Abstract
Bedforms related to supercritical flows and hydraulic jumps have been increasingly recognized in various depositional environments. However, few ancient examples have been investigated in terms of the variations and transitions of supercritical-flow deposits. We present a detailed analysis of the sedimentary facies and stacking patterns of cyclic step, chute-and-pool, and antidune deposits of a coarse-grained Gilbert-type delta in the middle Pleistocene Kamiizumi Formation, Shimosa Group, central Japan. The Kamiizumi Formation was deposited during marine isotope stages 8â7.5 in the paleo-Tokyo Bay basin. We describe the following four facies and two facies successions in the foreset beds exposed at the study site. Planar-parallel stratification represents antidune to subcritical deposits from high-density flows. Deep scours infilled by steeply dipping backsets are interpreted as chute-and-pool deposits. Shallow, lenticular troughs infilled with backsets and foresets are interpreted as chute-and-pool and unstable antidune deposits. Lenticular units of fine sand with low-angle backsets of considerably greater wavelength than the other three facies are indicative of cyclic step deposits. One facies succession is characterized by chute-and-pool deposits alternating antidune to subcritical deposits formed during progradation of foresets. The lateral cyclicity of coarser-grained chute-and-pool deposits and finer grained antidune to subcritical deposits is characteristic of autogenic flow changes on progradational steep delta slopes. The other facies succession represents an aggradational delta succession from cyclic step deposits, to chute-and-pool deposits, to antidune to subcritical deposits. The vertical changes indicate decreasing flow velocity and may be the result of autogenic flow transitions during rapid aggradation. These facies changes resulted from the intrinsic instability of supercritical flows on steep foreset slopes at high depositional rates.
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