Abstract

A model is developed to explain the wide range of sedimentary structures found in ancient storm-deposited beds. The model predicts the nature and association of internal sedimentary structures and sole marks that correspond to various storm flow conditions. Despite recent studies that reveal the signature of ancient storms with geostrophic flow systems like those documented from the modern, nonactualistic storm processes are needed to explain the unusual thickness and wider cross-shelf distribution of ancient tempestites. Mechanisms for storm transport of sediment onto the shelf are best recorded in sole marks, which appear to show a range of predepositional conditions from purely oscillatory flow to combined flow to purely unidirectional flow. The depositional phase of tempestites is also highly variable both from bed to bed and from basin to basin, as reflected in the wide range of vertical stratification sequences in the ancient. Most recent authors have considered excess-weight (density-induced) forces to have been relatively unimportant in ancient storm deposition. This view results from a major leap in understanding of modern storm processes, particularly the dynamics of combined-flow bottom boundary layers, during the last 15 years. It also comes from the unsubstantiated view that because the bottom slopes and measured storm-generated near-bottom sediment concentrations of modern shelves are presumably too low for autosuspension, such forces are unimportant. Experiments on the interaction of waves and density flows define the conditions under which mixing forces destroy density stratification, and also raise the possibility that with high Richardson numbers, wave-generated shear stresses may enhance turbulence just enough to raise sediment concentrations in the boundary layer and thus facilitate transport by excess-weight forces. We also believe that excess-weight forces are potentially important for the following reasons: (1) sediment concentrations during peak storms conditions exceed 1000 mg/l on inner shelves, and may therefore be nonnegligible and important for cross-shelf transport with or without currents and waves; (2) one cannot rule out catastrophic introduction of sediment by river floods, earthquakes, or other events that caused liquefaction during ancient storm events, particularly given the significant difference in maximum thickness between ancient and modern storm-generated beds; (3) the slopes of modern continental shelves may be anomalously low as a result of Holocene sea-level rise, and therefore poor analogs for many ancient storm-influenced settings. Higher slopes may have been the norm in a wide variety of ancient tectonic settings, thus providing greater offshore-directed driving force for sediment-rich, storm-generated suspensions.

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