Relict landscapes and fluvial landforms: Catastrophic outflow following a major Late Messinian base-level fall

Dia Ninkabou, Julien Gargani, Christian Gorini, Jean-Loup Rubino, Roger Urgeles, Romain Pellen, Damien Do Couto, Alexandre Lethiers, Bilal Haq, Jimmy Moneron

During the Messinian Salinity Crisis (MSC), the entire Mediterranean basin experienced deep canyon incision along its margins as the result of sea-level variations and rapidly increasing salinity. Yet, the processes and water sources capable of generating such dramatic incision have never been quantitatively demonstrated. Using high-resolution 3-D seismic reflection data and paleo-stream network geomorphic analyses, we reveal that a section of the Messinian Ebro valley is characterized by two distinct phases of fluvial dynamics. The initial phase is associated with an exceptional discharge of 240.103 m3/s, rapidly carving a 700-m-deep valley, and is followed by a period with more moderate discharge through the newly formed Messinian valley, forming alluvial terraces. Here we argue that the previously endorheic Ebro Basin was reached by regressive erosion during the MSC lowstand, triggering a major outburst flood and development of a wide and deep drainage system, transitioning to a more confined, meandering system. This study also refines the timing of this extraordinary outflow event to the MSC itself, contradicting earlier models that placed the Ebro Basin opening earlier. To the best of our knowledge, the complete drainage of an endorheic basin induced by a Messinian drawdown regressive erosion has not previously been described from the stratigraphic archive on Earth.