ERSC 2P16 Lecture 23: Bed forms and stratification under unidirectional flows pt 4
7 Aug 2015
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Due to textural and mineralogical variation of the sediment making up the cross laminae or beds. Well-sorted sand of only quartz may not display cross-stratification. Part a. coarsening upwards laminare may develop with avalanching down the bed form"s lee slope. Dispersive pressure, which acts upwards, away from the bed, is proportional to the size of the grains, pushing large grains further upward than fine grains. Fining upward cross-laminae may reflect pulses of deposition from suspension onto the bed form"s lee slope. Coarsest grains settle first followed by increasingly finer grains forming a layer that fines upward. Part c. layers enriched in particular mineral, particularly opaque, heavy minerals, may deposit episodically on the lee face as accumulations enriched in those minerals pass over the brink. Note that the form of internal cross-strata is determined by the geometry of the bed form. As a bed form migrates, there is erosion in the trough and stoss slope and deposition on the lee slope.
