Sedimentary processes and resulting continental margin configuration during large-scale sea-level drawdown: The Messinian Salinity Crisis in the Western Mediterranean Sea

Abstract

[eng] At the end of the Miocene (5.97-5.33 Ma), the Mediterranean basins underwent deep morphological and sedimentological changes as a result of the large-scale sea-level fall during the so-called Messinian Salinity Crisis (MSC) and subsequent fast sea-level rise in the 3 Zanclean. Whereas deep basins accumulated more than 1 million km of evaporites, continental margins recorded several erosion surfaces. Through the analysis of 2D and 3D seismic reflection data and 2D numerical modelling, this Thesis attempts to contribute to better understand the sedimentary processes and resulting continental margin configuration during the large-scale Messinian sea-level drawdown. Detailed mapping of the seismic stratigraphy from the continental margin to the deepwater basin of the Valencia Trough (Western Mediterranean) has revealed two Messinian depositional units (Complex and Upper Units) and four erosional surfaces (Margin Erosion Surface, and Bottom, Top and Intermediate Surfaces). The stratigraphic relationship and margin configuration of these ‘Messinian seismic markers’ suggest that during the Miocene and until late Tortonian, the Mediterranean passive margins were characterized by prograding depositional sequences in the context of a Highstand System Tract. At that time, a sedimentary- active continental slope and delta progradation developed in a normal regressive in the Ebro margin, suggesting context associated to a pre-Messinian Ebro River. During late Messinian, the large-scale sea-level drawdown resulted in the initial development of a Falling Stage System Tract. The subaerial exposure of the shelf and upper slope of the Valencia continental margin at the beginning of the sea-level drawdown – probably in combination with local factors – induced destabilization of the continental slope and deposition of major Mass-transport Deposits (MTD). Results obtained from 2D numerical modeling of coupled transient seepage and deformation analysis and stability analysis suggest a decrease in hydrostatic pore pressures and development of seepage and stress-induced excess pore pressures. As the sea-level dropped, depocenters displaced offshore, major Mediterranean rivers incise to adapt to the base level, and high sediment supply contributed to the development of submarine channels. Concurrently, a submarine paleo-Valencia channel formed in the deepwater Valencia Trough. The gradually slow-down of the sea-level fall allowed the development of a smooth surface at the top of the Lowstand System Tract, whereas the long- time exposure of the proximal shelf and upper slope resulted in a rough erosional surface deeply carved by drainage networks. The Margin Erosion Surface (MES) attained full development on the margins and the MTDs were partially eroded. The development attained by the Messinian Ebro network during the MSC corroborates that the capture of the Ebro Basin occurred prior to the MSC. A slow transgressive phase before the final rapid re-flooding, together with high evaporation rate, favored aggradational deposition of the Upper Unit in a shallow-water basin. At the end of the MSC, the reestablishment of the Mediterranean-Atlantic connection triggered the rapid sea-level rise and the return marine conditions, supported by the outstanding preservation of the fluvial deposits on the main valleys of the MES.