|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 533-539|
University of Houston, Houston, Texas
True stability was rarely attained, however, because relative sea level often changed, in a few cases by hundreds of feet but usually by some tens of feet. During each rise or transgression, limestone accumulated in a broad belt along the outer "drowned" undaform, occasionally developing a barrier reef behind which evaporites formed. Gradually pre-existing conditions were restored as increasing amounts of clay and sand arrived in the form of a smaller-scale embankment which advanced seaward across the lagoon which had developed over the older "drowned" undaform. When advance continued past the position of the older clinoform, it produced the "complete, normal" sedimentary cycle.
Occasionally regression modified the cycle by promoting deposition of coal or of red clay and evaporite facies on the emergent undaform, or in more extreme cases permitting erosion and channelling of the undaform and extensive deposition of sand and clay on the fondoform.
Much more commonly, however, the cycle was interrupted by renewed transgression long before the lagoon was filled, yielding an incomplete cycle which may not be widely recognizable. Repeated interruptions enabled a few undaform-edge limestone bodies to accumulate to thicknesses of several hundred feet, giving rise to composite cycles.
Each well-developed cycle should be recognizable in all environments; but part of the cycle may be represented by nondeposition in the deeper water environments, and by erosion on the undaform, especially near the sediment source areas. The base of each cycle is that of the bed deposited during the relatively rapid shift of facies that initiated the cycle, or the corresponding hiatus, both of which are practical "time surfaces." This rapid shift of facies was caused by some change in external conditions, ordinarily expressed by a rise in relative sea level, which upset the pre-existing gradational equilibrium. The rest of the cycle is the product of slow migration of contemporaneously adjoining environments past the point of observation as pre-existing conditions were gradually restored.
Cycles were caused, and preserved, by regional subsidence at a fairly uniform rate, upon which were superposed large and small fluctuations in sea level caused by waxing and waning of distant continental glaciers. Smaller cycles resulted, in part, from shifts in the positions of river mouths.
Numerous wells have been drilled through the Pennsylvanian and Permian sediments and logged electrically. Distinctive successions of responses on the electric logs make it possible to correlate some parts of the section in great detail so that lateral changes in the lithology of thin time-stratigraphic units can be observed on a much more extensive scale than is possible with other kinds of subsurface data or with outcrop sections. Unfortunately, however, intensive studies of such lateral changes in the lithology of restricted time units by use of cuttings and cores from numerous wells seemingly have not been made, or published. Despite this deficiency, certain broad relations are evident from electric logs plus scattered cores, samples, sample logs, and a few published descriptions. This approach brings out the unity of seemingly unlike cycles that formed simultaneously under dissimilar conditions.
Figure 1--Diagrammatic representation of uninterrupted complete "normal" sedimentary cycle in late Pennsylvanian and early Permian of western and central Texas. Shapes are distorted by thousand-fold vertical exaggeration of sediment body between surfaces A and E. A larger version of this figure is available.
The "typical" cycle of Figure 1 began with marine transgression which abruptly reversed the pre-existing conditions under which clay and sand had been arriving in sufficient amounts to build out the clinoform and undaform. In the absence of these terrigenous sediments indigenous lime-secreting organisms became the chief source of material deposited on the outer, "drowned," undaform. However, before the limy facies expanded into some localities, and beyond the limits of that facies, dark organic clay was the usual fondo deposit under the new sedimentary regime (where water depths were sufficient). On the deeper fondoform such clays tended to be highly siliceous, thus forming the most satisfactory stratigraphic markers in the deep fondo environment.
This state of affairs--deposition of limestone along the outer drowned undaform and fondo dark clay on both sides--changed as pre-existing conditions were gradually restored by arrival of increasing amounts of clay and sand. These sediments were deposited in the form of a smaller scale embankment, a clinoform-undaform, which advanced seaward (southwestward) across the lagoon which generally had developed over the drowned undaform. At first this advance simply restricted the area of "shallow" fondo deposition on the drowned undaform, then the embankment lapped onto the unda-edge limestone and in some cases completely covered it. If sand and clay continued to be supplied, the embankment built out beyond the pre-existing (drowned) clinoform and thereby restored conditions to those at the beginning of this "typical" cycle.
If no new cycle was started immediately, the clinoform-undaform continued to advance, expanding the coastal plain environments. In west-central Texas the two conspicuous sediment suites formed in such "extended" cycles were characterized by coal, and by red shale, which occasionally extended over much of the earlier limestone facies of the same cycle. Farther landward the coastal plain (as well as sediment source areas) was subject to erosion, which in some places formed a conspicuous unconformity similar to the channelled surfaces described on the outcrop by Lee and others (1938, Pl. 1,2,3).
During accumulation of the first red clay and evaporite section of regional extent, (Valera Anhydrite), the first fondo sand of regional extent (Dean Sandstone) was accumulating on the fondoform farther west, as demonstrated by well-log cross sections (Van Siclen, 1958, p. 1900). This association prompted the writer to suggest that extensive evaporite and fondo sand deposits may be the result of a relative drop in sea level which exposed part or all of the undaform. The result is an extreme modification of the basic cycle of Figure 1, with the cycle being terminated "prematurely" over most or all of the undaform while deposition is concentrated on the fondoform in what should perhaps be regarded as a new and different cycle.
However, most cycles were interrupted by marine transgression before the lagoon was filled and the cycle "completed." As a result, the area in which a cycle is expressed may be restricted to belts along each side of the lagoon. Along the seaward (southwest) side there is an alternation of limestone and shale; and along the landward side in the older (generally late Pennsylvanian) cycles alternations of clay and of sand occur, whereas in the younger cycles anhydrite and dolomite alternate with much thicker red shale and siltstone.
Repeated transgressions which kept the lagoon open continuously also enabled the limestone to become hundreds of feet thick over a few of the submerged undaform-edges, because this facies of successive small cycles continued to occupy the same geographic position. Time finally arrived (generally because of lowered sea level) when enough clay and sand were carried into the sea beyond the limestone belt to build the clinoform-undaform to a new position. The alternation of such outer undaform and upper clinoform limestone bodies, hundreds of feet thick, and belts of terrigenous clastic sediments, usually miles wide, gives rise to large-scale or composite cycles (Van Siclen, 1958, p. 1902).
The cycle began when some change in external conditions occurred, in these instances a rise in relative sea level, which favored certain gradational processes and environments. As a result, these environments developed and expanded rather quickly at the expense of other environments that were affected adversely. However, the pre-existing pattern was gradually restored by the continued action of the same processes. These processes tended to maintain dynamic equilibrium, while the changes in external conditions produced nonequilibrium, the alternation being responsible for cyclicity. Regarded in this manner, the ideal base of each cycle is the base of sediments that reflect the relatively rapid shift away from "equilibrium" under the influence of a change in external conditions (generally marine transgression).
To be recognizable throughout its area of occurrence, an individual cycle must include all sediments formed during an essentially uninterrupted interval, and only these. Its boundaries will then be thin zones representing rapid shifts of facies (and perhaps additional changes), or horizons representing fundamental reversals in conditions, which affect all environments and therefore are essentially "time surfaces." Each simple cycle so defined may be expected to vary by the addition of deeper water members at the distal extremity, and by addition of terrestrial members (if preserved) adjoining the source area. The systematic vertical changes in lithology represent principally the migration of a succession of adjoining environments past the locality of the observer.
In west-central Texas and adjoining regions, environments tended to migrate toward the southwest throughout the Pennsylvanian and Permian Periods. As a result, the stratigraphic section preserved at any particular locality displays various parts of many cycles; in the regressive part of the major cycle these generally begin with fondo deposits and end with terrestrial strata. This superposition of different parts of similar cycles in one general area facilitates reconstruction of the "complete, normal" cycle of Figure 1.
Sediments of the area under consideration are most simply interpreted as having formed under conditions of continuous, fairly uniform absolute subsidence; and continuous, fairly uniform absolute uplift of the terrigenous sediment source areas. Oscillatory uplift of the sediment source areas, or oscillatory subsidence of the depositional areas, is not necessary to explain the cycles. However, the occurrence of some such oscillations, and of sea-level changes caused by distant diastrophism, cannot be ruled out from evidence in the area under consideration.
Cyclicity is due largely, or entirely, to repeated drops in sea level caused by expansions of distant continental glaciers, plus local shifts in the position of river mouths, superposed upon the continuous regional subsidence. Evidence of repeated glaciation in the Pennsylvanian and Permian of Australia has been cited by Teichert (1941); this and additional evidence of extensive glaciation in the present Southern Hemisphere and in India was first applied in detail to the present problem by Wanless and Shepard (1936), and most recently by Wanless (1963). The shifts (avulsion) of river mouths is most conspicuously demonstrated by the Recent subdeltas of the Mississippi River, as described by H. N. Fisk (1944). For example, marine regression is resulting today from deposition around the active delta, while the older St. Bernard subdelta is sinking and the transgression has formed Chandeleur Sound.
Net changes in relative sea level associated with the undaform-edge limestone bodies hundreds of feet thick must have been of about the same magnitude-hundreds of feet. Stepdowns of the base of undaform-edge limestone body "X" southward across Kent and Scurry Counties, Texas (as shown by Van Siclen, 1958, p. 1907), from about 670 feet below the base of the Coleman Junction Limestone datum across Kent County to about 1,100 feet below that datum in the southeastern corner of Scurry County and vicinity, demonstrate about 430 feet lowering of sea level; there is no reason to believe that this lowering is close to the total. Sea-level fluctuations of such magnitude, in a region of so little deformation and so little change in general conditions of sedimentation must have external causes. Fluctuations occurred so frequently, and in a sense regularly, that the effects of distant diastrophism must be ruled out as the sole or chief cause. The remaining possible cause, the only one that to the writer seems adequate quantitatively, is distant continental glaciation.
If the sea-level changes were produced principally by waxing and waning of distant continental glaciers, it appears likely that sea level was high most of the time, and that the episodes of lowered level were relatively brief departures from the norm. At first glance this statement seems to invalidate the writer's previous statement that equilibrium conditions existed when mud and sand were arriving in sufficient amounts to build the undaform and clinoform, that transgression upset this equilibrium, and that the cyclic sediments represent gradual restoration of pre-existing equilibrium. The critical factor here is the rate at which equilibrium was attained at each extreme position of sea level, relative to the length of time each extreme existed. The writer's opinion is that erosion of weathered rock and recently deposited sediments, and deposition of the material, under conditions of lowered sea level is much more rapid than the same processes plus biochemical and perhaps chemical sedimentation during elevated sea level, so that equilibrium was much more commonly attained in the west-central Texas area during the lowered sea-level stages. This phenomenon is analagous to the present-day situation in which the continental shelf is adjusted to lowered sea level of a Pleistocene glacial stage, and has not been altered greatly by deposition since the Recent transgressions, except near the mouths of very large rivers and to a much lesser degree along coasts where barrier reefs occupy the drowned Pleistocene undaform-edge (like the Australian Great Barrier reef).
2. Such correlations disclose that the sediments deposited in this part of Texas in the regressive part of the major Pennsylvanian-Permian sedimentary cycle assumed a topographic form which in profile resembles the classic delta, with unda, clino, and fondo segments.
3. The normal situation, in terms of work accomplished if not in length of time, was for considerable clay and moderate amounts of sand from sources to the northeast to be carried across the undaform and deposited principally on the clinoform, extending the undaform (coastal plain) southwestward.
4. Each normal sedimentary cycle began with transgression of the sea, which prevented sand and appreciable clay from being carried onto the outer part of the "drowned" undaform and permitted biochemical limestone to accumulate there instead.
5. Small thicknesses of dark clay often accumulated on both sides of the undaform-edge limestone belt; that on the original fondoform tending to be highly siliceous (providing key beds).
6. Some undaform-edge limestone bodies developed into almost continuous barrier reefs which impeded circulation of sea water into the adjoining lagoon, and led to dolomitization of previously deposited limestone and accumulation of anhydrite and rarely halite.
7. Generally the lagoon that had developed from the drowned undaform was filled principally by clay and sand deposited in the form of a small-scale undaform-clinoform which advanced seaward in adjustment with the "new" higher sea level.
8. As long as conditions remained about the same the "new" undaform-clinoform continued to advance, first over the undaform-edge limestone of the same cycle, then into deep water beyond the former (drowned) clinoform. Such "complete" cycles display considerable seaward extension of terrestrial environments characterized by coal or by red clay.
9. Some cycles were modified by the effects of regression, which in certain instances led to extensive deposition of coal or of anhydrite and red clay on the emergent undaform, to conspicuous channelling, and generally to unusually extensive deposition of sand and clay over the fondoform.
10. More commonly the cycles were modified by renewed transgression even before the lagoon was filled, which greatly restricted the area in which each such cycle was clearly developed, and which enabled limestone bodies several hundred feet thick to accumulate over a few of the drowned undaform-edges, giving rise to large-scale or composite cycles.
11. Every well-developed "normal" cycle can be recognized in all environments (in principle), but part may be represented by nondeposition in the deeper water portion, and by erosion on the undaform, especially near the sediment source areas.
12. The base of each cycle should be defined as that of the bed deposited during the relatively rapid shift of facies that initiated the cycle, or the corresponding hiatus, both of which are practical "time surfaces."
13. This rapid shift of facies was caused by some change in external conditions, ordinarily a rise in relative sea level, which upset the pre-existing gradational equilibrium.
14. The cycle itself simply represents the slow migration of contemporaneous, adjoining environments past a point of observation, as the pre-existing conditions (equilibrium) are restored.
15. The various environments also had a net shift across the region toward the southwest during the Pennsylvanian and Permian Periods, so that at any locality various parts of many cycles are present.
16. The cycles developed, and were preserved, because the region subsided at a fairly uniform rate, and because large and small fluctuations in sea level were superposed on the subsidence.
17. The larger changes in sea level (to at least 400 feet), and many of the smaller ones, were caused by waxing and waning of continental glaciers in the present Southern Hemisphere and in India.
18. Small cycles resulted, in part, from shifts in the position of river mouths, and possibly from climatic changes.
19. There is no need for oscillatory uplift of sediment source areas, or oscillatory subsidence of the areas receiving sediments, or for sea level changes caused by distant diastrophism, although none of these can be ruled out as a contributing factor on the basis of evidence developed in the region under consideration.
20. Equilibrium was more commonly attained during the glacially lowered sea-level stages, which is analogous to the present-day situation where the continental shelf (a drowned undaform) is adjusted to glacially lowered late Pleistocene sea level. This situation is due principally to accelerated gradation upon lowering of base level and has little bearing on the relative length of the glacial and interglacial stages.
Fisk, H. N., 1944, Geological investigation of the alluvial valley of the lower Mississippi River: Miss. River Comm., Vicksburg, Mississippi, p. 1-78.
Lee, Wallace, and others, 1938, Stratigraphic and paleontologic studies of the Pennsylvanian and Permian rocks in north-central Texas: Texas Univ. Pub., no. 3801, p. 1-247.
Mear, C. E., and Yarbrough, D. V., 1961, Yates Formation in southern Permian Basin of West Texas: Am. Assoc. Petroleum Geologists Bull., v. 45, no. 9, p. 1545-1556.
Rich, J. L., 1951, Three critical environments of deposition and criteria for recognition of rocks deposited in each of them: Geol. Soc. America Bull., v. 62, no. 1, p. 1-20.
Teichert, Curt, 1941, Upper Paleozoic of Western Australia, correlation and paleogeography: Am. Assoc. Petroleum Geologists Bull., v. 25, p. 271-415.
Van Siclen, D. C., 1957a, Cenozoic strata on the southwestern Osage Plains of Texas: Jour. Geology, v. 65, no. 1, p. 47-60.
Van Siclen, D. C., 1957b, Organic reefs of Pennsylvanian age in Haskell County, Texas: Geophysics, v. 22, no. 3, p. 610-629.
Van Siclen, D. C., 1958, Depositional topography--examples and theory: Am. Assoc. Petroleum Geologists Bull., v. 42, no. 8, p. 1897-1913.
Wanless, H. R., 1963, Origin of Late Paleozoic cyclothems (abs.): Am. Assoc. Petroleum Geologists Ann. Prog., Houston, Texas, p. 58.
Wanless, H. R., and Shepard, F. P., 1936, Sea level and climatic changes related to Late Paleozoic cycles: GeoL Soc. America Bull., v. 47, no. 8, p. 1177-1206.