Original paper

Some new insights on fluvial dynamics and Holocene landscape evolution in the Nigerian Chad Basin

Gumnior, M.


The paper features some recent results on the Young Quaternary floodplain development in the middle to lower catchment of the Komadugu Yobe, a main northeast-Nigerian tributary to Lake Chad. The major aim of the study was the reconstruction of spatiotemporal changes in the fluvial response, the assessment of the system as one of the last untapped regional palaeo-ecological archives, and the chronological integration into the existing climato-stratigraphic models of the southwestern Chad Basin. For this purpose, a lithostratigraphical data base was created (41 sediment cores) and supplemented by age determinations (20 OSL datings). The morphogenetic information about ancient and modern depositional environments was derived from palaeo-geographical and -hydrological interpretations of diverse remote sensing data (air photographs, Landsat-5 imagery).Investigations focussed mainly on the large Komadugu terrace or palaeo-floodplain located upstream of the Bama Beach Ridge, which marks the high stand of Lake Chad during Holocene wet periods. It turned out that the ancient alluvium consists primarily of well-sorted fine and very fine sands, which occur both in structures of lateral accretion (e.g. point bars) and in channel fills. The palaeo-floodplain does not contain extensive stillwater facies, as could have been assumed in view of the supposed damming of Komadugu waters under humid conditions by the Bama Ridge. The lithofacial units correspond to the hybrid palaeo-channel morphology that combines meandering, braided and anastomosing drainage patterns with a high channel/floodplain ratio. The latter results from frequent avulsions in response to dominant downstream factors such as local tectonics and a high sediment load at a shallow and uneven floor. The datings reveal that alluvial deposition has typically occurred in transition periods. Especially the Pleistocene-Holocene transition is well represented, and it appears that extensive fluvial reworking has taken place between 13 and 10.5 kyr (approximately 11 and 9 14C kyr BP). The both spatially and temporally considerable morphodynamic activity was probably controlled by a lasting irregularity in rainfall and discharge, which is thought to have characterized the Komadugu catchment in the Early Holocene.