Pleistocene muddy forced-regression deposits on the Adriatic shelf: A comparison with prodelta deposits of the late Holocene highstand mud wedge

The upper Pleistocene-Holocene stratigraphy of the Adriatic margin consists of a succession of depositional sequences separated by shelf-wide unconformities. The most recent of these depositional sequences encompasses the last ca. 25 kyr, and consists of lowstand, transgressive and highstand units formed, respectively, during the Last Glacial Maximum lowstand, the subsequent sea level rise and the modem highstand. Older deposits are grouped into four depositional sequences, each recording ca. 100 kyr glacio-eustatic cycles during an interval between ca. 450 and 25 kyr BP. Each of these Pleistocene sequences is composed dominantly by muddy progradational units deposited during prolonged phases of sea level fall; therefore, these sequences represent a unique example of well-preserved muddy forced-regression deposits. Detailed seismic-stratigraphic analyses show that forced-regression deposits composing the four sequences are not directly connected to nearshore and/or fluvio-deltaic environments. In contrast, typical forced-regression deposits defined in outcrop and reported in the literature consist of sand-rich, nearshore/deltaic facies preserved as patchy remnants of ancient shorelines. We suggest that muddy forced-regression deposits on the Adriatic shelf represent the offshore counterpart of shoreface deposits recognised in outcrop as the product of typical forced regressions. Interestingly, muddy forced-regression deposits on the Adriatic shelf bear clear similarities with the late Holocene highstand mud wedge, suggesting a common depositional mechanism through lateral advection of fine-grained sediment. In this view, subaqueous progradation of muddy deposits occurs below storm wave base, with a subaqueous roll-over point that, during proceeding sea level fall, likely positioned at decreasing distance from the time-equivalent shoreline. (c) 2005 Elsevier B.V. All rights reserved.