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Journal Article

Biota of a Pennsylvanian Muddy Coast: Habitats within the Mazonian Delta Complex, Northeast Illinois

Gordon C. Baird, Charles W. Shabica, John L. Anderson and Eugene S. Richardson, Jr.
Journal of Paleontology
Vol. 59, No. 2 (Mar., 1985), pp. 253-281
Stable URL: http://www.jstor.org/stable/1305027
Page Count: 29

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Topics: Fossils, Braiding, Coal, Fauna, Taxa, Animals, Shales, Creeks, Strip mining, Geological facies
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Biota of a Pennsylvanian Muddy Coast: Habitats within the Mazonian Delta Complex, Northeast Illinois
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Abstract

The Mazon Creek biota (Westphalian D) is composed of plants and animals from terrestrial, fresh water and marginal marine habitats. Fossil animals, including jellyfish, worms, crustaceans, holothurians, insects, chordates, and problematica occur in sideritic concretions on spoil piles of more than 100 abandoned coal mines in a five-county region (Mazon Creek area) of northeast Illinois. These fossils record rapid burial and early diagenesis in a muddy, delta-influenced coastal setting submerged during marine transgression. Fossils are associated with laminar mudstone in the Francis Creek Shale Member of the Carbondale Formation, which records moderate to very rapid sedimentation in quiet water. Rapid engulfment of organisms is indicated by upright-standing trees, edgewise-buried plant leaves, and escape burrows with associated animals. Very regular sedimentary rhythms in the lower Francis Creek record daily sediment accumulation in a tide-influenced shallow bay or estuary. Major fossil associations include: 1) terrestrial plants and animals from coastal swamp, levee, and floodplain settings; 2) fresh water animals; and 3) euryhaline animals inhabiting waters near distributaries. Extensive fossil sampling shows a distinct regional boundary between marine (Essex) and nonmarine (Braidwood) faunal assemblages which parallels the ancient coastline. As this boundary is crossed, a low diversity nonmarine fauna is replaced by a diverse association of eurytopic marine benthonic and pelagic taxa. The faunal gradient between these associations as well as spatial changes in species dominance within the Essex fauna are attributed to salinity, turbidity, and oxygen depletion stress between localities. The Essex fauna is compared to coeval normal marine shelf faunas in western Illinois. Intermediate fossil groupings are absent in the Francis Creek, but one assemblage (Danville Fauna) in a younger Pennsylvanian unit clearly shows intermediate faunal composition and taphonomic characteristics; this delta margin biota is described and contrasted with Essex and normal marine faunas.

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