PETROGRAPHIC TRANSITION FROM CRATONIC TO MOBILE BELT PROVENANCE, ATOKA FORMATION (PENNSYLVANIAN), NORTHERN ARKANSAS SHELF

The Atoka Formation on the northern Arkansas shelf and throughout the northern part of the Arkoma basin is composed of alternating sandstone and shale units. The formation is informally divided into lower, middle and upper members by regionally extensive intervals of shale. The lower member ranges from 210 meters in thickness in the outcrop belt to 310 meters in the northern part of the basin. Seven sandstone units, from three to 36 meters in thickness, are prominent in the unit. Sandstone units are separated by intervals of shale. Lower member sandstone units are composed of fine-grained, submature to mature quartzarenite. Quartz grains are dominantly monocrystalline, although most display a degree of undulose extinction. Chert grains occur in trace amounts. Thin zones within several sandstone units are calcareous and contain calcite skeletal fragments. The lack of feldspar and the high ratio of unstrained to strained monocrystalline quartz indicate a mature and perhaps multicycle sandstone on the craton as the source. In contrast, sandstone units within the middle member of the Atoka range to 100 meters in thickness. They are composed of fine-grained, mature phyllarenite. Strained monocrystalline quartz, polycrystalline quartz, rock fragments of phyllite and schist and a trace of feldspar are the significant grain constituents. The composition of middle Atoka sandstone units suggests a reworked foreland uplift provenance or collision zone orogenic belt with minor contributions from the craton interior. The source became active after deposition of the lower Atoka and during deposition of the middle member. This activity could have included the unroofing of a metamorphic source or a change in the path of the transporting system. Uplift in the Ouachita or Appalachian orogenic is recorded in the changing compositions from the lower to the middle Atoka members.