TIDAL FACIES, MIDDLE ATOKA FORMATION, ARKOMA BASIN, ARKANSAS

The Arkoma basin of Arkansas and Oklahoma formed in the Ouachita foreland during Pennsylvanian time. The basin fill is dominated by strata of the Atoka Formation, a thick succession of sandstone and shale units that record a transition from a stable shelf to a foreland basin. Atoka strata thin from a few meters in the northern Arkansas to thicknesses exceeding 7000 meters in the south adjacent to the Ouachita fold belt. Much of the subsidence occurred during deposition of middle Atoka strata as the lower Atoka shelf was broken by large, normal faults with down to the south displacement.

Middle Atoka strata exhibit the greatest increase in thickness during basin subsidence but are not exposed around the basin margins and their character is known mainly from well data. Middle Atoka beds, in the central and southern part of the basin, crop out along the axial crest of anticlines but exposures are poor. A recent road cut in southern Sebastian County exposes a complete and continuous section exceeding 200 meters in thickness through a middle Atoka sandstone unit along the Backbone anticline. The interval is characterized by a coarsening upward sequence from shale at the base. Sandstone intervals are characterized by lenticular, wavey and flaser stratification in the intermediate part of the section. Channels with large scale cross stratification complete the section. The shale interval accumulated in an open shelf environment that was replaced by subtidal and adjacent tidal flat environments. Tidal channels complete the section. The shallow, tidally dominated intervals constitute the upper 170 meters of the section reflecting a high rate of subsidence as basin formation proceeded.