![]() ![]() Ripples, which are associated with the formation of cross-bedding, may be preserved on the surfaces of sedimentary beds. ![]() The upper few centimetres of fine clay may have accumulated over a few hundred years. The lower several centimetres of sand and silt probably formed over the duration of an hour. Figure 6.22 A graded turbidite bed in Cretaceous Spray Formation rocks on Gabriola Island, B.C. Most graded beds form in a submarine-fan environment (see Figure 6.17), where sediment-rich flows descend periodically from a shallow marine shelf down a slope and onto the deeper sea floor. Some graded beds are reversed (coarser at the top), and this normally results from deposition by a fast-moving debris flow (see Chapter 15). “Normal” graded beds are coarse at the bottom and become finer toward the top, a product of deposition from a slowing current (Figure 6.22). Graded bedding is characterized by a gradation in grain size from bottom to top within a single bed. Each ripple advances forward (right to left in this view) as more sediment is deposited on its leading face. #Diagram of undisturbed strata youngest rock series#Figure 6.21 Formation of cross-beds as a series of ripples or dunes migrates with the flow. Cross-bedding is a very important sedimentary structure to recognize because it can provide information on the direction of current flows and, when analyzed in detail, on other features like the rate of flow and the amount of sediment available. Each layer is related to a different ripple that advances in the flow direction, and is partially eroded by the following ripple (Figure 6.21). One bed dips in the opposite direction, implying an abnormal wind.Ĭross-beds form as sediments are deposited on the leading edge of an advancing ripple or dune. In most of the layers the cross-beds dip down toward the right, implying wind direction from right to left during deposition. Figure 6.20 Cross-bedded Jurassic Navajo Formation aeolian sandstone at Zion National Park, Utah. Cross-beds in streams tend to be on the scale of centimetres to tens of centimetres, while those in aeolian (wind deposited) sediments can be on the scale of metres to several metres. Some examples are shown in Figures 6.1, 6.8b, and 6.20. Bedding is defined by differences in colour and texture, and also by partings (gaps) between beds that may otherwise appear to be similar.Ĭross-bedding is bedding that contains angled layers and forms when sediments are deposited by flowing water or wind. Bedding can form in almost any depositional environment. Partings may represent periods of non-deposition that could range from a few decades to a few centuries. Bedding is an indication of changes in depositional processes that may be related to seasonal differences, changes in climate, changes in locations of rivers or deltas, or tectonic changes. By understanding the origins of these features, we can make some very useful inferences about the processes that led to deposition the rocks that we are studying.īedding, for example, is the separation of sediments into layers that either differ from one another in textures, composition, colour, or weathering characteristics, or are separated by partings - narrow gaps between adjacent beds (Figure 6.19). In addition to these principles that apply to all sedimentary rocks, a number of other important characteristics of sedimentary processes lead to the development of distinctive sedimentary features in specific sedimentary environments. The principle of faunal succession states that there is a well-defined order in which organisms have evolved through geological time, and therefore the identification of specific fossils in a rock can be used to determine its age.For example, the cobbles in a conglomerate must have been formed before the conglomerate. The principle of inclusions states that any rock fragments in a sedimentary layer must be older than the layer.The principle of superposition states that sedimentary layers are deposited in sequence, and that unless the entire sequence has been turned over by tectonic processes, the layers at the bottom are older than those at the top.The implication is that tilted sedimentary layers observed to day must have been subjected to tectonic forces. The principle of original horizontality states that sediments accumulate in essentially horizontal layers.Through careful observation over the past few centuries, geologists have discovered that the accumulation of sediments and sedimentary rocks takes place according to some important geological principles, as follows: Chapter 6 Sediments and Sedimentary Rocks ![]()
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