Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 607-621

Development of the Concept and Interpretation of Cyclic Sedimentation

by J. Marvin Weller

University of Chicago, Chicago, Illinois

Introduction

The idea of geologic cycles is an ancient one going back at least to the catastrophic theories of Cuvier and d'Orbigny (Weller, 1960, p. 1008, 1010), who believed that the earth witnessed a succession of organic creations and extinctions. In America, Amos Eaton, an adherent to Wernerian principles, outlined a five times repeated sequence of rocks termed Primitive, Transition, Lower Secondary, Upper Secondary, and Tertiary.

All geological strata are arranged in five analogous series; and that each series consists of three formations; viz. Carboniferous [more properly schist, slate, or shale], Quartzose, and Calcareous. (Eaton, 1830, p. 64.)

The geological deposits of this country (and probably those of the eastern continent), exhibit grounds for conjecture, if not absolute demonstration, that the surface of the earth has undergone five general modifications, which no animals survived. Four of these modifications were followed by as many new creations of animals. Also, that two new creations of animals succeeded the final deposition of all regular strata. In the whole, there appears to have been five creations of animals (perhaps ten) since the primitive mass of the earth was formed; and a long interval succeeded each creation. (Eaton, 1832, p. 48.)

Eaton's correlations were so confused that his formations cannot be identified in a way that has much modern meaning.

Systemic Cycles

Edward Hull in Britain, partly on theoretical grounds, outlined a natural sequence of sedimentary rocks related to distance from the shore and transgression and regression of the sea. Examples were provided by the Carboniferous System of England and Scotland as well as other strata.

We cannot fail to have observed that the many groups have a tendency to arrange themselves into threefold divisions, the upper and lower being composed of sands or clays, the middle of limestone. . . . Phenomena of so general a character cannot be accidental, but must be in accordance with the system of nature. (Hull, 1862, p. 134.)

In America, T. Sterry Hunt (1863, p. 166-167) recognized a sequence consisting of (1) basal siliceous strata, (2) dolomite with some gypsum and salt, (3) limestone, and (4) carbonaceous shale. This sequence was stated to be repeated three times in the Cambro-Silurian (Ordovician), Middle and Upper Silurian, and Lower Devonian of New York and adjacent regions.

In Nova Scotia, J. W. Dawson distinguished a general succession of Carboniferous rocks consisting of (1) coarse sediments deposited in shallow water at the base, (2) limestone and gypsum indicating the existence of deeper, clearer water, and (3) detrital sediments with coal indicating withdrawal of the sea. Wider application of the idea suggested by this succession led him to outline in tabular form a four times repeated "cycle" representing (1) subsidence and coarse sediment, (2) marine conditions and limestone, (3) elevation followed by slow subsidence, and (4) a subsiding basin filled with sediments.

. . . the Carboniferous period constitutes one of four great physical cycles, which make up the Paleozoic age in Eastern America--and each of which was characterized by great subsidence and partial re-elevation, succeeded by a second very gradual subsidence. (Dawson, 1866, p. 102; 1868, p. 137.)

Dawson's cycles corresponded with the Lower Silurian (Ordovician), Upper Silurian, Devonian, and Carboniferous Systems as then recognized. He also remarked that the Permian rocks of Europe suggest the existence of a fifth cycle.

Hull in a second article presented a classification of the stratigraphic column extending from the Upper Silurian through the Miocene in amplification of his concept of sedimentary sequences including middle limestone units. Ten sequences or groups were recognized, most of which corresponded to geologic systems except that the Jurassic was represented by three sequences.

. . . each natural group was composed of the representatives of three periods--the 1st, one of movement, accompanied by change of land and sea and much denudation; 2nd, a period of comparative repose, and a minimum of denudation; 3rd, of change, gradually increasing in intensity to the close of the epoch. (Hull, 1868, p. 144.)

At a meeting of the American Association for the Advancement of Science in 1860, Newberry (1874a, p. 186-187, 194, 196) read two papers that were never published (1861a, 1861b) in which he called attention to the Cretaceous section in the southwestern United States.

In this region I found the base of the Cretaceous system composed of coarse sandstone, sometimes conglomerate, containing everywhere the impressions of Angiospermous leaves, and in many places heavy beds of lignite. . . . Above this lies a laminated impure limestone, containing characteristic fossils. . . . Above the last mentioned group is a heavy mass of calcareous strata abounding in . . . characteristic Cretaceous mollusks. The fourth member of the series . . . . is formed by a group of calcareous sandstones and shales, with impressions of plants, sheets of lignite and some mollusks. . . . From this sequence of strata, I read the history of a submergence of the Triassic continent and an invasion of the sea which resulted, first, in the formation of a wide-spread sheet of beach sand and gravel . . . . Second a mixture of mechanical and organic sediments, constituting the offshore deposits of the invading sea. Third, a great calcareous mass, the organic sediments of the open sea during the long continued period of greatest submergence. (Newberry, 1874a, p. 185-186.)

Newberry's later work, particularly in Ohio, directed his attention to a generally similar "circle" of deposillon at was said to be repeated in the Paleozoic rocks of the eastern half of North America.

The history recorded in each case is the same, viz., a submergence of such portions of the continental surface as now carry the sedimentary strata enumerated: in the progress of each submergence, the spread of shore materials, over all the surface covered by the advancing sea; this sheet being followed first, by mixed mechanical and organic sediments, then by those almost purely calcareous deposits from the open ocean, and finally earthy limestones--mixed sediments--indicating a retreating, shallowing sea and a return to land conditions during which no deposition would be made on the surface, but which was the necessary starting point for a new circle of deposits. (Newberry, 1873, p. 64.)

In an elaboration of his "circle," Newberry emphasized the role of fluctuating sea level and the advance and retreat of shorelines on the type of sediment deposited. He summarized his ideas in a table (1874a, p. 193), which is reproduced here in slightly modified form (Table 1). He also referred to the observations of European geologists who had recorded the existence of strata exemplifying a similar succession of beds in the Permian and Triassic. Finally, he expressed an interesting and prophetic opinion.

In all our works on geology the Portage, Chemung and Catskill formations are included in the Devonian system, but in my judgment it would be better to consider the Portage sandstones . . . as the true base of the Carboniferous system . . . forming an indivisible mass of mechanical sediments . . . evidently the record of a new era in the geological history of the continent. (Newberry, 1874a, p. 192.)

Thus Newberry stated the idea that geologic systems should conform to a physical cycle recorded in the rocks.

Table 1--Newberry's "circles" of deposition. (Note: Some of the stratigraphic names have been modified to correspond more nearly to modern terminology. In the construction of this table, Newberry undoubtedly was influenced by Dawson's table of 1866.)

Periodicity

From this time onward, recognition of the proper limits of the geologic systems, by some of the foremost geologists in America, seems to have been influenced more and more by the idea of periodic sea level fluctuations or diastrophic pulsations.

There is a rhythmic relation between the successive grand subsidences and emergences of the interior of the continent that we believe should be the basis for a revised classification of the rocks of North America. (Ulrich and Schuchert, 1902, p. 659.)

We believe that there is a natural basis of time-division, that it is recorded dynamically in the profounder changes of the earth's history, and that its basis is worldwide in its applicability. (Chamberlin and Salisbury, 1906, v. 3, p. 192.)

Have diastrophic movements been in progress constantly, or at intervals only, with quiescent periods between? Are they perpetual or periodic? The latter view prevails, I think, among American geologists. (Chamberlin, 1909, p. 689.)

The North Atlantic, for instance, is bounded on the east and west by lands, which have been disturbed or have been at rest during the same epochs, . . . These cycles are indeed those on which the time-scale of geologic history is based, and each one corresponds in general with a standard period. . . . (Willis, 1910, p. 247.)

Assuming that baseleveling was in progress through all the ages, yet it seems that at certain recognizable times the process was strongly revived. . . . . Referring on this occasion only to the Paleozoic, they took place at what I conceive to have been rhythmically recurring intervals, corresponding essentially to the systemic divisions of the stratigraphic column as drawn in the revised classification to be proposed. (Ulrich, 1911, p. 313-314.)

Nature vibrates with rhythms, climatic and diastrophic, . . . which have divided earth history into periods and eras. (Barrell, 1917, p. 746.) The profound revolutions, . . . were relatively brief periods closing long eras marked by diastrophic quiet and low continental relief. (Barrell, 1917, p. 752.) It is clear that epochs of diastrophism are more or less closely correlated in widely different regions. Changes in sealevel are necessarily felt over the whole earth. (Barrell, 1917, p. 756.)

At these times ranges of mountains are slowly raised up near the margins of the continents . . . the "disturbances," which are coming more and more to be regarded as the basis for dividing the eras into periods of time. . . . (Schuchert, 1918, p. 70-71.)

As our study progresses, the fact becomes more and more evident that transgression and regression are primarily "hologeodetic"--that is, universal so far as the earth is concerned--and that, moreover, they proceed in a rhythmic manner. . . . (Grabau, 1936a, p. 540.)

A review of American geologic literature shows that the idea of more or less worldwide periodicity in diastrophism and marine transgression and regression was firmly held for many years. Attempts were made at first to equate the orogenic and marine cycles with the standard geologic systems. As important discrepancies came to light, however, several influential American geologists sought to revise the systems in conformity with their interpretations of the presumed cycles. To the five or six Paleozoic systems recognized by Europeans, Chamberlin and Salisbury (1906, v. 2, p. vi) added one, Schuchert (1910, p. 513-576) seven, Ulrich (1911, p. 379-380, Pl. 26) four, and Grabau (1936b, p. 27-44) at first six and then (1938, p. 22-23) one more system.

The possibility that diastrophic and marine cycles may not have been contemporaneous everywhere was considered, but generally this was not seriously entertained. Willis, however, seems to have disagreed.

Each region has experienced an individual history of diastrophism, in which the law of periodicity is expressed in cycles of movement and quiescence peculiar to that region. (Willis, 1910, p. 247.)

The logical reasoning of Chamberlin (1909) evidently satisfied most others in Europe as well as in America that the worldwide periodic theory was acceptable. Early objections received little notice and aroused less interest.

The writer has no desire to attack the well-established idea of peneplanation and relative quiescence locally over long periods, but . . . to replace this idea [worldwide periodicity] with the suggestion, that diastrophism has been continuous. . . . (Shepard, 1923, p. 599.)

The twentieth century was half gone before a vigorous and well documented reconsideration reversed this trend of thinking.

Worldwide orogenic revolutions do not appear to me to have been demonstrated. . . . It is my belief that such changes are not yet required by the evidence at hand. (Gilluly, 1949, p. 589.)

Grand Cycles

In the meantime attention had been called to the possible existence of supra-systemic cycles.

It is possible to recognize at least three grand cycles from the late pre-Cambrian to the close of the Mesozoic, each grand cycle consisting of a long period of activity [diastrophism] and a still longer time of relative quiet. . . . (Willis, 1910, p. 247.)

A two-fold division of the Paleozoic was made at first to correspond with the supposed tectonic history of New England. A break was provided by the Taconic "revolution" at the end of the Ordovician Period. Later, this was altered to conform to the better established structural development of Europe where a division was indicated by the Caledonian "revolution" at the end of the Silurian Period. Snider (1932, p. 72-76) presented this concept clearly. These cycles were marked off by orogenesis at (1) the late Precambrian, (2) the late Silurian, (3) the late Permian, and (4) the late Cretaceous continuing to the present. The epochs of Ordovician, Mississippian, and Cretaceous limestone deposition identified the quiescent phases of widespread marine transgression. The opinion now is common that these grand cycles, although much generalized, probably have more reality than the lesser systemic ones so far as the whole world is concerned.

Pennsylvanian Cyclothems

While some geologists theorized about the larger patterns of geologic history, others engaged in detailed field work noticed repeated associations of different kinds of rocks on a much smaller scale. The economic importance of Carboniferous coal beds led to some of the earliest observations of this sort. The intermittent occurrence of coal seams in a succession of dominantly detrital strata is obviously cyclic in some degree and had long been recognized. Different interpretations of these associations, however, introduced and perpetuated a longstanding controversy concerning the origin of coal. Were the seams formed in place by the local growth and preservation of vegetation, or do they consist of the remains of transported vegetable material? This question was not resolved in favor of local origin to most persons' satisfaction before the publication of an exhaustive review of all the evidence by Stevenson (1911-1913).

Early Notice in America

The sea cliffs at the South Joggins in Nova Scotia probably present the finest display of outcropping Carboniferous coal measures to be seen anywhere in the world. There, in a distance of about 7 miles, a nearly continuous, gently dipping section exposes more than 14,500 feet of strata containing at least 76 coal beds (Logan, 1845). Much of this section was carefully studied by Dawson. He observed the general presence of underclay with roots beneath the coals and argued strongly for the local origin of coal. He also noted the occurrence of thin limestones.

It is remarkable that in almost every instance the conditions requisite for the formation of these limestones and their allied Modiola-shales have followed immediately on the formation of layers of coal based on underclays. (Dawson, 1854, p. 15.)

This seems to have been the first report of the sequence that is so characteristic of many Pennsylvanian cyclothems.

In the United States, Newberry observed comparable associations in the Ohio coal measures.

. . . it will be seen that the elements composing the Coal Measures occur in an order of superposition that is so constant, or at least so frequently repeated, that it cannot be a matter of chance, but must be the expression of a general law. The order of sequence . . . is this, namely, that the coal strata almost invariably rest upon beds of fire-clay. They are also almost always covered with shale of greater or less thickness, and this in turn is overlaid sometimes with sandstone, more rarely with limestone; and thus each section is divisible into series of three or more members each, in which the elements hold nearly a constant relation to each other. (Newberry, 1874b, p. 114.)

Many discontinuous but relatively good Pennsylvanian outcrops occur in parts of western Illinois. By piecing them together, Udden built up a stratigraphic section in the vicinity of Peoria nearly 250 feet thick cosisting of plainly developed cycles (1912, p. 27, Fig. 2).

. . . the coal-bearing rocks present an unusual persistence of the 21 recognizable divisions [lithologic members] . . . grouped into an almost perfect quadruple repetition of a sedimentary cycle. Each cycle may be said to present four successive stages, namely: (1) accumulation of vegetation; (2) deposition of calcareous material; (3) sand importation; and (4) aggradation to sea level and soil making. (Udden, 1912, p. 47.) . . . the most remarkable feature bearing on the physical conditions prevailing at the time of the deposition of the coal measures of this quadrangle is the horizontal extent and uniformity in thickness of each deposit. (Udden, 1912, p. 50.)

These statements, together with the graphic section, were the clearest expression of the Pennsylvanian cycle yet published. In the following years several papers, in one way or another, mentioned comparable cyclic arrangements of strata. One curious publication by a Belgian geologist in Wales refers to "cycles" with coal beds in their midst. This author made casual mention of the writings of Dawson, Hull, and Newberry.

. . . all rocks of the coal measures form a succession of cycles of deposition, with the finest sediments in the center. . . . (Simoens, 1918, p. 7.)

In Ohio, Stout evidently was aware of the cyclic arrangement of Pennsylvanian strata.

In the coal formations of the United States and other countries, clay and coal occur as associated materials with great regularity; in fact, one is not often present without the other. This constancy of relations is so universal that it is not a matter of chance. (Stout, 1923, p. 533.) With few exceptions marine limestones lie on or not far above coal beds. (Stout, 1923, p. 538.)

It is a strange fact that the statements concerning the existence of a repeated Pennsylvanian sedimentary cycle by Dawson and Newberry and its clear description by Udden so generally escaped the attention of stratigraphers. The principal reasons for this neglect seem to have been: First, most Paleozoic stratigraphers were so accustomed to dealing with continuous marine strata that they failed to recognize or to be impressed by the repeated alternations of marine and nonmarine members in the Pennsylvanian. Even the presence of coal beds had little effect on their preconceptions.

. . . nearly all of the present Appalachian Coal Measures area bears the marks, either paleontological or stratigraphical, of the continued transgression of the sea. (White, 1904, p. 277; see also, Wilson and Stearns, 1960.)

Second, most coal geologists were so concerned with the economic aspects of their work that they neglected to comment on some rather obvious stratigraphic associations, if they noticed them at all. Third, the idea seems to have prevailed that Pennsylvanian strata are so variable and discontinuous that the local occurrence of different kinds of sediments and their associations are not significant.

Because of the lenticular character of the strata . . . a generalized section . . . is of little value. (Cady, 1919, p. 64.)

On account of this variable and lenticular character of the strata it has not been possible to identify any easily recognized stratigraphic units. (Savage and Udden, 1922, p. 143-144.)

Fourth, in many areas either marine beds or coals of noteworthy thickness are missing from the cycles. Finally, most theoretical speculation was devoted to the problem of the origin of coal.

Rediscovery

The Pennsylvanian cycle so well described by Udden was rediscovered in Illinois in 1926.

. . . each series of beds starts with an unconformable sandstone below, contains an underclay and coal horizon in the middle, and continues above with limestone and shale. Such a series of beds . . . may be observed in almost every section from the bottom to the top of the system as developed in western Illinois. (Weller, 1930, p. 102.)

These cycles were demonstrated in the Peoria area on a field trip following a meeting of the Association of American State Geologists in 1927. This evidently stimulated interest because cycles were soon distinguished in other states both to the east and west.

. . . each sandstone represents the definite termination of a stratigraphic substage or depositional cycle [and] it is evident that there have been nine such cycles in the history of Monongahela deposition . . . [in West Virginia]. (Reger, 1929, p. 136.)

Detailed stratigraphic studies of the Pennsylvanian deposits of Kansas, Nebraska and northern Oklahoma show that well marked rhythms or cycles of sedimentation characterize at least a considerable part of the section. . . . These sequences so closely duplicate one another that it is easily possible to mistake the identity of a formation. . . . (Moore, 1930, p. 51-52.)

An interesting feature of the detailed stratigraphy of the Millstone Grits and Lower Coal Measures, and to a less degree of the Middle Coal Measures, is the occurrence of a well-marked rhythm in the sedimentation. Throughout we find repeated in varying degrees of completeness the four-fold cycle [in descending order] Coal, Sandstone, Mudstone, Marine Band. (Wright and others, 1927, p. 9.)

A symposium (Weller and others, 1931) on Pennsylvanian stratigraphy was held in Urbana, Illinois, in 1930 which served further to publicize the renewed interest that had developed in Pennsylvanian cycles.

A historical resumé of the development of Pennsylvanian cyclic studies in the United States has been published (Weller, 1961, p. 130-135) and does not need to be repeated here. In 1932 the word, cyclothem, was introduced for Pennsylvanian cycles.

The word "cyclothem" is therefore proposed to designate a series of beds deposited during a single sedimentary cycle of the type that prevailed during the Pennsylvanian period. (Weller, in Wanless and Weller, 1932, p. 1003.)

Evidently, cyclothem filled a need because it was soon adopted and used by other geologists. Besides numerous individually published papers, symposia devoted to cyclic sedimentation, generally with emphasis on the Pennsylvanian, were held in London in 1948 (International Geological Congress, 1950) and in Houston, Texas, in 1963 (Society of Economic Paleontologists and Mineralogists, 1963). Pennsylvanian cyclic sedimentation formerly met with considerable skepticism, but it is now well known to be more or less characteristic of the Upper Carboniferous in many parts of the world.

Other Cyclothems

Cyclic sedimentation is not restricted to the Pennsylvanian. Although it occurs and has been recognized in rocks of other ages, it is, however, nowhere developed so clearly or so widely in other systems. For this reason, the Pennsylvanian cyclothem may be taken as a model with which other types of cycles can be compared.

Strata of the Mississippian, below the Pensylvanian in some areas, and those of the Permian, above it in others, are distinctly cyclic in their arrangement. For example, cycles are evident in some of the descriptions of stratigraphic sections in parts of England and southern Scotland that were published long ago.

. . . the most striking feature of the mountain limestone [lower Carboniferous] deposits of the North of England, [is] the repeated succession of nearly similar combinations ["terms," p. 182] of limestone, gritstone, and shale. . . . (Phillips, 1836, p. 175.)

Later accounts have noted them more specifically.

The alternations of these beds fall into a certain orderly sequence which marks them into zones and cycles . . . a complete cycle of deposition, of which the lower part is marine and calcareous, and passes up into mechanical sediment in a fine state of division; while the upper part is marked by a coarsening of the deposits, by an intermingling of plant remains, and by the presence (general), near the top, of Coal or the roots of Stigmaria in situ. . . . (Miller, 1887; Dunham, 1950, p. 47; see also, Peach, 1888, p. 17.)

. . . the general succession is one of shale, sandstone and limestone, repeated in the same order several times. . . . (Hudson, 1924, p. 125.) Occasionally this sedimentation culminates in emergence, and coal seams result either at the top, or near the top, of the sandstone. (Hudson, 1924, p. 132.)

The outcropping Chester Series (Upper Mississippian) of Illinois, Indiana, and western Kentucky also provides a good example. Stuart Weller (1920, p. 285) distinguished 16 alternating sandstone and limestone-shale formations.

Throughout the alternating succession of Chester formations, the several units should be considered in pairs, each pair consisting of a sandstone formation below, passing upward into a limestone-shale formation. . . . Each one of these pairs doubtless represents one oscillatory advance and retreat of the waters of the basin. (S. Weller, 1920, p. 414.)

Moreover:

In four or five of the units [pairs] thin coal beds are present between the sandstone and limestone-shale portions [formations] and thus these units resemble the cyclothems of the Pennsylvanian system. (Weller and Sutton, 1940, p. 847.)

Permian sedimentary cycles in Kansas generally lack sandstones and coals but otherwise they are not greatly different from some of those in the underlying Pennsylvanian.

. . . there is a strikingly remarkable recurrence in the same sequence of very similar strata. . . . there are eight or ten complete or nearly complete cycles of sedimentation, or cyclothems, each of which, where complete, has the following parts: (1) beds of varicolored [generally red] shales . . .; (2) a thin bed of limestone . . .; (3) highly fossiliferous . . . shale . . .; and (4) a series of limestone beds . . . much thicker than the [other] limestone . . . these rhythmic cycles are quite as persistent and regular . . . as are those in the Pennsylvanian of Kansas. . . . (Jewett, 1933, p. 138-139.)

The Devonian "Old Red Sandstone" of northern Scotland includes considerable thicknesses of nonred strata arranged in a recurrent sequence of (1) fine-grained, crossbedded sandstone at the base, (2) greenish, mudcracked shale, (3) dark-bluish to black, calcareous shale, and (4) dark, impure limestone with fossil fish (Crampton and Carruthers, 1914, p. 102).

. . . the rhythmic sequence is confined to continental, alluvial, and lacustrine deposits, and has resulted from variations in the height of the floodwater level on the one hand, and from a recurrence of periods of quiescence and crustal deformation on the other. (Crampton and Carruthers, 1914, p. 89-90.)

Twelve sedimentary cycles, which seem to be closely similar to those of the Pennsylvanian, have been described in Sweden in a section of uppermost Triassic or lowermost Jurassic beds 250 meters thick.

Each cycle begins with nonmarine beds, contains clays and coal in the upper part, and is terminated by a calcareous or ferruginous bed with marine fossils. (Troedsson, 1950, p. 64.)

Cycles of a somewhat different kind occur in the Upper Cretaceous of Utah and Colorado. There, the Mesaverde Sandstone, and its subdivisions, and the Mancos Shale intertongue extensively in the Book Cliffs and elsewhere. Excellent exposures permit the tracing of individual beds for long distances. Several coals are present.

A generalized cycle of four units can be recognized in these deposits: (1) basal marine shale, (2) littoral marine sandstone, (3) lagoonal rocks, (4) coal (Young, 1955, p. 177.)

Rather regularly spaced Tertiary coals in part of the Philippines are separated by intervals of clay which include, alternately, coral fragments grading laterally into limestone, and local sand bodies.

A pair of these contrasting intervals might be interpreted as a depositional cycle, and the two associated coal beds might have accumulated under transgressive and regressive conditions respectively. (Crispin and others, 1955, p. 15.)

Many other examples of cycles or cyclothemlike sequences in all parts of the stratigraphic section might be cited (Wells, 1960). Some undoubtedly record interesting recurrent series of conditions, but others are little more than the alternations of two kinds of strata. In the absence of both coal, or other surely terrestrial material, and sediments with marine fossils, comparisons with Pennsylvanian cyclothems are uncertain and interpretations in terms of geologic history are doubtful.

Cycles of Cyclothems

The idea of a cycle involves repetition because a cycle can be recognized only if units are repeated in the same order. The question that inevitably arises is: How closely similar must the repetition be? An answer seems to depend on two requirements: (1) nearly complete transitions between variants must be observed, and (2) a generalization must be made reducing the cycle to its simplest form by excluding all unessential details. The cycles, then, must be closely similar with respect to this simple form.

Some Pennsylvanian cyclothems are so similar that they are easily mistaken for each other but others are so different that their similarities are not immediately apparent (Wanless, 1947; 1950, p. 20; Weller, 1961, p. 142-148). When all common variants are considered, a single "ideal" cyclothem can be constructed that embodies most of the variant characters. This consists of ten or more members (Weller and Wanless, 1939, p. 1377), but all of these are not necessary for the cycle to be recognized. In its simplest or most reduced form, the Pennsylvanian cyclothem requires only three or four members (Weller, 1961, p. 142). Those likely to be most important are underclay, coal, and strata with marine fossils. In early cyclothemic studies, a marine limestone was observed to be a common development of the last (Weller, 1930, p. 102).

The Pennsylvanian cycle originally recognized in Kansas (Moore, 1930) contains several, up to five, marine limestones. Each of these limestones was believed to identify a different, simple but compressed cyclothem.

This repeated succession of cyclothems of different character indicates a rhythm of larger order than that shown in the individual cycles and suggests the desirability of a term to designate a combination of related cyclothems. The word "megacyclothem" will be used in this sense to define a cycle of cyclothems. (Moore, 1936, p. 29.)

Later field studies in Illinois showed that many seemingly simple cyclothems include two marine limestones. Therefore, the possibility was entertained that a megacyclothem might be only a further elaboration of this more complex sequence (Wanless and Shepard, 1936, p. 1202; Wanless, 1950, p. 21).

Detailed lithologic analyses of the Kansas megacyclothems have revealed the presence of rudimentary representatives of certain previously unrecognized members of the simple cyclothem in some, but not all, of the shaly intervals between the limestones (Weller, 1958, p. 198-201). This seems to confirm the original conclusion that the megacyclothem is a cycle of simple cyclothems. It also suggests that a cycle of cyclothems of a different kind recognized in Illinois (Weller, 1942) can be equated with the Kansas megacyclothem.

Further consideration of the Kansas section reveals a still larger, four times repeated cycle.

This great cycle may be termed a hypercyclothem. Each consist of four megacyclothems and an alternating detrital sequence of more than ordinary thickness and complexity. (Weller, 1958, p. 203-204.)

Other so-called cycles consisting of subordinate cyclothems have been reported (Young, 1955, p. 199-200; D. Moore, 1959, p. 523-524; Jablokov and others, 1961, p. 298-299). Mostly these seem to be groups of cyclothems which resemble each other in some of their characters rather than recurrences of similar sequences of cyclothems.

Interpretation of Cyclothems

Surely no single explanation can account for the origin of all sedimentary cycles (see, Robertson, 1948, p. 143). Primary attention in the following, therefore, is directed to Pennsylvanian cyclothems and other cyclothemlike stratigraphic sequences. These cycles have been observed most carefully and commented upon most commonly, and they, more than any others, have been productive of the most varied interpretations. At the very outset, the observation should be made that much remains to be learned about these cycles and it is safe to say that no interpretation yet made can be accepted more than very tentatively. Furthermore, the only truly scientific approach to a problem of this kind is one of critical appraisal and sympathetic skepticism.

Before proceeding with this consideration, several other general observations are in order: (1) The existence of sedimentary cycles of a particular type must be conceded. This is obvious but necessary because some experienced stratigraphers have denied the existence of cyclothems.

. . . were these recurrences rhythmic or cyclic in character? My own answer is "yes" to the extent of the coal and its underclay, and "no" beyond that. (Ashley, 1931, p. 241.)

It is quite natural that: (2) The opinions of every person are likely to be biased in a way that depends upon the region with which he is most familiar. Thus, most geologists working in the central United States are impressed by the great lateral persistence and uniformity of many cyclothems, or at least some of their principal members.

Individual phases of various Pennsylvanian and Permian cyclothems in the northern midcontinent have been traced along the outcrop for distances of 400 miles or more. Many of them have been identified down dip into basins for distances of at least 300 miles from outcrop. (Moore, 1950, p. 12.)

On the other hand, those whose experience has been gained at places closer to the source of sediments are more aware of lateral changes occurring in short distances and of possible confusing irregularities such as splitting coals (Thiadens and Haites, 1944). In the same way, views will differ depending upon whether marine strata are rare or dominant in the local stratigraphic section. Furthermore: (3) some of the Mississippian and Permian cycles that have been recognized differ more or less notably from the ordinary Pennsylvanian cyclothems. Good reasons, however, are believed to exist for concluding that these are all related in their origin and that they differ mainly because they represent somewhat different environments. Any adequate theory must take into account whatever evidence is provided by each variety of the cycle.

Theories interpreting the origin of Pennsylvanian-type cyclothems can be classified roughly in three groups, each of which includes several variations (Westoll, 1962, p. 767-768). These are (1) diastrophic theories--sinking basins, rising sedimentary source areas, either continuous, intermittent, or reversing, (2) climatic theories--glaciation producing sea-level oscillations, rainfall cycles and variable erosion, and (3) sedimentation theories--differential deposition related to depth of water, strength of currents, or distance from a river's mouth, compaction of sediments, etc. Most of these theories have been critically reviewed recently (Weller, 1956, p. 18-25; Wheeler and Murray, 1957, p. 1986-1998; Weller and others, 1958; Goodlet, 1959, p. 220-224; Beerbower, 1961) and details cannot be repeated here. All or most of these theories seem to be unsatisfactory in one or more respects.

Diastrophic Theories

Subsidence probably has prevailed in any basin where an appreciable amount of sediment accumulated and, in the long run, sediment thickness generally can be accepted as an approximate measure of the total amount of subsidence. Confidence in such a probability is increased if sedimentary characters indicate that deposition occurred close below or above sea level. Evidence of deposition within this narrow range is provided by all late Paleozoic sedimentary cycles. Likewise, elevation must have prevailed in the source area to account for a great amount of sediment contributed over a long period of time.

(1) Continuous subsidence of basins--If cycles developed in a continuously and evenly subsiding basin, changes in sedimentation must have resulted from other factors, particularly those controlling the kind and quantity of sediment provided. Therefore, diastrophism within the basin was not primarily important in their origin.

(2) Intermittent subsidence of basins--One of the oldest theories accounts for cyclothems by only intermittent subsidence. The origin of this rather obvious explanation is not known but it has been expressed many times.

. . . downward movement . . . was marked by pauses, long enough for the silting up of lagoons and the spread of coal jungles. (Geikie, 1882, p. 722.)

The movement was one of slight but rather rapid depression, followed by a pause, this order being repeated over and over for each succeeding cycle. (Stout, 1931, p. 211.)

Each subsidence was followed by accumulation of sediments, leading to the filling of basin and formation of a swamp. (Trueman, 1947, p. lvi.)

The simple idea involved here is that deposition lagged behind subsidence and that during the pause deposition occurred in progressively shallowing water. Nothing is inferred about conditions in the source area.

A somewhat more sophisticated idea has been presented.

The regularity of the pattern [of rhythms] throughout long periods of time, and its relationship to the present structures suggests that the pattern is due to areal stresses [resulting in subsidence in synclines], more or less constant in direction, deep seated in origin, and probably related to the contemporaneous Variscan orogeny. (Goodlet, 1959, p. 227.)

Intermittent subsidence may have produced cycles, but it need not have involved the sedimentary basins.

. . . the two processes of displacement of [ocean] water by sedimentation and accommodation of water by sea bottom movement [subsidence] as proceeding side by side and of the same order of magnitude, but by no means always counteracting each other [unsynchronized]. This would result in a continual oscillation of absolute sea level. . . . (Wells, 1960, p. 401.)

(3) Alternate subsidence and elevation--Some evidence has been considered to suggest that dominant subsidence in the basins was interrupted by periodic minor uplifts.

The alternations of limestones containing marine remains, and of sandstones, shales and coal-beds with no trace of a marine creature in them, are exceedingly remarkable, and seem difficult of explanation without calling in the aid of oscillations of the solid surface of the earth, by which very gradual rises and depressions are effected. (De la Beche, 1837, p. 264.)

. . . the Appalachian and Eastern Interior [coal] fields . . . were characterized by slow, widespread sinking with many long pauses and frequent slight uplifts followed by surface erosion. (Ashley, 1931, p. 245.)

. . . the depth of erosional entrenchment [below sandstones], . . . indicate a connection between erosional wash-outs and tectonic uplifts in the area of sedimentation. (Jablokov and others, 1961, p. 297.)

The kinds of sediments delivered to the basins can be explained if basin movements paralleled those of the adjacent uplands.

. . . cyclothems were developed by repeated [synchronous] oscillations [of both basins and source areas], each consisting of a long gradual subsidence followed by a short sharp uplift both centering in the area from which sediments were derived. (Weller, 1956, p. 47.)

. . . upward crustal movements . . . lead to the emergence of the coalbelt, and to the beginning of hinterland erosion. (Rutten, 1952, p. 534.)

Climatic Theories

Climate is a factor that may have influenced sedimentation importantly. Repeated glaciations could account for eustatic shifts in sea level. Rainfall cycles might affect erosion directly, or indirectly in a reverse way by contributing to the growth of vegetation.

(1) Glaciation--Late Paleozoic glaciation has been called upon to explain rise and fall of sea level in either a gradually or an intermittently subsiding basin.

. . . the depositional basins are thought to have subsided slowly as sediment accumulated, the cyclic fluctuations being due to the rise and fall of sea level. . . . (Wanless and Shepard, 1936, p. 1206.) . . . the cycles are inferred to be causally related to glaciation. . . . (Wanless and Shepard, 1936, p. 1202.) . . . widespread glaciation, particularly in the southern hemisphere . . . must have lowered sea level, and caused the temporary withdrawal of waters from large portions of shallow interior seas. (Wanless and Shepard, 1936, p. 1205; see also, Wanless, 1950; Beerbower, 1961.)

A variant of this theory seeks to relate a more complex double cyclothem to a pair of glaciations produced during a single solar radiation cycle (Simpson, 1934).

. . . a typical cyclothem records four distinct base-level reversals rather than two [i. e., sea level rose and fell twice] . . . . (Wheeler and Murray, 1957, p. 1993.) . . . Simpson's theory or its modifications demand eustatic fluctuations and climatic changes of the same magnitude and sequential order as those indicated by Paleozoic stratigraphy. . . . (Wheeler and Murray, 1957, p. 1985; see also, Wilson and Stearns, 1960.)

These theories obviously require a great number of glaciations continuously from the late Mississippian to the early Permian.

(2) Rainfall--Humid and arid epochs resulting from glaciation or some other cause have been suggested as either the main or a contributory factor in the origin of cyclothems.

The aridity, the cold and the temperature extremes [during a glacial epoch] would have tended to reduce the vegetation on the upland source area. As a result these areas would have been subject to . . . greatly increased erosion. (Wanless and Shepard, 1936, p. 1200.)

The major clastic pulse [sand deposition] is the result of a relatively arid climate in the source area. . . . With increasing humidity and more effective plant cover at the source, a smaller supply of fine sediment cannot keep up with the rate of basin sinking. (Swann, 1963.)

The opposite idea that increased rainfall resulted in more rapid erosion and greater amounts of detrital sediments also has been expressed.

If there was a cyclical climatic variation . . . it is likely that periods of aridity would be represented by the limestones and periods of humidity by the terriginous sediments and coal. . . . (Brough, 1928, p. 125.)

Sedimentation Theories

All sedimentation theories are more or less complex. They relate the production, delivery, or deposition of different materials to a variety of factors such as earth movements, fluctuating climate, physiographic development of the land, changing sea level, strength of currents, distance from source, compaction of sediments, and the building and breaking of barriers to the distribution of sediments. Several of these theories have been included in the foregoing.

(1) Peneplanation--Successive uplifts of the land each followed by peneplanation might account for an intermittent supply of variably coarse sediment.

The major succession of shale, sandstone and limestone can thus be read in terms of uplift and denudation, reduction of land level, and finally cessation of erosion of the land [i. e., peneplanation]. (Hudson, 1924, p. 135.)

Coarse grained detritus from the remnants of the hinterland hills is still being spread over the coal. belt area, up till the time when the entire region has been peneplained. (Rutten, 1952, p. 535.)

(2) Differential settling--Several theories relate coarseness of sediment to settling in different depths of water and explain vertical changes in the deposits by advance and retreat of shore lines and the resulting variation in water depth. One of them supposes that coal, next to limestone, records the deepest water environment.

. . . the rivers were carrying . . . all kinds of materials . . . and . . . debris of fragile plants. . . . All . . . were submitted to the action of separation in order of density, and . . . to the distance from shore. . . . (Simoens, 1918, p. 7.) . . . organic matter is able to travel [float] much farther out to the high seas. . . . (Simoens, 1918, p. 8.) The coal is a terriginous sediment which has been deposited in the sea very far from shore. . . . (Simoens, 1918, p. 9.)

(3) Currents--Strength of currents in the sea would have much the same effect as depth of water in influencing the kind of sediment deposited.

The sands alternate with slightly finer sediments that were, no doubt, actually suspended in the same currents. . . . In the deeper seas the currents might be stronger because less interrupted, and might more rapidly bring forward the continental deposits [particularly sand] from their evidently somewhat distant source. . . . (Udden, 1912, p. 49.)

(4) Shifting deltas--The distributaries in a deltaic area shifted from side to side so that detrital sediments were delivered to the sea now at one place and later at another where sand bodies were built up. There, the sediment surface rose to sea level and coal swamps may have formed. At the same time, normal marine conditions prevailed elsewhere. Lateral shifting of the actively growing delta front and submergence of its older parts resulted in the development of cycles.

Each major cyclothem is the result of interplay of two environments: a shallow epicontinental sea whose normal sediment was limestone, and the delta of a large river. . . . Regional subsidence, slower than delta deposition but continuing after the delta built up above sea level, gradually drew the [older] bypassed parts of the delta plain below the sea. (D. Moore, 1959, p. 522.) . . . the apparent periodicity in delta deposition . . . resulted from crevassing [breaking of natural levees] in the trunk river. . . . (D. Moore, 1959, p. 538).

A river pushes its delta hundreds of miles across a gradually subsiding basin, then is unable to keep up with the sinking, so that the sea reclaims the basin. Two factors seem to be involved--rate of basin subsidence and the rate of sediment supply, with cyclic development dependent upon the change of one with respect to the other. (Swann, 1963; see also, Goodlet, 1959; Duff and Walton, 1962.)

(5) Alluvial vs. deltaic environment--If strata below coal beds were deposited on an extensive alluvial or coastal plain, the channels cut below sandstones would be the result of subaerial erosion.

It is highly improbable that such channels could have been eroded in the bottom of a shallow epicontinental sea. (Weller, 1930, p. 116.)

The succeeding shale and coal [above the sandstone] are clearly continental in origin and indicate deposits made on an extremely low, flat coastal plain. (Moore, 1936, p. 25.)

In sedimentation cycles of coal measures alluvial [as distinct from deltaic] deposits are always beneath the coal seam with which they are paragenetically connected by a gradual transition from channel deposits through flood plain deposits and to bog deposits. (Jablokov and others, 1961, p. 296.)

The channels apparently are nonmarine and most of their fill material . . . is very likely nonmarine in origin. (Mudge and Yochelson, 1962, p. 98.)

Those who have favored eustatic or deltaic theories, however, generally considered the channels to be submarine.

These valleys . . . may have been cut in part below sea level. . . . (Wanless and Shepard, 1936, p. 1202.)

The writers believe that they [channel sandstones] mark submarine distributaries down which sediment-laden water moved from shallow to deeper water. (Wilson and Stearns, 1960, p. 1459-1460.)

Actually the base of the sandstone in most places was deposited well below sea level, and in some instances may represent deposition at the greatest depth of any of the parts of the cyclothem. (Swann, 1963.)

(6) Compaction--Differential compaction of sediment has been included as a more or less important part of several theories. For example, compaction of a thick peat bed might have had the same effect in a basin as subsidence or rising sea level.

. . . immediately after the deposition of the first sedimentary covering to the drowned peat, considerable sinking of the region must have taken place [because of compaction]. If the sea happened to be near at that moment, marine incursion naturally followed. (van der Heide, 1950, p. 40.)

(7) Barriers--Different deltaic enviroments were separated by barriers in the form of natural levees and coastal sand bars. When barriers were broken, sudden environmental change resulted.

. . . the barrier broke, and the swamp area was flooded. (Robertson, 1948, p. 168.) . . . a return to salt or brackish water conditions. . . was due to the breakdown of a barrier or levee. . . . (Robertson, 1948, p. 171.) The barrier was built up again by current action. . . . (Robertson, 1948, p. 168.) . . . the return to fresh water conditions was due . . . to the restoration of a levee or lagoonal bar which had shut off the mass of swamp vegetation from the sea or river. (Robertson, 1948, p. 167.)

Swamp vegetation also constituted barriers to the distribution of detrital sediment.

Plant growth holds back deposition of other sediments [acts as a filter] until subsidence [and compaction] has taken place to such an extent as to induce extensive and rapid flooding. (Robertson, 1948, p. 173.)

The vegetation prevents the sediment bearing stream from spreading its load all over the delta. (Robertson, 1952, p. 519.)

Conclusions

Almost every conceivable factor that might have influenced the development of late Paleozoic sedimentary cycles has been incorporated ingeniously into one theory or another. Many of the views expressed are irreconcilable, and it is obvious that no agreement is presently in prospect.

. . . the cycles represent recurrent submergences, alternating with periods during which the sunken areas were filled to the level of the surface of the sea. (Udden, 1912, p. 49.)

. . . the geosyncline [coal basin] emerges from the marine transgression through upward crustal movement, and not through sedimentary silting up . . . . (Rutten, 1952, p. 533.)

. . . progressive compaction of the sediments . . . must have played an important part. . . . (Trueman, 1954, p. 15.)

. . . compaction combined with strong differential subsidence--and not compaction alone--had caused the rhythmic development. (Fearnsides, 1950, p. 63.)

[It is] unnecessary to assume intermittent subsidence in order to account for the production of a rhythmic sequence of Coal Measures type. (Robertson, 1950, p. 62.)

. . . there remains clear evidence that the intermittent (cyclical) subsidences of the area of sedimentation were related to the wider tectonic events of the period. (Trueman, 1947, p. lxii.)

[It is not] necessary to invoke actual individual tectonic rhythms in any one place. . . . (Wells, 1960, p. 401.)

Only the theories of glaciation-eustatic fluctuations explain the localization of this type of cyclothem within the late Paleozoic. (Beerbower, 1961, p. 1048.)

The sedimentary cycles resulted directly from the pluvial-interpluvial alternations, and their relations to glacial cycles was secondary. . . . Neither worldwide eustatic sea-level change nor regional tectonism was needed for cyclic development. (Swann, 1964.)

It seems most unlikely that each rhythm could be the result of a series of complex diastrophic movements affecting large areas. Moreover, it seems highly improbable that each rhythm records a glacial and an interglacial episode in the Southern Hemisphere and it is almost inconceivable that it should record two each of such episodes. (Goodlet, 1959, p. 223.)

The relations between the rock types and faunal assemblages of the midcontinent area, attributed by some individuals to rapid fluctuations of sea level, might be explained equally well by combinations of other physical and chemical factors. Cyclic sedimentation may be as much a reflection of changes on the land as of changes in sea level. (Mudge and Yochelson, 1962, p. 115.)

. . . cyclic causes . . . all fail to account for one or another features of these cyclothems. (D. Moore, 1959, p. 538.)

When, if ever, the true explantion of late Paleozoic sedimentary cycles is discovered, it probably will embody parts of several of the theories that have been proposed. In the meantime:

No more fascinating field for research and speculation exists within the entire domain of stratigraphy. (Weller, ]956, p. 48.)

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Weller, J. M., and Wanless, H. R., 1939, Correlation of minable coals of Illinois, Indiana, and western Kentucky: Am. Assoc. Petroleum Geologists Bull., v. 23, p. 1374-1392.

Weller, J. M., Wheeler, H. E., and Murray, H. H., 1958, Cyclothems: Am. Assoc. Petroleum Geologists Bull., v. 42, p. 442-447.

Weller, J. M., and others, 1931, Studies relating to the order and conditions of accumulation of the Coal Measures (symposium): Illinois Geol. Survey Bull. 60, pt. 5, p. 161-289.

Weller, Stuart, 1920, The Chester series in Illinois: Jour. Geology, v. 28, p. 281-303, 395-416.

Wells, A. J., 1960, Cyclical sedimentation; a review: Geol. Mag., v. 97, p. 389-403.

Westoll, T. S., 1962, The standard model cyclothem of the Visean and Namurian sequence of Northern England: C. R. 4m Congr. Avanc. Étude Strat. Géol. Carbon., v. 3, p. 767-773.

Wheeler, H. E., and Murray, H. H., 1957, Base-level control patterns in cyclothemic sedimentation: Am. Assoc. Petroleum Geologists Bull., v. 41, p. 1985-2011.

White, C. D., 1904, Deposition of the Appalachian Pottsville: Geol. Soc. America Bull., v. 15, p. 267-282.

Willis, Bailey, 1910, Principles of paleogeography: Science, v. 31, p. 241-260.

Wilson, C. W., and Stearns, R. G., 1960, Pennsylvanian cyclothems in Tennessee: Geol. Soc. America Bull., v. 71, p. 1451-1466.

Wright, W. B., Sherlock, R. L., Wray, D. A., Lloyd, W., and Tonks, L. H., 1927, The geology of the Rossendale Anticline: England Geol. Survey Mem., Explanation of Sheet 76, p. 1-182.

Young, R. G., 1955, Sedimentary facies and intertonguing in the Upper Cretaceous of the Book Cliffs, Utah. Colorado: Geol. Soc. America Bull., v. 66, p. 177-202.

Kansas Geological Survey
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