At the confluence of the Nyabarongo River with the Mukungwa River, to the North of present Rwanda, there are thick alluvial sedimentary sequences, which can reach 30 m in thickness, where they are capped by tufa deposits. From these deposits, two different sedimentary sequences have been identified, with.(i) a first sedimentation stage characteristic of an obstructed valley controlled by contrasted seasonal flows during a dryer and cooler climate than at present, and (ii) on top of this unit a more recent sequence of alluvial terraces that were emplaced during a wetter and warmer context allowing the development and stabilisation of the rainforest. Palynological data and their comparison with a database at the East African regional scale have proven that the first rapid sedimentation stage started around 40,000 BP and might have ended abruptly around 14,000 BP. This process has been then followed by a natural embanking stage of the alluvial corridor. These different terraces have been dated thanks to a series of proxies: the discovery of a Stenoece animal remains in the upper part of a terrace confirmed the palaeo-origin of the sequence; a bone harpoon with typical manufacturing characteristics that indicated a human settlement around 9,000 BP on another terrace; and the tufa deposits provided a maximum age for the terraces below 7,000 years. This multi-proxy approach, therefore, provides an interesting series of benchmarks for the development of the palaeoenvironment in Northern Rwanda and is of high importance for the reconstruction of the river bio-geomorphological adaptation to climatic changes.
Wassmer, P; Schwartz, D; Gomez, C; Ward, S; Barrere, P
GEOMORPHOLOGY AND SEDIMENTARY STRUCTURES OF UPPER PLEISTOCENE TO HOLOCENE ALLUVIUM WITHIN THE NYABARONGO VALLEY (RWANDA). PALAEO-CLIMATE AND PALAEO-ENVIRONMENTAL IMPLICATIONS
Geografia Fisica E Dinamica Quaternaria