The Quaternary paleo-lacustrine deposits of Bamako and their significance for the development of the Niger River course

Sills play a crucial role in shaping river courses. They are typically natural dams that obstruct water flow and create lakes along the river. This study examines the case of the Sotuba sill in the Niger River's development in West Africa. It discusses four Quaternary outcrops in Bamako, the Niger Valley, composed of lacustrine deposits covered by fluviatile sediments, indicating a change in depositional environment. The Quaternary outcrops were mapped in the field; their azimuthal directions were recorded using GPS and a geological compass; their structures were described. Samples were collected from two outcrops for OSL dating, and a stratigraphical correlation was established. Their significance for the Quaternary development of the Niger River course was assessed. The results suggest that the Sotuba sill was a natural dam that blocked the Niger River’s flow from the upper course, leading to the formation of lacustrine deposits. It was only after about 176 ka that the flows managed to cross the dam and connect the two independent rivers, forming the current unified Niger course. The contact between the laminated lacustrine and fluvial sediments indicates that the lithofacies change was abrupt and catastrophic. Since seismic events commonly trigger this change and seismic structures (seismites) are widespread in the studied Quaternary sediments, this change likely reflects neotectonic events in the area.