Sedimentology and Stratigraphy(book review, Nichols)

TERRIGENOUS CLASTIC SEDIMENTS: GRAVEL, SAND & MUD

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.

INTRODUCTION

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 and1/16mm (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.

CLASSIFICATION OF SEDIMENTS AND SEDIMENTARY ROCKS

Four basic divisions are recognised:- clay (2.0 mm)

The phi scale is a numerical representation of the Wentworth Scale. The Greek letter M (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

M = - log2 (grain diameter in mm)

Using this formula, a grain diameter of 1mm is 0M: increasing the grain size, 2mm is -1M, 4mm is -2M, and so on; decreasing the grain size, 0.5mm is +1M, 0.25mm is 2M, etc.

THE UDDENWENTWORTH GRAIN-SIZE SCALE

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.

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.

GRAVEL AND CONGLOMERATE

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 polymictconglomerate 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.

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

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

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

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.

Shapes of clasts

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.

SAND AND SANDSTONE

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

Detrital mineral grains in sands and sandstones

- Lithic fragments- Biogenic particles- Authigenic minerals

Matrix Fine-grained material occurring between the sand grains is referred to as matrix

Other components of sands and sandstones

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.

Sandstone nomenclature and classification

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

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.

CLAY, SILT AND MUDROCK

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.

Definitions of terms in mudrocks

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

Clay minerals

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.

Median = 1.3

TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

The shapes of clasts, their degree of sorting and the proportions of clasts and matrix are all aspects of the texture of the material.

The fragments that make up a sedimentary rock are called clasts. They may range in size from silt through sand to gravel (granules, pebbles, cobbles and boulders). A distinction is usually made between the clasts and the matrix, the latter being finer-grained material that lies between the clasts.

Sorting is a description of the distribution of clast sizes present: a well-sorted sediment is composed of clasts that mainly fall in one class on the Wentworth scale (e.g. medium sand); a poorly sorted deposit contains a wide range of clast sizes.

TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

Kurtosis, a value that indicates

whether the histogram has a sharp

peak or a flat top

TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

Values

from

To Equal

0.41 0.67 very platykurtic

0.67 0.90 platykurtic

0.90 1.11 mesokurtic

1.10 1.50 Leptokurtic

1.50 3.00 very leptokurtic

> 3.00 extremely leptokurtic

Skewness of the distribution, an indicator of whether the grain-size

histogram is symmetrical or is skewed to a higher percentage of coarser

or finer material.

TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

Values

from

To Mathematically: Graphically

Skewed to the:

+1.00 +0.30 Strongly positive

skewed

Very Negative phi

values, coarse

+0.30 +0.10 Positive skewed Negative phi

values

+0.10 - 0.10 Near symmetrical Symmetrical

- 0.10 - 0.30 Negative skewed Positive phi values

- 0.30 - 1.00 Strongly negative

skewed

Very Positive phi

values, fine

Clast roundness: During sediment transport the individual clasts will repeatedly come into contact with each other and stationary objects: sharp edges tend to be chipped off first, the abrasion smoothing the surface of the clast.

Clast sphericity: In describing individual clasts, the dimensions can be considered in terms of closeness to a sphere.

TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

Roundness and sphericity estimate comparison chart (from Pettijohn et al. 1987).

The grain-size distribution is determined

to some extent by the processes of

transport and distribution.

Glacial sediments are normally very

poorly sorted, river sediments

moderately sorted and both beach and

aeolian deposits are typically well sorted.

TEXTURES AND ANALYSIS OF TERRIGENOUS CLASTIC SEDIMENTARY ROCKS

The texture of sediment or sedimentary rock can be used to indicate something about the erosion, transport and depositional history.

Any sandstone that is classified as a wacke is considered to be texturally immature. Arenites can be subdivided on the basis of the sorting and shape of the grains. If sorting is moderate to poor the sediment is considered to be submature. Well-sorted or very wellsorted sands are considered mature if the individual grains are angular to subrounded and supermature if rounded to well-rounded.

Maturity often increases downstream in a river and once the same sediment reaches a beach the high wave energy will further increase the maturity.

Maturity of terrigenous clastic material

Maturity of terrigenous clastic material

Flow diagram of the determination of the textural maturity of a terrigenous clastic sediment or sedimentary rock.