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ESC102
Sedimentary Rocks
Our keys to the past
Sedimentary Rocks
Sedimentary rocks are rocks that form through the
accumulation of sediment.
Divided into two major categories:
•Detrital or Clastic sedimentary rocks are rocks that are
made of the weathered remains of other rocks.
•Chemical sedimentary rocks are produced either
through life processes (biochemical/bioclastic) or
through precipitation of minerals in water
environments
Why is the study of
sedimentary rocks important
to understanding Earth
history?
Investigating Sedimentary rocks
In order to decode past environment using
sedimentary rock we need to pay particular
attention to:
textures
composition
fossils (if present)
thickness
relationships to other rocks
Preliminary interpretations in the field
- redish rocks may have been deposited on land
- whereas greenish rocks are more typical of marine
-
deposits
(caution: exceptions are numerous)
Detrital Sedimentary Rocks
Detrital sedimentary rocks are identified based on
mineral composition and texture.
Mineral Composition:
• Common minerals include quartzes, feldspars, and clay minerals
Mineral composition is only used to determine source rocks it does
not indicate the manner of transport or deposition
For example:
• Quartz sand may have been deposited
• in a river system
• on a beach or
• in sand dunes
Detrital Sedimentary Rocks
Texture refers to the grain size of the clasts that
form the sedimentary rock
Sediment Type
Sediment Description
blocks
Angular particles greater than 256 mm in size.
boulders
Rounded particles greater than 256 mm in size.
cobbles
Rounded particles between 64 and 256 mm in size.
pebbles
Rounded particles between 2 and 64 mm in size.
sand
Particles between .0625 and 2 mm in size.
silt
Particles between 2µm and .0625 mm in size.
clay
Particles less than 2µm in size.
Texture as an environmental indicator
Grain size gives some indication of the energy
conditions during transport and deposition
•
The larger the clast the greater the energy
needed to transport that clast.
• Sand and larger clasts require vigorous currents
(fast water= high energy)
• Silt and clay are transported by very weak
currents and and therefore only accumulate
under low-energy conditions such as in lakes
and lagoons
Textural characteristics
Sorting refers to the variation in
size of particles making up
sediment or sedimentary rocks
It results from processes that
selectively transport and deposit
sediments of particular sizes
If the size range is not very great,
the sediment or rock is well sorted
If they have a wide range of sizes,
they are poorly sorted
Other textural characteristics
Rounding is the degree to which detrital particles have
their sharp corners and edges warn away by abrasion
Gravel in transport is rounded very quickly as the particles
collide with one another
Sand becomes rounded with considerably more transport
Chemical Sedimentary Rocks
Divided into two categories:
Chemical sedimentary rocks are produced through
precipitation of chemicals in water environments as a result of
changing environmental conditions ie. temperature, pH,
oxidizing conditions
Biochemical sedimentary rocks are produced as a direct
result of life processes
Chemical Sedimentary Rocks
Composition of chemical sedimentary rocks is more
useful in revealing environmental information
Limestone is deposited in warm, shallow seas
although a small amount also originates in lakes
Evaporites such as rock salt and rock gypsum
indicate arid environments
where evaporation rates were high
Coal originates in swamps and bogs on land
Sedimentary Structures
Sedimentary structures are visible features that formed at
the time of deposition or shortly thereafter
• Represent manifestations of the physical and biological processes
that operated in depositional environments
• Structures seen in present-day environments or produced in
experiments help provide information about depositional
environments of rocks with similar structures
Sedimentary Structures
Sedimentary rocks generally have bedding or
stratification (they are deposited in layers)
Beds are layers that are thicker
than 1 cm common in rocks
with coarser grains
laminations are layers
less than 1 cm thick are
common in mudrocks
Sedimentary Structures
Graded bedding occurs when sediment loaded currents (turbidity
currents) experience a relatively quick drop in velocity.
what will happen to the
sediment as the
current slows?
Will all sediment sizes
behave the same way?
Sedimentary Structures
Cross-bedding forms when layers come to rest at an
angle to the surface upon which they accumulate as on
the downwind side of a sand dune
These herringbone
Tabular crossbeds:
crossbeds
have
foresetis
Trough
cross
bedding
shallow
dips
as in this
beds
that
dip
in both
produced
by such
the
image are migration
indicative of
directions.
downflow
of
currents
enter
still
bodies of
What
may
have
caused
lunate
dunes
in
both
water (Gilbertand
Delta). Steep
this?
subaqueous
foreset beds
are indicative
subaerial
environments
of eolian (wind) deposition
Sedimentary Structures
Small-scale cross bedding creates ripple marks. These
structures are common on bedding planes, especially in
sandstone
•
Current ripple marks form in
response to water or wind
currents flowing in one
direction
• have asymmetric profiles allowing
geologists to determine
paleocurrent directions
•
Wave-formed ripple marks
result from the to-and-fro
motion of waves
• symmetrical profile
Sedimentary Structures
When clay-rich sediments dry, they shrink and crack
into polygonal patterns bounded by fractures called
mud cracks
Mud cracks require
wetting and drying to
form so in what
environments would
you expect to find
them?
Sedimentary Structures
Biogenic sedimentary structures include
• tracks
• burrows
• trails
These structures are also called trace fossils
Extensive burrowing by organisms is called bioturbation
may alter sediments so thoroughly that other structures
are disrupted or destroyed
Depositional Environments
A depositional environment is anywhere sediment
accumulates
Three broad areas of deposition include
•continental
•transitional
•marine
each of these areas has several specific environments
with their own physical, chemical, and biological processes