater flow paths through deeper strata. The polygenetic nature of karst development in the Colorado Bend State Park has created an enhanced porosity structure which forms a complex modern groundwater flow network. and 30ºC respectively (Estaville and Earl, 2008). INTRODUCTION Annual precipitation averages 30 cm, with most Colorado Bend State Park (CBSP) is located precipitation occurring during Spring (March – on the northern edge of the Texas Hill Country May) and Fall (September – November). on the flank of the Llano Uplift (Figure 1, 2). More than 400 karst features have been Here Ordovician Ellenburger carbonates crop identified within CBSP and surrounding out along a highly entrenched segment of the properties, including more than 100 physically Colorado River, immediately upstream from mapped caves. Most caves exhibit Lake Buchanan. Proximal to the river, numerous characteristics of complex, polygenetic origins, springs discharge including subaqueous springs, springs within a few meters of the river and springs that discharge hundreds of meters from the river (Figure 1). Most springs discharge with normal epigenic karst chemistries; however, one spring in the region, Sulphur Spring, discharges with a slightly thermal component, an elevated sulfate content and easily discernable odor of hydrogen sulfide. CBSP is located approximately 180 kilometers northwest of Austin, Texas in San Saba and Lampasas Counties. The park covers 21.6 square kilometers including a seven kilometer long stretch of the Colorado River. The area is located along the boundary between subtropical steppe climate and subtropical subhumid climate, with Figure 1. Map of study site showing location of springs, average annual temperature of 20ºC outline boundary of CBSP in yellow and approximate and minimum and maximums of 8ºC location of study site with reference to the state of Texas. 152 development with abundant springs that discharge from subaqueous position in the Colorado River to more than sixty five meters above river level suggests a complex hydrogeologic system that has not completely equilibrated with current climatic and geomorphic conditions. GEOLOGIC OVERVIEW The Llano Uplift, and greater Texas Hill Country, is dominated by Precambrian (~1.0 bya - ~1.2 bya) basement rocks which form a large structural dome overlain unconformably by Cambrian and Ordovician clastics and carbonates (Figure 2) (Sellards and others, 1932). Ordovician strata are unconformably overlain by Carboniferous rocks that are subsequently overlain unconformably by Cretaceous strata that compose the northern extension of the Edwards Plateau (Rose, 1972). Precambrian strata were emplaced as part of the Grenville orogenic event, while these and overlying Paleozoic rocks in the study area were modified by tectonism associated with the Figure 2. Spatial distribution of Llano Uplift aquifers in relation Ouachita Orogeny, primarily to project study area (red box) with ~10X vertically exaggerated resulting in minor tilting and faulting cross section (modified from Preston and others, 1996). (Standen and Ruggiero, 2007). including many that exhibit classic hypogene Throughout the Cenozoic, all strata within the MSRF (Morphologic Suite of Rising Flow) region have seen additional brittle deformation (Klimchouk, 2007) characteristics with varying and minor tilting as a result of uplift of the degrees of epigenic overprinting, while other Edwards Plateau and down-dropping of the gulf caves are more purely epigene in origin. DeLeon coastal plain, with intense faulting along the (2010) showed that at least two thirds of caves Balcones Fault Zone, approximately 80 developed within the CBSP region exhibit kilometers east of the study area (Collins, 1995). hypogene origins with variable degrees of epigene overprinting, while less than one fourth Karst development within the study area is of all caves showed clear dominance of epigene largely limited to Ordovician Ellenburger origins. Eight known perennial springs discharge carbonates, including cave and spring subaerially within the park, while two other development in all three Ellenburger units, springs discharge upstream within the oldest to youngest — Tanyard, Gorman and commercially operated Sulphur Springs fish Honeycutt Formations. The Tanyard Formation camp (Figure 1). Stream mapping conducted by is ~170 meters thick and consists of fine- to Mitchell and others (2011) suggests that at least coarse-grained, irregularly bedded dolomite ten additional springs also discharge deposited as high-energy, restricted, subtidal subaqueously within the Colorado River in this facies, including common ooidic zones and region. The polygenetic nature of karst cryptalgal laminae (Kerans, 1990). The Gorman 153 Formation is ~130 meters thick and consists of others, 1996). The Marble Falls aquifer is micro-granular dolomite associated with developed in corresponding Carboniferous deposition in a low-energy, restricted-shelf limestone, which exhibits highly variable environment, including common macrofossils, permeability due to well-developed secondary distributed zones of intense burrowing and porosity. All three of the Llano Uplift aquifers cryptalgal laminae, as well as zones indicative of show gradual increases in total dissolved solids subaerial exposure that include rip-up clasts and in the down-dip direction away from the main siliciclastic sediments (Kerans, 1990). The Llano Uplift dome, with deep, distal components Honeycutt Formation is ~40 meters thick and containing total dissolved solids greater than consist of thinly interbedded limestones and 10,000 mg/L (Preston and others, 1996). dolomites that were deposited in an open, shallow-water shelf environment, including In the CBSP area, Ellenburger strata are common structures indicative of periods of brief exposed at the surface and most associated subaerial exposure (e.g. desiccation cracks, rip- springs exhibit normal, epigene karst up clasts) and structures indicative of variable chemistries, while one spring exhibits current energies (e.g. ooids, current ripples, anomalous characteristics. The seven springs cryptalgal laminae) (Kerans, 1990). that exhibit normal epigene karst chemistries, include Bear Spring, Gorman Cave Spring, Within the study area, three minor aquifers Gorman Falls Spring, Gorman Spring, Lemon collectively referred to as the Llano Uplift Spring, McLarrin Spring and Well House Spring aquifers are developed in the Paleozoic rocks, (Figure 1, Table 1). Sulphur Spring, as the name including from bottom to top: Hickory aquifer, implies, exhibits anomalous patterns. Bear Ellenburger-San Saba aquifer, and Marble Falls Spring discharges from a solutionally widened aquifer (Preston and others, 1996). The aquifer fracture in the Honeycutt Formation that is system dips gently into the subsurface away located four meters above and twenty five from the central Llano Uplift, dipping mainly to meters away from the river. Gorman Cave the north in the study area. The Hickory aquifer Spring is associated with Gorman Cave and is developed in the Cambrian Hickory discharges from the Gorman Formation. Gorman Sandstone, which is underlain by Precambrian Cave