The Upper Neoproterozoic lacustrine–fan delta depositional systems associated with braided alluvial fans in the Nubian Shield, Egypt

Abstract

The present paper affords new data and results that enhance and refine the lacustrine and alluvial fan depositional models in the Precambrian. Well-exposed siliciclastic profiles along the intermontane Kareim Basin in the Nubian Shield signify an excellent occurrence of the Late Neoproterozoic lacustrine/fan delta and alluvial fan settings. Detailed sedimentologic data are integrated to define thirteen depositional facies that belong to six main facies associations (FA1 to FA6). Incorporation of facies architectures, physical sedimentary structures, stratigraphic relations and depositional patterns is used to suggest depositional models that comprise three interrelated depositional phases. The subaerial humid to semi-arid depositional phase of the braided alluvial fan has been suggested to interpret the sedimentological pattern and facies architecture of the conglomerate and sandstone deposits (FA1 to FA3) which are attributed to debris flow, sheet flood and copious braided stream processes. The depositional pattern of the transitional phase is represented by distributary channels and mouth bar strata (FA4) of a shoal-water fan delta that has indistinctive prodelta deposits and a non-vegetated delta plain of the fringing alluvial fan. The subaqueous phase demonstrates accumulations of sandstone and mudstone-dominated deposits (FA5–6) in wave-dominated lacustrine nearshore and offshore, under humid to semi-arid climates and recurrent short-aridity periods. This Neoproterozoic lake was characterized by relatively large extent, shallow depths, an insignificant profound zone and remarkable starvation of stromatolites, evaporites and carbonate sediments. Allogenic factors such as differential uplift and subsidence, climatic fluctuations and varied sediment supply control and modulate the depositional patterns and facies models. Outcome datasets and results can be applied in distinguishing lacustrine from shallow-marine siliciclastics, and may be of further help in discrimination of humid/semi-arid lakes from arid lakes and playas in the Precambrian non-vegetated and unfossiliferous successions.