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Earth Materials: Sedimentary Rocks
• What we can learn from sedimentary rocks
• Types of sediment
• Classification of sedimentary rocks
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• Sedimentary processes: weathering, erosion, transport, deposition, etc.
• Depositional environments & sedimentary structures
Sediments and Sedimentary rocks
Why study sediments and 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.
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• Some natural resources are of sedimentary origin or are foundprimarily in sedimentary rocks: i.e. coal, oil, natural gas, etc.
Three main categories of sediments and sedimentary rocks:• Clastic (also called detrital)• Chemical• Biochemical
Rock composed of clastic sediment
Rock composed of chemical sediment
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Chapter 7 Understanding Earth
Rock containing biochemical sediment
Sediments and Sedimentary rocks
Classification of clastic sedimentary rocks:
Texture
Maximum grain size: fine, medium, coarse
S ti ll t d l t d
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Sorting: well-sorted, poorly-sorted
Grain shape: rounded, angular
Composition
what the grains are composed of:
i.e. quartz, feldspar, clay, rock fragments, etc.
Well-sorted, angular, quartz sand (magnified) Fig 7.2 Understanding Earth
Poorly sorted sediment (mixture of sand and gravel) with rounded fragmentsGeology - Chernicoff
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Sediments and Sedimentary rocks
Classification of clastic sedimentary rocks by texture:
Texture Rock Type
coarse rounded Conglomerate
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coarse, rounded Conglomerate
coarse, angular 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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Fig 7.16 Understanding Earth
Classification of chemical and biochemical sedimentary rocks by composition
Composition Rock Type
calcite (may also contain some mud) limestone (several types)
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plant remains coal or peat
silica chert
halite rock salt
gypsum rock gypsum
Three examples of limestone
Chalk - a type of fossil-bearing limestone
Fossiliferous limestone
Fig 7.19 Understanding Earth 9
Geology - Chernicoff
Rock salt (halite) - a chemical sedimentary rock formed by evaporation of water
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
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Fig 7.19 Understanding Earth
Fig 7.19 Understanding Earth
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Relative abundance of major sedimentary rock typesFig 7.14 Understanding Earth
Sedimentary processes:
Weathering•physical & chemical
•rates of weathering
•importance of minerals and climate
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Erosion and Transportation
Deposition
Burial and compaction
Diagenesis and lithification
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Processes involved in the formation of sedimentary rocks:Weathering, Erosion, Transportation, Deposition, Burial, and DiagenesisFig 7.1 Understanding Earth
The Grand Canyon, a landscape shaped by weathering and erosionand the source of a huge amount of sediment
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S.
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
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p y ( ) g
• chemical weathering
Erosion
- includes processes which remove weathered material from its source
- water, wind, and ice, for example, can erode weathered material
Weathering and Erosion
Physical (mechanical) weathering
- breaks rocks into smaller pieces
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Examples:
• abrasion by particles carried by wind, water, or ice
• fractures created or widened by changes in pressureor temperature or by root action
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Abrasion by glaciersGlaciers 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
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Abrasion by flowing waterWater 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 caused by growing rootsPlant roots can grow into tiny fractures in rocks. As the roots grow over time, the fractures become wider.Fig 6.12 Understanding Earth
Mechanical weathering by frost actionFreezing and thawing widens fractures over time. Pieces can then fall off the cliff and move down the slope.
Mount Rainier andesite
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lava flow at Burroughs Mountain, Mt. Rainier National ParkS. Kuehn
Animation:
Frost Wedging
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.
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Animation:
Uplift and erosion of a volcanic arc
.Photos: exfoliating granite in Yosemite National Park -
Fig 6.14 Understanding Earth (right) & S. Kuehn (left)
Weathering and Erosion
Chemical weathering
- breakdown of minerals by chemical reactions toform dissolved material and/or other minerals
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such as clay
Examples:
Dissolution - dissolves in water
Oxidation - oxygen is added
Hydration - water is added
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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
Rates of Weathering and Erosion
type of mineral or rock
surface area
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climate
acids
time
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Understanding Earth
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Understanding Earth
Time
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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
Weathering along natural fractures in rocks
Photos of weathered granite at Yosemite National Park
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Effects of sediment transport: Grain shape related to the duration of transport
Grains transported by water or wind become rounded over time
Angular grains Rounded grainsFig 7.3 Understanding Earth
Sediments and Sedimentary rocks
Depositional environments:
-where
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- what / how
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Some common sedimentary environments31
Fig 7.5 Understanding Earth
Interpreting the depositional environment
Grain size: fine, medium, coarse
Sorting: uniform size or mixed sizes
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Sedimentary structures:
- sedimentary layering – expressed as changes in sediment size and/or type
- ripples
- cross-bedding
- mud cracks
- burrows and tracks (bioturbation, trace fossils)
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Sedimentary layersCh 7 Understanding Earth
Modern ripples on a beach (below) and ancient ripples preserved in sandstone (left)Understanding Earth
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Cross-bedding in sandstoneThese sediments were deposited in desert dunes.Understanding Earth
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Mud cracks: modern (left) and ancient (right)Understanding Earth
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