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Variability of Wave-Dominated and Wave-Influenced Coastlines in the Holocene - Lessons for the Subsurface


Wave-dominated and wave-influenced systems form along most of the world's coastlines and represent an important reservoir type. Holocene analogues provide a valuable source of information on how such systems accumulate sediment. Information on the plan-view dimensions of depositional units and the distribution of erosional surfaces internal to such deposits is especially useful, since such data is difficult to obtain directly from subsurface studies or from outcrop analogues. Such data is critial for building suites of subsurface models. The link to local tectonic setting and fluvial drainage networks can also be readily determined. A major global coastline mapping initiative allows the comparison of a large number of wave-dominated and wave-influenced systems. Features recorded are distributions of beach and chenier ridges, distributions and types of active and abandoned channels, distributions of channel belts, shapes of channel scroll bars, distributions of water bodies, and geometires of mouth bars. The geomorphology of modern systems formed under different wave, tide and fluvial process regimes, as well as different geographic and tectonic settings, can thus be used to study the natural variability of coastal deposits and their subsurface equivalents. Holocene wave-dominated and wave-influenced systems appear more architecturally complex than what is typically assumed in the subsurface. Variables such as fluvial points sources and their avulsion frequencies, presence of barrier islands, direction of longshore currents, and the shape of the coastline can significantly affect deposition. Major depocenters at the shoreline can form away from river mouths due to along-shore transport. The shape of the coastline is an effective predictor of where new depocenters will develop. Depocenters can develop in association with both concave (landward curved) or convex (seaward curved) stretches of shoreline. Concave shorelines tend to be filled with wave-dominated sediment lacking internal complexity. Progradation tends to decrease shoreline curvature over time. Depocenters related to convex shorelines are more architecturally complex and often internally consist of distinct beach ridge element sets. The interaction of delta lobe progradation and erosion with along shore-driven sediment transport makes such systems dynamic and often difficult to split into distinct deltaic and shoreface components. Practical approaches to dealing with such complexity are discussed.