Abstract

Gravelly Gilbert-type progradational wedges 30-90 m thick developed during the Pleistocene in the Bradanic area (foredeep of southern Apennines). Facies within foresets and toesets are essentially included in two major groups: (1) massive or graded beds, and (2) beds with widespread internal lamination, including units with sheet-like geometry and planar lamination, lensing layers with broadly convex-up or wavy upper boundaries (with internal lamination ranging from planar or low-angle, boundary-unconformable, to curved, boundary-conformable), and scour-filling "backsets" (sets of upslope-dipping cross-laminae). It is suggested that the lamination reflects temporal and spatial variations in bed shear stress and lift forces acting on sediment particles in motion over the bed and related to burst/sweep cycles. The lenticular facies is inferred to be generated by migration and aggradation of long-wavelength antidunes. The backsets were probably produced by rapid upstream migration of submerged hydraulic jumps and related erosional pools, accompanied by sediment accretion on the downstream flanks of the pools. As the scours are infilled, a standing-wave condition may be established, generating wavy bedforms with long trochoidal profiles at the tops of backsets. In complete sequences a waning-flow trend is recorded by the transition from backset and standing-wave laminae to planar laminae, and then ripple cross-laminae. The presence of backsets on the foreset slope may reflect the spontaneous formation of hydraulic jumps without the need of break in slope or obstructions to flow. Although the various facies may individually be present everywhere on the foreset and toeset, with common short-distance mutual transitions suggesting high intrinsic instability of flows, a general pattern of downslope change exists, from planar sheetlike stratification and lamination to lenticular/undular units; the transition may reflect a sequence of upper-flow-regime bed phases with increasing densiometric Froude number down the foreset slope. Although typically developed on the toe, sets of backset cross-laminae are nearly ubiquitous, and their presence near the tip of the foreset slope suggests that flows responsible for sediment dispersal were commonly generated on the topset platform and were already turbulent and supercritical before spreading onto the foreset slope. The nearly ubiquitous presence of wave-worked topset gravels supports the hypothesis of the importance of storm-driven flows in promoting gravity resedimentation on the foreset slope. The source was probably a wave-dominated coastal belt with open-fetch, short-headed braid deltas highly affected by longshore drift.

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