BED-SCALE STRATIGRAPHIC STUDY OF THE STRUCTURALLY COMPLEX UPPER ATOKA FORMATION EXPOSED AT THE PARIS RESERVOIR, PARIS, AR

The Pennsylvanian-age Upper Atoka Formation is comprised of a mixture of interbedded sandstones and shales depicting a delta environment building out into the sediment-rich Arkoma Basin. A significant exposure of the Upper Atoka Formation is accessible at the Paris Reservoir Spillway in Paris, Arkansas. Also present in and around the spillway are numerous small thrust faults, strata with near-vertical orientation (both related to the Ouachita orogeny), and prominent facies and formation contacts. The purpose of this project was to determine if the rock layers exposed in the spillway could be measured on a bed-scale and if magnetic susceptibility values from those beds would represent the fine-scale changes in present sandstone and shale layers. Samples were collected at fixed intervals of 0.30 m (corrected for dip) for ~70 m of total section in order to correlate to a stratigraphic column, and, potentially, core. Magnetic susceptibility values for each sample fell into the range for marine rocks, and the high resolution data set showed recognizable trends between sandstone- and shale-rich portions of the section. Significant shifts in magnetic susceptibility values through the data set matched well with broad-scale lithologic changes and these were assigned as unit divisions. The resulting curve was detailed enough to correlate to well-log gamma-ray data from a nearby well. There were no problematic effects from sampling bare rock exposed to surface weathering, and the faulted zones were avoided by sampling in multiple areas of the spillway. Magnetic susceptibility values of rocks have proved to be a useful tool for correlating from outcrop to core or subsurface data sets at high resolution, and especially with shale-rich intervals that are usually avoided when interpreting basin-wide stratigraphy.