Cross-bedding
Cross-bedding (English cross - bedding ) is a term sedimentology and refers to the deposition of granular sediment (usually sand or gravel) in oblique incident layers. This type of deposit is found on the leeward side of dunes, as well as current and wave ripples in water. The cross-bedding in superposed layers can be obtained by changing flow conditions in the deposition of different or contradictory oriented ( cross stratification ).
Formation
Cross-bedding can be caused by both flowing water and wind. Their formation can be observed in deserts, rivers, lakes and seas, and is geologically young unconsolidated rocks and consolidated sedimentary rocks, eg Sandstone spread obtained. It is caused by the preliminary bed and deposition of sand grains in the lee of sandbanks or bumps. Cross-bedding is also located on the leeward side of gravel bars where the cobbles are adjusted imbricate.
Cross-bedding also occurs when loose sediment is transported up the windward side of a wave ripples in a river or the sea, a dune, mostly as a jumping freight ( saltation ), passes the ripple crest and then the leeward side slides down ( grain flow) or falls. The leeward side is always steeper than the windward side formed. By erosion processes, the crests of the wave ripple can be removed, so that only the lower portions of the cross-bedding body are retained.
Differences rather than morphological end members are tabular and bogige cross-bedding. The contact surfaces of the individual cross-bedding body are flat, curved in cross-bedding in the tabular arched. Cross-bedding occurs in different dimensions, that depends on the height of the cross-bedding of the micro- cross-bedding ( height up to 2 cm) over small ( 2-20 cm) and mitteldimensionale (20 - 200 cm ) for large dimension cross-bedding ( 2 to 20 m and more ) rich. The dimension reflects the flow rate of the respective transport medium.
The incident angle of the swash stratification is dependent on the cohesion of the transported material, and is due to frequent charging of the grains of sand dunes in about 30 °, in the water because of the lower cohesion, due to the transport medium such as 15 °.
The orientation of the preliminary bed can be used for determine the direction of the causal flow and plays a role in the reconstruction of surface shape and deposit transactions of the past era.