Cliff overstep model and variability in the geometry of transgressive erosional surfaces in high-gradient shelves: The case of the Ionian Calabrian margin (southern Italy)

CHIRP subbottom profiles performed on the Ionian Calabrian margin, southern Italy, allow the recognition of an irregular step-like seabed topography and a marked variability in the geometry of the post-Last Glacial Maximum (LGM) transgressive wave ravinement surface (WRS). The WRS truncates an older unit showing locally an opaque seismic character, and displays both concave-up and concave-down profiles in any dip section and a local stepped geometry, with gradients between 1 degrees and > 20 degrees. A younger unit, characterized by high-amplitude reflectors onlapping or draping the WRS, is interpreted as a sediment wedge accumulated during the post-LGM glacio-eustatic rise. The WRS crops out forming steep slopes up to 12 degrees, which are interpreted as relicts of palaeo-coastal cliffs generated by wave action during relative sea-level rise. In particular, the deeper slope is locally exposed between ca. 75 and 100 m water depth, corresponding closely with the depth range of melt-water pulse (MWP) 1A (14.3-14.0 ka BP), assuming no subsidence or uplift, and its generation is inferred to be related to this event. A transgressive model for high-gradient settings and stepped sea-level rise is proposed. Coastal cliffs develop and retreat due to wave erosion during phases of slow relative sea-level rise. During phases of very high rate of relative sea-level rise, coinciding with melt-water pulses, cliffs tend to be overstepped, drowned and not completely eroded by the WRS. Such a 'cliff overstep' transgressive model is the equivalent in high-gradient settings of the 'in-place drowning' model, developed for low-gradient shelves. The present model may be effective in reconstructing stepped sea-level rises and the evolution of shelf areas during Late Quaternary time. (C) 2011 Elsevier B.V. All rights reserved.