|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 275-281|
New York University, New York, New York
In a review of depth-related aquatic environmental factors and sedimentary records of depositional depth, cyclic marine depth changes, modified by climate and presence of physical barriers in water up to approximately 60 feet deep, are defended as the major causes of most Kansas Wolfcampian cyclothems. Eustatic sea-level changes of 60 feet or less are not extraordinary or inexplicably large. Fusulinids in Kansas Pennsylvanian and Permian cyclothems may have lived in waters much shallower than 60 feet, and need not be interpreted as representing times of deepest water and maximum marine transgression.
In 1937 Elias postulated that the cyclothemic Upper Pennsylvanian and Lower Permian rocks of Kansas owe their cyclicity of lithofacies and biofacies to cyclic changes of water depth in seas less than 180 feet deep. Such epicontinental seas must have been spread over a broad, flat, "shelf-basin," in communication with the open ocean. Between 1937 and 1959 many geologists used these concepts to explain the sedimentary facies of most Pennsylvanian and Permian cyclothems in the Oklahoma-Kansas-Nebraska area.
The first major departure from Elias' type of interpretation was offered by Imbrie, Laporte, and Merriam (1959), who sought to explain some Lower Permian (Wolfcampian) cyclothems in terms of more constant depths and shallower waters. They stated that depth changes alone were insufficient to account for the cyclothemic facies, and assigned more importance to rates of "terrigenous influx," to turbulence (related to depth), and to shoals that impeded free-water circulation to and from the open ocean. That is, Imbrie, Laporte, and Merriam (1959, p. 78) developed their interpretations in terms of slight depth changes, and consequently "no major advances and retreats of the shoreline," in contrast to Elias (1937, p. 428), who emphasized depth change as the controlling factor, with attendant "advance and retreat of sea."
Another major difference of interpretation between Elias (1937) and Imbrie, Laporte, and Merriam (1959) concerns the depth of deposition of fusulinid-bearing layers in the cyclothems. Elias (1937, p. 410) ascribed depths of 160 to 180 feet to fusulinids. Imbrie, Laporte, and Merriam (1959, p. 78), and Laporte (1962, p. 540) asserted that fusulinids may represent deposition in water "less than 30 feet deep" and "closer to 50 feet," respectively, and Tasch (1957, p. 396) assigned them depths of 5 to 50 feet. Aside from revising his fusulinid depths to a minimum of 100 feet, Elias (personal communication) has not changed his view substantially since 1937 and continues to defend the concept that fusulinids are "indicators of maximal depths at culminations of marine invasions" (Elias, 1962, p. 114). In 1962 Elias buttressed his own arguments with writings by Rauser-Chernousova (1951) which offer fusulinid depth interpretations similar to his own. Reconsideration of the various data suggests to the writer that the 5 to 50 foot range is most probable, though it is true that other factors, singly or in combination, may have masked the sedimentary effects of depth. Nevertheless, support is offered for Elias' concept that depth of deposition was the major control of most cyclothemic deposition. It is noteworthy that a change of fusulinid depth interpretation does not challenge Elias' depth-control concept for the cyclothems as a whole. Only some changes in the depth curves attending his cyclothems are called for, at least to the extent that fusulinids may not coincide with deepest waters.
Because of the significance of depth in cyclothemic sedimentation, it was thought that this paper should provide a general review of depth-related environmental factors. For the benefit of students, mention of elementary depth-related factors is included, with discussion of diagnostic factors, in the paragraphs that follow.
Ultimate causes and mechanisms of cyclothemic depth changes are not discussed in this paper, although some combination of geodetic and eustatic control theories seems most probable. Useful bibliographies and short reviews of prevailing hypotheses for the origin of cyclothems were published by Lowman (1959) and Beerbower (1961). Fairbridge (1961, p. 107-114) provided a thorough review of hypotheses for shoreline displacement, which are applicable to cyclothem discussion. His geodetic figure (1961, p. 109) is especially noteworthy.
Sunlight is absorbed by water so that darkness prevails below depths of approximately 300 feet. Blue light penetrates farthest, and most light of the red end of the spectrum is absorbed in the top fathom.
Pressure increases with depth, and solubilities of chemical compounds in water commonly increase with pressure.
Direct agitation of water by open sea waves becomes negligible at depths below one wavelength (Russell and Macmillan, 1953, p. 26). In some shallow waters wavebase can be lowered sufficiently during storms for bottom sediments to be disturbed. In such areas much of the vertically oscillatory wave energy is converted to translatory motion toward the shore. The sheetlike motion of such water tends to smooth the surface (depositional interface) of shallow bottom sediments. After waves surge over the shore, the sheetlike seaward backwash also may smooth sediments. The offshore position at which waves break can be related to this depth-controlled backwash as well as to bottom roughness, so that the smoother the bottom the more rapid the backwash current that may interfere with the lower parts of incoming waves, causing them to break farther offshore. Conversely, rough bottoms may slow the backwash, allowing unbroken waves to advance farther inshore because of less interference with the advance of their lower extremities (Russell and Macmillan, 1953, p. 81-85). Tidal currents also exercise smoothing action on bottom sediments; more effectively in shallow water than in deep water.
In shallow aquatic sedimentary environments most chemical and physical activities depend on sunlight for fundamental energy. Water depth is, therefore, a simply observable but fundamentally important sedimentary environmental control because of its regulation of solar energy made available to the sedimentary environment. Absorbed light energy, especially from red and infrared wavelengths in the upper few feet of water, changes largely to heat, which accounts for thermal stratification of water bodies. Temperature-sensitive plants and animals may come to live in, or avoid, upper warm water layers, so that pelagic biotic stratification commonly parallels thermal stratification.
Most aquatic organisms flourish best in warm water. Most natural chemical reaction rates increase with increase of temperature (for each 10°C increase, reaction rate roughly doubles), so that most chemical activity, especially biochemical, occurs in near-surface or shallow waters.
Photosynthesis and other natural photochemical reactions, some functioning within specific wave lengths, account for the abundant plants which serve as food for profuse faunas in shallow waters. The carbon dioxide used by plants in photosynthesis during the day is directly related to acidity in water, so that diurnal changes of pH (and, less directly, Eh) tend to occur in the shallower water layers. The pH tends to rise during the day when CO2 is consumed and drop at night. Despite these tendencies the pH changes are only large in the littoral zone, and in the open ocean are rather small because of strong buffering by sea water. Carbonates and borates are important buffers in sea water. Clearly, the above-mentioned environmental factors function within complicated and frequently reversible interaction systems.
Where waters are shallow enough for bottom sediments to be reached by wave-induced turbulence, bottom roughness may be correlated with water depth. 1f poorly sorted large- and small-grained, unconsolidated sediments are available on the bottom, the fines will be periodically winnowed away by turbulent water and carried to quiet-water bottoms elsewhere. The lag concentrates that survive winnowing may constitute a very rough bottom. Clearly, bottom roughness, which contributes to bottom turbulence, can be both the cause and result of winnowing under variable conditions of water agitation. That is, sediment texture and bottom roughness depend considerably upon water depth in both source and depositional areas.
Agitated waters reduce light penetration mainly by causing multiple reflection and refraction of light from and in the uppermost water layers, and by taking sediments into suspension. Agitated circulating water is important to aquatic organisms, especially sessile forms, for it carries nutrients and oxygen to them and carries wastes away. Oxygen is added to sea water only in the upper sunlit layers by photosynthesis and by absorption of air at the surface. Small amounts of oxygen can be acquired from air bubbles caught up by waves, but wave agitation can also cause oxygen losses. Equilibrium partial pressure between ocean and atmosphere is thus produced rapidly, with respect to both oxygen and carbon dioxide (Fairbridge, 1963, personal communication).
Thus, water agitation, which depends in several ways upon depth, can control the rate of accumulation of sediments (1) directly, by regulation of transportation and deposition velocities of sedimentary particles, and (2) indirectly, by control of turbulence, erosion, and the nourishment of organisms whose remams ultimately become sedimentary particles.
Not only does photosynthesis, the most important photochemical activity, depend on light; it also governs light penetration toward the bottom by its control of the growth of rooted plants and phytoplankton, which sometimes become so numerous that they obstruct the passage of light in the same way as suspended mineral particles.
Profuse bottom vegetation, which depends directly on light penetration, can reduce erosive water agitation and serve as a filter trap for moving sediments, causing accumulation of sedimentary mounds (Ginsburg and Lowenstam, 1958).
The heat that comes from light penetration especially enhances the growth of bacteria, most of which prefer temperatures above 60° F. More than any other organic group, bacteria control the chemistry of diagenetic sedimentary environments, and the preservation or destruction of organic and mineral detritus. In this respect bacteria influence the rate of accumulation of sediments.
The hydrostatic pressure that attends water depth also controls organic osmotic activities at various levels, as well as mineral solubilities and compaction of accumulating sediments.
Rough bottoms offer hiding places for animals and plants that require them and hard stable objects for attachment of sessile organisms. Moreover, large rock blocks, together with benthonic plants or animals, can aid in filter-trapping of finer sediments.
In shallow marine sedimentary environments water depth, mainly by its control of light penetration, is the major control of a complex system of physical and chemical interactions which are partly self-regulatory. [Note: The growth of phytoplankton provides a good example of one type of self regulation operating within the system. somewhat as follows: Light penetrates water and warms it. Warm temperatures favor growth of phytoplankton, which multiply aod thrive. Their great numhers subsequently obstruct light penetration. This restricts their growth or sometimes cause sudden mass mortality. In the ensuing clearer waters the phytoplankton tend to grow again, and so on.]
In extremely shallow waters the rate and amount of sedimentary accumulation directly affects the depth of the water (see, Sedimentoeustasy, Fairbridge, 1961, p. 113). Unless the basin of accumulation subsides as rapidly as sediments accumulate, waters become progressively shallower until they are forced out entirely, or are partially impounded and then stagnate. Conversely, waters deepen and epicontinental basins usually enlarge if accumulation lags behind subsidence. The accumulating sediments tend to record these depth changes in their composition, biota, textures, sequence, and sedimentary structures; and in their regional facies patterns.
(a) Desiccation cracks and crack fillings, footprints, tracks and trails, record temporary withdrawal of shallow waters.
(b) Extensive lamination and small-scale cross-lamination, and scour and fill structures within very thin, uniform, broadly distributed beds, suggest shallow, flat, sea bottoms within reach of wavebase.
(c) Knife-sharp boundaries (and bedding planes) between successive rock layers that are extremely widespread, uniform, and thin, indicate sudden changes of depositional conditions uniformly over vast areas. Such widespread, sharp, and uniform sedimentary changes are best explained by sudden depth changes.
(d) Remarkably uniform thicknesses of several beds in a given widespread succession, all separated by sharp boundaries, help to substantiate the concept of repeated and sudden depth changes all across broad flat "shelf-basin" areas.
(e) Most calcareous oolites are deposited in very shallow hypersaline waters (Freeman, 1962) . Some form in turbulent water; others do not. Many form in water less than 1 fathom deep.
(f) Shallow marine banks accumulate where fine sediments are filter trapped by upright algae or other vegetation (Ginsburg and Lowenstam, 1958; Harbaugh, 1959, 1960).
(g) Calcareous algal buns and mounds accumulate in very shallow waters (Logan, 1961).
(a) Calcareous algae (both green and red), often associated with oolite laminae and calcareous banks, mounds and buns, are usually most numerous in warm waters less than 60 feet deep (Newell and Rigby, 1957). They abound in waters less than 2 or 3 fathoms deep, and in some intertidal areas (Logan, 1961). However, some red algae (e. g. Lithothamnion) can form calcareous banks at depths exceeding 100 feet, and as far north as the Arctic Ocean.
(b) Shelly faunas rich in bryozoans, pelecypods, crinoids, and solitary corals develop best in water less than approximately 60 feet deep.
(c) Lingula is mainly a littoral and shallow neritic genus favoring organic-rich muddy sediments and warm waters.
(d) Most living larger benthonic Foraminifera favor waters less than 60 or 70 feet deep, although they can survive at greater depths. Consequently, all interpretation based on depths is tenuous.
Most of the aforementioned sedimentary structures and biological features are common in the cyclothems enumerated by Elias (1937, p. 407). Typical examples are cited in Table 1. These and other more local features, taken together, lead the writer to believe that most Kansas Wolfcampian cyclothems were deposited in warm waters (> 700 F.) less than approximately 60 feet deep. Moreover, the evidence taken from successions of strata containing associations of the tabled sedimentary features support Elias' contention that cyclic changes of depth were the major cause of the widespread cyclothemic rock sequences. However, modern ecological observations give reason to suggest that Elias (1937) may have overestimated his maximum (180 feet) depths of deposition, which he based on fusulinids. Indeed, there seems to be no need for depths much greater than 60 or 70 feet to explain the cyclothems studied by Elias, from the evidence of analogous modern sediments. Fusulinids, if they may be compared at all with most modern larger foraminifers, might have lived at depths less than 60 feet.
Table 1--Examples of formations in Kansas Pennsylvanian and Permian marine cyclothems that contain useful sedimentary indications of their depositional depths.
|Desiccation cracks and crack fillings.||Roca Shale1|
|Tracks and trails.||Glenrock Limestone1|
|Small-scale and larger cross lamination within thin, extensive beds.||Johnson Shale1
|Knife-sharp boundaries between widespread, uniform, thin beds.||Glenrock Limestone-Bennett Shale contact1
Leavenworth Limestone-Heebner Shale contact2
|Uniform bed thickness over very broad areas.||Glenrock Limestone1
|Algal buns (some Cryptozoon like)||Howe Limestone1|
|Marine limestone "banks"||Bennett Shale contact1
|Concentrations of Lingula (commonly with Orbiculoidea)||Black shales in:
|1Wolfcampian. 2Virgilian. 3Missourian.|
Unfortunately, direct application of the principle of uniformity to fusulinid interpretation is precluded by the lack of a modern foraminifer that is closely similar. Only Borelis is crudely comparable to fusulinids, and its ecology is imperfectly known from only one dredging noted in records of the Challenger expedition. Consequently, all interpretations of fusulinid depths are tenuous at best, and can be based only on indirect evidence, mainly by way of fossil associates of fusulinids, such as calcareous algae, and by other lithostratigraphic data.
It is mentioned above that Elias (1962) derives support for his revised (greater than 100 feet) fusulinid interpretation from interpretations of Russian fusulinids by Rauser-Chernousova that are based on some dubious paleoecological criteria, and local stratigraphic evidence from a thick (up to 3,800 feet) rock sequence in the Pre-Urals of Russia. [Note: As an example, Mrs. Rauser-Chernousova (1951, in Elias 1959. p. 55) mentions "algal submarine banks deeper than 40-50 meters (120-150 feet)." Such algae, which might be comparable to some types of modern Lithothamnion, do not seem to he typical of the Kansas Wolfcampian biota. Moreover, a number of authors have observed that most present-day marine calcareous algae thrive at depths much less than 120 feet.] There seems to be no compelling reason why Rauser-Chernousova's views should refute depth interpretations of fusulinids by Imbrie, Laporte, and Merriam, Tasch, and McCrone in a cyclothemic sequence in far away Kansas. Indeed, the Kansas Wolfcampian cyclothemic basin may have been much shallower and flatter than the one in Russia. Regardless of the possibility that fusulinids might have survived in Russia at the depths suggested by Rauser-Chernousova, such depths simply may not have been available in Kansas.
In one of the cyclothems enumerated by Elias (1937) the writer found (in several widely separated outcrops of a very thin, widespread limestone) profuse inflated fusulinids in association with crustose calcareous algae whose living analogues certainly dwell in waters less than 60 feet deep. It was this and other types of paleoecological evidence that led Imbrie, Laporte, and Merriam (1959) , Laporte (1962), and McCrone (1963) to suggest much shallower depths than Elias (1937, 1962) for Kansas fusulinids and Wolfcampian cyclothems. In fact, McCrone (1963) suggests that many sediments in the Red Eagle cyclothem may have been deposited in water less than 2 or 3 fathoms deep. In the same cyclothem the fusulinids that are found, seemingly in place, with numerous crustose shallow-water calcareous algae, especially near ancient "shoal" areas, also may contradict Elias' (1962, p. 114) contentions that fusulinids represent deepest cyclothemic waters.
In short, there seems to be little need to invoke depths greater than 60 feet to explain most Kansas Wolfcampian cyclothemic facies; and in many respects Elias' (1937) 180-foot depth range could be simply compressed into a 60-foot range, with no serious damage to his cyclic depth change concept as a whole.
It has been suggested above (together with the depth criteria) that the Kansas Wolfcampian shelf-basin was extremely flat. It seems certain that even with slight water depth changes the shorelines of such a flat sea basin would have transgressed and regressed great distances, thereby controlling the flow of terrestrial detritus to the central part of the basin and contributing to vertical lithologic changes.
Imbrie, Laporte, and Merriam (1959) stressed the role of physical barriers ("shoals") that interfered with free circulation between the Kansas shelf-basin and the open sea, and thereby controlled facies patterns within the Beattie, and possibly other, cyclothems. However, they also acknowledged the influence of shoals within a major "regression-transgression-regression" pattern for the Beattie cyclothem as a whole, which also implies a measure of cyclic sea-level change. There is scant reason to doubt that such positive tectonic features as the Nemaha Anticline, did cause local shallowing of waters (shoals) during Wolfcampian time. In this connection, McCrone (1963) went so far as to postulate absence of one sedimentary unit in the Elmdale, Saffordville, and Alma areas of Kansas because of erosion or nondeposition over positive tectonic features that seem to be associated with the Nemaha Anticline.
Notwithstanding the evidence for local shoals within Wolfcampian cyclothems, the broad pattern of most Permian and Pennsylvanian cyclothems in the Midcontinent appears to be the result of remarkably uniform sedimentation over wide areas of essentially uniform water depth. This accounts for units such as the Glenrock (Permian) and Leavenworth (Pennsylvanian) Limestones, which are about 1 foot thick over thousands of square miles of outcrop and subsurface area. Clearly, the flatness of the basin floor at the time of their accumulation is well established, so that changes of sedimentation must have been felt almost simultaneously all across the flat shelfbasin. This may account for "knife-sharp" unit boundaries such as the Glenrock-Bennett and Leavenworth-Heebner (Table 1) contacts that record abrupt changes from calcareous to black mud deposition throughout broad areas. Whether water depth changed abruptly, or whether something else changed the sedimentation, might be open to some argument, but this writer believes that although depth changes were paramount, physical barriers or "shoals" may have been significant also.
The close relations between physical barriers, water levels, and sedimentary changes may be understood by means of a simple analogy--the breaching or overtopping of a levee by a river during flood. Admittedly the analogy is not perfectly transferable to cyclothems (because the postulated shoals may not have been complete barriers), but it illustrates the point that physical barriers such as those pictured by Imbrie, Laporte, and Merriam (1959) could exercise sudden and major control over sedimentation within a cyclothemic shelf-basin, and perhaps account for some of the knife-sharp lithologic boundaries. That is, with a slow, steady, rise of sea level, not much would happen within a temporarily barred basin until the barrier were breached. Following the moment of effective breaching, sudden sedimentary changes would ensue throughout the basin. Conversely, with lowering of sea level rapid sedimentary changes could follow the emergence of a physical barrier. The suddenness of the breaching or reemergence of a barrier would determine the scale of the sedimentary gradations that record such events, but in any event the change of depth would be the prime control of sedimentation.
The fact that much evidence points to depositional depths (and therefore relief of the postulated physical barriers or "shoals" as well) less than approximately 60 feet calls attention to the likelihood that very ordinary changes of sea level may have produced the remarkable vertical sedimentary changes recorded in the Wolfcampian cyclothems. Indeed, most students of eustasy seem to agree that 60-foot sea-level changes are not extraordinarily large, and could easily result from moderate glacial activity, or minor geodetic changes such as shifts in the inclination of the earth's axis, or wandering of the magnetic poles (Fairbridge, 1961, p. 109). Moreover, the weight of water trangressing the cyclothemic shelf-basin also might have contributed slightly to depression of the basin floor, but maximum depression would have occurred somewhat later than the first moment of maximum eustatic transgression.
In conclusion it must be added that changes of climate, with associated inwashing of more or less clastic sediments, also may have influenced cyclothemic sedimentary facies significantly. The basic sedimentary patterns and cyclicity, however, suggest that climatic influences were not directly so important as depth. Clearly, the benthonic sedimentary effects of external climatic factors would vary with height or depression of water levels and the attendant distance or nearness of shorelines, and the extent of poorly vegetated tidal flat exposure.
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