Identifying Clastic Sedimentary Rocks

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Undergraduate Geology - Part 2 (Sedimentary Processes and Products) Note on Identifying Clastic Sedimentary Rocks, created by siobhan.quirk on 05/18/2013.

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Created by siobhan.quirk over 6 years ago
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BrecciaBreccia is a coarse grained (>2mm diameter) clastic rock. The clasts are angular, which means that they were not transported for very long. The coarse clasts are often mixed with finer grains and set in a matrix. This means that they are poorly sorted, making it possible to infer that the transporting medium lost energy quickly. Breccias commonly form as scree, alluvial fans and wadi deposits. Volcanic breccias are pyroclastic rocks from a volcanic vent.ConglomerateConglomerate is another coarse grained clastic rock. The clasts are rounded, which means that they have been transported for a longer period of time than the angular clasts in a breccia. The large clasts may be surrounded by a finer grained matrix and be poorly sorted. In some conglomerates, the clasts are held in place by mineral cement. Conglomerates commonly form as beach and river channel deposits.OrthoquartziteOrthoquartzite is a sandstone (0.0625 - 2mm in diameter). It consists only of quartz grains held together by quartz cement, so it is white or grey in colour. The grains are well sorted and well rounded, indicating transport over a long period of time. This indicates extensive weathering so that less stable minerals are not present. Orthoquartzites commonly form in beach and shallow marine deposits. SandstonesVery important group of sedimentary rocks, as they are one of the main oil reservoir rocks and aquifers for water supply, as well as being used as a building stone. They may be well cemented in which case they will have a low porosity, or be poorly cemented and have a high porosity. The main component is quartz, but other minerals low on Bowen's Reaction Series, such as muscovite, mica and K feldspar are also common. Desert SandstoneDesert sandstone is red in colour due to iron oxide coating the quartz grains. It is very well sorted, with very well rounded, medium sand sized (1mm) grains having high sphericity, sometimes referred to as 'millet seed sand'. The grains have a frosted appearance when examined using a hand lens. Unlike orthoquartzite, desert sandstone is not composed entirely of quartz due to the iron oxide coating around the grains. The clasts are cemented together by silica or iron minerals. These rocks commonly form in arid environments as wind blown sands. ArkoseArkose is a medium to coarse grained sandstone that contains at least 25% K feldspar. The other main mineral is quartz, though rock fragments and mica are also present. Arkose is often pink in colour due to the feldpsars and is usually moderately sorted. The grains will be subangular to subrounded, showing that the sediment has not been transported very far. It is commonly formed in alluvial fan environments in arid areas.GreywackeGreywacke substances are fine to coarse in grain size. They are dark coloured and ooorly sorted, with angular to sub-angular clasts that consist of rock fragments with some quartz and K feldspar. More than 15% of the rock is clay matrix. They are commonly formed as turbidite deposits, so often show graded bedding.

Agrillaceous rocks are very fine grained. This makes observation of their characteristic features difficult.MudstoneMudstones are dark grey, very fine grained clastic rocks. They contain clay minerals, mica and quartz, but you will not be able to identify individual minerals. The minerals do not have any preferred alignment and so mudstones are not layered. Also, mudstone does not have plasticity.ClayClay is very fine grained, more compacted than mudstone and contains 40% water. The grains are clay size (0.039mm) and mainly consist of clay minerals. Clay may have a variety of colours, including dark brown, red and green depending on the carbon or iron content. The higher the organic content, the darker the colour. Clay is plastic and can be moulded. It forms layers with distinct bedding planes.ShaleShale is a dark coloured, fine grained clastic rock that has distinctive layers, due to the alignment of minerals. The clay minerals are flat and platy and align parallel to the beds at 90 degrees to the pressure from the weight of overlying rocks. The rock easily splits along the layers or laminations and for this reason is described as fissile. It is composed of clay minerals, mica and quartz. Shale is not plastic but hard, brittle and impermeable. All of these argillaceous clastic rocks and sediments are deposited in low energy environments, commonly marine  but also on the flood plains of rivers or in lakes. They may contain fossils.

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Fine Grained Clastic Sedimentary Rocks