Download Earth Materials: Sedimentary Rocks

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Earth Materials: Sedimentary Rocks
Sediments and Sedimentary rocks
Why study sediments and sedimentary rocks?
• What we can learn from sedimentary rocks
• Most of the earth s surface is covered with sediments.
• They record past environmental conditions and much of the
history of the earth.
• They contain fossil evidence of past life.
• Some natural resources are of sedimentary origin or are found
primarily in sedimentary rocks: i.e. coal, oil, natural gas, etc.
• Types of sediment
• Classification of sedimentary rocks
• Sedimentary processes:
weathering, erosion, transport, deposition, etc.
• Depositional environments & sedimentary structures
Three main categories of sediments and sedimentary rocks:
• Clastic (also called detrital)
• Chemical
• Biochemical
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Rock composed of
clastic sediment
Rock composed of
chemical sediment
Sediments and Sedimentary rocks
Classification of clastic sedimentary rocks:
Texture
Maximum grain size:
Sorting:
fine, medium, coarse
well-sorted, poorly-sorted
Grain shape:
rounded, angular
Composition
what the grains are composed of:
Chapter 7
Understanding
Earth
Rock containing
biochemical sediment
i.e. quartz, feldspar, clay, rock fragments, etc.
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Well-sorted,
angular, quartz
sand (magnified)
Fig 7.2 Understanding Earth
Poorly sorted sediment
(mixture of sand and gravel)
with rounded fragments
Geology - Chernicoff
Sediments and Sedimentary rocks
Classification of clastic sedimentary rocks by texture:
Texture
Rock Type
coarse, rounded
coarse, angular
Conglomerate
Breccia
medium
Sandstone
fine
Siltstone, Mudstone,
Claystone, or Shale
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The four major types of sandstones:
an example of classification by grain composition
Classification of chemical and biochemical
sedimentary rocks by composition
Composition
Rock Type
calcite (may also contain some mud)
limestone (several types)
plant remains
coal or peat
silica
chert
halite
rock salt
gypsum
rock gypsum
Fig 7.16 Understanding Earth
Fig 7.19 Understanding Earth
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Rock salt (halite) - a chemical
sedimentary rock formed by evaporation
of water
Three examples of limestone
Chalk - a type of
fossil-bearing
limestone
Fossiliferous
limestone
Fig 7.19 Understanding Earth
Rock gypsum a chemical
sedimentary rock
formed by
evaporation of
water
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Chert - a sedimentary
rock formed from
silica sediment. Chert
may be chemical or
biochemical in origin
Fig 7.19 Understanding Earth
Geology - Chernicoff
Fig 7.19 Understanding Earth
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Sedimentary processes:
Weathering
•physical & chemical
•rates of weathering
•importance of minerals and climate
Erosion and Transportation
Deposition
Relative
abundance of
major sedimentary
rock types
Burial and compaction
Diagenesis and lithification
Fig 7.14 Understanding Earth
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The Grand Canyon, a landscape shaped by weathering and erosion
and the source of a huge amount of sediment
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Processes involved in the formation of sedimentary rocks:
Weathering, Erosion, Transportation, Deposition, Burial, and Diagenesis
Fig 7.1 Understanding Earth
S.
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Weathering and Erosion
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Weathering and Erosion
Weathering
- includes all processes which break down rocks at the
earth s surface
- there are two general types of weathering:
• physical (mechanical) weathering
Physical (mechanical) weathering
- breaks rocks into smaller pieces
• chemical weathering
Examples:
Erosion
- includes processes which remove weathered material from
its source
• abrasion by particles carried by wind, water, or ice
• fractures created or widened by changes in pressure
or temperature or by root action
- water, wind, and ice, for example, can erode weathered
material
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Abrasion by moving ice
Glaciers typically have many
fragments of rock embedded in
the ice. When the ice slides
along, these rock fragments grind
against the bedrock beneath the
glacier.
Breaking rocks through
temperature changes,
freezing and thawing
Freezing and thawing
widens fractures over time.
Pieces can then fall off the
cliff and move down the
slope.
Animation:
Frost Wedging
Abrasion by flowing water
Water typically carries sand and
gravel. These particles bump into
each other and larger rocks wearing
them down over time.
Mount Rainier andesite lava flow
at Burroughs Mountain, Mt.
Rainier National Park
Photo by S. Kuehn
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Rock expansion & cracking due to pressure changes
Exfoliation (sheeting) occurs as rocks expand when brought to the surface from deep in the crust where
pressures are much higher than at the surface.
Weathering and Erosion
Chemical weathering
- breakdown of minerals by chemical reactions to
form dissolved material and/or other minerals
such as clay
Examples:
Dissolution - dissolves in water
Oxidation - oxygen is added
Hydration - water is added
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Photos: exfoliating granite in Yosemite National Park Fig 6.14 Understanding Earth (right) & S. Kuehn (left)
Animation:
Uplift and erosion of a volcanic arc
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The brown stain on the surface of this sandstone is called desert
varnish. It is produced by chemical weathering and evaporation.
S. Kuehn
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Different rates of weathering of slate and marble. Marble is composed of calcite
which dissolves easily in acidic water. The faster weathering of the marble headstone
is apparent in the blurring of the text. Fig 6.1 Understanding Earth
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Rates of Weathering
Influence of
Mineral Type
type of mineral or rock – chemical stability and hardness
cracks and surface area
climate – wet vs. dry, warm vs. cold
acids
Understanding Earth
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Effect of Time
Weathering along natural
fractures in rocks
Photos of weathered granite at
Yosemite National Park
Understanding Earth
Time
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Effects of sediment transport:
Sediments and Sedimentary rocks
Grain shape related to the duration of transport
Grains transported by water or wind become rounded over time
Depositional environments:
-where
- what / how
Rounded grains
Angular grains
Fig 7.3 Understanding Earth
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Some common sedimentary environments
Interpreting the depositional environment
Grain size:
Sorting:
fine, medium, coarse
uniform size or mixed sizes
Sedimentary structures:
- sedimentary layering
– expressed as changes in sediment size and/or type
- ripples
- cross-bedding
- mud cracks
Fig 7.5 Understanding Earth
- burrows and tracks (bioturbation, trace fossils)
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Modern ripples on a beach
(below) and ancient ripples
preserved in sandstone (left)
Understanding Earth
Sedimentary layers
Ch 7 Understanding Earth
Cross-bedding in sandstone
These sediments were deposited in desert dunes.
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Mud cracks: modern (left) and ancient (right)
Understanding Earth
Understanding Earth
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Sediments and Sedimentary rocks
From sediment to rock, changes after deposition:
- dissolution
- re-crystallization
- compaction
- formation of new minerals
- cementation / lithification
Fig 7.13 Understanding Earth
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