Clastic Shallow Seas

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Undergraduate Geology - Part 2 (Sedimentary Processes and Products) Note on Clastic Shallow Seas, created by siobhan.quirk on 05/19/2013.

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Most shallow sea areas receive large amounts of clastic sediment transported from the land by rivers. Shallow seas extend from the extreme low water of the spring tide to the edge of the continental shelf about 200m below sea level. The extent of these shallow seas varies greatly and around the coast of Britain there are large areas.The littoral zone is defined as the area between the high water and low water marks and this includes beaches. It is covered by the sea for part of the time and this high energy tidal area accumulates sediment.Most sand and finer grained sediment deposited on beaches and in shallow seas has been transported to the sea by rivers. Coarser grained sediment may have been supplied by the erosion of coastal cliffs. Pebbles and cobble sized sediments are usually found at the back of a beach, to where they have been transported by high energy waves during storms. Transport in shallow seas may be by longshore drift or by rip currents. The currents uni-directional and take water and sediment along the coast or back out to sea. Tides are bi-directional.Beach EnvironmentsBeaches are mainly composed of sand and gravel, though some may contain mud. Sand grains are regularly picked and moved around by waves and this makes them well sorted. The grains are usually rounded as a result of attrition whilst being transported by waves and tides. The rocks commonly formed are light coloured orthoquartzites, as they are made entirely of quartz.A few specially adapted burrowing organisms such as bivalves can survive in these extreme environments, which vary between being land and covered by the sea, according to the tides. Tracks, trails and burrows of beach organisms as well as broken shells may be found scattered through the sand or in shell banks.The bi-directional movement of water by tides leads to the formation of symmetrical ripple marks with crests that are parallel to the crests of the waves.In high energy areas, beach gravel, pebbles or cobbles are deposited. These are less frequently worked than the beach sands because they are only affected by the highest tides and highest storm waves. They are less well sorted than beach sands. Traces of life are rare in this environment, though there may be some shell fragments washed in. The rocks commonly formed are conglomerates. The fact that they formed on a beach is usually shown by the presence of shell fragments.Shallow SeasThe material deposited in marine sediments has its origin on the land: rivers carry a suspended load of clay minerals, a dissolved load of salts, and some bed load of sand wind carries atmospheric dust, which can be deposited anywhere in the sea Below the littoral zone is the shallow water of the continental shelf, where the average water depth is 130m. Here sediments are mainly affected by the currents. In general, sediment size decreases as depth and distance away from the coast increases.Continental shelves are dynamic areas with a number of key variables that increase variety in sediment deposition: sediment supply controls the rate of sedimentation. It can be very low where little sediment is entering the sea, perhaps from a low lying landmass or an area with little erosion and weathering. Where erosion is high and large rivers are transporting sediment, the rate of deposition is high. Changes in sea level have a huge effect on the areas close to shore. A rise in sea level causes the sea to flood the land and the zones of sediment move inland so that mud may be deposited on top of the sand. This is called marine transgression. Submerged forests around the coast of Britain are evidence of the rise in sea level since the last glacial period. Fossil forests are where trees were submerged and preserved by replacement of the woody tissue by silica or calcite. A drop in sea level means that the sea retreats, causing a marine regression and renewed erosion on land takes plaxe bringing more sediment to the sea. sediment transport can be increased by changes in a current direction or rate of flow. Many offshore sand bars can move several hundred metres in a year, causing a danger to shipping. The sea water contains suspended sediment, reducing daylight penetration closer to shore. The abundance of life is closely linked to the availability of sunlight. The area of the continental shelf where the water depth is less than 100m has abundant life.Glauconite is a distinctive green potassium iron silicate mineral found in some sandstones formed in shallow seas. Uni-directional currents form asymmetrical ripple marks in sandstones. Mudstones and clays form in areas of lower energy offshore and commonly contain a wide range of fossils.

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