How to Use the Atlas
With the exception of this introductory section, the pictures in the atlas are labeled and numbered as plates. Each plate consists of one or more pictures related to the same subject, which gives it its title. A running head at the top of page recalls the type, or family, to which the example shown is assigned (e.g., erosional structures).
Types are defined in a broad way, and individual structures are not further classified or listed following a systematic order; rather, they are ordered in sequences that outline "routes" for reading. Starting with the basic or typical features, some variants are then shown to point out similarities and differences. Certain sequences zoom in the same object, or phenomenon, as in the case of stratification (from a wide panoramic view to a close-up of individual beds). In essence, an attempt is made here to interweave the threads of a series of topics, whereas the previous edition was organized more in the style of a deck of cards.
Because sedimentary features are found in a great variety of sizes, geologists usually place, within the picture frame, persons or objects to give an idea of the actual size or scale. For an average distance (middle scale), a geologist hammer is commonly used. in close-ups and hand specimens (small scale), a variety of objects, from a meter to a lens cap, a coin or a paper clip, may be used.
For readers to get a correct spatial perception of the structures shown in the photographs, which were shot from a variety of distances and visual angles, mostly in the field but also in the lab, something must be said about what the subjects are and how they show up:
Figure 1. Ripples on the sandy surface of a beach at low tide. These small-scale structures were formed by waves when the beach surface was under water (upper part of photo), and subsequently modified by the low tide. The ebb flow remolded the sand and made its own ripples, which have a different shape (asymmetrical) and orientation (from lower right to upper left) with respect to ripples produced by waves. Part of the surface was completely smoothed by shallowing water (upper right corner). Ripples are formed and canceled many times in a day or in a week; sometimes, but rarely, they can fossilize. Imagine that the sea level suddenly rises, submerging the beach surface to a depth out of reach for waves. The ripples will then be covered by mud and preserved. PHOTO G. C. PAREA 1970.
Figure 2. Lingering in the same place (more exactly, in the part of the beach that is normally emerged) and cutting a vertical section in the sand with a knife, one can observe the profile of several surfaces produced by sedimentation or erosion in recent times. Some of these surfaces are flat, others are wavy; the former reflect the smoothing effect of waves, the latter record partly preserved ripples. Darker and lighter bands indicate depositional laminae and sets of laminae (organic matter, iron minerals, or other peculiar grains are concentrated in dark laminae); their superposition underlines the vertical accretion of the beach.
The change in geometry of the buried surfaces suggest that the shoreline shifted a little during the time recorded by sedimentation, in such a way that the spot we are in remained alternately in submerged and emerged conditions. By comparing this photo with that of figure 1, you can see that the interfacial expression of ripples is much more evident than their cross-sectional view. Nevertheless, ripples can be recognized in section and this is important because scientists usually work on sections when studying Ancient sediments. PHOTO G. C. PAREA 1970.
Figure 3. Mold of a section to that of figure 2, dug in Recent sands of a Dutch tidal flat (North Sea). The laminated sand has been impregnated with resin, which has been taken away after drying. The structure shown is cross-lamination, changing in scale from the lower to the upper half. Small-scale cross-laminae reveal the formation and migration of ripples, while medium-scale laminae reflect similar but larger structures (subaqueous dunes). PHOTO A. COLELLA 1985.
Figure 4. Sediment cores collected in the Red Sea have been cut longitudinally to show the stratigraphy; the top of this composite section is to the upper right. Note the change from well-laminated (or thinly bedded) sediment in the upper portion to relatively homogeneous, structureless sediment. All these sediments are fine grained, and the bedding is caused by a different mechanism as compared with previous examples (fallout from a water column instead of lateral traction on the bottom). The dark layers have a high content in organic matter.
Cores are cylindrical, vertical samples wrapped by a plastic liner. Various types of sediment corers are in use; light corers (piston, gravity, etc.) are employed to sample surficial sediments of the sea bottom. They penetrate for some meters after being dropped from a boat. Hard rock cores are obtained by rotary drilling equipment that cross geologic formations hundreds of meters thick. PHOTO COURTESY OF INSTITUTE FOR MARINE GEOLOGY C.N.R. 1983.
Many outcrops are markedly bidimensional and can be called cross-sections. Sedimentary rocks can be recognized there because of their stratified, or bedded, character; visible lines represent the intersections between bedding planes and the plane of section. Stratification is not always apparent, and many outcrops look unbedded or poorly bedded. Except in arid zones, devoid of vegetation, outcrops are generally discontinuous and separated by areas where the rock is covered (by soil, detritus, plants, buildings, roads).
Bedding can be deformed or crushed by tectonic forces operating in the outer, rigid earth shell (crust and lithosphere) or in the deeper mantle; in the case of mild deformation, strata are simply tilted, or rotated around an horizontal axis, and appear inclined with respect to their original horizontal setting (figure 5). It is, in fact, assumed as a rule that depositional surfaces are flat and horizontal. A secondary inclination is, however, almost the norm for bedding as some tectonic stresses affect, or affected to a various degree in the past, every part of the earth's surface. In certain places, sediments are laid down on sloping surfaces, and the beds have a primary inclination. Secondary inclination must not be confused with the original one, which is also called clinostratification, or clinoforms.
Figure 5. How sole marks are formed and preserved (as molds). They can be used for way-up recognition in stratigraphy: 1. bottom erosion by a current; 2. deposition starts; 3. burial (several beds); 4. diagenesis (cementation); 5. tilting and emergence: 5a. overturning and emergence; 6, 6a. weathering and selective erosion. FROM RICCI LUCCHI 1970.
This tells us that the study of Ancient sediments cannot be done apart from fundamentals of geology: sedimentary rocks are just a type of rocks, which can have been involved in various geologic processes, and experienced many vicissitudes after their formation.
A knowledge of the principles and rules of stratigraphy is especially important for correctly reading and interpreting sedimentary phenomena. Beside the assumption of original horizontality of strata, the principle of superposition (beds at the bottom of a pile or succession are older than those at the top) and that of intersection (features and objects that cross or truncate others are younger: see, for instance, fractures versus bedding in figure 6) must be taken into account. Moreover, there are way-up criteria to establish whether beds are in normal position or upside down (figure 5) because of tectonic movements and rotations. Every time we geologists observe inclined strata, we should ask ourselves: are these beds upright or overturned? In other words: are they inclined less or more than 90° from their original position? The answer can be provided, in many cases, by sedimentary structures, if they are present within the beds or on their surfaces (bedding planes). If we are able to identify the structures, their shape and orientation give us the vertical orientation, or stratigraphic polarity, of the beds.
Figure 6. A sandstone bed in vertical position. The photo has been taken from above (it crops out on a littoral terrace); the original top is recognizable by the finer grain size and the presence of laminae. See network of fractures (secondary structures) almost at right angle with bedding (primary structure).
The way-up criterion works like this: in the normal position, the lower bedding plane, or base of a bed, faces downward, no matter how much it is inclined; if the position is inverted, the base faces upward. It is thus fundamental to determine whether an exposed bedding plane is the base or the top of a bed. If you look again at figure 5, and compare frames 5 and 5a (or 6 and 6a), this point will be made clear. Notice also, in the same example, that you are not observing the original structures, but molds or casts of them: depressions formed at the top of a bed appear as relieves at the base of the overlying bed, and vice versa. What is preserved is often the filling of a scour, not the scour itself. This depends on the different behavior of sediments (for example, sands interbedded with muds), when they are compacted, cemented and, later on, weathered and eroded. Remember, then, that it would be wrong to infer the type of structure from its present position and the orientation of the bed in which it is found. The right procedure is just the opposite: the original morphology and orientation of the structure must be recognized first, then the bed orientation can be inferred.
To understand a structure, one should see where, when and how it is formed in Modern environments, or try to reproduce it with experiments in controlled conditions (laboratories, flumes, wind tunnels, and so on). The implication is that one relies on the fundamental principle of uniformity, or actualism ("the present is the key to the past"), which is a guide for all branches of geology. That is not an absolute rule, and should not be applied too rigidly. Processes active today were certainly active in the geological past, but their intensity or duration may have changed. When the vegetation did not exist on land surfaces, the rate of erosion was higher because the bare ground had no protection from wind or flowing water. When the atmosphere contained little or no oxygen, the chemical alteration of rocks and the decomposition of organic matter, which influence the rock cycle and sedimentary processes, were different. With the evolution of the biosphere, the interactions between organisms and abiotic components of the environment, including sediments, became more and more intricate.
Other examples could be quoted, but these are sufficient to show that the rate of change in earth systems has not been uniform, and that their present condition is the result of their evolution in time. In some respects, science says that the past is the key to the present. It can be said geologists, endowed with imagination, ingenuity and spirit of observation, were able to infer, from the rocks themselves, processes that were detected or proven only later: catastrophic floods due to collapsing of massive ice dams (e.g., Lake Missoula in the western United States), desiccation of an entire sea (the Mediterranean at the end of Miocene), and turbidity currents and large submarine slides (from the observation of bedding and structures in sandstones of the Alpine fold belts).
Thus, we geologists are all uniformitarian if that means that "past geological events can be explained by phenomena and forces observable today" (AGI Glossary, 1987), as the basic laws of nature have not changed since the formation of our planet. We realize, however, that: 1) specific processes, which are a particular expression of these laws, have changed, especially in the biological and biochemical domains; 2) a certain dose of "catastrophism," the old archrival of actualism, must be admitted as far as natural phenomena are concerned.
The last point implies that actualism does not mean "every day" or "every year" events only; there are so-called rare events occurring sporadically, without a regular periodicity, which deliver enormous amounts of energy over restricted areas and during short-time intervals. These events, which are also called "catastrophic," recur over time spans that can exceed a human life span, several generations, or the whole human history. Large fluvial floods, tsunami waves, earthquakes of large magnitude, and exceptional volcanic eruptions are all cases in point. Some of these had a tremendous impact on early civilizations, and originated legends and myths such as the submergence of Atlantis or the Great Deluge. The human mind tends to record these natural events as unique and to charge them with meanings and purposes known only to God.
Modern statistics tell us that the rarer the events, the bigger they are; in other words, there is an inverse relationship between their magnitude and their frequency (Poisson's rule). That is why no human beings witnessed the collision of a large meteorite or comet body (but dinosaurs did, 65 million years ago). If the average recurrence time of such an event is, for example, 10 or 20 million years, should science say that it represents an actualistic phenomenon or not? By applying the concept too literally, one would be induced to consider as actualistic only the scattered rain of micrometeorites that fall continuously on Earth (see figure 13). On the other hand, there is a probability, however small, that a celestial body of considerable size will impact on our planet this year or next year: several near misses have already been reported in the last decades. It is, therefore, more reasonable to encompass all natural events and processes in a uniformitarian view of the world.
After this long digression occasioned by the problems of interpretation of Ancient sediments, I come back to more practical items by considering the last two points:
Sedimentary Structures: Preliminary Remarks
Deciphering the visible marks left by natural processes on rocks and sediments is a fascinating part of a geologist's work; physical processes such as currents and waves are responsible for most of these structures, but chemical reactions and biological activity also contribute. So, even where body fossils of organisms (shells, bones, plant remains) are lacking, the traces of their past existence can be preserved in sediments as a disturbance caused by their passage, metabolism, rest, or rooting. All these marks and traces can be encompassed under the phrase sedimentary structure; they consist of objects and forms that are produced by sedimentary processes and are preserved in rocks. That structures can fossilize or, in other words, have a preservation potential, make them interesting to geologists. Some structures are very decorative and can attract the attention of a passer-by, who perhaps attributes them to some prank of nature. Even among scientists, not many years ago many structures were called "hieroglyphs."
Apart from some ingenious intuitions expressed by Leonardo da Vinci, serious attempts to understand the meaning of structures were not undertaken until the nineteenth century when eminent geologists like C. Lyell, H. C. Sorby, and G. K. Gilbert realized that these morphological features of sediments and sedimentary rocks could offer clues to their origin. This in turn could contribute to finding useful materials and resources associated with sediments, such as coal.
A great impetus to the study of sediments and the founding of sedimentology as a special branch of earth sciences, came from the growing importance, after 1930, of the oil industry and mass motorization. Hydrocarbons are intimately associated with both our life and sediments: their source is a sediment rich in organic matter, and their ultimate repository, or reservoir, is made of porous sedimentary bodies. Structures are useful for understanding the environment and the tectonic setting in which sediments are deposited and transformed into rocks, coal, or oil. Sedimentary processes are responsible for deposition, diagenetic processes for post-depositional transformations.
To make the best use of structures as a tool, both their potentialities and their limitations must be known. Among the many structures produced in sediments, only a few are bound to preservation; most of them are canceled by the same sedimentary processes, or by prolonged exposure to weathering and erosion. The surface of a beach, for example, is a sort of palimpsest (figures 1 and 2): in the emerged part, ripples produced by wind or the tracks of our feet are flattened by high tides or cleaners; in the submerged parts, waves continuously remold the sandy bottom. In general, a newly deposited sediment is subject to remobilization by various agents, especially by cataclysmic events such as storms or gravity slides. Only when it is buried under other sediments, can it preserve its structures. How many and which structures will be found in the stratigraphic record depends not on a single event but on the whole history of the sediment, including diagenetic changes (diagenesis sometimes enhances the structures but in other cases efface them). This history can be synthesized in the following steps: primary deposition, remobilization, redeposition, burial. The loop remobilization-resedimentation can be repeated many times before arriving at the definite emplacement. When the loop is not activated, and deposition is immediately followed by burial, the chances of preservation of primary structures are maximized.
After burial, tectonic processes can add to diagenesis in modifying sediments and their structures, which react to tectonic stresses with various types of deformation. A brittle behavior is typical of an indurated, rigid sediment; it is dismembered into blocks or slices by ruptures and faults: within the blocks, no deformation occurs. A ductile behavior, often but improperly called "plastic," means that the sediment is still relatively soft (because of its fine grain size and water content) or was mollified by high temperatures and pressures. Folds and shear planes represent the deformation in this case. Shear planes can be discrete and far apart or closely spaced; in the latter case, cleavage is produced in the rock. Cleavage and small-scale folding are an example of pervasive deformation; an analog for brittle materials is given by comminute fragmentation, or brecciation. In pervasively deformed sediments, sedimentary structures can be completely obliterated and replaced by tectonic structures. In certain cases, small- and medium-scale tectonic structures mimic the sedimentary ones, and attention must be paid to avoid confusion (see, for instance, plates 177 and 178).
The static pressure due to the weight of overlying materials contributes to the physical aspect of diagenesis in buried sediments: water is squeezed out from pores, the solid particles come closer, and the sediment volume decreases. Thus the sediment is compacted; three-dimensional structures are flattened and squashed because of compaction. The chemical aspects of diagenesis include exchanges of matter and energy between sediment particles and interstitial fluids: the result may be dissolution of some materials, changes of composition, formation of new minerals, hydration or dehydration, cementation by salts deposited in pores and cavities. Overall, diagenetic processes tend to lithify a sediment, or transform it into a rock (somebody speaks of lithogenesis). Buried sediments can also be affected by heat sources, such as magma bodies, and be "cooked" or metamorphosed. The boundary between diagenesis, as part of the sedimentary or surficial processes, and metamorphism, is often subtle because temperature and pressure increase gradually with depth in the subsurface.
Not all of diagenesis is done underground, however. A fall in sea level or a crustal uplift may bring marine sediments, still uncompacted or unlithified, in the emerged domain, where erosional processes predominate and can exhume them. Materials escaping erosion may then undergo a subaerial form of diagenesis, consisting of dissolution and/or cementation. This vadose diagenesis is caused by drastic chemical changes in the sediment pore fluids, when meteoric water (i.e., slightly modified rainwater) replaces saltwater, and is transitional to soil formation (pedogenesis).
Bedding surfaces represent discontinuities in sedimentation; before diagenesis, they are not so obvious and the beds stick together; the effect of diagenetic processes (compaction, differential cementation, etc.) is to enhance the discontinuities as contrasts in physical properties. Beds can thus be separated by both natural and artificial causes; moreover, they send a more distinct signal when penetrated by acoustic waves. In that case, buried beds are (seismic) reflectors.
Several structures may be found on exposed bedding surfaces, which is no surprise because all these surfaces were, before burial, part of the bottoms of rivers, lakes, and seas (figure 7). In essence, they were depositional interfaces, either subaerial (between air and sediment) or subaqueous (between water and sediment).
Figure 7. Orthogonal sections of beds to show different views of sedimentary structures. In a,traces and burrows of organisms and a basal scour fill; in b and c, ripples, cross-laminae (foreset), and sole marks.
Structures present on bedding planes are also visible as lines in sections and trenches; the same object can consequently show us an interfacial or a transfacial (cross-sectional ) view. Remember that both are an expression of the same object. Ripple marks are among the most typical interfacial structures at the top of beds, as shown by figures 1 and 2; structures and traces occurring at the base of hard beds (generally as molds) are collectively called sole marks, or basal structures, and may show a profusion of shapes.
Other depositional surfaces can form inside the beds when sedimentation is not continuous but occurs in steps or pulses; laminae can thus be recognized. A bed can be entirely laminated or only in part; in the latter case, laminated is distinguished from nonlaminated, or structureless, portions. These portions are called intervals, horizons, or divisions. The effects of diagenesis are felt also by all surfaces internal to beds (intrastratal ), which become surfaces of weakness; laminated portions can thus be detached from the others, or split into individual laminae and sets of laminae. The rock is called fissile, or flaggy. Minor sedimentary structures can be observed on intrastratal surfaces, or parting planes.
Finally, diagenesis can produce structures of its own within sediments, e.g., concretions, nodules, cavity linings, or fillings. These structures are called secondary, or diagenetic, to distinguish them from the primary, or depositional ones. Secondary structures are discussed in this book, but selectively; only a few examples are shown. The reason is twofold: first, secondary structures are much more specific of a certain material, locality, or geologic unit than primary structures, and so are not easily amenable to categories. Only a few general types can be recognized. Second, they tell us less about the original environment of deposition, whereas primary structures also play the primary role in this respect.
As said before, both bedding planes (that delimit individual beds) and parting planes (that subdivide beds into constituent parts or laminae) are physical discontinuities. How, then, does one tell the one sort from the other? Is the distinction arbitrary (bedding surfaces separate beds by definition) or real? How is it applicable in outcrops and cores? To answer these questions, the concept of hierarchy must be introduced. Discontinuities in sedimentation can be arranged in order of importance according to the duration of interruption in deposition, the amount of erosion or dissolution that occurred along them, the evenness of the discontinuity surface, and other criteria.
In principle, bedding surfaces mark longer interruptions than lamination surfaces do, and are thus of higher rank; this should be reflected in their being more sharply defined in the field. Unfortunately, such is not always the case. A bedding surface separating two beds of the same lithology can be more subdued than the surface separating two laminae of slightly different material. It thus seems that some subjectivity inevitably colors any attempt to recognize a hierarchical order among objects that involves some degree of abstraction. If you are on a trip with a geological party, do not wait too long to test this statement: ask everybody, in front of a suitable outcrop, to make a distinction between groups of beds, individual beds, and bed portions, and to sketch them in notebooks. Repeat the test at several outcrops, and note how frequent are the disagreements between "observations" (or, better, observers).
In spite of this limitation, let us expand a little more on the matter of bedding hierarchy. The bed (layer, stratum) is our essential reference. It represents the base line for both stratigraphy (where it is the building stone of stratigraphic successions or sequences recognized. Second, they tell us less about the original environmen) and sedimentology (it is a container of sedimentological features, including structures, and a framework for their observation). In classical stratigraphy, the bed has two characterizations: one is geometrical, the other lithological. In terms of geometry, the bed is a three-dimensional object in which two dimensions prevail on the third one (thickness); on the other hand, it is formed by a definite rock type, which implies that changes of lithology occur along bedding planes. Though a geologist's eye may be struck, in different circumstances, more by one or the other of the two aspects (the container or the content), lithology is commonly privileged in descriptions. In a succession where, for instance, sandstones alternate with shales (figure 8A and B) or with conglomerates (figure 9), geologists speak of sandstone beds and shale (or conglomerate) beds.
Figure 8. Definition of stratification units (bed and layer ) in columnar sections of clastic sediments; numbers refer to beds, lowercase letters to internal subdivisions (when present) characterized by different structures. A and B, alternating sandstone and shale beds. C, sequence of amalgamated pebbly sandstone beds. Bedding surfaces are barely visible in C and do not constitute physical discontinuities (e.g., a barrier to vertical migration of fluids) as in A and B.
Figure 9. A set of amalgamated beds in Ancient fluvial deposits (Devonian Old Red Sandstone, Midland Valley, Scotland). Sandstones and conglomerates alternate vertically and, in part, laterally. Beds are roughly parallel but lenticular and wedge-shaped in detail. The hierarchical relation between bed and laminae can be appreciated here. The central bed of conglomerate, for instance, is subdivided by diagonal lines into crude, thick laminae. The laminae form a single set extending to the whole bed. The underlying bed of conglomeratic sandstone is composed of several sets of cross-laminae cutting each other, i.e., a coset. The generic term cross-bedding, or cross-stratification, can be applied to it (to indicate the style of bedding but not its hierarchical organization). PHOTO G. G. ORI 1992.
Sedimentologists, however, have introduced a genetic concept of bed and bedding based on interpretation of observable features like geometry, lithology, textures, structures, fabrics, and fossil content. In synthesis, these features constitute the facies of a bed or a group of beds. Facies is also a classic, albeit disputed, concept of stratigraphy, and has been continually refined with the progress of knowledge about sedimentary processes in Modern environments. Today scientists know a lot about these processes, whereby the definition of beds can lie on them, i.e., on the mode of bed emplacement. In this respect, a bed becomes an episode, an event of sedimentation: it is finite both in space (areal extent and thickness) and time (deposition occurs between two pauses). Its limits may coincide with lithological changes but are not defined by them; it is the process of emplacement, not the materials emplaced, that defines the base and top of a bed. Referring to figure 8, suppose that the process is represented by turbidity currents; these flows transport suspended particles of different size, which are stirred and mixed by turbulence. When the flow slows down, the particles settle in order of decreasing size and weight, forming a vertically sorted deposit, i.e., a graded bed. Sand is concentrated at the base, mud at the top; the lower part of the bed is sandy, the upper one is muddy. Diagenesis transforms them into sandstone and shale or mudstone, respectively.
The alternative view is to consider two distinct beds, one made of sand and the other of mud. Which is the best view? By applying the lithologic criterion, you have two beds for each sedimentary event; with the sedimentological method, there is only one bed, whose lithology changes gradually from base to top.
One can wonder whether this is a relevant problem, or a pure question of nomenclature. The important point here, even for practical purposes, is to know whether a sedimentary body was made by one or more events. If we know what the case is, we can more easily make predictions about its lateral changes of geometry and lithology, and give a more correct interpretation of its environment of deposition.
Assume, for example, that all mud drapes over sand beds were eroded prior to deposition of every next sand; what you eventually get is a monolithic body of sandstone made of similar, welded beds. If you trace it laterally, you can see shaly intercalations between sandstone beds appear and grow thicker and thicker; this is because you go from the place where all mud was eroded to that where the same mud was deposited. By comparing directly the successions at the two ends in lithological terms only, one would say that the sandy sequence is made of just one sandstone bed, the other of many sandstone beds. However, the number of depositional events is the same: only their mode of preservation changes from place to place.
The welding of beds of the same lithology requires that one looks at details and uses all criteria of facies differentiation to recognize bedding: every new bed can be marked, for example, by a change in grain size, or porosity, or cementation. This is not necessarily so, however: different beds can be made of the same materials, and be virtually indistinguishable. The phrase more commonly used for a pile of monolithic beds, usually separated by erosional surfaces, is amalgamated beds.
To avoid confusion in describing and commenting on the illustrations in this atlas, the following convention was adopted: when I speak of individual objects, bed is used in a lithologic or otherwise generic meaning, layer in the sedimentological meaning. The collective terms bedding, stratification, and layering are used interchangeably. A layer often corresponds to a lithological couple or, less commonly, a triplet; a classical turbidite, or a tidal layer (a deposit made by the waxing and waning of a tidal current), is thus composed of a sandy bed and a shaly bed.
A hierarchical classification of bedding has been set up by Campbell (1967); it has stood the test of time and is followed here. A bedset is the local, vertical expression of a sedimentary body made of several beds, or layers; when not otherwise specified, a sedimentary body is meant as a multilayered unit, i.e., a bundle or sequence of depositional events. If, observing the set from the bottom up, you notice a certain order, a kind of rule in the succession of strata, the set can be called a sequence or a sedimentary cycle (there is no well-defined and agreed-upon difference between these terms). A sequence can be defined by one or more parameters: if the thickness of individual beds is used, the sequence can be qualified as thinning-up or thickening-up (figure 10); if the grain size is preferred, analogous terms are fining-up (figure 11) and coarsening-up (figure 12). The combination of two bedsets with opposite trends gives a symmetrical sequence (-/+ or +/-; see figure 10), with an obvious cyclical pattern. On a descriptive basis, the asymmetrical bedsets could be termed sequences (upper row of figure 10), the symmetrical ones termed cycles.
Figure 10. Bedding sequences and facies sequences (facies are represented by types of beds) are bedsets with a certain vertical order within them. This vertical trend is shown by a single parameter (bed thickness, grain size) or more. Facies characters can be synthesized by particular indexes (e.g., ratios between parameters: sand to mud, clastic versus nonclastic components, etc.), which can also be useful in revealing trends. Here, the thinning-up trend is symbolized by a plus (+) ("positive" sequence), the thickening-up trend by a minus (-) ("negative" sequence).
Figure 11. A bedding sequence defined by grain size (fining-up, = FU) in alluvial sediments: it starts from coarse gravel, ends with silty sand ("dirty sand") and is capped by a soil horizon. Beds are amalgamated and almost undistinguishable in the lower part. Photo taken in quarried deposits of the river Reno near Bologna. PHOTO G. G. ORI 1992.
Figure 12.An example of thickening and coarsening-up ("negative") sequence. The story is complicated here by the occurrence of nonclastic deposits at the top (evaporitic gypsum in large crystals). Gypsum is present also in the middle but is detrital; thin beds at the base of the sequence are siliciclastic (siltstones and fine sandstones). Dome-shaped deformational structures are present at the top. This sequence is one of seven horizons of "Upper Gypsum" in the Messinian of western Sicily.
What you observe, however, is not necessarily a continuous record of sedimentation, but what is preserved of it; a complete cycle could have been deposited, then erosion took away the upper half. In this way, an asymmetrical sequence can actually record a truly cyclical phenomenon. This is probably the main reason why the difference between sequence and cycle has never been convincingly codified, regulated, and accepted by the geological community (in spite of many attempts to do so, "sequence stratigraphy" being the last and most comprehensive "legal" effort). Some ambiguity also exists because these terms are employed for both the real thing (the beds, the rocks) and the abstract concept (implying the controlling process or the geologic age, i.e., the time span during which the beds were deposited: eustatic cycles, tectonic cycles, lower Cretaceous cycles, etc.). Perhaps, it is more precise to use them as adjectives: sequential bedsets, sequential arrangement of beds, cyclical bedsets, and so on. Asequential bodies, by contrast, are those devoid of an evident vertical order in bedding; in them, changes of bed thickness, lithology or grain size appear to be random, at least in the field (subsequent processing of data by statistical methods can reveal a hidden cyclicity).
Sedimentary beds, layers, and strata cannot be described in a purely qualitative way--a specification of thickness, for instance, is required. This can be done by direct measurement or by referring to classes of thickness (see table 1); a scale, in centimeters or meters, must be chosen.
|Table 1. Thickness scale for beds
(layers and Laminae (in cm)
|Beds and Layers||Laminae|
|>300:||extremely thick a||3-10:||very thick b|
|100-300:||very thick c||1-3:||very thick|
|a Avoid the prefix mega
if you do not now the overall volume of beds; the estimate the volume,
you must have an idea about the areal extent of the bed and its thickness
changes. Do not judge the size of a bed from on outcrop!
b Use the term "faintly laminated," "diffuse lamination," "banding."
c In a certain usage, beds thicker than 1 meter are qualified as massive: this term is not recommended as it is also used to indicate beds devoid of internal structures.
d Many authors, following McKee and Weir (1953), put a lower limit of 1 cm to beds; units thicker than 1 cm are called laminae. Here, instead, a genetic-hierarchical criterion is adopted for distinction (a lamina is sort of a bed, i.e., a subvent or elementary unit.)
Parting surfaces within beds, although subordinate in importance to bedding planes, are sometimes sharp-cut and look like them; the distinction is not always easy, especially for beginners; some practice is needed to get a certain familiarity with these phenomena. Experience tells that the more you learn about sedimentary structures, the less difficult it is to understand bedding. A bed can be completely structureless or structured; in the latter case, structures appear in the form of laminae of variable geometry. Lamination can occur throughout the whole bed or be localized in laminated intervals; one or more of these intervals, or divisions, can exist within a bed. A single laminated interval often appears in the upper portion of a sandstone bed (figure 6), which is a useful way-up criterion where the strata are strongly inclined or subvertical.
In between the individual lamina and the laminated interval, there is an intermediate rank, occupied by the laminaset; this is a genetic package of laminae that is, anyway, distinguishable from adjacent ones on a reasonably objective basis (for example, by scour surfaces). The genetic link among laminae in a set is demonstrated chiefly by their parallelism, or conformity, which records an episode of continuous sedimentation. A single set of laminae can form the whole laminated interval (see figure 8.B2); otherwise, it consists of bundles of sets (cosets: figure 8.A1; figure 9, lower half).
Are there lower and upper size limits to sedimentary structures? The matter is not yet settled, and at the moment precise limits are not established. Ordinary structures are macroscopic characters, which means that they are recognizable with the naked eye. Morphological features that can be observed with the help of a hand lens or a microscope (for example, on the surface or interior of sedimentary particles: see figure 13) are more properly called microstructures. Only a passing mention is here made of them as they are the object of other kinds of atlases (thin sections of rocks, SEM images).
Figure 13. Magnetic spherules (micrometeorites) in a SEM (scanning electron microscope) image. Microstructures, related to melting, corrosion and rupture, are visible on their surface. The spherules were extracted from a pelagic rock of Mesozoic age. Meteorites of very small size survive the encounter with the terrestrial atmosphere better than larger ones do. They continually fall all over Earth but are widely dispersed and deteriorate. A concentration of micrometeorites can occur in pelagic sediments because of their slow rate of deposition. From Castellarin and others 1974; see plate 171.
Half-way between micro- and macrostructures one can consider the laminae, thin sheets of sediment that are often less than 1 mm thick (paper-thin). Laminae are the smallest sedimentation units; they represent an almost continuous deposition, interrupted only for some instants or just varying in rate.
As for the upper end of the size spectrum, no stakes have been posted. To get an idea of the largest objects to be found, imagine parachuting to a sandy desert (an erg: see plate 24). From the plane, before the launch, you see a rippled surface, quite similar to that you tread upon in a beach or tidal flat. The scale, however, is different. The relief, for example, is in the order of meters or tens of meters instead of centimeters, as you can see from the ground. This wavy topography, extended over wide areas, reflects the shapes of eolian dunes. Dunes are made of well-sorted fine sand (about the size classification of sedimentary particles, see table 2). The sand is transported and accumulated by the wind, whose power and persistence determine the dune size. On the upwind or stoss side of a dune, the sand grains are in transit, because a high shear stress is applied there and favors erosion or nondeposition; deposition occurs on the downwind or lee side, the steeper one. Overall, the dune surface is a depositional surface as the net result of the wind action is deposition, not erosion.
|Table 2. Grain size of sedimentary
particles: the Udden-Wenworth scale
|Size classes (mm)||Components||Aggregate|
|>256||cobbles and boulders||rubble|
|2-1||sand grains||very coarse sand|
|1-1/2||sand grains||coarse sand|
|1/2-1/4||sand grains||medium sand|
|1/4-1/8||sand grains||very fine sand|
|<1/256||clay particles (minerals)||clay|
Generally speaking, the morphology of sediment-fluid interfaces is ascribed to one or more types of sedimentary structures, in particular to so-called bed forms. In fluid dynamics language, the topography of a depositional surface is called roughness: it influences the behavior of a fluid moving over it and, at the same time, is influenced by the flow. But dunes are also a form of terrestrial relief, i.e., geomorphic features. Dunes are thus studied by sedimentologists, hydraulic engineers, and geomorphologists. Moreover, dunes have a considerable mass and thus can be regarded as sedimentary bodies: if you dig a trench in one of them, you will see several superposed beds. Sedimentary bodies are mappable units and are studied by stratigraphers.
In conclusion, what is a dune: a sedimentary structure? a geomorphic unit? a stratigraphic unit? It can actually fall into all these categories: it depends on the approach one chooses. The tendency to pigeonhole natural objects is in our brain and education, not in nature. In the academic world, disciplines and specialization proliferate and, consequently, it is normal that the same objects are contended between different specialists. No doubt, anyway, that dunes are sedimentary structures, among other things: they are formed by the same processes that produce the much smaller ripples. From the genetic point of view, there is only a difference of scale.
Problems arise, as usual, when one tries to define and classify things. In this case, you can see that, at the smaller end of the scale, structures are accessories, ornaments of beds; at the larger end, subaerial dunes, and even greater structures discovered under the sea, have the same extent of beds, and can bound many of them. For convenience, I shall speak of large-scale structures when I focus on the shape, the morphology of large sedimentary objects; I shall treat them as bodies or stratigraphic units when my focus is on their content, architecture (internal organization), and volume.
Some large-scale structures are erosional, and truncate previous deposits; such a truncation is shown by stratigraphic sections as an angular or irregular contact cutting through one or more beds. An erosional surface of large scale is a stratigraphic discontinuity whose rank depends on what happens above it: if it is covered by just one bed, i.e., a single sedimentary event, it is equivalent to a bedding surface. If, on the other hand, several beds abut against it, the surface has a higher rank, and must be regarded as an unconformity: it remains exposed for a significant geologic time before being buried by new strata. Unconformities have a regional extent: this means that a single erosional structure does not suffice to produce an unconformity. Several of them must be juxtaposed to form the complex topography that characterizes such regional surfaces; they are, in essence, subaerial or subaqueous landscapes of the past.
Large-scale structures and sedimentary bodies are rarely visible in their entirety in stratigraphic sections; the size of most outcrop is insufficient to show them. They are best displayed by seismic sections of the subsurface, although one must be aware that a seismic image has its own drawbacks: it is the result of instrumental processing, filtering, etc., of elastic waves passing through receivers, and is more or less distorted due to the use of different scales for height and distance ("vertical exaggeration").
When one looks at sediments from a short distance, one's attention is attracted not only by structures but also by textures, i.e., by various characters of the sediment components such as coarser and finer particles, their shape, orientation, and spatial arrangement. The main textural parameter, especially in clastic, or detrital sediments (those that are carried and deposited by mechanical forces) is the grain size: it can be inspected visually, by comparison with grains mounted on slides or printed silhouettes, to determine the size class of the most abundant (modal) grains. An appreciation of the size range, or sorting, can also be cursorily made. If one wants to be more precise, one will take samples to the laboratory and do grain size analysis with various methods.
Notice that the terms coarse, medium, and fine are not used loosely; they match precise size ranges according to the reference scale (table 2). This scale is based on powers of 2 and is employed also for materials erupted explosively by volcanoes (pyroclastic deposits), with some modifications for the specific setting (table 3). Pyroclastic materials, although volcanic in origin, are emplaced by gravity and superficial processes like normal sediments; beautiful structures can be found in them, of which several examples are illustrated in this book.
|Table 3. Grain-size classes of
pyroclastic deposits (from Schmid 1981, in Fisher and Schminke
pyroclastic particles (general)
tephra, pyroclastic deposit (rock), pyroclastite, tuff, tuffite (general)
|>64||blocks, bombs, cinder||agglomerate, pyroclastic breccia|
|64-21||lapilli||lapilli tephra, lapillistone, lapilli tuff, lapilli tuffite|
|2-1/16||grains, shards||ash, ash layer, tuff, tuffite|
|<1/16||shards||fine ash, fine tuff|
Sedimentary structures, although preserved in sediments, do not record only sedimentation events or depositional mechanisms. Even in sedimentary environments, where deposition is predominant, erosion can occur, sporadically or with a relative frequency. Erosional structures, consisting of surfaces of variable shape and size, are thus produced; eventually, they are mantled by sediment and fossilize. It is very important to recognize erosional surfaces, especially of medium to large scale, because they represent breaks in sedimentation and loss of stratigraphic record. These surfaces usually appear as lines in cross-sections; at places, they are also exposed at the top of substratal beds or, as molds, at the base of covering or filling beds. Remember that, for a correct description of rocks, surfaces must be discriminated from volumes; strata filling a fossil channel, or paleochannel, should be quoted as a channel fill, not simply as a channel, as is often incorrectly done.
In a stratigraphic section, geologists should speak of a channel, or valley, only with reference to erosional surfaces which should be: 1) visible on the outcrop and not just inferred, and 2) correctly interpreted (not all erosional features are channels). To recognize erosional phenomena in stratigraphic successions, the application of the actualistic principle is essential: one needs to observe Modern landscapes, the action of the various agents that model them (streams, wind, glaciers, waves), and the results of this modeling in terms of topographic and geomorphic forms.
Besides depositional and erosional structures, deformational structures are found in sediments. The deformations I am talking about are those occurring in the early stages of the burial history, when sediments have undergone little diagenetic changes and are still soft (they may be cohesionless, i.e., made of loose particles, or coherent when particles stick together). To distinguish this kind of deformation from that produced later, by tectonic stresses, scientists use the term soft-sediment deformation, or penecontemporaneous deformation. It can be caused, for example, by creeping or sliding on slopes, expulsion of water, liquefaction induced by seismic shocks, shrinkage due to dehydration, etc. Practical criteria for discriminating syn-sedimentary from tectonic deformation are difficult to generalize and are not discussed here; they will be recalled case by case in a section of the atlas. I only remark that deformational structures have the same variability of scale as other types of structure.
The last group of sedimentary structures includes those produced by the activity of organisms: they are called biogenic. Organisms play two contrasting roles concerning sediments: constructive and destructive (or deformative). On the one hand, they contribute to the accumulation of sediments with their remains, both hard (shells, skeletons) and soft (tissues, cells, organic matter). On the other, plants and animals use soil or sediment as their home or shelter, as a source of food or simply as a route or resting place: in some way or other, they disturb the sediment and often obliterate structures previously formed in them. This kind of sediment disturbance is generally called bioturbation (or bioerosion if it affects hard rock or lithified sediment). Organisms do not leave their remains but the traces of their activities; when they are identifiable, these marks are called trace fossils; when they are not (being muddled in a wholly mixed sediment), the generic term bioturbation, or bioturbated bed (deposit, facies), is used.
The constructive role of organisms can be active or passive in relation to accumulation of sediment and growth of sedimentary bodies. Coral and algal reefs are examples of active construction (bioconstruction); accumulations of shells and shell fragments (bioclasts), produced for example by storm waves and currents, represent passive growth. In any case, depositional structures can be produced, such as accretionary laminae (i.e., stromatolites, discussed later).
We have thus reviewed the principal groups of sedimentary structures, which correspond to the main divisions, or sections, of the book. Specific mechanisms and modes of formation of the structures will be discussed in the explanatory text accompanying the plates. It seems expedient, however, before concluding these introductory remarks, to give some general information about sedimentary processes and environments, which are responsible for the origin of sediments and their structures. Mention will also be made of the use of structures in "facies analysis," as the type of stratigraphic analysis aimed at reconstructing paleoenvironments of deposition and erosion is termed.
Among sedimentary processes, the most important are those called physical, or mechanical. They are almost ubiquitous over the Earth's surface, and move huge amounts of solid matter from erosional to depositional domains. Moreover, physical processes can entrain and remobilize sediments accumulated by other processes; every natural or artificial material can thus become a component of clastic sediments. It is, therefore, logical to presume that a large amount of structures preserved in sediments are physical in origin.
Chemical and biological processes must not be overlooked, anyway. Not only do they play an essential part in controlling the Earth's climate, the ecosystems and the interactions between biosphere, atmosphere, idrosphere, and geosphere (through biogeochemical cycles, for example, or the removal of carbon from the atmosphere), but also build up sediments. In the geologic past, accumulation of dead organic matter and biochemical reactions have formed imposing coal beds and oil-fields, and even larger masses of calcium carbonate have been extracted by organisms from the water of seas and lakes. Carbonate rocks may show specific structures beside the whole range of types usually found in clastic rocks. After being segregated from water and precipitated in the skeleton of organisms, calcium carbonate is subject like any other material to physical disgregation, transport and redeposition: it becomes a clastic carbonate sediment. The same can be said for evaporites, the main representative of chemical sediments: they form, in the first place, by precipitation of salts from sea or lake water, because of evaporation. After that, evaporitic salt crystals can be removed from their original place and resedimented as clastic particles. The finding of physical structures in evaporite beds gives us evidence that such a reworking indeed occurred.
Physical processes of transport and sedimentation are to be considered from several points of view. A first is that of the operator, or the transporting agent, such as wind, various types of aqueous currents, waves, and gravity flows. A transporting medium is not always needed; when gravity directly acts on solid particles, they fall, slide, or roll down slopes, and accumulate at their base. A planet devoid of atmosphere or with a thin one, like Mars, can have sediments of this type. On Earth, the atmosphere and the water masses can either act as working fluids that entrain and carry sediment to its resting place or as passive "spectators" of what gravity is staging (this means that part of the fluid is entrained along with the solid particles moved by gravity, while the rest of it stands still). In some way or the other, the energy that is dissipated by physical processes is gravitational, but a distinction is made between processes directly promoted by gravity (gravity-driven ) and all others, in which a fluid acts as a go-between (fluid-driven ). So-called sediment gravity flows belong to the first category.
You are now prepared to examine, more concretely, the ways sediments are removed, transported, and deposited: fast or slowly, abruptly or gently, a few particles at a time or en masse, etc.
In some cases, sedimentary particles travel at different velocities, or some are moving while others remain stationary: this occurs as well in a fluvial current, in a wind storm or under the waves approaching a beach. The main differentiation occurs between suspended load and bed load, i.e., between particles that have lost contact with the ground and are supported by fluid eddies, and particles that struggle along with difficulty near the bottom, pulled and pushed by the flow. The former are lighter, smaller and faster (they move at the same speed as the fluid, and make it turbid), the latter are heavier, coarser and much slower (also because of frequent collisions and strong friction). Individual particles in the bed load do not move continuously but come to rest temporarily; their movement can be described as a stop-and-go. They are also subject to a more intense wear (fragmentation due to collisions, abrasion). The moving grains can either roll, slide or jump; all these mechanisms (the last one is called saltation) are encompassed by the term traction, or tractive process.
Particles behaving differently in the same flow constitute distinct sediment populations and will be deposited in different places; the flow selects these populations from a parent stock produced by weathering in source areas. Not all processes have the same efficiency in doing this job: thereby scientists distinguish, among them, selective from more massive types. Sedimentary structures produced by selective flows (which can include waves) are called tractive structures if originated in the bed load, fall-out or settling-related structures if generated by a passing or stationary suspension. The former are usually found in coarse sediments (gravel to sand size), the latter in fine materials (silt to clay). Traction and fallout can also be combined during deposition of particles from highly turbulent suspension flows; the grain sizes involved are intermediate between the coarser and finer end members (fine sand to coarse silt). Traction-plus-fallout structures are common in turbidites and deposits of fluvial floods and melt water.
Mass transport processes put in motion in a short time, often instantaneously, consistent volumes of previously deposited sediment or detritus covering weathered rocks. This material can be quite heterogeneous, from stones to mud, to plant stems or branches and other debris, or more sorted (only mud or sand, for example). It can mix with water and form a viscous slurry, or move like a dry avalanche. Dry or almost so mass flows occur on the flanks of volcanoes, when ejected particles mix with hot gases to form dense clouds, heavier than the surrounding air and driven downslope by gravity. If water is involved in the eruption, it vaporizes.
Most mass flows are gravity-driven and occur on relatively steep slopes, both subaerial and subaqueous. Their speed is related, on one hand to the topographic gradient, or steepness of the slope, and to the mass involved; on the other, to the concentration of solid particles (the ratio between their volume and the volume of fluid), their specific weight (which regulates their buoyancy) and their relative, or specific surface (ratio between surface and volume, which increases with decreasing grain size and affects frictional resistance).
Flows in which solid particles are relatively diluted in the fluid mass move more easily, can develop turbulence and segregate their load into subpopulations of grains. Consequently, the ensuing deposit can be structured, at least in part: vertical size grading and laminae of different styles can be found in it (which led to the definition of the "Bouma sequence," figure 8.B). For higher concentrations of sediment, the movement is slowed down by the stronger friction, both within the mass and at its boundaries; the behavior tends to be that of a visco-plastic material, a sort of toothpaste, and is common to water-soaked rock debris, glacier ice and flows of viscous lava. The finer particles mix intimately with the fluid to form a cohesive phase (like mud in water) capable of supporting, with its strength, coarse and heavy particles: large blocks can thus be transported at the same speed as the smaller clasts and the mud (or equivalent matrix material). As all these particles are entrained all together, helter-skelter, they are also deposited in the same way. Nor is it matter of real deposition: the flowing mass reduces its speed and "freezes"; freezing is equivalent to the solidification of a lava, and is caused by a decrease in slope gradient, water expulsion, or both factors (the reason is that motive force wanes and internal friction increases).
Less dense and turbulent mass flows look like fast moving, turbid clouds: on land, they take the aspect of dry avalanches on mountain slopes, or of "nuées ardentes," pyroclastic flows and base surges on the sides of volcanic cones. Sand storms in deserts and major river floods (especially the "flash floods" of torrential streams) can have this character but are generally slower. Under water, these phenomena have been inferred or detected instrumentally but never observed, except on a small scale: sediment is remobilized in shallow water by strong storm waves or tsunami waves, and redeposited there as storm layers or entrained in deep water by a gravity-driven suspension cloud, a turbidity current. The excavation of deep-water channels and canyons, the buildup of sediment in their levees and the mantling of vast areas of the abyssal seafloor by sand and mud containing coastal and fluvial debris, are all effects of the action of these density currents, whose deposits are called turbidites.
The slower and more concentrated mass flows (sometimes called hyperconcentrated) can also occur both on land and under water: in certain cases, they are monogenic, i.e., made of a single component (mud, sand, stones, wet snow: the coarser the particles, the stronger is the solid friction, and the steeper the slope needed for movement), but more commonly they are constituted by a mixture of two or more textural types (modes ). In the latter case, the term debris flow is used, with the exception of pyroclastic deposits: hot ash directly emanating from volcanic vents is emplaced by pyroclastic flows (denser, highly concentrated variety), whereas cooler ash remobilized from slopes by rainstorms or simply gravity form so-called lahars.
Debris flows occur in both subaerial and subaqueous environments; in the first case, they can be directly observed, for example, on mountain slopes and streams after heavy rains and thunderstorms. Cohesive flows move like a paste, while a higher liquid content fluidizes the mass, which nonetheless keeps a high viscosity and a laminar behavior.
Mass flows pick up their material from a repository localized in a source area of some kind (snow on mountains, talus or scree debris on valley sides, river sediment on deltas). It is thus implicit that some other sedimentary processes (or gravity alone) accumulated this detritus in the first place. This is a first requisite for mass flows to occur: a causal factor acting in the long term. Another requisite is the potential instability of the accumulated material: the pull of gravity on slopes, the lubricating effect of melted snow, the excess pressure of water in pores, the delayed compaction due to rapid deposition are examples of factors that contribute to the instability of sediment. The last requisite is the "final push": a sudden overload, a seismic shock, a heavy storm, etc. This immediate cause, or "trigger," is not always necessary: the least tremor in the ground or perturbation in air or water can induce a catastrophic flow only because the other factors had prepared suitable conditions for it. The return or recurrence time of mass flows, in fact, has more to do with the size of the available reservoir and the energy accumulated in it (and hence the time required for its formation) than with the frequency of triggering events.
Mass flows fall in the category of catastrophic events. Some object to this term because of its affirmed anthropic implications (it alludes to damages inflicted on human communities) and prefer "episodic," "sporadic," "exceptional," or "rare." In addition, there is still, among geologists, a prejudice against catastrophism dating back to the old quarrel with actualism. To be a catastrophist implied a philosophical view by which a divine power bears the responsibility for major natural changes. Actualists fought this theory, among other reasons, because it would induce an attitude of passivity and resignation toward nature. Today, however, there is no reason to attach symbolic or religious meanings to natural catastrophes: they do not reflect God's wrath, but natural causes, and a better knowledge of these causes would certainly contribute to alleviate their impact on society. Structures indicative of catastrophic events (e.g., earthquakes) recorded in sediments are stressed in my text, with a hint to their potential utilization for natural hazard studies and risk analysis.
Tractive currents, fair weather waves, sluggish suspension flows, and wind are examples of "normal," i.e., typical "actualistic" processes; the marks left by them in sediments consist in more structures, more order, more sorting of materials, and less erosion with respect to products of mass flows. Normal processes operate almost continuously, though varying in intensity with regular (tides, seasonal changes) or irregular periodicity. The energy at stake can be much greater than that developed by catastrophic processes (think of the enormous amount of energy stored in seawater and dissipated by tides every day), but it is less concentrated in both time and space. It is thus less available (which means less valuable, in terms of quality ) to do mechanical work. That is the reason it is said that "normal" processes produce structures of lower energy than mass flows; this statement is not true in absolute terms, but relative to the energy spent on a particular sediment in a particular place.
The causal relationship between a sedimentary structure and a sedimentary process is not always simple or straightforward. This must be borne in mind to avoid mistakes and hurried conclusions. Some points are here suggested as caveats:
If the link between structure and process is complex and needs some thinking, even less definite is the link between structure and environment. Processes are just one of the factors and variables at play in sedimentary environments. In many of them, for example, fluids exert a tractive drag on sediment and form tractive structures; consequently, tractive structures, per se, are trivial (however, their presence rules out massive and chaotic processes). There are, fortunately, certain structures that are more characteristic and significant than others, and will be stressed as environmental indicators. This will be done on a case by case basis, assuming that the reader has already a summary knowledge of what is a river bed, a beach, or a glacier, i.e., a natural environment in the common geographical sense.
Beside structures, there are other features in sediments and sedimentary rocks that can be utilized as indicators: composition, texture, fabric, etc. All these elements, which are comprised in the concept of facies, are valuable tools for the reconstruction of Ancient environments and geography (paleoenvironmental and paleogeographical analysis). A geologist cannot reproduce a landscape that has vanished but can infer and reconstruct it piece by piece, with more or less confidence, by using fossil evidence as a clue. The more indicators that can be discovered, the more reliable the reconstruction is.
Information on paleoenvironments and paleogeography allows us to better understand the history of our planet and the work of its systems: geodynamics and internal processes (magnetism, magmatism, seismicity, plate movements), external processes and influences (solar radiation, atmospheric and oceanic heat machines, climate, weathering, erosion, sedimentation), biological processes, and the ecosphere. This is a noble goal in itself, but not the only one for research.
Sedimentology can be applied to find resources (minerals, water, energy). The analysis of sedimentary facies, for example, can elucidate the characteristics, properties, and history of sedimentary basins, where diverse types of economic materials can be found.
One cannot trace the subsidence history of a basin without knowing the depth below sea level at which some critical stratigraphic levels were deposited, and this depth must be inferred through appropriate indicators. It is part of the "search for the paleoenvironment" game. Basin analysis also requires that the sources, pathways, and sinks of sediments be located. A major contribution to trace the dispersal of sediment in Ancient basins can be given by the sedimentary structures and the geometry of sedimentary bodies (in conjunction with compositional data that testify to the provenance of clastic sediments). The orientation of structures and deposits must be measured in this respect, both in relation to present-day spatial coordinates and to the trend of Ancient shorelines, margins of continents, mountain chains, volcanic ridges, and so on.
The geometrical attributes of sedimentary structures (shape, asymmetry, elongation, etc.) can be used to get the directions of paleocurrents, paleowaves, and paleo-slopes. The orientation of structures is reported in maps and diagrams and is a current and essential tool in the sedimentologist kit for facies analysis. Since the classic book by Potter and Pettijohn (1963), the operational procedure is known, comprehensively, as paleocurrent analysis.
Let me conclude this introductory section with a remark that I deem necessary. The present book has been conceived to help you in identifying sedimentary structures, and to do so with a critical attitude. It is a guide to observation, not a catalog of identical, mass-produced objects. In geology, a recognition procedure is rarely easy and immediate because geological objects are complex, almost unique entities. They are very different from the more elementary objects of chemistry and physics: you cannot tell the difference between two atoms or molecules of a certain chemical species, but no two sand grains or two structures are ever exactly alike in all minimum details. Each of them has a certain degree of individuality, like persons.
For this reason, you must stop in front of each structure and think about it for a while. Do not be satisfied with the first explanation that comes to your mind, especially if it seems obvious. Make more than one hypothesis, weigh and balance them, use logic in discriminating between them. Make comparisons and use analogies (but do not rely only on them). Make sure that your interpretations are consistent with the facts by looking for more facts (even though they are never sufficient to definitely prove a point). Free your fantasy and do not be afraid to speculate, but check for internal consistency and coherence of your speculations. Several plates in this atlas are presented as examples to stimulate such a way of reasoning. Good reading, and good work!
Allen, J. R. L. 1968. On the character and classification of bed forms. Geologie en Mijnbouw 47: 173-85.
Allen, J. R. L. 1968. Current Ripples. Amsterdam: North Holland.
Allen, J. R. L. 1982. Sedimentary Structures: Their Character and Physical Basis. 2 vols. Amsterdam: Elsevier.
Bosellini, A., E. Mutti, and F. Ricci Lucchi. 1991. Rocce e successioni sedimentarie. Turin: UTET.
Bouma, A. H. 1962. Sedimentology of Some Flysch Deposits. Amsterdam: Elsevier.
Collinson, J. D. and D. B. Thompson. 1982. Sedimentary Structures. London: Allen & Unwin.
Campbell, C. V. 1967. Lamina, laminaset, bed, and bedset. Sedimentology 8: 7-26.
Cas, R. A. F. and J. V. Wright. 1987. Volcanic Successions: Modern and Ancient. London: Allen and Unwin.
Conybeare, C. E. B. and K. A. W. Crook. 1968. Manual of Sedimentary of Sedimentary Structures. Australian Dept. Nat. Dev. Bur. Mines Res., Geol. & Geoph. Bull. 102.
Fisher, R. V. and H.-U. Schminke. 1984. Pyroclastic Rocks. New York: Springer-Verlag.
McKee, E. D. and G. W. Weir. 1953. Terminology for stratification and cross-stratification in sedimentary rocks. Bull. Geol. Soc. Amer. 64: 381-90.
Pettijohn, F. J. and P. E. Potter. 1964. Atlas and Glossary of Sedimentary Structures. New York: Springer-Verlag.
Portmann, A. 1965. Aufbruch der Lebenforschung. Frankfurt am Main: Suhrkamp Verlag.
Potter, P. E. and F. J. Pettijohn. 1963. Paleocurrents and Basin Analysis. New York: Springer-Verlag.
Ricci Lucchi, F. 1970. Sedimentografia: Atlante fotografico delle strutture primarie dei sedimenti. 1st ed. Bologna: Zanichelli.
Shrock, R. R. 1948. Sequence in Layered Rocks. New York: McGraw-Hill.
Note. A more exhaustive list of references, based on specialized literature, was prepared but, for reasons of space, was not included here. It was published in the Italian journal Giornale di Geologia (vol. 52, 1991) and is available on request at the editor's address: Dipartimento di Scienze Geologiche (Department of Geological Sciences), Via Zamboni 67, 40127 Bologna, Italy. Every reference is accompanied by abbreviated information concerning the quality of illustrations, type of data on structures (outcrops, cores, seismic sections), sedimentary environment and facies of the unit where the structures come from, and possible experimental setting (flumes, etc.).
Some additional references will be found as endnotes in the plates section.