The Earth Through Time, 8e

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
  1. Dolostone


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.

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:


Origin of carbonate sediments


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.


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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

September 17, 2005