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Accommodation Space Controlled Delta Progradation in Arid Extensional Basins, El Qaa Fault Block, Suez Rift, Egypt

Abstract

Our current understanding of the sedimentary evolution in marine rift basins is biased towards examples developed under humid climatic conditions in which sediment is mainly sourced by perennial fluvial/alluvial systems. Such circumstances are seldom the case in arid to semi-arid basins, where the sources tend to be dominated by strongly seasonally controlled ephemeral processes. It is fundamental to take these factors into consideration in order to correctly asses the autogenic and allogenic controls affecting the depositional environments in arid to semi-arid settings. The spatial and temporal evolution of a prograding fan delta system and its interaction with coral-algal associations and evaporites is studied from Miocene exposures of the El Qaa half-graben, Suez Rift, Sinai Peninsula, Egypt. The results obtained constitute a valid depositional model comparable to other arid rift basins characterized by hyper-saline and marginal marine successions (e.g. pre-salt hydrocarbon plays in the South Atlantic). The eastern margin of the El Qaa half-graben is characterized by a coarse-grained deltaic succession with coral-algal deposits concentrated at the topset of the lobes. This succession progrades progressively on top of mudstone and evaporite dominated prodelta units towards the west, across the half-graben. The deltaic lobes evolve from east to west from 30 to 50 m thick conglomerate dominated units to up to 120 m sandstone dominated ones. Progradational-aggradational cycles, interpreted from the alternation of siliciclastic and carbonate bodies, appear punctuated by relative sea-level lowstands defined by the presence of the evaporites. Hangingwall subsidence and deformation is the primary control on the stacking pattern of the deltaic lobes. However, the internal architecture of the lobes reveals an interplay between the ephemeral alluvial input of sediment and its reworking in the coastal setting that proves to be a crucial factor determining the evolution of the individual lobes. The delivery of sediment from the continental source to the marine environment is determined by the magnitude of the ephemeral events. As the radius of a fan delta increases the magnitude of these events has to increase accordingly in order to be able to cross the subaerial portion of the fan delta and reach the coast. Consequently, the duration, paucity and magnitude of the alluvial events impose critical restrictions on the size of the resulting fan deltas in a depocentre.