Sediments and Sedimentary Rocks

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Sediments and Sedimentary Rocks

• Types of Sediments
• Lithification and Diagenesis
• Common types of Sed. Rock
• Environmental Clues in Sed. Rocks

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Sedimentary Rocks

• Controlled by the processes that occur at surface
• Water
• Wind
• Ice
• gravity

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Sedimentary Rocks

• Records conditions at the surface at the time of
  deposition

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River Bluffs
450 million years ago
St. Peter Sandstone Pure quartz sand Beach deposit
Platteville Limestone Deep water deposit
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Sedimentary Rocks

• Contain the Fossil Record

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Sedimentary Rocks

• Contains all the
• Fossil Fuel Supplies

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Sedimentary Rocks
weathering
Small rock fragments
Clastic sediments
Na, Cl-, ions
Chemical sediments
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Sedimentary Rocks
weathering
Small rock fragments
Transport Wind, water Ice, gravity
Na, Cl-, ions
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Sedimentary Rocks
weathering
Small rock fragments
Transport Wind, water Ice, gravity
Na, Cl-, ions
Deposition of sediment
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Sedimentary Rocks
weathering
Small rock fragments
Transport Wind, water Ice, gravity
Na, Cl-, ions
Deposition of sediment
Burial Lithification Sedimentary Rock
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Turning Sediment into Rock

• Diagenesis
• All chemical, physical, and biological changes
  after sediment is deposited
• Upper few kilometers of Earths Crust
• Temperatures lt 100 C

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Turning Sediment into Rock
Lithification processes? transform sediments
into rock
Cement agent
Pore space
Compaction
Cementation
After deposition
Precipitation- Crystals precipitate directly from
solution.
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• Stratification (Strata)

Distinct layering Beds
Sedimentary Rock
Bed 1
Bed 2
Different thicknesses Color, other
characteristics
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Figure 7.1
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• All sediment is derived from preexisting rocks
  and can be classified as
• Clastic Sediments
• Form from the combination of rock fragments.
• Chemical Sediments
• Comprised of mineral crystals precipitated from
  solution.
• Biogenic Sediments
• Composed of remains of dead organisms, seashells,
  plant fragments, and carbon.

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Clastic Sediments and Clastic Sedimentary Rocks
A. Sediments
B. Sedimentary Rocks
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Well sorted and well rounded Features of Clastic
Sediments
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Fluctuations in Energy of Transport
Sorting
Distance of transport
Shape Roundness
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Varieties of layerings

• Horizontal (rhythmic)

Sediment Cycles (Varves)
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Varieties of layerings

• Cross-bedding

Direction of wind or current
Cross beds
Particles drop out of flowing water or wind
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Varieties of layerings

• Graded Bedding
• Particles are sorted according to size

fine? top
Mudflow from Mt. St. Helens
Coarse? bottom
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Varieties of layerings

• Non sorted Sediments

Tillite, Ancient glacial deposit (S. Afrcia)
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Chemical Sediments

• Form by precipitation of minerals from solution
  in water.
• Inorganic reactions in the water.

Evaporites The most important salts that
precipitate from seawater are halite(rock salt
NaCl) gypsum (CaSO4.2H2O).
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Carbonate shelf in the Bahamas
Inorganic precipitation of Limestone (CaCO3,
calcite)
S. C. Porter
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Chemical Sediment

• Banded Iron Formation

Layers of iron oxide and SiO2 Some of the
Earths most important iron concentrations Formed
2.5-2.0 billion years ago
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Iron Formations and the Chemistry of the Ancient
Atmosphere
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Banded Iron Formations are a distinctive type of
rock often found in old sedimentary rocks. The
structures consist of repeated thin layers of
iron oxides, either magnetite or hematite, with
bands of shale and chert. Some of the oldest
known rock formations dated around 3,000,000,000
years before present, 3000MA, include banded iron
layers, and the banded layers are a common
feature in sediments for much of the Earth's
early history. Banded iron beds are less common
after 1800MA although some are known that are
much younger. The conventional concept is that
the banded iron layers are the result of oxygen
released by photosynthetic cyanobacteria,
combining with dissolved iron in Earth's oceans
to form insoluble iron oxides. The banding is
assumed to result from cyclic peaks in oxygen
production. It is unclear whether these were
seasonal or followed some other cycle. It is
assumed that initially the Earth started out with
vast amounts of iron dissolved in the world's
seas. Eventually, as photosynthetic organisms
pumped out oxygen, all the available iron in the
Earth's oceans was precipitated out as iron
oxides. The atmosphere became oxygenated.
Water flowing over iron rich beds Until fairly
recently, it was assumed that the rare later
banded iron deposits represent unusual conditions
where oxygen was depleted locally and iron rich
waters could form then come into contact with
oxygenated water. An alternate explaination of
these later rare deposits is undergoing much
research as part of the Snowball Earth hypothesis
wherein it is believed that an early equatorial
supercontinent (Rodinia) was totally covered in
an ice age (implying the whole planet was frozen
at the surface to a depth of several kilometers)
which corresponds to evidence that the earth's
free oxygen may have been nearly or totally
depleted during a severe ice age circa 750 to 580
million years ago (mya) (See Cryogenian period,
from 800 million years ago (mya, boundary defined
chronometrically) to approximately 635 mya) prior
to the Cambrian Explosion wherein the earliest
multicellular lifeforms appear. Alternatively,
some geochemists suggest that BIFs could form by
direct oxydation of iron by autotrophic
(non-photosynthetic) microbes. The total amount
of oxygen locked up in the banded iron beds is
estimated to be perhaps 20 times the volume of
oxygen present in the modern atmosphere. Banded
iron beds are an important commercial source of
iron ore.
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Biogenic Sedimentary Rocks
Sediment produced by the physiological activity
of organisms
Calcium Carbonate (CaCO3)
Foraminifera
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Biogenic Sedimentary Rocks

• Limestone (CaCO3)
• The most important biogenic rock.
• limestone accounts for a major proportion of the
  carbon dioxide CO2 stored in the Earth crust.

If all the CO2 in limestone was released into
atmosphere ?Like Venus

• Venus
• CO2-rich atmosphere
• greenhouse gas
• surface T about
• 400oC (750oF)
• above value
• without CO2

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Green River Oil Shale,Colorado
Biogenic Sedimentary Rocks Remains of plant
material Coal Oil shale
Fig. 6.17
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Environmental Clues inSedimentary Rocks
Grains size shape composition
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Environmental Clues inSedimentary Rocks
Grains size shape composition
BB-sized spheres
Mars
???
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Environmental Clues inSedimentary Rocks
Features on Bedding planes
Footprints
Mudcracks
Ripple marks
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Fossils
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Depositional Environments
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Sedimentary Facies Reflect Depositional
Environments
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End
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Turning sediments into rocks

• Lithification occurs by
• burial
• when additional sediment accumulates on top
• compaction
• reduction of the amount of pore space between
  particles
• because of the weight of overlying sediment
• cementation
• precipitation of minerals within pores
• binds sediment together
• calcium carbonate (CaCO3) cement is common
• silica (SiO2) cement is common
• iron oxide (Fe2O3) cement is less common

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Turning Sediment into Rock

• Lithification processes? transform sediments
  into rock
• Compaction sediments get compressed and
  compacted
• Cementation Small crystals precipitate and act
  as glue that holds the rock together
• Precipitation- Crystals precipitate directly from
  solution.

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Size
Rounding
Sphericity
Sorting