342 ILLINOIS STATE ACADEMY OF SCIENCE 



CORRELATIONS OF WELL DRILLINGS IN NORTH- 

 ERN ILLINOIS, WITH OUTCROPPINGS OF 

 EARLY PALEOZOIC BEDS IN WISCONSIN 



A. W. Thurston, University of Illinois. 



The correlation of the Lower Paleozoic formation in 

 Illinois is somewhat difficult because of incomplete data. 



The St. Peter sandstone outcrops at several places 

 along the LaSalle anticline and in LaSalle County where 

 its entire thickness is exposed. The Lower Magnesian 

 or Prairie du Chien group is seen only in a few scattered 

 outcrops and there only the top member is partially ex- 

 posed. The nearest exposure of Cambrian rocks is near 

 Janesville, Wisconsin, but most of the Cambrian forma- 

 tions outcrop near Madison Baraboo or farther north. 

 These are traced into northern Illinois, a distance of 250 

 miles, by means of deep well records. Both the Illinois 

 Geological Survey and the Wisconsin Geological Survey 

 try to procure records and, if possible, to secure samples 

 from the wells in their respective states. By this means 

 only can subsurface formations be traced with certainty. 

 The greater the number of well records available the 

 more complete will be the data at hand and hence the 

 more accurate will be the correlations that can be made. 



Fossils are ground up in the well cuttings. Therefore, 

 little or no paleontological evidence is offered to assist 

 in the correlation of well cuttings. 



The problem is entirely a stratigraphic one. The cor- 

 relation is made by means of "key" horizons, or hori- 

 zons which are recognized by certain peculiar character- 

 istics which are maintained over wide areas. Forma- 

 tions may change lithologically from place to place be- 

 cause conditions for sedimentation were not the same. 

 Nearest the shore the coarser materials are deposited. 

 Farther out on the mud flats shale is deposited, and still 

 farther out in quiet water lime mud which cements to 

 form limestone is deposited. During the Jordan sea in- 

 vasion central Wisconsin was near the shore, and the 

 formation is a sandstone there, whereas during the same 

 time northern Hllinois was farther from the shore where 

 clear water prevailed and limestone was deposited. For 



