MAGNETIC SUSCEPTIBILITY OF BEDS FROM THE UPPER LOWER ATOKA FORMATION

The upper Lower Atoka Formation is comprised of alternating medium- to thick-bedded sandstones and shale layers. These facies represent shallow shelf deposits, with alternating beds of sandstone and shale signifying shoreline shifts as a response to small changes in sea level. Magnetic susceptibility (χ) is a geophysical rock property that detects the concentration of paramagnetic materials, normally derived from the continent, compared to weaker diamagnetic components. About 20 meters of section was collected at a fixed interval of 0.10 m to track changes in χ to represent changes in relative distance to shore. The abrupt changes in the χ curve match the alternating bed lithologies. χ values for sandstone beds are uncharacteristically higher than the shale beds. Intense burrowing and alteration to hematite may have caused the sandstone beds to have a strong paramagnetic and even ferrimagnetic signature; future work on core may result in χ curve more closely matching the gamma-ray log. Strong χ values for medium-bedded sandstone layers allows for good correlation among outcrops at lower sampling resolution, and may prove to be a robust tool for recognizing, interpreting, and correlating Atoka Formation stratigraphy in outcrop and the subsurface throughout the Arkoma basin.