CENTRAL STABLE REGION OF THE UNITED STATES 



41 



The major unconformity in the Paleozoic sequence is at the base of the 

 St. Peter sandstone and the Trenton and Black River limestones. See 

 Figs. 5.4 and 5.5. The St. Peter sandstone is late Lower Ordovician, and 

 marks the time of uplift and erosion. When traced eastward from Indiana 

 to Ohio and northeastward into Ontario in well logs, the Lower and 

 Middle Ordovician formations rest successively across the several forma- 

 tions of the Upper Cambrian, and finally come to rest directly on the 

 Precambrian crystallines of the Canadian Shield. Through western 

 Ontario, the Cambrian beds are absent. 



Significant units in the Michigan basin are the evaporite series of the 

 Silurian and Devonian. A number of beds of salt are present throughout 

 much of the basin and southwestern Ontario which in places may aggre- 

 gate over 2000 feet in thickness. Porous dolomites in these evaporite series 

 are reservoir rocks for oil and gas, and many oil fields have been developed 

 in the basin. Very gentle folds or "highs" ripple the basin beds and 

 take an irregular northwest-southeast direction. They have served to trap 

 the oil (Fig. 5.4). 



In the Straits of Mackinac region, the most prominent outcrops are a 

 limestone breccia. It is noted for its resistance to erosion and forms the 

 scenic pillars and cliffs of the region. The map of Fig. 5.6 shows its known 

 distribution. 



The columns of breccia, according to Landes ( 1945 ) , may range up to 

 1500 feet in vertical dimension. The solution of Silurian salt has resulted 

 in subsidence and roof collapse, and the breccias are the result. Certain 

 blocks can be shown to have fallen or settled 600 feet. The formations 

 involved and the nature of the breccias are illustrated in the cross section 

 of Fig. 5.7. Supporting the salt solution and collapse theory is the map 

 showing the abrupt thinning of the Salina salt in the Mackinac Straits 

 region (Fig. 5.8). The solution of salt and the collapse of the overlying 

 layers of limestone and dolomite took place chiefly in pre-Dundee time 

 ( Middle Devonian ) , but even now some leaching may be occurring. 



Great Lakes Depressions 



The Salina salt emerges from the basin in a horseshoe-shaped pattern 

 that corresponds closely with Lake Michigan and Lake Huron. The out- 



ILLINOIS 



Fig. 5.2. Configuration of the Precambrian floor in the Michigan basin and adjoining areas. 

 Contours in thousands of feet. After Cohee, 1948. 



