Location: Illinois.
Age: Pennsylvanian, Carboniferous.
| Name: Mazon
Creek. Location: Illinois. Age: Pennsylvanian, Carboniferous. |
|
Palaeogeography:
Sediments of the Mazon Creek were deposited during the Middle
Pennsylvanian Westphalian D stage of the Late Carboniferous,
approximately 296 million years ago [1]. Palaeogeographic reconstructions
suggest that most of the North American continent was in tropical
latitudes, with Illinois a few degrees from the palaeoequator [2]
(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 [2].
Stratigraphy:
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 [1]. 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 [1].
Mazon Creek fossils are found in various localities of Northern
Illinois, including parts of Grundy, Will, Kankakee, Essex and
LaSalle counties.
Depositional Environment:
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 [2]. Cyclic
repetition of marine and non-marine strata led to the deposition
of sediments during the progradational advancement of an active
coastal deltaic system [2]. 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 [3]. 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].
