SEDIMENTARY ENVIRONMENTS/Storms and Storm Deposits 583 Figure Bed configuration stability diagram for combined flows in fine sand Based on published data and on data from Arnott RW and Southard JB (1990) Exploratory flow duct experiments on combined flow bed configurations, and some implications for interpret ing storm event stratification Journal of Sedimentary Petrology 60: 211 219 and Southard et al (1990) The left side of the diagram (up to UU of 25 cm s–1) is drawn from the data in Arnott and Southard (1990), and the right side is extrapolated clay even at mid-shelf depths (50–70 m) Such floods are potentially a geologically important process for cross-shelf sediment transport, although it is unknown whether they could transport very fine and fine sand, the grain sizes that dominate most ancient tempestites Facies models for some ancient sandstone tempestites cite nearshore sediment bypass (Figure 7), which is consistent with powerful unidirectional flows in very proximal positions, possibly associated with hyperpycnal oceanic floods In both sediment equilibrium and sediment disequilibrium conditions sediment may be transported in combined flows produced by a combination of excess-weight forces and storm waves Such wavemodified turbidity currents differ from classic turbidity currents in that storm waves enhance boundary-layer shear stress and eddy viscosity, which reduces deposition from suspension and thus enhances gravity-driven transport As long as turbulent diffusion by waves does not destroy the density stratification, it provides the energy necessary to maintain gravity-driven flow on relatively low gradients This process has been documented in the cross-shelf movement of modern fluid mud layers, but not for sand-sized sediment Ancient examples of wave-modified turbidites have internal sedimentary structures, including combinedflow ripples, that reflect the influence of both waves and gravity-driven flow (Figure 8) Whereas the preceding discussion has emphasized the range of possible processes responsible for tempestite deposition and thus has explained the wide variety of sole marks and internal sedimentary structures in the ancient, there are two important aspects of tempestites that remain controversial First, beds in the ancient that are interbedded with shale and were therefore deposited below the fair-weather wave base