Carbonates Recent and Ancient Lab

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Carbonates Recent and Ancient Lab 6

• Geol. 343
• Facies Models

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What is a carbonate?

• A rock composed primarly of carbonate minerals
  (usually CaCO3)
• Primary minerals
• Aragonite CaCO3 (primarily biogenic)
• Calcite CaCO3 (primarily chemical)
• Dolomite CaMg(CO3)2
• Can be biogenic or chemical in origin.
• Most form in marine environments

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Difference from Sandstones and shales.

• Most form in shallow seas and lakesnot eroded
  from highlands
• Mainly formed by chemical and biological means
• Mostly deposited close to where they formed
• May be lithified at or just below the surface

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Distribution and importance

• Mainly tropical
• Some poleward (mostly shells)
• Reefs maily 30 deg. N/S
• Limited areas of shallow water carbonates
• But this was not the case in the past
• 20 of all sediments/ sedimentary rocks
• Host 1/3 of all hydrocarbon production
• For agriculture Pb-Zn hosts building stone

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Depth distribution

• Aragonite and calcite are oversaturated in
  surface waters but undersaturated in deep waters
• Boundary is the compensation depth 1km for
  aragonite and 4km for calcite (CCD)
• Little carbonate sediment on the ocean floor at
  gt5km depth

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Carbonate grains

• Carbonate clasts (detrital grains)
• Rock fragments from erosion of ancient carbonates
  lithoclasts
• Lithified young carbonate materials intraclasts
• Skeletal particles
• Whole or broken fragments of fossils bioclasts

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More carbonate grains

• Ooids
• Small spherical grains formed by coating a
  nucleus
• Other coated grains
• Pisoliths gt2mm sediment binding by algae or pptn
  in lakes/hot springs
• Oncolites/oncoids large 1-2cm near spherical
  forms
• Peloids
• Small spherical or eliptical grains lacking
  internal structure formed as fecal pellets
• Aggregate grains
• Grains joined by mud matrix some resemble grapes
  (grapestone)

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Things to do

• Make a full description of the recent carbonates
  (grain size, type, fossil ID, sorting, etc.)
• Based on this information make a full description
  of the depositional environment (tell me a good
  story!)
• Look at the ancient carbonates choose three
  samples (describe in full!) use both the Folk and
  Dunham classification
• Come up with a complete environmental
  interpretation and if possible tell me which one
  of the recent samples (if any) it is most similar
  to, remember uniformitarianism the present is
  the key to the past.

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Some depositional environments

• Carbonate platforms or shelves
• Atolls reefs
• Tidal flats
• Deep ocean (but above the CCD)
• Non-marine lakes
• Hot springs
• Restricted bays
• Shallow arid seas