456 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The older Pennsylvanian deposits are confined to the southeastern part of the region, in 

 Arkansas and Oklahoma, and correspondingly in that region is found the greatest thickness of 

 rocks. Later a series spread northwestward and westward over Kansas, Missouri, Nebraska, and 

 Iowa. The Pennsylvanian of northeastern Missouri and southeastern Iowa would seem to have 

 been at one time connected with the "Coal Measures " of Illinois. Whether the series in Arkan- 

 sas has extended southeastward to join with the Appalachian field in Alabama is uncertain. 



SUBDIVISIONS. 



For purposes of study the region may readily be divided into four subareas. The Kansas- 

 Missouri-Iowa area is characterized by rocks nearly all of which are of post-Pottsville age, 

 that are easily divisible into the "Upper Coal Measures" and "Lower Coal Measures," by 

 common elements in their stratigraphy, and by* simple monoclinal and basin structure. The 

 Pennsylvanian of Oklahoma and Arkansas may be divided into two subareas by a great fault 

 or fault zone, extending from Atoka to the northeast and east, crossing Arkansas River about 

 40 miles above Little Rock. The area north and east of this fault, which may be called the 

 Arkansas Valley field, differs from the Kansas-Missouii-Iowa area in structure and in stratig- 

 raphy, there being a great thickening of the series from Kansas to the southwest, where the 

 strata are underlain by older Pennsylvanian rocks. In the main this area is gently folded and 

 faulted. The area south of the great fault, which may be known as the Ouachita region, differs 

 from the area last mentioned in its stratigraphy, notably in the presence of a great series of 

 shale and sandstone not found outside of this area, and in structure, its rocks having been 

 closely folded and markedly faulted, and possibly overthrust upon the rocks to the north. The 

 northern Texas area is cut off from the areas to the north by the Arbuckle uphft. It is plainly 

 a continuation of the northern Arkansas-Oklahoma area, though differing greatly in its strati- 

 graphic details. 



lOWA-MISSOUKI-KANSAS AREA. 



The Pennsylvanian in Iowa, Missouri, and Kansas outcrops over an area of 63,000 square 

 miles, distributed, according to Bain,^° as follows: Iowa, 20,000; Missouri, 23,000; Kansas, 

 20,000. Bain ^°* describes the "Coal Measures" in these States as follows: 



"The Coal Measures of the western interior field * * * pest unconformably upon the 

 rocks of the Lower Carboniferous or Mississippian series. They are covered in Kansas and 

 Nebraska by the Permo-Carboniferous beds, which are conformable with them. In Iowa 

 Cretaceous deposits rest unconformably on them, and in Iowa, Nebraska, northern Missouri, 

 and northeastern Kansas the drift and associated deposits of the Pleistocene cover the coal- 

 bearing strata. 



"The Coal Measures of this field include limestones, sandstones, shales, fire clays, and the 

 coal beds proper. Limestones are probably least abundant, and shales in their various phases — 

 argillaceous, bituminous, calcareous, and arenaceous — most abundant. In the northern portion 

 of the field hmestones and calcareous shales are notably more abundant in the upper or barren 

 portion, and less so in the lower or productive portion. In Kansas the distinction between the 

 two divisions is not so well preserved, and in the adjacent southwestern field it quite loses value. 

 In a general way there is a prevailing dip to the west of 10 to 20 feet per mile. In detail the dip 

 is south to' southwest in Iowa, west to northwest in Missouri, and predominantly northwest in 

 Kansas. 



"The Coal Measures of this field increase in thickness westward from their outcrop. There 

 is also a gradual increase in maximum thickness from north to south. In Iowa the maximum 

 thickness measured is 1,060 feet." In Missouri Winslow has estimated the total thickness at 

 2,000 feet. '' In Kansas Haworth gives 3,000 feet as the thickness. " 



"On the maps accompanying this report two divisions of the Coal Measures are repre- 

 sented. These are the Upper and Lower Coal Measures. In Iowa and Mssouri the terms 



= Norton, W. H., Iowa Geol. Survey, vol. 6, p. 333. 



6 Missouri Geol. Survey, vol. 1, Preliminary report on coal, p. 24. 



c Univ. Geol. Survey, vol. 3, p. 20. 



