Mazon Creek Flora

The Penssylvanian Flora of Mazon Creek Ana Maria Martinez Loma Linda University School of Public Health Department of Earth and Biological Sciences Professor: Roberto Biaggi Ph.D. December 10, 2012

The Pennsylvanian Mazon Creek Flora

The Pennsylvanian Flora of Mazon Creek

One of the most remarkable soft-bodied fossil deposits on Earth is the Pennsylvanianaged Mazon Creek flora innortheastern Illinois. It was named for the stream in whose bed and walls the first fossils were found (Savage, 1910). These fossils were destined to make this area the best known Pennsylvanian flora in North America (Darrah, 1970: Horowitz, 1979) with specimens found in all the great natural history museums of the world. In 1857, concretions containing plant fossils found in the bed of Mazon CreekRiver and from small scale coal mines were sent to the paleobotanist Leo Lesquereux. His classification and description of 140 species which around 50 were new brought this area to the attention of the scientific community.

The term Mazon Creek biota can be applied to all floras collected from the Francis Creek Shale of the Carbondale Formation (Moore et al., 1957). The Francis Creek Shale hasgenerally been considered unfossiliferous except at few localities including the Mazon Creek area. In the Mazon Creek area, the Francis Creek Shale consists of concretionary gray shale. The concretions are composed of siderite “ironstone” and can be fossiliferous or unfossiliferous. The fossiliferous concretions contain land plants and terrestrial & marine animals with an excellent preservation. TheFrancis Creek Member is considered Middle Pennsylvanian (Pfefferkon, Peppers and Phillips, 1971; Darrah, 1970) and on the basis of floral content the unit is correlated with Westphalia D levels of Europe and the age of the deposits is about 290 My (Baird, 1978).

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The Pennsylvanian Mazon Creek Flora Within the Mazon Creek area two biotas have been discovered. One, called the Braidwood Biotais of terrestrial and fresh-water fossils. It contains most of the plant fossils and occasionally terrestrial animals, such as millipedes, arachnids or insects. The other biota, named Essex, is of a near-shore brackish or marine environment, containing mostly marine fossil animals, like sea jellies, annelids, fish and Illinois’ state fossil the Tully Monster (Richardson and Johnson, 1971). Theplant fossils in this area are mostly small and poorly preserved. The dividing line in some areas is very clear; in other places the distinction is difficult to determine.

This paper will first describe the most important geologic features of the Mazon Creek flora, such as: location, stratigraphy, age, depositional environments and paleoecology. Second, it will describe and explain the compositionand diversity of this flora. Finally, it will present a detailed description of the most important plant fossils found in this flora.

Location

Ironstone concretions containing Mazon Creek fossils are found in outcrops of the Francis Creek Shale. The Francis Creek Shale is exposed along streams and in strip mines in LaSalle County between the Vermillion River and Marseilles (Baird, 1978). Ingeneral, most of the fossils come from the Mazon Creek area of Grundy, Will, Kankakee, and Livingston counties, northeastern Illinois (Figure 1). In this area fossils are recovered from natural exposures on Mazon Creek, from active and abandoned strip mines, from shaft mines, and from mine spoil piles. Fossils like those from Mazon Creek are recovered from a few other areas in Illinois.

3The Pennsylvanian Mazon Creek Flora

Figure 1. Location of the Mazon Creek area and strip mines where most of the fossils had been found (fromhttp://fieldmuseum.org/explore/our-collections/mazon-creek-flora). Stratigraphy and Age

The Francis Creek Shale Member is the first unit above the basal member (Colchester 2 coal) and before the deposition of the Mecca Quarry Shale Member, of the...