Graded bedding

Layering the graded structure is a feature of certain sedimentary rocks, in which the size of the particle sediment from the base layer toward the top decreases.

Formation

Many sediments and sedimentary rocks (gravel, sandstone, silt ) arising from sales of particles in water under varying flow conditions. The vertical differentiation of the graded stratification arises from the fact that the larger sediment fractions at a given flow rate fall to the ground faster than the smaller ones. Reduces the flow velocity, sediment always smaller particles to the underlying larger.

Addition of different flow rate can also be caused by change in the viscosity of the transport medium graded bedding; in purely geological processes, this effect is, however, less significant.

Geological conclusions

If graded bedding observed in a sedimentary rock, this allows - even in tectonically disturbed or overturned formations - a clear statement of the " top - down" the deposition sequence ( similar to the Geopetalgefüge and other top-bottom criteria).

Horizontal and vertical variation of grain size distribution allows conclusions to changes in flow conditions during deposition beyond.

Typical rocks

Graded bedding can occur in various clastic sediments. Typical representatives are greywacke turbidites and often occur in the Flysch.

In addition to the overall differentiation coarse-fine, it also happens that only the larger particles decrease upward, whereas finer, largely uniform grain forms a continuous from top to bottom base matrix. In this case, one speaks of " coarse tail grading".