TEMPESTITE DEPOSITION

A model is developed to explain the wide range of sedimentary structur es found in ancient storm-deposited beds, The model predicts the natur e and association of internal sedimentary structures and sole marks th at correspond to various storm flow conditions. Despite recent studies that reveal the signature of ancient storms with geostrophic flow sys tems like those documented from the modern, nonactualistic storm proce sses are needed to explain the unusual thickness and wider cross-shelf distribution of ancient tempestites. Mechanisms for storm transport o f sediment onto the shelf are best recorded in sole marks, which appea r to show a range of predepositional conditions from purely oscillator y flow to combined how 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 strat ification sequences in the ancient. Most recent authors have considere d excess-weight (density induced) forces to have been relatively unimp ortant in ancient storm deposition, This view results from a major lea p in understanding of modern storm processes, particularly the dynamic s of combined-how bottom boundary layers, during the last 15 years, It also comes from the unsubstantiated view that because the bottom slop es 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 Richards on 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 ex cess-weight forces are potentially important for the following reasons : (1) sediment concentrations during peak storms conditions exceed 100 0 mg/l on inner shelves, and may therefore be nonnegligible and import ant for cross-shelf transport with or without currents and waves; (2) one cannot rule out catastrophic introduction of sediment by river flo ods, earthquakes, or other events that caused liquefaction during anci ent storm events, particularly given the significant difference in max imum thickness between ancient and modern storm generated beds; (3) th e slopes of modern continental shelves may be anomalously low as a res ult of Holocene sea level rise, and therefore poor analogs for many an cient 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 su spensions.