Cambrian and Ordovician Rocks in Michigan Basin and Adjoining Areas

Abstract

Sandstone, limestone, and dolomite of Upper Cambrian and Ordovician age either are exposed at the surface or underlie glacial drift in the Northern Peninsula of Michigan. The outcrop of these rocks extends southward from northern Michigan into Wisconsin and Illinois. Eastward across the Northern Peninsula, Middle Ordovician rocks overlap progressively older rocks, and on Manitoulin and adjacent islands Black River rocks of Middle Ordovician age rest on pre-Cambrian. This same stratigraphic relationship is observed in other outcrop areas in Ontario and in deep wells that have penetrated the full section of sedimentary rocks. Pre-Black River erosion removed most of the older Paleozoic rocks from the underlying pre-Cambrian in southwestern Ontario and in some places only a t in remnant of Upper Cambrian sandstone and some dolomite occurs between Middle Ordovician and pre-Cambrian rocks. Cambrian and Lower Ordovician (sub-Trenton) sandstone and dolomite range in thickness from less than 500 feet to more than 3,500 feet in Michigan. The aggregate thickness of these rocks increases southwestward from Michigan toward Illinois and thins southeastward over the Findlay arch in southeastern Michigan and northwestern Ohio. The rocks increase in thickness eastward into the Appalachian basin from the Findlay arch. The widespread limestones and dolomites of Middle Ordovician age rest on eroded Lower Ordovician and Upper Cambrian rocks in Michigan, northern Indiana, and northwestern Ohio and range in thickness from less than 200 feet in the Northern Peninsula to more than 900 feet in southeastern Michigan. These rocks are thickest a short distance east of the center of the present Michigan basin and are thinner over major anticlinal folds. This indicates that the basin was depressed in an area somewhat similar to its present form as early as Middle Ordovician time. Oil and gas were commercially produced from the sub-Trenton rocks in Ohio on the east flank of the Findlay arch in several small areas. Showings of oil and gas in these rocks have been reported from wells in northeastern Indiana, southeastern Michigan, and southwestern Ontario. The Trenton limestone, which is the producing formation in the Lima-Indiana field, is productive in small areas in southeastern Michigan and southwestern Ontario.