2. Characteristics of
Sedimentary Rocks
Formed at or very close
to the earth’s surface
Deposited in layers or
beds - often horizontal
Frequently contain fossils
3. Classification of
Sedimentary Rocks
3 Groups recognised
according to mode of origin
Clastic/Detrital
Organic
Chemical Precipitates
4. 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
8. Clastic Rock Groups
Rudaceous - Coarse >2mm
Arenaceous - Medium 1/16 – 2mm
Argillaceous – Fine <1/16mm
9. 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
10. 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
11. 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
12. 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
13. 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
14. A Sediment Sorting Comparitor
Very Well Sorted Well Sorted Moderately Sorted
Poorly Sorted Very Poorly Sorted
15. 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.
16. 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
17. 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.
18. Clast/Particle Shape
c/b Zinng classification
Involves measuring a, b
Rod
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.
19. Conglomerate
Typical deposit of a high-energy
shallow marine environment-beach
Flint showing
conchoidal fracture
Clasts range in size
1mm – 3cm, poorly
Grey, cream,
sorted, polymodal
yellowish cement,
no acid reaction
probably quartz
Clasts are all flint
pebbles=oligomict
Mineralogically
Clasts all well rounded 1cm mature
texturally mature
20. Breccia All fragments are angular
texturally immature
Contains fragments of limestone,
basalt, slate and quartz = polymict
mineralogically immature
Produced by a flash Matrix is a micro-breccia
flood in a desert fine grained <0.25mm
environment
1cm
Red colour is haematite
(iron oxide) cement
Clasts range in size 1mm -
>3cm poorly sorted, polymodal
21. Limestone Breccia – Fault Breccia
Poorly sorted,
clasts 1mm – 7cm Calcite cement
Limestone reacts with acid
All clasts are limestone
therefore oligomict
Formed adjacent to
a fault plane, main
process is cataclasis Limestone
Zo
ne
of
Fau
lt B All clasts are
rec
cia very angular
Texturally very immature
4cm
22. Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified)
Wide range of particle sizes from Very poorly sorted, texturally and
clay <1/256mm to boulders >256mm mineralogically very immature
Direction of
Ice Flow 1m
Large boulder showing
All clasts are
glacial striations
very angular
Long axes (a) of clasts show Polymict and Polymodal
sub-parallel alignment
Produced by freeze-thaw, plucking,
glacial abrasion and attrition
Photograph courtesy A.Quarterman, Greenhead College
23. Greywacke/Turbidite or Muddy Sandstone
Polymict/polymodal
Clasts are angular rock and
mineral fragments 2-6mm
Texturally and Fossils
mineralogically Rare
immature
Angular quartz grain
Possible fining upwards
sequence/graded bedding
Comprises up to
40% muddy Poorly sorted 1cm
matrix
24. 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
25. Comprises angular feldspar and quartz grains Arkose
and is texturally and mineralogically immature
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.
26. 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
27. Desert Aeolian/wind blown deposit
Sandstone Formed in a desert
Grains well rounded and
texturally mature
Red/brown haematite cement
Cross bedding common
Grains have frosted/pitted surfaces but no fossils
due to constant
attritiom/abrasion
All grains are quartz
Poorly consolidated grains
mineralogically mature
rub off in the fingers
8mm Well sorted grains 0.25-0.5mm
28. Micaceous Sandstone (Flagstone)
Moderately well sorted, most Mineralogy is quartz and muscovite
grains 0.25-1.25mm in diameter
Bimodal grain size-mica
occurs as thin flakes,
quartz as sub rounded
to rounded grains
1cm
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
29. 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
Very resistant to mechanical
Absence of fossils due to and chemical weathering
long transport history and
prolonged erosion
Comprises over 95% rounded quartz grains
30. 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
31. Siltstone
Contains mainly clay minerals
such as kaolinite, illite, serecite 1cm
plus fine quartz particles
Reddish brown colour
implies haematite cement
Shows laminations-splitting
into layers <1cm thick
Grain size mainly Feels gritty when rubbed
1/16-1/256 mm gently on the teeth!
32. Mudstone
Deposited in a low
1cm energy environment
such as a river estuary
or marine harbour
Homogenous/structureless with
Grain size <1/256mm little evidence of laminations
Represents a clay that has
Feels smooth when rubbed been consolidated and the
on the teeth and implies water content reduced
absence of quartz
Comprises entirely clay minerals
such as kaolinite, illite and serecite
33. 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
34. Black Shale with Graptolites
Well laminated Composed of clay minerals
and carbonaceous material
Deep sea, low which results in dark colour
energy deposit
Well preserved Didymograptus (Tuning
Fork Graptolite) of Ordovician Age
Main clay minerals are
kaolinite and illite
Splits into thin 1cm
layers = Grain size <1/256 mm
Fissile
35. Depositional Environments – Sedimentary Rocks
5
4
1 3
2 6 7 8
9 10
13
11
12 15
14
Suggest an appropriate
sedimentary rock type that
may be forming in the areas
labelled 1 to 15 above
