Abstract

The Arkoma Basin, once regarded as a simple asymmetric basin of Atokan (Pennsylvanian) age, is now known to thicken rapidly towards its axis across a series of steplike down-to-the-south normal growth fault blocks of mid-Atokan origin. These faults are not apparent at the surface, and surface structure is therefore not always a reliable indicator of the structural attitude at depth.

The structural configuration in the Bonanza field area, Sebastian County, Arkansas, is an example of Atokan growth fault conditions which have been further complicated and obscured by an overthrust block of subsequent orogenic origin. Gas production in the Bonanza field has been obtained from the Siluro-Devonian Hunton Limestone, the Morrowan Hale Sandstone and the basal Atokan Spiro Sandstone. A detailed north-south structural cross-section through the field illustrates the following geologic features which are clues in unraveling the tectonic history of the Arkoma basin: (1) the relative uniform thickness of the pre-Atokan section as compared to the radical changes in the thickness of the Atokan attests to the fact that the Arkoma basin is tectonically Atokan in age; (2) the abrupt appearance of approximately three thousand feet of mid-Atokan section hints of a condition of growth faulting; (3) down-to-the-south normal growth faults of mid-Atokan age which terminate below the upper Atokan unit are not normally apparent at the surface; and (4) deep structure is further obscured by an overthrust block of subsequent origin.

The multiplicity of depositional environments and resultant hydrocarbon trap variations within the Atokan sedimentary "wedges" south of the growth faults have served as a strong stimulus for continued exploration in the Arkoma basin area.