17.2: The Origin of Planar Lamination in Sands and Sandstones

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Up until the late 1980s, the origin of planar lamination was an unsettled issue; a number of hypothesis had been proposed but none seems to have been widely accepted. Perhaps surprisingly, from the vantage point of the present, there were no reports of attempts to reproduce planar lamination experimentally in laboratory flumes, with the exception of the study by McBride et al. (1975). McBride et al. (1975), in shallow flow in a small flume, demonstrated that a variety of parallel lamination could be generated by the downstream movement of very low, depth-limited current ripples during slow overall aggradation of the sediment bed. They did not succeed, however, in producing parallel lamination under upper-flow regime plane-bed conditions—the flow condition under which it might be most natural to expect planar lamination to develop.

Early ideas on the development of planar lamination developed along two lines: downstream movement of very low-amplitude bed forms or bed waves under plane-bed conditions in the upper flow regime under conditions of slow aggradation of the bed, or the effect of features of the turbulence structure on sand transport. In a field study of a shallow sand-bed river, Smith (1971) was the first to report on development of planar lamination caused by downstream movement of low-amplitude bed forms. Models that invoke turbulence structure of the flow date back to that of Allen (1964).

The first attempt to develop a model that involves both of the effects describe above was that of Allen (1984). Later, on the basis of similar flume studies made at about the same time, Bridge and Best (1988) and Paola et al. (1989) (see also Best and Bridge, 1992, and Bridge and Best, 1997) developed a satisfying model in which the laminae are accounted for by downstream movement of very low-amplitude bed forms under upper-regime plane bed conditions, with the internal features of the laminae accounted for by the nature of the turbulence felt by the bed.