Development of submarine canyons after the Mid-Pleistocene Transition on the Ebro margin, NW Mediterranean: The role of fluvial connections
Résumé
After the Messinian Salinity Crisis (MSC), the Ebro Margin, like other Mediterranean deltaic margins, rebuilt through progradation and increasingly significant aggradation. The Plio-Quaternary transition from a ramp-like system to a “new” margin with large clinoforms is an opportunity to understand the processes that govern canyon initiation and evolution. We used a 3D seismic data set located at the outer shelf - upper slope of the Ebro margin. We tied major bounding surfaces to chrono-stratigraphic constraints from borehole data or, for the most recent interval, from averaged accumulation rates derived from borehole stratigraphy. The major shelfal erosion surfaces are interpreted as sequence boundaries, tied to major glacial maxima. Along these surfaces, seismic attributes characterize the fluvial/canyon connection, viewed as one of the key factors in canyon development. The first appearance of “proto-canyons” (dense networks of channels and gullies 50–100 m deep and 1–2 km wide) occurs in the late Zanclean. Their size increased progressively throughout the Pliocene and early Quaternary, in relation to the increase in clinoform heights. “True canyons” (with distinct interfluves, more than 200 m deep and 3–4 km wide) appeared during the Mid-Pleistocene Transition (MPT, 1250–700 ka BP). The first evidence of direct connection between canyon heads and fluvial systems is observed during Marine Isotope Stage 22, one of the most pronounced glacial stages of the Quaternary. Similar connections also existed, at least, during MIS 16 and MIS 12, which are also stages of pronounced low sea level. The topography of fluvial channels in the outer shelf is not imaged in detail in the picked horizons at the resolution of our seismic data, but sinuous fluvial systems are very well imaged through amplitude and coherency attributes. Direct connection of fluvial systems during and after MIS 22 also favored headward erosion and the formation of shelf-indenting canyons, probably during early phases of sea-level rise.