Hillslope seepage erosion, spring sapping, and knickpoint migration: evidence of groundwater sapping Middle Trinity aquifer, Honey Creek basin, Comal County, Texas by Kristin Miller White, B.A. Thesis Presented to the Faculty of the Graduate School of The University of Texas in Partial Fulfillment of the Requirements for the Degree of Master of Science in Geological Sciences The University of Texas at Austin May 2005 Hillslope seepage erosion, spring sapping, and knickpoint migration: evidence of groundwater sapping Middle Trinity aquifer, Honey Creek basin, Comal County, Texas Approved by Supervising Committee: John M. Sharp, Jr. Paul F. Hudson, Libby A. Stern. Dedication To my family, husband Kemble White, our son Kemble Thorn White, and daughter Carys Morgan White. Acknowledgements Thank you to thesis advisor John M. Sharp, Jr. for his guidance and for supervising the work. Thanks to Paul F. Hudson for helping to formulate ideas and for providing instructions to install the erosion pins. Thanks to Libby A. Stern for sharing her time and energy in the field and for her guidance. Thanks to the Texas Parks and Wildlife Department (TPWD) for providing permits to access Honey Creek State Natural Area and to George Kegley and David Riskind of TPWD for providing data collected by volunteers from central Texas speleological chapters of the National Speleological Society (NSS). The United States Geologic Survey (USGS, 2001) made provisional data at Honey Creek (water well levels, precipitation, and stream hydrograph data) possible. Honey Creek stream discharge measurements were measured by students during field methods classes (Geo 376L/328C), June 2001. Funding was made possible by the Geological Society of America, University of Texas Environment Science Institute Summer Research Fellowship (2001), Geological Society of America (2002), Ronald K. DeFord Field Scholarship Fund Award (2002), Arnold P. (Dutch) Wendler Professional Fund (2002 and 2003), and John A. and Katherine G. Jackson Fellowship in Geohydrology (2004). iv Abstract Hillslope seepage erosion, spring sapping, and knickpoint migration: evidence of groundwater sapping Middle Trinity aquifer, Honey Creek basin, Comal County, Texas Kristin Miller White, degree sought M.S. The University of Texas at Austin, 2004 Supervisor: John M. Sharp, Jr., Ph.D. Geomorphic features within Honey Creek basin are consistent with formation by spring sapping which is the erosion of soil and rock by groundwater. Geomorphic evidence includes: swallow holes (stream swallets) that pirate spring discharge into the subsurface, groundwater piping and seepage along weathered marly slopes, headward erosion at knickpoints and spring orifices, fracture controls on incised streams, and generation of alluvium from scarp collapse. Erosion pins were used to measure erosion and sediment accumulation on marly slopes. Aerial photograph interpretation, Arcview GIS, 3D Analyst, and Geoorient techniques were used to evaluate the physical hydrogeologic features (potentiometric surface, karst springs, recharge features, knickpoints, and fractures) and their relationship to surface erosion patterns. Honey Creek basin is underlain by interbedded marl and limestone units of the Cretaceous Middle Trinity aquifer. Springs and caves provide a window into subsurface processes, including flow direction along preferential flowpaths and perched water tables. Precipitation affects spring discharge and water table levels in both stratigraphically perched aquifers and deeper aquifers. Upland karst features allow recharge of surface water to focus flow into spring conduits that rapidly discharge into streams following intense precipitation. Spring conduits and upland creeks feed into intermittent tributaries, then into perennial channels of Honey Creek and the Guadalupe River. Perched aquifers focus flow toward intermittent springs, while perennial springs are supported by a deeper regional system. Transmissivity is high within rock units that contain solutionally enlarged fractures and spring conduits. Elsewhere, the transmissivity of limestone and marl is generally low so that preferred flow pathways concentrate spring discharge where hillslope erosion has intersected bedding planes, conduits, and fractures. As springs discharge into local surface water bodies, erosion occurs at the spring orifices causing headward erosion along the pathways. Dominant fracture trends within the basin are generally aligned with the northeast-trending Balcones Fault Zone and a secondary fracture distribution to the northwest. These trends strongly influence spring location and sapping. v Table of Contents Dedication.............................................................................................................. iv List of Tables ...........................................................................................................x List of Figures........................................................................................................ xi List of Figures........................................................................................................ xi List of Photographs.............................................................................................. xiii INTRODUCTION 1 Statement of Problem...............................................................................................6 Objectives ..............................................................................................................10 ENVIRONMENTAL SETTING 21 Land Use ................................................................................................................21 Physiography..........................................................................................................22 Climate...................................................................................................................23 Vegetation..............................................................................................................23 Soils 25 Topography............................................................................................................28 Lithology................................................................................................................28 Cow Creek Limestone...................................................................................28 Hensel Sand ..................................................................................................31 Glen Rose Limestone....................................................................................32 Basalt.............................................................................................................32 Quaternary Terrace Deposits ........................................................................32 Hydrogeologic Setting ...........................................................................................34 Hydrostratigraphy .........................................................................................34 vi Structural Controls........................................................................................36 Stratigraphic Controls ...................................................................................36 Regional Potentiometric Surface ..................................................................39 Potential Recharge ........................................................................................43 Springs ..........................................................................................................43 Hydraulic Properties .....................................................................................46 METHODOLOGY 47 Field Observations .................................................................................................47 Evaluation of Hillslope Properties.........................................................................48 Hillslope Erosion Pin Measurements............................................................48 Soil Measurements at Field Test Plots..........................................................51 Evaluation of Hydrogeologic Properties................................................................52 Stream Discharge Measurements..................................................................52 Comparison of Precipitation, Runoff, and Potentiometric Surface ..............54 Longitudinal Profile Development ...............................................................54 Statistical Analysis of Structural Controls.............................................................59 Stream Orientations ......................................................................................59 Karst Features ...............................................................................................59 Scanline Fracture Data..................................................................................60 Rose Diagrams..............................................................................................60 Evaluation of Hydrogeologic Data Using ArcView GIS.......................................61 Potentiometric Surface Map .........................................................................61 RESULTS AND DISCUSSIONS 65 Field Observations .................................................................................................66 Evaluation of Hillslope Properties.........................................................................66 Hillslope Erosion Pin Measurements............................................................67