Single-grain TT-OSL dating results confirm an Early Pleistocene age for the lower Moulouya River deposits (NE Morocco)

Abstract

The lower Moulouya River (NE Morocco) drains a tectonically active area related to the NW-SE convergence of the African and Eurasian plates. Fluvial deposits preserved in the lower Moulouya have been dated to ∼1.5–1.1 Ma, as part of a recent multi-technique geochronology study involving electron spin resonance and luminescence methods, in combination with Palaeomagnetism. The present work aims to verify and refine the existing Early Pleistocene ages for the Moulouya deposits using single-grain thermally transferred-OSL (TT-OSL) dating. The single-grain TT-OSL De distributions are characterised by high overdispersion (77–91%), significant negative skewness, and several discrete populations can be identified when applying the finite mixture model (FMM). The lowest FMM dose components of the TT-OSL datasets comprise relatively dim grains that have very slow decays. The Fast Ratio (FR) was therefore used to explore whether the presence of slower-decaying TT-OSL components might have exerted a significant effect on our De values. Our samples show a 40–50% increase in weighted mean De and a 50–100% decrease in overdispersion when applying a FR acceptance threshold of 2, resulting in the elimination of the lowest FMM component. Application of a higher FR value does not result in any additional change in TT-OSL De value. Dose recovery tests confirm the suitability of the single-grain TT-OSL protocol and use of an additional FR acceptance threshold of ≥2 for final age determination. Previous geomorphic interpretations suggested a capture event occurred at the Beni Snassen gorge between 1.04 and 1.36 Ma at the latest. This interpretation is supported by the newly obtained TT-OSL ages, which reveal that fluvial deposition occurred between ∼1.09 and ∼1.15 Ma.