Sedimentation and Stratigraphy Geology 5142 Dr' Thieme

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Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 9 Chemical and Biochemical Sedimentary
  Rocks

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Carbonates

• account for approximately 1/10th of the rocks
  exposed at the Earth's surface
• Lithologies include

• limestone
• oolite
• coquina
• travertine
• marl
• chalk

• dolostone
• marble

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Carbonates

• the principal rock-forming carbonate minerals
  are
• Calcite - CaCO3
• some substitution of Mg
• hence "high Magnesium" and "low Magnesium"
  calcite
• Aragonite - a "polymorph" (CaCO3)
• Dolomite - MgCaCO3
• might also see Siderite (FeCO3) , Magnesite
  (MgCO3), Rhodocrosite (MnCO3). Ankerite - Ca
  (Fe,Mg,Mn)(CO3)2

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Calcite Compensation Depth (CCD)
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Adjectives for Field Use

• Dolomitic - has the mineral dolomite in it
• Calcareous - has the mineral calcite in it (not
  used with limestone)
• Argillaceous - has clay present in it.
• Arenaceous - has sand size detrital grains
• Fossiliferous - "skeletal" material of animals
  and plants
• Oolitic - contains oolites (ooids)
• Pisolitic - contains pisolites (pisoids)
• Peloidal contains peloids
• Cherty - contains chert nodules
• Brecciated - the rock is broken in situ

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Dunham Classification

• can be used with hand samples
• mirrors the Pettijohn classification of
  siliciclastic sedimentary rocks into mudstone,
  wacke, and arenite

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Use of the Dunham classification makes it
possible to use a common scale on a graphic log
to illustrate both the carbonate rocks and the
siliciclastic rocks
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Chemical Rock Components

• Folk (1959, 1962) separated
• Allochems ("from outside") - carbonate grains
  transported and deposited as clasts
• Orthochems ("true") found where they formed
• micrite is fine-grained carbonate mud, the
  matrix in a "wackestone"
• spar is cement identifiable as clear
  interlocking crystals that "sparkle"

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Folk classification
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Allochems (Skeletal grains)
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Mollusc (Bivalves, Pelecypods)
A mollusc shell in the Castle Hayne ls
A mollusc shell replaced by spar calcite
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Brachiopods
Brachiopod shells form as calcite so the internal
structure preserves and they look very different
from mollusc shells (wavy or oblique layers
of calcite)
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Echinoderms
Echinoderms also make calcite, but only for a few
body parts, particularly the plates on the
outside of crinoids, sea urchins, and their
relatives.
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Ooids and Peloids
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Biolithite
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Biomicrite
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Biomicrudite
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Biosparite
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Pelsparite
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Biopelsparite