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SEDIMENTARY ENVIRONMENTS/Shoreline and Shoreface Deposits 573 Figure Longshore sediment transport (longshore drift) Waves approach the shoreline at an oblique angle Sediment is moved up and across the beach by the incoming wave; the returning flow is weaker and tends to be straight back (i.e., shore normal) The net result is that sediment is moved along the coastline Figure Plan view of the shoreface showing the distribution of bedforms resulting from the onshore approach of shoaling waves Reproduced with permission from Clifton NE, Hunter RE, and Phillips RL (1971) Depositional structures and processes in the non barred high energy nearshore Journal of Sedimentary Petrology 41: 651 670 produce a variety of current-generated structures, including ripples, plane beds, and dunes, producing trough and tabular cross-bedding Breaking waves generally produce upper-phase plane beds, although the returning swash current may also generate current ripples In the deeper part of the system, combined flow associated with very large storm-related waves produces compound, three-dimensional low-angle bedforms called hummocks, which in turn produce hummocky cross-stratification (see Sedimentary Environments: Storms and Storm Deposits) Classification and Geomorphology of Shoreline Systems Geomorphologically, coastal systems can be characterized by the medium- to long-term movement of the shoreline (Figure 1B) In transgressive systems, the shoreline moves towards the hinterland and land is lost to the sea, whereas in regressive systems, the shoreline moves basinward and new land is created on the coastal plain These movements are controlled by changes in sea-level and by the amount of sediment that is supplied to the system Sea-level change may occur due to global changes in sea-level (eustasy) or may be driven by local uplift or subsidence within the basin Commonly, a combination of the two forces is in play; this is termed ‘relative sea-level change’ If relative sea-level is falling, the shoreline will move basinward (termed a ‘forced’ regression) Sea-level rise does not always lead to transgression, because the movement of the shoreline also depends on whether enough sediment is supplied to the system to fill the new space that is created If more sediment is supplied than space is created, then the shoreline will prograde (i.e., will be regressive) If, however, the
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