gl4 e1 ki 2c sedimentary rocks exhibit differences in texture: –grain angularity –sphericity...
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GL4 E1 KI 2c
• Sedimentary rocks exhibit differences in texture:– Grain angularity– Sphericity– Size– Sorting
•Which reflect:– Derivation (original rocks)– Climate (during formation)– Post-depositional factors
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Texture
• T O S S S• Texture =
– Orientation (random/lined up), – Size (measurements, all same?), – Shape (rounded/angular), – Sorting (wellpoor)
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Differences in texture
• Sedimentary rocks show great differences in their texture
• This relates back to their mode of formation
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Grain shape
• Angular – little evidence of wear, sharp corners, little transport
• Increased sphericity - more spherical, rounded, corners smoothed off to broad curves, great amount of transport
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Grain shape
• Defined by ratio of dimensions of the fragment
• Length, breadth, thickness (a, b, c axes)
• Zingg classified shape into tabular, equant, blade and rod
• Some unusual: DREIKANTER/VENTIFACT – wedge shaped (wind transport, desert)
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Size
• Boulder >256mm• Cobble 64 - 256mm• Pebble 4 - 64mm• Gravel 2 - 4mm
• Sand 1/16 – 2mm
• Silt 1/256 – 1/16mm
• Clay <1/256mm
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Grain size 2• Sand – divided into coarse, medium
and fine• SIEVING to separate grain fractions
(technique demonstration)• Then weigh and calculate
percentages
• Geoscience page 89 figure 5.22 examine frequency plots for dune sand, beach sand, tidal mud, glacial till and river sand
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Sorting
• Well sorted = particles nearly all the same size– Current strength constant and for
long periods of time
• Poorly sorted = particles of a great mix of sizes – Current strength suddenly drops
and material is dumped
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Porosity/Permeability
• Obviously sorting links well to poroperm
• Well sorted, rounded, medium grain size has good pore spaces and will allow water to fall through quickly
• Poorly sorted, angular sediments have small pore spaces and trap water reducing permeability
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Quick practical
1. Measure a, b, c axes of 10 pebbles (from puddingstone conglomerate, best “guestimate” in some cases!) describe shape – Zingg analysis
2. Using hand lens, 30 grains of at least 3 sands (desert, glacial, beach from jars on windowsill) – shape analysis, size analysis, sorting analysis
(NB: replace sand into correct jars after examination!)
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So what do shape, size, sorting tell us?
they reflect:–Derivation (what were the original rocks)–Climate (during formation)–Post-depositional factors
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• Geoscience page 87• Copy figure 5.20
Characteristics of sedimentary grains
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Derivation
• what were the original rocks?• Lots of different rock particles
mean the HINTERLAND was a big area and very diverse
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Climate• Which existed during particle
accumulation• Sedimentary logs “what was the
environment of deposition?”• Sediment colour• Bed thickness (how long
conditions persisted for)• Grain size – Hjulstrom – current
strength
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• Coal – swamp• Limestone – cwsmas• Shale – slow currents (fine
material)• Red well sorted fine sandstone
(desert)• Conglomerate – dumped
material, sudden slowing of current
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Also need to observe structures
• Sole structures - base of bed – eg. erosion features (potholes) seen at Trevor quarry
• Current bedding (direction of flow)
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Post depositional factors
• Burrowing animals before loose sediment is turned into rock
• DIAGENESIS/LITHIFACTION• Turning the loose sediment into
a rock (hardening)• Compaction - pressure – weight
of sediments above, squeezes grains together (decrease in porosity)
• Loss of water (volume changes, water escape structures etc)
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Diagenesis 2• Mineral forms can change
(recrystallisation eg. aragonite to calcite)
• Minerals can be exchanged/replaced (dissolved and re-precipitated)
• Temperature increase with depth– Diagenesis/Metamorphism boundary
300C (temperature increases at 1C per 30mdepth; pressure increases 1atm per 4.4m depth)
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• Sand – fairly well compacted on deposition
• Muds – high water content
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Addition of CEMENT
• “glue” grains together• Calcite, silica etc• Deposition of minerals in pore
spaces• Produces rigidity in the rock• Can be simultaneous with
deposition (penecontemporaneous) or introduced later
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• Geoscience page 86• Copy figure 5.19 major
diagenetic processes
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Finally…
• Read - Chapter 5 in Geoscience as a summary