166 IOWA ACADEMY OF SCIENCE 



THE WESTERN INTERIOR GEOSYNCLINE AND ITS 

 BEARING ON THE ORIGIN AND DISTRIBU- 

 TION OF THE COAL MEASURES. 



FRANCIS M. VAN TUYL. 

 (ABSTRACT.) 



Late studies of the Mississippian formations of southeastern 

 Iowa for the Iowa Geological Survey have shown that these 

 formations were tilted to the southwestward and partly trun- 

 cated in late Mississippian time. There is convincing evidence 

 that this tilting was related to deformation over a wide area in 

 southern Iowa, southeastern Nebraska, eastern Kansas and north- 

 western Missouri which outlined a southwest wardly pitching 

 geosyncline in which the Coal Measures of the Western In- 

 terior coal field were deposited. This geosyncline was shallow 

 in early Pennsylvanian time and probably did not greatly ex- 

 ceed 700 feet in depth at the close of the Cherokee stage. 

 At the present time, however, it is approximately 2400 feet 

 deep at the deepest known point which is at McFarland, Kan- 

 sas. An important part of the deepening is believed to have 

 been brought about by subsidence during the post-Cherokee 

 stages of the Pennsylvanian. 



The magnitude and significance of the basin has been dem- 

 onstrated by the construction of 100 foot contours on the base 

 of the Coal Measures from data furnished by the reports of the 

 State Geological Surveys of Iowa, Missouri and Kansas. 



The presence of this basin not only explains the great dis- 

 similarity bet ween the Coal Measures of this field and those 

 of the Eastern Interior field which were undoubtedly deposited 

 in a distinct basin, but also explains the belted arrangement of 

 the outcrops of the Pennsylvanian formations, particularly in 

 Iowa. Missouri and Kansas, where the younger members are 

 approximately confined to the center of the basin, progressively 

 older ones being exposed towards its margins. The present 

 distribution has resulted from post-Paleozoic erosion of the dip- 

 ping beds but there are reasons for believing that the Missouri 

 formations were never as extensive as those of the Des Moines 

 and that the younger members of the Missouri itself were more 

 restricted than the older ones. 



Department of Geology 

 University of Illinois 



