Mazon Creek: Preservation
in Late Paleozoic Deltaic
and Marginal Marine Environments
Stephen A. Schellenberg
IN THE MAZON CREEK AREA OF NORTHEASTERN ILLINOIS (Figure 10.1), fossiliferous concretions of the Francis Creek Shale provide an unparalleled window into late Pennsylvanian life in deltaic and coastal environments. The exquisitely preserved, commonly soft-bodied fauna spans a wide range of life modes (flying to infaunal) and environments (terrestrial to freshwater to full marine) (Richardson and Johnson 1971), while the flora is among the most diverse known from North America (Darrah 1970; Horowitz 1979). Some Mazon Creek specimens represent the earliest or only known fossil occurrence of their higher taxa; a few remain systematic mysteries altogether (Nitecki 1979a). Many Mazon Creek organisms passed relatively unscathed through the fossilization barrier, starting with rapid burial within low-oxygen sediments, followed by early diagenetic encapsulation within iron-rich carbonate (siderite) concretions (Baird et al. 1986). Rapid fossilization is apparent by the preservation of ink sacs in cephalopods, yolk sacs in hatchling fish, and articulated setal hairs in polychaete worms. In some cases, preservation is threedimensional and color patterns are preserved. Thus, Mazon Creek concretions are considered to be an extreme form of conservation Lagerstätte (Seilacher, Reif, and Westphal 1985).
The first reported Mazon Creek fossils were collected by the local populace during the mid-nineteenth century from natural outcrops of the Francis Creek Shale. Subsequent development of regional strip and shaft mining of the underlying Colchester (No. 2) Coal (Ledvina 1997)