Decorah Formation



The Decorah Formation is the rock unit that I cut my teeth on some 33 years ago. Having completed my undergraduate Paleontology class several of us budding geologists set out to amass a Decorah Formation Type collection. It was on a Paleontology field trip to the Decorah Formation at Rochester that I found my first complete trilobite, a rolled Eomonorachus. I wish that I had logged my field collecting in those days. It would be an interesting read today.

We concentrated our collecting, at that time, over the area extending from the Type Locality in Decorah, Iowa, northwest through Rochester, Minnesota up to the Canon Falls, Minnesota area, an outcrop distance of roughly 100 miles long by approximately 25 miles wide.

Following the sea level high stand represented by the underlying carbonate Platteville Formation sea levels dropped in the UMV or more likely the Transcontinental Arch rose creating a renewed source of terrigenous sediment that swept in from the northwest into the area.

Seaward, toward the southeast into southwestern Wisconsin and northern Illinois and out of the Hollandale Embayment the Decorah remained in predominantly carbonate terrain but moving northwest the carbonates disappeared replaced by predominantly shale deposition. As the Galena transgression continued the shale facies, represented by the Decorah stepped further northwestward replaced by carbonates in the form of the Cummingsville member (Dunleith) of the Galena. The Decorah, therefore gets younger moving toward the northwest. In outcrops in Wisconsin and along the Mississippi River the formation is generally Kirkfieldian (lower Chatfieldian) whereas the upper Decorah at Canon Falls, Minnesota is Shermanian in age.

The formation thus correlates with different units depending on where one is in the stratigraphic section and where locality wise.

The Millbrig K-bentonite (an altered volcanic ash bed) has been correlated with the Kinnekulle K-bentonite in Baltoscandia and throughout eastern North America. Recent efforts seem to be unraveling the complex northeast (New York & Ontario). The Millbrig, now thought to represent several volcanic eruptions over a very short time period occurs within the lower Decorah formation and allows a fixed time reference of 454 million years. The Millbrig is now used to demarcate the lower boundary of the Chatfieldian stage.

Trilobite collecting in the Decorah Formation is generally restricted to collecting parts. The shallow water environment under which the formation was deposited was not conducive to preserving complete specimens. Though they do occur the formation produces primarily disarticulated parts.