Abstract

The shelf of the eastern Tyrrhenian Sea margin changes substantially in width, shelf-break depth, and sea-floor steepness over relatively short distances, largely owing to pronounced lateral changes in subsurface geologic structure. Remnants of late Pleistocene prograded coastal deposits are locally preserved on the middle and outer parts of the shelf. We recognize two major controls on the distribution, lateral extent, thickness, and preservation potential of these prograded deposits. First, prograded (downlapping) deposits formed only where the physiographic shelf break was deeper than the lowstand shoreline, thus providing accommodation space for the lowstand deposits. Second, the proximity and relative size of sediment sources and the local coastal dispersal system apparently influenced the geometry of the deposit. Midshelf and shelf-margin bodies composed of seaward-steepening downlapping reflectors were deposited as thin to thick continuous prograding sheets over an irregular eroded shelf surface and onto the shelf edge during the latest fall and lowstand of sea level. Decreased sedimentation at the end of this lowstand resulted in net erosion. During sea-level rise, shoreface erosion created a major marine erosional (ravinement) surface landward of the 120-m isobath, and much of the downlapping deposits were removed. Preservation of these downlapping deposits is largely a function of their thickness. Thick continuous deposits are common on the shelf edge, whereas on the midshelf only thin remnants are preserved locally where depressions or morphologic steps were present in the shelf surface.

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