DEEP-WATER LOWER ATOKA FORMATION, OUACHITA TROUGH, OK

Deep-water lower Atoka Formation (middle Pennsylvanian) crops out in road cuts along OK-2 south of Wilburton and OK-82 south of Red Oak, OK. Exposures are discontinuous and reveal complex structural geometry with in the Ouachita fold-thrust belt (Choctaw to Winding Stair faults). A composite section is being constructed from detailed measured sections and natural gamma-ray (GR) emission logs. The emphasis to date has been on OK-2 exposures south of the Ti-Valley fault system and continuing southward to the Winding Stair fault, which is ~13 km. south of Wilburton. Paleocurrent data indicate that coarse sediment was derived from several sources and delivered to the Ouachita trough mainly by turbidity currents.

From detailed studies along OK-2, mudrocks dominate the section with interstratified sandstone beds rarely exceeding 0.5-m thick. Both total and spectral GR logs have proven to be useful in correlating outcrop sections. Using conventional correlation approaches and total GR emissions, general correlation is possible, but ambiguous at the bed-to-bed scale. Because Th/K GR emissions is positively correlated with mica content in sandstones, detailed correlation of individual sandstone beds is unambiguous. Bouma Tc beds, with a thin Tb cap, are most common. Tc intervals are typically convoluted. Locally, Ta beds and thin Tb-c intervals are well developed. Paleocurrent indicators show a westward transport direction in the southern exposures and eastward transport direction in the northern exposures.

Exposures along OK-82 (~16 km. west of OK-2) are also being studied with large-scale correlation being possible using an airborne radar image. Some preliminary findings include the mapping of a very large submarine slump block(s). The southern section is much thicker and sandier compared the OK-2 and northern exposures; it also includes paleocurrent indicators showing northwestward transport. Other sections expose 1-m thick, Ta-e sandstones.