Hydraulic and Sedimentological Reconstruction of a Middle/Late Pleistocene Boundary Superflood (Ebro River, NE Iberian Peninsula)

Abstract

Fluvial superfloods are geomorphologically significant episodes due to their enormous energy and extremely low recurrence in the geological record. Their rarity is particularly notable in areas far from the major continental glacial masses. Hydraulic constrictions in the Lower Ebro River gorges caused the accumulation of fluvial deposits, as floodwaters from the Ebro River were forced upstream into the tributary valleys of the Móra Basin. The most illustrative example is the Comte Creek, where two depositional units of different ages show upstream flood accumulations (extending up to ~4 km) with very high-energy sedimentary structures, followed by downstream reworking during backflow. Two-dimensional hydraulic simulations indicate that peak discharges reaching 385,000 m3·s−1, with water depths of approximately 28 m in the confluence area, would have been required to transport sediments to the observed outcrop positions. Luminescence dating of the slackwater deposits indicates that the formative processes occurred during the latest Middle Pleistocene or the Last Interglacial (Eemian).