TOPOGRAPHIC INVERSION IN KARST TERRAINS: THE BRECCIA-PIPE HILLS OF THE WASHITA BATTLEFIELD NATIONAL HISTORIC SITE, OKLAHOMA, USA

The valleys of the Washita River and its tributaries in west-central Oklahoma, USA display a unique landscape defined by low bluffs, rolling hills, and peculiar conical hills. These peculiar positive relief structures have moderate to steep (45-80°) slopes and are composed of blocks of Permian Doxey Formation residing within the stratigraphic position of the underlying Permian Cloud Chief Formation.

In order to characterize these features and examine their origin, we conducted a field campaign in and around the Washita National Battlefield Historic Site that included geologic mapping with a differential GPS at a 1:10,000 scale on a LIDAR basemap and measuring 3 stratigraphic sections. Field examinations within 34 of these Doxey Formation hills revealed collapsed beds, slump blocks, and isolated boulders all suggesting a collapse origin. Minimum vertical elevation offset measurements of 8-53 m between the projected Doxey and Cloud Chief Formation contact and the peaks of individual knolls also support a collapse origin for these hills. The higher carbonate content in the Doxey Formation appearing as both crystals and cement may have preferentially preserved the Doxey hills during fluvial erosion of the surrounding friable Cloud Chief Formation.

The observed breccia-pipe hills are similar in size, slope, aspect, and erosion resistant carbonate content as the siliciclastic breccia pipes in the Jurassic Entrada Sandstone in Glen Canyon National Recreational Area, Utah. However, the vertical elevation displacement measured compared to reported subsurface evaporite thicknesses requires a dissolution front on multiple horizons and collapse through more than a single void.