Abstract

Laminated sand and laminated rhythmites are produced by a variety of physical processes and are hence known from several specific environments in association with a variety of structures and textures. Laminated sands associated with plane bed phases of the upper flow regime are mostly produced in shallow water environments like rivers, ephemeral streams, tidal flats, channels and gullies, but have also been found in deep-sea turbidites and in tsunami deposits. Laminated sands are characteristic of beach deposits where they are produced by the swash and backwash. On the shoreface they are produced by strong oscillatory flow and by deposition of sand clouds stirred up by breakers. Other important processes producing laminated sand are: grain fall deposition in the lee of transverse bedforms such as ripples and dunes, as well as other steeply sloping structures like beach ridges; sedimentation of sand clouds in currents experiencing sudden flow reduction due to enlarged channel cross sections, or on natural levees of rivers and tidal channels due to overbank flow; sedimentation of drift sand behind a negative step with flow separation (grain-fall laminae). An important mode of formation of laminated sand is by transcurrent processes, whereby laminae of finer sand alternate with those of coarser sand or heavier particles alternate with lighter particles. In the case of water ripples the coarser lamina is at the base, whereas in the case of migrating wind ripples the coarser lamina is at the top.

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