Ichnotaxonomic assessment of Mazon Creek area trace fossils, Illinois, USA

Abstract

The Francis Creek Shale Member (FCSM) of the Mid-Pennsylvanian Carbondale Formation along Mazon Creek in northern Illinois is known for soft-bodied organisms preserved within siderite concretions. Trace fossils, though less well known, also occur in concretions. Several hundred trace-fossil-bearing concretions were examined from collections at The Field Museum, Chicago, and The University of Kansas Natural History Museum, Lawrence. Identification was based on accepted diagnostic criteria and comparison with like specimens. Described were 24 ichnogenera and 28 ichnospecies, including a new ichnospecies of Pennatulites and a new ichnogenera of coprolite--Funiculichnus spiralis, unnamed trace fossils including a bivalve-locomotion trail, spiral and amorphous coprolites, and two possible galls, and several deformation structures and body fossils misidentified as trace fossils. Trace-fossil assemblages indicate a Skolithos-Cruziana ichnofacies transition from shallow, high-energy to deeper, lower-energy nearshore marine environments, which is consistent with previous interpretations that the FCSM was deposited in a nearshore, deltaic environment.