Mazon Creek Fossil Beds - Geology

Geology

The Mazon Creek fossils are found in the Upper Carboniferous Francis Creek Shale; the type locality is the Mazon River (or Mazon Creek), a tributary of the Illinois River near Morris, Grundy County, Illinois. The 25 to 30 meters of shale were formed approximately 300 million years ago, during the Pennsylvanian period. The fossiliferous concretions are usually found within the thickest deposits of Francis Creek. The concretions occur in localized deposits within the silty to sandy mudstones, in the lower four metres of the formation. The paleoecosystem is believed to be a large river delta system, deposited by at least one major river system flowing from the northeast. The sediments are believed to derive from the Appalachian orogeny events. The delta had a tropical climate, a result of the area being within 10° north latitude of the equator during the Pennsylvanian.

The remains of plants and animals were rapidly buried by the sediment deposited in the deltaic system. Bacterial decomposition of the remains produced carbon dioxide that combined with dissolved iron from the groundwater. This process formed siderite in the sediments surrounding the remains, forming detailed casts of their structure. Lithification of the sediments formed protective nodules of ironstone around the now fossilized remains. This mode of preservation is known as authigenic mineralisation.

The fossiliferous concretions are found in the Mazon River area of Grundy, Will, Kankakee, and Livingston counties. Additional fossils are found in LaSalle County, Illinois; between the Vermilion River and Marseilles, Illinois. The ironstone concretions are recovered from exposures along streams, roadcuts and in active or abandoned coal mine areas.

The site's importance was realized in the mid-nineteenth century: "the nodules of Mazon Creek, where fragments of plants, even of the softest texture, have been preserved in their integrity".