Download SGES 1302 Lecture16

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 16: Sedimentary Rocks
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Sedimentary Rocks – weathering & transportation
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Sedimentary rocks begin with the process of weathering.
Wheathering, which is a set of physical, chemical and biological processes
that breaks rock into smaller particles while some dissolved into solution.
Gravity and erosion agents remove the products of weathering and carry
them into a new location where they are deposited.
These small pieces of rock that are moved and deposited by water, wind,
and gravity are known as sediments.
During transportion, the particles are further broken down and abraded.
During transportation and deposition, the particles are also sorted, both in
terms of size and density.
Most sediments are deposited by water in the sea, river, swamp or lake.
Following deposition, the sediments may transform into rocks (litification) by
compaction and/or cementation.
Sediments produced by weathering and erosion form sedimentary rocks
through the process of lithification.
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Sedimentary Rocks - deposition
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Sediments are deposited when transport stops, eg. when river
enters a quiet lake or sea, or when the stream velocity decreases.
Fast moving water can transport larger particles better than slow
moving water.
As water slows down, the largest particles settle out first, then the
next largest, and so on, so that different-sized particles are sorted
into layers.
Such deposits are characteristic of sediment transported by water
and wind.
Wind, however, can move only small grains.
Glaciers move all materials with equal ease. Large boulders, sand,
and mud are all carried along by the ice and dumped in an unsorted
pile as the glacier melts.
Landslides create similar deposits when sediment moves downhill in
a jumbled mass.
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Sedimentary Rocks - lithification
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The weathering of bedrock and the transport and deposition of
sediments are continuous.
As piles of sediment accumulate, the materials near the bottom are
compacted by the weight of the overlying layers.
Water flowing through spaces between sediment grains may carry
dissolved mineral matter, and over time or changes in physicochemical conditions, the sediments can be cemented by
precipitation of minerals.
About 75% of the land surface is covered by sedimentary rocks,
however it make up only about 15% of the volume of the upper
crust.
A large part of the Earth’s history has been reconstructed based on
information obtained from sedimentary rocks.
Sedimentary rocks are also important economically (coal,
hydrocarbon, Fe, Al, Mn, etc).
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Features of Sedimentary Rocks
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Sedimentary rocks provide important evidence of Earth’s long history
They are formed at Earth’s surface, accumulate layer upon layer, each
records the nature of the environment at that time.
These layers are called strata or beds, are the most characteristic
feature of sedimentary rocks.
The beds are separated by bedding planes, each bedding plane marks
the end of one episode of deposition and the beginning of another.
The thickness of the beds varies from microscopic to tens of meter.
Bedding in which the particle sizes become progressively coarser toward
the bottom layers is called graded bedding.
When inclined layers of sediment are deposited across a horizontal
surface, crossed beddings are formed.
Other important features found in sedimentary rocks include fossils and
sedimentary structures such as ripple marks, mud cracks etc.
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Sedimentary Rocks – classification
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Sedimentary rocks are classified by their mode of formation.
There are 2 main categories of sedimentary rocks:
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Detrital sedimentary rocks: deposition of solid particles
Chemical sedimentary rocks: precipitation of dissolved substance
Detrital sedimentary rocks are distinguished based on particle size.
Particle Size
Sediment Name
Rock Name
Coarse (> 2 mm)
Gravel
Conglomerate /
Breccia
Medium (1/16 – 2 mm)
Sand
Sandstone
Fine (1/256 – 1/16 mm)
Mud/Silt
Siltstone
Very Fine < 1/256 mm
Mud/Clay
Shale / Mudstone
Although a wide variety of minerals and rock fragments may occur in
the sedimentary rocks, clay minerals and quartz dominates.
From particle size and other features in the rock, we can interprete the
environment of deposition.
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Conglomerate
Conglomerates have
rounded, gravel-sized
particles. Because of its
relatively large mass,
gravel is transported by
high-energy flows of
water, such as those
generated by mountain
streams, flooding rivers,
strong ocean waves, and
glacial meltwater. During
transport, gravel
becomes abraded and
rounded as the particles
scrape against one
another.
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Breccia
Breccias are composed of
angular, gravel-sized
particles. The angularity
indicates that the sediments
from which they formed did
not have time to become
rounded. This suggests that
the particles were
transported only a short
distance and deposited
close to their source.
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Sandstone
Stream and river
channels, beaches,
and deserts often contain
abundant sand-sized
sediments that lithified to
form sandstone.
Sandstone can have a
porosity of up to 30%,
which makes them
valuable as underground
reservoirs of oil, natural
gas, and groundwater.
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Shale / Mudstone
These rocks represent
environments which have still
or slow-moving waters (lakes,
lagoons, deep ocean basins),
are the most abundant
sedimentary rocks. In the
absence of strong currents and
wave action, these fine grained
sediments settle to the bottom
where they accumulate in thin
horizontal layers.
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Chemical Sedimentary Rocks
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During weathering, dissolved materials are carried into lakes and oceans.
Some organisms that live in the ocean use the calcium carbonate or silica
that is dissolved in seawater to make their shells.
When these organisms die, their shells settle to the bottom of the ocean
and can form thick layers of biogenic chemical sediment.
Chemical sedimentary rocks are distinguished based on composition.
Composition
Texture
Rock Name
Calcite (CaCO3)
Fine to coarse
crystalline
Limestone
Quartz (SiO2)
Very fine crystalline Chert (light coloured)
Flint (dark coloured)
Gypsum (CaSO42H2O)
Fine to coarse
crystalline
Rock Gypsum
Halite (NaCl)
Fine to coarse
crystalline
Rock Salt
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Chemical Sedimentary Rocks
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Limestone is the most abundant chemical sedimentary rock
Limestone commonly forms in shallow water environments where
coral reefs thrive.
Many types of limestone contain evidence of their biological origin
in the form of abundant fossils.
Not all limestone contains fossils. Some limestone has a
crystalline texture, some consists of tiny spheres of carbonate
sand, and some is composed of fine-grained carbonate mud.
Dissolved silica precipitates to form various types of
microcrystalline to amorphous rocks (chert, opal, etc). Could also
formed by biochemical accumulations of siliceous shells
fragments.
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Chemical Sedimentary Rocks
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Limestone & chert requires precipitation of a relatively insoluble
substance from aqeous solution or from biogenic accumulations.
Evaporite minerals such as gypsum and halite precipitated from a
saline solution that has been concentrated by evaporation.
Evaporites do no precipitate until evaporation has removed most
of the water.
Evaporites are forming today in desert basins and locally on
shallow sea margins in hot climates (sabkha).
If a column of sea water 1000 m thick is evaporated to dryness,
the precipitated salt deposit would be about 15 m thick. Of this,
0.5 m would be gypsum, 11.8 m would be halite, and the rest, 2.7
m, would be mainly salts of potassium and magnesium.
The evaporites are important economically.
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Chemical Sedimentary Rocks
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Coal is an unique chemical sedimentary rock. It is not precipitated
from dissolved substances.
It is formed by direct accumulation of organic matter, mainly plant
material.
Plant structures such as leaves, bark and wood may still be
identified in coal.
Coal is produced by the burial of large amounts of plant material
over extended periods.
Special conditions are required for the accumulation of large
quantities of plant material because plant debris normally
decompose when exposed to the atmosphere.
What is the ideal place for the accumulation of plant material?
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