Download Sedimentary Rocks - Earth and Environmental Sciences

Sedimentary Rocks
Deposited on or Near Surface of Earth by
Mechanical or Chemical Processes
Sedimentology is study of sediments. The Oxford English
Dictionary defines sediments as "that material which settles to the
base of a liquid." Liquids, in this case may be:
water - riverbed or beach deposition
air - sand dunes, aeolian dust
gas - volcanic ash flows
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Sedimentary Cycle
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What Rocks Tell Us
Rock Type
Igneous
Sedimentary
How Classified
Composition
Texture
Chemical
Composition
Grain Size
Composition
Metamorphic
Mineral Makeup
Texture
What it Tells Us
Tectonic Setting
Cooling History
Surface
Environment
Energy of
Environment
Original Rock Type
Temperature,
Pressure
Degree of Change
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Sedimentary Rocks are the
Principal Repository for
Information About the Earth’s
Past Environment
EARTH HISTORY!
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Environmental Clues in
Sedimentary Rocks
• Grain Size - Power of transport medium
• Grading - Due to sudden events followed by
waning energetics
• Rounding
} Transport, Reworking
• Sorting
• Cross-bedding Wind, wave or
current action
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Environmental Clues in
Sedimentary Rocks
• Fossils (some examples)
– Salt Water - Corals, Echinoderms
– Fresh Water - Insects, Amphibians
– Terrestrial - Leaves, Land Animals
• Color And Chemistry
– Red Beds - Often Terrestrial (oxidized iron)
– Black Shale - Oxygen Poor, Often Deep Water
– Evaporites – Arid Climates
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Two features characterize sediments and
sedimentary rocks: a detrital fabric and the
presence of layers, or bedding
Sedimentary Fabric
Settling produces a detrital fabric i.e. a fabric
with point contacts
between grains, rather
than a fabric of
interlocking grains
found in igneous or
metamorphic rocks.
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Bedding or Stratification
• Almost always present in
sedimentary rocks
• Originally horizontal
• Tilting by Earth forces later
• Variations in conditions of
deposition
• Size of beds (Thickness)
– Usually 1-100 cm
– Can range from
microscopic to 50m
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Graded Bedding Often largest particles settle
first, followed by successively
smaller ones so that the
particles are sorted more or less
according to size.
Normal grading - the coarsest
material on bottom, getting
finer grained towards the top
This suggests an initial
energetic event, followed by
waning energy
(e.g. a flood, turbidity currents)
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Bedding
• Bedding is a series of visible layers within the
rock. It is primarily due to episodic nature of
sedimentation where very fine particles are laid
down slowly between times of more rapid
deposition.
• Bedding planes are assumed to be originally
horizontal or nearly horizontal for water-laid
sediments.
• For aeolian (wind-laid or sub-aerial) more
commonly show cross-bedding, because the sand
can support steeper dune surfaces in air than in
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water.
Modern ripples in loose sediment
ancient ripples preserved in sandstone
Sedimentary
Structures
• Often, evidence of the
nature of the
sedimentary surface
are preserved in
sedimentary rocks
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Mud cracks on the bottom of a
modern puddle.
Mud-cracks form because clay
minerals may shrink by up to
15% in volume on drying out.
Mud-cracks can be preserved
and indicate a depositional
environment that is near shore
and periodically exposed to air.
Mud cracks in Cambrian rocks
in southwestern Virginia. Even
the curls of the flaking mud are
preserved.
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Sorting
• Sorting - is the measure of range of particle size of
sediments. A sediment having a wide range of
particle size is said to be poorly sorted and if the
range is small, the sediment is said to be well
sorted.
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Settling is the key process, and is dependent on :
size of grain (d)
density of grain (rg)
fluid velocity and turbulence
fluid density (rf) and viscosity (m)
Settling
Velocity
 r g  r f g  2

d

 18m 
(Stokes Settling Law)
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Sedimentary Rocks
Clastic Rocks
• Made of fragmentary
material
• Deposited by
– Water (most
common)
– Wind
– Glacial action
– Gravity
Biochemical
Sedimentary Rocks
• Evaporation
• Precipitation
• Biogenic sediments
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Clastic Rocks
• Clastic sediments are those derived from the
weathering, transportation and deposition of
older rocks. The term "clastic" comes from the
Greek klastos (= broken), indicating the broken
nature of the sediment. (An individual particle is
a clast)
• also known (somewhat confusingly) as detrital
sediment, or detritus, which is Latin for "worn
down"
• As erosion occurs predominantly on the land,
they are also termed terrigenous sediments (terra
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= Earth)
Clastic Rocks are classified by:
• Grain Size (Sand vs. Gravel vs. Silt, etc.)
• Grain Composition (Quartz vs. Feldspar, etc.)
• Texture (rounded grains, sorting, etc.)
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Sediment Sizes and Clastic Rock Types
Rock Type
Sediment
Grain Size
Shale
Clay
less than 0.004 mm
Siltstone
Silt
.004 - 0.064 mm
Sandstone
Sand
.064 - 2 mm
Conglomerate* Pebbles
2mm - 64 mm
Conglomerate* Cobbles
64 mm - 256 mm
Conglomerate* Boulders
> 256 mm
Sedimentary rocks made of silt- and clay-sized particles are
collectively called mudrocks, and are the most abundant
sedimentary rocks.
*
rocks with rounded clasts are conglomerates. Those with angular clasts are breccias
Conglomerate / Breccia (less commonly Rudite / Rudaceous Rocks)
Lithified gravel (grains over 2 mm) with rounded grains are called
conglomerates. The name can be modified by the size of the
predominant particles. Thus we can have a boulder
conglomerate, cobble conglomerate, pebble conglomerate or
granule conglomerate.
Conglomerate
Breccia
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Sandstone (less commonly Arenite / Arenaceous Rocks)
Lithified sand (grains between 2 mm and 1/16th mm) is called
sandstone, or, less commonly, arenite. The name may be modified
by the grain size (eg. very coarse sandstone through to very fine
sandstone) or grain composition.
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A sandstone composed of a significant amount (> 25%) of feldspar
is called an arkose (or arkosic sandstone).
If feldspar is present in significant quantities, but less than 25%, it
may be termed a feldspathic sandstone.
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A sandstone composed of mainly rock fragments is called a lithic
sandstone (or a litharenite).
A poorly sorted lithic sandstone with angular to sub-angular grains
with a fine, muddy or clay matrix is called a greywacke.
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Mudstone (Argillaceous Rocks)
Mudstones are fine grained clastic rocks, which may be
divided into siltstone and claystone.
Although mudstones are too fine to easily determine their
mineralogy, they are most commonly composed of quartz and clay.
If there is a significant proportion of quartz, the rock will be gritty,
whereas if it is nearly all clay it will feel slimy. The best way of
determining ‘grittiness’ is to scrape the rock on the underside of
your teeth! A gritty texture defines a siltstone, whereas a slimy
texture is a claystone.
The abundance of clay minerals (micas) means that the rock
may have a tendency to break along planes that are parallel to
bedding, due to the original orientation of the clays when they
settled. This property is called fissility. Fissile mudstones are 24
termed shales.
SHALES
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Clastic Sediment Composition
What the grains are composed of. Generally this involves
determining the amount of quartz grains, feldspar grains
Chemical Sediments
and rock fragments.
mineral aggregates precipitated out of natural bodies of water such as lakes, lagoons
and oceans. The components were transported in solution and then chemically
precipitated out of solution.
Biogenic Sediments
sediment composed mainly of fossil remains
Residual Sediments
products remaining after intensive chemical breakdown of rock exposed to attack by
natural agents such as oxygen, water and organic and inorganic acids.
Pyroclastic Sediments
result from violent volcanic eruptions - typically produces a mixture of angular
fragments of mineral grains, volcanic glass fragments, and volcanic rock fragments.
When deposited by primary volcanic processes = “pyroclastic.” If reworked by wind
or water it can best be called a “volcaniclastic sediment.”
Because quartz (SiO2) is abundant and relatively resistant to
chemical weathering it makes up the bulk of sand-sized
particles.
Fine particles (silt and clay) are carried to regions where the water
is still (off-shore environments). Clay minerals that are the
weathering products of feldspars and ferro-magnesian minerals
form the bulk of these particles.
Coarse products (boulders and cobbles) require fast moving water
(mountain streams) to be moved at all, and so are not
transported very far from their sources.
These particles may be unweathered and retain their source
mineralogy and chemistry. Intermediate-sized particles (sand)
are transported by rivers and wind and deposited on river
bottoms, at coasts or in deserts.
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Sediment Maturity
• Stability of Minerals (e.g. - feldspar, micas unstable;
quartz is very stable)
• Rock Fragments (most rock fragments not as
durable as individual grains)
• Rounding or Angularity (well rounded grains have
been through a lot of abrasion, angular ones are
“fresher”)
• Sorting (well sorted deposits have had processes
acting on them longer than poorly sorted ones)
Removal of unstable ingredients suggests Mechanical Working
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Shape - there are two main things to look at here: the grains’
roundness and sphericity
Sphericity
Roundness
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Diagenesis
(making a rock from loose sediment)
After sediments are deposited, they are commonly compacted by the
weight of overlying sediments.
Compaction
Cementing
• Quartz
• Calcite
• Iron Oxide
• Clay
• Glauconite
• Feldspar
Alteration
• Limestone - Dolomite
• Plagioclase – Albite
Recrystallization
• Limestone
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After sediments are deposited, they are
commonly compacted by the weight of
overlying sediments.
They may be lithified (solidified) by the
deposit of a cement or secondary mineral that
fills the pores. They may also be lithified by
recrystallization of the primary minerals.
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Circulating pore water contains dissolved minerals that may
precipitate out of solution, producing a cement.
Calcium carbonate is a common
cement.
Silica is not quite as common, but
forms an extremely tough
rock when it forms the cement.
Circulating pore water may also dissolve minerals from the rock.
Such dissolution is particularly common in carbonate rocks and may
lead to an increase in pore space.
Replacement involves the essentially simultaneous dissolution of
existing minerals and the precipitation of a new mineral.
Occasionally, delicate carbonate skeletons can be pseudomorphed by
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microcrystalline quartz crystals.
Chemical Sediments
Evaporites -Water
Soluble
• Halite
• Gypsum
• Calcite
Precipitates
Example: Ca(sol'n) +
SO4 (Sol'n) = CaSO4
• Gypsum
• Limestone
• Iron Formations
Alteration After
Deposition
• Dolomite
Biogenic Sediments
• Limestone - Shells,
Reefs, Etc.
Organic Remains
• Coal
• Petroleum
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Biogenic Rocks:
Limestone is the rock formed by calcite. Calcite
(CaCO3) is very near saturation in sea water and so
is used as shells material by marine organisms.
Most calcite rocks of Phanerozoic age (the last 600
million years) are of biological origin.
Dolostones (formed of dolomite (CaMg(CO3)2)
may also be formed in this way.
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Limestone
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Fossils often form a major component of limestones but not always.
Limestones are composed chiefly of calcium carbonate (CaCO3).
Calcium carbonate reacts with acids (e.g. - HCl) according to the
equation:
CaCO3 + 2HCl => CaCl2 + H2O + CO2 (gas)
and an audible and visible "fizzing" occurs as the CO2 is released.
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Limestone is also relatively easy to dissolve compared to other
lithologies.
When buried, dissolution can be initiated due to pressure. (more
soluble under high P)
Dissolution occurs along planes that have a jagged appearance to
them. Those planes are called stylolites and are frequently black
due to the accumulation of insoluble material such as clays and
organic detritus.
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Stylolites
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Some special kinds of limestones
• Chalk - a limestone
composed of the shells
of tiny, planktonic
one-celled organisms.
• Coquina - a limestone
composed exclusively of
large shell fragments.
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Oolites are small spherical concretions, commonly formed by calcite
that was deposited around a sand grain, shell fragment, or some
other foreign particle in shallow, wave-agitated water.
Oolites form where gentle or periodic wave action in warm marine
waters allow carbonate precipitation on all sides of a grain of sand or
shell fragment.
Large Oolites are called Pisolites or Peastone.
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Evaporites
• Dissolved material (Ca, Na, K, CO3, SO4, Cl) is
carried farthest and deposited where the ocean,
sea, or lake is evaporated off.
• Such rocks are called evaporites. As sea water is
evaporated, the sequence of minerals formed is
(
(
(
(
1) calcite (CaCO3)
2) gypsum (CaSO4.2H2O)
3) halite (NaCl) and
4) sylvite (KCl)
halite
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Evaporite Formation
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Some Weird Chemical Rocks:
Lake waters may have different chemistries, depending on the
source rocks in the catchment area for the waters. Salts that may
precipitate from lake waters include:
sodium carbonate (Na2CO3), (for example, lakes in the East
Africa Rift Zone)
sodium sulfate (Na2SO4)
and
borax (Na2B4O7.10H2O),
which have important commercial applications.
Phosphorous Deposits
The calcium phosphate, apatite (Ca5(PO4)3(OH,F), can also
precipitate from seawater if deep ocean waters, enriched in
phosphorous by the decay of marine animals, are brought to the
surface by upwelling currents and reach saturation in a shallow
basin. Such deposits may be economically valuable sources of
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phosphorous, which is used as a fertilizer.
Chert
When silica is precipitated, it forms chert. Chert may form
extensive, continuous bands or as nodules in carbonate rocks (flint).
(Many banded cherts have formed from the deposition of silica
organisms, so are classified as biogenic rocks rather than chemical.)
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Banded Iron Formations
In the past, Fe2+ has formed precipitates in remarkably continuous
bands or laminations, known as banded iron formations (BIF).
These formed when the atmosphere had very much less oxygen in it
than it does today, allowing the
dissolution and transportation of Fe2+.
Later oxidization of the iron transformed
it to Fe3+ which has a much lower degree
of solubility and so precipitated out of
solution.
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Banded
Iron
Formation
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Coal
• A biogenic sedimentary rock composed
essentially of lithified plant debris
• Delta, continental environments
• Carbonized Woody Material
• Often fossilized trees, leaves present
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Coal Seams, Utah
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Plant Fragments Are Often
Visible in Coal
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Coal forms from the accumulated remains of
plants that have not decayed. As the plant material
is buried and put under increasing heat and
pressure the non-carbon elements are driven off to
produce a sequence of substances increasingly
concentrated in carbon.
Need: Plant rich environments with little oxygen in
the water
(swamps, bogs --> peat --> coal)
(peat --> lignite --> bituminous coal --> anthracite)
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Thick coals require very thick peats, for which there are three
requirements:
• Steady, high groundwater levels. Water cover suppresses oxidation
and alteration of plant debris. The water level needs to be
maintained at the ground surface as subsidence is balanced by peat
accumulation. Rising waters flood peatland, forming a lake,
whereas falling water levels allow the drying of peat and erosion.
• No sediment influx. An input of terrigenous sediment will lead to
the formation of shale, rather than coal.
• Tectonic stability. A gentle subsidence is required to allow the
slow, uninterrupted accumulation of plant debris.
Tropical settings are favored, but not required, for peat formation
due to the more luxuriant growth of plants and the lower rate of
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decay.
Fossil Fuels
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Petroleum
A hydrocarbon molecule
What organisms make these?
Answer: None
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Petroleum
• Lots of organisms make these, however
• Fatty Acids
• Probable source: Marine plankton
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Petroleum Traps
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Facies Changes
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Sedimentary Environments
The fundamental property of any sedimentary rock is that grains have to be
transported from a source and deposited somewhere. One reason for studying
sedimentary rocks is to try to understand the conditions of the sedimentary
basin in which the sediments were deposited.
There are a relatively limited number of basic types of sedimentary environments.
Each depositional environment has its own distinctive sets of physical,
chemical and biological characteristics, that result in distinctive lithologies.
The characteristics of any particular environment is known as the facies. Adjacent
facies can merge into each other gradually or abruptly. By determining the
facies of ancient rocks, we can reconstruct the sedimentary environments that
were operating at that time.
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Let’s visit each of the environments and see at least
some of the types of deposits found there...
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Important concept: Migrating Facies
Sedimentary environments change over time with changing conditions. At the
shoreline, for example, there may be a river facies, beach facies and shallow
marine facies.
At any one time, sediments are deposited in these facies in a sub-horizontal
manner.
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If conditions were to change such as a rise in sea level, the facies would
migrate laterally.
Stream sediments would now be deposited further inland, beach
sediments would be deposited over the older stream sediments and
shallow marine sediments would be deposited on top of the older beach
sediments.
This is known as a marine transgression.
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With continued sea level rise (or the sinking of the land), the facies migrate
further towards the land. The end result may be a layer of river sediment
overlain by beach sediment that, in turn, is overlain by shallow marine
sediments. The diagrams shown here are vertically exaggerated. In reality, the
three facies form almost horizontal strata. However the layers were not
deposited in that order (stream, beach, marine), rather each layer represents
the change in the position of the facies over time - the layers were built
horizontally, rather than vertically.
Time lines
A vertical succession of sedimentary strata therefore represents facies
that occurred in laterally adjacent environments, provided that no
erosion events have occurred between the strata (Walther's Law Johannes Walthers, 1894).
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Terrestrial Sedimentary Environments:
Stream sediments (fluvial, alluvial sediments)
Streams are the principal means of transporting sediment across land,and stream
deposits can be seen nearly everywhere. A wide variety of depositional
environments gives a wide range of sediments in terms of composition, grain
size, sorting, rounding etc.
In general, where streams are close to their headwaters, they carry coarse, angular,
poorly sorted deposits. These regions are high energy regimes, however, where
erosion is dominant, rather than deposition, so the deposits are rarely
preserved. Where mountain streams emerge onto relatively flat plains, the
sediment is deposited in an alluvial fan.
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Alluvial Sediments
deposited by rivers deposited on
channel bottoms, on floodplain, and
on levees
Meandering Streams:
As river turns a bend, water velocity
is greatest on the outside of the
curve, and lowest on the inside therefore erosion is greatest on the
outside, and deposition is greatest on
the inside
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Point Bar
Cut Bank
Slowest
velocities
Fastest
velocities
Channel migration
THALWEG
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Meandering Stream Cross-Section
Deposition
Silt and clay
Erosion
muds
sand
gravel
sands
Gravels, rip-up clasts
Channel bottom deposits are the coarsest,
topped by sands, silts and clays (finer
sediments)
Overall fining upwards sequence
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Some features
commonly found
in channels
Ripples
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Cross-bedding
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Fining upwards sequence
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Oxbow Cut-offs
• fill with fine grained sediments
(clays and organic sediments)
• isolated “clay lenses” in sands and
silts
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Flood Stage
During floods - as flooding waters emerge from the channel,
they slow down and deposit silts and sands. Do this enough
years and you build up...
--> LEVEES (sands and silts, getting finer as you move away
from the channel)
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Eventually, the river surface may be above the surrounding
floodplain (especially during floods) (but possibly even
during nonflood times!)
The river may escape through the levee temporarily in what is
known as a Crevasse Splay, depositing sand and silt on the
neighboring floodplain.
Less commonly, the levee will be permanently breached, and the
river will abandon its old path and follow a new course
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AVULSION
Crevasse Splay
Avulsion - abandoning the old
channel and following a new
course. (e.g. - the Atchafalaya
River, Louisiana)
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FLOODPLAINS:
deposition of suspended mud during floods with later desiccation, soil
formation and plant growth.
a. sediment is thick beds of mud, especially clays
b.laminated to homogeneous mudstones with rootlets, soils
c. sheet units
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clay
Silt and clay
sand
gravel
x-section, looking “upstream”
map view
sand
Overall pattern:
Silt and
clay
Coarse channel and levee deposits
(which each individually fine
upwards) isolated in fine grained
floodplain deposits
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Braided streams
• A series of anastomosing channels
• Bifurcating and rejoining around
mid-channel bars
• These bars are typically coarse-grained
deposits, and variously eroded
and deposited in a complex pattern
• Braided streams form in environments
with high slopes and abundant
coarse-grained sediment supply.
• Therefore, generally coarse-grained
and very permeable
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Lake sediments (lacustrine)
Lakes are low energy environments, where the gradient and water
velocity is very low.
Deltaic sediments are deposited at the inflow, passing into well
sorted, finer, laminated layers.
Yearly cycles may produce varves of alternating finer and coarser
material.
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Varved clays Rhythmic deposition of clays and
silts/sands, especially in glacial lakes
Annual summer melting of glaciers
releases coarser material
During winter glaciers refreeze, and
only fine-grained material (still settling
out) is deposited
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Glacial sediments:
Debris eroded by a glacier is released by the melting at the snout or
base of the glacier.
Grain sizes vary from boulders to clay and are characteristically
unsorted, non-stratified and angular.
The unlithified sediment is known as till, and the resulting rock is
known as tillite.
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Aeolian sediments
Wind blown sediments are generally sand to silt sized. Sand may form
into dunes that are well sorted with bedding inclined downwind.
Aeolian silt (“loess”) is a common sediment, but is virtually unknown
as a sedimentary rock, presumably because it is so easily eroded.
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(Namibian coast satellite photo)
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Continental Shelf Environments:
Estuarine Environment
•Much of the load transported by a large river may be
trapped in the low energy environment of an estuary.
• Muds settle out as clays flocculate when saline waters are
encountered. Clay minerals have a surface negative charge
and tend to stay in suspension in fresh water as like charges
repel. When saline water is encountered, the cations are
attracted to the clay surfaces, neutralizing them. Adjacent
particles can then approach each other and aggregate.
• If the sedimentation rate is high and the land is slowly
subsiding, thick bodies of estuarine form. Estuarine
environments have increased from 18,000 years ago,
sediment can due to the flooding of river systems by rising
sea level since the last glacial maximum.
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Deltaic Environment
Marine deltas are built where large river systems deposit their load on
entering the sea. The coarsest sediments are deposited near the river
mouth and finer ones are carried further out. The size and shape of the
delta depends on the interplay of sediment supply by the river and
removal by waves and longshore drift.
Yukon
Delta
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Deltas:
When running water enters a body of still water, its momentum is
checked, velocity drops, and sediment is allowed to be
deposited
The coarsest-grained material drops out first, finest drops out last
(and therefore travels the furthest from the shore)
Topset beds
Sands and
gravels
Foreset beds
Sands and
silts
Bottomset beds
Silts and
clays
As the delta builds out into the standing body of water, the
sediment coarsens upwards
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Deltas often grade upwards into
stream deposits (either
meandering or braided - see
those descriptions) as the river
and deltas grow out into the
standing water body
Ganges Delta
Nile Delta
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Beach Environment:
Beach sediments are typically well sorted and rounded due to the winnowing
effects of wave action and the constant abrasion as the particles are washed
back and forth.
Sandy quartz beaches are the most common, but beaches many times consist of the
coarsest sized particles available, so cobble beaches also occur.
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Coastal areas are particularly dynamic environments
Waves affect only depths less than 1/2 their wavelength, so shallow
water is more energetic (and will have coarser grained
deposits associated with it)
Currently, the oceans are undergoing sea level rise, though recently
(12,000 ybp) sea level was much lower (-100m)
Sands (from beaches, barrier islands, etc.) interfinger with clays and
silts (from lagoons, off-shore areas) to form complex
stratigraphy
Barrier
Island sands
Offshore muds
Lagoon muds
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Offshore Environments (continental shelf):
Most coarse marine sediment is deposited within 5 to 6 km of the
land.
Shelf sediments are distributed over the entire shelf, however, as
sea level as a result of changing sea levels in the past.
Up to 70% of the sediment cover on continental shelves is probably
a relict of past conditions.
The great bulk of the Earth's sedimentary strata is formed from
continental shelf sediment, as only about 10% of the sediment
reaching the continental shelves remain in suspension long enough
to arrive at the deep sea.
The shelves effectively conserve continental crust which is
recycled with active tectonic conditions are generated.
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Carbonate shelf
Carbonate sediments are largely of biogenic origin where terrestrial input is
minimal and sea temperatures are warm enough to promote abundant
carbonate secreting organisms.
Carbonate shelves can accumulate thick deposits of fine, massive carbonates.
Coarser sediments are found near coral reefs in areas of higher turbulence and
strong currents. Carbonate platforms are relatively rare today, but were
much more abundant in the past, where higher sea levels inundated large
tracts of continental land masses.
Bahamas
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Spectacular coral reefs and atolls occur in the oceans growing in
4,000-5,000 m of water, yet coral growth is clearly limited (by
light) to less than 30m of water. Coral growth must be fast enough
to keep pace with the gradual submergence of the reef due to plate
tectonic movements and changes in sea level.
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Marine evaporite basins :
Saline waters occupying restricted basins in warm climates will evaporate, leading to
the saturation of salts and the formation of evaporitic mineral deposits.
A modern example of such a restricted basins is the Mediterranean Sea.
Warm surface waters from the Atlantic flow east and evaporate, increasing their
density. The saline waters sink and salts crystallize on the bottom, forming
evaporite deposits. Evaporites underlying the Mediterranean basin are 2 to 3 km
thick. If the Mediterranean became landlocked, it would precipitate a salt layer
about 70 m thick over the 1000 years it would take to evaporate. These relative
thicknesses indicate the importance of a continuous inflow and evaporation over
long periods of time to accumulate significant deposits.
Evaporation
ocean
Barrier
to
circulation
High salinity
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Continental Rise Environments:
Turbidites
Gravity driven turbidity currents consisting of dilute mixtures of
sediment and water flow down the continental slope. With mixing,
the mass expands, becomes more turbulent and erodes the material at
its base. Velocities of over 90 km/h have been recorded, and flows
can spread almost 1000 km from their source.
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Graded bedding
Sole marks
(rock flipped upside down
- looking at bottom
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Deep Ocean (Abyssal) Environments:
Deep-sea oozes:
In temperate oceans less than ~4 km deep, calcareous ooze forms as carbonate
secreting organisms die and settle to the bottom.
Cooler, deeper waters can dissolve more carbon dioxide, so carbonates particles
are dissolved in deeper waters.
Other parts of the ocean are mantled with siliceous ooze formed from siliceous
organisms. Siliceous ooze is most notable in the equatorial Pacific and
Indian Oceans and around the Antarctic, where upwelling deep ocean water
rich in nutrients leads to high biological activity.
Siliceous oozes are one theorized way for cherts to form.
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Manganese Nodules:
Concretions with elevated
concentrations of Mn, Fe, Ni, Cu, Co
Redox reactions:
• Reducing conditions in sediments
mobilize Fe, Mn, etc;
• Oxidizing conditions in ocean
waters precipitate Fe, Mn, etc.
oxides as nodules
Their growth is very slow
(mm per million years) - therefore,
they require a SLOW sedimentation
rate in order not to be buried by
sediments.
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- Far from continents!