Mid-Pennsylvanian Epoch - Carboniferous Period (approximately 280 million years ago)
The Mazon Creek is an important lagerstatte in that a great diversity of organisms are found in a remarkable quality of preservation, many times both hard and soft parts are preserved. Most fossils are found in concretions (siderite nodules) along the Mazon River area of the Illinois counties of Grundy, Will, Kankakee, Livingston and LaSalle, and are recovered from formation exposures along the river and nearby streams, roadcuts and within waste piles of abandoned and active coal mines. The most well-known location is "pit 11", which is largely inaccessable and mostly flooded and used as a cooling basin for the Braidwood nuclear reactor. Additional exposure areas are now state parks and managed by the Illinois Department of Natural Resources.
Fossils can be found in localized mudstone units of the Francis Creek Shale, and occur in iron carbonate concretions, created due to rapid burial of the organisms in siderite-rich sediments. The concretions contain detailed casts of a great number of flora and fauna, often times preserving soft parts not normally seen in fossilization.
During the Middle Pennsylvanian, modern day Illinois was then a tropical climate, located just a few degrees north of the equator. What now contains the siderite concretions, the Francis Creek Shale is due to deposits formed from a large marine delta / interdistributary bay created by at least one, but more probably many river systems, the greatest of which was flowing from the northeast. The swampy lowland environment / interchannel flood plain sustained a wide variety of plant and animal life, both terrestrial and aquatic.
The remarkable diversity of life represented by Mazon Creek concretions is well-known throughout the world (most major museums contain at least some Mazon Creek nodules in their collections), and represents both flora and fauna. Over 400 species and 130 genera of flora have been identified, although research is currently being done on distinguishing actual species from those specimens that merely represent differing examples within the same species. Represented examples include the seed ferns, relatives of the club mosses, horsetails, gymnosperms, fossilized bark and seeds / cones. Additionally, over 320 species of fauna have been identified so far. (...and the amount of flora and fauna keep increasing as new species are continually found)
Fauna is divided into two major sub-categories: Essex Biota and Braidwood Biota.
The Essex fauna is characterized by marine organisms that lived in the shallow bay of the deltaic system, while the Braidwood fauna consists of land and freshwater organisms that were most-likely washed into the bay by flooding events and other causes. Major fossil representations in these two faunas include:
The continents, as they were during the Carboniferous Period.
Collecting Mazon Creek Fossils
Active collecting of Mazon Creek nodules is still taking place, although many of the old collecting spots are gone or disappearing. Some access points are located on public lands, and eager collectors flock to the area when the river is shallow and banks are exposed.
1,017 acres of land once part of the old strip mining property where pit 11 is located is now managed by the Illinois Department of Natural Resources. Designated the Mazonia State Fish and Wildlife Area, fossil hunting is allowed from March 1 through September 30. Some paths are available throughout the area, but a good amount of hiking is generally involved. Although many of the spoil piles tend to be overgrown, some areas are occassionally plowed up by staff in order to expose new fossil nodules. Ask an employee when visiting to find out if any areas have recently been plowed.
A day permit is required to collect fossils. The permit can be obtained from the park office or from their website. http://dnr.state.il.us/LANDS/Landmgt/PARKS/R2/MAZONIA.HTM#Fossil
Fossils collected within the Mazonia State Fish and Wildlife Area must be reported following the collection day.
Some museums and various Illinois geology organizations offer fossil collecting field trip opportunities, many of them taking place on private land arranged through the owner.
For identification of fossils from the Mazon Creek, the best resource tends to be the two reference books written by Jack Wittry and others from the Earth Science Club of Northern Illinois (ESCONI):
The Mazon Creek Fossil Fauna and The Mazon Creek Fossil Flora
Both books can be purchased directly from the club's website:
Please visit the following pages if looking for additional resources on the Mazon Creek Lagerstatte:
Mazonia - Braidwood State Fish and Wildlife Area
The Vanishing Mother Lode of Mazon Creek
Great historical account of Mazon Creek fossil collecting.
Illinois State Museum - About the Mazon Creek Fossils and Deposit
University of California - Berkeley - Localities of the Carboniferous, Mazon Creek IL.
The Field Museum - Mazon Creek Fossil Invertebrates
Earth Science Club of Northern Illinois (ESCONI)
Fossil Museum.net - Fossils of Mazon Creek
Search Mazon Creek Fossil Galleries