MESIC VEGETATION COMMUNITIES WITHIN THE OWL CREEK AND BEAR CREEK WATERSHEDS AS EVIDENCE OF HYPOGENE KARST

The Owl Creek and Bear Creek watersheds are located in Bell and Coryell counties on the eastern peninsula of the Fort Hood Military Installation near the confluence of the Leon River. The landscape and its topography are largely controlled by the erosional behavior of the Lower Cretaceous limestones and marls of the Fredericksburg Group; the resistant Edwards caps the plateaus and the lower permeability of the Comanche Peak forces ascending fluids to flow laterally, incising slot canyons into the steep sided scarps. These canyons follow the trend of major conjugate joint sets where deep-seated fluids rise along solutionally widened flow paths to feed surface springs and seeps. Although the climate can be characterized as sub-humid to semi-arid, continued erosion along preferential flow paths has created north and east facing mesic slot canyons where relict forest species such as Acer grandidentatum continue to thrive.

Today, isolated populations of A. grandidentatum continue to exist as Pleistocene relicts in sheltered, incised canyons within the Owl Creek and Bear Creek watersheds in Bell and Coryell counties. These stands are isolated from larger populations by several hundred miles but continue to thrive as a result of moisture associated with ascending fluids. Permeability varies greatly over the boundaries of the interbedded Comanche Peak and Edwards; the formations have likely created a semi-confined aquifer system where deeper seated fluids migrate upwards through low permeability strata along preferential flow paths and communicate with meteoric waters near the ground surface, maintaining the moisture regime vital to the continued existence of mesic vegetation communities in the incised canyons.