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

Recent studies of marine shelf sediment dispersal show that wave-enhanced sediment-gravity flows are widespread phenomena and can transport large volumes of fluid 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 flow models, we predict that such flows in mud-dominated sediments will develop a three-part microstratigraphy produced by changing flow conditions, beginning with wave-induced turbulent resuspension, then development of a wave-enhanced sediment-gravity flow, prior to lutocline collapse and suspension settling. Petrographic examination of modern flow 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 flows 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. Identification of these recognition criteria now allows the products of this newly recognized sediment dispersal mechanism to be identified in other shale-dominated successions.