STRATIGRAPHIC ARCHITECTURE AND DEPOSITIONAL ENVIRONMENTS OF AN EARLY FORELAND BASIN SOURCE-TO-SINK SYSTEM: THE PENNSYLVANIAN ATOKA FORMATION, ARKANSAS

The Pennsylvanian Atoka Formation is a major reservoir unit in the Arkoma Basin and the Ouachita Mountains, one of the most prolific hydrocarbon basins in the US. The Atoka consists of sediments from deep to shallow water origins and recorded the transition from a rifted continental margin to a rapidly subsiding foreland basin. This study focuses on the lithofacies, stratigraphic architecture, paleocurrent, and ichnofacies early foreland basin based on 23 outcrops in western Arkansas.

The Atoka is informally divided into Lower, Middle, and Upper intervals in lithologic and chronostratigraphic sense. The Lower Atoka is a fine-grained, sand-rich deepwater complex. Both axial fan and transverse fan systems are predominantly fine-grained turbidite sandstones and mudstones. The main architectural elements are lobes, inter-lobes, MTD for the axial fan, and channels, levees and overbank for transverse fan. Net to gross is high for individual fan, but decreases westward. Paleocurrent shows overall E-W for axial fan, N-S for transverse fan. Trace fossils are identical of Nereites ichnofacies. The Middle and Upper Atoka are mud-rich shelf, deltaic and shallow marine deposits. They are predominantly fine- to medium-grained sandstones sandwiched in thick ripple- or planar-laminated mudstones, with some carbonaceous and fossiliferous horizons. The main architectural elements include sandstone and mudstone sheets, channels, bars. Combined influences of wave, tide, and traction currents are common. Paleocurrent shows bidirectionally N-S or E-W. Trace fossils are very abundant, mostly identical of Cruziana and Zoophycos ichnofacies.

The changes throughout the Lower, Middle and Upper Atoka indicate an evolutionary response to tectonic compression and subsidence, increasing confinement, localized accommodation, variations in basin geometry, natural maturing of the feeder systems during the progression of the transition from Ouachita Trough to Arkoma Basin. Comparative studies of the Atoka and the Jackfork in the same basin have important implications of deciphering deepwater successions of early foreland basin deposits. Such transitions are common throughout basin evolution records worldwide and the Atoka lends an opportunity for improved understanding of source-to-sink system response to such changes.