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:
S ti
Sorting:
Grain shape:
Chapter 7
Understanding
Earth
fine, medium, coarse
well-sorted,
ll
t d poorly-sorted
l
t d
rounded, angular
Composition
what the grains are composed of:
i.e. quartz, feldspar, clay, rock fragments, etc.
Rock containing
biochemical sediment
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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
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
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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
Three examples of limestone
Chalk - a type of
fossil-bearing
limestone
Fossiliferous
limestone
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sedimentary rock formed by evaporation
of water
Fig 7.19 Understanding Earth
Rock gypsum a chemical
sedimentary rock
formed by
evaporation of
water
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
E i andd Transportation
Erosion
T
t ti
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
Physical (mechanical) weathering
- there are two general types of weathering:
y
((mechanical)) weatheringg
• pphysical
- 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 glaciers
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. This wears down underlying rock and creates smoothed and
grooved surfaces. S. Kuehn
Abrasion by flowing water
Water typically carries sand and gravel.
These particles bump into each other and
larger rocks wearing them down over time.
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Mechanical
weathering by frost
action
Freezing and thawing
widens fractures over
time. Pieces can then fall
off the cliff and move
down the slope.
Mount Rainier andesite
lava flow at Burroughs
Mountain, Mt. Rainier
National Park
Mechanical weathering
caused by growing roots
S. Kuehn
Plant roots can grow into tiny
fractures in rocks. As the roots
grow over time, the fractures
become wider.
Animation:
Frost Wedging
Fig 6.12 Understanding Earth
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Mechanical weathering by exfoliation (sheeting)
Exfoliation 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
.
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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Rates of Weathering and Erosion
type of mineral or rock
surface area
climate
acids
The brown stain on the surface of this sandstone is called desert
varnish. It is produced by chemical weathering and evaporation.
time
S. Kuehn
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Understanding Earth
Understanding Earth
Time
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Weathering along natural
fractures in rocks
Photos of weathered granite at
Yosemite National Park
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
Effects of sediment transport:
Grain shape related to the duration of transport
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Sediments and Sedimentary rocks
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
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Interpreting the depositional environment
Grain size:
Sorting:
fine, medium, coarse
uniform size or mixed sizes
S di
Sedimentary
t
structures:
t t
- sedimentary layering
- ripples
- cross-bedding
- mud cracks
- burrows and tracks
Fig 7.5 Understanding Earth
– expressed as changes in sediment size and/or type
(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
Fig 7.13 Understanding Earth
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