Age: Pennsylvanian, Carboniferous.
Sediments of the Mazon Creek were deposited during the Middle Pennsylvanian Westphalian D stage of the Late Carboniferous, approximately 296 million years ago . Palaeogeographic reconstructions suggest that most of the North American continent was in tropical latitudes, with Illinois a few degrees from the palaeoequator  (Figure 1). Low-lying coastal regions, now covering parts of Illinois and some adjacent states were submerged by an epeiric sea, while further west in what is now the Great Basin of the United States .
The Francis Creek Shale is a member of the Carbondale Formation and is underlain by the laterally widespread Colchester (No.2) Coal Member. Strip and deep mining of this coal horizon created exposures and produced the spoil heaps from which the concretions were collected . The Francis Creek Shale is of variable thickness, reaching a maximum of 25-30 metres in NE Illinois. Where the Member is thin, it is overlain by the Mecca Quarry Member, a thin, fissile black shale unit. Fossiliferous siderite concretions are characteristic of the thickest Francis Creek deposits. They occur in highly localised bands within the silty to sandy mudstones in the basal 4 metres of the facies . Mazon Creek fossils are found in various localities of Northern Illinois, including parts of Grundy, Will, Kankakee, Essex and LaSalle counties.
The Francis Creek Shale of the Mazon Creek area comprises a series of discontinuous detrital mud, silt and sandstone units sourced from the then tectonically active Appalachian region . Cyclic repetition of marine and non-marine strata led to the deposition of sediments during the progradational advancement of an active coastal deltaic system . This transitional estuarine environment enabled deposition, which occurred both during and after various marine transgressive episodes of inundation by a large epicontinental sea that extended into the Mazon Creek area . A major river flowed from the NE and deposition of the Francis Creek Shale occurred following a marine transgressive event [3, 8]. Several connected distributaries episodically deposited organic-rich, fine muddy sediments following flood induced crevasse splays, infilling interdistributary estuarine bays [2, 3, 4].