Horizontal lamination and the sequence of bed phases and stratification under upper-flow-regime conditions

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ABSTRACT

Two forms of horizontal laminae have been produced under upper-flow-regime conditions in a flume by deposition on a plane bed and on low in-phase waves. Upper plane bed laminae consist of approximately equal numbers of fining-upward (FU) and coarsening-upward (CU) textural laminae; heavy minerals, when present, are typically within or form CU laminae. In horizontal laminae formed under low in-phase waves, FU laminae may predominate and are significantly thicker than CU laminae; heavy minerals are associated with the tops of some FU laminae and are absent from CU laminae. Deposits aggraded on plane beds and low in-phase waves both display low-angle (13–14°) upstream grain imbrication. Upper plane-bed horizontal laminae are attributed to the effects of the burst/sweep cycle on local bedload transport (bursts form FU laminae and sweeps form CU laminae). Some FU laminae in in-phase wave deposits are thought to form with downstream migration of bed waves on which sand size decreases from trough to crest. Heavy minerals are concentrated in the tops of these FU laminae due to their enrichment on the crests of the migrating bed waves. Other FU and CU laminae in in-phase wave deposits probably formed by the action of bursts and sweeps, as on Upper plane beds.

A sequence of bed phases and stratification formed under upper-flow-regime conditions is described based on the observations here combined with descriptions from the literature. With increasing flow strength the gradual sequence of bed phases is: (1) plane bed, (2) low relief, downstream-migrating inphase waves increasing in amplitude and wavelength with increasing flow strength, (3) stationary in-phase waves, (4) upstream-migrating (including breaking wave) antidunes. The sequence of stratification formed under these bed phases is: (1) upper plane bed horizontal lamination, (2) in-phase wave horizontal lamination to in-phase wave foreset cross-laminae, (3) in-phase wave drape laminae, (4) antidune backset cross-laminae.

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