Download Sedimentary Textures

GL4 E1 KI 2c
• Sedimentary rocks exhibit differences in
texture:
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–
–
–
Grain angularity
Sphericity
Size
Sorting
• Which reflect:
– Derivation (original rocks)
– Climate (during formation)
– Post-depositional factors
Texture
• T O S S
• Texture =
–
–
–
–
S
Orientation (random/lined up),
Size (measurements, all same?),
Shape (rounded/angular),
Sorting (wellpoor)
Differences in texture
• Sedimentary rocks show great
differences in their texture
• This relates back to their mode of
formation
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
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)
Size
•
•
•
•
•
•
•
Boulder
Cobble
Pebble
Gravel
Sand
Silt
Clay
>256mm
64 - 256mm
4 - 64mm
2 - 4mm
1/
16 – 2mm
1/
1/ mm
–
256
16
<1/256mm
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
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
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
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!)
So what do shape, size,
sorting tell us?
they reflect:
– Derivation (what were the
original rocks)
– Climate (during formation)
– Post-depositional factors
• Geoscience page 87
• Copy figure 5.20 Characteristics of
sedimentary grains
Derivation
• what were the original rocks?
• Lots of different rock particles
mean the HINTERLAND was a big
area and very diverse
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
• Coal – swamp
• Limestone – cwsmas
• Shale – slow currents (fine
material)
• Red well sorted fine sandstone
(desert)
• Conglomerate – dumped material,
sudden slowing of current
Also need to observe
structures
• Sole structures - base of bed – eg.
erosion features (potholes) seen at
Trevor quarry
• Current bedding (direction of flow)
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)
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
300C (temperature increases at 1C
per 30mdepth; pressure increases
1atm per 4.4m depth)
• Sand – fairly well compacted on
deposition
• Muds – high water content
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
• Geoscience page 86
• Copy figure 5.19 major diagenetic
processes
Finally…
• Read - Chapter 5 in Geoscience as
a summary