Rivers, oceans, winds, and rain runoff all have the ability to carry the
particles washed off of eroding rocks.
Such material, called detritus, consists of fragments of rocks and
When the energy of the transporting current is not strong enough to carry
these particles, the particles drop out in the process of sedimentation.
This type of sedimentary deposition is referred to as clastic
Another type of sedimentary deposition occurs when material is dissolved
in water, and chemically precipitates from the water.
This type of sedimentation is referred to as chemical sedimentation.
A third process can occur, wherein living organisms extract ions
dissolved in water to make such things as shells and bones.
This type of sedimentation is called biogenic sedimentation.
Thus, there are three major types of sedimentary rocks: that can be
grouped by the type of particle found in the rocks.
Siliclastic sedimentary rocks form by the accumulation of
mostly silicate mineral fragments. These include most
sandstones, mud rocks, conglomerates,
Biochemical sedimentary rocks consist of fragments of
particles produced by precipitation from once living organisms.
Most of these rocks are limestones and cherts.
Chemical sedimentary rocks are formed by direct chemical
precipitation from water. While some limestones and cherts may form in
this manner, evaporite deposits consisting of halite,
gypsum, and other salts are the
Because sediment gets buried, and if exposed gets eroded, older
sedimentary rocks show less exposed outcrop area than younger sedimentary
rocks. Over 40% of the exposed sedimentary rocks are younger than
Cretaceous in age.
Because of their detrital nature, any mineral can occur in a sedimentary
Clay minerals, the dominant mineral produced by chemical weathering of
rocks, is the most abundant mineral in mudrocks.
Quartz, because it is stable
under conditions present at the surface of the Earth, and because it is
also a product of chemical weathering, is the most abundant mineral in
sandstones and the second most abundant mineral in mudrocks. Feldspar is the most
common mineral in igneous and metamorphic rocks. Although feldspar
eventually breaks down to clay minerals and quartz, it is still the third
most abundant mineral in sedimentary rocks. Carbonate minerals, either
precipitated directly or by organisms, make up most biochemical and
chemical sedimentary rocks, but carbonates are also common in mudrocks and
Siliclastic sedimentary rocks can be classified on the basis of
their mineral composition In the charts Q=quartz F=feldspar
L=other fragments number are in percents
Minerals found in sedimentary rocks can be divided into 2 classes:
Allogenic minerals - These are formed elsewhere and
transported into the area of deposition.
Authigenic minerals - These are minerals that are formed at
the site of deposition, either by direct chemical precipitation or by
later diagenetic processes.
Any mineral can be an allogenic mineral, but some are more stable under
the conditions present at the Earth's surface than are others.
Conditions that are present at the Earth's surface and differ from those
where most minerals form are:
High free oxygen concentration
High amounts of free liquid water
Because these conditions differ from those under which most rocks form,
allogenic minerals can be classified based on their stability under near
surface conditions. Such a classification, with minerals listed in
order of increasing stability is as follows:
In this list, the igneous minerals have an asterisk (*). Note that
the order in which they occur is in the same order that occur in Bowen's
reaction series. Igneous minerals that crystallize at the highest
temperatures are most out of equilibrium at the Earth's surface, and are
therefore the most unstable.
Minerals that are very stable at the Earth's surface are minerals that
either form as a result of chemical weathering, or crystallize at the
Authigenic minerals can also be allogenic minerals, but some are formed
during diagenesis but not very stable in the transportation cycle either
because they dissolve readily in water or because they are easily abraded
during transportation. Thus we can divide authigenic minerals into
those that are stable during diagenesis and transportation, and those that
are unstable during transportation.
The longer a mineral is in the weathering and transportation cycles of
sedimentary rock forming processes, the more likely it is to break down to
a more stable mineral or disappear altogether.
Thus, we can classify sediments on the basis to which they have achieved
Mineralogically mature sediments and sedimentary rocks
consist entirely of minerals that are stable near the surface.
Such sediment is considered to have been in the weathering and
transportation cycle for a long amount of time.
Mineralogically immature sediments and sedimentary rocks
consist of a high proportion of unstable minerals.
Because such minerals will not survive for a long time in the
weathering and transportation cycles, sediments and rocks with high
proportions of these minerals must not have been in the
weathering/transportation cycle for long periods of time.
Since most sedimentary rocks are derived by processes of weathering,
transportation, deposition, and diagenesis, the textures we find in
sediment and sedimentary rocks are dependent on process that occur during
nature of the source rocks.This determines the original
shape of the grains and the mineralogical composition of the original
strength of the wind or water currents that carry and deposit
the sediment.This determines whether or not grains are transported or
deposition process also controls structures that could be
preserved in the sediment and thus give clues to the environment of
distance transported or time in the transportation
process. The longer grains are in the transportation process the
more likely they are to change shape and become sorted on the basis of
size and mineralogy. This also controls extent to which they break
down to stable minerals during the transportation process.
biological activity with the sediment prior to diagenesis.
Burrowing organisms can redistribute sediment after it has been
deposited, thus erasing some of the clues to the original environment
chemical environment under which diagenesis occurs.
During diagenesis grains are compacted, new minerals precipitate in
the pore spaces, some minerals continue to react to produce new
minerals, and some minerals recrystallize. What happens depends
on the composition of fluids moving through the rock, the composition
of the mineral grains, and the pressure and temperature conditions
attained during diagenesis.
Grain Size Clastic sediments and sedimentary rocks are classified on the basis of
the predominant grain size of clasts
Note how the particle sizes small than pebble size are defined. The
lower size limit of granules is 1/2 the lower size limit of pebbles, the
lower size limit of coarse sand is 1/2 the lower size limit of granules,
For this reason, grain size is often given in units called f (phi)
units, where f is defined as
f = -log2(d)
where d is the grain diameter in millimeters.
This is convenient for grain size analyses of sediment, or rocks if they
can be disaggregated, because sieves can be constructed where the opening
of each sieve is 1/2 the size of the sieve above.
Sediment put through such a set of sieves, will be trapped in different
sieves, and sorted by grain size. The amount of grains in each sieve
can then be weighed to give a quantitative measure of the size
distribution of sediment.
One precaution- note how the large grain sizes have negative f
Sorting refers to the uniformity of grain size in a sediment or
sedimentary rock. Particles become sorted on the basis of density
because of the energy of the transporting medium. High energy (high
velocity) currents can carry larger fragments.
As the energy or velocity decreases, heavier particles are deposited and
lighter fragments continue to be transported. This results in
sorting due to density. If the particles have the same density, such
as all grains of quartz, then the heavier particles will also be larger,
so the sorting will take place on the basis of size. We can classify
this size sorting on a relative basis: well-sorted to poorly-sorted.
Beach sands and dune sands tend to be well-sorted because the energy of
the waves or wind is usually rather constant.
The coarser grained sediment is not carried in because the wave or wind
velocity is too low to carry such large fragments, and the finer grained
sediment is kept in suspension by the waves or wind.
Mountain streams, because they have many turbulent eddies where the
velocity of the stream changes suddenly usually show poorly-sorted
sediment on the bottom of the stream channel. Similarly, glacial
till, because it is deposited in place as glacial ice melts, and is not
transported by water, tends to show poor sorting.
- During the transportation process, grains may be reduced in size due to
abrasion. Random abrasion results in the eventual rounding off of
the sharp corners and edges of grains.
Thus, the degree ofrounding of grains gives us clues to the amount of time
a sediment has been in the transportation cycle.
Rounding is classified on relative terms as well. Note that rounding is
not the same as sphericity.
is controlled by the original shape of the grain.
is the percentage of the volume of the rock that is open space (pore
space). This determines the amount of water or other fluids, like
petroleum, that a rock can contain.
In sediments or sedimentary rocks the porosity depends on grain size, the
shapes of the grains, the degree of sorting, and the degree of
Well-rounded coarse-grained sediments usually have higher porosity than
fine-grained sediments, because the grains do not fit together well.
Angular grains of fine grained sediment can be compacted to fit together
better, and thus porosity is reduced. Mudrocks, because of fine grain size, usually have
very low porosities.
Poorly sorted sediments usually have lower porosity because the
fine-grained fragments tend to fill in the open space
that takes place during diagenesis tends to fill in the pore space.
Thus, highly cemented sedimentary rocks have lower porosity than do
poorly cemented sedimentary rocks.
Packing of Grains
Packing refers to the arrangement of clastic grains entirely apart from
any authigenic cement that may have later crystallized between them. If
the clastic grains touch each other throughout, the rock is said to be
grain supported.If the rock is poorly sorted and the grains are separated
by a mud or silt matrix, the rock is said to be matrix supported. Matrix-
If the rock is poorly or moderately sorted, the percentage of matrix and
texture of the matrix should also be described. Induration
Induration refers to the hardness of the rock or how easily it breaks
Well indurated rocks are difficult to break with a hammer.
Moderately indurated rocks can be easily broken with a rock hammer.
Poorly indurated or friable rocks break apart easily in your hand.
The term non-indurated would describe a sediment that has not undergone
any cementation. Textural Maturity
The longer sediment is involved in the transportation cycle, the more time
it has to become well-sorted.
Similarly, the longer the sediment is transported, the more time is
available for grains to lose their rough edges and corners by abrasion.
Thus, we consider a texturally mature sediment to be sediment that is
well-sorted and well-rounded.
Note that sediment tends to become both texturally and mineralogically
mature the longer it is in the transportation cycle. Descriptions of Texture
A complete description of the texture of a sedimentary rock should include
statements about each of the factors discussed above.
To summarize, these are:
Size of the grains.
Degree of roundness and sphericity of the grains.
An estimate of the porosity of the rock.
Packing of the grains.
A description of the matrix.
Induration of the rock.
A statement about the textural maturity of the rock.
The process of deposition usually imparts variations in layering,
bedforms, or other structures that give clues to environment in which
deposition occurs. Such things as water depth, current velocity, and
current direction can be sometimes be determined from sedimentary
structures.Thus, it is important to recognize various sedimentary
structures so that we can interpret the clues that they offer to these
conditions.Also, because sedimentary rocks can be deformed by folding and
faulting long after the deposition process has ended, it is important to
be able to determine which was up in the rock when it was originally
deposited, especially if one is going to use the rocks to determine the
sequence of events that occurred or interpret the geologic history of an
Features that tell us which way was up are often referred to as top and
structures and features
Useful sedimentary structures and the stories they tell... Stratification and Bedding
One of the most obvious features of sedimentary rocks and sediment is the
layered structure which they exhibit. Compaction
as the mass of overlying sediment pushes down, water is sqeezed our and
layers are formed
The layers are evident because of differences in mineralogy, clast size,
degree of sorting, or color of the different layers. In rocks, these
differences may be made more prominent by the differences in resistance to
weathering or color changes brought out by weathering.
Layering is usually described on the basis of layer thickness
Distinctive types of layering
Planar bedding has little diagnostic importance but animal burrows
or bed form irregularities my give an indication of way-up and
environment. Current cross bedding gives clear way-up indication as bed are
truncated by the next influx of sediment. The orientation of the cross
beds can also indicate the direction of water or wind currents. Ripple-marked bedding indicates waves action and the ripple cusps
point in the younging direction. Wave action usually indicate shallow
marine conditions but ripples can be found in freshwater sediments also. Imbricate bedding is the overlapping of fossil shells or platey
pebbles and can be used as an indicator of current direction. Graded bedding usually fines upwards to give an indication of
way-up. Graded units represent individual pulses of sediment which may be
seasonal or indicative of some other cyclic process. Cut-and-fill bedding involves the reworking of sedimentary layers
by new water channels. They provide both way-up and current direction
indicators and are common in stream fluvial deposits.
- consists of alternating parallel layers having different properties.
This is sometimes caused by seasonal changes in deposition (Varves).
i.e. lake deposits wherein coarse sediment is deposited in summer months
and fine sediment is deposited in the winter when the surface of the lake
-Sediment showing a mixture of grain sizes results from such things as
rockfalls, debris flows, mudflows, and deposition from melting ice. Imbricate bedding
-Elongated grains can sometimes pile up on each other to form what is
called imbricate bedding. Note that imbricate bedding can be a
current direction indicator if some other means is present to provide
Surface Features Symmetrical ripple marks occur in environments where there is a
steady back and forth movement of the water. Such ripple marks can
still be used as top and bottom indicators.
Asymmetrical Ripples give current direction and create cross
- result from the drying out of wet sediment at the surface of the
Earth. The cracks form due to shrinkage of the sediment as it dries.
In cross section the mudcracks tend to curl up, thus becoming a good
The presence of mudcracks indicates that the sediment was exposed at the
surface shortly after deposition, since drying of the sediment would not
occur beneath a body of water.
- pits (or tiny craters) created by falling rain. If present, this
suggests that the sediment was exposed to the surface of the Earth, and
also are good top/bottom indicators.. Casts and Molds
-Any depression formed on the bottom of a body of water may become a mold
for any sediment that later gets deposited into the depression. The
body of sediment that takes on the shape of the mold is referred to as a cast.
Load Casts - These are bulbous protrusions that are formed when
compaction causes sediment to be pushed downward into softer sediment.
Flute Casts (Sole Marks) - Flutes are elongated depressions that
form on the bottom of a body water as the current erodes. The
flutes may form a mold into which new sediment is deposited.
Preservation of the overlying sediment as a cast result in flute
casts, which are sometimes referred to as sole marks. Flute
casts are excellent indicators of current direction and tops/bottoms
- Any object carried along by a current my gouge or scrape the sediment on
the bottom of the body of water. Depressions, scratches, or gouges are
called toolmarks and can be useful top/bottom indicators.
Tracks and Trails
-These features result from organisms moving across the sediment as they walk, crawl, or drag
their body parts through the sediment.
-Any organism that burrows
into soft sediment can disturb the sediment and destroy many of the
structures discussed above. If burrowing is not extensive, the holes made
by such organisms can later become filled with water that deposits new
sediment in the holes. Such burrow marks can be excellent top and
The carbonate rocks make up 10 to 15% of sedimentary rocks. They
largely consist of two types of rocks. Limestones which are composed mostly of
calcite (CaCO3) or high Mg calcite [(Ca,Mg)CO3], and
Dolostoneswhich are composed mostly of
Because carbonate minerals in general are soluble in slightly acidic
waters, they often have high porosity and permeability, making them ideal
reservoirs for petroleum. For this reason they are well studied.
Limestone can be easily recognized in hand specimen or outcrop because of
its high solubility in HCl. A drop of such acid placed on the rock
will cause it to fizz due to the generation of CO2 gas. A
dolostone, on the other hand, will not fizz until a fine powder is made
from the rock or mineral. Also, dolostones tend to weather to a
brownish color rock, whereas limestones tend to weather to a white or gray
colored rock. The brown color of dolostones is due to the fact that
Fe occurs in small amounts replacing some of the Mg in dolomite.
Folk Classification- The Folk classification, which we will use in lab, is shown below.
The classification divides carbonates into two groups:
Allochemical rocks are those that contain grains brought in from
elsewhere (i.e. similar to detrital grains in clastic rocks). Orthochemical rocks are those in which the carbonate crystallized
Allochemical rocks have grains that may consist of fossiliferous material,
ooids, peloids, or intraclasts. These are embedded in a matrix consisting
of microcrystalline carbonate (calcite or dolomite), called micrite, or
larger visible crystals of carbonate, called sparite.
Sparite is clear granular carbonate that has formed through
recrystallization of micrite, or by crystallization within previously
existing void spaces during diagenesis.
The Dunham classification is based on the concept of grain support;
further subdivided as to whether or not the grains are mud-supported or
The classification divides carbonate rocks into two broad groups called
boundstones (similar to biolithite of the Folk classification)
those whose original components were not bound together during
those whose original components formed in place and consist of
intergrowths of skeletal material.
If the rock consists of less than 10% grains it is called a mudstone
(potentially confusing if taken out of context).
If it is mud supported with greater than 10% grains it is called a wackstone.
If the rock is grain supported, it is called a packstone, if
the grains have shapes that allow for small amounts of mud to occur in
the interstices, and a grainstone if there is no mud between
Carbonate Depositional Environments
Most modern, and probably most ancient, carbonates are predominantly
shallow water (depths <10-20 m) deposits. This is because the
organisms that produce carbonate are either photosynthetic or require the
presence of photosynthetic organisms. Since photosynthesis requires
light from the Sun, and such light cannot penetrate to great depths in the
oceans, the organisms thrive only at shallow depths. Furthermore,
carbonate deposition in general only occurs in environments where there is
a lack of siliciclastic input into the water. Siliclastic input
increases the turbidity of the water and prevents light from penetrating,
and silicate minerals have a hardness much greater than carbonate
minerals, and would tend to mechanically abrade the carbonates. Most
carbonate deposition also requires relatively warm waters which also
enhance the abundance of carbonate secreting organisms and decrease the
solubility of calcium carbonate in seawater. Nevertheless, carbonate
rocks form in the deep ocean basins and in colder environments if other
conditions are right.
The principal carbonate depositional environments are as follows:
Carbonate Platforms and Shelves.
Warm shallow seas attached the continents, or in the case of
epiric seas, partially covering the continents, are ideal places for
carbonate deposition. Other shelves occur surrounding oceanic
islands after volcanism has ceased and the island has been eroded
(these are called atolls). Carbonate platforms are buildups of
carbonate rocks in the deeper parts of the oceans on top of
continental blocks left behind during continent - continent
separation.Reef building organisms from the framework of most of these
Tidal flats are areas that flood during high tides and are exposed
during low tides. Carbonate sands carried in by the tides are
cemented together by carbonate secreting organisms, forming algal mats
Carbonate deposition can only occur in the shallower parts of
the deep ocean unless organic productivity is so high that the remains
of organisms are quickly buried. This is because at depths
between 3,000 and 5,000 m (largely dependent on latitude - deeper near
the equator and shallower nearer the poles) in the deep oceans the
rate of dissolution of carbonate is so high and the water so
undersaturated with respect to calcium carbonate, that carbonates
cannot accumulate. This depth is called the carbonate compensation
. The main type of carbonate deposition in the deep oceans consists of
the accumulation of the remains of planktonic foraminifera to form a
carbonate ooze. Upon burial, this ooze undergoes diagenetic
recrystallization to form micritic limestones. Since most
oceanic ridges are at a depth shallower than the CCD, carbonate oozes
can accumulate on the flanks of the ridges and can be buried as the
oceanic crust moves away from the ridge to deeper levels in the ocean.
Since most oceanic crust and overlying sediment are eventually
subducted, the preservation of such deep sea carbonates in the
geologic record is rare, although some have been identified in areas
where sediment has been scraped off the top of the subducting oceanic
crust and added to the continents, such as in the Franciscan Formation
of Jurassic age in California.
. Carbonate deposition can occur in non-marine lakes as a result of
evaporation, in which case the carbonates are associated with other
evaporite deposits, and as a result of organisms that remove CO2
from the water causing it to become oversaturated with respect to
. When hot water saturated with calcium carbonate reaches the surface
of the Earth at hot springs, the water evaporates and cools resulting
in the precipitation of calcite to form a type of limestone called
Dolostones are carbonate rocks composed almost
entirely of dolomite - (Ca,Mg)CO3.
Although there used to be a common perception that the abundance of
dolostones increased with age of the rock, it is now recognized that
although no primary dolomite bearing rocks are being directly precipitated
in modern times, dolostones have formed throughout geologic time. This is
true despite the fact that modern sea water is saturated with respect to
dolomite. Still, most dolostones appear to result from diagenetic
conversion of calcite or high-Mg calcite to dolomite, after primary
deposition of the original calcium carbonate bearing minerals.
Two mechanisms of dolomitization of limestones have been proposed.
. This mechanism involves the evaporation of seawater to form a
brine that precipitates gypsum. After precipitation of gypsum,
the brine is both enriched in Mg relative to Ca and has a higher
density. If the brine then enters the groundwater system and
moves downward into buried limestones. This Mg-rich brine then
reacts with the calcite in the limestone to produce dolomite.
Mixing of Seawater and Meteoric Water.
This mechanism involves the mixing of groundwater derived from the
surface with saline groundwater beneath the oceans.
Dolomitization is thought to occur where the two groundwater
compositions mix with each in the porous and permeable limestone
within a few meters of the surface.
Evaporite minerals are those minerals produced by extensive or total
evaporation of a saline solution. Because such minerals dissolve
readily in less saline rich solutions, like most groundwater and surface
water, evaporite rocks rarely outcrop at the surface except in aid
regions. Evaporite rocks are common, however, in the
subsurface. Three different environments result in the deposition of
1. 2. 3.
Basins of internal drainage.
In arid regions with basins of internal drainage rainfall in the
adjacent areas is carried into the basin by ephemeral streams carrying
water and dissolved ions. The water fills the low points in the
basin to form a playa lake. These lakes eventually evaporate,
resulting in the precipitation of salts such as
and a variety of other salts not commonly found in marine evaporite
deposits, such as trona (NaHCO3.Na2CO3.2H2O),
nahcolite (NaHCO3), mirabilite (Na2SO4.10H2O),
and colemanite (CaB3O4(OH)3.H2O).
Restricted bays or seas.
In areas where there restricted input of fresh or marine waters into a
basin, coupled with extensive evaporation within the basin, dissolved
ion concentrations may increase to the point where form a dense
concentrated solution is formed near the surface.These dense saline
waters then sink within the basin, become oversaturated with respect
to salts like gypsum and
halite, and precipitate the salts on the floor of the basin.
Shallow arid coasts or sabkhas
Along shallow arid coastlines where input of fresh water is rare and
evaporation increases the salinity of the marine water, evaporation
may increase the salinity of the water to a point where evaporite
minerals like halite and gypsum
Chert is a mineralogically simple rock consisting of microcrystalline
There are three common occurrences of chert.
As nodules and silt-sized grains in carbonate rocks. Chert
nodules, as discussed previously, occur as structureless dense masses
within carbonate rocks. They range in size from a few
centimeters to many meters in length. The source of silica is
likely silica secreting organisms that include diatoms (Jurassic to
Holocene), radiolaria (Ordovician to Holocene), and sponges (Cambrian
to Holocene). But, these organisms are not preserved in the
chert nodules. Instead, the remains of these organisms were
likely dissolved by fluids flowing through the rock during
diagenesis. Most chert nodules are found along bedding planes in
the carbonate rocks, likely because these were zones along which
fluids that precipitated the microcrystalline quartz were able to
As bedded cherts that formed along tectonically active continental
margins. Bedded cherts occur in association turbidites, ophiolites,
and mélanges (oceanic trench deposits scraped off the seafloor at
subduction zones). The beds range in thickness from a few
centimeters to several meters, and are interbedded with siliceous
shales. Although thought to represent deep water accumulations
of silica secreting organisms, they may also form in warm nutrient
rich shallow water environments. Sometimes the remains of silica
secreting organisms, like radiolaria, sponge spicules, or diatoms are
preserved in the cherts, but most show a microcrystalline texture that
results from recrystallization during diagenesis
Associated with hypersaline-lacustrine deposits. Although less
common than the previously discussed occurrences of chert, some cherts
appear to form in a hypersaline environment where they are associated
with evaporite deposits. Such cherts may in fact form as a
result of replacement of sodium silicate evaporite minerals like
magadiite by the following chemical reaction:
=> 7SiO2 + 4H2O + Na+ + OH-
Since mechanisms 1 and 2 generally require the presence of silica
secreting organisms in order to form chert, the occurrence of chert in
Precambrian rocks is problematical because no such organisms existed prior
to the early Paleozoic. Such Precambrian cherts may have actually
formed by direct chemical precipitation from silica oversaturated
Mudrocks are fine grained sedimentary rocks consisting of mostly silt and
clay size fragments. They are sometimes called argillites Because of their
small grain size, they are difficult to study, even with the petrographic
microscope. But, they are important rocks because they are the most
abundant sedimentary rocks, making up over 65% all sedimentary rocks, are
likely the source rocks for
petroleum and natural gas, and are sometimes valuable ore
deposits. In addition, the mudrocks are the protoliths
(precursor rock) for aluminous metamorphic rocks, Classification
Classification of mudrocks is mainly based on observations one can make in
the field or at the level of a hand specimen. The classification depends
on the grain size of the minerals making up the rock and whether or not
the rock is fissile or non-fissile. A fissile rock tends to break
along sheet-like planes that are nearly parallel to the bedding
planes. Fissility is caused by the tendency of clay minerals to be
deposited with their sheet structures [(001) crystallographic planes]
parallel to the depositional surface.
Abundant silt sized grains visible with a hand lens
Elements of texture that can be observed in mudrocks include the shapes of
the grains, the fissility or lack of fissility, and laminations.
Clay minerals are generally angular and sheet like, reflecting their
crystal structure, which is also why most mudrocks are
Silt size quartz grains are usually angular or platy shaped.
Little rounding occurs because these minerals are so small that they can
be kept in suspension in very low energy currents, with little chance of
impact with other grains that would normally cause abrasion and rounding.
. Whether or not a mudrock is fissile or non-fissile depends on several
The abundance of clay minerals. The more clay minerals
contained in the rock, the more likely the rock is to be fissile.
The degree of preferred orientation of the clay minerals.
Small clay minerals tend to adhere to one another if they collide
during transport. The tendency is increased by increased
salinity of the water and the presence of organic matter in the water.
Adhesion of small mineral grains is referred to as
flocculation. If the clay minerals flocculate, then they are less
likely to have a preferred orientation, and thus less likely to form
rocks with fissility.
Bioturbation of organisms within or on the surface of the sediment
can disturb the preferred orientation of clay minerals, and thus lead
to a non-fissile rock.
If the clay minerals recrystallize during diagenesis they will tend
to do so with a preferred orientation with their (001) crystal planes
oriented perpendicular to the maximum principal stress
direction. (This process also results in slatey cleavage and
foliation in metamorphic rocks). If diagenesis occurs shortly
after deposition, then it is likely that the maximum principal stress
direction will also be oriented perpendicular to the bedding planes.
Laminations are parallel layers less than 1 cm thick. Such
laminations can be seen as differences in grain size of the clasts in
different laminae - due to changes in current velocity of the depositing
medium, or could be due to changes in the organic content and oxidation
conditions at the site of deposition of the different layers.
Environments of Deposition
Mudrocks represent texturally and mineralogically mature sediments
deposited in low energy environments. The fine grain size of these
sediments means that that the sediment can be suspended for long times in
relatively quiet, low energy currents. This results in deposition on
the abyssal plains of the oceans, at the distal ends of deltas, in quiet
lakes and swamps, and as wind blown dust. Large deposits of wind
blown dust are called loess, and consist of mostly silt-sized
fragments. While the small fragments can be transported into lakes
and oceans easily by streams, it is likely that large quantities of these
fragments could also reach the oceans by wind transport.
In order for sediment to be deposited, a low lying area, called a basin,
is required. The largest of basins are the ocean basins, which currently
cover about 70% of the Earth's surface. In the past, however, sea
level has often changed, resulting in episodes where even the continents
were covered by shallow seas, referred to as epiric or epicontinental
seas. When sea level rises to invade the continents it is referred
to as a transgression.
It is for these reasons that most of the sedimentary rocks preserved in
the geologic record are marine sediments.
Basins result from plate tectonics, and even the large transgressions
appear to be related to tectonic factors, as increased spreading rates in
the ocean basins can result in changing the configuration of ocean basins
that result in flooding of the continents.
A sedimentary facies is a group of characteristics that reflect a
sedimentary environment different from those elsewhere in the same
deposit. Thus, facies may change vertically through a sequence as a result
of changing environment through time. Also, facies may change laterally
through a deposit as a result of changing environments with distance at
the same time.
The structures and textures found in sedimentary rocks give clues to the
environment of deposition, as discussed above, and thus allow geologists
to assign parts of a deposit to a particular sedimentary facies.
Individual facies are generally described in terms of the environment is
which deposition occurred.
The following are facies or environments are among those possible: