Clay Mineralogy of Pennsylvanian Sediments in Southern Illinois

Abstract
Controversy over the origin of clay minerals in ancient sediments arises because it is difficult to segregate the factors that cause differences in clay mineral composition. A plan of sampling based on four contrasts was used to evaluate some of these factors. Contrasts studied were: (1) permeability contrasts (sandstone vs. shales); (2) environmental contrasts (cyclic sedimentation); (3) weathering contrasts (outcrop vs. core); (4) source area contrasts (orthoquartzite facies vs. subgraywacke facies).The resultant data indicate that clay minerals are both allogenic and authigenic. Allogenic clay minerals are the rule when inheritance from the source area dominates either postdepositional or environmental effect; authigenic clay minerals are found where the environment dominates. Rapid sedimentation favors a dominance of allogenic clay minerals, and major depositional environmental effects are required to produce appreciable change. Some of the factors that influence clay mineral composition include source area contribution, the depositional environment, postdepositional effects after burial, permeability, and postdepositional weathering effects.