Detrital Sedimentary Rocks
UNIT – 2
Clastic Sediments and Sedimentary Rocks
 Clastic sediments are loose materials, however when these sediments under
go lithification (diagenesis), they form sedimentary rocks.
 Clastic sedimentary rocks are also known as Detrital or Mechanical
sedimentary rocks.
 Mud, silt and sand are all loose sediments. The addition of stone to these
words (mudstone, siltstone and sandstone), indicates that these sediments
are lithified and are sedimentary rocks.
 A three fold classification based on the grain size is used as the starting point
for classifying detrital sediments and sedimentary rocks. They include



Gravel and Conglomerate : Size greater than 2mm
Sand and Sandstone: 2mm to 1/16 mm
Mud and Mudstone: Less than 1/16 mm
The Udden-Wentworth grain-size scale
 Known as the Wentworth scale, this is the most widely
accepted scale for classifying clastic sediments.
 The divisions on the scale are made on the basis of
factors of 2.



Example : Medium Sand : 0.25 to 0.5 mm
Coarse Sand: 0.5 mm to 1.00 mm
Very Coarse Sand : 1.00 mm to 2.00 mm
 The phi scale (φ) is the numerical representation of the
Wentworth scale
The Udden-Wentworth grain-size scale for clastic
sediments
Size (Millimeters)
Wentworth Grade
Phi (Φ) Scale
>256
Boulder
–8
256-64
Cobble
–6
64-4
Pebble
–2
4-2
Granule
–1
2-1
Very coarse sand
0
1-1/2
Coarse sand
1
1/2-1/4
Medium sand
2
1/4-1/8
Fine sand
3
1/8-1/16
Very fine sand
4
1/16-1/32
Coarse silt
5
1/32-1/64
Medium silt
6
1/64-1/128
Fine silt
7
1/128-1/256
Very fine silt
8
<1/256
Clay
>8
Sediment
Sedimentary Rock
GRAVEL
CONGLOMERATE
SAND
SANDSTONE
MUD
MUDSTONE
Gravel and Conglomerate
 Sediments/clasts more than 2 mm in
diameter are divided into
pebbles, cobbles and boulders.
 Consolidated
conglomerate.
gravel
is
granules,
called
Gravel
 The term breccia is used for conglomerate
made up of clasts that are angular in
shape.
Conglomerate
 Mixture of rounded and angular clasts are
sometime
known
conglomerate.
as
breccio-
Breccia
Composition of gravel and conglomerate

If all the clasts in a conglomerate are of the same material
(e.g. granite), then the conglomerate is called monomict.

A polymict conglomerate is one that contains of clasts of
many different lithologies/rock types.

Any kind of rock type may be found as a clast in gravel or
conglomerate.

Resistant rock type which are less susceptible to physical
and chemical weathering have a greater chance of being
preserved as clasts in conglomerate.

The factor controlling the type of clast found in a given
conglomerate depends on the type of bedrock being eroded
in the area.

If the bed rock being eroded is limestone then all the clasts
in the gravel or conglomerate will be made up of limestones.

Recognition of the clast type can help in determining the
source of the conglomerate.
Texture of Conglomerate

Conglomerate beds are rarely composed entirely of
gravel sized material.

Between the granules, pebbles, cobbles and boulders
finer sand or mud is always present.

This finer material between the bigger clasts is known
as the matrix.

If there is a higher proportion of matrix ( more than
20 %) then the conglomerate can be called as sandy
conglomerate
or
muddy
conglomerate
depending upon the grain size of the matrix.

The conglomerates can be clast supported or matrix
supported.

Matrix supported conglomerates are known as Paraconglomerates.

Clast supported conglomerates are known as Orthoconglomerates.
Matrix
Clasts
Para-conglomerates
Ortho-conglomerates
Conglomerate/Sandstone/Mudstone
Ternary Diagram
Shape of clasts
 The
shape of clasts in gravel and
conglomerates are determined by fracture
properties of the bedrock.
 Rocks with equally spaced fracture planes
in all directions form cubic or equant
blocks that form spherical clasts when the
edges are rounded off.
 Limestones and sandstones form clasts
with one axis shorter than the other two.
This is termed as oblate or discoid
form.
 Rod shaped or prolate clasts
are less
common and are formed mainly from
strongly foliated metamorphic rocks.
Shape of clasts
 When discoid clasts are moved in a
flow of water they are preferentially
oriented and stack up in a form
known as imbrication.
 The
direction of imbrication of
discoid pebbles in a conglomerate
can be used to indicate the direction
of flow that deposited the gravel.
Sand and Sandstones
 Sediment/clast
size
between 2 mm to 1/16 mm
are known as sand and the
sedimentary rock formed
from these clasts are
known as sandstone.
 Many
sanstone contain
mainly quartz grains, but it
is not always true. Some
sandstones have no quartz
at all.
Detrital Mineral grains in sands and sandstones
 A
very large number of
different minerals may occur in
sands and in sandstones. The
most common of them include:
 Quartz: Quartz is the most
common
mineral found in
sanstones.
 Quartz is a very stable mineral
and is resistant to weathering.
The quartz grains remain
intact over long distances and
long period of transport.
Detrital Mineral grains in sands and sandstones
 Feldspar is a common mineral and
is released in large quantities when
felsic igneous rocks and granitic
gneiss and schists breakdown.
 Feldspars are more susceptible to
chemical alteration and is found
only when the chemical weathering
of the bedrock has not been too
intense and the transportation
distance to the site of deposition is
short.
 Potassium
Feldspars are more
common in sandstones than
Calcium or Sodium Feldspar as they
are chemically more stable when
subjected to weathering.
Detrital Mineral grains in sands and sandstones
 The two common Mica
minerals, biotite amd
muscovite are relatively
abundant in sandstones.
 Muscovite
is
more
resistant to weathering as
compared to Biotite.
 They are derived from
intermediate
igneous
rocks and from schists
and gneisses.
Detrital Mineral grains in sands and sandstones
 Heavy
minerals:
The
common minerals in sands
have densities of around 2.6
to 2.7 g/cm3.
 Most sandstones contain a
small proportion, commonly
less than 1% of minerals that
have a greater density.
Zircon
Garnet
 Common
heavy minerals
include zircon, tourmaline,
rutile, apatite, garnet etc.
Tourmaline
Detrital Mineral grains in sands and sandstones
 Miscellaneous
minerals: Other
minerals rarely occur in large
quantities in sandstones.
 Most of the common silicate minerals
in silicate rocks such as pyroxenes,
amphiboles and olivines are easily
broken down by chemical weathering.
 Oxides of iron are fairly abundant in
sandstones.
 Local concentration of a particular
mineral may occur when there is a
near by source.
Other components of sands and sandstone
 Lithic Fragments: Breakdown of pre-existing, fine
to medium grained igneous, metamorphic and
sedimentary rocks result in sand-sized fragments.
 Determination of these fragments of rock usually
requires petrographic analysis by thin-section
examination to identify the minerology and fabric.
Other components of sands and sandstone
 Biogenic particles: Small pieces
of calcium carbonate found in
sandstone are commonly broken
shells of molluscs and other
organisms that have calcareous
hard parts.
 These
biogenic fragment are
common in sandstone deposited
in shallow marine environment.
 If these calcareous fragments are
more than 50 percent then the
rock would be called as limestone.
Other components of sands and sandstone
 Authigenic
minerals:
Minerals that grow as
crystals in the depositional
environment
are
called
authigenic minerals.
 Glauconite
and
many
carbonate minerals form in
this way.
Other components of sands and sandstone
 Matrix: Fine grained mineral occurring between
the sand grains is referred to as matrix.
 In sands and sandstone the matrix is typically silt
and clay sized material and it may wholly or partly
fill the space between the grains.
 Matrix is the material deposited along with the
grains whereas cement is chemically precipitated
after deposition.
Sandstone Classification
 Full description of a sandstone
about the type of grain presents.
requires information
 Sandstone containing mica may be called as micaceous
sandstone.
 Sandstone containing calcium carbonate may be called
as calcareous sandstone.
 For a formal classification of sandstone the Pettijohn
sandstone classification is used.
Pettijohn Sandstone Classification
Pettijohn Sandstone Classification
 Sandstone having less than 20 percent matrix is known as arenite.
 Sandstone having between 20 to 50 percent matrix is known as wacke.
 Sandstone with more than 50 percent matrix is known as mudstone.
 Each of this sandstone type is represented by a triangular plot and the
corners of the triangle are represented by Quartz (Q), Feldspar (F)
and Lithic fragments (L).
 So depending upon the compostion of Q, F and L sandstones can be
classified as

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•
Quartz Arenite, Feldspatic (Arkose)Arenite and Lithic Arenite
Quartz Wacke, Arkose Wacke and Lithic Wacke
Quartz Mudstone, Arkose Mudstone and Lithic Mudstone
Lithic Wacke and Arkose Wacke are also sometimes known as
Graywacke
Clay, Silt and Mudstone
 Sediment/clast size less than 1/16 mm are known as
mud and the sedimentary rock formed from these
clasts are known as mudstone.
Definitions of terms in mudstone
 Silt is defined as the grain size of materials between 1/16 mm
to 1/256 mm.
 This range is subdivided into coarse, medium fine and very
fine.
 Clay is defined as the grain size of material less than 1/256
mm. Clay minerals can not be seen with the naked eye and
can be seen only with high power optical microscope.
 Clay minerals are a group of phyllosilicate ( sheet silicates )
and are the main constituent of clay sized particles.
Definitions of terms in mudstone
 When clay and silt sized particles are
mixed in unknown proportion, the
resulting sediment is known as mud and
the rock is known as mudstone.
 If more than two-thirds of the sediments
are clay sized then the corresponding rock
is known as claystone.
 If more than two-thirds of the sediments
are silt sized then the corresponding rock
is known as siltstone.
 The term shale is applied to mudstone,
however it is best to use this term for
mudstones that show fissility.
Texture of clastic sedimentary rocks
Texture of clastic sedimentary rocks
 Clasts and Matrix: The fragments that make up
a sedimentary rocks are called clasts.
 Matrix is
the fine grained material that lies
between the clasts.
Clasts
Matrix
Texture of clastic sedimentary rocks
 Sorting is a description
of the distribution of clast
sizes presents.
 Sorting is a function of the
origin
and
transport
history of the clastic
sediments.
 With increased transport
distance the different size
of clasts get separated.
Texture of clastic sedimentary rocks
 Clast Roundness: Roundness of the clasts is a function of the
transport history of the sediments.
 Well rounded clasts in a sediment or sedimentary rock is an example of
long transportation history.
 Short transportation results in angular clasts.
 Sphericity is an inherited feature that is, it depends on the shape sof
the fragments which formed during weathering.
High Sphericity
Low Sphericity