INTRODUCTION TO THE MT. RORAH COAL BED AND MT. RORAH COAL BALLS AT THE CREEK PAUM MINE IN SOUTHWESTERN ILLINOIS, GEOLOGIC CONTEXT AND PALEOBOTANICAL UNIQUENESS

The early Desmoinesian Mt. Rorah Coal Bed, located in the upper Tradewater Formation, occurs throughout southern Illinois (outcrop and subsurface). It ranges from a thin streak to a reported 1.6 m in Jackson County, IL, and was locally mined. The Mt. Rorah is typically bright-banded, with a thin layer of claystone in the middle 1 – 30 cm thick. At Creek Paum Mine in southwestern Illinois, the Mt. Rorah coal is 12 – 15 m above the Murphysboro coal, is 30 – 60 cm thick with shaley partings, overlies a fossiliferous tidalite shale that transitions into the coal base, and has a marine roof.

We discovered a coal-ball mass (permineralized peat preserving original plant material) within the coal seam approximately 3 m in width and 8-9 m long, interbedded and coincident with a linear trending claystone dike, injected upwards from underlying strata. These are the only reported coal balls from the Mt. Rorah.

Forty two coal balls were collected, cut, and acetate peels prepared. Plant composition was quantified via a cm-square grid. Based on 9,830 cm² the peat consists of 35.3% lycopsids (largely Lepidophloios), 21.1% pteridosperms, 15.7% cordaitaleans, 13.9% ferns (mainly marattialeans), 6.7% sphenopsids, and 7.3% unknowns. Compared to stratigraphically adjacent coals (Murphysboro Coal of Indiana, and Secor Coal of Oklahoma), the Mt. Rorah is similar only in abundance of Cardiocarpus spinatus producing cordaitaleans. It differs in the abundance of Lepidophloios,Paralycopodites, Medullosa (dominant), and Psaronius. The assemblage is most similar to late Desmoinesian peat floras. Unusual are abundant arthropod fecal pellets, showing excellent preservation.

The paleobotanical uniqueness may reflect swamp conditions. Underlying strata are part of the Oroville Channel complex, an abandoned estuarine paleochannel. Numerous claystone partings observed in the coal suggest continued fluvial influence, introduction of nutrients and water into an environmentally stressed swamp habitat.

The coincident coal ball mass and clastic dike suggests a causative relationship. Upwardly injected claystone, and geometric relationship between the dike and linear coal ball mass could imply a spring bringing calcium-carbonate bearing water into the peat swamp, mineralizing peat in place. However, overlying marine rocks, another potential source of calcium-carbonate bearing water, makes interpretation of coal ball mineralization problematic.