Formation · Carboniferous
Mazon Creek (Francis Creek Shale)
Pennsylvanian unit, Illinois
Mazon Creek is the Pennsylvanian ironstone-concretion Lagerstätte of northern Illinois, split into a Braidwood (freshwater and terrestrial) and an Essex (marine) biota. Ferns and seed-fern foliage such as Neuropteris, Alethopteris and Pecopteris are common and inexpensive; the Tully monster (Tullimonstrum, Illinois state fossil), jellyfish, shrimp and insects are prized. Concretions are opened by freeze-thaw cycling, and both halves (part and counterpart) should stay together; be wary of 'rare' animals that are actually ambiguous blobs.
| Country | United States |
|---|---|
| Region | Illinois |
| Age | Moscovian, Pennsylvanian |
| Age range | ~309–307 Ma |
| Group | Carbondale Formation |
| Member | Francis Creek Shale Member |
| PBDB collections | 91 collections, 566 occurrences· Paleobiology Database |
Collecting and legality
Pit 11 (Mazonia–Braidwood Fish and Wildlife Area) is Illinois state land: collecting requires a free permit from the site office, is limited to the March–September season, and material may not be sold. Nearly all Mazon Creek material in trade is old collection stock from the strip-mining decades; document that history when you sell.
Lithology and setting
Grey silty shale of a river delta prograding into a coastal swamp; fossils are enclosed in siderite (ironstone) concretions that split along the fossil plane.
Commonly recorded taxa (PBDB)
Genera most often recorded from this unit in the Paleobiology Database (first 200 occurrences): Gerarus (17), Miamia (11), Eucaenus (9), Asterotheca (4), Eubleptus (4), Lithoneura (4), NO_GENUS_SPECIFIED (4), Gerapompus (3), Insecta (3), Mylacris (3), Prochoroptera (3), Stellatheca (3), Adiphlebia (2), Architarbus (2), Asemoblatta (2).
Source: Paleobiology Database
Collecting rules
Elsewhere
Cite this article
Stratum Study. "Mazon Creek (Francis Creek Shale)" (Formation). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/formations/mazon-creek-francis-creek-shale. Accessed 2026-09-20.
@misc{stratum-mazon-creek-francis-creek-shale,
title = {Mazon Creek (Francis Creek Shale)},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/formations/mazon-creek-francis-creek-shale}},
year = {2026},
note = {Formation article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, pbdb, commons. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.