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Classification
Major Divisions
The beds extending from the Mississippian System to the base of the Fort Scott formation (basal Marmaton group) in southeastern Kansas were originally designated the "Cherokee shales" by Haworth and Kirk (1894, p. 105-106). Since the definition of this succession, later called the Cherokee group (Moore, 1949), its original boundaries have been accepted consistently in geologic literature. Various beds have been widely traced, and stratigraphic boundaries corresponding to those of the Cherokee as originally defined have been established for many years in areas to the south where these strata rest on older Pennsylvanian beds rather than rocks of Mississippian age. Extensive study of pre-Marmaton beds of Desmoinesian age in the northern Midcontinent by Walter V. Searight, of the Missouri Geological Survey, has led to recognition of a significant time-stratigraphic break within the Cherokee. The Seville limestone and lower beds contain the brachiopods Marginifera missouriensis, Spirifer rockymontanus, S. occidentalis, and striated forms of Mesolobus, whereas stratigraphically higher Desmoinesian strata are characterized by the presence of Marginifera muricatina and smooth forms of Mesolobus. In addition, recent work in east-central Oklahoma (Oakes, 1953) indicates that important diastrophism is approximately contemporaneous with the faunal change noted in areas to the north. At the Nevada conference (Searight and others, 1953) the Venteran and Cygnian Substages were adopted as time-stratigraphic divisions of the Desmoinesian Series (upper Middle Pennsylvanian). The Venteran Substage includes rocks extending from the top of the Atoka formation upward to the top of the Seville limestone. The Cygnian Substage extends from the top of the Seville limestone to the base of the Missourian Series (base of Pleasanton group). Two new group names, Krebs and Cabaniss (Oakes, 1953), were adopted to replace the term Cherokee. In view of the undesirability of discarding the extremely useful and convenient term Cherokee, it was re-adopted and the Krebs and Cabaniss were relegated to subgroup status at a meeting in Lawrence on October 17, 1955. The Krebs subgroup includes all rocks of the Venteran Substage, and the term Cabaniss is applied to post-Venteran Cherokee strata. The Cabaniss subgroup and Marmaton group together constitute the Cygnian Substage.
At the Nevada conference, Searight also proposed division of the Krebs and Cabaniss subgroups into formations, each of which is composed of the deposits extending from the top of a specified coal bed to the top of the next higher coal bed, except in a few special cases. This proposal was accepted for the northern Midcontinent region but not for the area of thick basin-type sediments in southeastern Oklahoma, where such division was judged undesirable. Formations are named after the unit judged to be most prominent in the formation, regardless of lithology.
The classification, as adopted at the Nevada conference and later modified, is illustrated by Figure 5. Stratigraphic divisions of less than formation rank are informally regarded as members, and are described individually.
Figure 5--Classification of Middle Pennsylvanian beds in the northern Midcontinent.
| System | Series | Previous classification |
Present classification |
Formation | ||
|---|---|---|---|---|---|---|
| (Middle) Pennsylvanian |
Desmoinesian | Marmaton group |
Cygnian substage |
Marmaton group |
Holdenville Lenapah Nowata Altamont Bandera Pawnee Labette Fort Scott |
|
| Cherokee group |
Cherokee group |
Cabaniss subgroup |
Excello Mulky Lagonda Bevier Verdigris Croweburg Fleming Robinson Branch Mineral Scammon Tebo Weir |
|||
| Venteran substage |
Krebs subgroup |
Seville Bluejacket Dry Wood Rowe Warner Riverton |
||||
| Atokan | Atoka fm. (subsurface only) |
Atokan | Atoka fm. | Atoka | ||
Subdivisions of Formations
The formations extending from the top of a given coal bed to the top of the next higher coal bed are judged to be cyclic successions. Each is made up of several lithologic units. Five of these are common to most Cherokee formations, and include, from the base upward: dark shale and dark irregular limestone, gray shale, underlimestone and sandstone, underclay, and coal. Each of these types of rock, although subject to minor variation, occurs constantly in the same relative position in the several successive formations, and is informally regarded as of member rank. Field evidence indicates that nearly all other sorts of rock encountered are products of facies variation of those regarded as basic. The descriptions of lithology apply fully only to beds included in the Cabaniss subgroup. Specific differences or complete absence of some of these in the underlying Krebs subgroup are noted in the following brief discussion of the five major lithologic subdivisions of coal-measure formations.
Dark shale and dark, irregular limestone
Coal beds in the Krebs and Cabaniss subgroups are characteristically overlain by dark shale or limestone or both. In the Krebs subgroup, although dark shale lies on the coal beds, it is associated with beds of clay-ironstone more commonly than with limestone, such as occurs generally at this position in the overlying Cabaniss subgroup. Phosphatic nodules in platy black fissile shale, common in the Cabaniss subgroup, are not known in the Krebs subgroup. Evidence of a minimum of water circulation is the abundance of the iron sulfide minerals, pyrite and marcasite, and the presence of soft to fissile, thinly laminated, dark-gray to black, carbonaceous, calcareous, or phosphatic shales. Marine fossils of several kinds are common in the lower part of most Cherokee formations (Pl. 2). It is apparent that the presence of different kinds of fossils is controlled chiefly by slight variations in environment. Dark calcareous shale may be seen to grade laterally into dark impure limestone. Spirifers, productids, chonetids, and simple corals are normally the most abundant forms in the dark calcareous shales and limestones. Orbiculoids are the most common forms in phosphatic shales. At some places the dark shales overlying coal beds contain so much finely disseminated carbonaceous material that they are best described as carbonaceous shales. The three major types of dark shale (calcareous shale grading to limestone, carbonaceous shale, and phosphatic shale) suggest appreciable differences in depth and probably circulation of water by currents, but all seem to be variants of a lagoonal or near-shore environment, differing from that of coal only by presence of sea water having moderate depth. Clay-ironstone in layers and concretions is exceedingly common in this lithologic subdivision and also occurs in the lighter-colored superjacent shales. It is more prominently developed in the Krebs than in the Cabaniss subgroup.
Gray shale
Generally unfossiliferous, evenly bedded gray shale, which overlies dark shale and impure limestone, commonly forms the thickest lithologic unit in the formations. This shale seemingly would result from relatively rapid subsidence and concurrent deposition of silty muds in shallow marine water.
Underlimestones and sandstones
Either sandstone or impure nodular limestone characteristically overlies the evenly bedded, relatively thick shales just described, and lies below underclay associated with the coal above. The underlimestones, seemingly formed in very shallow lagoonal waters, are rare in Missouri, but become relatively common, especially in the Cabaniss subgroup, in Kansas and northern Oklahoma. They are regarded as marine because some have been found to contain marine fossils.
Sandstone at this position ranges from massive to shaly or thin bedded; most of it is fine grained, lenticular, and grades laterally into impure sandy limestone, which is a facies of the impure limestones called underlimestones. Underlimestones are in some places brecciated or conglomeratic, suggesting wave or current action. The good to excellent sorting of sand grains and the presence of marine fossils at a few localities, together are evidence of a marine environment of deposition of the sandstone. Underlimestones and sandstones are the most variable of the lithologic divisions. It is judged that the most likely point of introduction of nonpersistent cycles of deposition is below this unit and above the thick shale unit in a given cycle. It is noted that the Iron Post coal and overlying Kinnison shale, present in northern Oklahoma below the Breezy Hill limestone, are unknown farther north and east in Kansas and Missouri, where the Breezy Hill is much less regular and in general closely resembles underlimestones developed below underclays in other Cabaniss formations. Changes in lithology and stratigraphic relations of the Breezy Hill limestone from Oklahoma northeastward across southeastern Kansas are illustrated by Figure 6. Some, and perhaps all, underlimestones grade laterally from very impure, nodular, seemingly unfossiliferous rock to more even, less impure, abundantly fossiliferous limestone, as does the Breezy Hill. A second example of lateral gradation of this sort is found in the underlimestone of the Croweburg coal, which is represented in northern Oklahoma by the McNabb limestone.
Figure 6--Diagram showing southward change in Breezy Hill limestone, and introduction of additional beds between sandstone of Lagonda formation and Breezy Hill limestone in northern Oklahoma.
Underclay
Underclay is a characteristic associate of coal, and is one of the least variable lithologic types. This material, which is relatively even in thickness, commonly is somewhat silty and may be slightly calcareous. Carbonized plant material and fossil root impressions are present in most exposures. Slickensides are very common. Underclays in this region locally contain portions of fossil root systems, including stigmaria and attached rootlets, but whether or not underclays represent a true soil in which the plants form an entire coal bed is unknown. Nearly all such clays express the lithology of the underlying bed to a greater or lesser degree. Most beds of underclay next above sandstones are sandy or silty, and the upper surface of the subjacent sand may be unevenly leached and contain root markings in vertical position. Few, if any, of the underclays of this region are of fire-clay quality.
Coal
Coal is economically the most important rock in these strata. Lateral extent, thickness, and quality are variable. Many coal beds in this region are remarkably persistent throughout the area of this report.
The Croweburg-Verdigris relationship
An outstanding exception to the generalization in the preceding paragraphs is the Verdigris formation, which includes beds from the top of the Croweburg coal to the top of the Wheeler coal, where the succession is fully developed. The prominent and extremely widespread Verdigris (Ardmore) limestone, for which the formation was named, is probably not nearly so closely related to the Croweburg coal as the adopted classification implies. The Verdigris limestone and underlying black slaty shale find their closest lithologic counterpart in rocks within the lower part of the Marmaton group. They correspond very closely to the Blackjack Creek limestone and underlying black shale, or to the Myrick Station limestone and subjacent shale. It is here suggested that further study of this succession, possibly in the subsurface, will result in the discovery of an additional coal bed below the black shale associated with the Verdigris, and above the gray clay shale overlying the Croweburg coal.
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Kansas Geological Survey, Geology
Placed on web May 25, 2009; originally published October 1956.
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