Bedforms and associated sedimentary structures formed under supercritical water flows over aggrading sand beds

Bedforms and associated sedimentary structures, formed under supercritical water flow over an aggrading sand bed, were studied in a laboratory flume. Although the geometry and hydraulic characteristics of these bedforms (antidunes, chutes-and-pools) are well known, their internal structures are not. The objectives of the study were to: (1) describe the three-dimensional geometry of the sedimentary structures and examine their mode of origin; (2) develop a relationship between the geometries of the sedimentary structures and the formative bedforms and; (3) identify criteria that distinguish these sedimentary structures from similar types, such as hummocky and swaley cross-strata. Sedimentary structures associated with antidunes are primarily lenticular laminasets with concave-upward erosional bases (troughs) in which laminae generally dip upstream or fill the troughs symmetrically. These laminasets are associated with growth and upstream migration of water-surface waves and antidunes, and with surface-wave breaking and filling of antidune troughs respectively. In addition, sets of downstream-dipping laminae are produced by rapid migration of asymmetrical bedwaves immediately after wave breaking. Rare convex-upward laminae define the shape of antidunes that developed under stationary water-surface waves. The laminasets and internal laminae extend across the width of the flume, but vary in thickness and inclination, indicating that the antidunes have some degree of three dimensionality. The length and maximum thickness of the lenticular laminasets are approximately half of the length and height of formative antidunes, providing a potentially useful tool for palaeohydraulic reconstructions. The sets of downstream-dipping laminae formed under antidunes are distinctive and do not occur in hummocky and swaley cross-strata. Sedimentary structures associated with chutes- and-pools are sets of upstream-dipping laminae and structureless sand.