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Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 283-286 |
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U. S. Geological Survey, Denver, Colorado
Cyclic sedimentation, which results from alternation or repetition of environments of deposition, is common where certain naturally concentrated salts are deposited periodically with contemporaneously accumulating detrital deposits to form beds in a definite sequence. Some chemical sedimentary rocks included in cyclic sequences are limestone, gypsum, and chert, as illustrated by the Toroweap, Moenkopi, and Kaibab Formations, respectively, in northern Arizona. These cyclic sequences generally differ from those formed by changes in sea level in that they are mostly simple, usually involving only two or three lithologic units in each cycle, and in that they have relatively restricted geographic distribution. The cyclically deposited units may be repeated many times in a stratigraphic interval, however, and attain a large total thickness.
[Note: Publication authorized by the Director, U. S. Geological Survey.]
Cycles in sedimentation as represented by the classic cyclothems of the Pennsylvanian and Permian Systems (Weller, 1930; Moore, 1931; Wanless and Shepard, 1936) are generally conceded to result from changes in sea level, brought about either by climatic or by tectonic factors. Some other cyclic stratigraphic sequences of Paleozoic and Mesozoic age, however, consist of series of repetitive units apparently caused by periodic changes in the chemistry of the water in which they were deposited and are unrelated to depth variations. Excellent examples of this type of cyclic deposition are strata of the Toroweap, Moenkopi, and Kaibab Formations that contain chemically deposited beds that alternate with beds of detrital rock or of clastic carbonate rock, or both. Unlike cyclic deposits resulting from sea level changes, these have relatively limited geographic distribution, even though repeated many times in a stratigraphic section. Three examples will be discussed.
A cyclic sequence consisting of, from bottom to top, redbeds, aphanitic limestone, and gypsum (Fig. 1A), occurs 10 or more times within the upper member of the Permian Toroweap Formation in the Grand Canyon region (McKee, 1938, p. 121), and extends from the east-central to the western part of the canyon. This sequence is interpreted (p. 18, 25) as representing a regressive stage in the history of the Toroweap. The redbeds in this cyclic sequence generally are 3 or 4 feet thick, but, in some places, as along the eastern margins of the area delineated above, are as much as 8 feet thick. Conversely, the overlying limestone units commonly are 1 foot or less thick near the east margin of the area, but 2 to 4 feet thick farther west. The gypsum beds, although lenticular, also generally thicken, from about 1 foot at the eastern margin to about 4 feet 75 miles west.
Figure 1--Diagrammatic partial sections of (A) upper member of Toroweap Formation, Permian, and (B) Shnabkaib Member of Moenkopi Formation, Triassic, showing cyclic sequences in northwestern Arizona.
Paleogeographic considerations and texture trends (McKee, 1938, p. 100) indicate that detritus, forming redbed units in the Toroweap cyclic deposits, was transported from the east by streams. These detrital beds probably were deposited in shallow residual water bodies, such as playa lakes or lagoons (p. 109), because associated calcium carbonate and calcium sulfate were accumulating intermittently as chemical precipitates from concentrated saline waters. This member consists entirely of redbeds to the east, but within the belt of redbeds that alternate with chemical deposits, individual redbed units not only are progressively thinner toward the west, the direction of the geosyncline, but also show a gradation in texture in that direction from dominantly sand to dominantly silt or clay. These detrital sediments apparently were introduced into the basins more or less continuously, for along the eastern margin of the area of cyclic deposits many of the limestone and gypsum beds include various proportions of sand and mud. The chemical sediments, in contrast, must have been deposited rhythmically and at intervals of time when sufficient concentration of salts in the water, probably attained by evaporation, resulted in precipitation of calcium carbonate, then calcium sulfate. Thus, cyclic deposits in the upper member of the Toroweap seem to represent recurring periods of evaporite precipitation, followed by periods of no precipitation but continued accumulation of redbed detrital materials.
In the same general region as that occupied by the Toroweap cyclic deposits are others of somewhat similar character but of younger (Triassic) age. These are the repeated sequences of drab mudstone or claystone and gypsum (with calcium carbonate units lacking) that form the Shnabkaib Member of the Moenkopi Formation in the western part of northern Arizona and southern Utah (McKee, 1954, p. 47). The gypsum in this member occurs as beds, lenses, and concretionary layers that range in thickness from a few inches to at least 5 feet. In general, beds are regular in thickness and some can be traced a mile or more. The gypsum beds alternate with beds of olive-gray, gypsiferous, shaly mudstone and claystone (Fig. 1B). Such alternations of mudstone and gypsum are repeated many times in a stratigraphic sequence.
Because the Moenkopi cyclic deposits differ from those of the Toroweap in having no calcium carbonate unit in the sequence, they require a somewhat different interpretation of origin. The absence of limestone beds underlying the gypsum suggests either (1) that seawater entering ponds or lagoons where the gypsum formed lacked calcium carbonate, as these salts had precipitated out in advance; or (2) that the water containing calcium sulfate in solution was not from the sea but from streams deficient in calcium carbonate. The first explanation is essentially the "multiple basin hypothesis" of Branson (1915) and seems the more likely of the two because distribution of the Moenkopi evaporites was marginal to the sea on the west (McKee, 1954, p. 15-16) indicating that water, originally marine, probably was involved. The largely gray, olive, or yellow color of the closely associated shaly mudstones in each sequence indicates a reducing environment, probably beneath shallow water--in contrast to the oxidizing conditions represented by the red-brown but otherwise similar shaly mudstones elsewhere in the formation, which probably were of fluvial origin (p. 43).
Cyclic deposits of a different type, but also involving chemical sediments, form a narrow belt within the Kaibab Limestone in the eastern end of the Grand Canyon. Each cyclic sequence, where fully developed, consists of three units (from bottom to top): (1) sandy limestone largely barren of fossils, (2) fossiliferous sandy limestone, and (3) earthy bedded chert (Fig. 2). As typically exposed along the Bright Angel trail in Grand Canyon, each unit is 2 to 10 feet thick, the cyclic sequence being repeated at least 17 times in the middle member of the Kaibab. Eastward the limestone units extend for about 10 miles but are progressively more sandy until finally they are represented by calcareous sandstone. Westward from Bright Angel trail the limestone units are less sandy and within a few miles are largely free of detrital quartz grains. The bedded chert units are limited approximately to the area of sandy limestone. In sandstone to the east chert is absent and in the pure limestone to the west concretionary, rather than bedded, chert is present. Thus, a north-trending belt of cyclic deposits, 10 to 20 miles wide, extends across the eastern end of Grand Canyon (Fig. 3), parallel to what was the shore area of the Kaibab sea.
Figure 2--Schematic cross section of part of middle member of Kaibab Limestone near eastern end of Grand Canyon. B indicates brachiopod fauna, and M indicates molluscan fauna.
Figure 3--Generalized map showing facies distribution in lniddle member of Kaibab Limestone, Arizona and Utah.
The distribution of invertebrate fossils in the Kaibab Limestone shows a clear relation to the belt of bedded chert and sandy limestone just described. In the area along Bright Angel trail a normal marine fauna of brachiopods, corals, and bryozoans is represented in each cyclic sequence by a distinctive faunule, or combination of species (McKee, 1938, p. 90), whereas to the west in the pure limestone no such distinctive faunules occur and the entire fauna seems to be randomly distributed within the member. Eastward across the belt, the marine fauna is progressively more restricted both in species and number of individuals, and in very sandy parts it is absent; farther east in the sandstone a molluscan fauna, believed to be a brackish-water type, occurs. No fossils, so far as known, are common to both facies.
Interpretation of the cyclic deposits of the Kaibab Limestone must satisfactorily coincide with the interpretation of paleogeography, with the lithologic facies pattern, and with the faunal distribution pattern. The bedded chert, occurring in a narrow belt between areas of sandstone and limestone and being devoid of fossils, represents a sea environment, apparently unfavorable to life, in which water had become saturated with silica. The suggestion is made here that the sand facies with its fauna of probable brackish-water type to the east is evidence that streams may have entered the sea nearby and that these streams introduced sufficient silica periodically to form chert beds in an area where fresh water and salt water mingled. Furthermore, sea animals that lived in a normal marine environment to the west apparently were able from time to time to migrate into this adjoining belt and to flourish there during periods of calcium carbonate deposition. On the other hand, living conditions for these animals probably were intolerable during the times when silica was concentrated in the water and was accumulating on the sea floor.
Branson, E. B., 1915, Origin of thick salt and gypsum deposits: Geol. Soc. America Bull, v. 26, p. 231-232.
McKee, E. D., 1938, The environment and history of the Toroweap and Kaibab formations of northern Arizona and southern Utah: Carnegie Inst. Washington Pub. 492,268 p.
McKee, E. D., 1954, Stratigraphy and history of the Moenkopi formation of Triassic age: Geol Soc. America Mem. 61, 133 p.
Moore, R. C., 1931, Pennsylvanian cycles in the northern Mid-Continent region: Illinois Geol Survey Bull, v. 60, p. 247-257.
Wanless, H. R., and Shepard, J. P., 1936, Sea level and climatic changes related to late Paleozoic cycles: Geo. Soc. America Bull., v. 47, p. 1177-1206.
Weller, J. M., 1930, Cyclical sedimentation of the Pennsylvanian Period and its significance: Jour. Geology, v. 38, no. 2, p. 97-135.