Sedimentary Rocks Laboratory

Pamela J. W. Gore
Department of Geology, Georgia Perimeter College
Clarkston, GA 30021

Copyright © 1998-2009 Pamela J. W. Gore
See new version of this lab at http://facstaff.gpc.edu/~pgore/geology/historical_lab/SedimentaryRocks.pdf

This lab introduces sedimentary rocks. Sedimentary rocks are important because they contain the historical record of ancient environments and life on Earth. Throughout this course we will be studying sedimentary rocks, the fossils they contain, and the history that they record.
Sediment is loose particulate material, which can form in several ways. Sediment may be derived from the weathering and erosion of pre-existing rock (it is called terrigenous, or detrital, or clastic sediment). Sediment also may be formed from chemical, biochemical, or biological materials (such as minerals formed by the evaporation of sea water, sea shells, or plant remains). Sedimentary rocks are formed when sediment is compacted and cemented together. Approximately 75% of the rocks exposed at the Earth's surface are sedimentary rocks.
Sediment accumulates in sub-aqueous environments, such as lakes, rivers, bays, deltas, beaches, and ocean basins. Sediment also may be deposited in other types of environments, such as deserts or glaciated areas. The characteristics of the sediment (grain size, shape, sorting, and composition), and the sedimentary structures are clues to the environment in which the sediment was deposited.
IDENTIFYING SEDIMENTARY ROCKS
Sedimentary rocks are grouped according to their origin into (1) terrigenous sedimentary rocks (also called detrital or clastic sedimentary rocks), which form from fragments of pre-existing rocks, (2) chemical and biochemical sedimentary rocks, which form as chemical precipitates, or from the shells of organisms, and (3) organic sedimentary rocks, composed of organic matter or carbon.
I. TERRIGENOUS (CLASTIC OR DETRITAL) SEDIMENTARY ROCKS:

Terrigenous sedimentary rocks are those which are derived from pre-existing rocks. They are composed of rock fragments and mineral grains which have been weathered, eroded, transported, deposited, and cemented together to form a sedimentary rock. They are sometimes referred to as extrabasinal, because they are derived from rocks outside of the basin of deposition. The individual grains (or clasts) in these rocks are mechanically durable (to withstand abrasion during transport), and chemically stable. Typical clasts are made of quartz, feldspar, muscovite, clay minerals, or rock fragments.

Texture

There are three "textural components" to most clastic sedimentary rocks:

1. Clasts (gravel, sand, silt)
2. Matrix (fine-grained material surrounding clasts)
3. Cement (silica, calcite, or iron oxide - the "glue" that holds the rocks together).

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

Clast size

Most terrigenous sedimentary rocks are classified by the size of the clasts they contain. The size ranges of sedimentary grains are given below:

Gravel	> 2 mm
Sand	1/16 - 2 mm
Silt	1/256 - 1/16 mm
Clay	< 1/256 mm

Sedimentary rocks with gravel-sized clasts are sometimes referred to as rudites or rudaceous rocks. Rudite means "gravel". Arenaceous sedimentary rocks or arenites are those with sand-sized grains. Arenite means "sand". Argillaceous sedimentary rocks or argillites are those with mud. (Mud is defined as a mixture of silt and clay.) Argillite means "mud". In general, it takes higher energy (higher water velocity) to transport larger grains.
Clast shape

Shape of clasts is important in naming the coarser-grained sedimentary rocks (those with gravel-sized clasts). Gravel may be rounded or angular (based on the sharpness of the corners of the clasts). Gravel rapidly becomes rounded in the first few miles of transport.
Sorting

Sorting refers to the distribution of grain sizes in a rock. If all of the grains are the same size, the rock is "well sorted". If there is a mixture of grain sizes, such as sand and clay, or gravel and sand, the rock is "poorly sorted".

Classification of Terrigenous Sedimentary Rocks

(See Table 1)

Rocks with gravel-sized clasts

Conglomerate and breccia contain gravel-sized clasts surrounded by finer-grained matrix. Conglomerates have rounded clasts. If the particles are angular, the rock is a breccia. In a conglomerate, the larger clasts are generally more rounded than the smaller clasts.

Conglomerate

Breccia
Rocks with sand-sized clasts
Sandstones contain sand-sized clasts. Sand grains may be either rounded or angular, and they are generally more or less the same size (this is called well sorted). The sand grains are held together by cement, which may be silica (quartz), calcite, or iron oxide. (Calcite will fizz in hydrochloric acid; iron oxide makes the rock red, brown, or orange). (Arenite is another word for sandstone; the word is derived from the material that covered the floor of the Roman arenas where the gladiators fought.)
Sandstones are classified according to the composition of the sand grains into three main groups:
1. Quartz sandstone or quartz arenite is composed mainly of quartz sand grains.
  
  Quartz sandstone or quartz arenite
2. Arkose is composed mainly of pink or white feldspar grains, with quartz, and generally some muscovite mica or sand-sized rock fragments.
  
  Arkose
3. Litharenite (meaning rock-sand) or lithic sandstone or graywacke is predominantly composed of dark sand-sized rock fragments, with some mica, quartz, and feldspar grains in a clay-rich matrix. A wacke is defined as a "dirty" sand. The term "graywacke" is best used loosely; there is no strict definition of the term with which all geologists agree. A litharenite is more strictly defined as a rock primarily composed of sand-sized rock fragments.
  
  Litharenite or lithic sandstone
  sometimes called graywacke

· Rocks with silt-sized grains

Siltstone is intermediate in texture between sandstone and shale. The grains are difficult to see with the naked eye because they are so small, but the rock has a distinct gritty feel to the fingernails.

· Clay-dominated rocks

Shale or claystone is a fine-grained rock composed of tiny (less than 1/256 mm) clay minerals, mica, and quartz grains. The individual grains are too small to see with the naked eye or a hand lens, and the rock feels smooth to the touch (not gritty).

Shale and claystone differ in the way that they break.

· Shale is fissile; this means that it splits readily into thin, flat layers.

· Claystone, on the other hand, is not fissile, and breaks irregularly.


Shale is fissile	Claystone is not fissile (variety = kaolin or kaolinite)

The color of shale or claystone may reveal something about its composition.

Black shales contain organic matter (they are sometimes called bituminous shales).
Red shales contain iron oxide.
Kaolin, a type of white claystone mined in Georgia, is composed of the mineral kaolinite (used in the manufacture of china, coatings for glossy paper, rubber, etc.).

Mud is defined as a mixture of silt and clay. Rocks with both silt and clay are referred to as mudstones or mudshales, depending on whether or not they are fissile.

II. CHEMICAL AND BIOCHEMICAL SEDIMENTARY ROCKS

Chemical, biochemical, and organic sedimentary rocks are sometimes referred to as intrabasinal because they form within the basin of deposition, rather than being transported into it. They include chemical precipitates (such as rock salt and gypsum), as well as the accumulated remains of organisms which lived within the basin (such as limestones composed of fossil shells).

We will be dividing chemical and biochemical sedimentary rocks into four groups: evaporites, siliceous rocks, ironstones, and carbonate rocks.

A. Evaporites

Evaporites are chemical precipitates, which form by precipitation of dissolved minerals from water during evaporation. There are numerous evaporites, but we will concentrate on three:

1. Travertine (calcite - CaCO₃) - Travertine forms by evaporation of cave, spring, or river waters. It consists of intergrown calcite crystals, and fizzes in acid. Travertine is a dense, crystalline rock with tan and white color bands. It is especially common in limestone caverns where it forms flowstone and dripstone, including stalactites and stalagmites, recognized in the lab their cylindrical shape and internal "tree-ring-like" appearance. Travertine which forms around springs is a more porous, light-colored rock, which is generally called tufa. Because travertine is composed of calcite, it is also mentioned with the carbonate rocks, below.

2. Rock gypsum (gypsum - CaSO₄^.2H₂O) - Rock gypsum is softer than your fingernail (you can scratch it), and may be glassy with obvious cleavage, granular, or have a fibrous, satiny luster. It ranges from colorless to white to pink. Gypsum forms by precipitation from sea water after about 80% of the water has evaporated. Gypsum is altered to anhydrite (CaSO₄), by removal of water, generally caused by burial to depths greater than several hundred meters. For purposes of lab, we will not need to distinguish between gypsum and anhydrite.

3. Rock salt (halite - NaCl) - Rock salt is a glassy, crystalline rock which can be easily recognized by its salty taste. It ranges from colorless, to white, gray, pink or orange, due to impurities. Halite forms by precipitation from sea water after about 90% of the water has evaporated.

B. Siliceous Sedimentary Rocks

Siliceous rocks are dominated by silica (SiO₂), which precipitates from solution within the basin of deposition. (They do not include quartz sandstones which are extrabasinal in origin.) The most common siliceous sedimentary rocks are chert, opal, and diatomite.

1. Chert (microcrystalline quartz - SiO₂). Chert is a very fine grained silica sediment of chemical or biochemical origin. Some chert contains siliceous skeletons of micro-organisms known as radiolarians, which can be seen in thin section. Other chert forms through the replacement of limestone, often preserving carbonate textures such as oolites, although the rock has been completely altered to silica.

Chert can be recognized by its extremely fine grain size, smooth feel, and hardness (scratches glass). Chert varies in color, and may be black, white, tan, gray, or greenish gray. A red variety is called jasper. Opal is related to chert, but contains varying amounts of water, which produces the characteristic iridescence. Flint is sometimes used as a synonym for chert, but the term is used loosely and best reserved for artifacts such as arrowheads.

2. Diatomite (diatomaceous earth) is a soft, white, powdery rock of low density, composed of the siliceous (silica) skeletons of microscopic algae called diatoms. Diatomite can be distinguished from chalk because it does not react with hydrochloric acid. It can be distinguished from kaolinite by its low density (may float on water).

C. Sedimentary ironstones

Some sedimentary rocks are dominated by iron-bearing minerals such as hematite. Common examples of sedimentary ironstones include:

· Precambrian banded iron formations

· Paleozoic oolitic hematite or oolitic ironstone beds such as those exposed along Red Mountain in Birmingham, Alabama

· Iron oxide concretions in sandstone, commonly known as "bog iron ore"

D. Carbonate rocks

Carbonate rocks are made up of carbonate minerals. These are minerals which contain a carbonate (CO₃) group, such as:

Calcite	(CaCO₃)
Aragonite	(CaCO₃)
Dolomite	(CaMg(CO₃)₂)

Calcite and aragonite are polymorphs of calcium carbonate. Calcite is the stable form, and aragonite is metastable. Aragonite will alter to calcite over a long period of time. Rocks which contain abundant calcium carbonate are often referred to as calcareous rocks. Carbonate rocks most commonly form in warm shallow seas in areas such as southern Florida and the Bahamas. Dolomite frequently forms by chemical replacement of calcium carbonate in limestones after deposition.

Carbonate minerals are easy to identify because they react with hydrochloric acid. Calcite and aragonite effervesce (fizz) readily in hydrochloric acid. Dolomite will fizz weakly, only after it has been powdered. (It will not be necessary to distinguish calcite from aragonite in the lab.)

The rocks which contain carbonate minerals are:

Limestone (primarily composed of the minerals calcite and aragonite). Limestones are generally gray (but may be tan, pink, white, black, or other colors).
Dolostone (primarily composed of the mineral dolomite). Weathered surfaces of dolostones are commonly yellowish or brownish gray because of the presence of small amounts of iron associated with the magnesium in dolomite.

Textures of carbonate rocks

Terrigenous rocks are composed of clasts, matrix, and cement. Carbonate rocks have similar textural components, which go by the following names:

1. Allochems (Analogous to clasts.) Includes:

o intraclasts

o oolites

o fossils

o pellets)

2. Matrix (microcrystalline calcite or lime mud)

3. Spar (calcite cement).

The textures of carbonate rocks are best studied in thin section, however, some of the larger allochems such as intraclasts, oolites, and fossils are visible in hand specimens.

1. Coquina. Coquina is a type of fossiliferous limestone made up of fossil shells with little or no matrix. It is porous and light-colored, and the shells are frequently broken, abraded, and fairly well sorted. The shells are gravel-sized (greater than 2 mm), and coquina is a calcirudite.

Coquina

2. Chalk. Chalk is a type of fossiliferous limestone made up entirely of microscopic shells. These tiny shells are coccoliths, the remains of planktonic marine algae called coccolithophores. Coccoliths are too small to see using an ordinary light microscope; they can only be viewed with an electron microscope. The texture of chalk is similar to that of micrite or calcilutite, but chalk is white in color, less dense, and less compact than micrite. Chalk may be distinguished from other white fine-grained sedimentary rocks (such as kaolinite or diatomite) because it fizzes readily in hydrochloric acid.

3. Oolitic limestone. Oolites (or ooids) are tiny concentric spheres of calcium carbonate which range between 0.1 and 2.0 mm in diameter. On a cut or broken surface they look circular, and internal concentric laminations may be seen with a handlens or microscope. Oolites are not fossils! They form by the precipitation of aragonite under certain conditions in warm shallow seas, probably under the influence of blue-green algae. Because oolites are less than 2 mm in diameter, oolitic limestone is calcarenite. (Structures resembling oolites that are larger than 2.0 mm in diameter are called pisolites).

Oolitic limestone,
Lower Paleozoic, eastern Tennessee
Scale in cm

Oolitic Limestone

4. Intraclastic limestone. Intraclasts are flat, gravel-sized chips of limestone in a lime mud matrix. Intraclasts form when tidal flats covered by lime mud dry up, experience cracking, and break into flat, gravel-sized chips. These chips of lime mud are redistributed by the tides, and accumulate to form intraclastic limestone. Intraclasts may be internally layered, reflecting the layering in the tidal flat sediments. Because intraclasts are gravel-sized, intraclastic limestone is calcirudite. Intraclastic limestone is similar to conglomerate or breccia, but may be distinguished from them because both the clasts and the matrix of an intraclastic limestone are made of calcium carbonate, and will fizz readily in hydrochloric acid.

Intraclastic Limestone

5. Pelleted or peloidal limestone. Pellets are small (less than 1 mm) aggregates of microcrystalline calcite, many of which are fecal in origin. Unlike oolites, they have no internal structure. Pellets are so small that they generally cannot be seen in hand specimens, but they can be seen in thin sections using a microscope.

6. Crystalline limestone. Crystalline limestone generally consists of a coarse mosaic of intergrown calcite crystals, resulting from the post-depositional alteration of some other type of limestone. Allochems may or may not be visible.

7. Travertine. Travertine was discussed above under evaporites. It consists of a mosaic of intergrown calcite crystals. Travertine is a dense, crystalline rock with tan and white color bands. In lab, stalactites and stalagmites can be recognized by their cylindrical shape and internal "tree-ring-like" appearance. Texturally, travertine is essentially a carbonate rock made up entirely of spar.

8. Dolostone. Dolostone is made up of the mineral dolomite, a calcium-magnesium carbonate. Most dolostones form by the chemical replacement of calcium carbonate through the action of magnesium-rich fluids. A dolostone may retain the texture of the original limestone, but it is typically dense and compact with a fine-grained texture. Dolomite fizzes weakly in hydrochloric acid, and only after the rock is scratched or powdered.

III. ORGANIC SEDIMENTARY ROCKS

Organic sedimentary rocks are primarily composed of organic matter or carbon. In general, they do not contain minerals, because minerals are by definition inorganic.

Peat is a sediment composed of plant fragments. Coal is its lithified equivalent. The plant fossils in coal generally indicate deposition in fresh-water swamps. Peat is transformed by burial pressure and temperature to lignite (a soft, black or brownish, coal-like material). Lignite alters to sooty bituminous coal with greater depth and duration of burial, and higher temperatures (basically, low grade metamorphism). With increasing metamorphism, the bituminous coal alters to anthracite coal (a hard, shiny coal). (Many books consider anthracite to be, in fact, a metamorphic rock.)

Plant fragments
ß
Peat
ß
Lignite
ß
Bituminous coal
ß
Anthracite coal

Summary

Here is an outline of the major rock types mentioned in this lab.

1. Terrigenous (or clastic, or detrital) sedimentary rocks

o Conglomerate

o Breccia

o Sandstone (types of sandstone: quartz sandstone, arkose, litharenite)

o Siltstone

o Shale and claystone

2. Chemical and biochemical sedimentary rocks

o Rock gypsum

o Rock salt

o Chert

o Diatomite

o Ironstone

Types of ironstone:

§ banded iron ore

§ oolitic hematite or oolitic ironstone

§ bog iron ore

o Dolostone

o Limestone

Types of limestone:

§ Micrite

§ Fossiliferous limestone

§ Coquina

§ Chalk

§ Oolitic limestone

§ Intraclastic limestone

§ Pelleted or peloidal limestone

§ Crystalline limestone

§ Travertine

· Organic sedimentary rocks

Coal

Types of coal:

· Peat

· Lignite

· Bituminous coal

· Anthracite coal

» Metamorphic Rocks and Their Classification