DEEPWATER LOWER ATOKA SEDIMENTOLOGY AND STRATIGRAPHY IN THE FRONTAL OUACHITA OKLAHOMA: CHALLENGES TO THE PARADIGM

Deepwater Atoka in the Ouachita frontal zone has received a great deal of attention over the last many decades. The conventional view of the sedimentology and stratigraphy is that this interval represents a single very large submarine fan with sediment derived from the southern Appalachians and that this interval rests unconformably above the Johns Valley Shale. Detailed studies along highway road cuts and other locations indicate that this view is not completely supportable, especially for the lower Atoka.

Within the Ti Valley-Winding Stair structural block, the equivalent of the Spiro sandstone is recognized. In most locales, it is a thin, bituminous skeletal moldic fine sandstone. The unique lithologic character makes it useful as a marker bed. As such, it resides at different stratigraphic positions relative to the lithostratigraphic Atoka basal contact. South of Bengal (OK-82) the Spiro equivalent resides with in the Johns Valley Shale, providing evidence that the Johns Valley is at least in part a facies equivalent to Atoka deepwater sandstones and shales.

Detailed measured sections with paleocurrent indicators related to lithofacies provide evidence for the presence of several submarine fans with input points from the east, north and southwest. In addition, detrital zircon age dates point to basement sources as far away as the Yucatan for at least part of the Atoka exposed in the frontal zone.