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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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

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