Second Stop: Fithian cyclothem

Getting There:

Outcrop at Salt Fork of the Vermilion River near Fithian

The Outcrop:

The Fithian Cyclothem: Lithology

The Fithian cyclothem is shown diagrammatically on the left (sandstone thorough limestone layers).  The sandstone, mudstone, and coal layers formed in a freshwater environment while the limestone and shale formed in a marine environment.

Limestone: typically forms from skeletal fragments of organisms (fossils) in a warm shallow ocean environment
            It fizzes with acid as shown in picture below.
Shale: forms in deep ocean environment with little oxygen. It is fissile, or splits into thin sheets, and 
            carbonaceous (rich in organic carbon, black in color)

Coal: forms in a tropical swamp from plant matter buried in anaerobic conditions. Commonly contains pyrite, which weathers to iron oxide.
Mudstone: forms in quiet waters such as deep ocean, deep lake, swamp, or in this case floodplains. Shows effects of leaching due to absence of calcite and increasing clay abundance.
Mudstone with Paleosol: soil for plants in a tropical swamp. Contains wood debris, iron staining, and possibly root casts.
Sandstone: forms in active waters such as a beach, river, dune, or in this case a river channel  

Geologic History of the Fithian Cyclothem

Beginning in the late Mississippian Period and continuing through the Early Permian Period, the North American craton displayed a repetitive pattern of sedimentation. 

A typical sequence of sedimentary rock, called a cyclothem, consists of an unconformity, cross-bedded sandstone or conglomerate, mudstone, coal, marine limestone, and shale (As depicted in figure above). Cyclothems can be correlated over large distances. To the east they tend to thicken, have more coal, and less of a marine influence, while to the west they are thinner, have less coal, and more marine influences.  

Alternations of non-marine (mudstone, coal, sandstone) and marine (shale. limestone) strata are interpreted as having formed during marine transgressions, or times of relatively high sea level and marine regressions, or times of low sea level . The changing level of epicontinental seas during this time was due to the advance and retreat of continental glaciers.  The widespread effects of the transgressions and regressions are probably due to the fact that much of North America was flat and near sea level at that time.  Therefore, a small change in sea level would cause widespread transgressions and regressions.

Additional history:
Cyclothems were named in 1932 by Harold Wanless, a former professor in the Dept. of Geology at Univ. of Illinois at Urbana-Champaign.

The Salt Fork of the Vermilion River gets its name from saline seeps along the river southeast of Kickapoo State Park. Saline seeps represent rising salty groundwater located in bedrock that is close the surface.

Importance of Cyclothems

This outcrop contains coal, a major energy resource. The major coal seams in Illinois are numbered (#1 - 8) and named.  All occur in Pennsylvanian age rock. In this area of east central Illinois, the Danville #7 and Herrin #6 coal seams were mined. All of the coal in Illinois is bituminous rank (80% fixed carbon) and most of it has a relatively high sulfur content.  Sulfur in coal occurs mainly in the form of pyrite, or fool's gold.  When pyrite is exposed at the surface, it weathers quickly to iron oxides and sulfuric acid, which is damaging for many plants and animals. This is why strip-mines must be reclaimed, i.e., restored to the original look.  Kickapoo State Park and other areas around Danville, IL are reclaimed coal mines.

The Fithian Cyclothem outcrop also contains mudstone which in other areas has been mined and used for brick manufacturing. 

Additional Pictures of the Outcrop:  

Site created by: Rachel Vinsel in Spring of 2010