Geology Group Diary (25)

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    The Geology Group met at 10.30 am on Wednesday 8 November 2017 at Merlin's Bridge Village Hall. The topic for the meeting was
    Sediments can be deposited in rivers and on the sea floor; they can also be laid down by melting ice or by the wind in arid regions. Lithification is the process by which unconsolidated sediment is converted into a sedimentary rock. The sediment is first compacted by the weight of overlying sediment and water is squeezed out of the pores between the grains. Silica or calcite may be precipitated from solution to form a cement around the grains. Iron oxides also form cement and give the characteristic red colour to sandstones.
    There are two main groups of sedimentary rocks (i) Siliciclastic Rocks that are formed mostly of quartz grains or lithic fragments (clasts) and (ii) Carbonate Rocks that are formed by the  precipitation of carbonate minerals in shallow seas.

    Sedimentary Processes in the formation of SILICICLASTIC ROCKS

    1.Fast flowing mountain streams erode narrow V shaped valleys. Erosion of the channel occurs by abrasion and the fast flowing river transports  boulders, cobbles and pebbles as bedload particularly during floods. Finer sediment is carried in suspension downstream and deposited when current velocity is minimal in lower reaches of river.
    2.A lowland river meanders over a flood plain where deposition of silt occurs when water overflows the levees. Lag gravels and coarser sands accumulate on inside of meander bends (point bar) where current is slower.
    3.A delta may develop on a coast where a slow moving river is heavily charged with sediment and forced to deposit its load as current velocity drops. Fine silt may be transported many miles out to sea. When sea level fluctuations occur on a deltaic coast repeated fining up sequences or cyclothems of siltstone, shale, sandstone and coal seams are produced
    4.In contrast to a meandering river, a braided river will form where the current velocity is high and there is an abundant supply of coarse material forming the bedload. A braided river consists of many channels separated by islands of poorly sorted pebbles and cobbles. This often occurs where a river, fed by glacial meltwater, becomes braided as it emerges from the mountain tract on to the lowland.
    5.Sand and mud accumulate along the coast in sheltered bays or behind spits and in lagoons under the influence of long shore drift. Ripple bedding is produced either by wave action in shallow water (symmetrical ripples) or current action (asymmetrical ripples). Cross stratification (cross bedding) occurs as a result of the down current migration of sediment both in rivers or under shallow marine conditions..
    6.Turbidity currents occur where sediment is washed out over the continental shelves and descends rapidly down the continental slope through submarine canyons. Poorly sorted, slumped, coarse grained sediment will form greywacke whilst finer muds and sands will accumulate as a fining up sequence (Bouma units).
    7.Under aeolian conditions (strong winds) dune bedding (large scale cross stratification) occurs when barchans dunes migrate across a desert surface.
    8.Glaciers produce U shaped valleys and debris is transported by ice as it moves downstream. Glacial till (boulder clay) is deposited under an ice sheet as the ice begins to melt.
    Shallow shelf carbonate sediments are accumulating today in the Arabian Gulf and off the Bahamas but they were more common in past geological periods.
    Common carbonate rocks are:
    i).Bioclastic limestones
    These are calcareous rocks formed largely of the remains of marine animals such as bivalves, brachiopods and corals. The clasts are often cemented together by calcite. Lower Carboniferous bioclastic limestones were deposited as calcareous sediments on off shore ramps in shallow waters.
    ii) Oolitic limestones
    These are formed of tiny concentrically layered carbonate grains called ooliths. The calcium carbonate is precipitated in shallow shelf seas under tropical conditions. Today ooliths are being deposited in the Persian Gulf and off the coast of Florida, but in the past oolitic limestones were formed during the Jurassic (eg.Cotswold limestone and Portland limestone)
    iii) Chalk
    This is a very pure form of limestone made from microscopic skeletal plates (coccoliths) which accumulated as a carbonate rich mud on the sea bed in Upper Cretaceous times. eg the chalk of Beachy Head, Chiltern hills,etc. The Upper Chalk is characterised by the presence of flint nodules which may have been precipitated from silica rich ground waters percolating through the chalk. However, recent research suggests that the flint was formed by the sub surface breakdown of siliceous organisms such as sponges, radiolaria and diatoms during the deposition of the chalk.
    iv) Magnesian Limestone (Dolomitic limestone)
    This limestone is formed of the mineral dolomite or calcium magnesium carbonate. Most dolomitic limestones are formed by the replacement of the calcium ions in calcareous sediments by magnesium ions when water is evaporated. During Permian times dolomite was deposited as an evaporate mineral in the Zechstein Sea.

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