|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 239-247|
Northwestern University, Evanston, Illinois
The classical cyclothem, first studied in western Illinois, can be used as a norm for shelf areas of low relief to study cyclothem variants that occur in different tectono-environmental settings. This approach, developed some years ago, is set into a present-day framework of modal cyclothems statistically defined; of detailed environmental maps based on individual cyclothem members; of cyclothem models based on physical and chemical controls in sedimentation; and on basin facies models. These several approaches to cyclothem studies can be arranged into a hierarchy that relates them to the scale of phenomena involved.
A cyclothem in the classical sense (Wanless and Weller, 1932) is a "... series of beds deposited during a single sedimentary cycle of the type that prevailed during the Pennsylvanian period." In North America, cyclothems were first studied in western Illinois, where they characteristically consist of a lower sandstone changing upwards through siltstone to underclay beneath a coal bed. The beds above the coal commonly include black fissile shale, marine limestone, and gray shale. The particular beds present vary from one cyclothem to another, but their orderly succession led to the construction of Weller's idealized cyclothem, consisting of ten members as illustrated in Willman and Payne (1942, p. 86).
The term cyclothem has been extended to many kinds of cyclical successions, as Duff and Walton (1962) point out. This paper returns to the classical Pennsylvanian cyclothem; the purpose of the paper is to examine this as a response to environmental and tectonic conditions in a framework that includes the scale of phenomena observed. A single cyclothem member may be examined in great detail for its composition, texture, and faunal content in a limited outcrop area. A stratigraphic unit having a number of cyclothems, may be studied over a large area in terms of the behavior of the entire section in response to tectonic or other controls, without minute examination of each member in each cyclothem. Thus, a scale factor to be discussed later is normally included in studies of cyclical deposits.
Several points of view in cyclothem study are reviewed to show parallels and contrasts among them; these are then related to a hierarchy of scales of observation and to the class of cyclothem models involved in each.
Acknowledgments--The writer is strongly indebted to J. Marvin Weller for encouraging a continuing interest in cyclothems, and for many discussions through more than two decades. The report for Gulf Research and Development Company benefited from the helpful advice of Charles Ryniker, then with the Gypsy Division in Tulsa. Appreciation is expressed to Melvin J. Hill of Gulf Research and Development Company for permission to draw upon the company report in this discussion of frameworks for cyclothem analysis. Paul E. Potter of the University of Indiana read the manuscript critically and contributed several important suggestions.
The concept of an idealized cyclothem implies an orderly sequence of sedimentary beds formed in a succession of environments characteristic of marine transgression and regression, with the marine episodes in successive cycles separated by an interval of nonmarine sedimentation culminating in a coal bed. Sometimes the cyclothem boundaries are disconformable.
The thickness and composition of a cyclothem member, the number and kinds of members in one cyclothem, and the succession of cyclothems in a stratigraphic unit are subject to variations controlled presumably by shifting strandlines, by local and regional variations in environment, and by contemporary tectonism in source and depositional areas.
Conventionally, cyclothems are observed, measured, and recorded as they occur over some area of interest, and from these observations such substantive generalizations as an idealized cyclothem arise. An important forward step in this conventional procedure was introduced by Duff and Walton (1962), who pointed out that discussion of cyclothems is facilitated if the rock types observed in a cyclic succession are statistically analyzed and arranged into a modal cycle, which represents the group of rock types that occurs most frequently through the cyclical succession. A composite sequence of all rock types present may also be constructed that assesses the position and order in which the less common members tend to occur within the modal cycle. Duff and Walton emphasize that modal cycles and composite sequences are derived from observation, whereas the idealized cycle is an inferred succession of beds based on some theoretical model of sedimentation. In practice, an idealized cyclothem may also be constructed from qualitative appraisal of members present in many observed sections.
Duff and Walton (1962) used 41 stratigraphic sections in the East Pennine Coalfield, extending roughly from Nottingham to Leeds, covering an area of about 480 square miles. The sections furnished data on 1,200 cyclical units, which were statistically analyzed by constructing histograms showing the frequency of various cycle types, the thickness distribution of the cycles, and other summary attributes. From this analysis they developed their modal cyclothem and composite sequence for the area and stratigraphic interval involved. The way is thus paved, through formal statistical analysis, to focus attention on an average cycle and its variants as shown by the composite sequence. The mode is an average of position in a frequency distribution and refers to the most abundant cyclical form; this term was chosen by Duff and Walton to emphasize the relative frequency of occurrence, as well as the statistical basis on which the modal cycle is defined.
On a somewhat different approach, Ferm and Williams (1963) introduced a cyclothem model for the Appalachian region based on the distinction between physical and chemical controls on the deposition of cyclothems, with chemical rocks considered as limiting members. The model is effectively and compactly represented as extending from seaward (limestone as the extreme limiting chemical member) to landward, with coal (and its associated seatearth) as the extreme limiting member. Ferm and Williams' emphasis on a physical-chemical dichotomy represents a reorientation in cycle studies that opens the door for more systematic investigation of process and response in cyclical successions.
In 1963 Wanless and co-workers published a comprehensive set of sedimentary environment maps for Pennsylvanian cyclothems. Individual members in three cyclothems (Summum, St. David, and Brereton, comprising about a third of the Des Moines Group) were identified and measured at more than 1,000 control points spread over Illinois, Iowa, Missouri, and parts of Kansas and Oklahoma, an area of the order of 150,000 square miles. These members were then mapped over the area of study in terms of the inferred environmental conditions associated with each. The 29 maps so prepared furnish a succession of paleogeographic maps, each representing a relatively short interval of geologic time. The maps show the changing environmental patterns through time, indicate directions of sand inflow, and show the relation of the environmental patterns to tectonic elements that in large part controlled strandline position, source and depositional areas, and thicknesses of the deposits.
Potter (1962) laid his main emphasis on Pennsylvanian sandstones in the Illinois Basin. A thorough analysis of patterns of sandstone distribution, number of sandstones, and other attributes of the pre-Carbondale Pennsylvanian was made. Potter shows that the distribution of sandstone integrates well with other aspects of Pennsylvanian sedimentation. The results of the study were summarized in a basin model in terms of basin geometry, lithic fill, arrangement of lithic members, and tectonic setting of the depositional episodes. This model, though based on a specific basin, serves as a general Pennsylvanian model that can be used as a guide for studies of basins elsewhere.
Potter's model is mentioned after Wanless' maps because in Potter's study, the maps are a means for selecting the essential elements that facilitate description of the deposits in concise form; Wanless' maps show the diversity of environmental conditions that occur within the broader framework of the generalized model.
A somewhat different approach to cyclothem mapping involves selection of a stratigraphic unit that contains a group of cyclothems mapped on a broad regional scale to show systematic changes in the dominance or degree of development of particular kinds of cyclothems as a response to tectono-environmental conditions. This approach was used by the writer in a company report to Gulf Research and Development Company in 1945-46. The concept arose from many discussions of cyclothems with J. Marvin Weller. It was known at that time that cyclothems change as the stratigraphic section is followed away from western Illinois. Thus, equivalent cyclothems in Kansas show a stronger development of marine limestone, with a corresponding decrease in sandstone, underclay, and coal. Eastward toward the Appalachians the marine limestone and black shale decrease in comparison with basal sand and thick sequences of interbedded sandstone, siltstone, and shale. Similarly, toward the Ozark Uplift, near St. Louis, some cyclothems that are well developed farther north in western Illinois degenerate into relatively thin cyclical successions of clay shale and underclay, with occasional marine limestones and thin coals.
Study of stratigraphic sections involving groups of cyclothems mainly in the Cherokee Group and equivalents (Moore and others, 1944) in various parts of the United States led to a conceptual model that defined a "balanced cyclothem" for western Illinois, which became "unbalanced" as certain members assumed dominance under different environmental or tectonic conditions.
In Figure 1 the central diagram represents what would now be called the modal cyclothem of western Illinois, and its attributes would be derived by the formal statistical methods of Duff and Walton. Cyclothems in western Illinois have good representation of both marine and nonmarine phases, and they were accordingly designated as balanced cyclothems. The term does not imply that marine and nonmarine members are of equal thickness or represent equal intervals of time. Typically however, the nonmarine hemicycle is well represented by basal sand, underclay, and coal. The marine hemicycle characteristically has a marine limestone and shale, with minor though conspicuous occurrences of black fissile shale not represented in the diagram.
Figure 1--Balanced cyclothem and its variants. (See text for details, reproduced from company report by permission of Gulf Research and Development Company.)
The arrow to the upper right in Figure 1 shows dominance of the marine hemicycle, with marine limestone becoming increasingly important. The arrow to the lower right represents deterioration of the balanced cyclothem to the condensed sections characteristic of those near St. Louis in the Cheltenham clays. The arrow to the lower left shows dominance of the nonmarine hemicycle, with increasing importance of basal sand, and virtually no marine limestone. A fourth variant of the balanced cyclothem is shown in the upper left of Figure 1. In this cyclothem the marine hemicycle dominates, but the principal component is marine shale rather than marine limestone.
The original analysis was concerned with the relationship of oil and gas to particular cyclothem variants, as well as the relation of the variants to environment and tectonism. Thus, with changes in wording to conform with present usage, it was postulated that the balanced cyclothem is characteristic of (i. e., modal for) shelf conditions of low relief, in which successive transgressions and regressions of the sea produced roughly equal incidence of marine and nonmarine members. The dominance of marine limestone was taken to represent the characteristic cyclothem for more seaward conditions on a shelf, relatively remote from clastic sources. The dominance of marine shale was interpreted as the cyclothem response to increased tectonism, as in a subsiding cratonic basin dominantly marine, but within range of abundant fine-grained detrital materials that can be trapped in the basin. The condensed cyclothems were assumed to be modal for depositional conditions associated with the flanks of mild epeirogenic positive areas, such as the Ozark Uplift, Rock Island High, and northern part of the LaSalle Anticline, as exposed in sections near Ottawa, Illinois. Finally, increasing importance of basal sand implied deposition in a subsiding area close to a clastic source so that presumably the coarse clastic inflow inhibited the marine hemicycle leading to dominance of nonmarine deposits. An experimental pattern map, not included here, showed the area of occurrence of each cyclothem variant. The design of facies maps for this kind of study, expressed in present-day terms, is mentioned below.
In terms of Duff and Walton's quantitative statistical approach, the balanced cyclothem and its variants represent qualitatively defined modal cyclothems, each associated with some inferred combination of tectonism and environment. The map variables would probably now emphasize numerical data that can be contoured. For example, if the stratigraphic unit under study contains 30 cyclothems, a map of the number or proportion of occurrences of marine limestone, superimposed on a base map showing the total thickness of marine limestone in the section, would yield contour patterns showing how the content of marine limestone changes from areas of balanced cyclothems to other modal types. Many maps of this sort, based on the frequency, thickness, and order of occurrence of various members in modal cyclothems or in the composite sequence could be prepared. Current facies mapping techniques, as reviewed by Forgotson (1960), offer numerous possibilities. The contour-type maps can be further analyzed by trend-surface analysis to examine large and small scale (regional vs. local) changes in modal cyclothem attributes in response to inferred tectono-environmental conditions.
Cyclothems may be studied at several scales of observation and interpretation. An individual cyclothem member may be examined inch by inch vertically in one or several adjacent outcrops, or it may be examined in somewhat lesser detail as one component in a cyclothem or group of cyclothems studied over a broader area.
The scale factor in geological studies has been treated by tectonists (Turner and Weiss, 1963, p. 15), who distinguish among several levels of structural observation and generalization, ranging from thin sections to mountain ranges. Cyclothems also provide a natural hierarchy of scales of observation, ranging in vertical sequence from an individual cyclothem member to a multicyclical stratigraphic unit, and ranging in a lateral sense from a single outcrop to the total area of occurrence of a member, a cycle, or a group of cycles.
Table 1 shows the cyclothem hierarchy to which each of the methods of cyclothem study reviewed in previous sections can be related. Stratigraphic correlation at each level, lateral and vertical, is essential, and it is assumed here that stratigraphic position throughout the hierarchy is known. The vertical scale in the hierarchy is relatively straightforward; the thickness of a group of cyclothems is from a few hundred to the order of a thousand feet. A single cyclothem ranges in thickness from the order of 1 foot to 100 feet, and a single cyclothem member is measured in inches or feet. Some members, such as channel sands, may be 100 or more feet thick.
Table 1--Cyclothem hierarchy.
|Levels||Vertical scale||Horizontal scale*|
|III Group of cyclothems within a
correlated stratigraphic unit
|102 to 103 feet||101 to 103 miles|
|II One cyclothem||101 to 102 feet||101 to 102 miles|
|I Single cyclothem member||10-1 to 101 feet
(Channel sands to 102 feet)
|100 to 101 miles
(Persistent members to 102 miles)
|*The lower limit may be considered as a "local scale" of observation, whereas the upper limit
is a "regional scale" that extends along the area of occurrence of the unit of interest. The values in the
horizontal scale may be thought of as diameters of roughly circular areas.
The lateral scale, assumed horizontal, presents some difficulties in quantification, inasmuch as a study may be framed on a very local scale (such as adjacent outcrops along a valley wall), or on a regional scale that extends over the whole area of occurrence of the unit of interest. As expressed in Table 1, the horizontal scale may be thought of as the diameter of a circle that encloses an area of study. At the top level, for example, a local study of a group of cyclothems may cover an area of 102 square miles, whereas a regional study covering the whole area of occurrence of the stratigraphic unit may be of the order of 105 square miles or greater. At the other extreme, on level I, a local study may involve an area of only 1 square mile, and a regional study of a single cyclothem member may range up to the order of 103 or 104 square miles.
One implication of the hierarchy is related to the degrees of variability associated with each level. When interest is focused on a single cyclothem member at level I in neighboring outcrops or boreholes, as in a detailed cross section, the attributes of the member (thickness, texture, faunal content, etc.) as well as its persistence as a correlatable unit may change very rapidly. Less commonly, it may display similar attributes over an appreciable area. On level II, an entire cyclothem consisting of several members will normally retain its cyclic attributes--the order and succession of its members--over a moderately large area, even though individual members in the cyclothem may change. At level III, the multiplicity of cyclothems in a stratigraphic unit lends an identity to the stratigraphic unit over very large areas even though individual cyclothem members (or even whole cyclothems) change, enter the section, or drop out.
A question of some importance in cyclothem studies is the relation between the scale of observation and the scale of inference. Obviously the areal changes in a stratigraphic unit at level III require observations of the whole unit at a number of localities and cannot be seen from even a very detailed study of one section. In similar manner, the selection of a modal cyclothem on level II for a moderate-sized area requires examination of a group of cyclothems at level III. This interplay between scales of observation and scales of generalization can be seen by relating several methods of cyclothem study to the hierarchy of Table 1.
In terms of the preceding paragraph, Duff and Walton's (1962) approach centers on recognition of a modal cycle (level II) by analysis of an adequate number of samples spread through a succession of cycles within a specified area of moderate size (level III on a geographic scale of the order of 5 x 102 square miles). Ferm and Williams' (1963) model would also arise as the inferred succession of beds in an idealized cyclothem (level II), from study of a group of cyclothems (level III) on an intermediate geographic scale in an area relatively widely separated from western Illinois, where Weller's idealized cyclothem arose. As an additional element, however, Ferm and Williams emphasize primarily the physical and chemical controls, thus achieving a relatively compact model.
The cyclothem maps of Wanless and others (1963) were based on a group of three cyclothems (level III) as the main unit for correlation, and maps were prepared for each of 29 cyclothem members (level I) over a geographic area of the order of 105 square miles.
Potter's (1962) approach uses some sandstone maps on level I, and other maps, such as total sandstone thickness, on level III, and condenses the main attributes of the stratigraphic unit into their essential features on a basinwide level III geographic scale. The writer's approach is similar to Duff and Walton (though not statistically formalized), in that interest centers on a modal cyclothem (level II) within a stratigraphic unit (level III), each modal type associated with a geographic scale of the order of 102 to 103 square miles. Variations in the modal cyclothems in response to tectono-environmental controls are then used as map variables. These maps would mostly be on level II, displaying aspects of the characteristic cyclothem in a group of cyclothems.
In addition to similarities and differences in scales of observation and generalization, the several approaches to cyclothem studies involve various kinds of underlying models for analysis of cyclical deposits. The environmental maps of Wanless and co-workers (1963) are based on recording the observed attributes of a cyclothem member at each control point, with the boundaries between inferred environments interpolated among them. This is conventional interpretational or analytical practice in geological mapping. Ferm and Williams' model (1963) is analytical in that it expresses the interplay between mechanically and chemically formed rocks in a specified tectono-environmental setting. Potter's (1962) approach is statistical-analytical in its use of frequency data as well as maps for constructing his summarized basin model.
Duff and Walton (1962) designed their study explicitly in a statistical framework. The elements of their geological population could be defined as all cycles observed in all sections through some stratigraphic interval in some area of interest. The modal cycle is then that group of rocks that occurs most frequently in the succession, expressed in terms of kinds of rocks, order of succession, and number of members. Inasmuch as cyclothems have many measurable attributes, Duff and Walton's statistical model can be extended to studies at all levels of Table 1. At the individual member level the target population could be defined as all possible strips W inches wide and D inches deep through the member normal to its bedding, over the whole area of occurrence of the member. Alternatively, the population elements for a basal sandstone could be all the individual sand grains in that member; similarly, from a paleontologic viewpoint, the population of individuals in a limestone member could be defined as all specimens of species R in genus S.
The writer's approach, re-examined in the light of Duff and Walton's contribution, is a statistical model involving several populations of cycles, each associated with some inferred tectono-environmental state, and each having its modal cyclothem. Maps of the modal cyclothem attributes would then bring out systematic or other changes among these populations; or the several populations could be thought of as comprising a superpopulation with a trend or gradient over its geographic area of occurrence.
The purpose of a geological model, whether statistical or analytical, is to express some real-life geological phenomenon in words or symbols in a concise and systematic manner. The model is commonly less complex than the phenomenon itself, in that effort is made to reduce the phenomenon to its essentials without sacrificing geological meaningfulness. The tests of a model are then its agreement with real-life phenomena and its usefulness as a formal framework for further investigations.
One kind of model that is useful conceptually in sedimentary-tectonic studies is a process-response model that relates properties of the deposits to the geological processes that control them (Krumbein and Sloss, 1963, p. 236; 502). Cyclothems, in common with other ancient sedimentary bodies, are response elements of processes long since completed. The problem is to move logically from observations on rock bodies to inferences about the conditions that produced them. The scale factor enters this analysis in that complexities may arise in deriving generalities on one scale, from observations made on an entirely different scale.
Table 2 is a schematic arrangement of observable response elements that lead to inferred process elements and permit reconstruction of the environmental setting of a single cyclothem member. The model represents hierarchical level I in Table 1, over the whole area of occurrence of the member. It is the model that underlies the environmental maps of Wanless and others (1963).
Table 2--Process-response model at cyclothem member level.
Geometry of Deposit
Geometry of depositional area
Inferred sedimentary environment
Inferred source of detritals
Inferred ecological conditions
Inferred tectonic state
The left-hand block in Table 2 lists the observable attributes of the cyclothem member. These include gross geometry, composition, texture, and associated sedimentary structures. Evaluation of these observable features leads to the inferred process elements listed in the central block of Table 2. These inferences relate to the geometry of the depositional site, the materials present or carried in during deposition, the energy factors, and the biological complex. These process elements may advantageously be expressed primarily as physical and chemical controls on sedimentation, as proposed in Ferm and Williams' model. The right-hand block in Table 2 represents condensation of the process elements to the tectono-environmental setting of the member, as it may be displayed on a map.
A process-response model for an entire cyclothem may appropriately start with identification of the modal cyclothem and composite sequence from observations on a group of cyclothems in the manner specified by Duff and Walton. The succession and kinds of members present lead to an inferred succession of process elements in the central block, or alternatively to an interplay of physical and chemical controls on the modal succession of beds. The inferred process elements for the modal cyclothem, adapted to include additional rock types in the composite sequence, can be used as a basis for setting up an idealized cyclothem applicable to some specified combination of tectono-environmental controls.
At the top hierarchical level of Table 1 the observed attributes of cyclothems in a stratigraphic section may need to be summarized statistically in terms of relative frequency, thickness, and succession of members for some specified but limited area. This yields a set of modal cyclothems, one for each area, and in turn leads to a set of idealized cyclothems, as illustrated by Weller's for western Illinois, and Ferm and Williams' for the Appalachian region. This level of analysis also includes megacyclothems that represent larger sequences, into which individual cyclothems or alternations of members are nested. Alternatively, at the top level, a generalized basin model, such as that proposed by Potter may be developed.
Cyclical deposits have challenged geologists for many years. Recognition of recurrent patterns of particular rock types, complicated by deviations introduced by absence of some beds and introduction of others, has led to a variety of ways for studying and expressing the underlying cyclical phenomenon. Advances in understanding of relations between sediments and their environments, clearer recognition of physical and chemical controls on sedimentation, added ways for expressing geological observations quantitatively, and new mapping techniques have all contributed new ways for attacking old problems. These advances, coupled with statistical and analytical models implemented by the high-speed computer, point to continuing progress in understanding cyclical deposits.
Although several frameworks for study of cyclical deposits are discernible, numerous fairly basic questions remain. Relations between maps (with their accompanying trend surfaces and residuals), and summary models of the kind presented by Ferm and Williams and by Potter, need further study. Choice of statistical and analytical models also offers problems; the analytical model is directed toward understanding process and response in chemical and physical terms, but such models commonly rest on a background of statistical analysis that helps "sort out" meaningful variables and supplies information on the degrees of variability associated with each element in the model.
The influence of the scale of observation on the scale of inference and generalization appears not to have been examined in detail for cyclothems. In structural geology observations on a small scale commonly are useful for generalizations and inferences on a larger scale. Drag folds, for example, observed on a geographic scale of 10-2 mile, can directly support generalization on a scale of at least 101 miles, which is several orders of magnitude greater. It would be interesting to identify some class of local-scale observations on cyclical deposits that can similarly be used for generalizations on a regional scale. Potter and others (1958), for example, demonstrated that cross-bedding directions observed on outcrop in Chester (Mississippian) sandstones can be used as predictors of the orientation of sandstone bodies in the subsurface. On the vertical scale from cyclothem member to stratigraphic unit, it would appear that stratigraphic correlation, or adequate sample size, requires initial blocking out of cycle studies on a hierarchical level higher than the level of generalization. Thus, as stated, Duff and Walton's modal cyclothem involves observation of each bed in a stratigraphic unit at level III of Table 1, from which generalization at level II is obtained.
In addition to questions of technique and method are several fundamental questions relating to the ultimate "cause" of cyclical deposits-questions of diastrophic movement associated with each cycle as against eustatic changes in sea level, conditions that control symmetrical cycles as against asymmetrical ones, the problem of larger rhythms that develop recurring megacycles, and so on. There seems little doubt that cyclical deposits will continue to challenge geologists in the future as they have in the past.
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