EVIDENCE OF SEASONAL PRECIPITATION IN PENNSYLVANIAN SEDIMENTS OF THE ILLINOIS BASIN

Pennsylvanian-age, small-scale tidal bundle sequences discovered on th e eastern margin of the Illinois basin record years of continuous dail y tidal deposition and have been used to interpret the astronomical fa ctors influencing deposition. Because they also provide an absolute ti me scale of daily, monthly, semiyearly, and yearly resolution, these r ecords can be used to identify nontidal factors of short duration. Dev iations from the semiyearly and yearly sedimentation patterns predicte d from pure tidal theory were observed in some of these tidal bundle s equences, and we interpret them to be the result of seasonal rainfall variations that affected sediment discharge. The possibility that seas onal climates existed in humid tropical paleoclimatic zones during the Pennsylvanian has been previously discussed, but analysis of the semi yearly and yearly depositional patterns in tidal rhythmites of the Ill inois basin provides direct evidence for their occurrence.