|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 69-85|
University of Wisconsin, Madison, Wisconsin
Rhythmic deposition has occurred in mobile belts as well as on cratons, though it is far better known for the latter. Universal (essentially worldwide) climatic or eustatic oscillations obviously would affect both kinds of regions, but in mobile belts evidence is particularly obscured by local diastrophic and sedimentary effects. Many of these local effects were also more or less rhythmic, but had differing periods or frequencies.
The Pennsylvanian of northern Nevada illustrates clearly the range of possible superimposed effects of three different sediment sources and at least two different, more or less repetitive phenomena. Sediment included indigenous carbonate, cratonic-derived quartz arenite (from the east) and tectonic-land-derived chert conglomerate (from the west). Deposition was everywhere influenced by a presumably universal (perhaps glacio-eustatic) short-period pulsation. In central and western Nevada it was also influenced by three longer-period, less regular diastrophic pulses that produced two angular unconformities and several conglomerate wedges.
One of the greatest universal long-period effects was the Cretaceous transgressive-Cenozoic regressive cycle which is evident in mobile belts as well as cratons. This cycle conforms very closely in time with shallower ocean basins in medial and later Cretaceous time followed by widespread oceanic subsidence during Cenozoic. It is suggested that such oceanic epeirogenic warping caused many of the interregional and intercontinental long-period transgressive-regressive cycles revealed in rock-stratigraphic sequences. In practically all cases, however, "local" uplift or subsidence (particularly in mobile belts) produced shorter-period effects tending to either accentuate or nullify the more subtle universal influences. For example, the Permo-Triassic emergence of most continents apparently resulted from both oceanie downwarping and widespread continental upwarp.
It is thought that universal short-period climatic effects in the Cretaceous and early Tertiary also affected sedimentation in addition to eustatic and diastrophic influences.
Some rather universal episodic patterns such as eustatic changes and certain great orogenic acmes certainly exist in the stratigraphic record. This statement should not imply necessity of return to Chamberlain's diastrophic control concepts, however, but need for closer recognition both in mobile belts and cratons of the relative contributions of largescale universal phenomena and the generally more obvious, but commonly less profound, local ones.
Rhythmic patterns of sedimentation occur in geosynclinal successions in mobile belts as in strata of more stable cratonic regions, though in the former they are commonly less obvious and are therefore less studied. [Note: Mobile belt is preferred as the most fundamental term to describe overall, long-continued structural habit of linear and arcuate belts in contrast with long-stable cratons. Geosyncline is used bere only in the sense of tbe great orthogeosynclinal belts of Stille and Kay (Kay, 1951). As conceived by tbe present author, such a geosyncline represents a structurally mobile belt which had sufficient land sources within or nearby to supply abnormally thick sedimentation during a complex history of nearly continuous diastrophism. Thus a geosyncline is a filled mobile belt! Its fill may comprise any and all types of sediments. Mobile belt is a first-order tectonic term; geosyncline a primarily stratigraphic concept in adherence to its original conception by James Hall. An orthogeosyncline represents, therefore, a special sedimentologic-stratigraphic development within the broader structural framework of tbe former; it is dependent upon the former, but not vice versa. It is recognized that this viewpoint tends to exclude features such as the Gulf of Mexico continental shelf "geosyncline" as separate situations possibly not related to true mobile belts because of their relative lack of mobility (as suggested by Kay. 1951). It is sheer conjecture as to whether or not such features are destined to become mobile in the future, but known mobile belt histories suggest that great structural mobility was a prerequisite ond continuous accompaniment for all true orthogeosyncliues, not just a terminal "orogenic stage."] But, designation of deposits as cyclic is itself fraught with ambiguity and subjectivity. As Duff and Walton (1962) point out, some successions considered cyclic in fact possess such great lateral variability in detail that no universal control seems indicated, thus such "cyclicity" is more or less coincidental and of only local importance. Definition of what is and is not to be considered cyclic is by no means clear. While recognizing this most basic problem, the present paper is concerned with major, apparently widespread patterns from a conceptual standpoint in order to highlight importance of superimposed sedimentary patterns in the strata of mobile belts.
Brief perusal of most current texts of stratigraphy and sedimentation immediately reveals the multiplicity of so-called cyclic patterns from the smallest repetitions of microscopic laminae such as varves to the largest scale, generally supra-systemic, transgressive-regressive oscillations which produced the interregional, cratonic rock-stratigraphic sequences of Sloss (1963), stratigraphic stages of Gignoux (1955) , or oscillations of Beloussov (1962). Moreover, the list of probable causes of these varied cyclic patterns is at least as complex and varied. It is axiomatic, nonetheless, that some of these same kinds of patterns with some of the same underlying causes, particularly if more or less universal, must have affected mobile belts as well as cratons.
Many factors may contribute solely or in concert to cyclicity. Chief among these are pulsating diastrophism, fluctuating fluid dynamics at depositional sites, climatic oscillations and eustatic fluctuations of sea level. In any region, but especially in mobile belts, effects of any of these primary factors may be modified or even masked by other situations. For example, average interval below or above base level and hydrographic conditions will strongly influence the resultant cyclicity. In mobile belts this interval has varied enormously both in time and space. And, effects of local orogenesis may completely mask a more or less universal pattern which otherwise would, and in nonorogenic areas does, develop. Thus, the stratigraphic record is almost entirely one of superimposed effects which may be wholly unrelated. An obvious example is the long-period, late Cenozoic regression with, during the Pleistocene, superimposed short-period eustatic oscillations. Another is local regression on the Mississippi delta due to sedimentation during a general Holocene eustatic rise (transgression) followed by local "transgression" due to abandonment of previous distributaries during a general still stand of sea level (Lankford and Shepard, 1960). Full understanding of the record can come only if such separate effects can be adequately "filtered." Stratigraphic synthesis in mobile belts, particularly identification of major and minor cycles, has been retarded considerably by local "background noise" which is commonly very difficult to "filter" from some of the more universal patterns. [Note: Mobile belt effects, chiefly diastrophic, are termed local herein. Effects originating outside mobile belts, but potentially affecting them, are termed universal or regional. These local and regional effects have analogy with local anomalies and regional gradients in geophysics.] As Sloss (1963, p. 95) has remarked about cratonic, large-scale sequences, they are "commonly complicated by minor reversals in trend and by a host of local effects." He also points out that a maze of locally important unconformities commonly prove insignificant regionally. These observations for cratons seem all the more true for mobile belts.
Sloss (1962) has suggested utility of constructing conceptual stratigraphic models to aid in evaluating varied interacting effects and the results thereof in the stratigraphic record. This approach is valuable as a discipline in organizing stratigraphic-sedimentological analysis of any region, therefore some suggested partial "models" of geosynclinal sedimentation are reviewed here. Sloss' treatment of chief interacting factors leading to the geometry of sedimentary deposits is not entirely adequate for the present consideration of mobile belts. His materials factor (M) (i.e. texture and composition of material supplied), which logically was considered constant for his purposes, certainly cannot be so considered for these regions. Furthermore, quantity of sediment supplied (Q) does not always exceed rate of subsidence. Dispersal (D) also has varied greatly. Range of variability of these and other factors is, in fact, so great that a number of special models will have to be devised in order to cover the range of important conditions in mobile belts. This, however, is beyond the scope of the present paper, therefore only the two presently most germane petrologic factors will be considered. Because composition and texture are still the dual basis for classification and ultimate interpretation of sediments (and all rocks, for that matter), these both must be included in any analytical scheme. It has become increasingly apparent that textural and compositional maturity are nearly independent (Klein, 1963), though size, at least, imposes an important control upon mineral composition as shown by Kelling, 1962; Allen, 1962; and Koch, 1963. These two fundamental parameters always must be separately and equally treated. The chief source of sediments (reflected by composition) together with magnitude and especially constancy of energy conditions of the transportive and depositional media (reflected chiefly in textural maturity) are considered the chief parameters for the present discussion. Wheeler and Murray (1957) have re-emphasized the importance of changes of base level which are encompassed here largely in "environmental factors." Some, but not all, tectonic factors are included in both of these parameters in complex, indirect ways. Climate is a chief variable, sorely neglected by geologists in recent years, which can be included only in its indirect influence upon composition, for its effects are pervasive over practically all environments. Its influence must be separately evaluated for each specific situation.
In Figure 1 the major terrigenous source types for sedimentation in mobile belts are shown diagrammatically. Of course there are many overlapping complications possible. For example tectonic lands (Kay, 1951) very commonly contain important volcanic as well as sedimentary, metamorphic and plutonic rocks. But volcanic lands tend to be more pure in their contributions. Any one, two, or all three source types may have provided sediment at a given place and time. In Table 1 the major different terrigenous clastic sediment types expected from each of these three sources (as well as a situation with no important lands) have been organized under two extreme energy conditions. These types represent the most outstanding interrelations between source and environment as reflected in rocks. Such as it is, this is a generalization only and susceptible to all of the inevitable shortcomings, but also, hopefully, possessing some of the valuable attributes thereof. The possible range of sedimentary environments of deposition is so great, especially in mobile belts, that it is pointless to detail sediment characteristics for each of all possibilities. For example, consider the myriad environments represented in the modern Indonesian-New Guinea oceanic mobile belt (volcanic mountains, alluvial plains, deltas, trenches, organic reefs, etc.). The imperfections of the stratigraphic record and our present knowledge seem only to justify a conceptual framework. One can himself relate many common environments to the scheme in Table 1, and a few examples are discussed next which serve to illustrate its implications. They shall be described first, then possible causal relations will be examined.
Figure 1--Diagrammatic relationships of major types of terrigenous intra- and extra-mobile belt sediment sources.
Table 1--Relations of source and environmental factors to clastic sediments in mobile belts.
(chiefly function of quantity of detritus and dispersal)
|Increasing textural maturity --->|
|Low or intermittent energy||High or constant energy|
|Volcanic island source||Immature lithic (volcanic) wackes||Texturally mature lithic (volcanic) arenites|
|Tectonic land source||Variable lithic and feldspathic wackes||Texturally mature lithic and feldspathic arenites|
|Cratonic source||Compositionally mature quartz wackes
(many redeposited pure quartz sands)
|Supermature siliceous arenites|
|No terrigenous source||Carbonates (reefs if climate favorable)||Mud or ooze, possible bedded chert,
perhaps some carbonate
As in the famous cratonic Pennsylvanian, equivalent strata of the eastern Cordilleran geosyncline are notably rhythmic in gross aspect (Fig. 2; Bissell, this volume; Dott, 1958). Their outcrop characteristics do, in fact, prove to be repetitive. True cyclothems of the eastern interior type, however, are not present, for in the Cordillera, cycles developed almost entirely in marine carbonate-quartz arenite deposits. Because of the seemingly universal cyclic character of the Carboniferous through early Permian strata on most continents, it has been tempting to postulate some worldwide control such as eustatic oscillations of sea level, though there is by no means unanimity of opinion on their cause. Some even question that they are worldwide cycles.
Figure 2--A, Typical rhythmic appearance of Pennsylvanian-early Permian strata of Great Basin; Oquirrh Formation, northwest side of Oquirrh Range, north-central Utah. B, Characteristic rhythmic stratification in graded Cretaceous sandstone-mudstone succession, Coast Highway near San Pedro Point, 12 miles south of San Francisco.
For purposes of illustration, let us assume an unspecified worldwide cause and investigate some of the rhythmic interactions thus revealed in this region. Rate of subsidence is assumed to be essentially constant for northeast Nevada. Cyclic marine deposition proceeded with a cratonic source periodically furnishing quartz-rich sand and silt to the eastern geosyncline in alternation with reduced terrigenous input and relatively increased indigenous calcarenite production. Simultaneously a local (intra-mobile belt) tectonic land source also developed in central Nevada and Idaho. It suffered diastrophic pulses of deformation, elevation, and erosion genetically unrelated to the contemporaneous rhythmic sedimentation to the east. Diastrophism is reflected in northern Nevada by two major incursions of westwardly thickening and coarsening chert and quartzite conglomerate wedges within the Pennsylvanian, and by at least two major angular unconformities (Fig. 3, 4). [Note: Bissell (1962) questioned the important, readily accessible upper example in Carlin Canyon; however, there seems no question that this is an angular unconformable rather than a fault contact. To be sure, there has been subsequent local disturbance, but regionally there is adequate evidence of truncation and reworked basal detritus of subjacent materials. The entire argument by no means rests upon the accessible evidence in Carlin Canyon as many have supposed. Systematic westward truncation of progressively older biostratigraphic zones discussed by Dott (1955, p. 2248, 2253-2256, Fig. 3, 5) and presented here in Figure 3 was a major link in the original argument. An error exists in original mapping at the east end of Carlin Canyon. The "Tomera Formation" shown at the southeast end of the canyon and also northeast of the canyon along US Highway 40 opposite Tomera Ranch is in reality an unusual conglomeratic phase of the Moleen Formation. The Tomera lies farther to the northwest. Abundant conglomerate and structural complicatious in this area make the two difficult to distinguish.] Several less extensive, coarse conglomerate wedges also occur in the early Permian, notably 8 miles south of Carlin and near Eureka in central Nevada.
Figure 3--Paleogeologic map of Carlin Canyon-Grindstone Mountain area, north-central Nevada showing late Pennsylvanian westward overlap of progressively older biostratigraphic zones (see Fig. 4 for location).
Figure 4--Dual restored sections of (a) early Permian (Wolfcampian) and (b) medial Pennsylvanian (late Desmoinesian) stratigraphic relationships across northern Nevada (modified from Dott, 1955; Roberts, and others, 1958).
Figure 4 shows dual restored diagrammatic cross sections illustrating the stratigraphic relationships and superimposed effects of the major independent repetitive influences that led to the present stratigraphy. Note that the relationships in northwest Nevada are modified somewhat from those shown in Dott, (1955, Fig. 11) based upon further field work in 1960 and discussions with Ralph J. Roberts of the U. S. Geological Survey. As presently interpreted, there was general westward transgressive overlap of the eroding Antler orogenic belt rather than the more complex hypothetical overlap of a local, central Nevada positive element with much more tenuous facies relations surrounding it as shown in the previously published cross section. Figure 5 is a time-stratigraphic diagram of the same section as shown in Figure 4 and a graph of the superimposed rhythmic patterns. The regular, relatively short-period rhythms appear clearly in easterly areas, whereas in central and northwest Nevada, local, less regular and longer-period diastrophic pulses intermittently masked and even obliterated the "regional" (universal) effects. Even the westerly derived conglomerates, however, show some influences of regional cyclicity in their stratification characteristics. A graph of all cyclic influences operating here presents a spectrum of superimposed oscillations of varying frequency and magnitude.
Figure 5--Time-stratigraphic diagram and qualitative curve of superimposed long- and short-period rhythmic patterns for Pennsylvanian of northern Nevada (same line of section as Fig. 4).
The Cretaceous succession of the Rocky Mountains and adjacent plateaus region illustrates different patterns. From New Mexico into western Canada it has long been regarded as a region of oscillating transgressive-regressive events that produced coal-bearing cyclothems (Young, 1955; Weimer, 1961; Scruton, 1961). Ideal vertical successions include sandstone-coal-shale repeated several times. Laterally it has been shown that the sandstones represent great complex tongues penetrating eastward into a dominantly shaly lithofacies (or, strictly, lithosome; Fig. 6). Moreover, at the western limit of outcrop, chiefly in central Utah, coarse, thick conglomerate occurs. The source model for this region is a large and complex tectonic land or "borderland" in the central Cordilleran mobile belt that shed coarse terrigenous debris eastward into a parallel subsiding belt on the west edge of the pre-Cretaceous craton. This model is unlike the combined dominant cratonic and minor local tectonic source model for the Great Basin Pennsylvanian. The marginal cratonic subsidence, so-called "Rocky Mountain geosyncline," was ephemeral as compared to the previous geosynclinal development and is not to be regarded as of the same order of magnitude and tectonic importance. It was merely a transitory phase of the final destruction of the Cordilleran geosyncline during which the locus of maximum subsidence briefly shifted eastward. Vulcanism played a minor role in the north in western Montana and Alberta. Also, very minor detritus came from an eastern cratonic source, but practically none of this reached the heart of the present Rocky Mountain region. A series of clastic wedges were built eastward from the great meso-Cordilleran tectonic land with the more coarse material in the west and sandy and swampy deposits in the east forming alluvial plains chiefly below base level. Analogous thick, nonred, coal-bearing, alluvial plain deposits characterized the Pennsylvanian of the Appalachian region immediately prior to the final Permo-Triassic orogenic elevation of that mobile belt. Typical redbed clastic wedges such as the Catskill have had similar relations to diastrophism and tectonic lands, but deposition occurred chiefly at or even above base level under oxidizing conditions. Rate of supply and dispersal as well as climate and vegetation were also important factors in determining which type of clastic wedge resulted at a given time or place.
Figure 6--Diagrammatic cross section of Cretaceous facies relationships in central Utah and Colorado showing many sandstone tongues and oscillatory shoreline position (modified after Spieker, 1949; King, 1955)
The Cretaceous System throughout the world records one of the major, essentially universal transgressions. Local specialists disagree on the timing and even the number of maximum transgression(s} in their domains of authority. Nonetheless, only a glance at geologic maps and stratigraphic columns of all continents, except possibly Antarctica, is needed to see that Cretaceous strata, largely marine, unconformably overlap vast portions of the great cratons and occupy large parts of mobile belts. Detailed facies analysis reveals much local variation, but this is to be expected, as Sloss (1963) has shown. Still, a general pattern of maximum world transgression is found in medial Cretaceous time, chiefly concentrated in the Albian, Cenomanian, and Turonian stages. For example, in central North America, southward transgression from the present Arctic via western Canada merged with northward transgression from the present Gulf of Mexico area, producing a continuous epeiric sea by Cenomanian-Turonian time (Cobban and Reeside, 1952). In Albian-Cenomanian time, a maximum transgression also penetrated eastward deeply into the Oregon-Washington portion of the Cordilleran mobile belt, but the maximum was later in northern California, where several local oscillations can be recognized (Jones, 1960), and on the southwest Oregon coast (Dott and Howard, 1962; Fig. 7).
Assuming again some undesignated "regional," apparently worldwide, cause for rising sea level affecting the Cordillera as well as all other regions, we can discern at least two oscillatory effects of different period in the Rocky Mountain Cretaceous. Superimposed upon the relatively long-period transgressive cycle, were shorter-period fluctuations reflected in the sandstone tongues and associated coals. These fluctuations can be attributed to a variety of possible causes, including diastrophic and erosional pulsations in the tectonic source land to the west, diastrophic pulsation of subsidence in the depositional trough (favored by Young, 1955), climatic (and erosional) pulsations over the source, or eustatic pulsations.
Among the most striking rhythmic deposits are the now-familiar, graded sandstone-mud. stone strata described from many parts of the world. Many excellent examples of the strata are to be found in California and Oregon. Figure 2B is a photograph of a Cretaceous exposure along the coast south of San Francisco. Similar, more or less equivalent successions occur widely on the west coast, but, contrary to popular opinion, by no means all of the sandstones in such suites are graded "turbidites." In fact, some such assemblages are virtually all nongraded, for example Upper Cretaceous strata of the Cape Sebastian area on the southwest Oregon Coast (Dott and Howard, 1962). Similarly, the Paleocene-Eocene of the Pacific states contains rhythmic sandstone-mudstone successions, but some of them are nongraded, too. Lithic wackes are especially common in these successions (see, Table 1, "low energy" column).
The truly graded, repetitive successions seem generally referable to intermittent turbidity current action as a major mechanism of final deposition under conditions of rate of subsidence in excess of quantity of sediment supplied as postulated by Sloss (1962) for a turbidite model. This model probably represents an over-simplification to some extent, for it tends to obscure the many important complexities of hydrographic energy factors which the writer considers of dominant importance in the preservability of the record of turbidity current deposition regardless of depth of water (Dott, 1963). One may ask, however, why many graded successions are so rhythmic? Or are they? Doubtless this impression is partly illusory, but there remains a striking apparent rhythm to the Cretaceous and early Tertiary graded successions of many mobile belts. What could cause the currents to recur with such a seemingly uniform period and in many cases with such a near-constancy of volume of material each time (Fig. 2B)? Pleistocene and Holocene deep oceanic graded deposits are by no means so regular, and, though better established as truly turbidity current products, they are of a scale and general character different from the older, more rhythmic ones. Furthermore, equally rhythmic appearance of some clearly nonturbidite suites of equivalent age suggests some rather universal ("regional") controlling factor independent of the exact local depositional mechanisms and environments. Thus, not only in both types of deposits within the Pacific Coast region, but also in nonturbidites on the east side of the Cordillera in the Rocky Mountain Cretaceous, rhythmic lithologies occur. The explanation can not be simple, however, for the scale of alternating sand and shale tongues in the Rocky Mountains is much larger than the sand-shale units of the Pacific Coast exemplified in Figure 2B. However, some possible smaller-scale alternations also occur in portions of the easterly Mancos-Pierre shaly lithologies. Perhaps three or even more superimposed repetitive effects are represented here as well as spurious. purely local phenomena. As Spieker (1949) implied, we probably never can fully understand all of these.
Local masking of apparently rhythmic deposition occurs in the Cretaceous as well as Pennsylvanian of the Cordillera. Exceptionally coarse, thick gravels derived from active local tectonic lands completely blank out any record of possible rhythmic repetitions in the earliest Cretaceous of southwest Oregon, though repetitions of graded sandstone and mudstone ensued higher in the section after the local lands became more subdued (Koch. 1963). Volcanic piles also punctuate many mobile belt successions that otherwise might be expected to show rhythmic strata.
As was pointed out in the introduction, one must be specific in discussing particular cyclic patterns, for there are all scales of repetitions. Alternations are inevitable, for conditions sooner or later must change. Probably too much attention has been given to some such alternations which may have no more than very local significance (perhaps even in the present paper). Patterns which are unusually reproducible vertically and apparently synchronous over large regions, even between continents, capture our attention most strongly, and rightly so. Doubtless the economic aspect of the coal-bearing cyclothems has stimulated much of the special interest that these have enjoyed.
In seeking causes for rhythms of sedimentation within mobile belts, it is convenient to group them as in Table 2.
Table 2--Major causes of rhythmic patterns in mobile belts.
|Local causes within|
|1. Diastrophic pulsations||Possibly includes rhythmic earthquake triggering of mass movements and turbidity currents|
|2. Sedimentary oscillations||Shifting delta distributaries
Pulsating hydrographic factors (currents. etc.)
|Pulsating regression due to excess sedimentation followed by transgression|
|Universal ("regional") causes:|
|1. Climatic pulsations||Relatively short period|
|a. Glacio-eustatic||Clearly establishable only for Pleistocene and late Paleozoic|
|b. Nonglacial aridity-humidity or temperature cycles||Affecting rates of erosion and possibly hydrographic processes (may affect no. 2 above)|
|2. Nonglacial eustatic changes||Long period|
|a. Epeirogenic warping of ocean basins or cratons||Effects spanning more than one geologic period|
|3. Episodic orogenesis||Long period: only important if synchronous and very widespread|
|Chief effect upon sedimentation; indirect effects upon climate, hydrography and sea level|
Figure 7--Approximate shorelines for several different stages of Cretaceous of Oregon and northern California showing complex local variations (modified slightly from Jones, 1960).
There is considerable feelmg that diastrophic pulses (either orogenic or epeirogenic) have not generally been regularly rhythmic enough to explain such patterns as cyclothems, though Weller (1956) does favor pulsating epeirogenesis. While there is evidence of long-period, synchronous, "universal" acmes of orogenesis, termed granite episodes by Engel (1963), the detailed scale of resolution of most local stratigraphic analyses shows no very regular discernible diastrophic rhythm. Rather, in detail, diastrophism in mobile belts has been rather continuous temporally and complex in its geographic variability. Beloussov (1962), however, has returned to the notion that cratons and mobile belts have undergone more uniform oscillations.
Local shifts of sedimentary mechanisms can be important in producing alternations in deposition. Change of distributary outlets of rivers or shifting of bars may explain some so-called cyclic patterns, but can not cover those which are truly interregional or intercontinental. Likewise, it is difficult to conceive of rhythmic oscillations of oceanic currents and the like in a fashion capable of explaining very widespread rhythms. Therefore, for any truly continentwide or worldwide effects, one is almost forced to seek eustatic or climatic causes. For short-period eustatic effects, climatically controlled glaciation seems the most satisfactory explanation (see, Wheeler and Murray, 1957). Other climatic fluctuations such as aridity-humidity oscillations would also be felt in mobile belts, as elsewhere, and conceivably might produce important rhythms of deposition by influencing precipitation and runoff, vegetative cover, and temperature, or possibly by altering oceanic circulation in some oscillatory way. The relatively short-period rhythms so common in many Cretaceous-to-Eocene successions, for example, seem very probably referable to such a climatic control. This is especially likely because rhythms appear to characterize deposits representing a wide spectrum of depositional environments, processes and geographic areas. Nonetheless, local diastrophic pulses are also found to complicate the issue.
[Note: After this paper was written, an important one by A. Hallam, "Major epeirogenic and enstatic changes since the Cretaceous, and their possible relationship to crustal structure," Am. Jour. Sci., v. 261, p. 397-423, came to my attention. Hallam reaches many of the same broad conclusions about oceanic warping and eustatic changes, particularly for the Cretaceous and Tertiary.]
As shown for the seemingly universal Cretaceous-Cenozoic transgressive-regressive event, nonglacial eustatic effects can readily explain very long-period "megacycles," including many of the rock-stratigraphic sequences. Such effects must inevitably be felt in mobile belts as well as cratons just as would shorter-period general climatic oscillations.
An essentially universal transgression began in the Jurassic and reached its peak near the middle of Cretaceous time. Because of its apparent universality, one intuitively suspects a eustatic rise as the most plausible cause. And, we do not have to seek far to find strong confirmatory evidence. Charles Darwin (1837) early suggested a general large-scale subsidence of the Pacific sea floor to explain formation of coral atolls. Hess (1946) presented further evidence in the form of widespread submerged sea mounts and guyots. Drilling on western Pacific islands shows thick reef complexes ranging in age to middle and early Tertiary (Table 3; Fig. 8). The deepest hole (Bikini) reached Oligocene or Upper Eocene sediments at a depth of 2,556 feet, and seismic data suggests another 1,000-2,000 feet to the volcanic reef foundation (Emery and others, 1954). The many guyots of this region fall in median depth groups of 690 and 985 fathoms (Emery and others, 1954). Significantly, this support of the Darwinian hypothesis is correlated in time with the general regression of the sea from continents during the Cenozoic. As always, other effects were superimposed on this universal phenomenon to modify local continental patterns. Still, generally speaking, worldwide regression has characterized at least the latter two thirds to three fourths of Cenozoic time, whereas transgression had typified the Cretaceous.
Table 3--Known depths of Cretaceous to Holocene mid-oceanic reef carbonate columns (see Figure 8 for Pacific localities).
|Bikini||2,555 ft.||Oligocene or late Eocene||Emery and others, 1954|
|Eniwetok||1,285 ft.||Miocene||Emery and others, 1954|
|Funafuti||1,114 ft.||Pliocene||Emery and others, 1954|
|Hess guyot||5,740 ft.||Medial Cretaceous||Hamilton, 1956|
|Cape Johnson guyot||5,832 ft.||Medial Cretaceous||Hamilton, 1956|
|Kita-daito-jima||1,416 ft.||Early Miocene||Emery and others, 1954|
|Bermuda||380 ft.||Early Oligocene||Newell, 1955|
|Bahamas||14,500 ft.||Cretaceous||Newell, 1955|
Additional evidence was reported in 1956 by Hamilton. Medial Cretaceous (Aptian to Cenomanian or possibly Turonian) fossil reef organisms were dredged from the submerged, flat-topped "Mid-Pacific Mountains" west of Hawaii (Fig. 8) which lie today at depths of 940-972 fathoms (nearly 6,000 feet) below sea level. Here, during medial Cretaceous, reefs must have flourished in the surf zone along the crest of a volcanic ridge rising 10,000 feet above the adj acent abyssal plains. The period of reef growth on a large, shallowwater volcanic island mass correlates remarkably well in time with mid-Cretaceous continental flooding, as does the subsidence of this large area with subsequent Cenozoic continental draining.
Figure 8--Map of western Pacific Ocean showing where Cretaceous-Cenozoic subsidence has been measured (area in circle with 2,000 mile radius employed for calculations of crustal subsidence) and postulated fossil Darwin Rise of H. W. Menard (see text).
To test validity of this circumstantial Pacific evidence, the writer in 1961 computed the magnitude of worldwide sea-level change for an epeirogenic downwarping of the central Pacific on the order of a vertical mile. With the necessarily artificial assumption of uniform subsidence of a vertical cylinder of water 2,000 statute miles in radius enclosing the known subsided Tertiary and Cretaceous atolls (Fig. 8), and one statute mile high, a worldwide reduction of sea level of approximately 400 feet would occur. Obviously, this is a crude over-simplification. Subsidence certainly has not been so uniform; more likely the seamounts and guyot areas subsided isostatically much more than surrounding abyssal plain areas. Nonetheless, average crustal change of say only 1,500 feet would still produce more than a 200-feet eustatic effect. Significantly, today a rise of about 230 feet would submerge approximately one third of the total continental area, approximately the estimated order of magnitude of the Cretaceous transgression.
A more complete treatment has been given by H. W. Menard. In oral presentation at the 1962 American Association of Petroleum Geologists' meetings (San Francisco), he presented evidence of what he termed a fossil Darwin Rise extending northwest-southeast across the South Pacific and including most of the known subsiding atolls and guyots. His more precise analysis of water displacement indicated a eustatic lowering of 100 meters in the past 100 million years due to subsidence of this postulated oceanic rise, again a geologically very significant effect. [Note: Discussion of this and other factors affecting sea level are treated hy H. W. Mcnard in a forthcoming hook, Pacific Marine Gealagy, to be published by McGraw Hill (Menard, written communication, March, 1963).] Of course, elevation of the present rises should tend to counteract this regressive effect, but these rises are considered young. Furthermore, we have treated only the west-central Pacific for which more data exist. If other oceanic areas were simultaneously subsiding, the regressive tendency would be much greater. Meager evidence from the Philippine Sea and the western North Atlantic indicates that this was at least in part the case (Table 3).
For the Cretaceous-Cenozoic example, there is considerable evidence, but for older events evidence is too scant to allow convincing proof. Nevertheless, broadly synchronous, extensive emergence of continents from about the end of Paleozoic into early Triassic time is also suggestive of a large-scale oceanic downwarping and eustatic fall. This event probably was accompanied by continental diastrophic elevation as well. That rather large ocean basins were already in existence is suggested by the magnitude of this regression and previous regressions of the epeiric seas, unless a considerable volume of water has been added since the pre-Paleozoic, which seems very unlikely (Kuenen, 1950).
Thus, in spite of the crudeness of these analyses, results coincide with apparent magnitude of changes indicated for some of the major transgressive-regressive events in the geologic record, and tend to support the rather old idea that major transgressions reflect changes in ocean basin volume (discussed long ago by Bucher, Suess, Joly, and others; see Kuenen, 1950). Now that much new evidence is available from the deep oceans, it would seem that Kuenen's suggestion (1950, p. 549) that some intermittent subcrustal influence, such as convection currents in the mantle, can reasonably explain major eustatic changes of sea level. Emerging evidence of the behavior and characteristics of oceanic rises and ridges also proves that the ocean floors are not so permanent and immobile as geologists had long assumed. Epeirogeny and orogeny are as at home beneath the sea as on continents, in fact their effects should be far less clouded by erosion and deposition there.
At least some causal processes for rhythmic sedimentation, particularly the rather universal climatic and eustatic oscillations, affected both mobile belts and cratonic regions. Some very rhythmic geosynclinal successions (such as the Silurian of the Caledonides and Ordovician of the eastern Appalachians) do not appear to have any readily apparent worldwide explanation. Therefore, local effects alone apparently explain such cases. Carboniferous and early Permian rhythmic geosynclinal successions, however, would seem to have at least some indirect relation to the better-known rhythms of equivalent-aged cratonic strata of virtually all continents. The Ouachita Carboniferous terrigenous detrital deposits are rhythmic (Cline, 1960), and the Great Basin carbonate-quartz arenite suites are as well. General synchroneity of rhythmic patterns of many cratons and also several mobile belts of very different overall tectonic habit argues for a universal cause such as relatively short-period climatic and eustatic oscillations affecting nearly all depositional sites of the world. Admittedly, the evidence is circumstantial and, therefore, may be entirely coincidental, but the probability seems otherwise.
In Nevada and Idaho, the assumed externally-caused shorter period oscillatory pattern was modified by superimposed, local, longer-period pulses of diastrophism resulting in introduction of coarse terrigenous clastic debris from the west to be mixed with indigenous carbonate and cratonic-derived detritus. Here, a moderately complex stratigraphy whose deposition was influenced by essentially continuous subsidence and at least two oscillatory patterns of quite different origin contains detrital refuse of three different origins. Separation of their effects is fairly clear, but would be more difficult in more intensively and continuously deformed portions of mobile belts such as the Lower Cretaceous of southwest Oregon where local tectonic lands produced very thick, coarse, massive conglomerates.
For the combined late Jurassic, Cretaceous, and earliest Tertiary, there is clear evidence of a general worldwide transgressive-regressive cycle both of cratons and mobile belts, apparently related in part to a then shallower central Pacific Ocean floor. The maximum transgression, though variable locally, was generally reached in mid-Cretaceous time, or about 100 million years ago. Though important lesser transgressive-regressive oscillations occurred in Paleocene-Eocene time, there has been a gross worldwide regressive tendency as oceanic areas have subsided deeply. Long-period transgressive-regressive cycles related to large-scale oceanic epeirogenic warping probably account for at least several of the other unconformity-bounded, interregional stratigraphic sequences of Sloss (1963). Some have also been affected by intracontinental diastrophism as well, particularly the universal emergence of continents in Permo-Triassic time. [Note: Newell (1963) has presented impressive evidence of repeated extinction "crises" among marine invertebrates which he relates to repeated sharp changes of paleogeography, chiefly eustatic (or epeirogenic) in character. He also notes the probable importance of ocean basin changes in controlling continental transgressions and regressions. It is puzzling, however, that the organic crises coincide closely with most period boundaries, whereas physical stratigraphic evidence of probable eustatic changes do not seem to match well (see, Sloss, 1963; Wheeler, 1963).]
Oscillatory continental diastrophic or climatic effects have also contributed to rhythmic patterns of much shorter period superimposed upon the large transgressive-regressive cycles. The Rocky Mountain and Pacific Coast Cretaceous each contain many rhythmic-appearing deposits. In the former, deposition was largely shallow marine to nonmarine under moderately strong energy conditions, producing many arenites. On the Pacific side of the Cordillera, however, there apparently was more offshore, lower energy (perhaps deeper) deposition of sand and mud with considerable graded bedding. It is striking that over the earth, Cretaceous to Eocene geosynclinal deposits in many mobile belts are typically rhythmic in character. This is true, for example, of the Alpine and Carpathian flysch, of Venezuela, southern Chile, Alaska, and the U. S. Pacific Coast. The writer speculated elsewhere (Dott, 1963) that eustatic rise of sea level in the Cretaceous "drowned" subsiding parts of mobile belts and created optimum conditions for preservation of turbidity current deposits quite independent of local subsidence or orogeny. This suggestion was tentatively made in an attempt to account for the exceptionally widespread occurrence of such deposits in geosynclinal strata of Cretaceous age. However, their apparent cyclicity requires some other explanation. Because the cyclicity is commonly of such regular period and occurs so widely over the earth, a diastrophic control seems unlikely. And, short-period eustatic oscillations are not indicated either, for no contemporaneous continental glaciation occurred as happened in the late Paleozoic. Essentially worldwide climatic oscillations are, therefore, suggested as a likely cause, manifested as very rhythmic fluctuations in vegetative and erosional-depositional factors. Doubtless other plausible, though less obvious, mechanisms exist, and it must even be admitted that these rhythms actually may have no regional significance at all.
Table 4--Summary of inferred superimposed cyclic patterns discussed in text.
|Example||Long-period effects||Short-period effects|
|Late Cenozoic (worldwide)||Universal Cenozoic regression due to eustatic fall||Pleistocene eustatic oscillations|
|Cretaceous of the Cordillera||Universal transgression due to eustatic rise||Local diastrophic and sedimentary oscillations
Nonglacial climatic pulsations?
|Pennsylvanian of northern Nevada||Local diastrophic pulses||Universal (glacioeustatic?) oscillations|
No attempt has been made to describe or explain all possible rhythmic patterns found in geosynclinal stratigraphic successions. Only a few apparently outstanding types reflecting some of the more important causative factors have been chosen (Table 4) with the chief objective of highlighting the possible range of complexity of interactions in the stratigraphic records of mobile belts. It is clear that many more or less oscillatory mechanisms operate with different frequencies and produce superimposed effects. Separation is essential, but difficult. Figure 9 summarizes the chief possible interactions of universal ("regional") eustatic effects of ocean-basin warping and local diastrophic complications. For example, local uplift may nullify the effects of universal transgression, or conversely, local continental subsidence could reinforce the net effect of such transgression (see, Fig. 7). The longer-period phenomena, chiefly eustatic changes and widespread orogenic acmes, are typically difficult to "filter" from the shorter-period ones. Thus, the scale of oscillations in time and space is extremely important. It would be fruitful to investigate more supposed cyclic successions with statistical significance tests (see, Duff and Walton, 1962) or perhaps frequency spectrum analysis to rigorously define and discriminate suspected composite patterns.
Figure 9--Results of interaction of eustatic changes due to oceanic warping and "local" diastrophism within a mobile belt or craton.
Obscurities of the stratigraphic record prevent unequivocal demonstration of multiple universal, and periodic eustatic, orogenic and epeirogenic events to the degree claimed by such older concepts as the diastrophic control theory of T. C. Chamberlain. Though there are some undeniable, outstanding continent-wide, and even worldwide historical patterns, the writer doubts that all these can ever be rigorously "filtered" and precisely correlated because of many sorts of local aberrations--especially in mobile belts. Within local provinces the record clearly appears episodic and some of the orogenic pulses seen within mobile belts even can be related to epeirogenesis in cratons, as stressed by King (1955). But, considered on the larger geographic scale of an entire mobile belt, diastrophism appears more nearly continuous, for diastrophic pulses occurred in one area while another was quiescent. On a very coarse time scale diastrophism seems to take on universal significance (Engel, 1963). With all of the potential interactions envisioned herein, return to constrictions of the diastrophic control dogma is impossible, but perhaps we could profitably be more cognizant of events half way around the world which may have cluttered up our own back yards in subtle ways.
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