Backset lamination produced by supercritical backwash flows at the beachface-shoreface transition of a storm-dominated gravelly beach (middle Pleistocene, central Italy)

Bedforms and resulting sedimentary structures of interpreted upper-flow-regime origin are fairly common in various ancient and modern depositional settings, yet outcrop examples of shallow-water strata dominated by sedimentary structures arising from such flow conditions are rarely documented. This outcrop-based study presents the first detailed analysis of mid-Pleistocene beachface-shoreface strata containing sedimentary structures interpreted to have been formed by storm-generated backwash flows during the transition in flow regime from supercritical to subcritical. Here we propose that moving down the steep beachface, backwash flows rapidly accelerated and became thinner, experienced abrupt deceleration and flow thickening passing through an erosional hydraulic jump in the trough at the toe of the beachface, and then waned to thicker and subcritical conditions just downflow of the hydraulic jump. In this frame, landward-dipping backset laminae deposited on the downflow side of the trough evolved into thinner, upper-stage plane-parallel laminae further downstream. The transition from the proximal to the distal upper shoreface records a significant increase of preserved wave ripples and burrowed intervals, in concert with a progressive decrease in thickness and grain-size of individual sets of plane-parallel laminae and increase of gravel-filled gutter casts.