Eighth Edition
by Harold L. Levin

Chapter 5 - page 9

The Sedimentary Archives

Carbonate Rocks and Sediments

Carbonate rocks consist of limestone and dolostone. Limestones are the most abundant carbonate rocks.

Carbonate rocks are chemical or biochemical in origin. They form within the basin of deposition, and so are called intrabasinal rocks.

The minerals present in carbonate rocks are:

1. Limestone
• Calcite
• Aragonite

2. Dolostone
• Dolomite

Depositional Conditions

Most carbonate rocks form in the shallow marine environment. Some form in lakes (as well as in caves and hot springs).

Most limestones are the direct or indirect result of biologic activity.

• May contain shells or the remains of other marine organisms
• May precipitate from seawater as a result of biologic activity (photosynthesis by microscopic marine plants and algae removes CO₂ from seawater, leading to calcium carbonate precipitation.)

Carbonate sediment is forming today in southern Florida, Bahama Banks, and other areas.

The Bahama Banks are an example of a carbonate platform, a steep-sided, flat-topped plateau rising from the seafloor, and covered by relatively shallow marine water.

Characteristics of most marine carbonate environments:

• Warm water
• Shallow water (less than 200 m deep)
• Tropical climate (30 ° N - 30 ° S of equator)
• Clear water (low to no terrigenous input)
• Sunlight required for photosynthesis by algae

Origin of carbonate sediments

• Much lime mud forms from the disintegration of calcareous algae (such as Halimeda and Penicillus). When the calcareous algae die, their skeletons break down and disintegrate producing aragonite needle muds. These lime muds lithify to form fine-grained limestone.

Halimeda	Penicillus
Photos courtesy of Pamela Gore.

Under-water view of calcareous algae in Florida Bay. Several specimens of Halimeda and Penicillus are shown. Photos courtesy of Pamela Gore.

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


Sand-sized ooids. Width of field about 2 cm.


Oolitic limestone, Lower Paleozoic, eastern Tennessee. Scale in cm.
Photo courtesy of Pamela Gore.

• Accumulation of microscopic shells of marine organisms

  
  Microscopic shells of foraminifera.

• Precipitation of calcium carbonate from seawater as a result of biologic activity
• Abrasion of shells


Coquina, a carbonate rock made of abraded shells of molluscs. Scale in cm.
Photo courtesy of Pamela Gore.


Fossiliferous Limestone, Ordovician, northwestern Georgia. Scale in cm.
Photo courtesy of Pamela Gore.

• Accumulation of fecal pellets produced by burrowing organisms.

Ancient limestone deposits

During the Paleozoic Era, thick deposits of shallow water limestone were deposited on shallow carbonate platforms along the eastern margin of North America and in low-lying interior regions that were flooded by the shallow seas.

Thick limestone deposits are present in many areas in North America, including the Appalachian Mountains, the midwest, Michigan, western Texas and New Mexico, and Alberta, Canada.

Dolomite

Dolomite is a calcium-magnesium carbonate mineral (CaMg(CO₃)₂) that comprises the sedimentary rock dolostone. (Sometimes the rock is also called dolomite.)

Dolomite is interpreted to form when magnesium that has been concentrated in sea water replaces calcium in calcium carbonate in a previously deposited limestone. Today, dolomite (or high magnesium calcite) only forms in a few areas of the world where intense evaporation of seawater concentrates the magnesium.

Extensive areas of ancient carbonate rocks of Precambrian, Paleozoic, and Mesozoic age have been altered to dolomite. Dolomitization may be related to times of high sea level worldwide, under favorable climatic conditions.