Morphology and morphogenesis of modern subaqueous drainage systems: implications for the origin of the Mediterranean Messinian canyons

Abstract

The Messinian Salinity Crisis (MSC) has been an important ecological crisis, occurred in the Mediterranean area between 5.97 Ma and 5.33 Ma. Actually, two main models are proposed in literature to explain the accumulation of such an amount of evaporites and related erosional surfaces. The first model, named “Shallow Water – Deep Basin” (SWDB), concerns the deposition of this massive amount of evaporites implying the almost complete desiccation of the Mediterranean basin and its transformation into a giant salt desert with subsequent development of a widespread erosional feature (the so called “Messinian Erosional Surface” – MES) along the Mediterranean shelves and slopes. A second model, named "Deep Water - Deep Basin" (DWDB), theorizes that the Mediterranean sea-level remained similar or slightly lower than ocean mean sea-level and the evaporites precipitated on the bottom of a hypersaline basin, with the development of the MES as deep-sea originated major feature. Nowadays, the first model is considered in a likely dogmatic view, with the morphologies of the MES interpreted as originated under subaerial conditions. Nevertheless, the presence of subaqueous evidences, such as the likely submarine origin of basinal evaporites sampled from DSDP cores, ensures that the debate must be still heated. In the effort to give an answer to the “Messinian Salinity Crisis issue”, the main aim of this work is to compare erosional and depositional features originated under submarine conditions with the morphologies related to the MES, in order to discriminate their subaerial or subaqueous nature. The part of this thesis concerning the deep-sea channelized systems addresses the identification of the processes involved in the evolution of erosional and depositional features and related main differences, through the analysis and interpretation of modern and recent turbidite systems developed along the Eastern Sardinian Margin and of the Indus fan. The Sardinian turbidite systems, developed along a passive margin characterized by intraslope basins confined by structural highs, have been analyzed through high-resolution multibeam data and integrated with CHIRP seismic profiles and Side Scan Sonar profiles, all acquired by ISMAR (Institute of Marine Science) of Bologna during several cruises. The Indus channelized system have been studied through a 3D seismic dataset acquired by the British Gas and analyzed at the University of Aberdeen. From this work, a 5-step model illustrating the interactions between erosional processes acting on the slope and on the shelf during canyon development has been theorized. In its early stages, the model is supported by examples located along the Eastern Sardinian Margin. In the first step, localized erosional features develop along the shelf break, due to mass failures processes, shelf-sourced flows and/or sediments overloading, and along the base of the slope as consequences of mass wasting processes. Subsequently (step 2) the two erosional areas show a simultaneous entrenchment respectively basinward and landward, that eventually merge (step 3), generating a knickpoint morphology (step 4) that continues its retrogression up to the shelf break (step 5). Furthermore, three main types of depositional fans, such as first stage fan, transient fan and basin plain fan, have been described and, whenever possible, related to the different stage of development described in the 5-step model. Finally, through the analysis of erosional valleys and internal identifiable features, the buried part of the Indus canyon system is classified as a long-term expression of a mature (5th step) channelized system. Finally, the analysis of the Picocca, Capo Ferrato and Colostrai turbidite systems, located in the Sarrabus and Ogliastra basins (southern part of the Eastern Sardinian Margin), provided an example of evolutionary interference between well-developed channelized systems. The second chapter of the thesis focuses on the analysis of the erosional and depositional features associated with the Messinian Erosional Surface, while the comparison between deep-sea channelized systems and Messinian channelized systems is treated in the discussion chapter. The results of this research highlights how the erosional and depositional features of the MES usually identified as "subaerial" can be actually encountered in both fluvial and submarine environments, especially considering that the seismic profiles containing the MES surface have a too low resolution to allow the identification of the details needed for a definitive distinction. Furthermore, there is an important lack of knowledge about the pre-MSC topography and paleobathymetry as well as the absence of direct data (core samples and biostratigraphic data) inherent the sediments immediately under and above the MES. Although there is the tendency to interpret MES features as subaerial not through certain scientific data, the evidences available so far are actually insufficient to draw a definitive interpretation of what happened in that period, and the question remains opened until new data would give new hypothesis and certainty to work on.