Gilbert-type deltas recording short-term base-level changes: Delta-brink morphodynamics and related foreset facies

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Abstract

Gilbert-type deltas are sensitive recorders of short-term base-level changes, but the delta-front record of a base-level rise tends to be erased by fluvial erosion during a subsequent base-level fall, which renders the bulk record of base-level changes difficult to decipher from the delta-front deposits. The present detailed study of three large Pleistocene Gilbert-type deltas uplifted on the southern coast of the Gulf of Corinth, Greece, indicates a genetic link between the delta-front morphodynamic responses to base-level changes and the delta-slope sedimentation processes. Sigmoidal delta-brink architecture signifies a base-level rise and is accompanied by a debrite-dominated assemblage of delta foreset deposits, thought to form when the aggrading delta front stores sediment and undergoes discrete gravitational collapses. Oblique delta-brink architecture tends to be accompanied by a turbidite-dominated assemblage of foreset deposits, which are thought to form when the delta-front accommodation decreases and the sediment carried by hyperpycnal effluent bypasses the front. This primary signal of the system response to base-level changes combines further with the secondary ‘noise’ of delta autogenic variation and possible allogenic fluctuations in fluvial discharge due to regional climatic conditions. Nevertheless, the evidence suggests that the facies trends of delta foreset deposits may be used to decipher the delta ‘hidden’ record of base-level changes obliterated by fluvial topset erosion. Early-stage bayhead deltas may be an exception from the hypothetical model, because their narrow front tends to be swept by river floods irrespective of base-level behaviour and their subaqueous slope deposits are thus mainly turbidites.

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