Clastic sediments & their texture
Clastic sediments & their texture
" Particles (or  clasts ) are the basic elements of
any sediment
" Terrigenous clastic/detrital sediments:
" - 80-85% of the stratigraphic record
SEDIMENTOLOGY
SEDIMENTOLOGY
- consist of particles derived from pre-
3 CLASTIC SEDIMENTS
3 CLASTIC SEDIMENTS
existing rocks
- eroded on land & deposited in any
environment
- (Non-terrigenous clastic sediments, e.g.
carbonate intraformational breccia)
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TEXTURE 1: GRAIN-SIZE
TEXTURE 1: GRAIN-SIZE
Clastic sediments & their texture
Clastic sediments & their texture
(unconsolidated clastic sediments)
(unconsolidated clastic sediments)
" Texture
" Grain (particle/clast) size
" The Udden-Wentworth grain-size scale is based on
factors of two:
" Sorting & Clast/matrix relationships
Åš = -log2 (mm)
" Grain shape
" Mud (<63 µm; >4 Åš)
" Fabric
" Clay (<4 µm; >8 Åš)
" Lithology is the general characterization of a
" Silt (4 63 µm; 4 8 Åš)
sediment or a sedimentary rock (e.g., coarse
" Sand (63 2000 µm; -1 4 Åš)
sand, mudstone) " Gravel (>2000 µm; <-1 Åš)
" Grain-size (particle-size, granulometric) analysis
" The old-fashioned way: direct measurement (gravel) and
sieve/pipette analysis (sand and mud)
" The modern technology: laser particle sizing (sand and mud)
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The Udden-
The Udden-
Wentworth
Wentworth
grain-size scale
grain-size scale
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Equipment for laser particle sizing (sand
Equipment for laser particle sizing (sand
and mud)
and mud)
Unconsolidated clastic sediments
Unconsolidated clastic sediments
" The Udden-Wentworth grain-size scale is based on factors of
two: Åš = -log2d(mm)
Åš = -log2
" Mud (<63 µm; >4 Åš)
" Clay (<4 µm; >8 Åš)
" Silt (4 63 µm; 4 8 Åš)
" Sand (63 2000 µm; -1 4 Åš)
" Gravel (>2000 µm; <-1 Åš)
" Grain-size (particle-size = granulometric) analysis
" The old-fashioned way: direct measurement (gravel) and
sieve/pipette analysis (sand and mud)
" The modern technology: laser particle sizing (sand and mud)
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Unconsolidated clastic sediments: statistical
Unconsolidated clastic sediments: statistical
Graphic presentation of grain-size analysis:
Graphic presentation of grain-size analysis:
histogram, frequency distribution & cumulative frequency curve
histogram, frequency distribution & cumulative frequency curve
parameters
parameters
Moment measures
" First moment: mean (cf. median, mode)
" Premier measure of the grain size (x = frequency; n =
grain size class midpoint)
n
µ = x /n
" i
i=1
" Second moment: variance (cf. standard deviation)
" Measure of the degree of sorting
n
2 2 (Ã = standard deviation)
à = (x - µ) /n
" i
i=1
" Third moment: skewness
" Measure of the symmetry of the grain-size distribution
n
3
sk = (x - µ) /n
" i
i=1
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SKEWNESS
SKEWNESS
OF GRAIN-SIZE
OF GRAIN-SIZE
DISTRIBUTION
DISTRIBUTION Grain-size
Grain-size
compo-
compo-
sition:
Coarse grains prevail sition:
classifi-
classifi-
cation based
cation based
on three
on three
classes
classes
Fine grains prevail
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TEXTURE 3:
TEXTURE 3:
TEXTURE2:
TEXTURE2:
GRAIN SHAPE  sand grains
GRAIN SHAPE  sand grains
GRAIN SORTING
GRAIN SORTING
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CLAST ORIENTATION
CLAST ORIENTATION
TEXTURE CONTD.
TEXTURE CONTD.
" Grain shape
" E.g.: pebble imbrication as indicatro of
" Roundness (well rounded to very angular)
current direction
" Sphericity (high or low)
" Clast/matrix proportion
" The matrix is the relatively fine-grained material that lies
between the relatively coarse-grained clasts
" Clast-supported sediments (clasts are in direct contact)
" Matrix-supported sediments (clasts are entirely surrounded by
matrix)
" Fabric
" Preferential orientation of particles in a sediment or
tendency of a rock to break in specific directions
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Clastic sediments composition Clastic sediments maturity
Clastic sediments composition Clastic sediments maturity
" Sediment composition Sediment maturity (degree of change
compared to original bedrock: provides
" Detrital mineral grains (quartz, feldspar,
mica, heavy minerals)
evidence on the history of a sediment)
" Lithic fragments (grains or rock fragments)
" Textural maturity (mud content, sorting,
grain shape)
" Detrital mineral grains dominate in silts,
lithic fragments dominate in gravels
" Mineralogical maturity (proportion of stable
or resistant minerals)
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Petrographic classification of sandstones
Petrographic classification of sandstones
 note differences in (a) mineralogical maturity between some
 note differences in (a) mineralogical maturity between some Textural maturity
Textural maturity
classes of arenites, and (b) textural maturity between arenites &
classes of arenites, and (b) textural maturity between arenites &
wackes
wackes
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Clastic sediments: clay minerals
Clastic sediments: clay minerals
Clay minerals
" Clay minerals are phyllosilicates with layered crystal
structures
" Kandite group (two layers): kaolinite
" Smectite group (three layers): montmorillonite, illite, chlorite
" Key physical and chemical characteristics of clay
minerals
" Platy shape (easy to keep in suspension, very slow settling
rates)
" Strong cohesion due to electrostatic charge (relatively
difficult to erode, tendency to flocculate)
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