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.

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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.

The Upper Eocene – Miocene Cypress Hills Formation of the Cypress Hills plateau, in southwestern Saskatchewan and southeastern Alberta, is interpreted as a braidplain deposit. The regional paleoslope dipped to the northeast, and the east–west distribution of outcrop exposes facies representing lateral variation across the slope of the braidplain. Overall, the average clast size of the deposits decreases from west to east, with western area sediments dominated by boulder-sized gravels deposited on longitudinal bars. The eastern outcrop area contains deposits of braided channels cut into and interbedded with finer interchannel material including lacustrine marlstones, silcretes, and debris-flow deposits, the latter commonly containing abundant fossils.The gravels of the Cypress Hills Formation are multicyclic; they were originally derived from the western ranges of the Rocky Mountains during Laramide orogenesis and later shed farther into the basin during rebound due to unloading of the Laramide thrusts by erosion. Most recent transport resulted from uplift by intrusive activity of the Sweetgrass Hills, the Bearpaw Mountains, and the Highwood Mountains in northern Montana. Transport from the uplifted source areas was largely restricted to valley-confined rivers with the braidplains beginning beyond the valley termini. The lateral extent of the gravel braidplain was limited by the position of valleys and resulted in the observed variation in facies. Climate, as indicated by the sedimentology, faunal assemblages, silcretes, and palynology, was semi-arid with seasonal rainfall.