|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 623-630|
University of Washington, Seattle, Washington
The writer, among others in recent years, has attempted to demonstrate the importance of not only the general concept that stratigraphy is four dimensional, but also of devising a system in which the dimensional configuration of such entities as nondeposition and erosionally removed former deposits in the framework of area-time is no less essential to the ultimate interpretation of geologic history than is the configuration of the preserved stratal record in both geologic and stratigraphic space (Wheeler, 1958, 1959, 1964). Parenthetically, the fact that the appreciably compromised, and thus partly outdated, "Stratigraphic Code" (American Stratigraphic Commission, 1961) gives no consideration to most of these problems makes them no less paramount. The point is that although such units are all present-day abstractions, the genesis of each has comprised a continuous succession of real or tangible lithosphere surface-moments, without the envisagement (and at least general interpretation) of which, anything approaching a proper reconstruction of geologic history is impossible. Abstract and volumeless though they are in the fragmented stratal record of present geologic space, two of these entities, holostrome and hiatus, constitute the alternate depositional and nondepositional phases of the countless succession of cycles comprising the stratigraphic history of any given place on the earth's surface.
Proper appreciation for the constitution and role of these cycles may be enhanced by the consideration of a few stratigraphic principles, one of which involves the concept of the above-mentioned lithosphere surface-moments.
Once this restriction is accepted, not only does the law of superposition become universally operative, but any stratigraphically defined point at the base, the top, or within any stratal succession, either was located on the lithosphere surface or its being was directly related to that surface at the time of its genesis. The base of a detrital particle, organic fragment or lava flow, for example, rests upon the lithosphere surface (depositional interface) at the moment of its emplacement. Such a particle fragment or layer may or may not be in "depositional continuity" with the underlying surface on which it rests; and regardless of whether such entities are followed in "continuity" by succeeding entities or whether they subsequently are wholly or in part removed, the lithosphere surface is always at hand, either as a surface of aggradation or of degradation. Moreover, in such a world of "pure" stratigraphy the ceaseless alternation of deposition and hiatus prevails throughout, regardless of the scale at which one may choose to interpret or recognize "continuity" and "discontinuity."
In these terms it is true that many fossils and lithic entities do not occur in precisely proper depositional succession, even though no "reworking" or structural derangement has occurred. Boring animals, for example, live beneath the lithosphere surface in rocks of any older age, but their stratigraphic significance is not diminished when they are properly related to the inevitable overlying surface (and depositional interface) to which they directly relate; or a regolith or a residual soil, though nondepositional, relates temporally to the surface beneath which it is formed and thus also to the base of the stratal unit which directly succeeds it, rather than to the older rock body which it occupies and of which it is composed. If this logic is correct, it leads to the simple and universally valid concept that any physical or biostratigraphic datum or any temporally conceived surface in any stratal succession at any specific locality has meaning only in the sense that it either coincides with the lithosphere surface or is directly relatable to that surface at the time of its origin. And it is equally obvious that any "continuous" stratal accumulation is a "complete" physical manifestation of a succession of lithosphere surface-moments.
But what of stratigraphic discontinuities as manifestations of nondeposition and accompanying erosion? Here we pass into the no less important but completely abstract, area-time framework, in which a discontinuity takes on volumetric configuration in the form of the lacuna, which in turn consists of hiatus and erosional vacuity. Here again, however, though now undergoing degradation, it is the evolution of the lithosphere surface which not only determines the configuration of the hiatus and consequent erosional vacuity, but which also culminates in the depositional interface at the base of the next succeeding deposition at any given locality. It is true, of course, that subsurface events may profoundly effect the evolution of the lithosphere surface, but the consequent subsurface-emplaced bodies themselves do not belong to the stratigraphic succession in this properly restricted sense of the term. From this it follows that any ultimate interpretation of the ebb and flow of events in area-time (geologic history) must attempt to envisage the succession of lithosphere surfaces relative to all other geological entities, as the only universal physical geologic "datum" surfaces with stratigraphic implication. This implies the following stratigraphic principle, which may be called the law of surface relations: time as a stratigraphic dimension has meaning only to the extent that any given moment in the earth's history may be conceived as precisely coinciding with a corresponding worldwide lithosphere surface and all simultaneous events either occurring thereon or directly related thereto (Wheeler, 1964).
At any given moment the earth's lithic surface is divisible into innumerable areas, each of which belongs to one or the other of two categories--depositional or degradational. The boundary between any two of these is baselevel, the concept of which, though basically simple, has tended to remain unnecessarily elusive among geologists.
The writer suggests the simple notion of a single, ever-present, worldwide, baselevel "sphere," constantly undulating and "vibrating" in response to the ever-changing patterns of sediment supply-energy relationship--rising above the surface wherever accumulation is initiated, and dropping beneath the surface as degradation commences, without regard for whether the increment is volcanic or sedimentary; marine or nonmarine; chemical, biogenic, or detrital or for the nature of the degradational process. It seems useful to envisage the supply factor as a buoyant force and the energy factor as a depressant force on baselevel. It thus intersects the lithosphere surface at all points of equilibrium, and its momentary "depth" beneath or "height" above the surface at any locality depends, not on the kind of increment, but on the relative "values" of supply and energy. Better understanding of the foregoing may be facilitated by diagrammatic representation (Fig. 1). Both cross sections show the same seawardly inclined lithosphere surface profile (S-S'), at a particular "moment," from an upland area to beneath sea level, along which two segments are undergoing erosion and two undergoing deposition.
Figure 1--Relation of baselevel to erosional and depositional surfaces: A, if baselevel is considered a "horizontal plane" (in part from Dunbar and Rodgers, 1957), and B, if baselevel is considered a single, ever-present, and worldwide "sphere" in response to changing patterns of sediment supply energy relationships. A larger version of this figure is available.
The lower part of the profile in Figure 1A is taken from Dunbar and Rodgers (1957, p. 129), in which baselevel (A-A') is shown as a horizontal plane. In addition, the upper part of the profile shows a subsiding interior basin in which deposition is occurring. If the notion of A-A' is valid, then so is that of another baselevel plane represented by B-B'. Similarly, there should be such a plane in the position of C-C'. In this part of the profile, however, as is often the case along exterior drainage profiles, it will at once be seen that any such horizontal surface as C-C' does not obtain, for the depositional segment would be above such a "baselevel," and the erosional segment would be below it. (This is only one among the demonstrable disparities inherent in such a system of "horizontal baselevel surfaces.")
It is evident that the true baselevel surface in case C must have a downgrade slope, the inclination of which exceeds the surface slope, in the manner of C''-C'''. From this it may be readily seen that this C''-C''' surface will merge with the B-B' surface in the up-grade direction, and with the A-A' surface in the down-grade direction; and if this logic is sound, an undulating surface approximated by B-C''-C'''-A' represents the gross (not detailed) form of baselevel for the moment under consideration. The appropriate segment of this worldwide abstract surface (baselevel) is shown as B-B' in Figure 1B. It is intrinsically represented, however, only at points 1, 2 and 3 on the profile.
Baselevel at any moment may thus emerge from and drop beneath the lithic surface many times on any profile, regardless of whether that profile includes marine, nonmarine, or both environments. Wherever and whenever baselevel in this sense intersects the surface in its transit either up or down, it marks a boundary between the two phases of the cycle under discussion.
Although in somewhat less explicit terms, this kind of cycle involving any two successive (and therefore opposite-direction) transits of baselevel through the lithic surface was defined by Wheeler (1959) as the "stratigraphic cycle." However, it is now evident, as it then should have been, that there are other cycles which are no less stratigraphic, and which may involve baselevel oscillation, but whose patterns may lie entirely within either the supra- or the sub-baselevel phase of the cycle considered herein. In other words, such cycles differ in the important respect that no baselevel-lithosphere surface intersection or transit takes place. Because this distinction is significant when expressed in terms of the ebb and flow of events in area-time (geologic history), the kind of cycle under present discussion may be more appropriately called the baselevel transit cycle.
In concept these cycles may be of any conceivable magnitude. Their depositional phases may vary in order of magnitude from the area and thickness of a clay particle to, say, a "continuously deposited" pan-continental sequence measuring thousands of meters in thickness; and their nondepositional phases may vary from similarly minute areas through a few successive moments of time, to vast interregional hiatuses with durations measured in many millions of years. Those of grosser magnitude, of course, may occur as cyclic entities only in a relative sense in which the lesser ones are deemed nonexistent (ignored). Such appeal to the principle of "existentialism" is admissible, however, only insofar as the cycles of great magnitude may generally differ in scale from the "insignificant" ones by at least several orders of magnitude.
Dunbar and Rodgers (1957) emphasize this ordinate distinction in their discussion of diastems and unconformities.
As clearly seen by James Hutton, an unconformity records a change in the overall conditions . . . involving, at the least, regional uplift and erosion if not tectonic disturbance or metamorphism of the rocks that were formed before the break. Diastems, on the contrary, are smaller breaks that occur without any basic change in the general regimen . . . breaks resulting from fluctuations of stage. . . . The distinction can thus be generalized from the sediments [and discontinuities] of shallow marine waters to sediments [and volcanic accumulations (and associated breaks)] of all environments. Of course, there are many borderline cases where . . . judgments by competent observers will differ, but the two main groups of breaks are different enough in character [magnitude] and origin to warrant separation.Although an appreciable range of magnitude characterizes each of these two categories, they generally differ, not only in order of magnitude, but also in the marked difference in the degree of their deformational and consequent erosional differentials. In other words, the widespread depositional "constants" which commonly characterize the major "conformable" successions are seldom "damaged" to any appreciable extent by the erosional effects of the myriads of included diastems or lesser baselevel transit cycles; while the original depositional patterns which represent the sub-baselevel phases of the major cycles are markedly fragmented by differential erosion during their ensuing hiatal or supra-baselevel phases. Appreciation for this distinction has led to the delineation of the preserved record representing the depositional phases of several of the major North American baselevel transit cycles as sequences (Sloss and others, 1949; Sloss, 1959, 1963; Wheeler, 1956, 1960a, 1960b, 1963; Shannon, 1962; Wheeler and Mallory, 1963). Sloss (1963) currently subdivides the Phanerozoic record of the North American cratonic interior into six major sequences-Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas. Except for the writer's interpretation that the "Kakaskia" comprises two sequences, Piankasha and Tamaroa (Wheeler, 1963), and that there are breaks of comparable magnitude in addtion to the sub-Zuni and sub-Tejas uncoformities in the Mesozoic and Tertiary, his patterns are otherwise in remarkable agreement with those derived by Sloss.
In addition to naming the sequences, which designations apply equally well to their respective holostromes or depositional cyclic phases, Wheeler (1963) extends the common practice of naming the major "breaks," which names in each case apply equally well to the unconformity, the hiatus or nondepositional cyclic phase, and the orogeny, if any. Thus, any contiguous pair such as Sauk-Owl Creek, Tippecanoe- Wallbridge, and Piankasha-Acadian, when viewed in area-time, comprises a complete cycle.
The significant question here, however, is not whose current interpretations and nomenclatural treatment are the more valid, but rather the correctness and implications of this general approach to stratigraphic integration and historical interpretation, as opposed to the traditional one in which the preserved record and deposition tend to be regarded as synonymous, and in which the arbitrary "standard" column is generally regarded as essentially complete.
Because of the deep-rooted misapprehensions inherent in the traditional approach, it has taken several generations to begin to overcome the erroneous notion of a genetic relationship between the "standard" temporally conceived units (especially era and period) and the actual stratigraphic patterns in North America. Moreover, as a consequence of our slowness in developing the bases for interpreting the fragmentary unconformity-bounded sequences, not to mention the markedly episodic baselevel transit cycles which they partially represent, much of the reaction in recent years has taken the form of the equally erroneous notion that the stratigraphic patterns (and hence the tectonic history) have been continuously differential. Although the advocates of this newer notion (of whom the writer was one for some years) have generally recognized ephemeral local order, and sometimes regionally extensive orderly patterns, most of the current interpretations continue to envisage the differentials in the stratal record as the consequence of differential tectonic fluctuation accompanying deposition, commonly to the exclusion of differential deformation and erosional removal during the suprabaselevel cyclic phases. Even cursory studies of such features as the Appalachian, Michigan, Illinois and Williston Basins, and the Cincinnati, Nashville and Ozark Positives, among many others, in the light of the major holostromes (depositional cyclic phases) indicates that these structures, as such, had little, if any, significant influence on the constitution of the strata of which they are mainly composed. Such an integrated approach based on an adequately conceived, multi-dimensional, stratigraphic system is indeed in its infancy, but it is nevertheless abundantly clear that unless we employ and improve these presently demonstrable cyclic patterns which are best displayed at a scale of at least continental framework proportions, we can only continue the common but unrewarding practice of attempting to interpret the past parade of events directly from the thoroughly fragmented record, and thus to continue the relative stagnation that has beset stratigraphy for the past several decades.
Are these problems which relate to the interpretation of the major episodic framework patterns characteristic only of the regions outside the tectonic realm of the European systemic types? If the writer and associates can justifiably judge from their preliminary efforts to interpret a few of the major cyclic patterns on the basis of interregional relationships in Great Britain and elsewhere in western Europe, they are not. Contrary to popular opinion, there appear to be amazingly similar disparities between at least a number of the baselevel transit cycles and the systemic subdivision of the "standard" column in that region also. However, the statistical bases for these interpretations are not yet sufficiently adequate to justify a reporting of cases. Significant progress in these terms has been made in the Soviet Union by Schatsky (1958) and others in recent years. It will be interesting to learn more of the specific experiences and evolution of concepts which have led to these developments in that region.
The foregoing is the writer's response to his invitation to participate in this symposium on cyclic sedimentation. It is obvious that this brief paper does not precisely comply with the specifications, for the cycles discussed here, as a category, cannot be classed as purely sedimentologic in any normal or even acceptable sense. Nevertheless, like all stratigraphically derived cycles, they are ultimately based upon the presently preserved, surface-deposited record. But, unlike the elements of cyclic sedimentation in its proper sense, these cycles may be delineated only in the area-time dimensional framework, and thus exist only as abstractions; moreover, cycles of this kind are universally present in the sense that any stratal succession comprises the preserved record of the depositional phase of one or more of them. They are also universally present in the further sense that although the stratal record may be removed at any place, such cyclic patterns, insofar as they may be interpreted from surrounding relationships, are nonetheless present. In fact, the common failure to interpret the patterns of this "lost" record has resulted in innumerable naive or otherwise anomalous interpretations of geologic history. Regardless of these fundamental distinctions, it is equally obvious that many sedimentary cycles in the more restricted sense also involve the baselevel transit cycle. In the last analysis, however, it is on the basis of the fundamental stratigraphic principles which these and the other kinds of cycles share in common that these words may seek an appropriate place in the symposium. Even so, the writer is fully aware that there are presently many who will regard them at best as a mental exercise in abstract, "theoretical" stratigraphy, and therefore of little "practical" importance. He is also aware, however, of the growing minority who realize that progress in stratigraphy has long been inhibited by the self-imposed restriction of most of our thinking and practice to that which is both tangible and dimensionally simple.
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