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. The present work aims to verify and refine the existing Early Pleistocene (∼1.8–0.8 Ma) 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.