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Petroleum Geological Survey
Published in Muhammad Abdul Quddus, Petroleum Science and Technology, 2021
Graded bedding strata is defined by coarse-grained heavier sediments at the lower base and fine-grained light sediments at the top. Graded bedding rock is created by the principle of ‘sediment size and gravity sorting’. Bedding rock is formed by flowing turbid water. The turbid water is saturated (loaded) with sediments. When the motion of the turbid water (turbidity current) drops, the unloading (settling) of sediments takes place. The sequence of settling follows the order of the size and weight of the sediments. The largest and heavier sediments are the first to settle down, followed by the medium sized, and finally the smaller sediments separate out and settle down. The bottom bed contains larger particles, the middle bed has medium-sized particles and the top bed light particles.
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.
Geological setting of the Moorowie Formation, lower Cambrian Hawker Group, Mt Chambers Gorge, eastern Flinders Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2020
T. J. Mount, J. B. Jago, N. R. Langsford, C. R. Dalgarno
Limestone beds with scattered dolomitic siltstone intraclasts are widespread, as for older units of the Mernmerna Formation. Graded bedding is common and is evidence for sediment distribution by turbidity currents. Oscillation ripple marks, parallel to N 48° E are a feature of the mid-section of Unit 6, south of Mt Daily.