Abstract

Devonian rocks of the north-central region occur in an irregular southeast-northwest band extending from the western slope of the Cincinnati arch in Indiana to the eastern slope of the Cambridge arch in central Nebraska. Northward they are bounded by the Sioux arch-Canadian Shield-Wisconsin arch complex. On the south they are limited by the Ozark uplift, the Pascola arch and the Nashville dome.

The eastern third of the area is dominated by the Illinois basin in which there are more than 1700 feet of marine Devonian sediments representing a virtually complete record correlated to the Devonian standards of New York and western Europe by means of conodonts and brachiopods. In the western part of the area, deposition occurred in a broad, shallow basin centred in central and southwestern Iowa. More than 700 feet of sediments accumulated mainly during middle and late Devonian time with probable Lower Devonian rocks confined to the deep part.

A broad arch involving the Lincoln fold of northeastern Missouri and the Sangamon arch of Illinois formed a divide over which no Lower and few Middle Devonian rocks were deposited but which was blanketed by argillaceous sediments in late Devonian time.

Lithologically, the Lower Devonian is dominated by siliceous carbonates. The Middle consists mainly of sandy fossiliferous limestone but also includes nonfossiliferous lithographic limestone, gypsum, and sandstone. The Upper Devonian consists of widespread black and grey shales with prominent limestone facies in Iowa.