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Sedimentary rocks (SR) - Coggle Diagram
Sedimentary rocks (SR)
Definition
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primary source of salt deposits, coal, oil, gas are extracted from them, only they contain fossils
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Sediments = fragments of preexisting minerals, rock, organic matter. ZB beach sand, gravel, particles of desert dunes
Formation Sediments
Weathering
Physical weathering
agents: wind (dust carried abrades surface), water (zB waves) & ice (through frost wedging). Also plants (root wedging)
Chemical weathering
rainwater + CO2 from carbonic acid (H2CO3), dissolves CaCO3. products (Ca2+ , HCO3- and CO3 2- ) are transported by ocean, where animals might form calcareous parts from it
silicate minerals react with the acid as well, products of reaction include dissolved ions and clay minerals, sea creatures might make mineralized parts of body from it
erosion & transport
erosion agents: wind, water, ice, then transported to deposition areas
no active removal, downslope movement by gravity = mass wasting
sediment deposition
water: strong currents with speed of >50cm/s: all sizes up to boulder-size sediment (>25.6cm). Moderately strong 20-50cm/s: carry sand (62,5 um - 2mm). Weak currents carry silts & clays (<62.5 um)
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Transformation
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soft sediment accumulate, get buried. physical & chemical changes after burial = diagenisis. result: transformation into SR = lithification
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Properties
Grain size
indication of current that has transported them. zB sand grained had stronger current than clay grained.
Sorting
= measure of variation in grain-size within sediment in SR. Aprox. same size: well-sorted. Level of sorting provides info about strength & consistency of current. If strength of current was consistent: uniform.
Grain morphology
during transportation grains are abraded & rounded. Roundness provides info about distance & duration of transport
basins
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Largest: ocean basins. Size & morphology controlled by plate tectonics zB East African Rift: corresponds to boundary where two plates pull apart. Rift contains many lakes, where sediments accumulate. -> if plates keep moving apart: cont. lithosphere will get thinner -> rift will grow larger -> more sediments accumulate.
subsidence: downward movement of lithosphere as it's stretched thinner. Is accentuated by weight of sediments pushing on it -> ultimately, seawater will flood it, making it into an ocean basin
thermal subsidence: two continental margins that are moving apart cool, the more they move apart from the heat source -> lithosphere gets denser, sinks progressively. sediments can accumulate in thick piles along continental margins -> key process of formation of continental shelves
Types
Siliciclastic SR
composed of fragments of rock, comp. is dominated by silicate minerals like quartz/ feldspar. Classified according to grain size (coarse = conglomerate, medium = sandstone, fine = shale) or grain comp. (arkose = sandstone containing >25% feldspar)
Biochemical SR
formed by accumulation of hard parts of marine organisms zB beach deposit consisting of bivalve shells (bioclasts) or coral reefs. diatomite & radiolarate: SR composed of accumulation of microorganisms producing tiny shells
Chemical SR
formed by mineral precipitation without biological activity zB minerals at bottom of saline lake after intense evaporation -< rock called evaporite zB NaCl
Organic SR
formed by accumulation of organic matter. if a lot accumulates in short time with low oxygen levels. zB peat: in wetlands, dead plant matter, gets buried -> lignite (compressed peat). more depth, pressure, heat -> coal. more pressure -> anthracite
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