SEG - First issue (2)

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Academic Articles II- Depositional Environment - Lake - As one of many ecosystems lakes have sedimentological characteristics that distinguish it from other terrestrial sedimentary environments but first what is a lake exactly ?? a lake is a large body of water that occupies an inland basin A related term would be pond but the difference between the two that lakes are great in size and have depth while ponds are shallow water bodies with small sizes How do lakes form ? lake can form from a variety of ways like 1- ancient glaciers sliding and carving the land and then melting so the water fills the land carved by the glaciers , this is comment in the northern hemisphere 2- basins created by tectonic movement buckling and breaking the earth , these basins are then filled with water from rainfall or streams 3- volcanoes are also another mean of creating a basin for water to fill , after the volcano has become inactive it leaves a crater that would be filled with water coming from rain or melted snow some times the volcano would erupt or the top would collapse and it would leave a depression called caldera that then would also be filled with water . 4- rivers can make lakes when rivers are meandering and have a sudden change of flow (flood) it may make a short cut and leave a body of water Isolated from the river that kind of lake is then named oxbow lake because it takes the shape of the U frame that is put around ox's necks 5- land slides could cause the creation of lakes due to barricading the flow of a stream or a river creating a dam tat would prevent the water from flowing 6- artificially made lakes made by humans digging and letting the man made basin fill with water from streams or rivers and then using the lake to produce electricity and other uses for mankind The water in the lakes comes from rainfall or melted snow, stream and ground water seepage hence most of the lakes are fresh water lakes . there are two types of lakes open lakes and closed lakes open lakes have an outlet that lets it's water flow from it closed lakes have no outlet so they tend to be more saline because the still water evaporates and leaves behind salts . lakes sedimentation and sediments lakes sediments comprise of silts clays and sand sizes which are clastic elements , it is also has organic matter chemically precipitated material and the ratio between these elements depends on the drainage basin ,climate and the age of the lake Page 4
Academic Articles 1- clastic sediments Large sediments roll due to the river moving it along it's bed ( bed load ) and when the river connects with the lake it's speed drops and it then drops the bigger sediments and then finer sediments which have been suspended till now settle to the bottom ( clays and silts ). the bottom of a lake will have mainly clays and silts when surveying away from the shore or the river mouth . big sediments will reveal their own origin from nearby source and will show current and wave directions that lead them where they have been deposited . volcanic ash may deposit and like the ash from the Pleistocene epoch it could be used as a stratigraphic marker . varve deposits are an annual deposit that indicates the change of seasonal change , it contains coarse grains and finer grains that alter between each other . varves are common especially when the water comes from ice sheets and glaciers .organic matter comes from plants, Bactria , small insects and algae which thrives of nitrogen and phosphorus and other neutrons . 2- chemical deposits In the lake compose of mainly calcium, sodium, and magnesium carbonates and dolomite, gypsum, halite, and sulfate salts. calcium carbonate precipitates as calcite or aragonite while the lake is saturated with calcium and bicarbonate ions. algae can also generate the right conditions for calcium carbonate to precipitate like raise the ph of the water from 7 to 9 which makes it alkaline. dolomite is found in very alkaline water where calcium carbonate and magnesium carbonate combine. also in saline lakes gypsum occurs with the presence of sulfate, calcium, and hydrogen sulfide with certain concentrations. lakes with high concentration of sodium sulfate are called bitter lakes and lakes that contain high concentrations of sodium carbonate are called alkali lakes . magnesium salts can be found in the same sediment as the sodium salts. lakes could have other salts precipitating like borates, nitrates, and potash also a small portion of borax is found in various lakes through out the world Lakes characteristics according to geological sediments : Page 5 The gradual thickening of sediments might put the lake under the danger of extinction. 1- sediments become more fine toward the center of the lake 2- lakes tend to have Low energy, poor oxygenation: bottom sediment generally poorly disturbed so lamina are present and commonly made of shale 3- excellent preservation of fossils due to low energy and low disturbance 4- edges of lakes have bigger sediments, bi-directional ripple marks and mud cracks 5- lake shale may get disrupted by bigger sediments (storm deposits (tempestites ))
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SEG - First issue (2)

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