Load cast

A load is a trademark deformation in unconsolidated sediment. This rather common structure on the bank side is the result of a Rayleigh-Taylor instability. The instability occurs when a layer of higher density is over a layer of lower density, and the sediment is liquefied under the influence of pressure. The trigger for the earthquake liquefaction are considered.

Disambiguation

The term stress mark ( engl. load cast, sometimes load structure ) refers to the force exerted by a heavier upper class load, which results in the lighter sub- layer to deformation, the brand. Closely related to stress brands are mass displacements such as " flame structures " (English flame structures), " stress waves " ( engl. load waves ) and so-called " anti Crests ". An extreme development are "pseudo tubers " (English pseudo- nodules ) and " stress balls " (English ball -and- pillow structure ) that is very deep ( up to 6 meters ) can sink into the lower class.

History

Load casts have been described scientifically for the first time in 1895 by T. Fuchs, who at that time still called Fließwülste. There he also managed to create the structures experimentally. Later important studies submitted by HC Sorby from 1908, by P. Kukuk from 1920 and RR Shrock from the year 1948.

Description

Load casts form ( sand, coarse sand or gravel consisting ) which overlies a less dense, hydro- plastic sub- layer ( consisting of clay, silt or fine sand) on the underside of a denser upper class. The resulting shapes are dents, calluses, sack, wart or protuberanzartig. Viewed in profile they appear to be lined up, flattened praise, which protrude into the lower layer. Which are arranged at the same distance lobes have approximately the same size and shape. Between the praise penetrate from the lower class lower density on flame-like tongues. Viewed in three dimensions resemble the praise round to oblong pillows that are separated by narrow grooves. The sequence of praise and tongues can be idealized by a series of semi-ellipses that touch each other at each of the tongue tips. The semi-ellipses can then be assigned to a characteristic wavelength L. This characteristic wavelength here is the respective density and viscosity difference between the two layers depends. Typically, it has a value between a few millimeters to 10 centimeters, but it can grow in extreme cases up to 10 yards!

Occurrence

Load casts occur in very different Faziesräumen. Most often they are in turbidites, but they also occur in fluvial and shallow marine environments. Occasionally, they are also found in lake sediments. Even in layered igneous intrusions ( engl. layered igneous intrusion ) and pyroclastic sequences can be observed. Very good examples of load casts are from the Borrowdale Volcanic Series in the Lake District and from the Bude Formation in the south west of England.

Formation

Load casts occur in muddy - sandy deposit areas, if different powerful, sandy deposits underground press in different degrees. Even larger rock fragments or other entrained in the sediment forming turbidity current material can produce these pressure differences at the sediment base.

This load casts may occur, an inverse density storage must be available. Such arrangements are in the gravitational field of the earth unstable, because their potential energy has no minimum. Load casts are an example of the instability of boundary layers in perturbed density profiles. Instability is known as Rayleigh-Taylor instability. It is driven by the higher buoyancy of the lighter bottom layer compared to the upper class.

This instability is but initially only potentially present. You also need to fluidization of the sediment, to actually appear. The fluidisation means a substantial, if not complete loss of cohesion ( engl. yield strength ) for the affected layer. That is, it must be applied to deform the material no particular minimum force. This additional condition has been recognized by Sorby in 1908 ( and later of Shrock in 1948 ) .. The fluidisation is most often triggered by shock-like movements as they occur during earthquakes. So Sims could bring in 1975 the formation of load casts in lake sediments with historical earthquake events in conjunction.

Importance

Load casts are Geopetalgefüge and thus good indicators of Hangendrichtung of the respective sediment.