The history of cyclic sedimentation and the early signifi- cance of vertical profiles is discussed in Sect. 1.2.2. The importance of Walther’s law and of the vertical profile in modern facies analysis is discussed in Sect.3.4.1. Methods of statistical analysis of cyclic sedimentation are also described briefly in this section. The recognition of cyclic sequences has become one of the most widely used tools for reconstructing depositional environments in the subsurface. One of the reasons for this is that cyclic changes are com- monly readily recognizable in petrophysical logs and are often interpreted without access to core or well cuttings. Whether such interpretations are always correct is another question. The sedimentological literature is full of shorthand references to fining-upward or coarsening-upward cycles or to fining-and-thinning or coarsening-and-thickening upward. Grain size, bed thickness and scale of sedimentary structures are commonly correlated in clastic rocks, so that the cyclicity may be apparent from several types of obser- vation. French sedimentologists tend to use the terms posi- tive and negative cycles; dipmeter analysts recommend coding cyclic changes diagrammatically in red and blue, but I am embarrassed to have to admit that I am not sure if fining-upward cycles are positive and red or negative and blue or the opposite and (or) vice versa. Such terms are obviously not helpful if one cannot remember which way they are used, and a simple descriptive terminology seems preferable.
An important warning: cyclicity may be in the eye of the beholder. Many facies interpretations in sedimentology place considerable emphasis on the recognition of repeated, ordered vertical change in facies characteristics, and many well-known facies models have been built upon such inter- pretations. But, as Hiscott (1981) demonstrated, with respect to submarine fan deposits, the presence of cyclicity may be based on very subjective interpretations. Anderton (1995) argued that most submarine fan deposits are non-cyclic. He stated“the human brain is very good at pattern recognition.
It seldom misses a real pattern and can often see patterns in random noise. Thus, objective correlation tends to be over-correlation.” Wilkinson et al. (1998) took issue with the interpretations of cyclicity in certain shelf-carbonate suc- cessions, descriptions of which have formed the basis for some important studies in sequence stratigraphy. They argued that other natural, random processes can be demon- strated to generate very similar patterns. The solution may be to apply rigorous statistical tests to the data. Harper (1998) provided such tests for the examination of thickening- and thinning-upward patterns.
There are two common basic types of cycle, those indi- cating an increase in transport energy upward and those demonstrating a decrease. Both types can be caused by a variety of sedimentary, climatic and tectonic mechanisms. Beerbower (1964) divided these into autocyclic and allo- cycliccontrols. The terms autogenic and allogenic are now preferred, in order to avoid the connotation of cyclicity. Autocyclic mechanisms are those that result in the natural redistribution of energy within a depositional system. Examples include the meandering of a channel in a river, tidal creek, or submarine fan; subaerial flood events; sub- aqueous sediment gravity flows; channel switching on a subaerial or submarine fan or a delta (avulsion); storms; and tidal ebb and flood. All of these can potentially produce cyclic sequences. Allocyclic mechanisms are those in which change in the sedimentary system is generated by some external cause. Tectonic control of basin subsidence, sedi- ment supply, and paleoslope tilt, eustatic sea-level change, and climatic change, are the principal types of allocyclic mechanisms. These are large-scale basinal sedimentary controls. A sedimentary basin may be affected by several of these processes at the same time, so that it is not uncommon tofind that there are two or three scales of cyclicity nested in a vertical profile. Allocyclic cycles tend to be thicker and more widespread in their distribution than autocyclic cycles. The latter are generally formed only within the confines of the subenvironment affected by the particular autocyclic process. This assists the geologist in distinguishing and interpreting sedimentary cycles, a matter of some importance in the definition of various scales of sequences and parase- quences, but such interpretations may be far from easy.
Vertical profiles formed a crucial component of the first facies models, including the point-bar model of Bernard and Major (1963) and Allen (1964), the barrier island model of Bernard et al. (1959), and the Bouma (1962) turbidite sequence (Sects. 1.2.2, 1.2.4, and 1.2.7). They were the focus of a classic paper on facies models by Visher (1965), and their recognition in the subsurface has been covered in many articles and textbooks (e.g., Pirson1977; Fisher et al.
1969; Sneider et al.1978; Selley1979). They still provide an essential basis of many modern facies models (Reading
1996; James and Dalrymple 2010), because they provide a
simple way of synthesizing many different types of obser- vation, including all the environmental criteria described in this chapter.
Vertical profiles have found an important use in recent years in the definition and documentation of stratigraphic sequences. As sea-level rises and falls, coastal and shallow-marine depositional environments may migrate landward and seaward, respectively. The result is a corre- sponding vertical succession of facies assemblages recording the drowning and reestablishment of the various coastal environments. Coarsening-upward andfining-upward cycles may demonstrate the presence of deltaic, estuarine, or beach-barrier systems at the shoreline, depending on the local paleogeography. Shoaling-upward successions boun- ded by marineflooding surfaces have been termed parase- quencesby sequence stratigraphers, and the nature of their vertical stacking is an important component of sequence analysis (van Wagoner et al.1987; Catuneanu2006). Where younger successions extend successively further into the basin they are said to exhibit a progradational stacking pattern, and this is interpreted to indicate a rate of sediment input more rapid than the rate at which depositional space (accommodation) is being generated by basin subsidence or sea-level rise. Aggradational (vertically stacked) and ret- rogradational (backstepping) patterns indicate a balance between sediment delivery and accommodation, and a deficit in sediment delivery, respectively. The latter condition is particularly common during transgression (Sect. 5.3.1). Lithostratigraphic and chronostratigraphic techniques may be used to correlate sequences and their component facies successions across a basin in order to determine the regional history of sea-level change (Chaps.5and 6).
Coarsening-and-thickening-upward cycles are the most varied in their origins. Several distinct types are produced by coastal regression and progradation (lateral accretion), where there is a gradation from low-energy environments offshore to higher energy in the shoaling wave and intertidal zones. Examples are illustrated in Fig.3.58. Other types are formed where there is a steep slope and abundant sediment supply, and theflow system that develops over it attempts to grade itself to a balance byfilling in the basin margin with a wedge of sediment. Coarsening-upward cycles are formed under these circumstances by prograding submarine fans and alluvial fans, particularly where the relief is maintained or even increased by active tectonic uplift. Other examples of coarsening-upward cycles (not illustrated) are those pro- duced by crevasse splays in fluvial and deltaic settings, washover fans building from a barrier landward into a lagoon, and fluvioglacial sequences formed in front of an advancing continental ice sheet.
Fining-and-thinning-upward cycles commonly occur in fluvial environments as a result of lateral channel migration (point-bar succession) or vertical channel aggradation.
Alluvial fans may also show fining-upward cycles where they form under conditions of tectonic stability. These three types are shown in Fig. 3.59. Other examples are the tidal-creek point-bar and intertidal beach progradation sequences. Sediments deposited by catastrophic runoff events, including fluvial flash floods and debris flows and
many types of subaqueous sediment gravityflows, also show afining-upward character.
Many of these cycles are superficially similar, and it may require careful facies and paleocurrent studies to distinguish them. Information on lateral variability may be crucial but this may be difficult to obtain in the case of subsurface studies.
Fig. 3.59 Typical examples of thinning-and-fining-upward profiles from James and Dalrymple (2010); Vertical scales approximately the same; atidal-inlet channel-fill succession (Boyd2010, Fig. 23, p. 277); b tidal channel or tidal bar succession (Dalrymple2010c, Fig. 20, p. 213);
cmeandering-river point bar or channel-fill (Dalrymple2010b, Fig. 6, p. 64)
Fig. 3.58 Typical examples of thickening-and-coarsening-upward profiles from James and Dalrymple (2010). A,B,C: Examples of delta-front successions (Bhattacharya2010, Fig. 19, p. 246); D: Shoreface-offshore profile (Dalrymple2010a, Fig.5, p. 9)
For carbonate environments, less emphasis has tradi- tionally been placed on the vertical facies succession or profile, and more on the grain type, fauna, and structures of individual beds. Assemblages of such attributes are com- monly environmentally diagnostic, whereas in the case of siliciclastic sediments, much ambiguity may be attached to their interpretation, and such additional features as vertical profile and lateral facies relationships assume a greater importance. The range of environments in which carbonates are formed is much narrower than that of siliciclastics; they are mainly confined to shallow continental shelves, plat- forms, or banks and adjacent shorelines and continental margin environments. Yet enormous variability is apparent in these various settings, particularly in shallow-water and coastal regions, and this is another reason why standard vertical profile models have not become as popular as they have with clastic sedimentologists.
Ginsburg (1975), James (1984b) and Pratt (2010) dis-
cussed shoaling-upward succession formed in
shallow-subtidal to supratidal settings. These are common in
the ancient record, reflecting the fact that the rate of carbonate sedimentation is generally much greater than the rate of subsidence. Shallowing-up sequences therefore repeatedly build up to sea level and prograde seaward. Lateral shifts in the various subenvironments are common. Pratt (2010) offered three generalized sequences as models of vertical profiles that could develop under different cli- matic and energy conditions (Fig. 3.60). Ginsburg and Hardie (1975) and Ginsburg et al. (1977) developed an exposure index representing the percent of the year an environmental zone is exposed by low tides. By studying tide gauges and careful surveying of part of the modern Andros Island tidalflat, they were able to demonstrate that a variety of physical and organic sedimentary structures is each present over a surprisingly narrow tidal exposure zone. This idea has considerable potential for interpreting shoaling-upward sequences, as demonstrated by Smosna and Warshauer (1981).
Carbonate buildups or reefs may contain an internal cyclicity that is the result of upward reef growth. James and
Fig. 3.60 Examples of meter-scale shoaling-upward carbonate succession formed in shallow subtidal to intertidal environments (Pratt2010, Fig. 18, p. 416)
Bourque (1992) suggested that the vertical profile may show an upward transition from an initial pioneer or stabilization phase to colonization, diversification and domination phases, characterized by distinctive textures and faunas. James and Wood (2010) described the response of reefs to sea-level change in terms of keep-up, catch-up or give-up modes, reflecting the rate of accumulation relative to the rate of sea-level change. In practice, most ancient reefs are the product of numerous sedimentation episodes separated by diastems or disconformities, attesting to fluctuating water levels (e.g., Upper Devonian reefs of Alberta: Mountjoy
1980). Analysis of vertical profiles of repeated cyclic pat- terns may therefore not be very helpful for basin-analysis purposes, although such work may be useful for docu- menting small-scale patterns of reef growth (e.g., Wong and Oldershaw1980).
Deep-water carbonates comprise a variety of allochtho- nous, shelf-derived breccias and graded calcarenites, con- tourite calcarenites and hemipelagic mudstones, cut by numerous intraformational truncation (slide) surfaces (Cook and Enos 1977; McIlreath and James 1984; Coniglio and Dix 1992; Playton et al. 2010). Slope sedimentation is commonly most rapid during times of high sea level, when abundant carbonate material is being generated on the plat- form and shed from the margins, the process termed high- stand shedding (Schlager 1991). The lithofacies assemblages are distinctive, but variations in slope topog- raphy and the random occurrence of sediment gravityflows seem to preclude the development of any typical vertical profile. Organic stabilization and submarine cementation of carbonate particles probably prevent the development of carbonate submarine fans comparable to those formed by siliciclastic sediments, with their distinctive channel and lobe morphology and characteristic vertical profile.
Cyclic sequences are common in evaporite-bearing sedi- ments, reflecting a sensitive response of evaporite environ- ments to climatic change, brine level, or water chemistry. Vertical profile models are therefore of considerable use in environmental interpretation. One of the most well known of these is the coastal sabkha, based on studies of modern arid intertidal to supratidalflats on the south coast of the Persian Gulf (Shearman1966; Kinsman1969; Kendall1992,2010). Coastal progradation and growth of displacive nodular anhydrite results in a distinctive vertical profile that has been widely applied (indeed, overapplied) to ancient evaporite-bearing rocks. Kendall (1992) discussed variations in this profile model, reflecting differences in climate and water chemistry that arise in other coastal and playa lake margin settings.
As noted elsewhere, evaporites can occur in a variety of other lacustrine and hypersaline marine settings. They mimic many kinds of shallow- to deep-marine carbonate and sili- ciclastic facies, and a range of sedimentary criteria is
required to demonstrate origin. The vertical profile is only one of these, but may be useful particularly when examining subsurface deposits in cores. For example, sulphates that
accumulate below wave base commonly display a
millimeter-scale lamination interbedded with carbonate and organic matter and possibly including evaporitic sediment gravity flow deposits (Kendall1992). The latter may even display Bouma sequences (Schreiber et al. 1976). Shoaling-upward intertidal to supratidal cycles have been described by Schreiber et al. (1976) and Vai and Ricci-Lucchi (1977) in Messinian (Upper Miocene) evap- orites of the Mediterranean basin (Fig. 3.52). Caution is necessary in interpreting these cycles because they may not indicate a buildup or progradation under stable water levels, but instead they may be the product of brine evaporation and falling water levels. Many cycles of recharge and evapora- tion have been proposed for major evaporite basins such as the Mediterranean (Hsü et al.1973).
Lacustrine environments are in general characterized by a wide variety of vertical profiles, reflecting many cyclic processes involving changes in water level and water chemistry. Many of these contain a chemical-sediment component. Lakes are highly sensitive to climate change, and their sediments have, therefore, become important in the investigation of orbital forcing mechanisms (Miall 2010). van Houten (1964) described a shoaling-upward, coarsening-upward type of cycle in the Lockatong Forma- tion (Triassic) of New Jersey. The cycles are about 5 m thick and consist, in upward order, of black, pyritic mudstone, laminated dolomitic mudstone, and massive dolomitic mudstone with desiccation cracks and bioturbation. Chemi- cal cycles which have an upper member of analcime-rich mudstone are also present. The cycles are interpreted as the product of short-term climatic change, with differences between the two types of cycle reflecting a greater tendency toward humidity or aridity, respectively. Eugster and Hardie (1975) described transgressive-regressive playa-margin cycles in the oil-shale-bearing Green River Formation of the Rocky Mountains. Donovan (1975) erected five profile models for cycles that occur as a result of changes in lake level and fluvial-deltaic lake-margin progradation in the Devonian Orcadian Basin of northern Scotland. Numerous other examples could be quoted.
Much has been written on the recognition of cyclic sed- imentation from petrophysical logs (Fisher et al. 1969; Skipper 1976; Pirson 1977; Cant 1984, 1992). At present, the technique is best suited to the study of clastic cycles. As explained in Sect.2.4, the gamma ray, spontaneous poten- tial, and resistivity logs are sensitive indicators of sand-mud variations and so are ideally suited to the identification of fining- and coarsening-upward cycles. These appear as bell-shaped and funnel-shaped log curves, respectively, and various subtleties of environmental change may be
detected by observing the convexity or concavity of the curves, smooth versus serrated curves, the presence of nested cycles of different thicknesses, and so on. The technique has been referred to as log-shape analysis (Fig. 3.61). Fisher et al. (1969) published a series of typical profiles for coastal and marginal-marine clastic environments based on exam- ples from the Gulf Coast (Fig.3.62). Other examples from the Beaufort-Mackenzie Basin (from Young et al.1976) and elsewhere are illustrated in Figs.3.63,3.64,3.65and 3.66. These curves are commonly interpreted in the absence of cores or cuttings. As should be apparent from the preceding pages similar cycles may be produced in different environ- ments, and so this is a risky procedure. However, by paying close attention to appropriate facies models and scale con- siderations (cycle thickness, well spacing) good paleogeo- graphic reconstructions can be attempted. The availability of cores in a few crucial holes may make all the difference.
Figure 3.65 illustrates wireline logs showing the typical bell-shaped log profile, together with two examples. These are bothfluvial examples. Figure3.66a illustrates a variation on the channel-fill log shape, the so-called blocky log response, indicating a sand-fill with little vertical variation in
mud content or grain size. Finally, Fig.3.66c, d illustrates a log response (and example) of a clastic succession showing no cyclicity, in this case a suite of turbidite sandstones.