New insights into the age and formation of the Ankarokaroka lavaka and its associated sandy cover (NW Madagascar, Ankarafantsika natural reserve)

Description

Lavaka represent a typical erosional landform in Madagascar. The chronology of their formation remains, however, under discussion. Our research focuses on the Ankarokaroka lavaka, a spectacular landform located in NW Madagascar (Ankarafantsika natural reserve), which is characterized by the presence of sandy units of regional extension at its top. The two main units correspond to white and red sands, and are closely associated with specific vegetations (dry dense forest for the white sands, savannah grasslands for the red sands). We applied a geochronological approach based on Optically Stimulated Luminescence (for the coversands) associated to radiocarbon dating performed on archaeological remains found at the contact between the sands and the lavaka. The combination of this approach with field work and sedimentological analyses makes it possible to show that the sands experienced a complex history, both in terms of sedimentation and post-sedimentary pedogenesis (podzolisation of the white sands, rubefaction of the red sands). The numerical ages furthermore indicate that the Ankarokaroka lavaka formed between 18.5±2.3 ka ago and the 14th century AD. The present study demonstrates that this lavaka has a climatic origin, and highlights the potential of OSL to date sediments associated with Madagascar lavaka.

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