Quaternary fluvial environments in NE Morocco inferred from geochronological and sedimentological investigations

Abstract

The investigation of fluvial archives in NE Morocco is of high interest for unravelling palaeoenvironmental changes linked to Quaternary climate fluctuations, long-term tectonic activity and/or human influence. The prehistoric site of Ifri n'Ammar is situated in NE Morocco and represents a key location in unravelling the history of anatomically modern humans (AMH) in northern Africa as it reveals Middle and Late Palaeolithic occupation phases since ~170 ka. Therefore, this study uses two fluvial systems of different nature – the ephemeral stream Wadi Selloum and the perennial Moulouya River – in order to reconstruct the varying environmental conditions for the last ~170 ka, the time when AMH started to disperse into the region. Both fluvial systems provide valuable insights into the geomorphic evolution of the study area. It has been shown that the two fluvial systems responded to different environmental triggers: the small catchment of the Wadi Selloum is highly affected by the sensitive ecosystem of the Mediterranean region. The ephemeral stream is characterised by a discontinuous and heterogeneous sediment record caused by short-term climatic shifts and human influence. In contrast, tectonic activity appears to be the main driver for the evolution of the lower Moulouya terraces, rather than climate cyclicity during the Quaternary. Establishing chronostratigraphies of river sedimentary sequences always remains challenging. However, based on different luminescence dating techniques (OSL, pIRIR, TL), electron spin resonance (ESR) dating using the multiple centres approach (Al and Ti centres of quartz), and palaeomagnetic analyses, geochronological frameworks for the two fluvial archives located in the direct vicinity of Ifri n'Ammar could successfully be established. First numerical ages in this area yield burial ages of the lower Moulouya terraces and the Wadi Selloum deposits dating back to Early Pleistocene and Late Pleistocene/Holocene times, respectively. In addition to the use of absolute and relative dating techniques, laboratory (sedimentology, geochemistry, mineralogy) and microscopic (micromorphology) analyses have been carried out in order to identify periods of enhanced flooding in both fluvial systems on the one hand, and periods of pedogenesis on the other. The Wadi Selloum gives information about morphodnamic phases in the time of AMH settling: periods of enhanced aggradation occurred around ~100 ka, ~75 ka, ~55 ka, after the LGM, and during the Holocene, whilst sedimentation ended after ~1.3 ka. Pedogenesis may be used as environmental indicator for more humid climate conditions during MIS 3 (palaeo-Calcisol), the early Holocene (Calcisol) and the late Holocene (Fluvisol). Although palaoenvironmental implications should be taken with caution due to the discontinuity of the ephemeral stream system, it appears that more humid and warmer climate conditions favoured human settling in this area. This study thus provides first insights into the palaeoenvironmental changes around the rock shelter of Ifri n'Ammar during the last glacial-interglacial cycle.