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37 Cards in this Set
- Front
- Back
What is diagenesis
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Any physical, chemical or biological alteration of sediment after initial deposition and during and after its lithification.
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What is lithification
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Conversion of sediment into rock.
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What is the upper boundary on temperature associated with diagenesis?
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< 300 C
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What is the geothermal gradient?
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temperature change per unit length of depth into the earth.
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What is the geostatic or lithostatic gradient?
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the rate of increasing P with respect to increasing depth in Earth’s interior.
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What is the hydrostatic gradient?
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the rate of increasing P with respect to increasing depth in water.
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What is the minimum burial depth and time associated with the onset of diagenetic processes?
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1-10^5 years and <1-100m
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What is mechanical compaction?
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compaction—the loss of porosity by rearrangement of the grain framework due to overburden pressure.
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What is the connection between compaction and thinning of deposits?
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the more compact a deposit becomes the thinner it gets.
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What is the connection between compaction and expulsion of pore fluid?
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compaction require sediment deposit to expel water that is filling pore space. this causes thinning of the deposit.
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What is the connection between under-compacted deposits and excess hydrostatic pore pressures?
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under-compacted sedimentary deposits ten to have pore fluid pressure greater than the hydrostatic pressure. these excess pressures drive subsurface fluid flow and destabilize deposits.
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12. How does cementation affect porosity?
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cementation decreases porosity.
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What is pressure solution? How does pressure solution at grain contacts affect porosity?
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pressure concentrated at points of contact between grains. causes solution and migration of ions away from points of contact. to lower pressure areas were the dissolved phase can be reprecipitated.
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What causes grain deformation and fracturing?
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high pressures.
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What are concavo-convex grain contacts?
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grains that start to dissolve in the pore waters at their contacts.
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What are sutured grain contacts?
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irregular grain boundaries.
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What is a styolite?
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A Stylolite is a sutured boundary made visible by insolube material trapped and concentrated in the zone of pressure solution. More common in carbonates.
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What are Cementation, Dissolution, Authigenesis, Recrystallization, and Replacement?
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cementation: nucleation and precipitation of a mineral the cements the rock.
Dissolution: a mineral dissolves, creating secondary porosity Authigenesis: a 'new' mineral that grows in a pore space is authigenic Recrystallization: the crystal favric changes, but the mineralogy stays the same. Usually calcite. Replacement: one mineral dissolves, another forms in its place. Usually taking the old shape. |
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What are the two most common pore-filling cements?
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Carbonate minerals, (calcite & aragonite) 2) silica.
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Under what pH conditions does carbonate dissolve and precipitate?
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carbonates precipitate from alkaline solutions greater than 7
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Under what pH conditions does silica dissolve and precipitate?
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Silica cements precipitate from acidic solutions with a pH less than 7.
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What are common sources of ions for calcite cement?
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calcium carbonate
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What are common sources of ions for silica cement?
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dissolution of silica dust and biogenic silica.
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What is the relationship between temperature and silica solubility?
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as temperature increases amorphous silica increases
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What are quartz overgrowths?
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the development of quartz cement around quartz minerals
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What are the most common authigenic clay minerals?
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Clay minerals that precipitate in place from pore fluids. Most common, illite and kaolinite
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What are clay rims?
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clay cements typically formed around mineral grains. typicaaly and early or first diagenetic event.
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What does occlude mean?
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to completely fill pore space
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What do smectite and kaolinite alter to with burial depth?
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with increasing depth of burial and temperature, the dehydration of smectites and kaolinite causes alteration to illite and chlorite.
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What is the most common clay mineral found in deeply buried and very old sedimentary rocks?
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illite
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What is the significance of an “embayed” sand grain?
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It is partially dissolved away
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What is secondary porosity?
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inhibition of cementation with the dissolution of minerals. This creates a new porosity in the sandstone deep underground.
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Dissolution of grains of what mineral produces the greatest amount of secondary porosity?
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feldspar
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What is vitrinite reflectance?
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resistant plant cells altered under T&P, and reflect more light the higher the rank. Coal.
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What are zeolites?
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hydrous aluminosilicates alteration
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What are fluid inclusions and how are they used to assess diagenetic conditions?
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fluid inclusion preserves a sample of the pore fluid at the time of cement or overgrowth formation. analyzed to find out diagenetic processes and rates.
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What is differential compaction?
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spacially varying compaction over a layer of inconsistent thickness.
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