Messinian subaerial erosion of the proto-Nile Delta

Abstract

The evaporitic drawdown and subsequent dessication of the Mediterranean basins during the Messinian salinity crisis has been confirmed by detailed seismic mapping of the ensuing erosion surface in the Nile Delta region. The basal Messinian unconformity, determined by seismic analysis and borehole correlation, extends from almost sea level in the southern delta area to depths of at least 5 km in the Nile Cone. The onlap of the contemporaneously deposited fluvio-deltaic clastics of the Qawasim Formation occurs at between 2.5 to 4 km below present. Gradual initial drawdown of the Mediterranean Sea with occasional sudden sporadic falls, is implied by the presence of entrenched meanders and impaired rejuvenation terraces in the Nile Canyon, formed as the proto-Nile River cut down into the pre-existing Tortonian delta complex. The former Tortonian delta slope became an extensive linear erosional front with over 3 km of vertical relief, the uppermost levels of which were protected by tabular Oligocene basalt flows, preserved as mesas and buttes. The lower slopes were the site of coalescing piedmont fans fed by upper slope wadi systems. The piedmonts merged into the Qawasim fluvio-deltaic deposits of the proto-Nile fluvial flood plain. The subcrop beneath the basal Messinian unconformity reveals a remarkable diversity of rock type, each with its own distinctive control on the geomorphology and geometry of the ensuing Messinian erosional surface. Fracture patterns within the Oligocene basalts produced a trellis drainage network which became later captured by a dendritic system developed on less resistant prodelta shales of the former Tortonian delta slope. Close study of seismic profiles clearly demonstrates that the basal Messinian erosional disconformity can be traced northwards to depths of at least 2.5–3.0 km before becoming affected by later Pliocene faulting. This validates a deep dessication model for the evolution of the Messinian evaporites.