UPPER-FLOW-REGIME STRATIFICATION TYPES ON STEEP-FACE, COARSE-GRAINED,GILBERT-TYPE PROGRADATIONAL WEDGES (PLEISTOCENE, SOUTHERN ITALY)

Gravelly Gilbert-type progradational wedges 30-90 m thick developed du ring the Pleistocene in the Bradanic area (foredeep of southern Apenni nes). Facies within foresets and toesets are essentially included in t wo major groups: (1) massive or graded beds, and (2) beds with widespr ead 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 su ggested that the lamination reflects temporal and spatial variations i n bed shear stress and lift forces acting on sediment particles in mot ion over the bed and related to burst/sweep cycles. The lenticular fac ies is inferred to be generated by migration and aggradation of long-w avelength antidunes. The backsets were probably produced by rapid upst ream migration of submerged hydraulic jumps and related erosional pool s, accompanied by sediment accretion on the downstream Banks of the po ols, As the scours are infilled, a standing-wave condition may be esta blished, generating wavy bedforms with long trochoidal profiles at the tops of backsets. In complete sequences a waning-flow trend is record ed by the transition from backset and standing-wave laminae to planar laminae, and then ripple cross-laminae. The presence of backsets on th e foreset slope may reflect the spontaneous formation of hydraulic jum ps without the need of break in slope or obstructions to flow. Althoug h the various facies may individually be present everywhere on the for eset and toeset, with common short-distance mutual transitions suggest ing high intrinsic instability of flows, a general pattern of downslop e change exists, from planar sheetlike stratification and lamination t o lenticular/undular units; the transition may reflect a sequence of u pper-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 ne ar the tip of the foreset slope suggests that Bows responsible for sed iment dispersal were commonly generated on the topset platform and wer e already turbulent and supercritical before spreading onto the forese t slope. The nearly ubiquitous presence of wave-worked topset gravels supports the hypothesis of the importance of storm-driven flows in pro moting gravity resedimentation on the foreset slope. The source was pr obably a wave-dominated coastal belt with open-fetch, short-headed bra id deltas highly affected by longshore drift.