UNLOCKING THE UPPER ATOKA SEDIMENTATION HISTORY USING THE ROCK PROPERTY OF MAGNETIC SUSCEPTIBILITY

The Pennsylvanian-age Upper Atoka Member of the Atoka Formation is shale-rich and represents shelfal facies in the Arkoma Basin. A section of significant thickness is exposed as roadcut along the Backbone Anticline in west-central Arkansas. Magnetic susceptibility values of the shale-rich interval indicate a statistically significant cyclicity of sediment input. At 0.129 cycles/meter, this is the first high frequency cycle detected in the Arkoma Basin. A resampling at finer-resolution directed towards the same interval resulted in detection of a finer eccentricity and two precessional Milankovitch periodicities at 0.5, and 2.4 and 3.167 cycles/meter, respectively. The powerful signals reported in the Fourier transform appropriately scale to predicted frequencies for Milankovitch cyclicities at 305 Ma. Correlation of these data to gamma-ray logs from cores in the region corroborates the coarser sedimentation cycles. Sediment accumulation rate preserved in the shelfal facies is calculated to be ~0.02 m/kyr, two orders of magnitude less than the broad sedimentation history assigned to the Atoka Formation. Stratigraphic work undertaken in this research demonstrates that magnetic susceptibility is a robust tool for understanding fine-scale sedimentation history in the Arkoma Basin.