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Low Stand Shelf-Edge Deltas and Linked Slope Depositional Systems, on a Steep, High-Relief Rifted Margin (Northeast Sicilly, Southeastern Thrrhenian Sea)

Abstract

Shelf-edge deltas develop preferentially during low stand of sea level, when the coastline is close to the shelf break, and are in general linked to slope and base-of-slope depositional systems. Shelf-edge deltas have been described along ancient and modern margins but examples from actively rifting, steep, high relief margins, like those described in this paper, are missing. The study area is located along the northeastern Sicilian margin and has been investigated through the interpretation of multibeam and chirp subbottom data and aims at illustrating shelf-edge delta characteristics and their linkage to sedimentary processes and depositional architecture in the adjacent slope. The shelf-edge deltas formed in the study area during the last low stand of sea level and turn up as seaward protruding bulges at the present day shelf break. The shelf-edge deltas form a discontinuous belt elongated parallel to the shelf break and were possibly formed by wave reworking of riverborne sediment. The shelf-edge delta bulges are split in coincidence of the slope channel heads, which during the last low stand of sea level were connected with incised valleys in the outer shelf. A dominance of fluvial processes prevented sediment accumulation at the river mouths and initiated slope channel excavation and sediment bypass to the slope. The shelf-edge deltas make up the external part of the shelf and the upper slope, comprising a relatively gentle delta front and a steep delta slope both floored by coarse grained deposits. Further downsolpe, in the middle slope, fine grained sediment prevail. In the upper and middle slope, channels do not develop levees and show a cut and fill history. Their fining upward infill reflects the progressive landward retreat of the coastal systems during the initial transgression of the shelf. In the lower slope, in coincidence with a gradient decrease coinciding with an extensional fault, channels develop laterally extensive levees. Landslides are widespread but do not reach the shelf edge, thus they do not appear to play a principal role in channel nucleation. However, landslide scars are exploited as route for the slope channels, that in turn contribute to the infill of the evacuated areas. In general our study serves as an analogue for interpreting the distribution and the extent of the various depositional bodies that make up low stand depositional systems developed at the shelf break and in the slope of steep, rifted margins.