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8/10/2019 The Distinction between Grain Size and Mineral Composition in Sedimentary-Rock
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The Distinction between Grain Size and Mineral Composition in Sedimentary-RockNomenclatureAuthor(s): Robert L. FolkSource: The Journal of Geology, Vol. 62, No. 4 (Jul., 1954), pp. 344-359Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/30065016.
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2/18
THE
DISTINCTION BETWEEN GRAIN
SIZE AND
MINERAL
COMPO-
SITION IN
SEDIMENTARY-ROCK
NOMENCLATURE1
ROBERT
L.
FOLK
University
of
Texas
ABSTRACT
A
system
of grain-size nomenclature of
terrigenous
sediments
and
sedimentary rocks
is
introduced
wherein
fifteen
major
textural
groups are
defined
on
the
ratios
of
gravel,
sand,
silt,
and
clay.
Further
subdivi-
sion of
each
class
is
based on
the
median
diameter
of
each
size
fraction
present. Next,
the
mineral
composition
of
terrigenous
sedimentary
rocks
is
considered. A
triangular
diagram
is
used
to
define
eight
rock
types
(orthoquartzite,
arkose,
graywacke,
and
five
transitional types)
based
on
the
mineralogy
of
the
silt-sand-
gravel fraction and
ignoring
clay
content.
The writer
contends
that
the
current
practice
of
calling
all
clayey
sandstones
"graywackes"
is
not
valid,
inasmuch
as it represents
a
confusion
of
texture
with composition.
It
is
suggested that sedimentary rocks may be best defined by the use of a tripartite name, based on the following
pattern-(grain
size):
(textural
maturity)
(mineral
composition).
I.
INTRODUCTION
Terrigenous
sedimentary
rocks
are
characterized
by
extreme
variation
in
composition
and
in
grain
size;
and,
with-
in
certain
limits,
any
mineral
composi-
tion
can occur
with
almost any
grain
size.
It is consequently
very difficult
to
devise
a
nomenclature
that
will,
in
one
or
a
few words,
define
the
essential
prop-
erties
of
a given
specimen and
differen-
tiate
it
from
its
neighbors. Furthermore,
grain
size
must
be described
independ-
ently
of
mineral
composition if
sedimen-
tary
petrography
is
ever
to
reach a
foot-
ing
as
secure
as
that
of
igneous
petrog-
raphy.
Confusion
of
grain size
and
min-
eral
composition is
especially
pro-
nounced
in
the
usage
of
the
term
"gray-
wacke,"
which
has
been
applied
indis-
criminately
to
all
clayey
sandstones
(a
textural
category),
regardless
of
the
min-
eral
composition
of
the
sand
portion.
In
addition to
mineral
composition
and
grain
size,
there
is
a
third
important
descriptive
property,
defined
as
the
textural
maturity
of
the
rock,
which
has
been
discussed
elsewhere
in
more
detail
(Folk, 1951).
According to
this
concept,
1
Manuscript
received July
10,
1953.
sediments
pass
sequentially
through
four
stages
of
textural
maturity,
depend-
ing
on
the
stability
of the
depositional
site
and
the
input of
modifying energy:
(1) the
immature
stage,
in
which
the
sediment
contains
a
considerable
amount
of
clay
and the sand
grains
are usually
poorly
sorted
and
angular; (2) the sub-
mature stage, in which the clay has been
winnowed out, but
the
silt-sand-gravel
fraction
remains
poorly sorted
and
angu-
lar;
(3)
the
mature
stage,
in
which
the
grains
are
well
sorted
but
still
show
low
roundness;
and,
finally,
(4) the
super-
mature
stage, in
which
the
grains
are
well
sorted
and
also
well
rounded.2
The
present
paper
is
concerned
first
with
the
grain
size
of
sedimentary
rocks;
fifteen major textural groups are defined,
depending
upon
the
proportions
of
grav-
el,
sand,
and
mud
(defined
here
as
the
2
Boundary
lines
between the
four
stages are
now
(1954)
defined
as follows:
immature
sediments
con-
tain
over
5
per
cent
clay and very
fine
micaceous
hash;
the
submature stage
is
attained
when
the
clay
content
drops
below 5
per
cent; the
sediment
becomes
mature
when
there
is
a
range
of less
than
1.0
f
unit
between
the
16th
and
84th
percentiles
of
the
grain-size
distribution
(i.e.,
o
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