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Sedimentation & Stratigraphy
( G 202 )
Course Coordinators:
Dr. Ahmed Sadek
Dr. Ahmed El-Sabbagh
Geology department, Faculty of Science, Alexandria Univ.
Part I
( Sedimentation )
Introduction to
Sediments & Sedimentary Rocks
Dr. Ahmed Sadek
Course objectives
This course aims at:
• Providing an introduction to the basics of sedimentation
processes and sedimentary deposits.
• Generate in students an appreciation of the importance of the
sediments and sedimentary rocks.
• Assessing the textural and mineralogical properties and
structures of sediment and sedimentary rocks.
• giving a survey of the description and classification of sediment
and sedimentary Rocks.
• Providing a brief review of the full range of sedimentary
environments.
Course Contents
Part I: Sedimentation
1. Introduction (definition of sediments and sedimentary rocks
and their importance)
2. Nature, sources and formation of sediments & sedimentary
rocks, sedimentary rock cycle
3. Texture of sedimentary rocks (mechanical analysis -grain
fabric, roundness, sphericity and surface textures of
grains, porosity & permeability) and its importance
4. Chemical and mineralogical composition and its
significance
5. Simple genetic classification of sedimentary rocks
Schedule of Course Assessments
Methods of Assessment
Weighting of
Assessments
Time
Semester Work
Mid-Term writing exam
Final practical exam
Final-Term writing exam
Oral Exam
5%
10 %
15 %
60 %
10 %
Weeks 4 & 11
Week 8
Week 16
Week 17
Week 17
Total
100 %
List of References
- Blatt, H. Middleton, G.V. and Murray, R.C. (1980), Origin of sedimentary Rocks, 634 p.,
Prentice-Hall, New Jersey.
- Tucker, M.E. (1982), sedimentary Petrology An Introduction, 251 p., Blackwell
Scientific Publications
- Levis, D.W. and Mc Conchie, D. (1994), Practical Sedimentology. Chapman & Hall,
N.Y.
- Sam Boggs (2006): Principles of Sedimentology and Stratigraphy, 4th Ed. Prentice Hall,
662 pp.
Introduction to
Sediments & Sedimentary Rocks
WHAT ?
Definition of sediments &
sedimentary rocks
WHY ? Importance of sediments and
sedimentary rocks
HOW ? Sedimentary rocks are formed?
WHAT ? Textures and Mineralogy of
sediments & sedimentary rocks
HOW ? Sedimentary rocks are classified?
Introduction & Definitions
Sedimentology is the branch of the geological science which deals with
the study of sediments, sedimentary deposits and sedimentary rocks.
All rocks at the Earth's surface (igneous, metamorphic and sedimentary)
disintegrate slowly by chemical and physical weathering. The products of
weathering are both solid particles, such as grains of sand, and ions
dissolved in water. These weathering products erode and are carried
away by running water, wind, glaciers, and gravity to lower elevations
where they collect and deposit physically, chemically or biologically.
These loose, unconsolidated particles are referred collectively by
sediment. When the sediment is lithified and cemented together and form
new consolidated materials, it is called sedimentary rock.
Stratified
Sedimentary
rocks
Sediment
Introduction & Definitions
So, Sediments are loose, unconsolidated particles formed at or near the earth's
surface under low T & P, in response to processes of chemical and physical
weathering, erosion, transportation and deposition. Sediments may be:
• A fragment of rock, mineral or shell (clastic sediments)
• Crystals which chemical precipitated directly from water (chemical sediments)
• Shells of marine organisms (biochemical sediments)
• Reef (Growth in position by organisms)
Shell fragments
Shells
Rock fragments
crystals
Quartz grains
reef
The clasts sediments include:
• Terrigenous clastic sediments: The solid grains derived from
weathering of previous rocks (sedimentary, igneous,
metamorphic).
• volcaniclasts sediments: those generated by volcanic eruptions.
• bioclastic sediments: those generated by the mechanical,
chemical and biological breakdown of skeletal parts.
- All these grains can be transported, redistributed and deposited by
surface processes to form clastic sedimentary rocks
Chemical sediments: derived from precipitation of authigenic
minerals from solutions by biological and inorganic chemical
processes to form rocks without the transportation of the particles
and form chemical and biochemical sedimentary rocks.
Growth in position by organic processes (e.g., carbonate reefs,
coals)
Definitions
Sedimentology is the science of study of sedimentary deposits and
rocks.
Sedimentary Rocks are secondary rocks formed from preexisting
rocks. They are layered or stratified rocks formed at or near the
earth's surface under the low temperatures and pressures
normally characteristic of this environment, in response to
transportation, deposition and lithification of loose sediment.
Sedimenatry deposit is a body of sediments accumulated at/or near
the earth’surface within a specific sedimentary environment
Sedimentary petrology is that branch of petrology, which deals with
the genesis of the sedimentary deposits and rocks.
Sedimentary petrography is the description of the sedimentary rocks.
Sedimentation is the process of sediment accumulation in the earth
surface under the influence of certain forces (e.g. gravity, winds,
streams currents, glaciers, tubidity currents, ..) and is primarily
applied to the settling of solid particles from a fluid.
Abundance of Sedimentary Rocks
- Cover 75 % of the Earth's surface
- Make up about 5% of the total volume of the Earth'crust.
- Have an average thickness of about 1800 m on the continents
Continental Sediment
& Sedimentary Rocks
Marine Sediment
& Sedimentary Rocks
Why ?
Importance of sediments and sedimentary rocks:
Academic importance:
deduce the environments and processes of deposition,
palaeogeography and palaeoclimatology
understanding of the Earth's geological history.
Stratigraphic correlation.
Why ?
Importance of sediments and sedimentary rocks:
Economic importance:
• Source rock for petroleum oil and
gas.
• Reservoirs for the petroleum oil, gas,
freshwater
• Cap rocks (Seal) for reservoirs such
as evaporites, mudrocks.
• Raw materials for the Mud Fluids
(e.g. Bentonite (clay minerals).
•Source of ores of many metals such as: iron, manganese, lead ores,
and others.…
• Raw materials for the ceramic and Portland cement industries.
• Some of them are used as fertilizers such as phosphates, some
nitrates.
• Some of them used as raw materials for building as sand and gravel.
How a sedimentary rock is formed?
Sedimentary Cycle
• It is a part of the major rock cycle which shows the manner to form new rocks
(igneous, metamorphic and sedimentary rocks) from old ones through geologic time.
• Sedimentary cycle takes place at or near the surface of the earth under normal
conditions of temperature and pressures.
Sedimentary
Cycle explains:
How a
Sedimentary
rock is formed?
Weathering - it is the decomposition and breakdown of rocks and
minerals at the earth's surface by mechanical and chemical processes.
Erosion - it is the removal and movement of weathered material from the
site of weathering by moving water, wind, glaciers, or gravity.
Transportation - The weathering products may still located in situ or
undergone variable long or short transportation. Transporting medium
are water (including river, streams, waves, ..), wind, glaciers and gravity.
Deposition – the accumulation of transported sediment in the basin of
deposition (ocean, lakes, lagoons, inland valleys) as layers of loose
sediment.
Lithification - it is the process of transforming a loose sediment into a hard
sedimentary rock.
- It involves different processes: cementation, compaction,
recrystallization,
Uplift - It the process of uplifting of the rocks by the effect of tectonic
movements.
- It leads to the exposing of the rocks whose are subjected to the
effect of weathering processes again.
2
3
1
4
Sedimentary Cycle
I. WEATHERING PROCESSES
Weathering is the breakdown and decomposition of rocks and minerals (at or near
the Earth's surface) by mechanical, chemical and biological processes into products
that are more in equilibrium with the conditions found in this environment.
Weathering itself involves little or no movement or removal of the decomposed rocks
and minerals. They accumulate where they form and overlie unweathered bedrock
A) Physical (Mechanical) Weathering
rock disintegration (physical breakdown)
It is the breakdown of rock materials into smaller and smaller pieces with no change in
their chemical and mineralogical composition. Processes of Mechanical Weathering
serve to increase surface area, which enhances chemical weathering.
A) Physical (Mechanical) Weathering
Exfoliation and spheroidal weathering (“sheeting”)
-Process in which Slab-like
layers/sheets of rock are split
from the main rock mass due
to reduction in pressure as a
result of removal/ erosion of
overlying rocks
-Exfoliation is largely restricted
to granitic rocks.
Pressure release fracturing
- As uplifting proceeds, the overlying
rocks gradually erode, and thus the
pressure on the underlying rocks
decreases. This unloading of pressure
causes the rocks to expand and fracture
“exfoliation domes”
Expansion/contraction of rock
Alternate thermal heating/cooling as a
result of daily and seasonally changes in
temperatures for long time can cause
fracturing and exfoliation of the rocks. The
surface of the rock expands more than its
interior.
Abrasion & Collision
Abrasion is the mechanical grinding of rock surfaces by friction and impact of
rock fragments which are carried by moving current and glaciers. Also the
collision between different particles with each other cause their abrasion and
wearing.
Frost wedging
-Repeated cycles of freezing & thawing in Mountainous regions can
break rock into smaller fragments
- It develops large accumulations of loose angular sediments
beneath cliffs (talus slopes)
Talus Slope
Salt cracking
• It is formed when salt water penetrates into cracks and evaporates.
This leads to growing of salt crystals in the cracks which causing
expansion of these cracks and pushes the rock apart.
• It is common in desert environment.
B) Chemical Weathering
Rock decomposition (Chemical alteration)
• It is the decomposition of rocks by chemical agents that can alter and
change both their chemical and mineralogical composition.
• New minerals may be formed.
• Most important agents in chemical weathering: Water, Carbon dioxide and
Oxygen
Factors Controlling Chemical Weathering:
(Rates of weathering)
1. Bedrock characteristics
a. Rock Composition:
the rate of weathering of rocks is related to
relative stability of their mineral composition
to chemical weathering.
Ex. Limestones (calcite) & Sandstones (quartz).
The more susceptible parts of the rock will weather faster than the more
resistant portions of the rock. This will result in differential weathering.
b. Particle size
Smaller the particle size the greater the surface
area and hence the more rapid the weathering.
c. Structure:
Bedding planes, joints, and fractures, all provide
pathways for the entry of water. A rock with these
features will weather more rapidly than a massive
and structureless rock.
2. Topography e.g., gentle or steep slope
On gentle slopes water may stay in contact with
rock for longer periods of time, and thus result in higher weathering rates.
3. Climate: (Temperature, moisture)
Processes of weathering are more rapid in humid and hot climate than
cold or very dry one.
Ex. limestones in a dry desert climate are very resistant to weathering, but
limestones in tropical climate weather very rapidly.
4. Amount of vegetation and organic materials
The more organic mass is in the zone of weathering the more
intensively the chemical decomposition of rocks takes place.
Main processes of chemical weathering:
1. Dissolution
Dissolving of minerals by a liquid agent (i.e. water)
example:
-Halite dissolution NaCl = Na+ + Cl-CaSO4. 2H2O (gypsum) = SO42- + 2H2O
2. Hydration
Hydration involves attachment of H2O or OH- ion to a mineral, resulting
in formation of a new mineral.
CaSO4 (anhydrite) + 2H2O
CaSO4. 2H2O (gypsum)
Fe2O3 (hematite) + H2O
2FeOOH (geothite)
Dehydration involves removal of H2O or OH- ion from a mineral.
3. Hydrolysis (water)
• Hydrolysis represents a substitution of H+ or OH- of water for an ion in
the mineral.
• It is especially effective in the weathering of common silicate minerals.
2KAlSi3O8 + 2H+ + H2O → Al2Si2O5(OH)4 + 2K+ +4H4SiO4
(K-Feldspar)
kaolinite (clay mineral)
Kaolinite
4. Carbonation (Carbon dioxide Hydrolysis, type of dissolution)
It is a dissolution due to the reaction of the minerals with carbonate and
bicarbonate ions formed as a result of dissolving of carbon dioxide in water:
CO2 + H2O ↔
H2CO3
H+ +
(carbonic acid)
Example: Calcite dissolution
CaCO3 + CO2 + H2O
HCO3(bicarbonate)
Ca2+ + 2 HCO3- (bicarbonate)
5. Oxidation
• It is the reaction that occurs between compound and oxygen.
• It may react with minerals to change the oxidation state of an ion, which
causes the structure to be less rigid and increasingly unstable.
• This is more common in Fe & Mn bearing minerals.
Examples:
1. Rusting: Iron combines with oxygen (dissolved in water)
to form iron oxide
-4Fe° + 3O2 = 2Fe 2O3 (hematite)
2. Oxidation of organic matter: CH2O + O2 → CO2 + H2O
6. Reduction
It is simply the reverse of oxidation, and is thus caused by the
addition of one or more electrons producing a more stable
compound (reaction with organic carbon.
Fe2O3 . H2O (limonite) + C → FeCO3 (siderite)
C) Biological Weathering
Weathering of rock from activities of:
organisms
plants roots
burrowing animals
humans
Products of Weathering
1. Solid particles
Formed from mechanical weathering of parent rocks
differ in grain size (gravel, sand, silt, clay).
Accumulations of these products called clastic or detrital sediments.
They include quartz and feldspar and all types of rock fragments.
They form clastic rocks such as sandstones, conglomerates, mudrocks
or form soils.
Gravel
Conglomerate
Sand
Sandstone
Mud
Shale
Products of Weathering
2. Soluble materials
Ions or molecules such as (Ca2+, Na+, CO32-, SO42-,..) dissolved in water.
They removed through chemical weathering
They produce some chemical and biochemical rocks such as:
evaporites, chert, limestones.
Ca+2
Na+
Mg+2
Fossiliferous L.S.
CO3-2
Coral L.S.
Flint
Rock Salt
Products of Weathering
3. Secondary minerals
Neoformed minerals, which are formed after chemical weathering and
alteration of some minerals.
Ex., chemical weathering of feldspars produces clay minerals, aluminum
hydroxides, ferric oxides.
2KAlSi3O8 + 2H+ + H2O → Al2Si2O5(OH)4 + 2K+ +4H4SiO4
(K-Feldspar)
(kaolinite)
Oxidation of Iron to hematite, limonite,..
4Fe° (Iron) + 3O2 = 2Fe 2O3 (hematite)
Products of Weathering
4. Soil
loose, uneroded sediment material
(residual materials).
It is a product of mechanical and chemical
weathering of rock plus addition of organic
material.
Soil Profile
O Horizon – decayed and loose organics (topsoil)
A Horizon - inorganic mineral particles mixed with
some organics
B Horizon – clays with little organics
C Horizon – transition between bedrock and soil
The most common residual deposits are
- Bauxite (aluminium ore),
- Laterite (iron-rich soil).
-Calcrete, Ferricrete
Decomposition of Main Minerals in Igneous Rocks
Original Minerals
Weathering
Process
New Mineral
Material
Washed Away
In Solution
Fe, Mg minerals:
olivine, pyroxene,
amphibole
H2CO3 alteration,
oxidation
Clay minerals, Mg, Ca, Si
Fe-oxides
Feldspars
H2CO3 alteration
Clay Minerals
Quartz
Little change, some none
dissolution
Si
Calcite
Dissolves easily
Ca
none
K, Na, Ca
Weathering
of Granite
Quartz
Clay Minerals Clay Minerals
+
+
K, Na, Ca
Mg, Ca, Si
II. Erosion and Transportation
* Erosion is the removal of weathered material from the site of
weathering.
* Transportation is the movement of weathered material from
the site of weathering to the site of deposition.
• The weathering products may still located in situ and not
transported or undergone variable long or short transportation.
• Mostly, no chemical changes take place for the sediments
during transportation processes but many physical changes may
occur as in rounding of particles, sorting of sediments and
particle sizes.
The major natural agents of erosion and transportation are
gravity, wind, running water, glaciers, waves, and rain.
Wind
Gravity
Rockslide
Glacial
Rock fall from a
steep slope or cliff.
River
Beach
Transport Mechanisms:
Suspended Load: Fine-grained sediment (clay and silt)
transported in suspension due to turbulence
Bed (or traction) Load: Coarse-grained sediment (sand and
gravel) transported on the bottom of the stream
bed by rolling and sliding
Saltation: Sediment (typically sand) transported by intermittent
jumps
Solution: as dissolved particles (soluble ions and molecules in
water)..
Factors Affecting Transportation of Sediments:
1. Agent of transportation:
- Glaciers transports all sizes together and deposit angular and poorly
sorted sediment.
- Wind transports clay-sand size sediments and deposits rounded and
well sorted sediments.
2. The average velocity (speed) of the flow.
As the velocity of the flow increases, the size of the particles carried in the
flow also increases.
3. Distance of transportation
Long transportation of sediments by water and air causes:
more abrasion of the grains (more
rounded).
well sorting (coarse particles are
transported for short distance and
deposited near the source, but the
fine particles are transported for long
distance and deposited far from the
source).
III. Deposition
Deposition is the accumulation and settling of sediment in the basin of
deposition (ocean, lakes, lagoons, inland valleys) as layers of loose
sediment. This process is called a sedimentation
Depositional environment :
- A geographic setting where sediment is accumulating (deposited).
- Each setting is characterized by a particular combination of geologic
processes
- The geologic setting may change with time (e.g. marine to continental)
- By studying present day environments, geologist can more easily
interpret the rock record along the geologic time.
How can determine the type of Depositional environment?
It can be determined by looking at
– Texture (grain size, shape and composition, etc..)
– sedimentary structures
– fossils content,
– bed shape and vertical sequences within the sedimentary layers
• Sedimentary Facies
It is a body of sedimentary rock accumulated and modified in a
particular environment.
Each facies is characterized by features (sediment composition
(lithology), grain size, texture, sedimentary structures, fossil content
and colour) that distinguish it from other facies
How can Depositional environment affect the rock Facies?
physical attributes such as:
• Water type and depth
• degree of agitation
• salinity
Chemical factors such as
•
Eh and pH of water
• Shape and location of
basin of deposition
• Plate tectonic
• topography
• affect and control the living
organisms or type of the
sediment.
• affect organisms and
control mineral
precipitation.
• affect the
composition, fossil
contents, and
textures
Depositional Settings
Continents: Desert, glacial, fluvial (rivers), lake, swamp, cave
Mixed (Transition zone): Lagoon, river delta, beach, tidal flat
Marine : Reefs, continental shelf, continental slope, deep water
Continental Sedimentary Environments
1. Glacial -- deposits may have wide range of grain sizes (poorly sorted). made of
glacial till.
2. Fluvial -- migration of ripples/dunes on riverbed produces X-stratification in
deposits (mud and sand).
3. Lacustrine -- deposition of mud, thin layers on lakebed; in arid regions forms
evaporites
4. Aeolian -- large wind-blown sand dunes produces thick cross-stratification in
deposits
Marine (Nearshore) Sedimentary Environments (Transitional)
1. Deltas -- where rivers meet the sea -- clastic sediments are deposited
2. Beaches -- deposits of sand (siliciclastic and carbonate ) at the coast
3. Shelf -- may be sand and mud or carbonate sediments
4. Reefs -- build-up of limestone from coral skeletons
Marine (Offshore) Sedimentary Environments (Deep Sea)
1. Shelf -- carbonates
2. Slope and rise -- clastics and mixed carbonates/clastics
3. Deep marine -- finely layered mud
Delta
Desert
Beach
Ocean
Swamp
Lakes
Deposition of sediment occurs in a wide variety of
geological environments.
Streams deposit clastic sediments in streambeds, in
floodplain, and on deltas.
Wind deposits sand and silt on land surfaces and form
dunes.
Glaciers deposit large volume of angular and unsorted
sediment of gravel, sand and clay, where they melt.
Dissolved ions (Ca, silica,..) in water may be carried to
the ocean where they are absorbed by some organisms to
form shells and skeleton of calcite or silica,... When the
organisms die, the skeletal materials deposited and form
bioclastic sediment.
IV. Lithification
Turning sediment into rock
Diagenesis = chemical, physical, and
biological changes that take place to
sediment after it is deposited. It includes
low temperature near-surface processes to
higher temperature subsurface processes
(<300C and 1-2 kb)
Lithification is a diagenetic process that refers collectively to all the
processes that convert loose sediment to hard sedimentary rocks.
loose sediment
hard sedimentary
rock
IV. Lithification
Turning sediment into rock
Main Lithification Processes:
1. Compaction
for sediments with clastic texture
2. Cementation
3. Recrystallization
for chemical crystalline sediment
• Lithification requires
increased pressures
and temperatures
(with depth)
Burial
1. Compaction
• It is a physical process of lithification by which buried sediment is
reduced in volume by pressure from the weight of overlying sediments.
•With compaction:
- the air and water between the grains are
expulsed
- the pore space is reduced
-the loose sediment is converted into more
cohesive rock.
•Compaction affects all sediments, but
changes are most pronounced in finegrained clastic sediments, such as clays
and silts.
•Burial of a clay may result in
a 40% reduction in volume
Burial
2. Cementation
• Cementation is the most important process for turning sediments to rock.
• It is the process by which sediment grains are bound together by minerals
chemically precipitated from water circulating through sediment.
• These cementing materials are precipitated in the pore spaces and cement
the grains together.
• Coarse-grained sediments, such as gravels and sands, are more likely to be
cemented than fine-grained sediments.
• The most cementing materials are: Silica, Calcium carbonate, Iron oxides.
3. Crystallization & Recrystallization
The process of formation of the chemical rocks with interlocked crystalline
texture is called crystallization process.
Recrystallization is a process in which the texture
within the less stable minerals reorganizes and
develop into new, more stable and more interlocked
crystals with more size in solid state by the effect of
heat and pressure with depth and the movement of
fluids within pore spaces .
Examples:
• Amorphous silica to coarse crystalline quartz
• fine lime mud into coarse sparry calcite
• recrystallization of aragonite (unstable form of CaCO3) to calcite (more
stable form of CaCO3).
Aragonite
Calcite
V. Uplift
It the process of uplifting of the rocks by the effect of tectonic
movements. It leads to the exposing of the rocks to the effect of
weathering processes again.
Simple Model for the Evolution of Sedimentary
Rocks
1. Weathering
2. Erosion
3. Transport
4. Deposition
5. Compaction / Cementation
6. Uplift