The Messinian Ebro River continental margin (NW Mediterranean) from 3D seismic reflection data: timing of erosion and preservation of clastic deposits

Abstract

Partial desiccation of the Mediterranean during the Messinian Salinity Crisis (MSC) implied the subaerial exposure of the continental margins for a relatively short time span (>1 Myr). This was enough for a complex subaereal drainage network to develop and erode continental margin sediments creating a regional unconformity known as the Messinian Erosional Surface (MES). 3D seismic data from the Ebro Margin, NW Mediterranean Sea, shed new light on the processes that formed the Messinian erosional Surface and the areal extent of the clastic deposits associated with the erosion. The 3D seismic data provide an unprecedented and detailed view of the MES and display characteristic features of subaerial incision and fluvial deposition, including a drainage network with tributaries of at least five different orders, terraces and meandering channels. The Messinian landscape presents a characteristic stepped-like profile that allows the margin to be subdivided in three different regions roughly parallel to the coastline. No major tectonic control exists on the boundaries between these regions. The boundary between the two most distal regions marks the location of a relatively stable base level, and this is used in backstripping analysis to estimate the magnitude of sea level drop associated with the MSC on the Ebro Margin (1.3 km). The MES on the Ebro Margin is dominated by a major fluvial system, that we identify here as the Messinian Ebro River. In the relatively short time period of the MSC the Ebro River was able to carve a 1.3 km deep subaerial canyon. Coupled isostasy and river incision and transport modeling offer constraints on the evolution of the Ebro River. Flexural isostatic compensation of sediment and water column changes indicate that lithospheric elastic thickness (Te) had a major role in determining the direction adopted by the main river within the subaerially exposed delta. The meandering nature and low gradient of the Ebro River long profile suggest that equilibrium conditions were reached. The 3D seismic data, onshore geology and modeling results suggest that the Ebro River drained the Ebro Basin well in advance of the Messinian. Preservation of the Messinian Ebro River fluvial deposits imply that the erosional surface was formed during sea level drawdown or during the lowstand stage and that inundation of the margin during the Zanclean was extremely rapid. This in turn indicates that the high-porosity/permeability facies of Messinian fluvial deposits (potential reservoirs) are well preserved along the margins of the Western Mediterranean Sea