|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 449-459|
Northwestern University, Evanston, Illinois
(Note: Since the submittal of the above contribution in 1963 a significant and pertinent paper by H.E. Wheeler has been published ("Post-Sauk and Pre-Absaroka Paleozoic stratigraphic patterns in North America," Am, Assoc. Petroleum Geologists Bull., v. 47, p. 1497-1526, 1963. See also discussion ibid., v. 48, p. 122-124, 1974).
It is Wheeler's thesis that the writer's Tippecanoe and Kaskaskia sequence should each be divided into two sequences. The additional sequence bounding unconformities are recognized by Wheeler below Early Silurian and latest Devonian strata, respectively. These are the positions of Stage 3 of the epeirogenic sequence model noted above. It is the present writer's contention that Wheeler is confusing temporary stabilization of the craton and consequent local and trivial unconformity witht he uplift anbd accompanying cratonwide erosion between sequences.)
Figure 1 illustrates a portion of a model continent characterized by a mobile belt (eugeosyncline) at the left and a continental craton to the right. The craton is dominated by stable shelf areas bearing a thin veneer of sediments. Toward the continental interior, the shelf passes without significant break into broad areas of exposed basement crystallines, a typical shield. Elsewhere, the shelf is interrupted by gently positive (or slightly less negative) elements termed epeirogenic uplifts. These elements may bear exposures of basement rocks as do the Ozark Dome and Llano Uplift, or they may be characterized by slower rates of sedimentation and more frequent episodes of erosion without basement exposure in the manner of the Cincinnati and Sweetgrass Arches. Interspersed with epeirogenic uplifts are ovate areas, the interior basins, of dominant negative tectonic behavior.
Figure 1--Portion of a model continent showing relationships of depositional sites to tectonic framework (Sloss, 1962).
These cratonic elements, shelves and shields, epeirogenic uplifts and interior basins, have individual histories of great longevity, extending over hundreds or multiples of hundreds of millions of years. In striking contrast are the cratonic orogenic uplifts and their complementary yoked basins. Cratonic orogeny is relatively rare and is characterized by a dominance of vertical uplift, commonly involving high-angle faulting; discordant and concordant plutonic activity is a common accompaniment of cratonic orogeny but is rarely important volumetrically. Extrusive activity ranges over a wide petrologic variety and may be an important factor in the fill of yoked basins or may be insignificant or lacking. Individual orogenic uplifts and yoked basins have relatively short periods of duration measured in tens of millions of years or less. The uplifts and adjoining basins are typically in the range of one hundred miles in length and may have their long dimensions oriented in apparent random fashion, en echelon, or in interrupted linear chains (such as the Laramie-Front Range-Sangre de Cristo) with as much as five hundred miles of continuity of direction.
Topographically similar but genetically remote from cratonic uplifts are the orogenic elements of the extracratonic mobile belt. Here the emphasis is on compression and regional metamorphism; granite emplacement, both magmatic and metasomatic reaches enormous volumes. Mobile belt trends are long in comparison to cratonic uplifts, extending over thousands and multiples of thousands of miles. Although tectonic activity in mobile belts is virtually continuous, it is subject to intense episodes of regional metamorphism, emplacement of granitoid masses, and pronounced vertical uplift. Where uplift produces large volumes of rock above base level, mobile-belt troughs and adjoining areas are the sites of accumulation of very large volumes of immature sediments and volcanics. Between the mobile belt and the craton lies a transition zone commonly called the miogeosyncline. The writer avoids this term and its connotations for a number of reasons. During much of the history of such zones they are the depositional sites of sediments indistinguishable from those of cratonic interior basins. The rates of subsidence are of the same order of magnitude and their tectonic behavior during sediment accumulation is typically cratonic in habit. Further, during times of relative quiescence in the adjoining mobile belt, much of the sedimentary clastic fill of the so-called miogeosyncline is of interior derivation, that is, it is derived from erosion on, and transportation across, the craton. The term miogeosyncline carries with it an implication that it is half of a couple which, with the eugeosyncline, constitutes the ortho-geosyncline. This concept evokes a picture of a continous miogeosynclinal trend everywhere parallel to the eugeosyncline and linked closely to it. In actuality, the subsiding portions of this trend form noncontinuous chains of basins more closely related to, and more difficult to distinguish from, elements of the craton than to the strikingly different tectonic habit of the eugeosyncline. For example, the Arkoma, Ardmore, and Anadarko Basins were a single tectonic province in Ordovician time. The basin in question extended from south-eastern Colorado, surrounded by hundreds of miles of cratonic shelf, to southeastern Oklahoma, where it approached the mobile belt of the Ouachita trend and where it would be considered part of the miogeosyncline. If the concept of the miogeosyncline is adhered to, the Permian of West Texas-New Mexico and the Mississippian of Montana offer similar terminologic problems. Reference to subsiding elements in the transition zone between mobile belts and continental cratons as marginaI basins avoids these philosophical questions.
The Allegheny and Warrior Basins of the Appalachian trend are examples of relatively narrow marginal basins with histories strongly affected by behavior of the adjoining mobile belt. Each significant pulse of orogenic activity in the mobile belt is marked by the spread of clastic wedges of externally derived material across much of the width of the marginal basin. In such narrow basins, diastrophic activity of the mobile belt is commonly exerted laterally as far as the basin axis to form parallel folds and thrust faults affecting the sedimentary fill. In Nevada and Utah the Oquirrh Basin, which existed from late Precambrian to mid-Jurassic, is an example of a marginal basin of mainly cratonic habit tectonically unaffected by orogenic events of the Cordilleran trend and almost free from adulteration by externally derived sediment. For hundreds of millions of years this basin behaved as a cratonic element. Ultimately, in Late Jurassic time, it was overwhelmed bv externally derived clastic wedges and by mid-Cretaceous time became involved in diastrophism related to the spreading mobile belt. Much of the Gulf Coast Basin, from Late Jurassic to present, provides an example of a marginal basin completely unaffected by mobile-belt tectonics, either as a sediment source or by compressional diastrophism.
This brief discourse of the writer's views on continental tectonics has a dual purpose. First, it attempts to provide a terminology of tectonic elements with which it is possible to consider the periodicity of tectonic behavior and resulting sedimentary patterns on the craton. Secondly and more importantly, the point is made that although orogenesis and resulting sediment supply may occur in mobile belts and on cratons, there is a fundamental distinction in tectonic habit, and no necessary relationship exists in time or space between these two very different patterns of diastrophic activity. In many other respects cratons are independent of mobile belts; however, investigation of the craton is not complete without consideration of the marginal basins. Here, the interplay of cratonic and extracratonic influences is manifested and requires analysis.
Figure 2--Tine-stratigraphic relationships of sequences in North American craton. Black areas represent non-depositional hiatuses; white and [tan] areas represent deposition (Sloss, 1963).
Thus, the diagram (Fig. 2) charts the approximate shifting of base-level position on the North American craton during the past six hundred million years or so of geologic time. It can be seen that the marginal basins tended to remain below base level and were the sites of accumulation of sediment for the greater part of this time span. The interior of the craton shows a centripetal tendency toward persistence above base level, remaining for long periods in a nondepositional regime, and the least negative areas are ouly occasionally the loci of sedimentary accumulation.
The record as interpreted on the time-length diagram is not, of course, preserved in auything like this form. Each of the nondepositional episodes was accompanied by extensive erosion introducing a severe loss to the preserved record, thus requiring a very considerable degree of interpretation and reconstruction. Therefore, readers may well find grounds to dispute a number of the details of the writer's reconstruction. However, the gross form of cratonic history from the late Precambrian is clear. The times of maximum transgression of the cratonic interior are approximately those indicated and the episodes of maximum regression, nondeposition, and erosion cannot depart widely from the pattern as presented. In other words, let us not here argue the validity of the sequence concept, but, rather, proceed to a consideration of the alternation between depositional and non depositional episodes of the craton and the significance of these events. Are the successive episodes repetitions of the same behavior? If differences exist, is there a systematic pattern of recurrence? What relation.ship, if any, appears to exist between cratonic events and those occurring in neighboring mobile belts?
Figure 3--Distribution of lithologic associations. Size of circles indicates relative prominence of association in each sequence.
The term clastic-wedge association, following the usage of Pettijohn (1957) and King (1959), is applied to wedge-shaped accumulations of sediment derived from extracratonic mohile belts. The Queenston "delta" and Alpine molasse are familiar examples. Coal-cycle associations are those characterized by a repetitive complex of depositional types and environments involving both marine and nonmarine deposition. Coal-cycle associations are dominated by sediment of cratonic interior derivation and are difficult to recognize among older sediments, if lacking land plants to clearly identify them as nonmarine members of the association. The deltaic association includes masses of cratonic interior derivation which, in contrast to clastic wedges, thicken in the direction of sediment transport and direction of reduction of average particle size. The still-stand sand association has much the same geometry but is characterized by mature sandstones representing reworking along strand lines, while the deltaic association preserves the record of the complex of environments observed on many modern delta systems.
Among carbonate-evaporite associations only the lentiform carbonates require explanation. These, the carbonate fills of subsiding basins, conform to the geometry of the basins which they occupy, and thus form lens-shaped masses of broad regional extent.
Returning to Figure 3, the available data on the several successive sequences, when organized in this fashion, do not reveal an order or a system to suggest a cyclical or repetitive pattern. The earlier three sequences, for example, have much in common but differ in important details. All are dominated by blanket, lentiform, and carbonate bank associations, but reef and evaporite associations are important only in the Tippecanoe and Kaskaskia. Blanket sands of high maturity are characteristic of the Sauk and Tippecanoe but occupy little of the total volume of the Kaskaskia. Clastic wedges are virtually absent from the Sauk assemblages but are important in the marginal basin accumulations of the Tippecanoe and Kaskaskia. The three later sequences are dominated by clastic sediments, including clastic wedges in the Absaroka and Zuni and an abundance of wedge-arkose associations in the Absaroka and Tejas. Thus, although the observed sedimentary associations of the several sequences serve to illustrate their similarities and differences, there is no readily detectable orderly succession in the occurrence of major associations. If a systematic pattern exists, it is not made evident by this analytic approach to the cratonic record of the past 6OO-odd million years.
Figure 4--Tectonic activity of adjoining mobile belts (upper plot) as indicated by spread of clastic wedges to cratonic margins, and tectonic activity of interior of craton (lower three plots) in terms of yoked basins, interior basins, and relative position with respect to base level. A larger version of this figure is available.
The Sauk sequence presents a simple pattern. It is assumed that the late Precambrian subsidence of the marginal basins in the Cordilleran and Appalachian regions was accompanied by parallel, though markedly less extreme, subsidence of the continental interior. The continuity of supply of coarse clastic detritus from the cratonic interior indicates that this subsidence did not approach base level, and the relatively slow transgression of the seas in Early and Middle Cambrian time over the cratonic margins suggests an approach to tectonic stability at supra-base level positions in the interior. The rapid advance of Late Cambrian sedimentation over the cratonic interior is clear evidence of renewed subsidence extending below base level and climaxing in very widespread seaways in the Early Ordovician. Relatively abrupt uplift above base level is marked by the sub-Tippecanoe unconformity, but the comparatively low volume of Sauk sediments removed indicates that pre-Tippecanoe uplift was not extreme.
In general form, then, the broad pattern of cratonic-interior behavior during Sauk deposition may be characterized by the following stages:
Parallel behavior during deposition of Tippecanoe and Kaskaskia sediments is less obvious because the cratonic interior tended to reach sub-base level positions earlier and remain there longer than during Sauk and Zuni accumulation. Nevertheless, broadly similar relative movements can be discerned. Stage 1 is obvious in both Tippecanoe and Kaskaskia episodes. Stage 2 is represented by the transgression of Chazy and Trenton sediments of the Tippecanoe and by parallel onlap of Middle and Upper Devonian Kaskaskia strata. Stage 3, the stable phase, is characterized by the sheetlike geometry of deposits of latest Ordovician-Early Silurian (Tippecanoe) and latest Devonian-Early Mississippian (Kaskaskia) age. There is evidence of minor uplift above base level during this stage in both sequences. It is possible that such positive movement is typical of the stage, but it is difficult to identify in the less complete Sauk and Zuni records. Stage 4, the climax of cratonic subsidence, is indicated by the wide distribution of Middle and Upper Silurian sediments without evidence of littoral deposition except under the influence of mobile-belt sources, and in the Kaskaskia, by the extraordinarily wide distribution of mid-Mississippian carbonates. Again, the evidence for emergence in Stage 5 is undeniable.
The interpretation of the record of the Absaroka sequence stands in decided contrast to those described above. Although much of the Absaroka sequence has been removed or deeply eroded by the severity of pre-Zuni erosion, it seems clear that no simple five stage phenomenon is indicated. Rather, if consideration is directed toward those areas not affected by cratonic orogeny, and thus comparable to the tectonic states of the previously described sequences, it seems clear that the most significant negative movement of the cratonic interior was climaxed very early in the history of the sequence before the close of Pennsylvanian time and in the first one-fourth of the time span of the sequence. For over one hundred million years thereafter the record indicates innumerable minor positive and negative fluctuations whose algebraic sum is a persistent positive trend leading to slow regression. If the sequence were to follow the pattern of the Sauk, Tippecanoe, Kaskaskia, and Zuni, the maximum subsidence and transgression of the cratonic interior should be within the Triassic Period. Although preservation of the Triassic System is limited, the areas of preservation demonstrate a lesser degree of subsidence and transgression than in the Pennsylvanian and Permian.
The record of the Tejas sequence may be clear when it can be seen in the proper time perspective. So far as behavior of the cratonic interior can be interpreted, the pattern seems to be much like that of the Absaroka and distinct from the pattern of the other sequences. Again, there is evidence of early subsidence leading to long-continued emergence with numerous minor oscillations and at least one major positive episode in the Miocene.
The second plot from the bottom of Figure 4 is an interpretation of the degree of differentiation of the craton as indicated by the development of interior basins. In this plot, the height of the curve is a measure of the degree of differentiation of basins and adjacent shelf areas. Note the striking similarity of the Tippecanoe and Kaskaskia patterns. Each has two peaks, Trenton and Salina in the Tippecanoe, Middle Devonian and Meramecian in the Kaskaskia. Each peak of interior basin development coincides with the time of subsidence of the cratonic interior as a whole (stages 2 and 4) and are separated by the mid-sequence episode of cratonic stability (stage 3). A very similar pattern is apparent in the Zuni sequence wherein stage 2 is the time of major basin-center evaporite deposition of the Jurassic. Stage 4 is the Late Cretaceous phase of basin development, as in the Denver-Julesburg and Hanna Basins of the Rocky Mountains-Great Plains region.
The possible affinity of the Sauk sequence with this pattern of cratonic differentiation is less clear. The lack of siguificant cratonic-interior Sauk record for anything except the Late Cambrian and Early Ordovician precludes observational data of stage 2, which might be expected to occupy a position in time immediately preceeding the beginning of the Cambrian Period. Great thicknesses of youngest Precambrian sediments in the marginal basin areas of Nevada, British Columbia, and Tennessee strongly suggest rapid subsidence and differentiation representing a marginal basin counterpart of stage 2 interior basin development. However, if there were a concurrent development of basins within the cratonic interior, there is no decipherable record within middle North America. Nevertheless, the writer feels that the numerous points of similarity between the Sauk and the other five sequences demands placement of the Sauk in the same tectonic frame.
The timing of major clastic wedges and the maxima of mobile-belt activity which they represent appear to have no correlation with other tectonic events of the Sauk, Tippecanoe, Kaskaskia, and Zuni sequences. Rather, externally derived sediment in these sequences appears as an addition or intrusion into quite unrelated circumstances. Thus, the development of the Queenston wedge coincides with parts of stages 2, 3, and 4 of the Tippecanoe sequence; the Catskill wedge overlaps stages 1,2, and 3 of the Kaskaskia sequence; and the Blairmore-Frontier-Mesaverde wedges span most of the time of the Zuni sequence.
A vague relationship is seen between mobile belt and cratonic orogenic episodes. In each case, cratonic orogenesis is initiated some hundred million years after the extension of clastic wedges to the cratonic border. However, even in geologic terms a hundred million years is very long for the maintenance of a control and response relationship. Furthermore, the discrepancy in the timing of the maxima and minima of activity is such as to suggest to the writer that no genuine interrelationship exists except, possibly, that the 450 million years which include times of both extracratonic and cratonic orogenesis may represent a phase of a very much longer cycle of tectonic activity in the crust and mantle.
The Epeirogenic Model
The five tectonic stages identified earlier in this paper, in terms of the responses observed in the Sauk sequence, can now be slightly recast as a more general statement of the epeirogenic sequence model as presented in Table 1. The model is best exemplified by the Tippecanoe and Kaskaskia sequences, is recognizable in the Zuni sequence, and by reconstruction of the hiatal record, admits reference of the Sauk sequence. The tectonic elements of the epeirogenic model are typically long-lived features prominent in the framework of earlier and younger sequences. Externally derived clastic wedges may intrude upon the normal cratonic sedimentary succession, but the external sediment is deposited under tectonic states dictated by a cratonic pattern and there is a lack of evidence to suggest external control of cratonic tectonic behavior.
The epeirogenic model exhibits a reasonably well defined cyclical pattern of tectonic states and stratigraphic responses. Invasions of extracratonic sediment, variations in physical enviromnent, and the nature and abundance of sediment-contributing organisms introduce a number of significant modulations of the observable stratigraphic response. Yet, the five-fold tectonic cycle remains apparent where the preserved record is adequately complete.
Table 1--Stages in the cycle of the epeirogenic sequence model
|Stage||Tectonic state||Stratigraphic response||Examples|
|1||Slight tectonic differentiation of craton; slow subsidence from supra-base level condition at close of preceding cycle.||Wideapread nondeposition and erosion of cratonic interior; blanket sands and carbonates in marginal basins, onlapping to adjacent shelves.||Tapeats
|2||Accclerated subsidence of entire craton; differentiation of interior basins and positives.||Broad transgression of cratonic interior; lentiform carbonates, basinal evaporites and euxinic shales.||Meagher (?)
Gros Ventre (?)
|Lower Red River
|3||Return to atability and lesser differentiation; brief elevation of more positive elemente above base level.||Blanket sands, shales, carbonates; minor nondeposition and erosion.||Reagan
|4||Renewed subsidence with extreme development and differentiation of interior basins.||Maximum transgression of craton; maximum facies differentiation of basins with emphasis on restricted deposits, including basin-center evaporites.||Arbuckle
|5||Return to undifferentiated craton; elevation, eventually bringing axes of marginal basins to supra-base level positions at close of cycle; reinitiation of nondeposition and erosion.||Regression and offlap; blanket clastics where preserved.||Swan Peak
The Orogenic Model
The orogenic sequence model, exemplified by the Absaroka and Tejas sequences, does not conform to any discernable cycle of tectonic events and responses. In tectonic terms, the characteristic feature is the abrupt appearance of orogenic positive elements and complementary yoked basins; some are accelerations or exaggerations of the tectonic habits of pre-existing elements and some are new elements unheralded by earlier patterns of behavior. From time to time during the span of the orogenic model, there is, at least locally, a cessation of violent uplift and downwarp, and the yoked basins take on the tectonic habit and sedimentary responses of normal interior basins. However, there is no evidence of cratonwide synchronism and no suggestion of conformance with the epeirogenic model during times of reduced orogenic activity. Therefore, no orderly systematic pattern of tectonic events and stratigraphic responses is apparent. Perhaps, if there were available for study more than a single complete orogenic sequence and part of a second, a more systematic picture could be developed.
The cyclical nature of the North American sequences has been recognized for some years (see, Sloss, 1950, p. 450). Other contemporary approaches to the definition of cratonic tectonic cycles of the same order of magnitude have developed out of efforts toward improved tectonic maps of major continental areas. The tectonic maps of Mexico (de Cserna, 1961, reviewed by King, 1962) and the U.S.S.R. (explanatory notes by Shatzki and Bogdanoff, 1957) are examples. A similar approach has been adopted in current projects leading to tectonic maps of Canada (Neale, Beland, Potter, and Poole, 1961) and of Europe (Bogdanoff, 1962). The most complete presentation of the European and Soviet point of view is given by Bogdanoff (1962), and a number of contrasts with the writer's thesis can be noted.
Bogdanoff recognizes a hierarchy of tectonic units which may be summarized as follows:
First order units: Megacomplexes. Cratonic crystalline basement is considered as one megacomplex; all sediments above, the "cratonic cover," form another.
Second-order units: Fold complexes. These are major tectonic subdivisions in time and space, each characterized by definable areas of occurrence, by typical orientation of fold and fault systems, and by a particular structural habit or style. Examples are Caledonian and Variscan complexes; the same terms are applied to both ancient mobile belts and cratons.
Third-order units: Structural stages. The stages are described as components of fold complexes, encompassing the rocks of a single geologic system or parts of two systems. Typically, three stages (phases of a tectonic cycle) are recognized within each fold complex in mobile belts and their structurally related margins. Difficulties appear to be encountered in identification of fold-belt stages on cratons.
The writer's sequences are most closely related to Bogdanoff's second-order units, the fold complexes. It is implicit however, in the latter taxonomy that cratonic history is controlled by events in the adjacent mobile belts; there appears to be little confirmation of this thesis in an investigation of the North American craton.
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