Processes, facies and architecture of fluvial distributary system deposits

Abstract

There is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down-flow and transitions from proximal, channelised to distal, unconfined flow. These deposits form fan-shaped bodies several tens of kilometres in radius, and their stratigraphic architecture is aggradational, with no evidence of deep incision driven by base-level fall. The fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today: an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation. A conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation. Proximal areas are characterised by amalgamated coarse, pebbly and sandy channel deposits with little preservation of overbank facies. Channel dimensions are generally smaller in the medial areas, but sizes are variable: deposits are of braided, meandering and simple channels which show varying degrees of lateral migration. The channel-fills may be mud or sand, with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion. The proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow. Interconnection of sandstone bodies is poor in the distal areas because channel-fill bodies are sparse, small and are not deeply incised. The radial pattern of the sediment body forms by the repeated avulsion of channels: active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas. The term ‘fluvial distributary system’ is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand.