Sedimentoloy of Abu Habil Terminal Alluvial Fan, Kordofan State, Western Sudan

Abstract

This study investigates the sedimentology, processes, facies, fluvial architecture and evolution of Abu Habil tenninal in the semi-arid areas of Western Sudan. The study methods used included surface and subsurface facies analysis, grain size and textural analysis, heavy and clay minerals analysis and hydrochemical analysis. Abu Habil terminal fan is seasonally fed from both major point source and other multi-discharge sources from the northern Nuba Mountains. The fan shows a characteristic gradual down fan change in morphology, channel pattern and type, facies as well as fluvial and aeolian processes. The fan displays three down fan zones namely are upper zone (tributary main feeder channels), middle zone (distributary zone) and the lower or distal zone (basinal zone).

The tributary feeder channels zone is dominated by channelized flows of low sinuosity channels. Deposition occurs as channel fills, bars and interchannel and overbank fines. Aeolian sanddunes and sand sheets are also reported in thin zone. The gravel, sand and clay ratios in this zone are 1: 15: 4 respectively. Channel types in this zone are influenced locally by the clay substrate, sediment load and vegetation leading to development of small, narrow high sinuosity channels. The distributary zone is characterized by channelized and sheet flows and a network of low sinuosity channels which show gradual decrease in width and depth. Active aeolian processes contribute sand dunes and sheets in this zone and in some parts the channels were completely burried and abandoned. The gravel, sand and clay ratios in this zone are 1: 48 and 22 respectively. The basinal zone shows the dominance of fine facies and sheet flooding over channelized flows. The small high sinuosity narrow ephemeral channels usually vanishe or terminate eventually into shallow clay dessicated ephemeral ponds. Aeolian processes also contribute sand dunes and sheets in this zone. The subsurface fine facies in this zone show interbedded salts and evaporite facies which may suggest playas or saline lakes setting. The surface and subsurface facies indicates that Abu Habil facies may represent the final stage of basin fill development of the White Nile rift sub-basins during late Miocene - Recent. Paleo Abu Habil fan appears to have passed through an earlier phase of a distributary system which terminated into playa or saline lakes. The present distributary system of the fan represents the late stage of development where aeolian processes dominate. Change in climate from humid to dry conditions appears to have played a major role. This is similar to climatic change control on river basins of central Sudan during the late Pleistocene to early Holocene Time. The grain size analysis reflects the down fan decrease in grain size and the dominance of fine facies in the basinal zones. The heavy mineral distribution and abundances show the dominance of the ultrastable over the meta-stable assemblage which reflects the variation of the source rock geology of the Nuba Mountains, dispersal patterns from both point and multi-sources, as well as the contribution of aeolian transported recycled detrital constituents to the fan sediments. The clay mineral assemblage consists in decreasing abundances of kaolinite, smectite, illite and chlorite. The clay mineral assemblage reflects processes operating under both humid and arid climate where conditions vary from intensive weathering, leaching and evacuation to poor and limited weathering, leaching and evacuation in poorly drained areas. Clays may include both inherited and authigenic types.