Understanding Sediment and Sedimentary Rock Textures

 
Textures and Structures of
Sediment and Sedimentary
Rocks
 
Sediment Texture
 
Sediment texture refers to the shape, size and three-dimensional
arrangement of the particles that make up sediment or a sedimentary
rock.
Textures can be 
clastic or crystalline
, it is 
clastic 
when they are
composed of grains from pre-existing rocks and 
crystalline
, when the
crystals grew from a fluid producing an interlocking mosaic of crystals.
Textures are 
primary or secondary
, it is 
primary 
where the grains possess
their arrangement that existed after they came to rest (or after
precipitation in the case of crystals).
In sedimentary rocks, however, textures are commonly 
secondary
,
because they have been altered in some way from their original
condition.
The most common effect is compaction, where the weight of overlying
sediments causes the component grains to rearrange themselves or even
become fractured.
 
Sedimentary rocks exhibit differences
in 
texture
:
 
Definition of texture:
 
“the size and shape of the particle that make up the rock.”
Grain angularity
Sphericity
Size
Sorting
Which reflect:
Derivation (original rocks)
Climate (during formation)
Post-depositional factors
 
Texture
 
T
 
 O
 
 S  
 
S
 
S
Texture =
Orientation (random/lined up),
Size (measurements, all same?),
Shape (rounded/angular),
Sorting (well

poor).
Sedimentary rocks show great differences in their texture
This relates back to their mode of formation
 
Grain Size
 
Particles are generally measured by their maximum grain diameter.
The most common classification is use is the Wentworth scale.
In this classification, terms such as pebble, sand and gravel have well-defined
limits.
Many statistical methods can be applied to particle size distributions (e.g.,
median, mean, skewness, kurtosis, 
etc
.) to try to characterize the depositional
processes that produced them (e.g., wind, swash, etc.), and even to distinguish
depositional environments.
The Wentworth scale is a geometric scale--adjacent orders differ be a factor of
two (either doubling or halving).
A common practice in studying grain size distribution is to use the 
phi scale
. As
well as mm, the boundaries between each class can be expressed by the
negative logarithm of this dimension to the base 2: (phi) 
Φ = -
log2 
d
where 
d 
is particle size in mm. Phi values are dimensionless, but because they are
based on mm, the scale is metric.
 
This figure
shows how
clastic sediment
of various sizes
will, after
compaction and
cementation,
form different
types of
detrital
sedimentary
rocks
.
 
The process of
sediment
turning into
rock is called
lithification
.
 
Shape
 
Particle shape is difficult to quantify or describe.
Shape is commonly described with reference to three axes at 90°
to each other that can theoretically be placed inside any particle.
The longest axis is 
a
, shortest 
c 
and the intermediate is 
b
.
In a sphere or cube, those axes intersect at the centre and have
equal length.
By comparing the ratios of the axes, it is possible to describe four
basic "shapes" of particles: oblate, equant, bladed and prolate.
Shape is some cases reflects erosional processes (how the rock
broke up with weathering and transportation), but may also reflect
structures and fabrics present in the parent rock.
Shale, for example, will commonly produce flat particles, whereas
quartzite (metamorphosed sandstone) may be more equant.
 
The four main classes of grain
shape, which are based upon
the ratios of the long,
intermediate and short
diameters of any particle (after
Zingg 1935).
 
Angular – little evidence of wear, sharp corners,
little transport
Increased sphericity - more spherical, rounded,
corners smoothed off to broad curves, great
amount of transport
 
Roundness
 
Roundness 
(or angularity) refers to how smooth or sharp are the edges of
particles.
It is usually measured with reference to a comparative chart (The commonest
is: Powers, M.C., 1953, A new roundness scale for sedimentary particles. 
Journal
of Sedimentary Petrology
, 23: 117-119)
Roundness and sphericity are not the same thing.
A grain with highly irregular shape can be well rounded; a cube has high
sphericity but very low roundness..
 
Roundness and Sphericity
 
Increasing Roundness=increasing maturity
Increasing Roundness=increasing maturity
 
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Maturity of Sediments
 
Sorting
 
Sorting
: 
increases with length of transportation history (weaker minerals  broken
 
down).
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
Sorting of the sediments also suggest the mode of deposition and
transportation.
Long distance transport= 
well-rounded and well-sorted
sediments,
 
Short distance transport =  poorly sorted angular grains.
Also helps in knowing the energy conditions of the river.
 
Sorting
 
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)
 
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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Discover the textures and structures of sediment and sedimentary rocks, including clastic and crystalline textures, primary and secondary textures, and the effects of compaction. Learn about sediment texture, differences in sedimentary rock texture, and the classification of grain sizes using the Wentworth scale. Explore how particle size distributions can reveal information about depositional processes and environments. Gain insight into the grain-size classification for clastic sediments, from boulders to clay particles.


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  1. Textures and Structures of Sediment and Sedimentary Rocks

  2. Sediment Texture Sediment texture refers to the shape, size and three-dimensional arrangement of the particles that make up sediment or a sedimentary rock. Textures can be clastic or crystalline, it is clastic when they are composed of grains from pre-existing rocks and crystalline, when the crystals grew from a fluid producing an interlocking mosaic of crystals. Textures are primary or secondary, it is primary where the grains possess their arrangement that existed after they came to rest (or after precipitation in the case of crystals). In sedimentary rocks, however, textures are commonly secondary, because they have been altered in some way from their original condition. The most common effect is compaction, where the weight of overlying sediments causes the component grains to rearrange themselves or even become fractured.

  3. Sedimentary rocks exhibit differences in texture: Definition of texture: the size and shape of the particle that make up the rock. Grain angularity Sphericity Size Sorting Which reflect: Derivation (original rocks) Climate (during formation) Post-depositional factors

  4. Texture T Texture = Orientation (random/lined up), Size (measurements, all same?), Shape (rounded/angular), Sorting (well poor). Sedimentary rocks show great differences in their texture This relates back to their mode of formation O S S S

  5. Grain Size Particles are generally measured by their maximum grain diameter. The most common classification is use is the Wentworth scale. In this classification, terms such as pebble, sand and gravel have well-defined limits. Many statistical methods can be applied to particle size distributions (e.g., median, mean, skewness, kurtosis, etc.) to try to characterize the depositional processes that produced them (e.g., wind, swash, etc.), and even to distinguish depositional environments. The Wentworth scale is a geometric scale--adjacent orders differ be a factor of two (either doubling or halving). A common practice in studying grain size distribution is to use the phi scale. As well as mm, the boundaries between each class can be expressed by the negative logarithm of this dimension to the base 2: (phi) = -log2 d where d is particle size in mm. Phi values are dimensionless, but because they are based on mm, the scale is metric.

  6. Grain-Size Classification for Clastic Sediments Name Millimeters 4,096 256 64 Micrometers Boulder Cobble Pebble Granule Very Coarse Sand Coarse Sand Medium Sand Fine Sand Very Fine Sand Coarse Silt Medium Silt Fine Silt Very Fine Silt Clay 4 2 1 0.5 0.25 0.125 0.062 0.031 0.016 0.008 0.004 500 250 125 62 31 16 8 4 (modified from Blatt, 1982)

  7. This figure shows how clastic sediment of various sizes will, after compaction and cementation, form different types of detrital sedimentary rocks. The process of sediment turning into rock is called lithification.

  8. Shape Particle shape is difficult to quantify or describe. Shape is commonly described with reference to three axes at 90 to each other that can theoretically be placed inside any particle. The longest axis is a, shortest c and the intermediate is b. In a sphere or cube, those axes intersect at the centre and have equal length. By comparing the ratios of the axes, it is possible to describe four basic "shapes" of particles: oblate, equant, bladed and prolate. Shape is some cases reflects erosional processes (how the rock broke up with weathering and transportation), but may also reflect structures and fabrics present in the parent rock. Shale, for example, will commonly produce flat particles, whereas quartzite (metamorphosed sandstone) may be more equant.

  9. The four main classes of grain shape, which are based upon the ratios of intermediate diameters of any particle (after Zingg 1935). the long, short and Angular little evidence of wear, sharp corners, little transport Increased sphericity - more spherical, rounded, corners smoothed off to broad curves, great amount of transport

  10. Roundness Roundness (or angularity) refers to how smooth or sharp are the edges of particles. It is usually measured with reference to a comparative chart (The commonest is: Powers, M.C., 1953, A new roundness scale for sedimentary particles. Journal of Sedimentary Petrology, 23: 117-119) Roundness and sphericity are not the same thing. A grain with highly irregular shape can be well rounded; a cube has high sphericity but very low roundness..

  11. Roundness and Sphericity

  12. Increasing Roundness=increasing maturity

  13. Maturity Maturity is a function of sediment transport Textural maturity refers to: The degree of roundness of the grains The amount of sorting of the grain sizes Texturally mature sandstones have well-rounded and well- sorted grains, immature if not Mineralogical maturity refers to the percentage of quartz grains Feldspars break down with transport Quartz grains more resistant Mineralogically mature sandstones have mostly quartz grains Arkose is mineralogically immature

  14. Maturity of Sediments

  15. Sorting Sorting: increases with length of transportation history (weaker minerals broken down). 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 Sorting of the sediments also suggest the mode of deposition and transportation. Long distance transport= well-rounded and well-sorted sediments, Short distance transport = poorly sorted angular grains. Also helps in knowing the energy conditions of the river.

  16. Sorting

  17. 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)

  18. 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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