Cyclic laminations in gray shales are noted above many major coal seams in the Illinois Basin. These features, which are termed “tidal rhythmites,” indicate significant tidal influence during deposition of the roof strata. Many existing models, however, suggest fluvial dominance and crevasse splay deposition for such gray shales.

Based on sedimentological and paleogeographical considerations, appropriate deposition models require micro- to mesotidal environments, limited wave reworking, and a mud-dominated, tropical setting. Following these requirements, tidally influenced estuarine/deltaic models can be generated for roof strata of many Illinois Basin coals. This model is based upon modern mud-rich tidally influenced estuaries and deltas in near equatorial settings and can be used to explain occurrence of shales that lack obvious marine influence (based upon paleontology) above low-sulfur coals in the Illinois Basin.