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SEDIMENTARY STRUCTURES
1. INTRODUCTION 1.1 You might have heard us define structure in rocks as rock geometry on a scale much larger than grains. This is a singularly unilluminating definition, be- cause it doesn't conjure up in the mind of the uninitiated any of the great variety of interesting and significant geometries that get produced by the physical, chemical, and biological processes that operate on sediments during and after their deposi- tion.
1.2 One qualification to the foregoing definition is that the term structure is used in two different senses :
θ For features, on the scale of hand specimens to large outcrops, produced within a depositional environment , during or (usually) not long after de- position. These are usually prefaced by the adjective sedimentary. θ For features, on the scale of hand specimens to whole regions, produced by deformation associated with regional rather than local deforming forces , folding and faulting being perhaps the most obvious examples. This stuff is not the province of sedimentologists or stratigraphers, although they have to be prepared to deal with it. These could be prefaced with the adjective tectonic.
1.3 Study of sedimentary structures is important because they are far and away the most valuable features for interpreting depositional environment. We know a lot about how most structures are formed, so finding them in the rocks can tell you a lot about the conditions of deposition. They're much more useful than textural things like grain-size distribution and grain shape.
2. CLASSIFICATION 2.1 It's not easy to classify sedimentary structures, because both their origins and their geometries are so highly varied. Two reasonable ways of classifying them are on the basis of: kind of mechanism that produces them ( physical sedimentary structures, chemical sedimentary structures, and biogenic sedi- mentary structures) and time of development relative to time of deposition ( primary sedimentary structures and secondary sedimentary structures).
2.2 Figure 3-1 is a pigeonhole chart showing most of the important struc- tures in terms of such a twofold classification.
2.3 Physical primary structures are certainly the most common and widespread and striking , and I think it's fair to say that in general they're the most useful in interpretation. Most are related to transportation and deposition of sedi- ment particles at a fluid/sediment interface. Such structures can be classified further on the basis of their relationship to transportation (the movement of sediment past a point on a sediment bed by currents) and deposition (the increase in bed elevation at a point with time). Figure 3-2 is an unofficial classification of this kind. It doesn't serve very well as a catalogue, but it should help to get your thinking organized.
3.2 Terminology 3.2.1 Stratification is officially subdivided into bedding and lamination , de- pending upon the thickness of the strata, and bedding and lamination are in turn subdivided according to thickness. Figure 3-3 is a chart that gives you all the official terminology. Get used to using this terminology in your descriptions of strata.
3.2.2 With that said, I suppose I should point out that in everyday sedimen- tological and stratigraphic usage, people commonly use the term bedding as a synonym for stratification rather than just in its technically restricted sense.
3.2.3 Also, stratification is often hierarchical , in that beds commonly show internal lamination on a much finer scale.
3.2.4 One of our little terminological peeves is that people sometimes use the term lamination not just for the phenomenon but for the object , instead of lamina. That's not good practice, and I want you to avoid it. It makes you sound uncultured.
3.3 Parting 3.3.1 Keep clearly in mind the distinction between stratification and parting. Parting is the tendency for stratified rocks to split evenly along certain stratifi- cation planes. (The word is also used for the plane itself along which parting has developed.) The approximately planar-tabular units developed by parting are usually just called beds, but it might be better to think of them as parting units.
3.3.2 There's official terminology for parting units , corresponding to that for stratification, although it's not in as common use; see Figure 3-4.
3.3.3 The problem with making a big deal of parting is that it depends not only upon the underlying existence of weaker bedding planes but also upon the extent and nature of weathering : A freshly blasted outcrop usually won't show any parting at all, but if you go back to the outcrop years or decades later, it might show well developed parting.
3.4 Origin 3.4.1 Here are three major "scenarios" for the origin of stratification. These are the broad ways loose sediments get deposited.
3.4.2 Quiet-fluid deposition of particles by settling: ocean bottom (plus lakes) mainly; low-velocity currents carrying a supply of suspended sediment from upcurrent; usually fine-grained but not always; usually thin lamination, because deposition rate is slow relative to the slight changes in settling regime; usually nearly or perfectly even and planar, unless later deformed. Often such deposits are later bioturbated to the point that none of the original lamination remains.
3.4.3 Deposition of particles by tractional currents: deposition onto a well defined fluid-sediment interface during bed-load (or bed-load plus plus sus- pended-load) transport by moderate to strong currents; stratification thick to thin depending on nature of variations in sediment supply, currents, and deposition rate; even stratification and cross stratification can both be important; usually fairly coarse sediment, coarsest silt size into gravel range.
3.4.4 Mass deposition of coarse and fine sediment mixtures (or only fine sediment, or rarely only coarse sediment) by sediment gravity flows (high- concentration sediment-water mixtures flowing as a single fluid) coming to rest without differentiation or particle-by-particle deposition; usually thick-bedded, with little or no internal stratification.
change in lithology. Figure 3-5 shows three common examples. In each example, the asterisk lies within a single set. The laminae within the sets may be planar or curving. Concave-up laminae are more common than convex-up laminae. The orientations of the truncation surfaces are usually different from the orientations of the laminae within the sets. Commonly the lateral scale of the sets may be not much greater than the vertical scale, or it may be much greater. In some cases, there are no truncation surfaces within the cross-stratified deposit; Figure 3- shows a common example.
4.1.7 Another thing you should be thinking about is one's view of cross stratification. Usually it's seen on a fracture surface, weathered or unweathered, nearly normal to the overall stratification. Some cross stratification is approxi- mately isotropic with respect to direction in the plane of overall stratification (the geometry of cross stratification looks about the same in differently oriented sec- tions), but most is anisotropic (the geometry of cross stratification commonly looks different in differently oriented sections normal to the overall plane of
stratification), so try to see the cross stratification on as many differently oriented planes normal to bedding as you can , because it might look quite different depending on the direction. Sometimes, but not often, you get to see what the cross stratification looks like on a plane within the cross-stratified bed parallel to overall stratification.
4.1.8 A final note on terminology: just as with stratification in general, you can think in terms of cross stratification as the general term, and cross-bedding and cross-lamination according to the thickness of the strata within the sets. People tend not to adhere rigorously to these distinctions, however.
4.1.9 Often a given cross-stratified bed may represent not just one deposi- tional event but two or more separate depositional events, each one superimposed on the the previous one. Such beds are said to be amalgamated. Sometimes it's easy to recognize the individual depositional events within the amalgamated bed; the stratification within each part of the bed can then be studied separately. But sometimes it's difficult to determine whether or not the bed is amalgamated.
4.2 How Bed Forms Make Cross stratification 4.2.1 In general terms, the fundamental idea about bed-form-generated cross stratification is easy to state (Figure 3-7): as bed forms of one kind or other pass a given point on the bed, both the bed elevation and the local bed slope change with time. Consider a short time interval during the history of decrease and increase in bed elevation. After a temporary minimum in bed elevation is reached, deposition of new laminae takes place for a period of time, until a temporary maximum in bed elevation is reached. Then, as the bed elevation decreases again, there's complete or partial erosion of the newly deposited laminae and formation of a new truncation surface. After the next minimum in bed elevation, another set of laminae is deposited.
4.2.2 The preceding paragraph is still too general to give you a concrete idea about how moving bed forms generate cross stratification. Now I'll be more spe- cific. Take as an example a train of downstream-moving ripples in unidirectional flow. (The picture would be qualitatively very similar for dunes.) Each ripple moves slowly downstream, generally changing in size and shape as it moves.
4.2.6 If no new sediment is added to the bed while the ripples move, the average bed elevation doesn't change with time, and the invisible plane that repre- sents the average bed surface stays at the same elevation. On the average, the foresets deposited by a given ripple are entirely eroded away again as the next trough upstream passes by (Figure 3-11). But if new sediment is added every- where to build the bed upward, the ripples no longer move parallel to the plane of the average bed surface but instead have a component of upward movement (Figure 3-12). The resultant direction of ripple movement is described by the
angle of climb , denoted by θ in Figure 3-12. The tangent of θ is equal to the
average rate of bed aggradation divided by the ripple speed.
4.2.7 As the ripples climb in space, as described above, their troughs climb with them. So the erosion surface associated with the downstream movement of the low-point curve in a given trough passes above the erosion surface that was formed when the preceding trough passed by. The lowest parts of the foresets deposited by the ripple that was located between those two troughs are then pre- served rather than eroded entirely (Figure 3-13). This remnant set is bounded both above and below by erosion surfaces.
4.2.8 Figure 3-14 shows cross stratification in an ideally regular deposit produced by low-angle climb of a train of ripples. The heavy lines are erosion surfaces, and the light lines are foreset laminae. The profile of the ripple train as it existed at a given time is shown also. The upper parts of each ripple in the train, underneath the dashed part of the profile, were eliminated by later erosion. In real cross-stratified deposits of this kind, the erosion surfaces are irregularly sinuous because trough geometry changes with time, and the sets tend to pinch out both upstream and downstream because the ripples exist for only a finite distance of movement.
individual sets are bounded by erosion surfaces. The general nature of such stratification is common to moving bed forms of all sizes, from small current ripples to extremely large subaqueous or eolian dunes. Important differences in the details of stratification geometry arise from differences in bed-form geometry and how it changes with time.
4.3 Important Kinds of Cross stratification
4 .3.1 Introduction
4.3.1.1 Here I'll present the substance of what the major kinds of cross stratification in the sedimentary record look like. They conveniently fall into (i) unidirectional-flow cross stratification , on a small scale corresponding to ripples and on a larger scale corresponding to dunes, and (ii) oscillatory-flow cross stratification. Unfortunately there's little I can say at present about combined- flow cross stratification. I'll make a few comments about that in the section on oscillatory-flow cross stratification.
4 .3.2 Small-Scale Cross stratification in Unidirectional Flow
4.3.2.1 Small-scale cross stratification formed under unidirectional flow is associated almost entirely with the downstream movement of current ripples. In accordance with the discussion of how moving bed forms produce cross-stratified deposits, discussed above, the general features of the cross-stratification geometry depend on (i) the geometry of the ripples themselves , as well as how that geometry changes with time as the ripples move, and (ii) the angle of climb.
4.3.2.2 For small angles of climb, the general geometry of the cross-strati- fied deposit is shown by the block diagram in Figure 3-16. In addition to the actual rippled surface, Figure 3-16 shows a flow-parallel section and a flow- transverse section perpendicular to the overall bedding. Figure 3-16 is the real-life counterpart of Figure 3-14.
4.3.2.3 In sections parallel to flow (Figure 3-16) you see sets of laminae dip- ping mostly or entirely in the same direction (which is the flow direction), sepa-
rated by truncation surfaces. The height of the sets is seldom greater than 2-3 cm, because it's always some fraction of the ripple height, which itself is seldom greater than 2-3 cm. The set boundaries are sinuous and irregular, because of the changes in the ripples as they move. Sets are commonly cut out at some point in the downstream direction by the overlying truncation surface. This is a reflection of either (i) locally stronger erosion by a passing ripple trough or (ii) disappear- ance of a given ripple as it moved downstream, by being overtaken or absorbed by another faster-moving ripple from upstream. New sets also appear in the down- stream direction, reflecting the birth of a new ripple in the train of ripples.
4.3.2.4 In sections transverse to flow, the geometry of cross stratification is rather different (Figure 3-16): you see nested and interleaved sets whose lateral dimensions are usually less than something like five times the vertical dimension. Each set is truncated by one or more truncation surfaces. These truncation surfaces are mostly concave upward. The laminae within each set are also mostly concave upward, but the truncation surfaces generally cut the laminae discor- dantly.
4.3.2.5 The key to understanding this cross-stratification geometry lies in the geometry of ripple troughs. Remember that fully developed current ripples have strongly three-dimensional geometry, and an important element of that three- dimensional geometry is the existence of locally much deeper hollows or swales or depressions in ripple troughs, where the separated flow happens to become con- centrated (because of the details of the ripple geometry upstream) and where scour or erosion is much stronger. As one of these swales moves downstream, driven by the advancing ripple upstream, it carves a rounded furrow or trench, oriented parallel to the flow, which is then filled with scoop-shaped or spoon-shaped laminae which are the foreset deposits of the upstream ripple. Eventually the resulting set of laminae is partly or mostly or even entirely eroded by the passage of a locally deeper swale in some later ripple trough. This accounts for both the geometry of the sets and their irregular interleaving.
4.3.2.6 On the rare occasions when you're able to see a planar section through the deposit parallel to the overall stratification, you see a geometry which looks like Figure 3-17, which shows the truncated edges of sets of laminae that are strongly concave downstream, separated laterally by truncation surfaces. This has been called rib and furrow (not a very descriptive term). It's an excellent pale- ocurrent indicator.
and fairly straight, and the elevations of the crests and troughs are nearly uniform in the direction transverse to flow. On the other hand, at relatively high flow velocities the dunes are moderately to strongly three-dimensional, in much the same way that ripples are three-dimensional. You should expect the geometry of cross stratification to vary greatly depending on whether the dunes were two-di- mensional or three-dimensional.
4.3.3.3 Three-dimensional dunes produce cross stratification that's quali- tatively similar in geometry to the small-scale cross stratification produced by ripples. You might reread the earlier section and apply it to the stratification produced by three-dimensional dunes.
4.3.3.4 Figure 3-19 is a block diagram of cross stratification produced by three-dimensional dunes in unidirectional flows. It shows the dune-covered bed surface and sections perpendicular to the overall plane of stratification and parallel and transverse to the flow direction. Most of what I said about the analogous section in Figure 3-16 for cross stratification produced by ripples at low angles of climb is applicable to Figure 3-19 as well. Set thickness ranges from less than 10 cm to as much as a few meters.
4.3.3.5 Figure 3-20 is a corresponding block diagram of cross stratification produced by almost perfectly two-dimensional dunes in unidirectional flows. The stratification geometry is rather different from that in Figure 4-19: in flow-parallel sections the sets extend somewhat farther and the set boundaries are less sinuous, but the biggest difference is in flow-transverse sections , where both the sets and the truncational set boundaries are much more extensive and show much less upward concavity. This is because of the absence of locally strong scour swales in the troughs of the dunes.
4.3.3.6 There's a whole spectrum of intermediate cases for which the cross- stratification geometry is less regular than the extreme case shown in Figure 3- but not as irregular as in Figure 3-19.
4.3.3.7 In both Figure 3-19 and Figure 3-20, the angle of climb of the dunes is very small. Dunes sometimes climb at higher angles, but that's not nearly as common as for ripples , because it's uncommon for fairly coarse sediment to be settling abundantly out of suspension over large areas to build up the bed rapidly. In the very few cases I've seen, the geometry of cross stratification is very much like that shown in Figure 3-18.
4 .3.4 Cross stratification in Oscillatory Flow
4.3.4.1 Remember that in truly symmetrical oscillatory flow at low to moderate oscillation periods and low to moderate oscillation speeds, the bed configuration is symmetrical two-dimensional oscillation ripples. Under these conditions, the sediment transport is also strictly symmetrical in the two flow di- rections. You might expect the ripples to remain in one place indefinitely. Then if sediment is supplied from suspension to build up the bed, symmetrical oscillation- ripple cross stratification with vertical climb would be produced (Figure 3-21). Although this kind of stratification is present in the sedimentary record, it's not common , presumably because even in purely oscillatory flow there's usually a minor degree of asymmetry of sediment transport which causes the ripples to move slowly in one direction or the other.
well understood, because there have been no studies in natural environments in which first the bed configuration was observed while the flow conditions were measured and then the bed was sampled to see the resulting deposit. And until only recently there had been no studies under laboratory conditions. Another el- ement of complexity is that in the natural environment the oscillatory flows are likely to be more complicated than the regular and symmetrical bidirectional os- cillatory flows that were assumed above, and essentially nothing is known in detail about the stratification types produced by these more complicated oscillatory flows.
4.3.4.6 In the face of this seemingly hopeless situation, I'll take the following approach. I'll describe in a general way a common style of medium-scale to large- scale cross stratification, called hummocky cross stratification , which is generally believed to be produced by some kind of oscillatory flow, and I'll present what evidence I can for the kinds of flows that might produce hummocky cross stratification.
4.3.4.7 Figure 3-23 is a block diagram of one of the common styles of cross stratification that's been called hummocky cross stratification. It shows sets of laminae that are both concave upward and convex upward, bounded by broad truncation surfaces which themselves may be either concave or convex upward. Two characteristic small-scale features of the geometry of stratification are (i) the fanning of truncation surfaces laterally into conformable sequences of laminae (Figure 3-24) and, where the thickness of the bed is great enough to observe this, (ii) a tendency for convex-up sets of laminae to be succeeded upward by concave- up sets, and vice-versa (Figure 3-25).
4.3.4.8 Note that the two normal-to-bedding faces of the block are shown to have about the same style of stratification, and on each face there's no strongly preferred dip direction. In the rare cases where you can make serial sections of the deposit to ascertain the entire three-dimensional geometry of the deposit, it's clear that there's no preferred dip direction in the entire deposit. This is the kind of stratification I call isotropic.
4.3.4.9 The upper surface of the block diagram in Figure 3-23 is shown to be a bedding surface with a bed configuration that could be described as a collection of hummocks (locally positive convex-up areas) and swales (locally negative con- cave-up areas). Sometimes, but not often, the upper surface of a bed with hum- mocky cross stratification can be seen to have just this bed geometry. The general belief is that isotropic hummocky cross stratification is produced by this kind of bed configuration , although it's seldom possible to actually demonstrate this.
4.3.4.10 Very recent preliminary experiments have shown that bed configu- rations which in their general features are like those just described are produced by symmetrical bidirectional oscillatory flows at long periods and high oscillation velocities. This suggests that at least some isotropic hummocky cross stratification is produced by such flows. But it also seems likely that more complex oscillations with more than one oscillatory component would also produce qualitatively similar bed configurations and therefore similar cross stratification. Much more work needs to be done before the origin of hummocky cross stratification is well understood.
4.3.4.11 This brings us to the problem of combined-flow cross stratification. Unfortunately there's an almost complete lack of observational information on the origin of combined-flow cross stratification, so we have no actual models to guide interpretations. Up to now the recognition of combined-flow cross stratification has been a strictly deductive matter.
