© 2014 Pearson Education, Inc.Detrital Sedimentary Rocks • Detrital Rocks – form from sediments that have been weathered and transported – Mostly clay minerals, quartz, feldspars, and m
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The Importance of Sedimentary Rocks
• Sedimentary rocks cover ~ 75% of Earth’s surface
• ~ 5 % (by volume) of Earth’s outer 10 miles
• Contain evidence of past environments:
• Important resource
– Coal, oil, and other fossil fuels
– Groundwater resources
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Origins of Sedimentary Rock
• Products of mechanical & chemical weathering
• Sediments & soluble constituents are transported down slope by gravity
• Sediments are deposited & buried
• Deposition causes lithification
• Types of sedimentary rocks:
– Detrital
– Crystalline
– Chemical/Organic sedimentary rocks
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Detrital Sedimentary Rocks
• Detrital Rocks
– form from sediments
that have been weathered and transported
– Mostly clay minerals,
quartz, feldspars, and micas
– Particle size is used to
distinguish among the various rock types
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Detrital Sedimentary Rocks
• Shale
– Silt & clay-sized particles
– Form from settling of
sediments in quiet,
non-turbulent environments
– Sediments form in thin
layers (laminae)
– Has fissility (rock can be
split into thin layers)
– Most common sedimentary
rock
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Detrital Sedimentary Rocks
• Sandstone
– Sand-sized particles
– Forms in many environments
– Common sedimentary rock
– Quartz is the most abundant
mineral
• Quartz sandstone (quartz)
• Arkose sandstone (feldspar)
• Graywacke contains rock fragments and matrix, in addition to quartz and sandstone
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Detrital Sedimentary Rocks
– Sorting can help decipher the depositional
environment of the rock
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• Conglomerate and Breccia
– Conglomerate consists of rounded, gravel-sized sediments
– Breccia consists of angular, gravel-sized sediments– Both types of rocks are poorly sorted
Detrital Sedimentary Rocks
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Chemical Sedimentary Rocks
• Form from precipitated material that was once in solution
• Precipitation of material occurs by:
– Evaporation
– Organic processes from water-dwelling organisms form biochemical sedimentary rocks
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Chemical Sedimentary Rocks
• Limestone
– Most abundant chemical sedimentary rock
– Mainly composed of the mineral calcite
– Can form from inorganic and biochemical origins
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Chemical Sedimentary Rocks
• Biochemical limestone forms from shells of marine organisms
– Large quantities of marine limestone are formed from corals– Corals secrete a calcium carbonate skeleton and create
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Carbonate Reefs
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Coquina
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The White Chalk Cliffs
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Chemical Sedimentary Rocks
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Coal: An Organic Sedimentary Rocks
• Stages of Coal Formation
– Accumulation of plant remains
– Formation of peat
– Formation of lignite and
bituminous coal– Formation of anthracite coal
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Turning Sediments into Sedimentary Rock:
Diagenesis and Lithification
Many changes occur to sediment after it is deposited
Lithification—unconsolidated sediments are transformed into sedimentary rocks
Compaction—as sediments are buried, the weight of the overlying material compresses the deeper
sediments
Cementation—involves the crystallization of minerals among the individual sediment grains
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Classification of Sedimentary Rocks
• Sedimentary rocks are classified according
to the type of material
– Two major groups
• Detrital
– Has clastic texture, composed of discrete fragments
cemented together
• Chemical/Organic
– Has nonclastic or crystalline texture, where the
minerals form patterns of interlocked crystals
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Identification of Sedimentary Rocks
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An environment of deposition or a sedimentary
environment is a geographic setting where sediment is accumulating
Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)
Sedimentary Rocks Represent Past
Environments
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Sedimentary Rocks Represent Past Environments
Types of Sedimentary Environments
Three broad categories
ContinentalMarine
Transition
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Continental Environments
Dominated by stream erosion and deposition
Streams are the dominant agent of landscape alteration
Glacial
Deposits are typically unsorted mixtures of sediments that range from clay to boulder-sizedWind (eolian)
Well-sorted, fine sediments
Sedimentary Rocks Represent Past
Environments
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Marine Environments
Shallow marine (to about 200 meters)
Borders the world’s continents Receives huge quantities of terrestrial sediments Warm seas with minimal terrestrial sediments have carbonate-rich muds
Deep marine (seaward of continental shelves)
Primarily fine sediments that accumulate on the ocean floor
Turbidity currents are the exception
Sedimentary Rocks Represent Past
Environments
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Sedimentary Rocks Represent Past
Environments
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Sedimentary Environments
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Sedimentary Structures
• The layers of the sedimentary rocks are called strata or beds
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Sedimentary Structures
• Graded beds - sediments in a strata gradually change from coarse at the bottom to fine at
the top
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Sedimentary Structures
– Ripple marks are small waves that are lithified in the sedimentary rocks
– Mud cracks indicate sediments form in an
alternatively wet and dry environment
– Fossils evidence of prehistoric life
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The Carbon Cycle and Sedimentary Rocks
• CO2 is one of the most active parts of the
carbon cycle
– Plants absorb CO2 through photosynthesis
– When plants die, some of the CO2 is deposited in the sediments
• Over geologic time, considerable amounts of plant biomass
is converted into fossil fuels
• When fossil fuels are burned, that CO2 is released back into the atmosphere
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End of Chapter 7