POSSIBLE TOPOGRAPHIC INVERSION OF KARST FEATURES IN THE EASTERN ARBUCKLE MOUNTAINS, SOUTH-CENTRAL OKLAHOMA

Numerous circular-shaped hills are present in the eastern Arbuckle Mountains of south central Oklahoma. These circular hills are composed principally of Simpson Group (Ordovician) limestone and clastic strata that appear to be more deformed than the surrounding outcrops of Simpson Group strata. The tops of these hills are about 15-75 m higher than the relatively flat topography that surrounds them and they range in diameter from about 75-250 m. Previously, they have been mapped generically as “areas of brecciated rock showing chaotic structure” and have been interpreted as klippen in a previous study. The consistent circular form (in map view) of all of these hills is odd with regard to a klippen (thrust-related) origin for these hills. Furthermore, the klippen interpretation requires an inferred thrust fault at the base of all of the individual circular hills, but the stratigraphic relations across those inferred thrust-fault remnants typically are younger-on-older rather than the expected older-on-younger relations for thrust faults. The circular geomorphic expression resembles hills that have been referred to by several names, including “haystack hills,” in karstic carbonate terrains around the world. Haystack hills represent deeply eroded karst in these regions and are said to be the result of differential weathering of relatively flat-lying limestones that are jointed in large rectangles. Although the circular hills in south-central Oklahoma morphologically resemble haystack hills, the deformed strata that form and characterize these hills suggests they are not simply the result of differential weathering controlled by fractures. An alternative interpretation for their origin is that the deformation resulted from karst-related collapse and that these hills are actually erosional remnants of exhumed large sink holes. The implied topographic inversion of these features might result from post-collapse fluids that could have altered the collapsed strata in sink holes and made it more resistant to erosion than the adjacent strata.