Pennsylvanian-age, small-scale tidal bundle sequences discovered on the eastern margin of the Illinois basin record years of continuous daily tidal deposition and have been used to interpret the astronomical factors influencing deposition. Because they also provide an absolute time scale of daily, monthly, semiyearly, and yearly resolution, these records can be used to identify nontidal factors of short duration. Deviations from the semiyearly and yearly sedmentation patterns predicted from pure tidal theory were observed in some of these tidal bundle sequences, and we interpret them to be the result of seasonal rainfall variations that affected sediment discharge. The possibility that seasonal 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 Illinois basin provides direct evidence for their occurrence.

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