Graded bedding

In geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Normally graded beds generally represent depositional environments which decrease in transport energy as time passes, but 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 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. It is also observed in eolian ripples. 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 will result in a separating of the detritus settling with respect to size. This will result in less dense, higher porosity clasts at the top and denser, less porous clasts 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 will precipitate from. Steno’s Principle of Original Horizontality explains that rock layers form in horizontal layers over an underdetermined time scale and depth. Nicholas Steno first published his hypothesis in 1669 after recognizing that fossils were preserved in layers of rock (strata).

For materials to settle in stratified layers the defining quality is periodicity. There must be repeated depositional events with changes in precipitation of materials over time. The thickness of graded beds ranges from 1 millimeter to multiple meters. There is no set time limit in which the layers will be formed. Uniformity of size and shape of materials within the bed form must be present on a present or previously horizontal plane.

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