THE BEAR CREEK UPLIFT DEFINED BY GEOLOGIC MAPPING AND MAGNETOTELLURIC SOUNDINGS IN THE WESTERN BUFFALO RIVER REGION, NORTHERN ARKANSAS

Geologic mapping and magnetotelluric soundings in the western part of the Buffalo River watershed of northern Arkansas document large thickness variations (from < 5 m to > 150 m) of Upper Mississippian strata that record tectonic activity of the latest Mississippian-earliest Pennsylvanian Bear Creek uplift within the southern Ozark Dome. The Upper Mississippian stratigraphic interval in the area includes Batesville Sandstone and Fayetteville Shale capped by Pitkin Limestone (that is variably preserved beneath a regional Pennsylvanian unconformity). The Bear Creek uplift is defined by an elongate northwest-trending isopach of minimum Upper Mississippian thickness. Pitkin Limestone is absent over the uplift axis and geologic mapping demonstrates that the zero-thickness pinch-out line is concentric around the southeastern, southwestern, and northwestern margins of the uplift. These relations suggest that the Pitkin Limestone was removed by pre-Pennsylvanian erosion over the uplift.

Whereas the southeastern margin of the Bear Creek uplift is well defined by the northeast-vergent Kyles Landing reverse fault, the northeastern margin of the uplift is uncertain because it is buried beneath younger Pennsylvanian strata. Magnetotelluric data were collected at 6 sites along a southwest-northeast profile of about 7 kilometers across the inferred northeastern margin as an additional constraint for the concealed lithology. The magnetotelluric method is a passive surface geophysical technique that uses the Earth's natural electromagnetic fields to investigate the electrical resistivity structure of the subsurface from depths of tens of meters to tens of kilometers. Preliminary 2-D resistivity inversions of the magnetotelluric data image a conductive unit that is predominantly Fayetteville Shale which drastically thickens to the northeast along the profile, better constraining the buried, northeastern margin of the Bear Creek uplift.