ENIGMATIC NODULES IN THE MID-PENNSYLVANIAN MCALESTER FORMATION OF WESTERN ARKANSAS

The mid-Pennsylvanian McAlester Formation (MF) in western Arkansas and eastern Oklahoma consists predominantly of shale with some sandstone and coal beds. The Hartshorne Coal at the base of the MF is an important source of coal-bed methane, and sandstone members of the MF are reservoirs for natural gas. In the lower MF southeast of Fort Smith, Arkansas, we have recognized a 4-cm thick sideritic? marker bed overlying at least four distinct horizons of sideritic? nodules. The marker bed and nodular horizons are surrounded by dark-gray, fissile shale, and separated stratigraphically from each other by 10 to 30 centimeters. XRD analyses are underway to determine the exact mineralogy of the bed and nodules, but they are very fine-grained, black with hematized margins, weakly magnetic, and significantly more resistant to erosion than the surrounding shale. Some of the nodules have pyritized cores. The nodules can be placed in one of three groups based on their shape: 1) “tubes”; 2) “pancakes”; and 3) “donuts”. The tubular nodules are circular to elliptical in cross-section, up to 12 cm in diameter, and form branching networks parallel to bedding. Individual tubes can be traced for up to 1 m and some tubes appear to cross over other tubes. The “Y”-shaped tube branches and tube terminations frequently have a larger diameter than the tubes themselves. The “pancake”-shaped nodules, less abundant than tubes, are up to 40 cm in diameter with long dimensions parallel to bedding. “Donut”-shaped nodules are least abundant, but largest. One example is approximately 1 m in diameter with fissile shale filling the central hole. The origin of these nodules is uncertain, but presumably they are chemical precipitates that formed relatively soon after deposition. The tubular nodules resemble the open-framework burrow systems of Thalassinoides and similar ichnospecies.