Classification of the Pennsylvanian Rocks in Relation to the Carboniferous System
On pages 8, 13, 15 and elsewhere in this report the Pennsylvanian rocks are designated as a geologic system. This classification conforms to my long-established views inculcated by early training, and is in accord with usage employed by most American geologists of the present day. The Pennsylvanian is classed as a system in my textbook on historical geology, as in the majority of other American textbooks in this field [Moore, Raymond C., Historical Geology, 673 pp., McGraw-Hill, New York, 1933; see pp. 281-315, esp. p. 282]. Judgment that the Pennsylvanian rocks should be given systematic rank rests chiefly on the widespread, mostly well defined nature of the break which separates these rocks from underlying Mississippian deposits, and general distinctions in the lithologic character and fossils of these two stratigraphic divisions. Separation of the Pennsylvanian from Permian rocks is more difficult, especially in the Mid-Continent region, but because the Permian is recognized generally as a system in other parts of the world, corresponding treatment has been given commonly in this continent.
A different classification of the late Paleozoic rocks that has long been adopted by the United States Geological Survey designates all of the rocks between Devonian and Triassic as the Carboniferous system, and divides this system into three series that are named (in upward order) Mississippian, Pennsylvanian, and Permian. This usage of terms represents acceptance of the well established European name, Carboniferous, for rocks next younger than Devonian, but unlike the definitions of Carboniferous and Permian that are uniformly understood in Europe, the "Carboniferous" of this American classification is expanded to include Permian, which is reduced to subordinate rank. The evident basis for the lower rank assigned to Permian is the inconspicuous nature of the boundary between Pennsylvanian and Permian rocks in certain North American areas. Places are now known on this continent, however, where the base of Permian rocks is marked by a very important unconformity. In any case, it is very clearly undesirable to use the term Carboniferous to include Permian, both because the Permian rocks of the type region for the Carboniferous, in England, are definitely excluded in the original definition of this term," and because Permian is uniformly excluded from Carboniferous in subsequent usage of Europe and other continents, excepting the classification here discussed [Conoybeare, W. D., and Phillips, W., Outlines of the Geology of England and Wales, pp. vii, 278, 320-364, 1822]. If Carboniferous is adopted as a systematic unit in North America, it should apply only to rocks that are equivalent to the European Carboniferous which is limited by the Devonian below and the Permian above. Since rocks of Permian age are definitely recognized on this continent and since Carboniferous is the only European systemic term of the entire geologic column that is not now commonly employed in North America as in other continents, it seems desirable to make our classification of late Paleozoic rocks conform to that of Europe. The geologic systems defined in Europe have come to be accepted as the standard divisions of the column, and the only valid basis for modification of this classification is general agreement that a different definition of certain divisions better suits European and world conditions. Probability of such agreement is exceedingly remote.
Study of almost any system reveals that no matter how clearly evident the defined boundaries may be designated type regions, there are many places where such clear definition of boundaries does not exist. The limits of a system in any given region must, therefore, be determined essentially by comparison with standard European sections, and if this were not done the so-called systematic units of different areas would surely not agree. This means that our geologic systems, established on the basis of conditions in one part of the world and to a large extent fixed by long usage, are divisions that fail ideally to fit conditions in all continents. They are inheritances which are serviceable to geologic science, but definition of boundaries of these inheritances is measurably arbitrary considering their application to the entire globe.
Examination of late Paleozoic sections in various parts of the United States indicates rather clearly that divisions of strata such as the Chester beds, Morrow beds, and the divisions described as series in this report (Des Moines, Missouri, Virgil), which are delimited by extensive unconformities below and above, are really the most significant stratigraphic elements of these sections. Each such division represents essentially uninterrupted sedimentation during a considerable length of geologic time, and it is characterized by distinguishing features of the paleontologic record as well as associated differences of lithology and structural relations. These unconformity-bounded units are the essential "building blocks" of the geologic column, for they reflect the lowest order of major sea and land movements. Accordingly it seems that these divisions are properly classified as series. It is undoubtedly true that the boundaries between the Morrow and Des Moines series and between the Missouri and Virgil series are in some places very much more conspicuous than the lower or upper boundary of the Pennsylvanian rocks in some places.
Acceptance of the conclusion that divisions such as Des Moines, Missouri and Virgil are properly classed as series does not require abandonment of any grouping of these series that is convenient or useful. The rocks called Pennsylvanian, in fact, comprise such a group of series which for various reasons, including established custom, it is desirable to set apart from a lower group of series that is collectively termed Mississippian. Neither Pennsylvanian nor Mississippian is represented by subdivisions of the European Carboniferous that are equivalent in span, the Pennsylvanian corresponding to Stephanian, Westphalian and upper Namurian, and the Mississippian to lower Namurian, Visean and Tournaisian. The Upper Carboniferous of Europe includes Stephanian, Westphalian, and Namurian. The lower Carboniferous comprises Visean and Tournaisian.
It appears desirable to recognize the Carboniferous system and the Permian system in North America, with definition of boundaries as in Europe. The Pennsylvanian and Mississippian may be classed as subsystemic divisions of Carboniferous. The Carboniferous system is represented in Kansas by the Waverly series of the Mississippian subsystem, and by the Des Moines, Missouri and Virgil series of the Pennsylvanian subsystem. The Permian system in Kansas contains the Big Blue and Cimarron series. The boundary between the Virgil and Big Blue series seems well defined, but it is recognized that certain questions of definition of Permian depend on clarification of boundaries in Russian sections.
August 15, 1936.
RAYMOND C. MOORE.
Kansas Geological Survey, Geology
Placed on web Sept. 28, 2016; originally published November 15, 1935.
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