340 W. I. ROBINSON 



a lack of data makes it impossible to use a contour interval which 

 would reveal the smaller and more common folds. Perhaps the 

 most typical example shown is that of the St. Clair area {A). 

 The shape and orientation of the fold in the Saginaw area (B) are 

 typical, but the position of this fold is exceptional in that it lies 

 near the center of the basin. The smoothness and regularity of 

 the contours in the northwestern part of the state are due to a lack 

 of data from deep wells. However, folds of the radial linear type 

 are known from surface observations in the Northern Peninsula 

 in the northwest quadrant of the basin. The location of one of 

 these folds is shown in Figure 2 which is drawn to show the theoreti- 

 cal direction and extent of the radial linear folds of the basin. 



The Michigan basin is exceptional in its lack of distortion by 

 tangential forces, and therefore has a more conspicuous development 

 of these folds, but it is believed that such folds are a feature to be 

 sought for in every basin of persistently negative tendency. It is 

 conceivable that under some conditions such folds might indicate 

 favorable areas for prospecting for oil and gas. In case of uncon- 

 formity, and with impervious sedimentary beds deposited over 

 such folds after the deformation but before any great migration of 

 contained fluids, this would certainly be true. 



CONCENTRIC TERRACE FOLDS 



A basin of persistently negative tendency, in which the only 

 movements are slight and long continued, would be expected to 

 develop terraces parallel to the margins of the basin and concentric 

 around the central area, especially in the parts of the basin which 

 are bordered by elements of a positive tendency. Such terraces 

 have developed in the Michigan basin in the southern part, bordering 

 the positive area which has been influenced by the Cincinnati 

 anticline. These terraces which are shown in Figure i are found, 

 by a comparative study of well records, to be developed at least 

 as far down in the stratigraphic column as the Dundee (Onondaga- 

 Hamilton). Such structures are important economically, and the 

 significance of an understanding of their mode of origin and typical 

 arrangement in regard to negative and positive areas, will be at 

 once obvious to petroleum geologists. 



