STRATIGRAPHY AND SEDIMENTATION
SECTION A
Rock Weathering and Sediment Formation
This Section introduces the products of rock weathering. Weathering is important because it is
the process through which rocks are broken down and sediment is formed. Sediment is loose
particulate material which becomes cemented and compacted to form sedimentary rocks.
TYPES OF WEATHERING
There are three major types of weathering:
1. physical
2. chemical
3. biological
Physical weathering breaks rocks down into smaller pieces. Types of physical weathering
include frost wedging, exfoliation, and thermal expansion.
Chemical weathering breaks rocks down chemically adding or removing chemical elements,
and changes them into other materials. Chemical weathering consists of chemical reactions, most
of which involve water. Types of chemical weathering include:



dissolution
hydrolysis
oxidation
Biological weathering is the breakdown of rock caused by the action of living organisms,
including plants, burrowing animals, and lichen (a crusty, rubbery, light green organic material
that grows in patches on rocks as well as on wood). Lichen is a combination of fungus and algae,
living together in a symbiotic relationship. Lichens can live on bare rock, and they break down
rocks by secreting acids and other chemicals. The fungal part of the association secretes the
acids, which react to dissolve the minerals, which are then used by the algae. Later, water seeps
into the crevices etched by the acid, and assists in the breakdown through freezing (frostwedging) and chemical weathering.
TYPES OF PHYSICAL WEATHERING
1. Frost wedging - water expands when it freezes, breaking rocks into angular fragments
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Talus slope, Lost River, West Virginia
The above photograph illustrates a sediment source area. The bedrock is being broken down into
sediment of a variety of sizes, primarily by physical weathering processes.
2. Exfoliation - the bedrock breaks into flat sheets along joints which parallel the ground
surface. This phenomenon is caused by the expansion of rock when the pressure of overlying
rock is removed by erosion. It is sometimes called unloading.
Exfoliation of granite at Stone Mountain has produced a rounded mountain. At the time the
granite body cooled, it is calculated that the land in this area stood about 10,000 ft higher than at
present. Over the past 325 million years, this 10,000 ft of rock has been eroded away.
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Stone Mountain, Georgia. Stone Mountain is a granite body which is a sediment source
area. The second image shows active exfoliation.
3. Thermal expansion - heat causes expansion; cooling causes contraction. Different minerals
expand and contract at different rates causing stresses along mineral boundaries. Repeated daily
heating and cooling of rock causes the rock to break down.
TYPES OF CHEMICAL WEATHERING
1. Dissolution alters rocks by removing soluble minerals. Minerals such as halite, gypsum, and
calcite are soluble (dissolve) in water (especially water that is slightly acidic). When the minerals
react with water, ions (such as Ca and Na) are released. The ions are carried as "dissolved load"
by rivers flowing to lakes or to the sea. As lake or sea water evaporates, the dissolved minerals
precipitate or crystallize out as solid minerals. (Examples: halite, gypsum, or calcite. These types
of minerals that form from the evaporation of sea water are called evaporites). Minerals may
also precipitate or crystallize from ground water in and around springs (particularly hot springs),
and in caves (example - travertine).
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2. Hydrolysis is the process by which feldspar (and some other aluminum-bearing silicate
minerals) are weathered to form clay. For example, potassium feldspar weathers to form the
mineral kaolinite.
2KAlSi3O8
potassium
feldspar
+
2 (H+ + HCO3- )
carbonic acid
+ H20 => Al2Si2O5(OH)4 +
water
kaolinite
(clay formed
through
weathering)
2K+
potassium
ion
(dissolved
in water)
+ 2HCO3- + 4SiO2
bicarbonate
ion
(dissolved
in water)
silica
(dissolved in
water)
In humid climates (such as the southeastern United States; West, East and Central parts of
Africa), most of the feldspar in rocks such as granite will weather to form clay.
Nearly all of the minerals in the common rocks of the Earth's crust will weather to form clay
(with the exception of quartz). Because of this, clays make up nearly half of the sedimentary
rocks on Earth.
3. Oxidation is the process by which iron-bearing minerals weather to produce iron oxides (or
"rust"). Iron-bearing silicate minerals which also contain aluminum (such as pyroxene,
amphibole, and biotite) undergo both oxidation and hydrolysis, forming both iron oxides and
clays. Iron-bearing alumino-silicate minerals weather to form the red clayey soils, such as are
found in Georgia, as well as lateritic soils formed in more tropical areas.
MINERAL STABILITY IN THE WEATHERING ENVIRONMENT
Some minerals weather more quickly than others. A few minerals are readily soluble in slightly
acidic water, whereas others weather to produce clay, and still others are very resistant to
weathering, and persist for a long time without alteration. One of the controls on the weathering
of minerals is the temperature at which the minerals originally formed when they crystallized
from magma or lava.
Minerals which formed at high temperatures and pressures are least stable in the
weathering environment, and weather most quickly. This is because they are farther from
their "zone of stability", or the conditions under which they formed. On the other hand, minerals
which formed at lower temperatures and pressures are most stable under weathering
conditions.
The order in which minerals tend to weather is related to the temperature at which they
crystallized. You may remember Bowen's Reaction Series, which described the order in which
minerals crystallize from magma. There is a similar ordering of minerals as related to their
weathering rates, and it is called the Goldich Stability Series.
The order of mineral stability in the weathering environment is the same order as Bowen's
Reaction Series.
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Least stable (high temperature minerals)
Olivine
Ca plagioclase feldspar
Pyroxene
Amphibole
Biotite
Na plagioclase feldspar
Potassium feldspar
Muscovite
Quartz
Most stable (low temperature minerals)
WHAT HAPPENS WHEN GRANITE IS WEATHERED?
Unweathered granite contains these minerals:
1.
2.
3.
4.
5.
Na plagioclase feldspar (white)
K feldspar (pink, but may be white in other granites)
Quartz (gray)
Small amounts of biotite and/or amphibole (black)
and sometimes muscovite (not shown)
Here is what will happen to each of the mineral constituents in a granite under warm, humid
weathering conditions:
1. The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions
2. The sodium and potassium ions will be removed through leaching and will be carried in
solution in running water
3. The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to
form iron oxides.
4. The quartz (and muscovite, if present) will remain as residual minerals because they
are very resistant to weathering.
Under warm, humid conditions, the granite bedrock will weather in place until the feldspars alter
to soft clay. The weathered rock is called saprolite, a term meaning "rotten rock". In areas of the
southeastern U.S. which are underlain by granite (and other igneous and metamorphic rocks), a
thick soil zone of weathered rock or saprolite has developed. Where the bedock contained ironbearing minerals (such as biotite, amphibole, or pyroxene) which weathered to iron oxides, the
saprolite has been stained a deep red color. (This is the same principle by which one red sock in a
load of laundry can stain all of the clothes red).
What happens after the rock has been weathered to saprolite?
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1. The clays will be eroded and transported by running water to the sea. Clay is fine-grained
and remains suspended in the water column. The clay may ultimately be deposited in
deep quiet water far from shore.
2. As the soft clay is removed, the unweathered, residual quartz grains will be released from
the saprolite by erosion. The quartz in granite is sand-sized, and it becomes quartz sand.
The quartz sand is ultimately transported to the sea, where it accumulates to form
beaches.
3. The dissolved ions (sodium and potassium) will be transported by rivers to the sea, and will become part of
the salts in the sea.
THE CHARACTERISTICS OF SEDIMENT
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 sometimes called terrigenous,
or detrital, or clastic or siliclastic 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).
The grain size of sediment depends on the types of rocks in the source area from which the
sediment was derived. The textures and mineralogy of the rocks in the source area control the
grain size and composition of the resulting sediment.
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.
DESCRIBING THE TEXTURE OF SANDS
Texture refers to the size and shape of the grains in a sediment.
Sediment can be separated into four main groups based on grain size. These four size groups are
gravel, sand, silt, and clay. Some of these groups (gravel and sand) can be further subdivided.
The sediment grain size scale is known as the Wentworth Scale.
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Particle name
Gravel Boulders
Sand
Particle diameter
> 256 mm
Cobbles
64 - 256 mm
Pebbles
2 - 64 mm
Granules
2 - 4 mm
Very coarse
sand
1 - 2 mm
Coarse sand
0.5 - 1 mm
Medium sand
Fine sand
Very fine sand
0.25 - 0.5 mm
0.125 - 0.25 mm
0.0625 - 0.125 mm
Silt
1/256 - 1/16 mm
(or 0.004 - 0.0625 mm)
Clay
< 1/256 mm
(or < 0.004 mm)
Gravel forms through physical weathering of rock. A piece of gravel is usually a "rock
fragment" composed of more than one mineral. Sometimes a piece of gravel is a single mineral,
most commonly quartz. This is because quartz is sometimes present as veins, which may be
several inches wide (or more), thus producing gravel-sized clasts.
Sand forms through the breakdown and disintegration of rocks which have sand-sized (1/16 2mm) grains, such as granite and gneiss.
In humid climates, quartz sand grains are released from granite after the feldspar grains alter to
clay by chemical weathering (hydrolysis). In more arid areas, granite breaks down by physical
weathering (such as frost wedging), releasing both feldspar and quartz grains.
Silt originates from the chipping of coarser grains during sediment transport, or from the
disintegration of fine-grained crystalline rocks (such as slates, phyllites, and schists).
Clay originates primarily through chemical weathering of feldspars and other alumino-silicate
minerals (those which contain aluminum and silicon). The term "clay" refers to a particular size
of sediment particle, which could be a quartz grain or a clay mineral flake, or some other very
small mineral fragment. The term "clay" is also used to refer to a group of minerals. There are a
number of clay minerals, including kaolinite (the white clay mined in central Georgia and used
for shiny coatings on paper, and additives to rubber), illite (which contains potassium), and
montmorillonite or smectite (a group of clays which can take in large amounts of water, and as
a result these clays are commonly referred to as "swelling clays").
SORTING
Sorting refers to the range in grain sizes in a sediment or sedimentary rock. Sediment (or rock)
which is well sorted will have most of the grains roughly the same size. A poorly sorted
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sediment or rock has a wide range of grain sizes. Sorting can be estimated using a visual
comparison chart.
ROUNDNESS
Roundness is a measure of the sharpness or roundness of the corners of a sedimentary particle.
Roundness is determined by comparing the sand grains with a visual comparison chart.
As sediment is transported, it undergoes abrasion by coming into contact with the stream
bottom, sea-floor, or other grains of sediment. The abrasion tends to "round-off" the sharp edges
or corners. Rounding is also related to the size of the grains. Boulders tend to round much more
quickly than sand grains because they strike each other with much greater force.
SPHERICITY
Grains of sediment are three dimensional. Sphericity refers to "equal dimensions". Is the
sediment particle elongated (one dimension longer than the other two), flattened or sheet-like
(one dimension much smaller than the other two dimensions), or is it spherical (its three
dimensions roughly the same length)? Sphericity can be described as high or low. According to
this definition, a ball would have highly sphericity, but so would a cube (high sphericity, but low
roundness). In contrast, a submarine sandwich would have low sphericity, but high roundness. A
shoebox would have both low sphericity and low roundness. Sand grains may have high or low
sphericity. Some minerals may produce elongated or flattened grains, depending primarily on
original crystal shape and cleavage.
Be careful not to confuse rounding with sphericity. A well-rounded grain may or may not
resemble a sphere. And a spherical grain may or may not be well rounded.
INTERPRETING THE TEXTURE OF SANDS
Texture is an indicator of energy levels in the environment of deposition (the place where
sediment accumulates, perhaps a beach, a riverbed, a lake, or a delta).
Moving water (such as waves or currents) is considered to be a high energy environment.
Quiet water or still water (water without waves or currents) is considered to be a low energy
environment. Deep water environments commonly have quiet water, because wave motion is
restricted to the upper part of the water column.
HOW DO YOU DETERMINE ENERGY LEVELS
ENVIRONMENT FROM LOOKING AT SEDIMENT?
IN
THE
DEPOSITIONAL
Grain size
Coarse-grained sediments (sand, gravel) indicate high energy environments. A large amount of
energy is required to transport gravel-sized clasts, and moving water is required to transport
sand.
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Fine-grained sediments (clay or silt) indicate low energy environments. There is insufficient
energy to bring larger clasts into the environment. Aso, if the water were moving, the clay would
not be able to settle out and be deposited on the bottom.
Sorting
Well-sorted grains indicate that the sediment was probably transported for a long time in a fairly
high energy environment (waves or currents). The finer grains were probably washed or
winnowed away.
Poorly sorted grains indicate that the sediment has not been transported very far from the source
area. It also suggests fluctuating energy levels, and a fairly short time in the depositional
environment.


Good sorting implies consistent energy (washing)
Poor sorting implies inconsistent energy (dumping)
Grain shape
A well rounded sand grain indicates that the sediment has been transported far from the original
source area, and that it has been in the depositional environment for a long time.
The environment of deposition is also a factor in sand grain roundness. Sands from desert
environments tend to be more rounded than sands from beaches.
Angular sand grains have probably only been transported for a short distance from the source
area, or they have been in the depositional environment for a short time.
TEXTURAL MATURITY
Textural maturity is a concept which proposes that as sediments experience the input of
mechanical energy (the abrasive and sorting action of waves and currents), they pass through a
series of four stages.
1.
2.
3.
4.
Stage 1- Immature - Sediment contains mud (clay and/or silt)
Stage 2 - Submature - Poorly sorted sediment with no mud
Stage 3- Mature - Well sorted sediment with no mud
Stage 4 - Supermature - Well sorted and rounded sediment with no mud
Three steps are involved:
1. Winnowing or washing out of fines - makes an immature sediment
become submature
2. Sorting of grain sizes - makes a submature sediment become mature
3. Rounding - makes a mature sediment become supermature
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DESCRIBING THE MINERALOGIC COMPOSITION OF SANDS
The minerals in sands (and in sandstones) can be identified using a microscope (or a hand lens if
a microscope is not available). Identifying the minerals present is important because sandstones
are classified based on the composition of their grains.
Three components are considered when naming sandstones:
1. Quartz grains
2. Feldspar grains
3. Fine-grained rock fragment grains. Possibilities include shale, slate,
phyllite, basalt, rhyolite, andesite, chert, and possibly schist. Limestones
would not be included usually because they dissolve so readily.
The three major types of sandstone are:
1.
2.
3.
Quartz sandstone (also called quartz arenite) - which is dominated by quartz
Arkose - which is dominated by feldspar
Litharenite or lithic sandstone (commonly but imprecisely called graywacke) - which
is dominated by rock fragment grains.
Other minerals may also be present in sands and sandstones. In fact, in some areas, sands may be
composed almost entirely of minerals other than quartz and feldspar. For example, at White
Sands National Monument in New Mexico, the sands are composed of gypsum grains. There is a
beach on the southern end of the Big Island of Hawaii that has green sand composed of olivine
grains. There are beaches in tropical areas in many parts of the world that are composed almost
entirely of the sand-sized shells and shell fragments of marine organisms (made of calcium
carbonate - calcite or aragonite).
It is important to keep in mind that "sand" is a texture term, not a composition term. A sand can
be composed of any types of sand-sized mineral or rock-fragment grains.
In addition to the major constituents in sand, there is often a suite of heavy minerals (those with
high specific gravity - greater than 2.85), which may consist of less than 1% of the sand grains to
perhaps several percent (or more). Examples of heavy minerals include rutile, tourmaline,
zircon, garnet, kyanite, staurolite, apatite, olivine, pyroxene, amphibole, magnetite,
ilmenite, hematite, pyrite, and others. The particular types of heavy minerals present depend on
the composition of the rocks in the source area. For example, garnet, kyanite, and staurolite are
metamorphic minerals, whereas olivine, pyroxene, and amphibole are constituents of mafic
igneous rocks (gabbro and basalt). Heavy minerals are important indicators which can tell us the
type of rocks that existed in the sediment source area.
IDENTIFYING MINERALS IN SANDS
The following is a handy dandy guide to identifying sand grains under the stereomicroscope or
hand lens.
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Grain type
Identifying features
Quartz
Glassy, gray or white (may be covered by brownish iron oxide stain), lacks cleavage
Feldspar
Has cleavage (look for flat surfaces or square corners), usually white or pink in color
Rock fragments
fine-grained, commonly dark gray or black, may be coated with iron oxide stain
Muscovite
Silvery color, flat sheets, shiny, may look sub-metallic
Magnetite or
ilmenite
Black, opaque. magnetite is magnetic.
Rutile
Deep red or yellow, may look opaque, generally elongate and well-rounded
Tourmaline
Elongated with triangular cross-section, dark color
Zircon
Colorless, elongated crystals
Garnet
Most commonly pale pink or red, no cleavage
Staurolite
Brown to yellow, elongate, may be filled with tiny inclusions to resemble swiss
cheese
Apatite
Colorless, rounded or elongated
Olivine
Olive green, glassy, no cleavage, may be rounded
Pyroxene
Stubby, angular cleavage fragments, gray or greenish to colorless
Amphibole
Elongated to fibrous, greenish
Biotite
Brown, flat sheets, shiny
Hematite
Red
Pyrite
Brassy gold, metallic,
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SECTION B
Sedimentary Rocks
This Section 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.
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.
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).
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Clasts and matrix (labelled),
and iron oxide cement
(reddish brown color
surrounding clasts)
1.
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 sandsized 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.
2.
Clast shape
Shape of clasts is important in naming the coarser-grained sedimentary rocks (those with gravelsized 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.
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3.
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
A. 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
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Breccia
B. 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.)
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Quartz sandstone or quartz arenite
Arkose
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.
2. Arkose is composed mainly of pink or white feldspar grains, with quartz, and
generally some muscovite mica or sand-sized rock fragments.
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.
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Litharenite or lithic sandstone sometimes called graywacke
C. 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.
D. 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,
 Claystone, on the other hand, is not fissile, and breaks irregularly.
Shale is fissile
flat layers.
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.
INTERPRETING
SANDSTONES
THE
MINERALOGIC
COMPOSITION
OF
SANDS
AND
Each type of sandstone implies something about depositional history:
1. Quartz sandstone implies a long time in the depositional basin.
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2. Arkose implies a short time in the depositional basin (because feldspar typically weathers
quickly to clay). Arkose also implies rapid erosion, arid climate, tectonic activity, steep slopes.
3. Litharenite implies rapid erosion, temperate or arid (not humid) climate
As noted above, the particular suite of heavy minerals present in sand also can tell a lot about the
source area from which the sediment is derived.
READING THE RECORD IN THE ROCKS: A SANDSTONE INTERPRETATION
GUIDE
One of the goals in Historical Geology is to try to interpret the depositional conditions of the
sedimentary rocks that make up the geologic record.
Sandstone textures and compositions may be used to interpret many things about the history of
the sand, including source area lithology, paleoclimate, tectonic activity, processes acting in the
depositional basin, and time duration in the basin. Remember that the source area is the land
which is weathering and eroding to supply terrigenous debris to the depositional basin.
SOURCE AREA LITHOLOGY
Composition gives the key information (minerals or rock fragments present). Remember that
quartz sandstone or quartz arenite is dominated by quartz grains; arkose is dominated by
feldspar grains (usually potassium feldspar); and graywacke is dominated by rock fragment
grains.



Sand-sized quartz grains could come from the weathering of source area rocks such as
granite, gneiss, or other sandstones which contain quartz (recycled sandstones).
Sand-sized feldspar grains could come from the weathering of source area rocks such as
granite or gneiss.
Sand-sized rock fragment grains come from the weathering of fine-grained source rocks.
Possibilities include shale, slate, phyllite, basalt, rhyolite, andesite, chert, and possibly
schist.
PALEOCLIMATE
Paleoclimate refers to the climate which existed in the source area. We are particularly
concerned with weathering rates here. Remember that in humid climates, feldspar weathers to
clay by hydrolysis. Other minerals also weather to clay (with associated iron oxides), such as
olivine, pyroxene, and amphibole.
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Also remember the difference between weathering (BREAKDOWN of rock by hydrolysis,
dissolution, oxidation, exfoliation, frost wedging, or freeze thaw), and erosion
(TRANSPORTATION of particles).



If feldspar is present in your sand, it indicates that the climate was probably arid. (Or that
erosion rates were very rapid, and that tectonic activity was extremely high - lots of
uplift,and steep slopes.)
If quartz is the dominant mineral in the sand, the climate was probably humid (all of the
feldspars weathered away to clay).
If rock fragments are present in your sand, it helps to know what lithology they are. If
they are rock types which would weather rapidly (such as basalt or limestone fragments),
the climate was probably arid. If they are rock types which would be relatively stable
(shale, slate, or chert), the climate may have been temperate to humid. (remember
Bowen's Reaction Series and the Goldich Stability Series to determine what is stable or
unstable). If rock fragments are present and no rock types are given, a good compromise
answer would be temperate climate.
TECTONIC ACTIVITY IN THE SOURCE AREA
We are basically classifying tectonic activity as "active" or "passive". For a good model,
consider the west coast of the US as tectonically active - steep slopes, mountains close to the
sea, lots of earthquakes, tectonic uplift, and volcanic activity. On the other hand, consider the
east coast of the US as tectonically passive - broad, flat coastal plain, few or no earthquakes, no
uplift, and no volcanic activity.


If a sand has a lot of feldspar or rock fragments, it probably indicates high tectonic
activity.
If a sand has a lot of quartz, it probably indicates low tectonic activity - a passive
setting.
Tectonic activity also influences sorting, time duration in the depositional environment (and to
some extent, compositional maturity). High tectonic activity might produce rapid dumping of
sediments into the basin with little or no time for sorting. Low tectonic activity means little
uplift, low erosion rates, and therefore little sediment supplied to the basin; what sediment that is
there is likely to wash around for a long time and become well sorted and rounded, and grains
other than quartz are likely to be destroyed (by abrasion or chemical weathering).
PROCESSES ACTING IN THE DEPOSITIONAL BASIN
This refers to energy levels ("high" vs. "low") and consistency of energy. Texture gives the key
information.
Grain size:

Coarse sediments generally indicate high energy, and fine sediments indicate low energy.
Sorting:
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

Well sorted sediments indicate consistent, fairly high energy levels. (Winnowing and
washing.)
Poorly sorted sediments indicate inconsistent energy levels - rapid dumping (which
might involve short episodes of high energy), followed by low energy conditions.
TIME DURATION IN THE DEPOSITIONAL ENVIRONMENT
Both mineralogy and texture can be used to determine time in the depositional environment.
A sand with abundant quartz grains suggests a long time in the depositional environment. Quartz
is more resistant to abrasion than feldspar or rock fragments.
A sand with abundant feldspar or rock fragment grains suggests a short time in the depositional
environment.
Textural maturity is also useful in interpreting time in the depositional environment. Immature or
submature sediments probably spent only a short time in the basin before burial. Mature or
supermature sediments were probably rolling around in the basin for a long time before burial.
Roundness is a good clue to a long time in the depositional environment. Rounding of grains
takes a long time; it is more likely in a tectonically passive situation. Desert sands are often well
rounded because of the "sandblasting" process of wind transport. Hence, in an arid desert, it is
possible to get a well-rounded (supermature) arkose.
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.
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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.
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 microorganisms 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
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Carbonate rocks are made up of carbonate minerals. These are minerals which contain a
carbonate (CO3) group, such as:
(CaCO3)
Calcite
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.
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:




intraclasts
oolites
fossils
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.
A. CLASSIFICATION OF CARBONATE ROCKS
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There are numerous types of carbonate rocks, and they can be classified by their textures and the
allochems they contain. Several different classification schemes for carbonate rocks are in
existence. One of the simpler classifications is based on texture alone, and uses the following
terms:
Calcirudite Limestone dominated by gravel-sized particles
Calcarenite Limestone dominated by sand-sized particles
Calcisiltite Limestone dominated by silt-sized particles
Calcilutite Limestone dominated by mud- or clay-sized particles
This classification scheme is fairly general, and does not specify anything about the types of
allochems present. A more detailed list of types of carbonate rocks is below.
1. Micrite. Micrite is an abbreviation for microcrystalline calcite. "Microcrystalline" refers to
the texture, which consists of clay-sized particles of lime mud. Basically, this is a rock that is
all matrix with no allochems or spar. Micrite results from the lithification of lime mud, most
of which originates from the breakdown of the hard "skeletons" secreted by calcareous algae
which live in warm, shallow seas. Calcilutite is another name for a limestone dominated by
lime mud.
We can recognize micrite by its fine grain size and reaction to hydrochloric acid. The color of
micrite is variable, ranging from gray to tan, or other colors.
2. Fossiliferous limestone. Fossiliferous limestone (sometimes called skeletal limestone)
contains fossils, or the remains of ancient plants or animals. Many organisms have calcareous
shells or skeletons, and their remains may accumulate in lime mud to form fossiliferous
limestone.
Fossiliferous Limestone,
Ordovician, northwestern Georgia
3. 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
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fairly well sorted. The shells are gravel-sized (greater than 2 mm), and coquina is a
calcirudite.
Coquina
4. 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.
5. 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
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Oolitic Limestone
6. 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
7. 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.
8. 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.
9. 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
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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.
10. 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
Peat
Lignite
Bituminous coal
Anthracite coal
SUMMARY
Here is an outline of the major rock types mentioned in this section.
1.
Terrigenous (or clastic, or detrital) sedimentary rocks



Conglomerate
Breccia
Sandstone
Types of sandstone:





2.
quartz sandstone
arkose
litharenite
Siltstone
Shale and claystone
Chemical and biochemical sedimentary rocks
 Rock gypsum
 Rock salt
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 Chert
 Diatomite

Ironstone
Types of ironstone:





banded iron ore,
oolitic hematite or oolitic ironstone,
bog iron ore
Dolostone
Limestone
Types of limestone:









Micrite
Fossiliferous limestone
Coquina
Chalk
Oolitic limestone
Intraclastic limestone
Pelleted or peloidal limestone
Crystalline limestone
Travertine
3. Organic sedimentary rocks
Coal
Types of coal:




Peat
Lignite
Bituminous coal
Anthracite coal
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