THE WESTERN INTERIOR GEOSYNCLINE AND ITS 



BEARING ON THE ORIGIN AND DISTRIBUTION 



OF THE COAL MEASURES 1 



FRANCIS M. VAN TUYL 

 University of Illinois 



While occupied with a study of the stratigraphy of the Missis- 

 sippian formations of Iowa for the Iowa Geological Survey the 

 writer has been attracted by the regular and nearly uniform gentle 

 dip of these formations to the southwestward, and he has recently 

 attempted to ascertain the age and significance of the deformation 

 which gave rise to this. Investigation soon showed that the tilting 

 was related to deformation over a wide area in southern Iowa, south- 

 eastern Nebraska, eastern Kansas, and northwestern Missouri 

 which outlined a great southwestwardly pitching geosyncline in 

 which the Coal Measures of the western interior coal field were 

 deposited. The evidence is clearly in favor of the view that the 

 movement took place, in part at least, during pre-Pennsylvanian 

 time, as shown by the fact that the Coal Measures now rest upon 

 trunkated Mississippian formations successively younger in age 

 toward the southwest. This belted arrangement of the Mississip- 

 pian deposits beneath the Coal Measures cannot be accounted for 

 on the assumption that the distribution of the former is original, 

 since the beds often consist entirely of nearly pure limestone up to 

 their very boundaries and show no indications of shore facies. 



That the geosyncline was shallow in early Pennsylvanian time 

 is indicated by the fact that the maximum known thickness of the 

 deposits of the Cherokee stage, which probably represents the time 

 of greatest sea extension in this basin during the Pennsylvanian, is 

 only 712 feet. At the present time, however, it attains a known 

 depth of approximately 2,400 feet at McFarland, Kansas, and 

 future drill records will probably show it to be considerably deeper 

 than this to the southwest. The deepening is believed to have 



1 Published with the permission of the Director of the Iowa Geological Survey. 



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