||Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 441-447
Stratigraphic Sequences in the Pennsylvanian of Nebraska and Their Relationships to Cyclic Sedimentation
by E. C. Reed and R. R. Burchett
Nebraska Geological Survey, Lincoln, Nebraska
This discussion is limited to the rocks of Late Pennsylvanian age in Nebraska. Older Pennsylvanian rocks do not outcrop in the state, although they are present at depth. Upper Pennsylvanian rocks outcrop in southeastern Nebraska (Fig. 1) and have been studied by many stratigraphers and paleontologists through the years.
Figure 1--Location map showing outcrops of Pennsylvanian rocks in Nebraska. A larger verison of this figure is avaiable.
Early Stratigraphic Studies in Nebraska
The earliest detailed studies were those of Condra and Bengtson in 1915 which recognized certain sequences and established a beginning in detailed correlation. Condra continued his interest in the Pennsylvanian of Nebraska and published a more detailed report in 1927 which included revisions in classification and many detailed measured sections. He continued his interest in Pennsylvanian stratigraphy throughout his life and published many additions and revisions with his co-workers and participated in joint studies with geologists of adjoining states through the years. He was most persistent in pursuing these studies and contributed more than any other person to establishing the geologic framework of Nebraska's Pennsylvanian and correlations with similar sequences iu other states in the northern Midcontinent region. The task of studying Pennsylvanian rock in Nebraska has been a difficult one because of the lack of exposure continuity and the presence of a thick Pleistocene mantle that obscures this sequence in many localities. The paleontologic studies of the fusulinids and brachiopods by Carl O. Dunbar (Dunbar and Condra, 1927; Condra and Dunbar, 1932) have been outstanding contributions to the correlation of Nebraska's Pennsylvanian rocks. In 1959, Condra and Reed published a revised classification of the geological section of Nebraska.
Graphic Representation of Stratigraphic Sequences
The stratigraphic sequence of the Upper Pennsylvanian in the outcrop area of Nebraska is shown graphically in Figures 2 and 3 and comprises about 810 feet of sediments. The general lithologic and paleontologic character of all formations and members is illustrated and the percentages of acid soluble material in each zone are shown, these being determined from lithologic collections made at the indicated locations. In addition, all limestones are classified according to their general characteristics as related to probable depositional environment.
Figure 2--Stratigraphic chart of Wabaunsee Group in Nebraska. A larger verison of this figure is avaiable.
Figure 3--Stratigraphic chart of Shawnee, Douglas, Lansing, and Kansas City Groups in Nebraska (see Figure 2 for key to lithology and environment). A larger verison of this figure is avaiable.
Cyclothemic Classification of Moore
The cyclothemic and megacyclothemic classification of the Upper Pennsylvanian limestones as developed by R. C. Moore has been very helpful in understanding the environmental conditions during deposition of these sediments and has been a considerable aid in precise correlations, but it also leaves something to be desired in a complete understanding of all varying and variable conditions. Moore's classification also has resulted in some errors in correlation where all parts of a typical megacyclothem are not represented. Regional studies indicate that there is strong tendency to "lose" lower or upper parts of some megacyclothems in a few localities. Significant changes in upper beds of some megacyclothems reflect different conditions of deposition than are represented in other stratigraphic sequences.
The most persistent and reliable lithology in the Upper Pennsylvanian of the northern Midcontinent region is Moore's "middle limestone," a thin, comparatively pure limestone usually with a fusuline fauna overlain by a zone of black fissile shale carrying Orbiculoidea and Lingula. Overlying the black shale is Moore's "upper limestone" which tends to be comparatively thick and which includes fossils indicative of marine sediment deposited under initially disturbed conditions with progressive quieting and clearing of the sea. The sequence often ends with a lithology suggestive of quiet, shallow-sea conditions.
"Lower limestones" of Moore's typical Shawnee megacyclothems appear to be faunally and lithologically similar to his "upper limestone" except that they are not underlain by black fissile shale and "middle limestone" and typically are of intermediate thickness. Limestones classed as "super limestones" are variable in nature. Typically, they are thin to moderately thick and contain a molluscan fauna indicative of marginal marine to brackish conditions; but some of the "super limestones," at least in Nebraska, are more like thinner "upper limestones" with a good marine fauna. This description is true of the South Bend, Avoca, and Kereford Limestones. Other "super limestones" are oolitic or "Osagea"-bearing, suggestive of deposition in quiet, shallow seas. The Farley Limestone and the upper part of the South Bend Limestone, in the Lansing-Kansas City Groups, are examples of this lithology, and the Sheldon Limestone, in the middle part of the Topeka Formation, Shawnee Group, has a similar lithology.
There are strongly developed oolitic or "Osagea"-bearing zones at the top of many of the "upper limestones," although occasionally this oolitic rock is separated from the main mass of the "upper limestone" by shale thus becoming a "super limestone." Locally there may be oolitic developments at the tops of some "lower limestones" of the megacyclothems, further evidence that "lower" and "upper" limestones are similar except for thickness and absence of black fissile shales and thin limestones below the "lower limestones." These are some of the problems that arise in the use of megacyclothem classification.
In general, the Kansas City Group consists of repeated sequences of the "middle-upper" limestone succession of Moore's megacyclothems with absence of "lower" and "super" limestones. The Lansing Group, in its entirety, is more similar to the Shawnee megacyclothems.
The Douglas Group is typified by shales in its outcrop area in Nebraska with the persistent Cass Limestone, a "middle" and "upper" limestone separated by a black fissile shale, occurring near the middle part of the group. Locally, a lower limestone, the Nehawka, is developed in the lower part of the Douglas Group. This limestone, at its type locality, is relatively nonfossiliferous, appears to be brecciated, and has a thin-bedded appearance on vertical, weathered faces. In other areas the Nehawka is a coquina with, again, a thin-bedded appearance on weathered, vertical faces. In some places, the Nehawka Limestone is represented by a concretionary zone in the Plattford Shale. Local names for these two limestones in the Douglas Group have been preserved because their precise correIation with the Kansas section is in question, but it is believed that the Haskell Limestone of Kansas is a part of the Cass Limestone of Nebraska.
In large part, the Shawnee Group is typified by "lower"-"middle"-"upper"-"super" limestone sequences with some complications in the upper parts, as mentioned above. The Wabaunsee Group consists of major shales, sandy shales, or sandstones separated by thin limestones. The middle limestones, and black fissile shales of older Pennsylvanian sequences are missing, and limestones generally are thin and occur in pairs, at least in Nebraska. Generally, these pairs consist of marine limestone of thin to medium thickness below, and a thin, fragmental to impure limestone above. This sequence seems to be abbreviated "upper"-type limestones below separated from thin "super" limestones above. There is a tendency to "lose" the upper "super" limestones locally, and many of these "super" beds apparently are absent farther south in Kansas. These absences complicate correlation of these units. Typical pairs of this type are the Tarkio-false Maple Hill,* the Dover-Morton, the Palmyra-Jim Creek, and the Nebraska City-Greyhorse.
(*A thin fragmental limestone is present in some exposures and occurs just above the Tarkio. Condra enrroneously correlated this limestone with the Maple Hill of Kansas.)
Environmental Classification of Intervals
Some of the limestone beds of megacyclothems differ from others mainly in thickness but not in features that suggest greatly different environments. Therefore, it is proposed to use subnumbers for these limestones indicative of thickness only. The subnumber 1 applies to all limestones 3 feet or less in thickness; subnumber 2 is applied to limestones more than 3 feet and less than 10 feet in thickness; and subnumber 3 is used to designate limestones with thicknesses of 10 feet or more. Thickness, in itself, is only a measure of time involved without radical changes in environment of deposition.
Capital letters A to F are applied to limestones, depending upon probable depositional environments irrespective of thickness, and significant thicknesses of shale, within the general limestone sequence, are also assigned capital letters based on probable depositional environments. In addition, subletters are applied to limestones depending upon the disturbed (d) or quiet (q) sea conditions during deposition.
Problems Involved in Use of Cyclothemic Classification
An examination of all sequential relationships in the Upper Pennsylvanian of Nebraska indicates that the ideal megacyclothemic relationships, as typified by Moore's Shawnee megacyclothem, may be part of an interrupted and incomplete sequence at the base, overlain by a more complete, normal sequence. There seems to be considerable evidence that Moore's "lower" limestones are somewhat thinner equivalents to his "upper" limestones, and that the normal expectancy would be a sequence starting with the "middle" limestone followed by the "upper" and "super" limestones with possibility of losing parts of this sequence from either end. Therefore, we designate Moore's middle limestone as the "A" zone, the black fissile shale as the "B" zone, the "upper" limestone as the "C" zone, the upper somber-colored shale as the "D" zone, and the "super" limestone as the "E" zone, providing that it is truly a "super" limestone with a molluscan fauna or fragmental in nature. Nonmarine shales that are red in some localities, indicating more rapid accumulation, are designated as "F" zones. Some nonmarine shales are equivalent to sandy shale or sandstone zones at other localities and some include true coals in their sequences.
It would be helpful if we could apply con-sistently some designation to limestones indicating probable depth of sea, but this presents some real problems. It is generally agreed that fusulines and many brachiopods indicate comparatively deep water and that oolites are deposited in shallow, quiet seas; but there are some limestones in which these elements are commingled and about all that can be said with assurance is that the sea was quiet and generally free from significant contributions from land areas.
Acid Insoluble Residues as an Aid in Stratigraphic Studies
The use of insoluble residue studies in the Upper Pennsylvanian rocks of Nebraska has been helpful in many ways. These studies were started in the 1930's by A. C. Hornady of the Nebraska Geological Survey, continued by the senior author, and supplemented by recent work of the junior author. Complete data for all parts of the Wabaunsee Group are not now available because of poorly exposed sequences. A knowledge of the percents of acid insoluble material in the various zones contributes to more impersonal description of lithologies, and occasionally types of residues are helpful in correlating outcrops. However, residue studies carried out without determination of percentages of residue are of limited use because of the tendency for repetition of residue types.
Calcium Carbonate Percentages as a Measure of Geologic Time
An examination of residue percentages of many of the thicker limestones suggests that the amount of noncarbonate materials, both argillaceous and siliceous, probably derived from the erosion of land areas, progressively decreases upward. This fact suggests that the percent of carbonate may be some comparative measure of geologic time and it also suggests that Upper Pennsylvanian rocks might have been a complete limestone sequence had it not been for the periodic influx of siliceous and argillaceous impurities. An excellent example of this exists in the Weeping Water, Nebraska, area where the Plattsmouth-Kereford interval is approximately 24 feet of uninterrupted limestone, opposed to a normal limestone thickness of 15 feet. It is believed that the Weeping Water locality was protected from the normal influx of siliceous and argillaceous impurities, giving rise to continuous carbonate deposition and resulting in an over-thickening of the Plattsmouth-Kereford limestone.
If we are to assume that percentages of residue are some measure of geologic time it would appear that the Lansing-Kansas City Groups represent 41 percent, Douglas Group about 5.5 percent, Shawnee Group about 26 percent, and Wabaunsee Group about 27.5 percent of Upper Pennsylvanian time. This, of course, would be assuming that deposition was essentially continuous.
Ideal Sequential Relationships
An ideal sequential relationship, represented by the A-B-C-D-E-F sequence of beds, may be interpreted as beginning with a sudden marine invasion under conditions of quiet seas and comparatively deep water. Inland swamps developed and expanded in the nearshore land areas as A bed was deposited. Headward erosion of valleys rather precipitously opened the swamps and caused draining of the swamps into the sea. Great contribution of humic material under conditions of poor circulation and sudden, quite complete cessation of carbonate deposition formed the B (black fissile shale) zones. This deposition was followed by progressive clearing of sea water as thicker C beds were formed, climaxed by quiet sea conditions with little or no contribution from the land surface and possibly with a shallowing of the sea, as evidenced by the deposition of oolitic limestone. Carbonate deposition of the C horizons was generally abruptly interrupted hy the influx of argillaceous and siliceous impurities from the land, resulting in the deposition of the D shales under conditions of comparatively slow accumulation. This accumulation was followed hy a progressive shallowing of the seas resulting in a more brackish environment with small to moderate contribution from the land areas as the E limestones were formed. These limestones were followed hy progressively more rapid deposition of argillaceous and siliceous material under progressively more nonmarine conditions climaxed by the progressive reduction of nearshore land areas and culminated by development of swamps in which coals were formed. This sequence of events seems to be typical of much of Late Pennsylvanian time.
Critical Level Conditions Suggested by Sequences
Much of the Upper Pennsylvanian deposition, as evidenced in Nebraska, appears to have taken place at critical levels where small changes resulted in signficant differences in depositional environment. Also, the geologic column appears to represent a number of in. complete ideal sequences where certain conditions persisted locally or were interrupted sooner than in other localities. This results in "losing" parts of the ideal sequence from both the base and top of the sequence. There seems to bave been a tendency to "lose" the upper parts of the ideal sequence during much of Lansing-Kansas City time and a strong tendency to lose the lower parts of the ideal sequences during Wabaunsee time.
Condra, G. E., 1927, The stratigraphy of the Pennsylvanian System in Nebraska: Nebraska Geol Survey Bull 1, 2nd ser., p. 1-291.
Condra, G. E., and Bengtson, N. A., 1915, Pennsylvanian formations of southeastern Nebraska: Nebraska Acad. Sci., v. 9, no. 2, p. 1-60.
Condra, G. E., and Dunbar, C. O., 1932, Brachiopoda of the Pennsylvanian System in Nebraska: Nebraska Geol Survey Bull 5, 2nd ser., p. 1-377.
Condra, G. E., and Reed. E. C., 1959, The geological section of Nebraska: Nebraska Geol Survey Bull. 14A, 2nd ser., p. 1-82.
Dunbar, C. O., and Condra, G. E., 1927, Fosulinidae of the Pennsylvanian System in Nebraska: Nebraska Geol Survey Bull. 2, 2nd ser., p. 1-135.
Moore, R. C., 1936, Stratigraphic classification of the Pennsylvanian rocks of Kansas: Kansas Geol Survey Bull. 22, p. 1.256.
Kansas Geological Survey
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Web version Feb. 2003. Original publication date Dec. 1964.