174 c. r. keyes — a depositional measure of unconformity 



Introductory 



An unconformity at the base of the Coal Measures in the upper Mis- 

 sissippi valley has long been known. In the writings of the early geo- 

 logical explorers are recorded observations showing that a physical break 

 in the stratigrapliic sequence was recognized at various localities between 

 the coal-bearing strata and the underlying Carboniferous limestones. In 

 most cases it was regarded as local discordance in sedimentation. Hall * 

 appears to be the first to note the wide extent of the unconformity. 

 Later, White f ^Iso emphasized its importance as a stratigraphic plane. 



Of recent years much information bas been obtained regarding the na- 

 ture of this junction of the coal-bearing Middle Carboniferous strata and 

 the formations on which they rest. The details of the unconformity 

 plane have been so nicely worked out in so many localities that we have 

 now a very good idea of its character, its real significance, and even of 

 its relief features now so long buried. 



This plane of unconformit}'' has proved to be of wide extent. It is 

 found stretching northward from the Ozarks beyond the south boundary 

 of Minnesota. It bevels all the Lower Carboniferous formations, the 

 Devonian, Silurian, Ordovician, and probably the Cambrian. 



The plane represents undoubtedly an old land surface. Data regard- 

 ing its depositional e(iuivalent have been too incomplete to even sug- 

 gest what it might include or where it might be represented ; only 

 recently X has any indication been given as to the possible representa- 

 tive sediments. It is to this phase of the subject that attention is here 

 called. 



Carboniferous and its Subdivisions in the Mississippi Valley 



Each of the states occupied b}^ Coal Measures of the Western Interior 

 basin has formed a general section of its formations that has been diffi- 

 cult to parallel with those of the adjoining states in any but an approx- 

 imate way. Of late years the Carboniferous terranes of Iowa, Missouri, 

 and Kansas have been brought into close accord. Those of Arkansas 

 and Indian territor}'' have also been subject to comparison ; but no 

 direct and exact correlation has been undertaken between the sections 

 of the north and the south regions. 



North of the Ozarks the classification of the median Carboniferous 

 may be considered according to the following table : § 



* Geology of Iowa, vol. i, 1858, p. 128. 



t Geology of Iowa, vol. i, 1870, p. 226. 



+ Journal of Geology, vol. viii, I9(J0, p. 28'). 



g American Geologist, vol. xxiii, 1899, p. 298. 



