CHAPTER-6
CLASSIFICATION
m
1
CHAPTER-6
CLASSIFICATION
6 : 1
INTRODUCTION :
The
properties of
for classification
because of sim ilarities
other ch aracteristics.
d ire c tly
in
The
origin
genetic classification .
d escrip tive
elements
which
may
are
be
Sandstones
predominate.
range
of
used
to
the
The sand
sizes, which
basis
the
be
used
conditions
of
to develop
relationship exist between genetic and
essential
are
or
may
from the properties
of sedimentary rocks
in
order
place
classification (Krukbien and Sloss 1963)
sand
,
factors in the classification
d escrip tive
criteria
^ dose
a
texture , composition or
properties
classification
may be inferred
rocks provide
in
observable
d escrip tive
sedimentary
sedimentary
rocks
grains
a
to
that
specimen
in
. The
measurable
its
proper
.
in
which
may
have
bind together grain
detrital
a single
against grain
grains
size
of
or
a
to form a
frame work that provides the primary solid support in the deposit.
Again,
mineral
composition
of
the
framework
grains
reflects
in a
general way the paleogelogy, paleogeography, and paleoclimate of the
region from which
the sand was derived
. It provides the primary
basis for classifying sandstones (Williams et_ a l . , 1984)
The last few
years have seen
.
several classifications of
sandstones though
there
hasbeen no overwhelming acceptance of any
one
so
Sedimentalogist
classification
classification
scheme
sediments
rock
or
classification
fa r.
which
(Dean
must depend
can describe the actual
et_ a l . ,
1985)
has been reviewed by many
.
The
on
such
composition
status of
of
sandstone
authors (Me Bride
1963 j
Okada 1971 j Pettijohn et a l . , 1973 , from Pettijohn 1984, P. 210)
.
2
Many
some based
schemes
on theoretical
of
classification
princip les,
composition, and others devised
have
has
sedimentologists
composition
,
e x p lic ity
Mineralogical
to
in
classification
been
recognise
the
up,
em pirically fo r convenience in fie ld
however,
prefer
drawn
some emphasising the mineral
and labortory description (Jt Greensmith, 1984)
It
been
observed
texture
classif icatory
may
.
that
and
scheme
most
mineralogical
of
sandstones.
provide useful information about the
source rock as because source determines the mineral composition of
the
rock.
reflect
On
the
other
hand
,
source
rock
(Provenance)
w ill
in directly the tectonism which affect the mineral composition
(Pettijohn
, 1984)
.
Folk
(1954 and
1980)
, prefers a classification
based
on textural data and pointed out that this type of classification can
provide its genetic significance (Dean et a l . , 1985)
Doeglas ( 1968)
based
.
on the grain size
This
indices
classification
In the
of
Dott
Two
(1964)
main
and
gives
present case,
of
a
N iggli after Atterberg
satisfactory
description
of
.
, modified
groups
recognised
, has proposed a sandstone classification
indices proposed by
environment of deposition
.
by Pettijohn,
sandstones,
separated
the classification adopted is that
on
Potter and Siever ( 1973)
namely
the
arenites
basis
of
their
and
wackes,
matrix
.
are
content.
Arenites and wackes are then sub-divided on the grounds of various
mineral and rock particle attributes .
Any
c rite ria ,
constituted
is
classification,
necessary
reflects
and maturity of the
to
based
appreciate
on chemical or mineralogical
as
eventually
the composition of the source rocks,
the nature
weathering
that
processes,
the
rock
the quantity and quality
of diagenetic changes and the presence or absence of biochemical or
Wy
3
other contaminants (Jt Greensmith 1984)
6 : 2
M ETH 0D 6 OF S T U D ?
The
.
:
sandstones
of
the
Surma
Group
and
Tipam
Group
have been classified on the basis of texture, grain size indices and
mineralogical composition .
The
friable
parameters
sandstone
obtained
samples
of
the
from
Surma
the
size
and
analysis
of
Groups
of
Tipam
Sandstones have been grouped into sand, silt and clay classes (Table
No.
18 and
triangular
(1980)
No.
diagram
.
of
These
sand,
three
silt
groups
and
clay,
were
as
then plo^pd
suggested
in a
by
Folk
.
As
show
19)
the
the
sandstones
presence of gravel
grains more than 1^ (P h i)
of
the
because
present
two
the sediments
diameter and for this
groups
did
do
not
not contain
reason the 3 tier
classification of Gravel - Sand- Mud was not done.
The
proposed
by
grain
Doeglas,
size
1968
indices
,
classification
with
the
,
was
Mj and
done
as
Qj values,
obtained from mechanical analysis for both the groups of sandstones,
and
were
median
plotted
were
quartile
(
in
the
plotted
)
on
(Q ^
the
)
diagonals,
, and third Quartile
(
diagram.
while
),
the
to find
grain
size
indices,
of
of
fir s t
vertica lly
The values of
for each sample
,
(Table-20 and
F j$ .6 ;5 ).
After completion of the plot,
sandstone
values
were also rounded off as suggested by Doeglas (1968)
out the
Table
values
were plotted
above and below respectively on the median point.
, M^ and
The
samples has
been
suggested by Doeglas ( 1968)
made on the
the nomenclature of
basis
. Doeglas (1968)
of
the
each
indices,
as
, has not suggested any
4
method for grouping the whole rock unit.
For this requirement the
following procedure was tried and a group-nomemcLature was adopted.
The values of first quirtile (
(
Qj, )
were
averaged
) , median (M^), and third quartile
separately.
By
values,
a new values were obtained,
indices
value
of
the
whole
rock
rounding off
which
group,
those
average
was considered as the
and
they
were
named
accordingly.
The percentages of QFL components (Table 22 and
23)
, were plotted in the standard Q.F.L. diagram of James et al.
( 1986)
and Q.F.L.
diagram of
Potter and Siever ( 1973)
Dott
. There
(1961)
is
slight
modified
by
Pettijohn,
variation only in the
grouping and nomenclature between these two diagrams. Of late, the
Q.F.L.
diagram
of
Dott
( 1964),
has
been
used
extensively
for
mlneralogical classification of sandstone. But James et a l., 1986, has
used
a
slightly
classification,
different
which
Q.F.L.
plot
for
the
purpose
he regarded as standard diagram.
of
Therefore,
both the diagrams were tried for the present work. The nomenclature
of the rock types was done according to their positions in these two
Q.F.L. plots.
The component used for mineralogical classification
were
1.
Quartz (Q) - Total quartz and chert.
2.
Feldspar (F) - Feldspar and igneous rock fragments.
3.
Lithic (L) - Mica + metamorphic rock fragments .
Their recalculated individual percentages were used
for the purpose.
6 : 3
6 :3
OBSERVATION :
A - SURMA GROUPS OF SANDSTONES :
5
6:3
A.
Surma Group sandstones :
The
Group of
(1980),
silty
sandstones,
the
sand
sand,
silt,
when
plotted
point concentrated
)
and
in sZ
34.07% to 77.25% , silt
clay
vary
in the
(Fig.
(i.e.
percentages
6:1)
sandy
ternary
of
the
Surma
diagram of
Folk
in the fields of zS; ( i . e .
silt)
.
the
sand
vary
from 21.87% to 63.96% and clay
from
ranges
from 0.45% to 6.30% .
The
granulometric
when plotted according to Doeglas
more than l(one)
6:2)
.
ranging
Plotting
from
deduced
for
shows
coarse
that
sandy
silt
'
the
Surma
-
Fine
to
'Very
different
samples
analysis
reveal
Granulometric
(1968)
of
0
, M , and Q ,
I d
3
, it was found that often
sample fall in the same point of the diagonal (Fig.
'Medium
’ Medium sandy
values
have
that
Sandstone
consist
sand1 to ‘ Very
coarse
been
majority
silt'
given
of
fine
.
in
of
sand'
and
Nomenclatures
Table
Surma
grains
-20
sediments can
be nomenclatured as 'medium sandy-very fine sand " and" fine sandy
coarse silt"
, followd by
'ver y fine sandy fine sand . "
The values of first quartile (Qj ) varies from 1(| to
3j)
, median (
Md) , ranges from 2$) to 4|) and third quartile
varies from 2(> to 6{)
and 6^
.
For the
. Only two samples had Q
whole
Md=3{) and Q = 40
r
3
r
of sandstones.
rock group of Surma,
sandstones were calculated (Table-22)
on the
Dott (1964)
.
Q.F.L.
values between 5j)
shows
that Q^=2{)
,
Hence Q -Md-Q indices is 234 for Surma Group
1
3
The percentages of Q.F.L.
made
(Q^ )
triangular
component of the Surma
. Plotting of these values were
diagram
(Fig.
6:3
a)
proposed
by
6
From the Table-22,
has
maximum
percentages,
it has been found that Q pole
followed
by
L
and
F
respectively.
Q
varies from 72.61% to 82.10%, L varies from 7.10% to 17.12% and F
varies
from
triangular
fa ll
on
9.00%
plot
fa ll
12.12%
within
the fille d
standard Q .F.L.
(1986),
to
.
the
alioted
Majority
sublithic
majority
of
the
points
fa ll
the
points
in
the
wing,
only three samples
But
when plotted in the
for subarkose.
diagram (F ig . 6:3 b ),
of
proposed by James et. a l.,
within
the quartzose
arenite
wing ; only three points concentrates on the field marked for lithic
arenite.
6 : 4
INTERPRETATION AND CONCLUSIONS
::
From the textural analysis of the Surma Sandstones,
it may be interpreted that the sediments of the sandstones belong to
the "silty sand" and "sandy silt" groups indicated by their location
in "zS" and "sZ" field s. The 'zS'
tire
indicate
the
presence
of
tier indicate silty sand and ' sZ1
silt
along
with
the
sand
in
the
sediments.
It indicates the predominance of sand size grains in the
sediments,
but
with
the
introduction of
minor amount of silt
size
grains to the dominant sand population, the sand becomes silty sand
(Folk 1980), and can be designated as 's ilty sandstone1 (point lying
on zS tie r)
s ilt1 (
. On the other hand, a small portion belong to
Point lie within
'sandy siltstone'
sZ tier)
'sandy
group and can be dtsignated as
. The clay components in the sediments are very
nominal to affect the bulk constituents.
From
this,
it
may
finally
be
concluded
that
the
Surma group of sandstones predominantly contains, sand followed by
silt
size
materials,
vrw dc1 &tely well
belong to Sub-mStufe stage and are moderately to
SOJtect TiatufC^this
also
supports
the
presence
of
considerable proportion of silt in the sediments.
The grain size
the
Surma group
of
sandstones
indices classification
consists
of
sediments
indicates
that
ranging from
7
'medium to find sand' to 'very fine sand' and 'medium sandy coarse
silt' to 'very coarse silt'. As a whole, the sediments of the Surma
sandstones
can
be
designated
as
'medium
sandy-very
fine
sand'
followed by 'fine sandy coarse silt'. The average indices value for
the
group
is
nomenclature
indices
found
for
the
to, -be
sediments
classification
may
be
'234'.
of
Therefore,
the
Surma
suggested
as
the
Group
'medium
general
according
sandy
to
- very
ffine sand'.
The
sandstones
of
mineralogical
the
Surma
classification
Group,
belong
indicates
to the
that
sublithic
the
arenite
clan as the majority of the samples lie in the sublithic arenite
wing.
Only
three
hand,
when
the
points
Q.F.L.
lie
in the subarkose wing.
values
plotted
in
the
On the other
standard
diamgram,
proposed by James et_ a^. , (1986), majority of the samples lie in
the quartzose arenite wing. Only three points lie within the lithic
arenite. Therefore, from the mineralogical classification it may be
concluded that the Surma group of sandstones belong to the lithic
group. The rock types are 'sublitharenite' or 'quartzose arenite' a
sub-group straddling the boundary between lithic arenite and quartz
arenite.
From the above studies it may be concluded that the Surma
group
of
sandstones
are
sub-mature.
They
consist
mostly
of
sand
size grains with small amount of silt and clay. The clay fraction
is scanty in the samples.
Mineralogically
it belongs to sublithic
arenite clan and texturally falls under silty sand and sandy silt
class.
The
sandstones
grain
are
size
'medium
indices
sandy
classification
to
very
fine
indicates
sandy';
the
that
the
indices
values also indiicate fluvial environmental condition of deposition
for
the
Surma
sandstones.
The
sediments
exhibits
moderately
to
moderately well sorted character, which might be due to selective
sorting of the transporting media.
8
6 : 3 B.
TIPAM
SANDSTONE : :
:
Ths percentages of sand, silt and clay obtained for
the
sandstones
diagram
of
of
the
Folk,
Tipam
1980 (F ig .
points concentrate in the
few
points
also
group
lie
' zS'
within
(Table-19)
.
tire
of the sand-silt-clay
tie r.
is
found
plotted
6:4)
'S'
It
are
Only
three
that
most
in
the
of
the
diagram.
points
lie
A
within
1sZ’ tire . Ths sand percentages 'sZ' tire. The sand percentages vary
from 38.34% to 92.22% , silt vary from 7.51%
to 61.20%, and clay
from 0.10% to 3.10% .
Ihe granulometric values of
, Ml, and Q^(Table-21)
are plotted on the Q^-Md-Q^diagram (F ig . 6:5) .
often more than
Plotting
'coarse
shows
sand
It
is
found
one sample fall in the same point of the
that the
1 to
sandstone
'v e r y fine
consists
sand'
.
of
The
grains
that
diagonal.
ranging
granulometric
from
analysis
shows that the
values of Qj vary from 2|) to 3() , Md from
2|) to 4j)
and Qj from 3|)
to 4{) . Only two samples had Md values between 2|)
and 4{) . For the whole rock type of the Tipam group, shows
that
Q|=2^ , Md=3^ and Q^ = 3{) . Hence , Qj - Md - (^in dices is 233.
The percentages of Q .F .L .
sandstones
diagram,
(Table-23),
proposed
concentrate,
when
on
plotted
( 1986),
shows
arenite'
wing
the
in
when
plotted
in
Dott
( 1964)
,
by
fie ld
the
alloted
Q .F .L .
that majority
j and few
arenite group (F ig .
for
diagram
of
points
the
lie
6:6 a and 6:6b).
components of the Tipam
the
Q .F .L .
shows
that
sublithic
proposed
points
in the
lie
field
the
arenite
by
triangular
points
group.
James
But
et_ a l.,
within
'quartzose
alloted
for
lithic
9
From the
found
that
Q pole
compositional
has maximum
studies
percentages,
(Table-23)
, it
followed by
L and
is
F
resp ectively. Q varies from 73.00% to 82.60", L varies from 9.00% to
16.5% andFvaries from
6 : 4
8.37% to 13,00% .
INTERPRETATION AND CONCLUSION
:
From the textural analysis and its ternary
classification
for the Tipam group of sandstones,
that the majority
's ilt y
sand'
belong to
(Points
group
the sediments of the
(point
lying
on
'sand' group (point lying
lying
predominance
presence
of
on
of
of
'sZ' tie r )
sand
size
minor amount
population,
the
components
in
sand
the
.
of
silt
becomes
in the
size
are
A small
portions
tie r)
,
' zS'
fields indicate
and
and 'sandy s ilt'
sediments,
grains
silty
sediments
sandstones belong to the
on 'S'
grains
it has been found
1zS1 tie r)
'S'
plot of
sand
but
the
with
the
to the dominant sand
(F o lk ,
nominal
to
1980)
affect
.
Clay
the
rock
composition .
From this
group ^ predominantly
,
, 'w e ll
it
can
sorted silty
by 'fine sand' and sandy-silt stone'
The grain
the
Tipam
groupof
sample
shows
samples
analyses
reveal
nomenclatured
to
"mediumsand"
different
as
have
sand' .
This
the
.
consist
grade
been
.
of
indicates that
sediments ranging
Nomenclatures
given
of
Tipam
is followed
'fine sandy-very fine sand'
that majority
"medium
that
size indices classification
sandstones
'Medium sandy-fine sand'
beinferred
in
Table-21.
the
and
. Only one
deduced
for
Granulometric
Tipam sediments
sandy-fine sand"
from
can
be
"medium sandy-very
fins sand" .
From the textural classification a general name may
be
suggested
for
the
Tipam
sandstones.
The average
indices
value
for the sandstone
is found to be
can
as
bo
designated
'233'. Therefore the rock group
'mod iurn-f in e sand'.
The
indices
value
also
indicates its deposition under fluvial environment.
The mineralogical
indicates
that
it
classification of the Tipam sandstones
belong
to
'sublith
arenite'
or
'quartzose
arenite1 group.
Therefore,
from
the
overall
textural
and
mineralogical
studies it may be concluded that the Tipam group of sandstones are
sub-mature one. They are composed mostly of sand size grains with
major amount of silt and clay. Mineralogically it belong to sublith
arenite
followed
class
by
and
fine
texturally
sand
and
falls
sandy
mainly
silt
under
class.
silty
Grain
size
sand
and
indices
classification indicates that the sandstones are medium-fine sand.
The indices values also suggest fluvial environment for the present
Tipam sandstone under study. The sediments shows mainly moderately
sorted
nature,
which
transporting media.
may
be
due
to
selective
sorting
by
the
Sand
F iq .6 :lT e x tu ra l c la ssific a tio n of the S u rm a Group of sandstones.
( A f t e r Fo lk .1 9 8 0 )
0
1
2
3
4
5
6
7
8
9
FiQ-C:3 Indices classification of the Tipam sandstone.
cs
MS
FS
VFS
VCSi
CSI
MS
FSI
VFS I
Pigor<al -
value
Flg.6:tlndices classification of the Surma group of
sandstones.
Q
Fig.6*»Classification of the Tipam sandstone (After Dott. Journal of Sedimentary
Petrology, Vol. 3 4 (1 9 6 4 ), P.629 from Pettijohn,1984).
100%
Quartz Arenife
Quartzase Arenife