Make your own free website on Tripod.com

Pennsylvanian
Geology

 

THE PENNSYLVANIAN PERIOD IN ILLINOIS  

GEOLOGY

DELTA FORMATION

The ancient Michigan River was building immense deltas out into the shallow Chicago sea, carrying in sediment from southeast Canada. While sand, silt and lime were accumulating in the sea, forming sandstone, shale and limestone, coastal coal swamps flourished on the adjacent lands along the deltas. The plant debris from these swamps accumulated in thick mats of peat and eventually turned into Illinois coal.

It has been estimated that it took between 5 and 10 feet of plant material to make each foot of coal in the Illinois Basin, thus, requiring a deposit of 35 to 70 feet of plant material to produce the 7 foot tall seam of the Herrin #6 coal, found in this area.

Figure from: A View of the Past - An Introduction to Illinois Geology, Christopher J. Schuberth, Illinois State Museum

THE ILLINOIS BASIN

In Illinois the youngest Pennsylvanian strata lies in southeastern Illinois and is surrounded by progressively older strata. Such an age distribution, with the young- est in the center, and progressively older strata surrounding it results from the broad downwarp of the Illinois basin. Almost all of Illinois, Southwest Indiana and western Kentucky are part of the Illinois Basin, a sub circular or spoon shaped depression 250 to 300 miles in diameter.

The above figure is a structure map of Cambrian strata that allows one to "see" below the ground surface to the top of the Cambrian strata.


Figure from: A View of the Past - An Introduction to Illinois Geology, Christopher J. Schuberth, Illinois State Museum

In north-central Illinois it appears on or near the surface, but in southeastern Illinois it lies nearly 7,000 feet underground. This basin structure is where most of the coal in the state is located.


A similar structure can be seen in the figure to the below. This is a map of the bedrock geology of the state and you can see the semi-teardrop shaped, bright green area in the southeast which represents the youngest Pennsylvanian strata.


Map from the Illinois Department of Natural Resources


The map to the left shows the distribution of coal within the state of Illinois. This includes all coal, whether or not economically mineable.

Map from the Illinois Department of
Natural Resources

PAGE LAST UPDATED: MARCH 4, 2002