Report of Investigations No. 180 Geomorphic Processes and Rates of Retreat Affecting the Caprock Escarpment, Texas Panhandle Thomas C. Gustavson and William W. Simpkins* Bureau of Economic Geology° 1989 d W. L. Fisher, Director . The University of Texas at Austin Austin, Texas 78713 *Department of Geology and Geophysics University of Wisconsin Madison, Wisconsin 53705 CONTENTS A b stra ct ................................................................................................... I In trodu ction .............................................................................................. 2 G eologic S ettin g ......................................................................................... 3 Structural D evelopm ent .................................................................................. 3 S tratigraphy ............................................................................................. 4 R egional H ydrology ..................................................................................... 5 Geom orphology and Physiography ....................................................................... 05 C lim a te .................................................................................................. 5 R un off ................................................................................................ .. 8 Relation of Dissolution-Induced Subsidence to the Caprock Escarpment ....................... 10 Stratigraphy beneath the High Plains, Caprock Escarpment, and R olling Plains ................................................................................... 10 Structure on the Tubb Interval ........................................................................ 10 Salt T h ickness ........................................................................................... 13 Structure on the Alibates Form ation .................................................................. 13 Middle Tertiary Erosional Surface ............................................. 13 Relation of Post-Ogallala Lake Basins to the Caprock Escarpm ent ......................................................................... 18 Fracture Systems and Landform Development .................................................... 18 Systematic and Nonsystematic Fractures .............................................................. 18 C lastic D ikes .......................................................................................... 19 Ground-Water Flow beneath the Caprock Escarpment ........................................... 20 Conceptual Model of Ground-Water Flow beneath the Caprock Escarpment....................................................... 22 "x Numerical Model of Ground-Water Flow beneath the Caprock Escarpm ent ................................................................................ 24 M odeling Results ....................................................................................... 25 Surface Processes Causing Scarp Retreat ........................................................... 27 Surface Runoff .......................................................................................... 27 Drainage D evelopm ent ................................................................................. 27 Thresholds for Sedim ent M ovem ent ...................................................................... 28 Spring Sapping and Seepage Erosion .................................................................. 29 R ockfalls ................................................................................................. 31 Rotational Slum ping ................................................................................... 31 P ip in g .................................................................................................... 34 Stages in the Physiographic Development of the Eastern Caprock Escarpment ..................................................................... 35 Rates of Retreat of the Caprock Escarpment ....................................................... 38 Rates of Interstratal Salt Dissolution along the Caprock Escarpm ent ............................................................................ 38 Procedure for Calculating Salt Dissolution Rates ......................................................... 38 Salt Dissolution Rates .................................................................................. 39 Surface Erosion R ates ................................................................................... 41 Effect of Diminished Spring Discharge on G eom orphic Processes .............................................................................. 42 Sum m ary and Conclusions ............................................................................. 44 Acknow ledgm ents ...................................................................................... 45 R eferen ces ............................................................................................... 45 iii Figures 1. Location of the study area in the eastern Texas Panhandle ............................................... 2 2. Major structural elements, Texas Panhandle and surrounding area ....................................... 3 4 3. Stratigraphic nomenclature of Permian and younger strata, Texas Panhandle ............................ 6 4. Topography along the Caprock Escarpment .............................................................. 7 5. Views of the Eastern Caprock Escarpment in Palo Duro Canyon State Park .............................. 6. Three-day total rainfall distribution, June 24-26, 1965, 8 along the Eastern Caprock Escarpment ................................................................. 9 7. Intensity-duration profiles of four storms along the Caprock Escarpment ................................. 8. Regional stratigraphic cross section illustrating zones of section lost owing to dissolution of salt and collapse of overlying strata beneath the 11 Southern High Plains, Caprock Escarpment, and Rolling Plains ......................................... 12 9. Structure-contour map of the top of the middle Permian Tubb interval .................................... 10. Net-thickness map of salt in the Permian upper Clear Fork Group and 14 San Andres, Seven Rivers, and Salado Formations ...................................................... 15 11. Net-thickness map of salt in the Salado and Seven Rivers Formations .................................... 16 12. Structure-contour map of the top of the Upper Permian Alibates Formation ............................... 17 13. Structure-contour map of the base of the High Plains aquifer (Ogallala Formation) ....................... 20 14. Head map of the unconfined aquifers in the Palo Duro Basin ............................................. 21 15. Potentiometric-surface map of lower Dockum Group ground water ....................................... 22 16. Map of physiographic units, major streams, and brine emission areas in the study area ................... 17. Conceptual model of ground-water flow and its impact on Permian bedded salt beneath the High Plains Caprock Escarpment and Rolling Plains ....................................... 23 24 18. Finite-element mesh design based on cross section shown in figure 8 ..................................... 19. Steady-state hydraulic heads and recharge/discharge rates computed by ground-water flow model ..................................................... 26 20. Plot of maximum rainfall intensity versus percent of erosion pins showing erosion at the 29 Buffalo Lake National Wildlife Refuge station .......................................................... 30 21. Area of spring sapping at the head of Red Canyon, Palo Duro Canyon State Park ........................ 30 \ 22. Detail of the spring discharge area illustrated in figure 21 ................................................ 23. Rockfall consisting of blocks of massive channel sandstones, Upper Triassic Dockum Group at the head of Tub Springs Draw, Palo Duro Canyon State Park .......................................................................... 31 24. a. Fracture in caprock caliche of the Miocene-Pliocene Ogallala Formation at Fortress Cliff, Palo Duro Canyon State Park ........................................................ 32 32 b. Fracture in sandstone of the Upper Triassic Dockum Group in Tub Springs Draw ...................... 25. Distribution of rotational slumps and landslides in Palo Duro Canyon, Palo Duro Canyon State Park .......................................................................... 33 26. Three segments of rotational slumps exposed along the northeastern side of Palo Duro Canyon ............................................................................... 34 27. Entrance to a large pipe in Caprock Canyons State Park ................................................. 35 28. Outlet for a large pipe in Palo Duro Canyon State Park .................................................. 35 36 29. Stages in development of drainage at the Caprock Escarpment ........................................... 40 30. Drainage basins and gauging stations in the Rolling Plains .............................................. Tables 9 1. Rainfall intensity-duration-frequency relationships, Amarillo, Texas, 1904-1951 ........................... 2. Hydrostratigraphic units and hydraulic conductivities used in the ground-water flow m odel .......................................................................