Evoluzione dei bacini profondi del Mediterraneo documentata dalle variazioni nelle velocità di sedimentazione nel Plio-Pleistocene

The data base for this study is represented by 46 DSDP-ODP drillsites that penetrated the sediments of the various Mediterranean basins for several hundred meters. Sedimentation rates were calculated for three discrete, well defined stratigraphic intervals extending from 5.33 Ma to time zero. A great dispersal of values, ranging from less than 5 up to 30 cm/1000 y is recorded in the Pleistocene, with maxima measured in the Nile cone, as a result of a high sediment supply from the longest river in the world, and in the Marsili basin, the deepest and youngest subbasin of the rapidly subsiding Tyrrhenian back-arc basin. In the late Pliocene the values recorded on structural highs are approximately half of those from basinal settings, whereas drillsites on basin margin often display depositional gaps. Starved basin conditions characterize the early Pliocene from 5.33 to 3.9 Ma, where sedimentation rates are consistently low, both on highs and on lows. We interpret this as a response to the rapid sea-level rise following the Messinian «salinity crisis» (Pliocene transgression). During the substantial draw-down when the water level in the playa lakes was two to three kilometers lower than world-wide sea-level, the major tributaries of the Mediterranean overincised their riverbed, and sedimentary aprons were formed in the deep basins. No sediment supply was available soon after the rapid sea-level rise, and approximately one million years elapsed before a new equilibrium was re-estabilished.