SEDIMENTARY ROCKS

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SEDIMENTARY ROCKS
Chemical Precipitates
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SEDIMENTARY ROCKS
Chemical Precipitates
Evaporites

• more than 70 known evaporite minerals
• halite most common overall, gypsum most common
  in
• outcrop
• Why does gypsum dominate some beds, halite
  others?
• evaporation of seawater
• to 1/3 original volume gypsum
• to 1/10 original volume halite
• to lt1/20 original volume others
• halite / gypsum ratio due to reflux

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SEDIMENTARY ROCKS
Chemical Precipitates
Evaporative reflux

• constant recharge
• constant evaporation
• loss of brine

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SEDIMENTARY ROCKS
Chemical Precipitates
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SEDIMENTARY ROCKS
Chemical Precipitates
Cherts
Age pC C to Tertiary
Phanerozoic convergent plate margins
turbidites, ophiolites, generally deep water
origin radiolaria, sponges,
diatoms Proterozoic generally shallow
marginal marine origin??? no siliceous
skeletons then
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SEDIMENTARY ROCKS
Chemical Precipitates
Precambrian Banded Fe Fms
Age 3800 800 m.y. most 2,600 1,800
m.y. Found on all continents marginal-marine
environments No modern counterparts!! Origin???
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SEDIMENTARY ROCKS
Chemical Precipitates
Phanerozoic Bedded Fe Fms

• Phanerozoic in age
• gt 15 Fe
• Often contain ooids, marine fossils
• Definitely shallow marine origin
• Resemble carbonates in texture
• Origin replacement of carbonates

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SEDIMENTARY ROCKS
Chemical Precipitates
Phosphorites

• important source of fertilizer, uranium
• Phosphoria Fm of Wyoming
• Often contain peloids ooids
• Resemble carbonates in texture
• Origin replacement of carbonates