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Sediments and Sedimentary Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Many clastic rocks display another common texture—called graded bedding—characterized by a systematic change in grain size from coarse at the base of the bed to fine at the top. Figure 8.46 shows a good example. Clasts in the conglomerate in this figure vary (grade) from coarse at the bottom to fine at the top. Sometimes graded bedding involves very fine grains or is so subtle that we can only see it with a hand lens. Graded bedding generally forms because coarse, heavy grains are deposited from suspension before lighter grains are, so sediments pile up with coarse grains on the bottom and fine grains on the top. Grading forms, for example when river velocity slows and grains of different sizes, starting with large followed by small, settle to the river bed. This kind of texture may develop when submarine sediments slide down a continental slope and slowly settle in deeper water. The coarser material settles first and finer sediment is deposited on top. Repetitive marine deposits of this sort are termed turbidites. For example, the strata shown in Figure 8.42 are turbidite deposits and, if examined with a hand lens, exhibit graded bedding. (At the scale of the photos shown, however, the grading cannot be seen.) Some rare rocks exhibit inverse graded bedding, with coarse material on top of fine. This type of bedding is most commonly associated with debris flows related to kinds of mass wasting. Figure 8.15 shows one example of inverse grading created when submarine landslides deposited poorly sorted material in undersea canyons.
Minerals, rocks and sediments
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
For general purposes, the two major types are graded bedding and cross-bedding. Graded bedding (Figure 4.12A) is a very common form of stratification in which grain size increases towards the base of each bed. Such vertical grading may be produced in a number of ways, such as by differences in the settling velocities of sedimentary particles in a given sedimentary ‘flush’, by rapid pulsating sedimentation under stable conditions, settling from turbidity currents within water or air (e.g. nuees ardente), or a general decrease in grain size in the provenance during slower sedimentation over a longer period of time. Such a consideration of sedimentation mechanisms shows the wide range of sedimentation rates which are possible, varying from the virtually instantaneous deposition by floods, mudflows, or sand dune invasion to much slower rates under offshore marine conditions. Some thin beds and laminations can be directly related to individual seasonal (e.g. varves) or tidal variations in sedimentation.
Distribution of rocks at and below the surface
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
Original layering in sediments is called stratification or bedding. (Different terms are used for the layering produced by other igneous and metamorphic processes.) Each layer is referred to as a stratum (plural, strata) or bed, though the latter term is normally applied only to sedimentary rocks and volcanic ash layers. It is never applied to the layer formed by a basaltic or rhyolitic lava flow. The interfaces between beds are called bedding planes. The outcrops of major bedding planes that separate thick beds of different rock type are depicted on geological maps. Bedding planes may also occur within a thick layer of sandstone, where minor changes of composition or texture, or even a break in deposition, are present. These are planes of weakness, or potential planes of failure, and their presence produces a change of mechanical properties of the rock with the direction in which they are measured (that is, the bulk properties of the rock are anisotropic). Minor bedding planes are abundant in micaceous shale, because mica flakes lie in the bedding direction as they grow in the rock or settle in water. Rocks that are devoid of bedding and other planes of weakness such as joints (see Section 4.4.4) are said to be massive.
Evidence for non-marine Jurassic to earliest Cretaceous sediments in the pre-breakup section of the Mentelle Basin, southwestern Australia
Published in Australian Journal of Earth Sciences, 2020
C. C. Wainman, I. Borissova, D. L. Harry, R. W. Hobbs, D. J. Mantle, A. Maritati, E. Y. Lee
Facies Association B comprises a combination of sandstones (very fine to medium-grained), silty sandstones, silty claystones and claystones that are found in both fining and coarsening-up successions (e.g. 508–512 mbsf). Bedding contacts are typically diffuse and gradational. This facies association exhibits a variety of sedimentary structures including ripple cross-lamination, parallel lamination/horizontal bedding, wavy bedding and convoluted bedding with roots. Between 440.30 and 441.60 mbsf in the U1515A core, the convoluted bedding is characterised by (1) small-scale furrows/depressions (2–3 cm wide), which are underlain by (2) decimetre-scale compaction/deformation suggesting a deformation force from above, and (3) sediment onlap/infills of the depressions/furrows (Figure 6). Bioturbation is overall absent to rare in this facies association.