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ABSTRACT: Geometry, Lateral Variability, and Preservation of Downlapped Regressive Shelf Deposits, Eastern Tyrrhenian Margin, Italy

Michael E. Field, Fabio Trincardi

The shelf of the eastern Tyrrhenian margin changes substantially in width, shelf-break depth, and sea-floor steepness over relatively short distances, largely due to marked lateral changes in geologic structure. Remnants of late Pleistocene prograded coastal deposits are locally preserved on the middle and outer parts of this complex shelf. Through our studies of these prograded deposits we recognize two major controls on the distribution, lateral extent, thickness, and preservation potential. First, prograded (downlapped) 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 sy tem influenced the geometry of the deposit.

Mid-shelf 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 last fall and lowstand of sea level. A dearth of sediment at the end of lowstand conditions led to a switch from deposition to erosion.

During sea level rise, shoreface erosion produced a major marine erosional (ravinement) surface landward of the 120-m isobath, and much, and in many places all, of the downlapping deposit was removed. Preservation of downlapping deposits is largely a function of their thickness. Thick continuous deposits are common on the shelf edge, whereas on the mid-shelf only thin remnants are preserved locally where depressions or morphologic steps were present in the shelf surface.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990