582 SEDIMENTARY ENVIRONMENTS/Storms and Storm Deposits Figure Heuristic model for the origin of stratification and sole markings for various possible combinations of storm processes The diagram has three corners, representing density induced flow due to excess weight forces, geostrophic currents, and wave oscilla tions The potential history of a storm event, in this three part system, is extremely variable The deposits of flows near the corners are well understood, but the central triangle and intermediate fields are poorly understood since so little is known experimentally about these fields, except for some information on simple rectilinear combined flows HCS, hummocky cross stratification Reproduced from Myrow PM and Southard JB (1996) Tempestite deposition Journal of Sedimentary Research 66: 875 887 to purely oscillatory flow, including a spectrum of combined flows (Figure 4) The most diagnostic feature of storm deposition is HCS at various scales (Figures and 6) The characteristics of HCS are well described and have been studied experimentally and with grain fabric analysis There does not appear to be a size break in the bedforms that create HCS, namely three-dimensional, symmetrical to weakly asymmetrical, subaqueous dunes Bedforms on modern shelves with HCS-like lamination have been attributed to combined flows, and experimental work has shown that hummocky bedforms are stable in complex oscillatory flows and wave-dominated combined flows (i.e low velocities of current component) Details of the geometry and internal structure of ancient tempestites indicate that beds range from those deposited in single events to those representing amalgamated accumulations produced by multiple events The latter include thick beds that grade laterally into thinner beds separated by thin shale beds These suggest that transport of sand onto shelves is in some cases incremental and involves a succession of storms Some beds that appear to result from single events are potentially remobilized sand that simply records the last stage of transport and deposition Other tempestite beds show little or no evidence of amalgamation and are suggestive of powerful offshore-directed unidirectional flow during the erosional and initial depositional conditions of the storm The nature of such flows is difficult to determine from the ancient In many cases they may have been driven, at least in part, by excess-weight forces, caused by high near-bed suspended-sediment concentrations (SSCs) ‘Sediment equilibrium’ conditions exist when SSCs are caused by resuspension of sediment by storm-generated currents and waves Very high SSCs (up to g l 1) and resulting powerful seaward-directed sediment-laden flows have been documented from shorefaces and inner-shelf settings It is less clear to what distances such flows can carry sand offshore on the relatively low gradients common to modern continental shelves Excess-weight forces can also be generated extrinsically (‘sediment disequilibrium’ conditions), for example when rivers discharge sediment-laden floodwater into standing bodies of water and thus produce hyperpycnal (bottom-hugging) flows termed oceanic floods Oceanic floods on the continental shelf of northern California adjacent to the Eel River have high SSCs (up to 2.5 g l 1) of silt and