Biogenic and organic rich sediment

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Sedimentary Resources
Biogenic and
organic rich sediments
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High Corg sediment
Chert
Phosphorites
Coal
Oil Shale
Hydrocarbons
Biogenic and organic rich sediment
Type
• High Corg sediment
• Sapropel
• Chert
• Phosphorites
• Coal
• Oil Shale, Black Shale
• Hydrocarbons
Marine settings
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Occur along continental margins
Upwelling zones
Anoxic regions
Oil shales
• Black shale
• Chert
• Phosphorite
• Sapropel
Organic-rich sediment follows
surface production patterns
Low Corg
in most
Deep
Ocean
Enviroments
High Corg
in select
Marine
Ocean
Enviroments
Modern example:
Oxygen Minimum Zone sedimentBlack shale in the making?
Oceanic silica cycling and
chert formation
Sources of dissolved Silica:
• Weathering of continental rocks (e.g. feldspars)
• Riverine input
• Dissolution in water column or sediment
• Hydrothermal reaction with sediment and basalt
Sinks of dissolved Silica:
• Growth of radiolarian, silicoflagellate, diatom, skeletons and sponge spicules
• Inorganic reprecipitation of biogenic silica
• Inorganic adsorption on sediment
Classification of Chert
• Diagenetically altered and lithified biosilica
(Opal-A, Opal-CT, “porcellanite”, chert)
• Bedded Chert - primary
• Reprecipitation in place
• Fossils may be apparent or cryptic
(viewed by HF etching)
• Nodular Chert - diagenetic replacement in
• Carbonate matrix
• Clay matrix
Bedded or primary chert
Nodular Chert
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Diagenetic reprecipitate
Formed by sediment contact with silica-rich pore waters.
Deposition can follow or cut bedding, may be associated with burrows.
Range of white/gray/black color due to organic content (“Fresh” material may be light green, tan, or
even purple!)
Mechanisms of formation are not clearly understood (What P, pH, T, fluid Si concentration)
Mechanisms of chert formation?
Maturation theory • Stepwise remineralization
• Carbonate is replaced by various silica polymorphs in succession:
• Chalcedony/Chalcedonic quartz
• Cristobalite
• Chert
• Variable rate of formation depending in part on Si supply rate
Mineral association theory • Specific Si polymorphs are associated with certain lithologies:
• Chalcedony/Chalcedonic quartz in carbonates
• Cristobalites in clays
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Ratio of Si/Metal cations in sediment determines polymorph formed
Phosphorites
• Rare apatite deposits contain as much as 50% phosphate mineral by wt.
(fluorapatite, hydroxyapatite, or collophane)
• Source of P in fertilizers
• Nodular phosphorites form on outer continental shelves
• Guano and Phosphatized Limestone
• Phosphatic placers deposits (reworked bone fragments)
Phosphoria
Formation,
Rocky Mts.
Organic-rich sediment and sedimentary rocks
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Organic rich sediment (humus, peat, sapropel)
Coal
Oil shales, including black shales
Concentrated hydrocarbons (oil, gas)
Asphalt and tar
Form in both terrestrial and marine environments
Coal genesis
• Abundant plant matter
• Rapid suboxic to anoxic deposition
• Little dilution by terrigenous or other material
• Environments: swamp, marsh, delta, coastal plains
• Often laid down in thin sheets or seams
• Deposition my be rythmic (Cyclothems)
Classic
Pennsylvanian
Coal Cyclothem
• Organic rich swamps form during marine regression and progradation of
deltas
• Rapid transgression buries organic rich swamp beneath marine sequence
of shale and limestone.
Classes of Hydrocarbon
Traps
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Folds
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Faults
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Unconformity
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Stratigraphic trap
Hydrocarbon reservoir genesis:
• High T, P
• Migration from high organic source rock
• Porous and permeable reservoir rock
• Impermeable cap rock
Next
lecture:
Shallowwater
carbonates
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