Maquoketa Group

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The Maquoketa Group is an assemblage of several geologic formations. It is Upper Ordovician in age and named for the Maquoketa River in Iowa. It exists in Missouri, Wisconsin, Iowa, Illinois, and Indiana. It is equivalent to the all but the basal formations of the Cincinnati Group in Ohio. Illinois and Indiana are the only states where the Maquoketa is considered a group. In other states it is a formation. The Maquoketa was deposited in a shallow intercontinental sea. Most of the clastic sediments are derived from the Appalachians, which were being raised at the end of the Ordovician. To a lesser degree some of the sediments were derived from the Ozarks.

1 Scales Formation
2 Fort Atkinson Formation
3 Brainard Formation
4 Neda Formation
5 Economic uses
6 References

[edit] Scales Formation

In Illinois the basal formation is called the Scales Formation and consists mainly of dark brown and gray dolomitic shale. In north central Illinois the bottom half is dominantly a yellowish gray coarse crystalline dolostone (rock form of dolomite which is related to calcite). It doesn't contain many fossils, although large trilobites and large cephalopods are known from Elmer-Lason Quarry. It is typically 75 to 100 feet thick. It is the only formation of the Maquoketa Group that is not exposed in Northeastern Illinois.

The type section for the Scales Formation is in Jo Daviess County, Illinois. In Illinois the Formation is divided into two members, the lower Elgin Member (mostly dark gray to brown and black shale with dolostone) and the upper Clermont Member (upper gray pure shale) both with type sections in Fayette County, Iowa. However, in Iowa the Scales, Elgin, and Clermont are all members of the Maquoketa Formation (the Maquoketa is not considered a group in Iowa). Over the years this has slowly led to the Scales being used in Eastern Iowa for the dark shale and the Elgin used in Western Iowa to refer to the dolostone that grades laterally into the Scales. The difference in terms between Iowa and Illinois has made the present nomenclature all but impractical. There is an active effort to revise the nomenclature. Present studies suggest that the Maquoketa should remain a group and not a formation for practical purposes. It is better to have a group with several formations, each with one or two members than it is to have a single formation with a dozen members.

This table below is a work in progress of updating the nomenclature for the entire Maquoketa Group and is NOT a finished version of the chart.

This photo is of a rock core taken in 2004 near Aux Sable Creek in Illinois. It shows the typical color of the Scales Formation. The numbers at the top are the depths in feet below the surface that the cores were taken from.

[edit] Fort Atkinson Formation

The overlying formation is known as the Fort Atkinson Formation and has been informally called the Middle or Divine Limestone by drillers. The Fort Atkinson is a dominantly a red, gray, and white coarse crystalline dolostone with thin beds of green and gray shale. It contains many invertebrate fossils, such as brachiopods and abundant corals. It ranges from 0 to 60 feet in thickness but is typically 40 to 50 feet thick.

This photo is of a piece of the Fort Atkinson along Aux Sable Creek in Illinois. It shows the coarse texture and multitude of colors.

[edit] Brainard Formation

The formation above the Fort Atkinson is the Brainard Formation. The Brainard is generally a greenish gray to gray dolomitic soft to hard shale. It contains abundant fossils near the base. It ranges from 0 to 100 feet thick. The variable thickness is due to an erosional unconformity caused by the an ice age at the end of the Ordovician, which lowered global sea level.

The photo below was taken in early 2008 at Waubonsie Creek in Oswego, Illinois. The greenish rock under the concrete block at top center is the base of the Brainard and is dominantly a sticky green shale with thin beds of purplish gray dolostone..

[edit] Neda Formation

The top and youngest formation is the Neda Formation. It exists in small patchy areas and was extensively eroded at the end of the Ordovician. It exists in large isolated patches in the subsurface in Lake, Cook, Will and Du Page Counties in Illinois. In Illinois it outcrops only at Kankakee River State Park. It is a reddish purple to brown oolitic shale and is sometimes cemented with iron and dolomite. The oolites are black and made of iron instead of the typical calcareous oolites. They are about 0.5 millimeters in diameter. There are no known fossils. In Illinois, it can be as much as 16 feet thick but it is typically no more than 8 feet thick.

It's origin is still debated. We do know that it was deposited near or on an ancient shore and is a sedimentary rock. However, iron oolites are not being actively deposited anywhere in the world today. It is possible that it was originally deposited as a calcareous oolite and hematite later replaced the calcite. Or it could have been deposited directly as an iron oolite. There is strong evidence for both theories. The world's surface has changed dramatically since the end of the Ordovician. There is something present today that was not present at the time that the Neda Formation was deposited and that is land plants. There were no bushes, trees, ferns, flowers, grass, or any other plants on land at the time of deposition.

This picture was taken in 2006 at Kankakee River State Park. The black pencil near the right center of the picture is near the Silurian-Ordovician contact.

[edit] Economic uses

The Maquoketa has almost no economic use. Since it is dominantly a week shale interbedded with thin dolostone, it isn't good for track ballast or concrete aggregate and is often stripped and piled as waste in open-pit quarries. The main exception is at the Neda's type section in Wisconsin, where it was once mined for iron ore. In Illinois the Neda is too thin and patchy to be economical. The Brainard is too soft to be used as anything other than fill soil. The Fort Atkinson is used to a limited degree as a decorative stone. The Scales is buried too deep and contains too much dolostone to be used for oil shale.

The Maquoketa group is also a major aquitard. This means that water cannot be removed from it for drinking or industrial uses.

Although not useful to industry it does contain rare and diverse fossils, some of which are only known in Illinois such as Tentaculites oswegoensis, a small unclassified fossil named after Oswego, Illinois and found in the basal Brainard.