Plio-Quaternary geological events in the Western Mediterranean Sea: focus on the West Sardinian margin and adjacent oceanic basin

Abstract

The Plio-Quaternary sedimentary deposition in the Western Mediterranean Sea was strongly influenced by the Messinian Salinity Crisis (MSC) and by the consequences of the oceanic opening that produced regional fault systems and the following thermal subsidence, still largely active during the Plio-Quaternary (PQ). We analyse the PQ events that occurred in the West Sardinian margin and in the adjacent deep basin by integrating vintage and more recently acquired seismic data, obtaining the currently most complete regional seismic grid in the study area. The base of the PQ (“Ms” reflector) represents the top of the Messinian evaporites in the lower slope and deep basin and the Messinian erosional truncation in the continental upper slope and shelf. Two units have been recognized within the PQ sequence: the low amplitude lower Plio-Quaternary unit (l-PQ) and the high amplitude upper Plio-Quaternary unit (u-PQ), separated by the “A0” reflector, for which we assume an age of 2.6 My (near Quaternary base), through the correlation with the published ECORS profile. The thermal subsidence, related to the Oligo-Miocene (OM) oceanic opening, produced the increased inclination of the slope and, coupled with the halokinetics of Messinian evaporites, triggered most of the geological processes in the study area. In the lower continental slope, rollover structures are produced by salt sliding, which is related to the increased deepening of the slope, while in the deep basin typical sub-vertical faults developed above the salt diapirs: these processes, that continued throughout the entire PQ slowing down in the Quaternary, influence the thickness and distribution of the PQ sequence. Faults usually act as a preferential path for magma upwelling and gas rising: fault systems developed during the OM produced some large volcanoes at the boundary between slope and deep basin, while on the continental shelf and upper slope the main volcanic buildings are ascribed to the later Pliocene magmatic phase and are related to fault reactivation caused by the PQ thermal subsidence. On the tilted continental outer shelf, OM faults reactivation led to gas rising phenomena and related pockmarks, generated from the Early Pliocene until Present. During the Quaternary, the accentuated tilting of the continental slope triggered erosional processes that led to the formation of three new canyon systems, not inherited by the Messinian erosion as often hypothesized; at the same time, erosion of the onshore area led to a high sediment supply, responsible for the widening of the clinoforms on the inner shelf. In this study we analyse the evolution of the different PQ process that affected the West Sardinian margin and their relationships with previous regional events occurred in all the West Mediterranean Sea: the objective is to create basic information to subsequently compare with other passive margins of the sea.