Tutorial:Coastal sediments

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Coastal sediments
Coastal sediments include beach, estuarine, aeolian and deltaic deposits. Sediment sources are from cliff erosion, platforms or submarine outcops by wave action; biogenic sources such as shell and coral; and rivers, glaciers, waste disposal by man, and aeolian action. Coastal sediments are largely of marine origins at lower elevations, and of alluvial-colluvial-deltaic origin further inland. Coastal plains recieve abundant rainifall and are therefore mostly acid, low in content of weatherable minerals, and kaoliniteic with some hydroxiy interlayered vermiculite, except for the marine clays that are montmorillonitic. The sediments are low in content of iron having been subjected to reducing conditions at the time of deposition.

Beaches can consist of material ranging in size from fine sand to boulders; many beaches also contain high quantities of carbonate material in the form of shells. Beach form is influenced by the size and shape of material, the tidal range and wave processes. The beach gradient can vary between 2o in fine sand, to 20o in pebbles where water infiltrates rapidly rather than washing back out (backwash) as it does on sandy beaches. This also means that pebble beaches are often better sorted and graded than sand beaches. Sandy beaches provide the parent material for beach dunes which are formed by aeolian processes.

In some circumstances, most commonly in tropical environments, the beach particles can be cemented together by calcium carbonate to form beach rock. Care should be taken not to confuse this with eroding conglomerate stone on rocky coasts.

In estuaries low-gradient tidal flats of silt and clay can develop. These flats are formed from clay and silt carried to the coast by rivers. When the rivers reach the sea, the salt in the seawater causes these fine particles to clump together (flocculate), the larger clumps can then settle out in the sheltered waters of the estuary.

Where rivers carrying large amounts of sediment reach the sea, the river water decelerates and its sediment is deposited. This localised deposition forms a large fan of material (delta) that protrudes beyond the coast.

Deposition and coastal reclamation has meant that some archaeological sites which today are inland were originally in coastal locations. Anthropogenic remains found in coastal sediments include shells and fishbone, which often form middens; exotic objects such as foreign pottery or beads indicating trade; and building remains of quays, warehouses and boats.