Tide-dominated deltas responding to high-frequency sea-level changes, Pre-Messinian Rifian Corridor, Morocco

Abstract

ABSTRACTThe Rifian Corridor was an ancient sea strait that connected the Mediterranean Sea with the Atlantic Ocean during the Miocene. Key outcrop exposures of this corridor's sedimentary fill are exposed at the Ben Allou, El Adergha, and Driouate localities, in the Fez–Meknes region, Morocco. These display cyclic successions that formed immediately before the disappearance of the Rifian Corridor, and the associated isolation and desiccation of the Mediterranean Sea. Sedimentary cycles at Ben Allou consist of: facies 1, Organic-matter-bearing blue claystones; facies 2, gray marlstones intercalated with turbidites; and facies 3, yellow-brown, coarse-grained calcarenites. Based on their coarse grain size, presence of reactivation surfaces, bidirectional current structures, and mud drapes as well as microfossil, macrofossil and trace fossil assemblages, we interpret the calcarenites (facies 3) as prograding, strait-adjacent tide-dominated deltas transitioning from large subtidal compound dunes to intertidal sand sheets that are composed mostly of authigenic carbonate sediment. The two fine-grained facies were deposited in deeper prodelta (facies 2) and shelfal (facies 1) environments, as shown by our combined sedimentological and paleontological evidence. Cross-sectional channel geometries provide a means for reconstructing the delta's paleo–tidal range, suggesting that it was macrotidal, and likely amplified by the paleogeometry of the Southern Rifian Corridor, to at least 4.2 m. The cyclic succession of corridor fill exposed here likely is the result of three, roughly 70 m fluctuations of eustatic rise and subsequent fall, possibly linked to ∼ 100,000-year glacial–interglacial climate fluctuations. Broadly similar, contemporaneous sedimentary successions from the outcrop at El Adergha, 40 km east of Ben Allou, show that these sea-level variations affected a wide range of the Corridor, while rocks at the locality of Driouate, 9 km south of Ben Allou, show evidence for lagoonal environments on the landward side of the corridor, which were subject to periodic marine regressions and floodings. These results are reinterpretations of previous work on these sediments, with implications for depositional processes in the Rifian Corridor, Miocene paleoclimate, and the Messinian salinity crisis.