Clastic/Detrital
Sedimentary Rocks
I.G.Kenyon
Characteristics of
Sedimentary Rocks
Formed at or very close
to the earth’s surface
Deposited in layers or
beds - often horizontal
Frequently contain fossils
Classification of
Sedimentary Rocks
3 Groups recognised
according to mode of origin
Clastic/Detrital
Organic
Chemical Precipitates
Clastic Sedimentary Rocks
Clastic is derived from
the Greek for ‘broken’
Clastic rocks represent the
accumulation of weathered and
eroded fragments of older,
pre-existing rocks of all types
Organic Sedimentary Rocks
Formed from the remains
of once-living organisms
Chemical Sedimentary Rocks
Rocks precipitated
directly from solution
Particle Sizes of Clastic Rocks
>256mm Boulder
256mm – 64mm Cobble
64mm- 4mm Pebble
4mm- 2mm Granule
2mm – 1/16mm Sand
1/16 – 1/256mm Silt
<1/256mm - Clay
Clastic Rock Groups
Rudaceous - Coarse >2mm
Arenaceous - Medium 1/16 – 2mm
Argillaceous – Fine <1/16mm
Rudaceous Rocks
Over 50% of the clasts (particles)
are over 2mm in diameter
Primarily consist of rock fragments
If particles rounded = Conglomerate
If particles angular = Breccia
Arenaceous Rocks
Over 50% of the particles
are 1/16mm to 2mm in diameter
Comprise a high percentage of quartz grains
These rocks are generally called sandstones
However, a wide variety of sandstones occur
Desert Sandstone, Arkose, Greywacke
Micaceous Sandstone, Orthoquartzite, Grit
Argillaceous Rocks
Over 50% of the particles
are <1/16mm in diameter
Consist of clay minerals
and small quartz grains
Rock types are Siltstone,
Mudstone, Clay and Shale
Clastic Rock Terminology 1
Phenoclast–A large clast/rock fragment
Matrix–the finer material often sand,silt
and clay surrounding the phenoclasts
Cement–material precipitated
from solution to stick the sediment
together.This is often quartz,
calcite
or haematite
Clastic Rock Terminology 2
Well Sorted – all of the clasts are
very similar in size (unimodal)
Poorly Sorted – clasts show a wide
range of particle sizes (polymodal)
Oligomict – all clasts are of the same type
Polymict – clasts are of a variety of types
A Sediment Sorting Comparitor
Very Well Sorted
Well Sorted
Poorly Sorted
Moderately Sorted
Very Poorly Sorted
Textural & Mineralogical Maturity
If a rock is texturally and mineralogically
mature it has undergone extensive transport
and erosion. It is a STABLE sediment and
would not change markedly in character if
it were to be transported and eroded further.
If a rock is texturally and mineralogically
immature it has only been transported a
short distance and suffered limited erosion.
It is an UNSTABLE sediment and would
change in character significantly if it
were
to be transported further.
Clastic Rock Terminology 3
Mineralogically Mature – the rock
consists of clasts of just one type
Mineralogically Immature – the rock
consists of a wide range of clast types
Texturally Mature – all of the
clasts are well rounded
Texturally Immature – all of the
clasts are very angular
Clast/Particle Shape
Individual clasts can be assigned to one of six classes
based on visual observation of the clasts in the rock.
(After Tucker 1982) Can be subjective as one person’s
subangular could be another person’s subrounded.
Clast/Particle Shape
c/b
Rod
Zinng classification
Involves measuring a, b
and c axes of clasts
a axis is longest
dimension on the clast
b axis is widest dimension
at right angles to a axis
c is shortest axis on which
the clast often sits vertically
b/a
b/a and c/b axial values
are plotted as co-ordinates
to identify individual
clasts as spheres, discs,
rods or blades.
Conglomerate
Typical deposit of a high-energy
shallow marine environment-beach
Flint showing
conchoidal fracture
Clasts range in size
1mm – 3cm, poorly
sorted, polymodal
Clasts all well rounded
texturally mature
1cm
Grey, cream,
yellowish cement,
no acid reaction
probably quartz
Clasts are all flint
pebbles=oligomict
Mineralogically
mature
Breccia
All fragments are angular
texturally immature
Contains fragments of limestone,
basalt, slate and quartz = polymict
mineralogically immature
Produced by a flash
flood in a desert
environment
Matrix is a micro-breccia
fine grained <0.25mm
1cm
Clasts range in size 1mm >3cm poorly sorted, polymodal
Red colour is haematite
(iron oxide) cement
Limestone Breccia – Fault Breccia
Poorly sorted,
clasts 1mm – 7cm
Limestone
Calcite cement
reacts with acid
All clasts are limestone
therefore oligomict
Formed adjacent to
a fault plane, main
process is cataclasis
Zon
e
of F
a
ult
Limestone
Bre
cci
a
Texturally very immature
4cm
All clasts are
very angular
Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified)
Wide range of particle sizes from
clay <1/256mm to boulders >256mm
Very poorly sorted, texturally and
mineralogically very immature
Direction of
Ice Flow
Large boulder showing
glacial striations
Long axes (a) of clasts show
sub-parallel alignment
1m
All clasts are
very angular
Polymict and Polymodal
Produced by freeze-thaw, plucking,
glacial abrasion and attrition
Photograph courtesy A.Quarterman, Greenhead College
Greywacke/Turbidite or Muddy Sandstone
Polymict/polymodal
Clasts are angular rock and
mineral fragments 2-6mm
Fossils
Rare
Texturally and
mineralogically
immature
Angular quartz grain
Possible fining upwards
sequence/graded bedding
Comprises up to
40% muddy matrix
Poorly sorted
1cm
Greywacke/Turbidite/Muddy Sandstone
A sandstone with a muddy matrix of up to 40%
Comprises a wide range of angular
rock and mineral fragments
Graded bedding common, fossils quite rare
Forms in subsiding marine basins of deposition
Texturally and mineralogically immature
Turbidity currents (water-laden sediment flows)
on the continental slope cause large volumes of
sediment to be deposited rapidly at the base of the
continental slope in broad fan-shaped structures
Comprises angular feldspar and quartz grains
and is texturally and mineralogically immature
Arkose
Absence of fine material and
mica as blown away by the wind
5mm
Pinkish/purple colour due to
high percentage of feldspar
and iron oxide cement
A sandstone containing over 25% feldspar, produced by
mechanical weathering of granite/gneiss under arid conditions.
Main processes exfoliation and granular disintegration.
Millstone Grit
Most grains between 1 and 4mm,
but still rudaceous and polymodal
Texturally and
mineralogically immature
Graded bedding
Fining upwards
sequence
Well cemented together
by a silica cement
5mm
Comprises sub angular to sub rounded
grains of quartz and feldspar, polymict
Desert
Sandstone
Aeolian/wind blown deposit
Formed in a desert
Grains well rounded and
texturally mature
Red/brown haematite cement
Grains have frosted/pitted surfaces
due to constant attritiom/abrasion
All grains are quartz
mineralogically mature
8mm
Cross bedding common
but no fossils
Poorly consolidated grains
rub off in the fingers
Well sorted grains 0.25-0.5mm
Micaceous Sandstone (Flagstone)
Moderately well sorted, most
grains 0.25-1.25mm in diameter
1cm
Mineralogy is quartz and muscovite
Bimodal grain size-mica
occurs as thin flakes,
quartz as sub rounded
to rounded grains
Well cemented
by quartz
Mica deposited from
suspension when energy
conditions reduced
environment was a delta
with a marked seasonal
fluctuation in river flow
Splits into layers quite readily 2-5cm in
thickness where mica concentrations occur
Orthoquartzite
Texturally and
mineralogically mature
Oligomict and unimodal
Quartz cement results
in very low porosity
Well sorted most grains
0.25 to 0.5mm
2mm
Absence of fossils due to
long transport history and
prolonged erosion
Very resistant to mechanical
and chemical weathering
Comprises over 95% rounded quartz grains
Orthoquartzite (Greensand)
Texturally and mineralogically very mature
1cm
Moderately well cemented by the
pale green mineral glauconite
Oligomict and
unimodal
Comprises entirely quartz grains
0.50 to 0.75mm in diameter
A very stable
sediment
Siltstone
Contains mainly clay minerals
such as kaolinite, illite, serecite
plus fine quartz particles
1cm
Reddish brown colour
implies haematite cement
Shows laminations-splitting
into layers <1cm thick
Grain size mainly
1/16-1/256 mm
Feels gritty when rubbed
gently on the teeth!
Mudstone
1cm
Deposited in a low
energy environment
such as a river estuary
or marine harbour
Grain size <1/256mm
Homogenous/structureless with
little evidence of laminations
Feels smooth when rubbed
on the teeth and implies
absence of quartz
Represents a clay that has
been consolidated and the
water content reduced
Comprises entirely clay minerals
such as kaolinite, illite and serecite
Clay
Particles <1/256mm
This specimen has dried out
and has zero plasticity so is
more appropriately called
a claystone or mudstone
Deep Sea or Lake deposit where
energy conditions are very low
Comprised of clay minerals, chiefly kaolinite
1cm
Black Shale with Graptolites
Well laminated
Deep sea, low
energy deposit
Composed of clay minerals
and carbonaceous material
which results in dark colour
Well preserved Didymograptus (Tuning
Fork Graptolite) of Ordovician Age
Main clay minerals are
kaolinite and illite
Splits into thin
layers = Fissile
1cm
Grain size <1/256 mm
Depositional Environments – Sedimentary Rocks
5
4
1
2
3
6
7
13
8
9
10
11
15
12
14
Suggest an appropriate
sedimentary rock type that
may be forming in the areas
labelled 1 to 15 above
The End