Pennsylvanian coalified rhizomorphs in Illinois: evidence for non-compressive coalification to bituminous coal rank

Irregular and discontinuous coalified objects, here termed rhizomorphs, occur above the Summum (No. 4) Coal Member (Carbondale Formation) at one locality in La Salle County, Illinois. These vitrinite structures, varying between 3 mm and 4 cm in diameter, occur in a blocky to structureless mottled claystone characterized by numerous carbonaceous streaks and small-scale slickensides. The rhizomorphs twist and bend in a tightly coiled and kinked pattern, and may extend laterally through the claystone for 40 cm or more before bifurcating or terminating. The rhizomorphs display little or no evidence of compressive distortion. They are composed of two types of material: structureless bright conchoidal collinite; and cellular woody structure. One or more vascular bundles occur as discrete steles within a groundmass of collinite gel; these are generally aligned longitudinally within the coal except where steles diverge laterally as offshoots. The rhizomorphs are believed to be a diagenetic derivative of rooting structures; this is based on their occurrence in an underclay-like sediment and on their irregular, kinked form, which differs from that of most vertical supportive plant stems. Non-compressive coalification of the plant material may have occurred through: (1) aerobic alteration of cell walls to non-compressive inertinite; (2) centripetal contraction and selective degradation of plant tissue; and (3) impregnation of more resistant stelar elements by organic gel derived from degradation of thin-walled tissue.