Carbonate rocks are "Chemical or Biochemical" in origin
They form within the basin of deposition, and are called Intrabasinal Rocks.

Composition

1. Limestones
   • Calcite
   • Aragonite

2. Dolostones
   • Dolomite

Depositional Conditions

• Warm, shallow seas - South Florida, Bahamas, etc.
• Tropical climate
• 30 ° N - 30 ° S of equator
• Low to no terrigenous input
• Light required
  • Algae are photosynthetic

  • Corals are animals with symbiotic algae (dinoflagellates) living in their tissues, called zooxanthellae. This benefits both parties. It protects the dinoflagellates from being eaten,and they have access to the by-products of coral metabolism (waste products containing phosphorus, nitrogen, and carbon dioxide). The coral benefits from the removal of its wastes, and from the oxygen and organic nutrients that are produced by the photosynthetic algae.

  • Much lime mud forms from the disintegration of calcareous algae (such as Halimeda and Penicillus). The lime mud forms micrite or calcilutite. When the calcareous algae die, their skeletons break down and disintegrate producing aragonite needle muds.

  • Blue-green algae are involved in the formation of oolites (or ooids). Oolites form in warm shallow seas with constant wave agitation.

Parts of the reef

• Back-reef (lagoon) - low energy, lime muds; bordered by tidal flat on landward side
• Reef - high energy, "boundstone"
• Fore-reef (deep water) - turbidites, breccias, grading seaward into organic-rich lime mud

Types of plankton

1. Foraminifera - CaCO3, animal-like (such as Globigerina ooze)
2. Coccolithophores - CaCO3, plant-like (forms chalk)
3. Diatoms - SiO2, plant-like (forms diatomaceous earth - diatomite)
4. Radiolarians - SiO2, animal-like

Carbonate compensation depth or CCD

• Above this depth, water is warmer, and precipitation of CaCO3 is greater than dissolution. Calcarous plankton can be found in the water column, and on the bottom, if water depth is shallower than the CCD.

• Above the CCD, bottom sediments can consist of calcareous sediments forming chalk or limestone.

• Below this depth, water is colder, and CaCO3 tends to dissolve (dissolution is greater than precipitation). Tiny shells of CaCO3 dissolve, and do not accumulate on the bottom if water is deeper than the CCD.

• Below the CCD, the bottom sediments consist of:
  1. Clay
  2. Silica shells of plankton (diatoms, radiolarians)

• At the CCD, the rate of precipitation of CaCO3 = the rate of dissolution of CaCO3

• Approximately 4000 - 5000 m deep in Pacific

• What would happen near the mid-ocean ridge? As spreading proceeds?

• How would you interpret a rock consisting of lime mud? (chalk or micrite?)
  1. Climate?
  2. Water depth?
  3. Energy levels?

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This page created by Pamela J. W. Gore
pgore@gpc.edu
DeKalb College, Georgia

Created Fall 1995
Last modified October 15, 1996