Earth image Sedimentary Rocks

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  1. Identify sedimentary rocks by their physical and chemical properties.
  2. Explain the formation of sedimentary rocks in terms of the rock cycle.
  3. Discuss several ways in which sedimentary rocks form.
  4. Describe the ways in which sedimentary rocks are lithified.
  5. List and describe the characteristics of various clastic sedimentary rocks, including textures and composition.
  6. List and describe the characteristics of various non-clastic sedimentary rocks, including textures and composition.
  7. Explain the origin of organic sedimentary rocks.
  8. Explain how sedimentary rocks are classified.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E5b. Classsify rocks by their process of formation.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Rock is composed of different combinations of minerals. Smaller rocks come from the breakage and weathering of bedrock and larger rocks. Soil is made partly from weathered rock, partly from plant remains-and also contains many living organisms.
  • Sediments of sand and smaller particles (sometimes containing the remains of organisms) are gradually buried and are cemented together by dissolved minerals to form solid rock again.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Land forms are the result of a combination of constructive and destructive forces. Constructive forces include crustal deformation, volcanic eruption, and deposition of sediment, while destructive forces include weathering and erosion.
  • Some changes in the solid earth can be described as the "rock cycle." Old rocks at the earth's surface weather, forming sediments that are buried, then compacted, heated, and often recrystallized into new rock. Eventually, those new rocks may be brought to the surface by the forces that drive plate motions, and the rock cycle continues.

Sediment and Sedimentary Rocks

Sediment = loose particulate material (clay, sand, gravel, etc.).
Most sediment is derived from the weathering (breakdown) of pre-existing rocks.
Some sediment is formed through chemical and biochemical processes acting in the depositional basin, For example, broken shell fragments or lime mud formed from calcareous algae would be sediment formed through biochemical processes.

Sediment becomes sedimentary rock through lithification, which involves:

  1. Compaction due to pressure or weight of overlying sediments
  2. Cementation by deposition of minerals in pore spaces from waters carrying ions in solution
Sedimentary rocks also form as a result of precipitation of minerals from solution in lakes, in the sea, in caves, and in hot springs. Examples include:
  1. Evaporites (which form from the evaporation of water), such as rock salt and rock gypsum
  2. Travertine, which is a type of limestone that forms in caves as a result of deposition of calcite from groundwater (stalactites and stalagmites)

Coal, an organic sedimentary rock, is made up mostly of organic matter, primarily plant remains which accumulated in a swamp with stagnant, oxygen-deficient water.

Types of sedimentary rocks

Overview

A. Clastic sedimentary rocks (also called terrigenous or detrital)

Clastic sedimentary rocks are derived from the weathering of pre-existing rocks, whch have been transported to the depositional basin.

They have a clastic (broken or fragmental) texture consisting of:

  1. Clasts (larger pieces, such as sand or gravel)
  2. Matrix (mud or fine-grained sediment surrounding the clasts)
  3. Cement (the glue that holds it all together), such as the following minerals:
    1. calcite
    2. iron oxide
    3. silica

Clasts and matrix (labelled),
and iron oxide cement
(reddish brown color)

Clastic sedimentary rocks are classified according to their texture (grain size, as well as shape for gravel-sized clasts):

B. Non-clastic sedimentary rocks (also called chemical and biochemical sedimentary rocks)

This group includes the evaporites, the carbonates (limestones and dolostone), and the siliceous rocks.

These rocks form within the depositional basin from chemical components dissolved in the seawater. These chemicals may be removed from seawater and made into rocks by chemical processes, or with the assistance of biological processes (such as shell growth). In some cases it is difficult to sort the two out (in carbonates or some siliceous rocks, for example), so they are grouped together as chemical/biochemical.

  1. Evaporites - The evaporites form from the evaporation of water (usually seawater).

    1. Rock salt - composed of halite (NaCl).


      Salt flats of Great Salt Lake, Utah.
      The lake bed is covered with rock salt.

    2. Rock gypsum - composed of gypsum (CaSO4.2H20)


      Layered gypsum of the Castille Formation,
      Carlsbad, New Mexico


      Gypsum crystals, Marion lake, Australia

    3. Travertine - composed of calcium carbonate (CaCO3), and therefore, also technically a carbonate rock; travertine forms in caves and around hot springs.

      Travertine deposits in cave in Virginia.

  2. Carbonates - The carbonate sedimentary rocks are formed through both chemical and biochemical processes. They include the limestones (many types) and dolostones.

    1. Two minerals are dominant in carbonate rocks:
      1. Calcite (CaCO3)
      2. Dolomite (CaMg(CO3)2)
      3. Remember which of these fizzes readily, and which of these must be scratched or powdered!

    2. Carbonate rock names:

      1. Micrite (microcrystalline limestone) - very fine-grained; may be light gray or tan to nearly black in color. Made of lime mud, which is also called calcilutite.
      2. Oolitic limestone (look for the sand-sized oolites)

        Oolitic limestone

      3. Fossiliferous limestone (look for various types of fossils in a limestone matrix)

        Fossiliferous limestone

      4. Coquina (fossil hash cemented together; may resemble granola)

        5. Coquina

      5. Chalk (made of microscopic planktonic organisms such as coccolithophores; fizzes readily in acid)
      6. Travertine (see evaporites)

  3. Siliceous rocks - The siliceous rocks are those which are dominated by silica (SiO2). They commonly form from silica-secreting organisms such as diatoms, radiolarians, or some types of sponges. Chert is formed through chemical reactions of silica in solution replacing limestones.

    1. Diatomite - looks like chalk, but does not fizz in acid. Made of microscopic planktonic organisms called diatoms. May also resemble kaolinite, but is much lower in density and more porous). Also referred to as Diatomaceous Earth.
    2. Chert - Massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.

C. Organic Sedimentary Rocks (Coals)

This group consists of rocks composed of organic matter (mainly plant fragments). Because of this, they lack minerals (which must be inorganic, be definition). These are the coals. In order of increasing depth of burial (temperature and pressure):

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

Page created February 25, 2005
Links updated October 13, 2008