Autogenic cyclicity of foreset sorting in experimental Gilbert-type deltas

Abstract

Gilbert-type deltas are commonly characterised by steep foreset bedding. Changes in the foreset characteristics such as grain size, grading, alternating sand and gravel are often interpreted as records of changes of base level or sediment input due to diurnal, seasonal or climatic forcing. To aid such interpretations, an important question is to what extent cyclicity in the foreset characteristics can be explained by autogenic processes rather than exogenic forcing. Experimental deltas were generated with gravelly sand under constant boundary conditions and a varying width–depth ratio of the feeder system. The foresets of the narrow feeder system are regular and show gradual upward fining. The foresets of the wide feeder system, in contrast, show quasi-cyclic patterns of coarsening and fining during the delta progradation. The cyclicity is caused by the emergence of a channelised point feeder system with migrating gravel side bars, which distributes the sand and gravel laterally in a non-uniform manner. This spectacular change of foreset architecture is fully explained by the autogenic response of the feeder system to the width. These experiments are contrasted with experiments and data from literature in the framework of the delta classification of Postma [Postma, G., 1990. Depositional architecture and facies of river and fan deltas: a synthesis. In: Colella, A. and Prior, D.B. (eds), 1990. Coarse-grained Deltas. Spec. Publ. 10 of the Int. Ass. of Sedimentologists, Blackwell Scientific, Oxford, UK, 13–28]