Coasts are the areas of interface between the land and the sea, and the coastal environment can comprise a variety of zones, including coastal plains, beaches, barriers and lagoons. The shoreline is the actual margin between the land and the sea. Coastlines can be divided into two general categories on the basis of their morphology, wave energy and sediment budget. Erosional coastlines typically have relatively steep gradients where a lot of the wave energy is reflected back into the sea from the shoreline: both bedrock and loose material may be removed from the coast and redistributed by wave, tide and current processes. At depositional coastlines the gradient is normally relatively gentle and a lot of the wave energy is dissipated in shallow water: provided that there is a supply of sediment, these dissipative coasts can be sites of accumulation of sediment.
Erosional coastlines
Exposure of bedrock in cliffs allows both physical and chemical processes of weathering: oxidation and hydration reactions are favoured in the wet environment, and the growth of salt crystals within cracks of rocks sprayed with seawater can play an important role in breaking up thematerial. Material accumulates at the foot of cliffs as loose clastic detritus and occasionally as large blocks when whole sections of the cliff face are removed. Cliff erosion may result in wave cut platforms of bedrock eroded subhorizontally at beach level. Wave action, storms and tidal currents will then remove the debris as bedload, as suspended load or in solution. This contributes to the supply of sediment to the marine environment, and away from river mouths can be an important source of clastic detritus to the shallow marine realm.
Depositional coastlines
A coastline that is a site of accumulation of sediment must have an adequate supply of material to build up a deposit. The sources of this sediment are from the marine realm, either terrigenous clastic detritus reworked from other sources or bioclastic debris. The terrigenous material ultimately comes from rivers, with a small proportion of wind-blown origin and from direct erosion of coastlines. This sediment is brought to a depositional coastline by tidal, winddriven and geostrophic currents that transport material parallel to shorelines or across shallow seas. Wind-driven waves acting obliquely to the shoreline are an important mechanism of transport, creating a shore-parallel current known as longshore drift. Shallow seas are generally rich in fauna, and the remains of the hard parts of these organisms provide an important source of bioclastic material to coastlines. The form of a depositional coastline is determined by the supply of sediment, the wave energy, the tidal range and the climate. Climate exerts a strong control on coasts that are primarily sites of carbonate and evaporite deposition, and these environments. Along clastic coastlines a beach of sandy or gravelly material forms where there is a sufficient supply of clastic sediment and enough wave energy to transport the material on the foreshore. The form of the beach, and the development of barrier systems and lagoons, is dependent on whether the coastline is in a micro-, meso- or macrotidal regime. Sea-level changes also strongly influence coastal morphology. In the following sections the processes related to beach formation are first considered, followed by a description of the morphologies that can exist in wave-dominated and tidally influenced coasts.
Beaches
The beach is the area washed by waves breaking on the coast. The seaward part of the beach is the foreshore, which is a flat surface where waves go back and forth and which is gently dipping towards the sea. Where wave energy is sufficiently strong, sandy and gravelly material may be continuously reworked on the foreshore, abrading clasts of all sizes to a high degree of roundness, and effectively sorting sediment into different sizes. Sandy sediment is deposited in layers parallel to the slope of the foreshore, dipping offshore at only a few degrees to the horizontal (much less than the angle of repose). This low-angle stratification of well-sorted, well-rounded sediment is particularly characteristic of wave-dominated sandy beach environments. Grains are typically compositionally mature as well as texturally mature because the continued abrasion in the beach swash zone tends to break down the weaker clasts. On gravel beaches the water washed up the beach by each wave tends to percolate down into the porous gravel, and the backwash of each wave is therefore weak. Clasts that are washed up the beach will therefore tend to build up to form a storm ridge at the top of the foreshore, a back-beach gravel ridge that is a distinctive feature of gravelly beaches. The clast composition will vary according to local sediment supply, and may contain terrigenous clastic, volcaniclastic or bioclastic debris. At the top of the beach, a ridge, known as a berm, marks the division between the foreshore and backshore area. Water only washes over the top of the berm under storm-surge conditions. Sediment carried by the waves over the berm crest is deposited on the landward side forming layers in the backshore that dip gently landward. These low-angle strata are typically truncated by the foreshore stratification, to form a pattern of sedimentary structures that may be considered to be typical of the beach environment. The backshore area may become colonised by plants and loose sand can be reworked by aeolian processes. Wave action in the lower part of the foreshore can rework sand and fine gravel into wave ripples that can be seen on the sediment surface at low tide and can be preserved as wave-ripple cross-lamination. However, wave-formed sedimentary structures on the beach may be obliterated by organisms living in the intertidal environment and burrowing into the sediment. This bioturbation may obscure any other sedimentary structures.
Beach dune ridges
Aeolian processes can act on any loose sediment exposed to the air. Along coasts any sand that dries out on the upper part of the beach is subject to reworking by onshore winds that may redeposit it as aeolian dunes. Coastal dunes form as ridges that lie parallel to the shoreline and they may build up to form dune complexes over 10 m high and may stretch hundreds of metres inland. Vegetation (grasses, shrubs and trees) plays an important role in stabilising and trapping sediment. The limiting factor in beach dune ridge growth is the supply of sand from the beach. They commonly form along coasts with a barrier system, but can also be found along strand-plain coasts. In a sedimentary succession these beach dune ridge deposits may be seen as well-sorted sand at the top of the beach succession. Some preservation of the roots of shrubs and trees that colonised the dune field is possible, but the effect of the vegetation is often to disrupt the preservation of well-developed dune cross-bedding.
Coastal plains and strand plains
Coastal plains are low-lying areas adjacent to seas. They are part of the continental environment where there are fluvial, alluvial or aeolian processes of sedimentation and pedogenic modification. Coastal plains are influenced by the adjacent marine environment when storm surges result in extensive flooding by seawater. A deposit related to storm flooding can be recognised by features such as the presence of bioclastic debris of a marine fauna amongst deposits that are otherwise wholly continental in character. Sandy coastlines where an extensive area of beach deposits lies directly adjacent to the coastal plain are known as strand plains. Along coasts supplied with sediment, beach ridges create strand plains that form sediment bodies tens to hundreds of metres across and tens to hundreds of kilometres long and progradation of strand plains can produce extensive sandstone bodies. The strand plain is composed of the sediment deposited on the foreshore and backshore region. The backshore area merges into the coastal plain and may show evidence of subaerial conditions such as the formation of aeolian dunes and plant colonisation.
ليست هناك تعليقات:
Write التعليقات