Clear distinction is recognized between divisions of Pennsylvanian rocks in the midcontinent region that are based on relation to boundaries having significance as markers of geologic time, and those that are defined primarily by lithologic characters. The boundaries of time-rock units, which are determined primarily on the basis of sedimentation, interruptions of sedimentation, and other features of historical geology, are obscure but geographically extensive disconformities that separate successions of Pennsylvanian strata having different paleontological attributes. Greatest change in the components of fossil faunas and floras closely coincides with the horizons of these disconformities, which are inferred, therefore, to denote relatively prolonged interruptions in sedimentation. Erosion of previously accumulated deposits, which is indicated at many places as accompaniment of nondeposition, ranges from barely perceptible - to quantitatively very considerable.
The deposits occurring next above the disconformable surfaces, which mark boundaries between time-stratigraphic divisions of the Pennsylvanian System, are prevailingly clastic. They include much sandstone and there are local or widespread conglomerate beds. The clastic sediments increase in prominence as one proceeds outward from basin areas, as in the Forest City and Salina basins, and especially as source areas of sediments are approached. All known features of surface and subsurface stratigraphy of Pennsylvanian rocks in Kansas and neighboring States point to, or are in accord with, the recognition of at least four major time-rock units of equivalent rank, and the Pennsylvanian column in the whole region is judged to be divisible into six such time-rock elements. These have been named (in upward order): Springeran, Morrowan, Atokan (Derryan, or Lampasan), Desmoinesian, Missourian, and Virgilian. All except the first named are identified in Kansas at outcrops or buried beneath the surface.
The rank which should be assigned to the units just mentioned hinges on collateral observations and considerations. If Pennsylvanian rocks are designated as a system, as nearly all American geologists are agreed that they should be, divisions such as Desmoinesian and Missourian may be treated appropriately as series; they have been so classified in previous reports of the Kansas Geological Survey and of surveys of adjoining States such as Missouri, Nebraska, and Oklahoma. The Nebraska Geological Survey (Condra & Reed, 1943) treats Pennsylvanian as a subsystem of the Carboniferous System, which permits designation of divisions like Missourian and Virgilian as a series. On the other hand, if the Pennsylvanian rocks as a whole are classed as a series division of the Carboniferous System, in accord with usage of the U. S. Geological Survey, units having the span of Desmoinesian, Missourian, and Virgilian must be designated as subseries or stages. Likewise, if Pennsylvanian deposits are designated as a system and divided into series in the manner advocated by Moore & Thompson (1949), it is necessary to treat main subdivisions of these series (Ardian, Oklan, Kawvian) as subseries or stages.
The Kansas Geological Survey, conforming to classification agreed to by the interstate conference of May 1947, continues to designate Desmoinesian, Missourian, Virgilian, and units of similar rank as series (Fig. 2). Consequently, the divisions called Ardian, Oklan, and Kawvian are not employed, despite judgment as to their usefulness in taking account of major features of Pennsylvanian historical geology.
The succession of stratified deposits belonging to the respective series of the Pennsylvanian System, as seen in Kansas, is each composed of varied sorts of sedimentary materials, partly marine and partly nonmarine. The lithologic characters provide basis for classification that serves requirements for geological description and mapping. The named rock units are ranked as groups, formations, and members; two of the Pennsylvanian groups are divided into subgroups and there are a few named beds, subordinate to members.
Evidence of cyclic sedimentation is varyingly clear throughout the Pennsylvanian System in all the northern midcontinent region, but only in minor degree and rather exceptionally do boundaries of the deposits belonging to one cycle of sedimentation (called a cyclothem, Weller, 1931) coincide with the limits of rock units defined as members or formations (Moore, 1936). Reason for this is that precise delimitation of individual cyclothems in most places is impossible, either because the nature of well exposed beds between adjacent cyclothems does not afford conclusive indication of a boundary, or because critical parts of the section are concealed. Accordingly, the recognized rock units comprise lithologic entities having clearly marked physical limits, identification of which almost any two or more geologists can agree on readily. Commonly, these boundaries are well expressed topographically.
The general basis for differentiating an assemblage of beds classed together as a formation or group is dominance of a particular type of rock, at least in the area of the type section. Thus, a limestone formation, such as the Stanton, is composed predominantly of limestone members which are separated by thin shale members. A shale formation normally consists mostly of shale, but it may contain subordinate limestone or sandstone or both, and laterally the shale may diminish in importance as other rocks increase, until designation of the unit as a shale is no longer reasonable. The stratigraphic name may be retained because equivalence of span is definitely the same. The type Cherryvale shale section lacks limestone beds, whereas Cherryvale strata in Nebraska (as defined by interstate classification) consist mostly of limestone. Likewise, two or more successive formations that stand apart from adjoining parts of the column because of contrasting lithologic features, topographic expression, and other attributes, may be defined as groups. The Shawnee group, characterized by relatively thick, escarpment-forming limestones, differs markedly from the underlying Douglas group and overlying Wabaunsee group.
In some parts of the Pennsylvanian succession of Kansas, cyclothems of different sorts are grouped by their arrangement and recurrence in constant order. Such sets of repeated cyclothems are called megacyclothems (Moore, 1936, pp. 26-38). Megacyclothem relationships as well as occurrence near together of similar. kinds of rocks layers, are now understood as having importance in classifying the midcontinent Pennsylvanian deposits. For example, persistent thin limestone beds that characteristically underlie black platy shale are properly associated with limestone overlying the black shale, rather than treated as a minor element of a shale formation which includes the black shale.
Accordingly, formations are defined, as previously, to consist of an assemblage of rocks layers, which is judged to constitute a convenient and natural unit for purposes of description and geologic mapping. Generally, the strata thus classed together have similar composition or they are dominantly of similar lithology.
Members are subdivisions of formations which are found to be laterally persistent and are judged to be important enough for separate designation. They are not mapped as individual units.
Groups are assemblages of two or more formations which are judged to have similarities or a closeness of association sufficient to call for putting them together. They are a sort of superformation. Subgroups are somewhat arbitrarily differentiated portions of a group.
Kansas Geological Survey, Geology
Placed on web Jan. 26, 2009; originally published Nov. 1949.
Comments to email@example.com
The URL for this page is http://www.kgs.ku.edu/Publications/Bulletins/83/04_defin.html