Wave-enhanced sediment-gravity flows and mud dispersal across continental shelves: Reappraising sediment transport processes operating in ancient mudstone successions

Abstract

Recent studies of marine shelf sediment dispersal show that wave-enhanced sediment-gravity fl ows are widespread phenomena and can transport large volumes of fl uid mud rapidly across low-gradient shelves. Flow evolution is controlled by sediment supply, seabed gradient, and spatial distribution of wave energy at the seabed. Using existing fl ow models, we predict that such fl ows in mud-dominated sediments will develop a three-part microstratigraphy produced by changing fl ow conditions, beginning with wave-induced turbulent resuspension, then development of a wave-enhanced sediment-gravity fl ow, prior to lutocline collapse and suspension settling. Petrographic examination of modern fl ow deposits collected from the Eel Shelf reveals that resultant beds possess a microstratigraphy consistent with our hypothesis: a silt-rich basal subunit with curved ripple laminae, abruptly overlain by a subunit composed of continuous intercalated silt/clay laminae, and an upper clay-rich drape. Analyses of beds from ancient mud-rich outer-shelf and basinal successions (Cleveland Ironstone, Jurassic, UK, and Mowry Shale, Cretaceous, United States) show that they too contain beds with this three-part organization, suggesting that such fl ows were active in these ancient settings too. Recognition of these microstructures in these ancient mud-dominated successions demonstrates that sediment in these settings was commonly reworked and transported advectively downslope by high-energy events, contrasting with previous interpretations of these units that deposition was dominated by quiescent suspension settling. Identifi cation of these recognition criteria now allows the products of this newly recognized sediment dispersal mechanism to be identifi ed in other shale-dominated successions.on June 5, 2015 geology.gsapubs.org Downloaded from

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