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Concept
Graded bedding
Summary
In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Such a bed is also described as fining upward. Normally graded beds generally represent depositional environments which decrease in transport energy (rate of flow) as time passes, but these beds can also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits.
In reverse grading or inverse grading the bed coarsens upwards. This type of grading is relatively uncommon but is characteristic of sediments deposited by grain flow and debris flow. A favored explanation for reverse grading in these processes is kinetic sieving. It is also observed in aeolian processes, such as in pyroclastic fall deposits. These deposition processes are examples of granular convection.
Graded bedding is a sorting of particles according to clast size and shape on a lithified horizontal plane. The term is an explanation as to how a geologic profile was formed. Stratification on a lateral plane is the physical result of active depositing of different size materials. Density and gravity forces in the downward movement of these materials in a confined system result in a separating of the detritus settling with respect to size. Thus, finer, higher-porosity clasts form at the top and denser, less porous clasts are consolidated on the bottom, in what is called normal grading. (Inversely graded beds are composed of large clasts on the top, with smaller clasts on the bottom.) Grades of the bedding material are determined by precipitation of solid components compared to the viscosity of the medium in which the particles precipitate.
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