Introduction to Sedimentary Rocks

Photo by W. W. Little
What is a Sedimentary Rock?
A sedimentary rock is a rock that formed through the
accumulation of particles derived from preexisting rocks,
including dissolved ions that recombine by chemical and
biological processes.

Also called:
• Siliciclastic (from silicate minerals)
• Terrigenous (from the earth)
• Detrital (disintegrated or weathered)
• Epiclastic (from the surface)

Compositional Classification
Terrigenous
Clastic sediment derived through extra-
basinal weathering and erosion of
subaerially-exposed rock bodies. E.g.
gravel, sand, and mud.
Allochemical
Reworked intrabasinal sediment produced
through chemical or biochemical
precipitation. E.g. bioclasts, ooids, and
intraclasts.
Orthochemical
Insitu intrabasinal sediment produced
through chemical or biochemical
precipitation. E.g. micrite, phosphate,
halite, gypsum, and chert.
T: Terrigenous
A: Allochemical
IA: Impure allochemical
O: Orthochemical
IO: Impure orthochemical

Sediment Production
Sediment is produced by the weathering and erosion of
preexisting rock bodies. Once formed, sediment is
transported to a new area where it is deposited as layers
that are subsequently lithified to form sedimentary rocks.

Weathering
Weathering is the breakdown or decomposition of
a rock body.

Erosion refers to the removal (entrainment) of
weathered material (sediment).
Erosion

Transport
Once sediment has been eroded, it is moved to a new location by
water, wind, ice, or mass movement.

Deposition
Eventually, sediment is dropped to form a sedimentary deposit, in
this case, a point bar.

Lithification
Through burial and processes related to groundwater,
sediment is converted to rock.

Diagenesis
Diagenesis includes all
changes (physical &
chemical) that occur to
sediment following
deposition, including
compaction, cementation,
and dissolution.
Lithification is part of the
diagenetic process.

What is Weathering?
Weathering is the breakdown or decomposition of a rock body.

Physical vs. Chemical Weathering
Chemical weathering refers to the
decomposition of rocks and minerals
as chemical reactions alter them into
new substances.
Physical weathering refers to the
disintegration or disaggregation of
rocks by physically breaking them
apart
Photo by W. W. Little Photo by W. W. Little

Physical Weathering
Physical weathering breaks rock into smaller pieces but does not
change its composition.

Chemical Weathering
Chemical weathering destroys or alters existing minerals,
changing the rock’s composition and reducing its size.

Sediment Entrainment & Deposition
Hjulstrom & Sundborg showed a critical current velocity is required to both
entrain and deposit sediment of a specific grain size for a fixed water depth.
Sediment entrainment is also found to be dependent on sediment cohesion and
consolidation and deposition is influenced by grain shape. The critical
velocities for entrainment and deposition are not the same, particularly for fine-
grained sediment.
Hjulstrom Diagram

Entrainment Forces
Two sets of opposing forces operate in determining whether or not
a particle begins to move. Opposing transport are friction and
gravity. For entrainment, these must be over come by lift
(Bernouilli Effect) and drag (shear stress).

Reynolds Number
The Reynolds Number is used to distinguish between laminar and
turbulent flow conditions. Turbulence occurs under conditions of
high flow velocities, great depths, and low viscosity. Laminar flow
takes place under the reverse. Wind and water flow are nearly
always turbulent. Ice and mud are mostly laminar.

ul
Re 
Re = Reynolds Number
u = flow velocity
l = flow depth
v = kinematic viscosity (dynamic viscosity/fluid density)
Re > 2000 = turblent flowRe < 500 = laminar flow
Re = 500 to 2000 = mixed or transitional flow

Froude Number
The Froude Number is used to distinguish between subcritical
flow (lower flow regime) and supercritical flow (upper flow
regime), which determines whether or not the flow is too high to
produce bedforms. Supercritical flow exceeds wave propagation
velocities.
Fr = Froude Number
u = flow velocity
d = flow depth
√gd = celerity (wave velocity)
Fr > 1 = supercritical flowFr < 1 = subcritical flow
Fr = 1 = the “hydraulic jump”
gd
u
Fr 

Stokes Law
Stokes Law indicates the rate at which fine-grained sediment
settles in a static fluid and appears to be valid only for spherical
grains with a density similar to that of quartz. Theoretically, the
larger or more dense the grain, the more rapidly it settles.
Density and grain size also affect entrainment
Vg = settling velocity
D = grain diameter
Pg = grain density
Pf = fluid density
µ = dynamic viscosityμ18
)ρ(ρgD
v fg
2
g



Fluid Viscosity & Competence
Competence is a measure of a transport medium’s ability to
entrain and carry particles of a given size. The greater the
viscosity, the larger and denser the particles can be. For
example, water has a greater competence than air and ice can
transport larger particles than water. Likewise, a mudflow can
move larger grains than pure water.

Grain flow: cohesionless sediment
movement
Debris flow: viscous sediment movement
Liquefied flow: over-pressured interstitial
fluid movement
Density/turbidity flow: slurry movement
driven by differential density
Sediment Transport Mechanisms
Fluid flow
Traction: grain rolling/sliding along substrate
Saltation: grain hopping along substrate
Suspension: ‘permanent’ grain entrainment
Solution: chemical transport as ions
Gravity flow

Types of Fluid Flow
Waves (water): wind-generated oscillatory motions of water. Height dependent upon wind
strength, duration, and fetch. Wave ripples tend to be symmetrical.
Swash (water): generated by breaking waves. Water becomes turbulent, then laminar as
sediment is carried onto and then away from a beach.
Marine currents (water): generated by temperature and salinity (thermohaline) contrasts in
combination with the Coriolis Effect. Transport mud in suspension and rework sand along
deep sea floor.
Gravity: slope failure due to earth’s gravitational pull. Enhanced by high relief and high water
content. Can occur in both subaerial and subaqueous environments.
Chennelized (water): water derived through subaerial precipitation moves down-hill under the
influence of gravity. Depending upon discharge, flow velocity, and nature of sediment, a wide
variety of bedforms and a full range of ripple size can develop. Ripples tend to be highly
asymmetrical.
Wind: generated by atmospheric pressure gradients. Transports fine-grained sediment – up
to medium sand by traction/saltation and mud by suspension. Ripples of all sizes form and
are highly asymmetrical.
Ice: Down-slope movement of ice under the influence of gravity. Highly competent. Creates
very poorly-sorted deposits with few sedimentary structures.
Tides (water): generated by Lunar and Solar gravitational attraction. Tidal cycles are semi-
diurnal, diurnal, and annual (neap and spring). Tidal ripples tend to be symmetrical.

Compaction: Caused by overburden pressure from sediment burial. Decreases
rock volume by expelling fluids and forcing grains closer together. Particularly
important in clays and organic materials. Reduces porosity. Extreme compaction
causes pressure solution and the development of stylolites.
Cementation (authigenic mineralization): Different minerals are stable under
different combinations of temperature and pressure; therefore, minerals that
dissolve under one set of conditions may precipitate under another and vice versa.
Relative dating principles applied to changes in mineralogy and to
precipitation/dissolution features can be used to reconstruct burial history.
Lithification/Diagenesis

Chemical
(mostly evaporites)
(aka orthochemical)
Sedimentary Rock Classification
Sedimentary rocks are first divided on the basis of the major
set of processes responsible for their formation.
Clastic
(mostly siliciclastics)
(aka terrigenous)
Biochemical
(mostly carbonates)
(aka allochemical)

Siliciclastic Classification
Siliciclastic sedimentary rocks form through the physical
accumulation of particles (clasts) and are subdivided on the basis of
particle (grain) size.

Uden-Wentworth Scale
The Wentworth Scale is
the standard
classification for clastic
sediment and rocks.
Subdivisions are
measured in both
millimeters and in Phi
units (log base 2). Class
sizes closely reflect
natural variation.

Conglomerate
Conglomerate is a siliciclastic sedimentary rock made up of
at least 30% gravel size particles.

Breccia
Breccia is a conglomerate in which most clasts are
distinctively angular.

Sandstone
Sandstone is a siliciclastic sedimentary rock made
up of sand size particles.

Siltstone
Siltstone is a siliciclastic sedimentary rock made up of silt
size particles.

Claystone
Claystone is a siliciclastic sedimentary rock made
up of clay size particles.

Coal
Coal is a rock made up primarily of
plant debris and is found in
association with some siliciclastic
rocks.

Composition
Composition (provenance) tells us where the sediment
came from. The study of rock composition is referred to
as petrography.

Provenance
Provenance studies focus on
composition are designed to
determine the nature of a
sediment source area,
particularly source rock,
tectonic setting, and climate.

Example from North America
Sandstones from North America show a northward increase in feldspar content for three
reasons:
• Source rock: Much more granite is exposed in the northern part of the continent.
Bedrock in southern regions consists mostly of Paleozoic sedimentary units.
• Climate: A colder, drier climate in north favors physical weathering; whereas, warmer,
wetter conditions in south enhance chemical weathering.
• Transport distance: Lack of glaciation in south has resulted in longer, better-developed
fluvial systems than in the north.

Texture
Texture (size, shape, & arrangement of particles) can tell
us how the sediment was carried (water, wind, or ice) and
how far it traveled.

Sedimentary Rock Components
Sedimentary rocks are made up of three main components, framework
grains (called allochems in carbonate rocks), matrix, and cement.
Framework Grains
(allochems)
Matrix
Cement

Framework Grains
Framework grains are the larger clasts that are used to define the rock.

Grain Contacts
The nature of grain
contacts is influenced by
mineralogy/rock type and
can be an indicator of
burial depth
Point
Sutured
Convavo-convex
Long
Long

Grain Form
Grain form refers to the relative length of three perpendicular
axes and is a weathering characteristic that is strongly controlled
by mineralogy, rock type, fractures, bedding planes, etc. Grain
form impacts both entrainment and settling.
Equant
PlatyProlate

Surface Texture
Surface texture is a weathering characteristic that is strongly influenced
by mineralogy, rock type, and weathering history.

Matrix
Matrix is composed of smaller particles that occupy some of the space
between framework grains. It can be deposited together with the
framework grains or physically infiltrated later.

Cement
Cement (authigenic mineralization) is chemically precipitated
between framework grains and matrix particles and is useful for
establishing burial history.

Common Authigenic Minerals
Carbonate cement (calcite,
dolomite, ankerite, siderite)
Clay minerals (kaolinite, illite)
Quartz
Feldspar (albite) and zeolite are also found
in small amounts.

Heavy (trace) Minerals
• Tourmaline
• Apatite
• Garnet
• Zircon
• Rutile
• Magnetite
• Cassiterite
• Monazite
• Rarely gold
Heavy minerals typically have specific
gravities between 3.0 and 5.2. They are
mostly found within quartz-rich
sandstones but rarely make up more
than 1% of the rock. Heavy minerals
are useful in provenance studies.

Porosity
Porosity (Ø) is empty space within a rock and can be primary or
secondary in nature. Some of the factors that influence porosity include
grain size (mean, median, and sorting), shape (rounding & sphericity),
composition, compaction, degree of cementation, dissolution.
Grain shape can
greatly influence
packing.

Primary porosity in
sandstone.
Secondary porosity in
sandstone.

Permeability
Permeability (K) is a
measure of how well fluid
can migrate through a rock
and is dependent upon the
same factors as porosity.

Sedimentary Textures
Sedimentary texture refers to the size, shape, and arrangement of
clasts and provides information pertaining to the nature and
distance of sediment transport.

Roundness
Roundness is a measure of surface irregularity and refers to
whether or not the constituent grains have sharp corners.

Sphericity
Sphericity indicates how closely the grains approach a state
where all three dimensions (length, width, and height) are the
same.

Roundness vs. Sphericity
The degree of roundness and sphericity for grains in sediment or a
sedimentary rock are usually determined by comparing grain shapes to
published charts that are based on statistical studies. The degree of
rounding and sphericity both tend to increase with transport distance and
with time. They are also impacted by grain composition.

Sorting
Sorting indicates how uniform or diverse particles are in size. Well-sorted refers
to sediment or rocks where all particles are about the same size; whereas, poorly-
sorted samples have a variety of grain sizes. Overall, sorting is better in finer-
grained samples and in sediment that has been transported a long distance.
Source material and transport medium are also important.
Well-sorted Sandstone Poorly-sorted Conglomerate

Sorting Classification
• Skewness: a measure of whether or not a particular grain size is favored.
Distributions can be multi-modal.
• Kurtosis: A measure of the flatness or peakedness of the size distribution
curve.

Sediment Maturity
As particles are transported from a source area to a basin, they become overall
smaller in size, better rounded, more spherical, and better sorted. This is
referred to as textural maturity. Concurrent with this is a change in
composition as easily weathered minerals decrease in abundance, leading to a
compositionally mature deposit.
Sorting SphericityRoundness

Textural Maturity
Sediment becomes finer-grained, more rounded, more spherical, and better
sorted with downstream transport.
Demir, 2003 Demir, 2003
Gomez et al. 2001

The Bowen Reaction Series demonstrates the order in which minerals
crystallize from a melt. An approximate reverse of the Bowen series seems
to occur under atmospheric conditions, with quartz being the most stable and
mafic minerals the least.
Compositional Maturity

Maturity Classification
There are four stages of sediment maturity, depending upon (in order of
decreasing importance) clay content, sorting, and rounding.

Sediment Maturity & Environment
Sediment maturity can be tied loosely to depositional environment, reflecting
distance of transport and energy conditions of the system.

Sedimentary Structures
Sedimentary structures form as sediment is deposited and are
indicators of past depositional conditions, such as flow velocity, flow
depth, flow direction, and the transporting agent. They can also
provide significant clues as to paleoclimate.

Depositional Structures
Depositional structures form as sediment settles from movement
during or following fluid flow.
Ripples Cross-bedding
Graded bedding Biological structures
Photo by W. W. LittlePhoto by W. W. Little

Erosional Structures
Erosional structures are in two forms: diastems that represent short-term
pauses between depositional events and form a normal part of a depositional
system or unconformities that represent long-term cessation of deposition.
Fluvial or tidal channels Cut-and-fill structures
Flute casts Unconformities

Post-depositional Structures
Following deposition, sediment can be altered by number of processes, such
as dessication, rainfall, sediment flow, loading, and burrowing.
Mudcracks & rain imprints Distorted/contorted bedding
Load casts Bioturbation

Diagenetic Structures
Upon burial, sediment can be further altered by compaction, cementation,
dissolution, and mineral replacement.
Nodules & Concretions
Disolution Pressure solution
Mineral replacement
(irregular) (spherical)

Fossils
Fossils are indicators of ancient environments and can
give clues regarding past climates.

Clay Minerals
Clay minerals have properties that make them both relatively slow to erode and
to settle. A strong electrostatic charge increases cohesion, inhibiting erosion. A
platy shape and low density lead to low settling rates; although, the electrostatic
charge can cause flocculation and more rapid settling.

Chemical and Biochemical Rocks
Chemical and biochemical sedimentary rocks are composed
of single-mineral precipitates.
Calcite
Limestone

The Carbonate Factory
Whereas siliciclastic sediment is produced primarily outside of the
depositional basin and then transported to the basin, most carbonate
sediment originates within the basin itself (primarily by biological
processes) and undergoes little, if any, significant movement.

Aragonite
High-Mg Calcite
Calcite
Dolomite
Carbonate Classification
Carbonate sedimentary rocks form through the chemical or
biochemical combination of Ca and Mg with the carbonate
anion and are subdivided on the basis of mineral composition.
Increasing Mg content
Slide modified from Gahn, 2006
LIMESTONE DOLOSTONE

Limestone
Limestone is a biochemical rock composed of the
mineral calcite or high-Mg calcite.

Dolostone
Dolostone is a biochemical rock composed of the mineral
dolomite.

Carbonate Rock Components
Like siliciclastics, carbonate sedimentary rocks are made up of three
main components, framework grains (allochems), matrix (mud or
micrite), and cement.
Allochems
Matrix
Cement

Carbonate Mud (Micrite)
Carbonate mud has two main sources, calcareous green algae and
fecal material.
Penicillus Halimeda Carbonate Mud

Allochems
Allochems are grains larger than mud size and can include fossils, ooids,
pellets, intraclasts, etc.

Carbonate Sediment Production
It has been said that “Carbonate sediments are born, not made.” (James,
1984), as the vast majority of carbonate sediment is of organic origin.

Biogenic Carbonate Mineralogy
• Aragonite
– bivalves, gastropods, cephalopods
– calcareous green algae
– modern reef corals
• Calcite
– Brachiopods
– Planktonic brachiopods
• High-Mg calcite
– Echinoderms
– Benthonic foraminfera
– Coralline red algae
Most Aragonite and high-Mg are altered to calcite upon burial. Slide modified from Gahn, 2006

Carbonate Organisms Have
Changed Through Time
The organisms responsible for biogenic production of carbonate
sediment have changed through geologic history.

Chert
Chert is a form of quartz that occurs in both siliciclastic and carbonate
rocks, usually in the form of discontinuous beds, lenses, or nodules.
It’s origin can be either biologic (mostly siliciclastics) or diagenetic
(mostly carbonates).

Evaporites
Evaporites form through the inorganic precipitation of various
minerals, mostly through high evaporation rates in environments with
restricted circulation. They are composed of a single mineral and
often have a crystalline texture.

Evaporitic Sequence
Carbonates (mostly dolomite)
• Form when original seawater volume is reduced by
50%
Ca Sulfates (gypsum and anhydrite)
• Form when original seawater volume is reduced to
20%
Chlorides (halite, sylvite, etc.)
• Na salts form when the original seawater volume is
reduced to 10%
• Mg and K salts form when the original seawater volume
is reduced to 5%
No modern equivalent exists for thick evaporite successions found in
the ancient rock record. However, through laboratory experiments, a
similar pattern has been produced.

Ironstones and Iron Formations
Iron-rich sedimentary deposits, such as the Banded Iron Formation
(BIF), formed commonly in the Pre-cambrian, prior to the advent of
an oxygenated atmosphere. Such deposits are no longer produced.

Depositional Environments
Putting it all together, we can use the features of sedimentary
rocks to interpret the past geological history of an area.

Bouma Sequence
Some sedimentologic patterns are highly diagnostic of a specific
depositional setting, such as the “Bouma Sequence” that is
indicative of deposition by turbidites.

Genetic “Classification”
Some geologists “classify” rocks based on genetic origin. This is not
a classification in the sense of a rock type, it is an interpretation and
should not be used in the same manner as generic terms.
“Tillite” “Turbidite”
“Tempestite” “Tidalite”

Common Clastic Environments
Each depositional environment has a unique set of characteristics that
distinguish it from all other sedimentary environments.

Introduction to Sedimentary Rocks

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