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5-Sedimentation_jb - 1st Midterm Exam Sedimentation Bisti...

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3/1/11 1st Midterm Exam
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3/1/11 Sedimentation Bisti Wilderness, New Mexico
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3/1/11 The Rock Cycle
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3/1/11 Significance of Sed Rocks Provide a record of ancient tectonic events (e.g., mountain building and erosion) Fossil record: evolution of life Major reservoirs of groundwater, oil, coal
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3/1/11 Sedimentary Processes Weathering: process by which rocks are broken down 1) Physical: mechanical fragmentation; chemical composition does not change 2) Chemical : minerals in rock are chemically altered or dissolved Erosion and Transportation (plate tectonics N gravity > water)
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3/1/11 Sedimentary Processes
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3/1/11 Sediment Types 1) Clastic Sediments – physically transported rock fragments produced from weathering of preexisting rocks, typically silicate minerals or rock fragments Examples: sandstone, shale 2) Chemical and Biochemical Sediments – direct, chemical deposition (either by chemical reaction or biotic production), typically
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3/1/11 Water Transport The speed of a current determines what size particles (and their density) it can carry. 1) Fast currents = large and small particles 2) Slow currents = small particles The speed of river and stream currents decreases with distance from the sediment source. Picture 3 Cramer, S. Steep and fast gentle and slow
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3/1/11 Sedimentary Transport Sediments are modified in several ways as they are transported downhill from their source: 1) Mafic minerals weather more quickly than felsic minerals ° mafic minerals tend to disappear first. 2) Sedimentary particles are abraded along the transport path ° smaller and more rounded with increasing distance from their source 3) Large boulders can only be moved by glaciers and fast moving currents in mountain streams. Only smaller particles are carried to the ocean. Sediments can be sorted by grain size as current strength changes.
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3/1/11 Sedimentary Transport Sediment MATURITY increases downstream 1) Irregular shaped clasts > rounded clasts 2) Mixture of particle sizes : all clasts about the same size (well-sorted and small) 3) Mixture of clast composition° only stable clasts (e.g., quartz)
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3/1/11 Rounding
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3/1/11 Sorting selection of clasts according to particle
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3/1/11 Weathering Some minerals are more susceptible to weathering than others.
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