Abstract

Introduction

Middle and Upper Ordovician formations are well developed in Minnesota, where the following classification is usually recognized:

Cincinnatian

Maquoketa shale

Mohawkian

Galena limestone

Stewartville dolomite

Prosser limestone

Decorah shale

Platteville limestone

Glenwood beds

Chazyan

Saint Peter sandstone

The Glenwood beds are generally of slight thickness and rest disconformably on the Saint Peter sandstone. Although these beds are not very fossiliferous, their contained fauna is sufficiently distinctive to cause them to be included in the Mohawkian series.

Kay has established a three-fold division of the Decorah formation 2 in Iowa. These divisions can be recognized to some extent in Minnesota as well, but they are less distinct than farther to the south. In fact, the formation in Minnesota is more truly a shale, and many of its limestone layers are lenticular. Faunally, however, subdivision is possible. The Decorah shale is easily separated from the Platteville limestone, but it grades upward . . .

First Page Preview

First page PDF preview
You do not currently have access to this article.