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Sedimentary Rocks
What is a sedimentary rock?
• Sedimentary rocks result from mechanical
and chemical weathering
• Comprise ~ 5% of Earth’s upper crust
• About 75% of exposed rocks
• Contain evidence of past environments
• Record how sediment is transported
• Often contain fossils
What is the economic importance of
sedimentary rocks?
• They are important for economic reasons
Remember this
because they contain
when we talk about
correlation. Note
• Coal
how beds pinch out
or are offset by
• Petroleum and natural gas
faults
• Iron, aluminum, uranium and manganese
• Geologists use them to read Earth’s history
How does sediment become rock?
• Diagenesis – chemical and physical changes that take
place after sediments are deposited
• Diagenesis varies with composition
Chesapeake Bay from Skylab
False color image
www.nasa.gov
Turning sediment into rock
– Diagenesis includes:
–Recrystallization – growth of stable
minerals from less stable ones
–Lithification – loose sediment is
transformed into solid rock by
compaction and cementation
Natural cements: calcite, silica, and iron
oxide. Formed from ions in solution in
water.
Lithification
• Compaction: As more sediments are piled on
top, compaction drives out the excess water.
• Cementation: Precipitation of chemicals
dissolved in water binds grains of a sediment
together.
• Remember where the dissolved chemicals
come from?
Types of sedimentary rocks
• Chemical rocks – sediment from
ions that were once in solution
• Detrital rocks –sediment
transported as solid particles
Detrital sedimentary rocks
• Rocks made of grains
• Constituents of detrital rocks can
include
• Clay minerals
• Quartz
• Feldspars
• Micas
• Particle size is used to distinguish
among the various types of detrital
rocks
Ions weather out of rock,
are transported by
groundwater to sediment
layers below
Sediment grains moved to ocean by streams
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Ions transported
to lake or
Water enters pore
ocean
spaces between
sediment grains
Ion-rich
groundwater
Dissolved ions precipitate
to form cement between
sediment grains
Detrital sedimentary rocks
• Mudrocks: less than .06 mm
–1. Mud: small particles easily kept in
suspension
– Settles in quiet water
– Includes Shale: mud-sized particles <.004
mm deposited in thin bedding layers
called laminae
Most common sedimentary rock
2. Larger mudrock grains called silts
silt-sized particles .004-.06 mm
Gritty grains can be felt
Detrital sedimentary rocks
• Sandstone
–Made of sand-sized particles .064 – 2
mm
–Forms in a variety of environments
–Sorting, angularity and composition
of grains can be used to interpret the
rock’s history
–Quartz is the predominant mineral
(due to its durable nature)
Classifying Sandstones
Plagioclase grain
Photomicrograph of quartz rich sandstone (Arenite)
Grains subangular to subrounded, sandstone is poorly sorted
Making thin sections
Types of Sandstone
• Quartz Arenite >90% quartz grains
– Beach and dune deposits
• Arkoses >25% feldspar, angular, poor sort.
– Transform boundaries; exposed granites
– Any felsic rock eroded, not transported far
• Graywackes Quartz, feldspar, volcanics
– Port sorted, angular
– Erosion of Island Arcs
– Rift Valley Sediments
Detrital
sedimentary
rocks
• Conglomerate and breccia
–Both composed of particles > 2mm in
diameter
–Conglomerate consists largely of rounded
clasts. Rounded pebbles in high velocity areas
–Breccia is composed of large angular
particles Breccia is made of shattered rock that accumulates at the base of a cliff
Energy
• Coarse sediments are deposited in high
energy (fast water) environments such as
under breaking waves at the beach, or in
the beds of fast streams.
• Fine sediments are deposited in low
energy environments, e.g. the slow water
of deep lagoons, the abyssal plain, etc.
Outcrop of conglomerate
with cobble-sized clasts
interbedded with sandstone
Conglomerates are fastwater sediments
“High Energy”
K.E. = 1/2mv2
In fast water, smaller sizes
swept away
Chemical sedimentary rocks
• Precipitated material once in solution
• Precipitation of material occurs two
ways:
• Inorganic processes: the minerals
precipitate out of water
• Organic processes: animals and plants
precipitate the minerals to use as shells
or skeletons
http://www.ucmp.berkeley.edu/collections/micro.html
• Common chemical sedimentary rocks
• Limestone
–Most abundant chemical rock
–Made of the mineral calcite CaCO3
–Marine biochemical limestones form
as coral reefs, coquina (broken
shells), and chalk (microscopic
organisms)
–Inorganic limestones include
travertine (caves) and oolitic
limestone (Bahamas)
http://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpg
• Common chemical sedimentary
rocks
•Evaporites
–Evaporation triggers
deposition of chemical
precipitates
–Examples include rock salt
and rock gypsum
Chalk Outcrops in SE USA
Chalk
Hand Specimen
Inorganic Chemical Sediments
Ooids under microscope
Oolitic Limestone - Bahama Shoals
Oolitic Limestone - Hand Specimen
• Other chemical sedimentary rocks
• Chert
– Made of microcrystalline quartz
– Usually deposited as siliceous ooze in deep
oceans (can be diatomaceous)
chert
Diatomaceous chert
• Other chemical sedimentary rocks
– Dolostone (made of mineral Dolomite)
– Like Calcite, but some Ca is replaced by Mg
The Dolomites, sediments thrust up when the Alps formed
Evaporation
How to make Dolomite
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Limestone
Seawater enriched
in Mg2
Mg2 -rich seawater circulates
through porous limestone
Dolostone
Mg2 replaces some of the
Ca2 in limestone
Chemical Sediments: Coal
Sedimentary environment determines roundness sorting, mineral diversity
Character of detrital sediments depends on time,
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distance, and energy. For example, in streams:
Particles are large and
irregular, and consist of
a variety of lithologies,
including the least
resistant.
Particles are mid-sized
and of intermediate
sphericity, and include
resistant and nonresistant
lithologies.
HIGHLANDS
LOWLANDS
Particles are small and
nearly spherical, and
consist mainly of the
most resistant lithologies,
such as quartz.
NEAR-COASTAL
• Sedimentary Facies
• Different sediments accumulate
next to each other at same time
• Each unit (called a facies) possesses
a distinctive characteristics
reflecting the conditions in a
particular environment
• The merging of adjacent facies
tends to be a gradual transition
Some Facies in an oversimplified drawing
Abyssal Ooze
Stillwater muds
Nearshore sands
A picture glossary of sedimentary
Turbidite: underwater landslide
= graded bedding
environments
Sedimentary structures
• Tell us something about past
environments
• Types of sedimentary structures
•Strata, or beds (most
characteristic of sedimentary
rocks)
-bedding planes that separate
strata caused by variation in
deposition
Strata- Bedding Planes
Fine Scale Bedding- Lamellae
Graded bedding
Floods change the local
conditions
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Fine-grained sediment
On floodplain
Older sediment
1
Pre-flood
Bounders on
bottom, sands and
muds suspended
Flood water
Erosion of uppermost
fine-grained sediment
2
Flood stage
Waning flow
Fine-grained above
Coarse-grained
below
Bedding plane
3
Post-flood
Graded Beds – grains fine upward
Note: Beds were tilted from horizontal after deposition
Recognizable Sedimentary Structures
• Ripples
• Irregularities in bottom sediment lead to
ripples
• Asymmetric types indicate flow
direction.
• Symmetric types formed in tidal areas
Slabs of eroding sandstone with ripple marks
Cross Beds are ripples in cross section
• Irregularities lead to ripples, dunes,
sand bars.
• In cross section these look like lines
at an angle to the horizontal – “cross
beds”
• Ripples can indicate direction of air or
water flow if asymmetrical, a tidal
environment if symmetrical. Size and
shape indicate fluid velocity.
Cross bedding in Sand Dune deposits
Navaho Sandstone
Sandstone deposited
in ancient sand dunes
Frosted Grains, well sorted
Mud Cracks: clay layer shrinks during drying, curls
upward; cracks fill next flood. Useful for right-side up
Sedimentary Environments
• Sediments are formed in many different
environments
• Each have characteristic appearance
today, features that allow them to be
recognized in the geologic record
Fresh Water Facies
Streams (includes big Rivers), with
floodplains and levees, called fluviatile. The
Point Bar Sequence is typical for meandering
streams. Cutoffs generate Oxbow deposits.
•
High gradient streams with high sediment
load are Braided.
• Lake deposits called lacustrine, generally
still waters, often varved deposits if winters cold
http://hays.outcrop.org/gallery/rivers/arid_meander?full=1
Point-bar Sequence:
Meandering
Stream
Deposits Associated with
Meandering Streams
OxBow
Floodplain
http://hays.outcrop.org/gallery/rivers/arid_meander?full=1
Point Bar Sequence
Fines of Floodplain
Crossbeds of Bar
Gravel of bed
Erosion
Terms for Marine (i.e. Ocean) Environments
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and some characteristic
sediment facies
Continental
shelf
Continental
slope
Abyssal
Plain
Submarine
volcanoes
Facies changes due to rising sea level - water getting deeper everywhere
River
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Direction
of migration
of shoreline, and landward
shift of sedimentary facies
Time B
Shoreline at
time B
Time A
Shoreline at
time A
Shallow
marine
Beach
Sea level
rising
Deep
marine
Shallow
marine
River
Beach
Comparison of sediments deposited
Deep
marine
Shallow
marine
Deposited
at time A
Deposited
at time B
REMEMBER: the facies
follow the shoreline
Fossils are traces of prehistoric life generally
preserved in sedimentary rock
Dinosaur footprint in mudstone
End of Sedimentary
Rocks