sedimentologi sains laut 2 terrigenous clastic sediments gravel, sand mud

Sedimentologi
Kamal Roslan Mohamed

TERRIGENOUS CLASTIC
SEDIMENTS: GRAVEL,
SAND & MUD


INTRODUCTION
Terrigenous clastic sediments and sedimentary rocks
are composed of fragments that result from the
weathering and erosion of older rocks. They are
classified according to the sizes of clasts present and
the composition of the material.


INTRODUCTION
The proportions of different clast sizes and the
textures of terrigenous clastic sediments and
sedimentary rocks can provide information about
the history of transport of the material and the
environment of deposition.
Terrigenous clastic is material that is made up of
particles or clasts derived from pre-existing rocks.
The clasts are principally detritus eroded from
bedrock and are commonly made up largely of
silicate minerals: the terms detrital sediments and
siliciclastic sediments are also used for this material.

CLASSIFICATION OF SEDIMENTS AND SEDIMENTARY ROCKS
A distinction can be drawn between
sediments (generally loose material) and
sedimentary rocks which are lithified
sediment: lithification is the process of
‘turning into rock’ (18.2). Mud, silt and
sand are all loose aggregates; the
addition of the suffix ‘-stone’ (mudstone,
siltstone, sandstone) indicates that the
material has been lithified and is now a
solid rock.
Coarser, loose gravel material is named
according to its size as granule, pebble,
cobble and boulder aggregates, which
become lithified into conglomerate
(sometimes with the size range added as
a prefix, e.g. ‘pebble conglomerate’).



CLASSIFICATION OF SEDIMENTS AND SEDIMENTARY ROCKS
A threefold division on the basis of grain size is
used as the starting point to classify and name
terrigenous clastic sediments and sedimentary
rocks:
• gravel and conglomerate consist of clasts
greater than 2mm in diameter;
• sand-sized grains are between 2mm and
1/16mm (63 microns) across;
• mud (including clay and silt) is made up

of particles less than 63 mm in diameter.
There are variants on this scheme and there are
a number of ways of providing subdivisions
within these categories, but sedimentologists
generally use the Wentworth Scale to define and
name terrigenous clastic deposits.


THE UDDEN–WENTWORTH GRAIN-SIZE SCALE
Four basic divisions are recognised:
- clay (<4 um)
- silt (4 mm to 63um)
- sand (63 mm or 0.063um to 2.0 um)
- gravel/aggregates (>2.0 mm)
The phi scale is a numerical representation
of the Wentworth Scale. The Greek letter ‘’
(phi) is often used as the unit for this scale.
Using the logarithm base two, the grain size
can be denoted on the phi scale as
= - log2 (grain diameter in mm)
Using this formula, a grain diameter of 1mm
is 0: increasing the grain size, 2mm is -1,
4mm is -2, and so on; decreasing the grain
size, 0.5mm is +1, 0.25mm is 2, etc.


Conglomerates &
Breccias

Sandstones

Mudstones


Nomenclature used for
mixtures of gravel, sand and
mud in sediments and
sedimentary rock.


Histogram, frequency distribution and cumulative frequency curves
of grain size distribution data. Note that the grain size decreases
from left to right.


GRAVEL AND CONGLOMERATE
Clasts over 2mm in diameter are
divided into granules, pebbles,
cobbles and boulders. Consolidated
gravel is called conglomerate and
when described will normally be
named according to the dominant
clast size:
if most of the clasts are between
64mm and 256mm in diameter the
rock would be called a cobble
conglomerate.
The term breccia is commonly used
for conglomerate made up of clasts
that are angular in shape.

Composition of gravel and conglomerate
If all the clasts are of the same material (all
of granite, for example), the conglomerate
is considered to be monomict. A polymict
conglomerate is one that contains clasts of
many different lithologies, and sometimes
the term oligomict is used where there are
just two or three clast types present.


Texture of conglomerate
Conglomerate beds are rarely composed
entirely of gravel-sized material. Between
the granules, pebbles, cobbles and
boulders, finer sand and/or mud will often
be present: this finer material between the
large clasts is referred to as the matrix of
the deposit.

A clast-supported conglomerate: the pebbles
are all in contact with each other.

- sandy conglomerate
- Muddy conglomerate
- intraformational conglomerate
- clast-supported (orthoconglomerate)
- matrix-supported (paraconglomerate)

A matrix-supported conglomerate: each
pebble is surrounded by matrix.


Shapes of clasts
The shapes of clasts in gravel and
conglomerate are determined by the
fracture properties of the bedrock they
are derived from and the history of
transport.
- cubic or equant
- oblate or discoid
-Rod-shaped or prolate
When discoid clasts are moved in a
flow of water they are preferentially
oriented and may stack up in a form
known as imbrication.


Shapes of clasts
When discoid clasts are moved in a
flow of water they are preferentially
oriented and may stack up in a form
known as imbrication.


SAND AND SANDSTONE
Sand grains are formed by the breakdown
of preexisting rocks by weathering and
erosion, and from material that forms

within the depositional environment. The
breakdown products fall into two
categories: detrital mineral grains,
eroded from pre-existing rocks, and
sand-sized pieces of rock, or lithic
fragments.
Grains that form within the depositional
environment are principally biogenic in
origin, that is, they are pieces of plant or
animal, but there are some which are
formed by chemical reactions.


Detrital mineral grains in sands and sandstones
A very large number of different minerals
may occur in sands and in sandstones, and
only the most common are described here;
- Quartz
- Feldspar
- Mica
- Heavy minerals
- Miscellaneous minerals


Other components of sands and sandstones
- Lithic fragments
- Biogenic particles
- Authigenic minerals
Matrix – Fine-grained material occurring
between the sand grains is referred to as

matrix


Sandstone nomenclature and classification
The Pettijohn sandstone classification
combines textural criteria, the
proportion of muddy matrix, with
compositional criteria, the
percentages of the three commonest
components of sandstone: quartz,
feldspar and lithic fragments. The
triangular plot has these three
components as the end members to
form a ‘Q, F, L’ triangle, which is
commonly used in clastic
sedimentology.

The Pettijohn classification of sandstones,
often referred to as a ‘Toblerone plot’
(Pettijohn 1975).


CLAY, SILT AND MUDROCK
Fine-grained terrigenous clastic
sedimentary rocks tend to receive
less attention than any other group
of deposits despite the fact that
they are volumetrically the most
common of all sedimentary rocks
types.

The grain size is generally too small
for optical techniques of mineral
determination and until scanning
electron microscopes and X-ray
diffraction analysis techniques were
developed little was known about
the constituents of these
sediments.


Definitions of terms in mudrocks
Silt is defined as the grain size of material
between 4 and 62 microns in diameter. This
size range is subdivided into coarse, medium,
fine and very fine. The coarser grains of silt
are just visible to the naked eye or with a
hand lens. Finer silt is most readily
distinguished from clay by touch, as it will feel
‘gritty’ if a small amount is ground between
teeth, whereas clay feels smooth.
Clay is a textural term to define the finest
grade of clastic sedimentary particles, those
less than 4 microns in diameter. Individual
particles are not discernible to the naked eye
and can only just be resolved with a high
power optical microscope.


Clay minerals
Clay minerals are a group of

phyllosilicate minerals that are the main
constituents of clay-sized particles.
Clay minerals commonly form as breakdown
products of feldspars and other silicate
minerals. They are phyllosilicates with a
layered crystal structure similar to that of
micas and compositionally they are
aluminosilicates. The crystal layers are
made up of silica with aluminium and
magnesium ions, with oxygen atoms linking
the sheets.
-Kaolinite
- montmorillonite
- illite
- Chlorite


TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

Histogram, frequency distribution and cumulative frequency curves
of grain size distribution data. Note that the grain size decreases
from left to right.


TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

Nilai median = 1.3