Messinian badlands on the southeastern margin of the Mediterranean Sea

Abstract

Substantial subaerial denudation of the North African continental margin took place during the Late Miocene salinity crisis. The Nile drainage was deeply entrenched as far as 1200 km inland, upto the Aswan cataract. Beneath the modern birdfoot delta the thalwegs of ancient Messinian river valleys lie as much as 2.5 km below present sea level. The relief between the top of the incised Late Miocene coastal plain and the axis of excavated channels exceeds 1.2 km. In the subsurface of the lower reaches of the modern Nile Cone a network of bifurcating entrenched channels descends to 4.6 km below sea level to become covered by the southern limit of the Messinian Evaporite Formation. A paleodrainage system is also present beneath the Levantine margin. Seismic reflection profiles document an extremely rough and steeply gullied morphology which is analogous to the badlands of South Dakota. In addition, reflection profiles reveal uniform and gentle seaward gradients of the preerosion (Jurassic to Late Miocene) and posterosion (Pliocene and Quaternary) sedimentary successions beneath the coastal plain and continental shelf. The closely comformable nature of all these depositional surfaces strongly indicates that the badland topography does not owe its origin to an epiorogenic uplift and tilting of the continental edge, but, instead, to a lowering of base level brought about by an unprecedented eustatic drop in water level within isolated Mediterranean basins. Calculations which consider the progressive isostatic loading of both the onshore and offshore sedimentary prism indicate a fall of the base level of at least 3.5 km. This accounts for the observed depth in the Levantine Basin at which Messinian-age shallow-water evaporites lap against the contemporaneous North African subaerial erosional surface.