Graduated layering
The graded stratification is a structural feature of certain sedimentary rocks in which the size of the sediment particles decreases from the layer base to the top.
Emergence
Many sediments and sedimentary rocks ( gravel , sandstone , silt ) are created by the sedimentation of particles in water under changing flow conditions . The vertical differentiation of the graded stratification results from the fact that the larger sediment portions sink to the bottom faster than the smaller ones at a given flow velocity. If the flow speed decreases, smaller and smaller particles sediment on the larger ones below.
In addition to different flow rates, graded stratification can also be caused by a change in the viscosity of the transport medium; however, this effect is less important in purely geological processes.
Geological conclusions
If graded stratification is observed in a sedimentary rock, this allows a clear statement about the "above - below" of the deposition sequence - even in the case of tectonically disturbed or overturned formations (similar to the geopetal structure and other above-below criteria).
Horizontal and vertical variation of the grain size distribution also allows conclusions to be drawn about changed flow conditions during the deposition.
Typical rocks
Graduated stratification can occur in various clastic sediments . Typical representatives are often in flysch occurring greywacke and turbidites .
In addition to the overall coarse-fine differentiation, it also happens that only the larger particles decrease towards the top, while finer, largely uniform grains form a basic matrix that is continuous from top to bottom. In this case one speaks of "coarse tail grading" .
literature
- Hans Füchtbauer: Sediments and Sedimentary Rocks (= Sediment-Petrology . Volume 2 ). 4. Completely reworked. Edition. Schweizerbart'sche Verlagbuchhandlung, Stuttgart 1988, ISBN 3-510-65138-3 .
- Dieter Richter: General Geology . 4th improved and enlarged edition. de Gruyter Verlag, Berlin / New York 1992, ISBN 3-11-010416-4 .
Individual evidence
- ↑ flow ripple. ( Memento of the original from June 10, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Course: Sediment transport and sediment structures, Institute for Planetology, University of Stuttgart
