EAE 07 Sedimentery Rocks (11) Flashcards
What is a Sedimentary Rock?
- Any rock that has formed out of fragments of pre existing rocks or minerals, or has precipitated from water
- Many types, depending on:
- what they are made of
- where they formed
- how they formed
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How do you make a sediment?
The basic steps in the formation of sedimentary rocks:
* Weathering * Erosion * Transportation * Deposition * Lithification weathering
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What types of weathering are there?
- Physical Weathering
- Chemical Weathering
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How does physical weathering occur?
“mechanical” weathering breaks rocks apart
Produces detritus (fragments of pre existing rocks)
* "patterned ground" caused by freeze thaw * Frost wedging (e.g. Antarctica)
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How does Chemical weathering occur?
Chemical reactions change or destroy minerals within a rock, generally when it is in contact with water or air
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What is erosion?
Physical processes that loosen rock or regolith, separates it from the substrate, and carries it away
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What are the agents of erosion?
- Heat & cold - expansion and contraction break surfaces
- Wind - erodes surfaces and removes fines
- Water - erodes surfaces and removes blocks and detritus
- Ice - erodes surfaces and removes everything in its path
- Gravity - causes things to fall
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What is the grain size of boulders?
>256 mm
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What is the grain size of cobbles?
64~256 mm
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What is the grain size of pebbles?
2~64 mm
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What is the grain size of sand?
2 ~ ¹⁄₁₆ mm
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What is the grain size of silt?
¹⁄₁₆ ~ ¹⁄₂₅₆ mm
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What is the grain size of clay?
< ¹⁄₂₅₆ mm
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What impacts the transport of sediment?
Grain shape
* angularity or roundness
Grain size
* finer grain means it can travel farther from its source
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What does transportation of sediment influence?
Causes sorting of grains →
- Uniformity of grain sizes
“Maturity” of the sediment →
- Removal of easily weatherable sediments over time
- Think of Bowen’s Reaction Series
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How does the wind transport sediment?
Generally transports only fine particles (sand, silt, clay)
Produces well sorted deposits = “aeolian”
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How does the water transport sediment?
- Transports all sizes of material
- Produces a range of deposits, generally well sorted
* Sorting improves with distance transported * For rivers and streams it "**fluvial**" transport
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How does the ice transport sediment?
Ice = Glaciers
- Transport all sizes of material
- Produce poorly sorted deposits “glacial till”
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How does gravity transport sediment?
- Transports all sizes of material
- Produces poorly sorted deposits “slumps” or “slides”
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What are Clastic sediments?
- Fragments of pre existing rocks
- Deposited from: Wind, Water, Ice, Gravity
- When the transport system is no longer able to carry them away
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What are Chemical sediments?
- Form by over saturation of minerals in water = Chert
- Form by evaporation of water = “Evaporites”, e.g., rock salt
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What are depositional environments?
- Fluvial (stream)
- Deltaic
- Lacustrine (lake)
- Desert
- Beach
- Coastal swamp
- Coral reef
- Continental shelf
- Ocean basin … etc.
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What are the methods of lithification?
- Compaction = Grains are compressed and pore space is reduced
- Cementation = Growth of new minerals binds the grains together
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What are the types of Sedimentary Rocks?
- Biochemical
- Organic
- Chemical
- Clastic/detrital
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Describe
Biochemical Sedimentary Rocks
Composed of skeletal or shell fragments
- Limestone
- Chalk
- Chert
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Describe
Organic Sedimentary Rocks
Made up of carbon rich plant material
- Coal = ancient swamp
- Oil shale = ancient lake or ocean basin
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Describe
Chemical Sedimentary Rocks
Composed of minerals that precipitate (crystallize) from water solutions i.e. salt lakes, caves
- travertine
- rock salt
- gypsum
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Describe
Clastic Sedimentary Rocks
Composed of grains of pre existing rocks
- Shale
- Siltstone
- Sandstone
- Conglomerate
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Describe
Shale
Shale is made of clay
The grains are too small to see ∴ ancient lake or ocean
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Describe
Claystone (mudstone)
Like shale, the clay size grains are too fine to see
Claystone has a massive habit, rather than layered ∴ river or swamp
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Describe
Siltstone
Similar to shale, the grains are too fine to see, but it has a gritty texture ∴ river or swamp
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Describe
Sandstone
Sand size fragments (grains) ∴ beach, desert, river
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Describe
Breccia
Angular fragments of pre existing rocks ∴ mountain river or stream
Some fragments (grains) are pebble, cobble, or boulder size
“Breccia” means broken rock
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Describe
Conglomerate
Rounded fragments of pre existing rocks ∴ plains river or stream
Some fragments (grains) are pebble, cobble, or boulder size
Grain size reflects the energy of the transportation and deposition of these sediments
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Core Term
Deposition of Sediments
- Sediments are deposited in layers, or beds = “strata”
- Beds tend to be relatively flat, or horizontal, when deposition occurs
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Key points
The Principle of Superposition
In a stack of sedimentary rocks, the rock layer on top is younger than the one below it.
The lower layer had to be there before something could be deposited on top.
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