ecimens to whole regions, produced by folding and faulting being perha - PDF document

ecimens to whole regions, produced by folding and faulting being perhaps the most obvious examples. This stuff is not the province of sedimentologists or 2. CLASSIFICATION It's not easy to classify sedimentarand their geometries are so highly varied howing most of the important struc-tures in terms of such a twofold classification. 73 , and I think it's fair to say that in general they're the most at a fluid/sediment interface. Su(the movement of sediment past a point on a sediment bed by currents) in bed elevation at a point with time). Fithis kind. It doesn't serve very well as 74 3. STRATIFICATION 3.1 General Stratification is by far the most important sedimentary structureMost, although not all, sedimentary rocks are stratified in one way or another. can be defined simply as being more generally used for rocks in bodies with approximately planar-tabular shape. I suppose it's obvious, but I'll say it anyway: 3.1.3 In dealing with stratification, there are two separate but related matters you have to worry about: What it was about depositional conditions that changed with time to give rise to stratification? makes the stratification manifest? conditions didn't vary much or because the rocks have been messed up since, or on, always think in terms of Here's a list of things that tend to obvious differences in grain size obvious differences in composition caused by slight differences in composition (subtle composition/texture; these range from gross to subtle; rwise homogeneous sediment; conglomerates) 75 3.2 Terminology nd bedding and laminaofficial terminology. Get used to using this terminol point out that in everyday sedimen- 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. That's not good practice, and I want It makes you sound 3.3 Parting developed.) The approximately planar-tusually just called beds, but it might There's stratification, although it's not in as common use; see Figure 3-4. 76 The problem with making a big de: A freshly blasted outcrop usually won't show s or decades later, it might 3.4 Origin are the broad ways loose sediments get deposited. lakes) mainly; low-velocity currents caspended sediment from upcurrent; usually fine-grained but not always; usually thin lamination, that none of the original lamination well defined fluid-sediment interface duripended-load) transport by moderate to str sediment, or rarely only coarse sediment) by sediment gravity flows (high-concentration sediment-water mixtures flowing as a single fluid) coming to rest 77 4. CROSS STRATIFICATION 4.1 Introduction the geometry of the deposi-what's meant by the scales ofscales ranging from centimeters to hundreds of meters.) Usually one or more beds in some part of a section show cross within the beds, or, if it's the same everywhere within those beds, then you can see that the orientation is different from ththe orientation is different from what The of cross stratification varies from , and the geometry is infinitely varied. Cross stratification comes about by deposition upon a sediment surface that isior to or concurrent with deposition. Some terminology: . (But as far as I know, there's configurations produced by fluid flows over loose beds of sediment. But there's an additional factor at work here too: some cross stratification comes about not from the movement of individual bed forms in usually large, which usually come under tion geometry in ancient rocks, studies of modern depositional environmenin tanks and channels. . That's why I'm em to you to be inordinate space in these notes. give you only a minimum of purely descriptive terminology and classification. I think it's better for you to get used to the which are closely bound up with mechanics of origin, and then deal with examples in the context of these styles. That's th Here's some geometrical terminology. More commonly than not, other sets (or beds without sets) by su 78 . Figure 3-5 shows three common minae within the sets may be planar or curving. Concave-up laminae are more common than convex-up laminae. The usually different from it may be much greater. In some cases, . Usually it's seen on a fracture Some cross stratification is approxi-geometry of cross stratification looks about the same in differetions), but most is anisotropic (the geometry of cross stratification commonly sections normal to the overall plane of 79 , because it might look quite different depending on the direction. Sometimes, but A final note on terminol ratified bed may represent . Sometimes it's sometimes it's difficult to determine whether or not the bed is amalgamated. 4.2 How Bed Forms Make Cross stratification In general terms, the fundamental idea about bed-form-generated cross as bed forms of one kind or other pass a bed elevation. After a temporary minimum in bed elevation isbed elevation decreases again, there's on of the newly deposited laminae and formation of a new laminae is deposited. about how moving bed forms generate cross stratification. Now I'll be more spe-downstream-moving ripplesmoves slowly downstream, generally changing in size and shape as it moves. 80 Sediment is stripped from the upstream In your imagination, cut the train of ripples by a large number of verti-cal sections parallel to the mean flow which I'll unofficially call the dimensions. The low-point curve moves downstream with the ripples, and it changes its shape as it moves, like a wr nstream, it can be viewed as having seems to shave off immediately downstream for removal by erosimmediately upstream. ons and sediment size, trough downstream at a small angle (Figure 3-laminae are always deposited directly on the erosion surface that's formed (as just described above) by the downstream moveme 81 If no new sediment is added to the bed while the ripples move, the average bed elevation doesn't change with trough upstream passes by (Figure 3-11). But if new sediment is added every- no longer move paralle 82 As the ripples climb in space, as described above, their troughs climb ciated with the downstream movement of deposited by the ripple that was located between those two tr (Figure 3-13). This remnant set is bounded produced by low-angle climb of a train of eliminated by later erosion. In real upstream and downstream because the ripplesmovement. 83 It's significant that If you compare the height of the cross-sets 14, you can see that for low angles of climb, the set height is only a small fraction of the bed-form height. rate at which sediment is transported froripple. The differences in geometry between Figure 3-14 and Figure 3-15 seem great, but keep in mind that the differences in environmental conditions are not large. The only difference is in the value of the angle of climb. . Examples with angle of climb so small that the contacts between sets are erosional (as in Figure 3-14) might be climbing-ripple cross stratification, and examples with angle of climb large enough for preservation 3-15) might be called climbing-ripple cross stratification. es by net addition of sediment to some of climb of the ripples depends on the ratio of rate of ripple movement. At high angles of climb, the entire angles of climb, only the lower parts of 84 stratification is common to moving bed foripples to extremely large Important differences in the details of stratification geometry arise from differences in bed-form geometry 4.3 Important Kinds of Cross stratification 4 .3.1 Introduction Here I'll present the substance of , on a small scale corresponding to ripples . Unfortunately there's little . I'll make a few comments about that in the section on 4 .3.2 Small-Scale Cross stratification in Unidirectional Flow Small-scale cross stratification formed under unidirectional flow is es of the cross-stratification geometry , as well as how that geometry For small angles of climb, the general geometry of the cross-strati-m in Figure 3-16. In addition to the gure 3-16) you see sets of laminae dip-ping mostly or entirely in the same direc 85 The height of the sets is seldom greater than 2-3 cm, because it's always some fraction of thgreater than 2-3 cm. The set boundaries ey move. Sets are commonly cut out at some point in the downstream direction by the overlying trance of a given ripple as it moved downstream, by being overtaken or absorbed by another faster-moving ripple from upstream. stream direction, reflecting the birth of a new ripple in the train of ripples. e geometry of cross stratification is dimensions are usually less than something like five times the vertical dimension. Each set is truncated by one or more ces generally cut the laminae discor- ss-stratification geometry lies in strongly three-dimensional geometry, and an important element of that three-dimensional geometry is the existence of locally much deeper hollows or swales or to become con-or erosion is much stronger. As one of these swales moves downstream, driven by the advancing ripple upstream, it carveslaminae which are the foreset deposits of the upstream ripple.geometry of the sets and their irregular interleaving. ratification, you see a geometry which ed edges of sets of laminae that are (not a very descriptive term). It's an excellent pale- 86 For large angles of climb, the general geometry of the cross-strati-m in Figure 3-18. In addition to the the overall bedding. Compare Figure 3-18 see mostly continuous laminae whose es which were moving downstream while sediment was added to the bed. The local angles of climb vary from place to place unless the overall angle of climb is veryparticular ripple moved temporarily at laminae which reflect the changing flow-tra Remember that angle of climb increases, the density don't vary greatly in either size or geomet 4 .3.3 Large-Scale Cross stratification in Unidirectional Flow formed under unidirectional flow is ometry of the dunes and the angle of climb. tendency to be two-dimensional: their 87 elevations of the crests and troughs are nearly uniform same way that ripples are three-dimensiona the geometry of mensional or three-dimensional. produced by three-dimensional dunes. Figure 3-19 is a block diagram three-dimensional dunes in unidirectional climb is applicable to Figure 3-19 as we k diagram of cross stratification produced by almost perfectly two-dimensisections the sets extend somewhat fartthe truncational set boundaries are much more extensive and show much less There's an the extreme case shown in Figure 3-22 88 nd Figure 3-20, the angle of climb of the dunes , because it's uncommon for faire areas to build up the bed rapidly. In the very few cases I've seen, the geometation is very much 4 .3.4 Cross stratification in Oscillatory Flow configuration is symmetrical two-dimensional oscillation ripples. Under these conditions, the sediment transport is also rections. You might expect the sediment is supplied from susp would be produced (Figure 3-21). ication is present in the sedimentary record, it's oscillatory flow there's usually a minor degree of asymmetry of sediment move slowly in one direction or the other. 89 ripple movement and (ii) the rate of cal axis, for zero ripple movement, is might be expected deductively. The chevripple crests, shown schematically, results on. This is shown by the first box from the top in box from the top in Figure 3-22). If the deposited laminae, and (see the third box from the top in Figure 3-22). This last type is the most common in the sedimentary record. Noes, from low-angle climbing-ripple cross (see the bottom box in Figure 3-22). remain the same for long. of laminae dipping more or less ran-domly in both directions. 4.3.4.5 The origin and classification of stratification produced by oscillatory flows at longer oscillation periods and higher oscillation velocities is much less 90 mmetrical bidirectional os- 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-me kind of oscillatory flow, and I'll present what s that might produce hummocky cross diagram of one of the common styles of cross stratification that's been called hummocky crlaminae that are both concave upwarbe either concave or convex upward. 91 have about the same style of stratifica there's no strongly n make serial sections of the ional geometry of the deposit, it's clear cave-up areas). Sometimes, but not often, the upper surface of a bed with hum-ve just this bed geometry. The general , although it's seldom possible to actually demonstrate this. . This suggests that at least some isotropic hummocky cross stratification of hummocky cross stratification is well This brings us to the problem of Unfortunately there's an almost complete lack of observational information on the so we have no actual models to guide the recognition of combined-flow cross stratification 92 It seems convenient to think separately about the relatively small same scale as current ripples, there's a kind of ripples: they're on the same scale as unidirectional-flow ripples, but more nearly two-dimensional. Actual experiments indicate that only a very small unidirec-tional component is needed to make such ripples noticeably asymmetrical. separate scales, and there's a complicated components, and the stratification they tion is probably what maanisotropic hummocky cross stratification. ponent. This situation must be important in natural environments, but systematic studies have yet to be made in either the field or the laboratory. 4.3.5 Eolian Cross stratification . But behind the gross similaritythe differing details of geometry of the dunes themselves, and of the sediment transport over them. looseness of terminology here) that's difficuplastering of new trough laminae not just on the mean-upcurrent side, as is usually ation, but on the lateral and mean- Eolian cross stratification is more likely to show greater dispersion of dip directions of cross-sets, because of the greater variability of wind directions than of subaqueous current directions. (But this is not as strong a tendency as you might think, because most of the major eolian sand bodies preserved in the 93 sedimentary record were probably produced The in eolian cross-sets tends to be different from sic kinds of laminae in cross-sets are: movement of grain —grain-fall laminae, produced by the rain of sand grains onto the foreset slope after they are carried across the brink in saltation , produced by the movemeclimb of ripples on sand surfaces th The first two kinds of laminae are common to both subaersubaqueous environments the scale and movement of ronments is such as to produce recognizable small-scale cross-lamination rather than laminae so thin that th 4.3.6 Cross stratification Not Produced by Climbing Bed Forms After all of the foregoing voluminous material on how to deal with it's important to point out here that although we think it's fair to say that mo 4.3.6.2 One case in point is pretty obvious, and has been touched upon in the earlier part of this chapter: bed forms is produced by a neutral flow (by "neutral" we mean that there's neither bed forms is mantled or draped by sediment deposited bed forms (by fallout without traction, for the most on might be termed, unofficially, . It's common in both oscillaflow. Depending on the thickness of movable sediment the flow has to operate on, and the size of the bed forms the flow wants to make, the train of bed forms may climb up one another 94 kinds—but sometimes dunes have large spacings and small height-to-spacing ratios, there's the added complication that you may on the outcrop set with uniform thickness (Figure 3-26). from looking at an outcrop like Figure 3-26, was—or even if I'm really dealing with a train of dunes in the first place! 95 at shown in Figure 3-26, there's also the problem . You might find features at the aller bed forms. Although that's not scribed above (a small part would be impossible without a degree of 5. PLANAR STRATIFICATION There is not as much ication, and even more in gravels and conglomerates, in particular. Planar stratification is also common in carbonate We need to make a distinction here between planar (the geometry of such a hand (this might best be called kind of planar stratification usually comprise. (Keep in mind the terminological 96 planar internal lamination } planarlaminationbed basea planar bedbed Top Figure by MIT OCW. 5.3 What is the origin of planar lamination? rocks, it must certainly be the outcome of deposition by differences might be in particle size or in composition. learn more about such event beds later. Intum (usually referred flows, river floods, or shallow-marine storms. is the outcome of discussion of modes of dedune stratification In what seems to be a flow environment in which the flow . To put that another way, with increasing particle size it becomes less carrying enough sediment in suspension to form a planar-laminated deposit just In planar lamination in sands and sa(heavy minerals; scraps of organic matter) highlight the lamination; usually, howevemean size and/or sorting. Such diffement surface by drying by the wind. For a long time the origin of the slilaminated sandstones was poorly understood. Laboratory experiments in recent that much if not most such planar lamination is generated by the downstream movement of very low-amplitude bed waves (akin to very low, shingle-like dunes) on an almost planar transport surface. 97 6. SOLE MARKS Sole marks are another important kind of sedimentary structure, less on a sediment bed by erosion by a strong current and sole marks is not entirely sharp, but with sole marks we're dealing with , usually of mud. (if you mention sole marks to most soft-rock geol-ogists they'll think of turbidites), but and shallower downstream. They thus make excellent pafrom the ancient record, but usually the strong current that makes the marks later deposits a bed of sand (or For this reason you often see the term I won't illustrate sole marks here, because I'll have slides for you later. 7. SOFT-SEDIMENT DEFORMATION 7.1 Introduction The only other kind of sedimentary structure I'll talk about here is mildly to grossly deformed strata, on scalesvarious kinds of evidence must have happened only shortly after burial, when the and buried less than a few meters. Some deformation can be shown to have anics involved would take a lot of time and space. All I'll do here is point out what must tion of the most common kinds. 98 grain-by-grain depositional mechanisms are ity is greater, and the number of grains sturbance to the sediment bed, like an earthquake, can s through a more or less large volume of the sediment. , which can do nothing but drain more through the surrounding porous sediment. to the smallness of the pore passageways, thstate for some time—long enough to deform under the influence of whatever small 7.2 Styles of Soft-Sediment Deformation a sedimentary sequence is gravita-a tendency for the material of the overlbed and (usually more diffusely) for the material of the underlying bed to rise up into the overlying material. This phenomenon is called cal and partial downward motion of the In more extreme examples of loading, whole masses of the overlying bed sink down into the underlying material. Usually these 99 Figure 3-28: Loading before after Figure by MIT OCW. masses end up with concave-up stratification that is terminated abruptly around Another kind of soft-sediment deformation is slump folding (Figure 3-30). When sediment on a slope al and recumbent. Scales range up to similar sedimentary material, emphasizing thwhen it is deformed. Such erosion is presumptive evidence of penecon-temporaneity (but you have to make sure that what you're thinking is an erosion surface isn't instead a gently dipping local 100 Figure 3-30: Slump folding 5010 cm Figure by MIT OCW. Figure by MIT OCW.