UNIT – 2Detrital Sedimentary Rocks

Clastic Sediments and Sedimentary Rocks

Clastic sediments are loose materials, however when these sediments under go lithification (diagenesis), they form sedimentary rocks.

Clastic sedimentary rocks are also known as Detrital or Mechanical sedimentary rocks.

Mud, silt and sand are all loose sediments. The addition of stone to these words (mudstone, siltstone and sandstone), indicates that these sediments are lithified and are sedimentary rocks.

A three fold classification based on the grain size is used as the starting point for classifying detrital sediments and sedimentary rocks. They include

Gravel and Conglomerate : Size greater than 2mm Sand and Sandstone: 2mm to 1/16 mm Mud and Mudstone: Less than 1/16 mm

The Udden-Wentworth grain-size scale

Known as the Wentworth scale, this is the most widely accepted scale for classifying clastic sediments.

The divisions on the scale are made on the basis of factors of 2. Example : Medium Sand : 0.25 to 0.5 mm Coarse Sand: 0.5 mm to 1.00 mm Very Coarse Sand : 1.00 mm to 2.00 mm

The phi scale (φ) is the numerical representation of the Wentworth scale

The Udden-Wentworth grain-size scale for clastic sediments

Size (Millimeters)

Wentworth Grade Phi (Φ) Scale Sediment Sedimentary Rock

>256 Boulder –8

GRAVEL CONGLOMERATE256-64 Cobble –6

64-4 Pebble –2

4-2 Granule –1

2-1 Very coarse sand

0

SAND SANDSTONE1-1/2 Coarse sand 1

1/2-1/4 Medium sand 2

1/4-1/8 Fine sand 3

1/8-1/16 Very fine sand 4

1/16-1/32 Coarse silt 5

MUD MUDSTONE

1/32-1/64 Medium silt 6

1/64-1/128 Fine silt 7

1/128-1/256 Very fine silt 8

<1/256 Clay >8

Gravel and Conglomerate

Sediments/clasts more than 2 mm in diameter are divided into granules, pebbles, cobbles and boulders.

Consolidated gravel is called conglomerate.

The term breccia is used for conglomerate made up of clasts that are angular in shape.

Mixture of rounded and angular clasts are sometime known as breccio-conglomerate.

Gravel

Conglomerate

Breccia

Composition of gravel and conglomerate

If all the clasts in a conglomerate are of the same material (e.g. granite), then the conglomerate is called monomict.

A polymict conglomerate is one that contains of clasts of many different lithologies/rock types.

Any kind of rock type may be found as a clast in gravel or conglomerate.

Resistant rock type which are less susceptible to physical and chemical weathering have a greater chance of being preserved as clasts in conglomerate.

The factor controlling the type of clast found in a given conglomerate depends on the type of bedrock being eroded in the area.

If the bed rock being eroded is limestone then all the clasts in the gravel or conglomerate will be made up of limestones.

Recognition of the clast type can help in determining the source of the conglomerate.

Texture of Conglomerate

Conglomerate beds are rarely composed entirely of gravel sized material.

Between the granules, pebbles, cobbles and boulders finer sand or mud is always present.

This finer material between the bigger clasts is known as the matrix.

If there is a higher proportion of matrix ( more than 20 %) then the conglomerate can be called as sandy conglomerate or muddy conglomerate depending upon the grain size of the matrix.

The conglomerates can be clast supported or matrix supported.

Matrix supported conglomerates are known as Para-conglomerates.

Clast supported conglomerates are known as Ortho-conglomerates.

Matrix

Clasts

Ortho-conglomerates

Para-conglomerates

Conglomerate/Sandstone/Mudstone Ternary Diagram

Shape of clasts

The shape of clasts in gravel and conglomerates are determined by fracture properties of the bedrock.

Rocks with equally spaced fracture planes in all directions form cubic or equant blocks that form spherical clasts when the edges are rounded off.

Limestones and sandstones form clasts with one axis shorter than the other two. This is termed as oblate or discoid form.

Rod shaped or prolate clasts are less common and are formed mainly from strongly foliated metamorphic rocks.

Shape of clasts

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

The direction of imbrication of discoid pebbles in a conglomerate can be used to indicate the direction of flow that deposited the gravel.

Sand and Sandstones

Sediment/clast size between 2 mm to 1/16 mm are known as sand and the sedimentary rock formed from these clasts are known as sandstone.

Many sanstone contain mainly quartz grains, but it is not always true. Some sandstones have no quartz at all.

Detrital Mineral grains in sands and sandstones

A very large number of different minerals may occur in sands and in sandstones. The most common of them include:

Quartz: Quartz is the most common mineral found in sanstones.

Quartz is a very stable mineral and is resistant to weathering. The quartz grains remain intact over long distances and long period of transport.

Detrital Mineral grains in sands and sandstones

Feldspar is a common mineral and is released in large quantities when felsic igneous rocks and granitic gneiss and schists breakdown.

Feldspars are more susceptible to chemical alteration and is found only when the chemical weathering of the bedrock has not been too intense and the transportation distance to the site of deposition is short.

Potassium Feldspars are more common in sandstones than Calcium or Sodium Feldspar as they are chemically more stable when subjected to weathering.

Detrital Mineral grains in sands and sandstones

The two common Mica minerals, biotite amd muscovite are relatively abundant in sandstones.

Muscovite is more resistant to weathering as compared to Biotite.

They are derived from intermediate igneous rocks and from schists and gneisses.

Detrital Mineral grains in sands and sandstones

Heavy minerals: The common minerals in sands have densities of around 2.6 to 2.7 g/cm3.

Most sandstones contain a small proportion, commonly less than 1% of minerals that have a greater density.

Common heavy minerals include zircon, tourmaline, rutile, apatite, garnet etc.

Tourmaline

GarnetZircon

Detrital Mineral grains in sands and sandstones

Miscellaneous minerals: Other minerals rarely occur in large quantities in sandstones.

Most of the common silicate minerals in silicate rocks such as pyroxenes, amphiboles and olivines are easily broken down by chemical weathering.

Oxides of iron are fairly abundant in sandstones.

Local concentration of a particular mineral may occur when there is a near by source.

Other components of sands and sandstone

Lithic Fragments: Breakdown of pre-existing, fine to medium grained igneous, metamorphic and sedimentary rocks result in sand-sized fragments.

Determination of these fragments of rock

usually requires petrographic analysis by thin-section examination to identify the minerology and fabric.

Other components of sands and sandstone

Biogenic particles: Small pieces of calcium carbonate found in sandstone are commonly broken shells of molluscs and other organisms that have calcareous hard parts.

These biogenic fragment are common in sandstone deposited in shallow marine environment.

If these calcareous fragments are more than 50 percent then the rock would be called as limestone.

Other components of sands and sandstone

Authigenic minerals: Minerals that grow as crystals in the depositional environment are called authigenic minerals.

Glauconite and many carbonate minerals form in this way.

Other components of sands and sandstone

Matrix: Fine grained mineral occurring between the sand grains is referred to as matrix.

In sands and sandstone the matrix is typically silt and clay sized material and it may wholly or partly fill the space between the grains.

Matrix is the material deposited along with the grains whereas cement is chemically precipitated after deposition.

Sandstone Classification

Full description of a sandstone requires information about the type of grain presents.

Sandstone containing mica may be called as micaceous sandstone.

Sandstone containing calcium carbonate may be called as calcareous sandstone.

For a formal classification of sandstone the Pettijohn sandstone classification is used.

Pettijohn Sandstone Classification

Pettijohn Sandstone Classification

Sandstone having less than 20 percent matrix is known as arenite.

Sandstone having between 20 to 50 percent matrix is known as wacke.

Sandstone with more than 50 percent matrix is known as mudstone.

Each of this sandstone type is represented by a triangular plot and the corners of the triangle are represented by Quartz (Q), Feldspar (F) and Lithic fragments (L).

So depending upon the compostion of Q, F and L sandstones can be classified as

Quartz Arenite, Feldspatic (Arkose)Arenite and Lithic Arenite Quartz Wacke, Arkose Wacke and Lithic Wacke Quartz Mudstone, Arkose Mudstone and Lithic Mudstone

• Lithic Wacke and Arkose Wacke are also sometimes known as Graywacke

Clay, Silt and Mudstone

Sediment/clast size less than 1/16 mm are known as mud and the sedimentary rock formed from these clasts are known as mudstone.

Definitions of terms in mudstone

Silt is defined as the grain size of materials between 1/16 mm to 1/256 mm.

This range is subdivided into coarse, medium fine and very fine.

Clay is defined as the grain size of material less than 1/256 mm. Clay minerals can not be seen with the naked eye and can be seen only with high power optical microscope.

Clay minerals are a group of phyllosilicate ( sheet silicates ) and are the main constituent of clay sized particles.

Definitions of terms in mudstone

When clay and silt sized particles are mixed in unknown proportion, the resulting sediment is known as mud and the rock is known as mudstone.

If more than two-thirds of the sediments are clay sized then the corresponding rock is known as claystone.

If more than two-thirds of the sediments are silt sized then the corresponding rock is known as siltstone.

The term shale is applied to mudstone, however it is best to use this term for mudstones that show fissility.

Texture of clastic sedimentary rocks

Texture of clastic sedimentary rocks

Clasts and Matrix: The fragments that make up a sedimentary rocks are called clasts.

Matrix is the fine grained material that lies between the clasts.

Clasts

Matrix

Texture of clastic sedimentary rocks

Sorting is a description of the distribution of clast sizes presents.

Sorting is a function of the origin and transport history of the clastic sediments.

With increased transport distance the different size of clasts get separated.

Texture of clastic sedimentary rocks

Clast Roundness: Roundness of the clasts is a function of the transport history of the sediments.

Well rounded clasts in a sediment or sedimentary rock is an example of long transportation history.

Short transportation results in angular clasts.

Sphericity is an inherited feature that is, it depends on the shape sof the fragments which formed during weathering.

Low Sphericity

High Sphericity