Sedimentary Rocks-Clastic
Sedimentary Rocks
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Sedimentary Rocks
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» lithification of sediments
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sediments - unconsolidated particles from the weathering of the earth’s
surface
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Lithification
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Process whereby sediment is converted to rock through compaction and cementation.
Sedimentary Rocks
Cements
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Common Cements
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Silica - SiO2
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Calcium carbonate - CaCO3
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Iron oxide - Fe2O3•H2O
Types of Sedimentary Rocks
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Types of Sedimentary Rocks
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Clastic (Detrital) - composed of particles derived directly from rock weathering
and transported to site of deposition
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Organic - lithified remains of plants and animals (coal, chert)
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Chemical - abiotic precipitation of dissolved ions (evaporites)
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Bioclastic - clasts of biological origin (originally shell fragments)
Clastic Sediments and Rocks
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Clastic Sedimentary Rocks
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>90% of all sediments
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Composed of chemically resistant mineral and rock fragments
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The composition and texture of a detrital rock are determined by its history
Clastic Sediments and Rocks (cont.)
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Texture
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Grain Size
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Grain Shape
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Sorting
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distribution of grain sizes
Clastic Sediments and Rocks (cont.)
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Grain shape
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Grain sorting (see transparency)
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poorly sorted
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intermediate sorted
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well sorted
Clastic Sediments and Rocks (cont.)
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Composition
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composed of minerals with slow rates of chemical weathering, or the products
of chemical weathering
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Composition varies with weathering intensity or length of transport
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low - qtz, feldspar (mostly K-spar), rock fragments, mica, amphibole, pyx
(?)
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medium - qtz, K-spar, mica, clays, rock fragments
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high- qtz, clays, K-spar, and clay minerals dominate
Clastic Sediments and Rocks (cont.)
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Sediment transport
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moving water, wind, or ice
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fluid flow velocity - energy - controls ability to move particles (competency)
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slowing causes coarser particles to drop out (i.e., be deposited)
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as rivers enter lake or ocean, coarsest particles drop out at mouth; finer
particles carried further
Clastic Sediments and Rocks (cont.)
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Clastic Sedimentary Rocks
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conglomerate - contains granule-sized grains and larger; grains rounded
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breccia - angular granule-sized and larger grains
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sandstone - 20% of all sedimentary rock
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quartz sandstone - mostly quartz; well sorted, well rounded
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arkose - angular grains; feldspars and rock fragments common
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graywacke - “dirty sandstone”; contains quartz, feldspar, rock fragments
and clay
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siltstone - mostly silt
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mudstone - silt and clay; no shaly parting (tendency to split along bedding)
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shale - most common sedimentary rock (70%); mostly clay, shaly parting
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color give clue to environment of deposition:
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red - oxygen present (oxic)
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gray or black - oxygen absent (anoxic)
Clastic Sediments and Rocks (cont.)
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Mature vs immature sediments - determining environment of deposition
Clastic Sedimentary Environments
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Terrestrial
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alluvial (rivers)
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lake
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desert (wind)
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glacial
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Marginal marine
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beach (waves, tides)
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delta (rivers, tides)