Mazon Creek-type fossil assemblages in the U.S. midcontinent Pennsylvanian: their recurrent character and palaeoenvironmental significance

Abstract

Terrestrial plants, the non-marine Braidwood Fauna, and the euryhaline Essex Fauna, best known from the Francis Creek Shale Member within the Mazon Creek area of northeastern Illinois, are found to recur in analogous deltaic lithofacies elsewhere both in Illinois and in adjacent states. An important new Essex-type fossil locality is reported from deposits coeval with the Francis Creek Member in Missouri, and occurrences of Braidwood animals from additional stratigraphic units are discussed. These assemblages occur in estuarine-deltaic deposits juxtaposed on coals; the significance of the association of these fossils with transgressive inundation of coastal peat swamps is discussed and a predictive model for the occurrence of Mazon Creek-type assemblages is presented. We believe that fossil associations comparable to those at Mazon Creek, occur in certain coastal deposits ranging in age from Pennsylvanian to, at least, the Triassic.