![]() ![]() They are a product of deposition from a slowing current (Figure 6.4.4). “Normal” graded beds are coarse at the bottom and become finer toward the top. Graded bedding is characterized by a gradation in grain size from bottom to top within a single bed. (On each ripple the last deposited layer is represented by small dots.) ![]() Each ripple advances forward (right to left in this view) as more sediment is deposited on its leading face (small arrows). Figure 6.4.3 Formation of cross-beds as a series of ripples or dunes migrates with the flow. Cross-bedding is a very important sedimentary structure to be able to recognize because it can provide information on the process of deposition, 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 direction of flow, and is partially eroded by the following ripple (Figure 6.4.3). In most of the layers the cross-beds dip down toward the right, implying a consistent wind direction from right to left during deposition.Ĭross-beds form as sediments are deposited on the leading edge of an advancing ripple or dune under steady state conditions (similar flow rate and same flow direction). Figure 6.4.2 Cross-bedded Jurassic Navajo Formation aeolian sandstone at Zion National Park, Utah. Cross-beds formed 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 6.0.11, 6.1.7b, and 6.4.2. 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 within otherwise horizontal beds, and it forms when sediments are deposited by flowing water or wind. Bedding can form in almost any sedimentary depositional environment. ![]() Partings may represent periods of non-deposition that could range from a few decades to a few millennia. 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 6.4.1). In addition to these principles, that apply to all sedimentary rocks (as well as volcanic rocks), a number of other important characteristics of sedimentary processes result in the development of distinctive sedimentary features in specific sedimentary environments.
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