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How they form, how they are classified, and why they are important.
Sedimentary Rocks How they form, how they are classified, and why they are important.
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Objectives Explain how sediments are produced
Describe and discuss siliciclastic rocks Discuss how sedimentary rocks are classified Describe and discuss sedimentary structures Explain why sedimentary rocks are significant
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Sedimentary Rocks Sediments produced by: Siliciclastic rocks
Weathering, erosion of other rocks Crystals precipitated from seawater Skeletal debris from organisms Siliciclastic rocks Sedimentary rocks composed of clasts of silicate minerals Quartz is most resistant to weathering Mafic minerals less stable at Earth’s surface
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Sedimentary Rocks
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Siliciclastic Rocks Classification based on grain size and shape:
Gravel >2 mm diameter Granules, pebbles, cobbles, boulders Conglomerate Rounded grains Breccia Angular grains
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Siliciclastic Rocks (cont.)
Sand 1/16-2 mm diameter Often quartz Sandstone Silt 1/256-1/16 mm Clay Smaller than 1/256 mm
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Siliciclastic Rocks (cont.)
Muds form mudstones Shale Fissile mudstone Fissile breaks along bedding surface Sediment aligned horizontally during deposition
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Sorting Grains settle out of suspension
Coarse, dense material settles first
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Sorting Measure of similarity in grain size Poorly sorted Well sorted
Mixed sizes Well sorted Similar sizes
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Siliciclastic Rocks: Sandstone
Not always quartz Arkose Feldspar dominated rock Pinkish color Graywacke
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Lithification Process by which siliciclastic sediments become rock
Primary process is compaction Cementation Chemical process in which minerals crystallize from solution that percolates through the grains of sediment Iron oxide Red beds
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Evaporites Form from evaporation of seawater
Anhydrite Gypsum Halite Readily formed, readily dissolved
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Other Chemical Sedimentary Rocks
Chert Flint Extremely small quartz crystals precipitated from watery solutions Brown, gray, or black Impurities Banded Iron Formations (BIF)
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Carbonate Rocks Limestone Dolomite
Chemical and biogenic bodies of rock Dolomite Carbonate mineral Uncommon in modern rocks Common in ancient rocks Dolostone
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Carbonate Rocks (cont.)
Carbonate sediments Often skeletal particles Carbonate muds Mainly aragonite needles Direct precipitation Collapse of carbonate algal skeletons
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Carbonate Rocks (cont.)
Oolites Nearly spherical sediment Produced in shallow water Formed by rolling and accumulating aragonite needles
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Carbonate Rocks (cont.)
Cross-section of oolith Concentric structure
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Sedimentary Structures
Distinctive arrangements of grains in sedimentary rocks Reflect modes of deposition Useful tools for interpreting ancient environment of deposition Graded Bed Grain size increases from top to bottom Normal
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Sedimentary Structures
Ripples Structures formed by wind or water Sediments accumulate downslope (lee) Symmetrical → Bidirectional current Asymmetrical → Unidirectional current Cross-bedding or cross-stratification Mudcracks
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Significance of Understanding Sediments and Sedimentary Rock
Sediment is a non-point source pollutant in California Sedimentary rock contains oil and natural gas Coal is a sedimentary rock and Groundwater is found primarily in sedimentary rock
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Sediment laden drainage, Betsiboka River, Madagascar
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Sedimentary Rock and Oil Formation
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Coal Formation Coal and Depth Peat Lignite Bituminous Coal
Anthracite Coal
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Groundwater Resources
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Summary Sediments form from detritus – broken down rocks
3 types of sedimentary rocks: 1) detrital; 2) chemical; and 3) biochemical or bioclastic The environment in which sedimentary rocks form control the different types found Sedimentary rocks are significant since they contain oil, natural gas, coal, and groundwater; sediments can also be considered pollutants
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