Sedimentary Rocks

Pamela J. W. Gore
Georgia Perimeter College

Objectives:

1. Explain the formation of sedimentary rocks in terms of the rock cycle.
2. Explain and give examples of the various environments of deposition of sediment.
3. Discuss the textural characteristics of sediments
4. Briefly explain why sedimentary rocks are important in the study of Earth history.
5. Contrast the basic groups of sedimentary rocks.
6. List and briefly describe the principle characteristics of the common sedimentary rocks.
7. Describe the ways in which sedimentary rocks are lithified.

Sediment = loose particulate material (clay, sand, gravel, etc.)

Sediment becomes sedimentary rock through lithification, which involves:

Compaction
Cementation
Recrystallization (of carbonate sediment)

Types of sedimentary rocks

Overview

Terrigenous (detrital or clastic)

Conglomerate or Breccia
Sandstone
Siltstone
Shale

Chemical/biochemical

Evaporites
Carbonate sedimentary rocks (limestones and dolostone)
Siliceous sedimentary rocks

Organic (coals)

Other - ironstones

Sedimentary Structures

Sedimentary Environments

Fossils

A. Terrigenous (also called detrital or clastic)

Terrigenous 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:

Clasts (larger pieces, such as sand or gravel)
Matrix (mud or fine-grained sediment surrounding the clasts)
Cement (the glue that holds it all together), such as:

calcite
iron oxide
silica

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

Terrigenous sedimentary rocks are classified according to their texture (grain size):

Gravel: Grain size greater than 2 mm

If rounded clasts = conglomerate
If angular clasts = breccia

Conglomerate

Breccia

Sand: Grain size 1/16 to 2 mm

Sandstone
If dominated by quartz grains = quartz sandstone (also called quartz arenite)
If dominated by feldspar grains = arkose
If dominated by sand-sized rock fragment grains = lithic sandstone (also called litharenite or graywacke)

Quartz Sandstone

Arkose

Graywacke (lithic sandstone)

Silt: Grain size 1/256 to 1/16 mm (gritty)

Siltstone

Siltstone

Clay: Grain size less than 1/256 mm (smooth)

Shale (if fissile)
Claystone (if massive)

Shale - fissile

Claystone
(kaolinite)
- not fissile

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B. Chemical/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.

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

Rock salt - composed of halite (NaCl).
Rock gypsum - composed of gypsum (CaSO4.2H20)
Travertine - composed of calcium carbonate (CaCO3), and therefore, also technically a carbonate rock; travertine forms in caves and around hot springs.

Bonneville Salt Flats of the Great Salt Lake, Utah. The lake bed is covered with rock salt which gives it the white color. The salt is mined by the Morton Salt Company.

Layered gypsum of the Castille Formation, Carlsbad, New Mexico

Gypsum crystals, Marion lake, Australia

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

Two minerals are dominant in carbonate rocks:

Calcite (CaCO3)
Dolomite (CaMg(CO3)2)
Remember which of these fizzes readily, and which of these must be scratched or powdered!

Carbonate rock names:

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.
Oolitic limestone (look for the sand-sized oolites)
Fossiliferous limestone (look for various types of fossils in a limestone matrix)
Coquina (fossil hash cemented together; may resemble granola)
Chalk (made of microscopic planktonic organisms such as coccolithophores; fizzes readily in acid)
Crystalline limestone
Travertine (see evaporites)
Others - intraclastic limestone, pelleted limestone

Stromatolitic limestone

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.

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.
Chert - Massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.

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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):

Peat (porous mass of brownish plant fragments resembling peat moss)

Lignite (crumbly and black)

Bituminous coal (dull to shiny and black; sooty; layers may be visible)

Anthracite coal (extremely shiny and black, may have a slight golden shine; low density; not sooty; technically a metamorphic rock due to high temperatures and pressures to which it has been subjected)

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