Grain size and transport regime at shelf edge as fundamental controls on delivery of shelf-edge sands to deepwater

Abstract

Relative sea-level fall and high sediment supply can be decisive in driving shelf-edge sands to form deep-water fans. However, a closer look at a series of lowstand shelf-edge deltas on flattish or downward prograding shelf margins yields two further important insights on this paradigm. Firstly, among 42 reviewed shelf-edge delta examples, only 24 river-, or wave-dominated shelf-edge deltas with supply of sand-dominated sediment fit sequence-stratigraphic models that link relative sea-level fall to submarine-fan growth. Contrary to conventional lowstand models, it is virtually impossible for lowstand shelf-edge deltas with supply of mud-dominated sediment (as evidenced by 18 reviewed examples without fan growth) to partition large volumes of shelf-edge sands into deep-water sites to form sandy basin-floor fans, even under the scenarios of river-dominated process regimes and sufficient sea-level fall. Grain size of the supply sediment (dominantly sandy or muddy, represented by the presence or absence of sandy upper delta fronts) therefore also plays a pivotal but underappreciated role in driving shelf-edge sands into deepwater, further modulating the conventional lowstand sand delivery concept. Secondly, contrary to recent suggestions that link basin-floor fan growth mainly to river-dominated shelf-edge process regimes, wave-dominated shelf-edge deltas with either high or low supply of dominantly sandy sediment (6 of 42 examples) can also foster sandy basin-floor fans, in conditions of direct linkage between deltas and slope-channel heads. These exceptional conditions, although long known, are less rare than believed, thus further modifying the evolving delivery paradigm.