Morphodynamics and sedimentary structures of bedforms under supercritical-flow conditions: new insights from flume experiments

27 Supercritical flow phenomena are fairly common in modern sedimentary environments, yet their 28 recognition and analysis remain difficult in the stratigraphic record. This fact is commonly 29 ascribed to the poor preservation potential of deposits from high-energy supercritical flows. 30 However, the number of flume datasets on supercritical flow dynamics and sedimentary 31 structures is very limited in comparison with available data for subcritical flows, which hampers 32 the recognition and interpretation of such deposits. The results of systematic flume experiments 33 spanning the full range of supercritical flow bedforms (antidunes, chutes-and-pools, cyclic steps) 34 developed in mobile sand beds of variable grain sizes are presented. Flow character and 35 related bedform patterns are constrained through time-series measurements of bed 36 configurations, flow depths, flow velocities and Froude numbers. The results allow the 37 refinement and extension of some widely used bedform stability diagrams in the supercritical38 flow domain, clarifying in particular the morphodynamic relations between antidunes and cyclic 39 steps. The onset of antidunes is controlled by flows exceeding passing a threshold Froude 40 number. The transition from antidunes to cyclic steps in fineto medium-grained sand occurs at 41 a threshold mobility parameter. Sedimentary structures associated with supercritical bedforms 42 developed under variable aggradation rates are revealed by means of combining flume results 43 and synthetic stratigraphy. The sedimentary structures are compared with examples from field 44 and other flume studies. Aggradation rate is seen to exert an important control on the geometry 45 of supercritical flow structures and should be considered when identifying supercritical bedforms 46 in the sedimentary record. 47