Erosion and Sedimentary Textures The specification says: Explain the influence of gravity, wind, ice, the sea and rivers on the parameters of grain size,

Erosion and Sedimentary Textures

The specification says:

Explain the influence of gravity, wind, ice, the sea and rivers on the

parameters of grain size, shape, roundness and degree of sorting.

We will start the textures first before looking at erosive agents.

Strictly the length of its intermediate axis determines the size of a sedimentary grain.

With large grains, such as pebbles, this can be easily measured.

However, for finer grained sediments, such as sands, it would be a very difficult process.

An easier method is by visual comparison with sand of known grain sizes.

The grain sizes can be printed on a "grain size comparator”.

Sediment grain size is described according to the "Wentworth Scale" .

Coarse >2 mm Rudaceous

Medium 1/16 - 2 mm Arenaceous

Fine < 1/16 mmArgillaceous

The term used to describe the overall shape of a grain is sphericity (how like a sphere it is).

Another, and separate, aspect of grain shape is the degree of roundness.

This relates to the extent to which originally angular edges and corners have been rounded off.

The range of terms used to describe the rounding of grains is : very angular subangular subrounded rounded well rounded

Very often, sediments contain grain sizes belonging to several different classes on the Wentworth Scale.

Sorting is an expression of the overall grain size of sediment.

If most of the grains in sediment are of a similar size the sediment is well sorted.

If the range of grain sizes is great then the sediment is poorly sorted.

Poorly sorted sediments were usually deposited rapidly whilst well sorted have taken time to be deposited.

COMPONENTS OF A SEDIMENTARY ROCK:

There are three types of grain in a clastic sedimentary rock:

1. Clasts: these are the dominant larger fragments that usually make up the majority of the rock.

2. Matrix: This is made up of finer grains that occur between the clasts.

3. Cement: Often there is a third type of grain present, too small to be seen by the naked eye.

Erosion Processes

We will now look at the various agents of erosion and look at how they transport

sediment and how they affect the nature of the sediment as required by

the syllabus.

Any material moved by rainwater can end up in a river and is then carried by the river as "load" as:

Solution: dissolved material. HCO3 is the

most abundant with Ca and Si less abundant and Na and Cl least abundant.

Suspension: Small particles held in water and do not touch the riverbed.

Bed load: rolled along the riverbed. Travel by bouncing, rolling and sliding across the riverbed.

Various processes occur which shape the riverbed and the material.

Processes Involved:1. Attrition: clasts knock against each other

and get smaller.

2. Abrasion: Clasts dragged along the river bed strike the bottom and sides of the channel and slowly grind them away.

3. Corrosion: water dissolves the rocks to varying degrees.

Sea erosion is caused by a number of processes:

1. Action of water and spray.

2. Abrasive action of clasts carried by the water.

3. Water can trap air in crevices, which is compressed. The rapid decompression as the wave recedes causes the rock to break and may be sucked out "Hydraulic Action".

4. Some rocks can suffer solution from sea water i.e. limestone.

The sea is very efficient at eroding rocks along the coast.

Much material is quickly removed by the sea water:

Dissolved: Obviously NaCl. Suspension: arming the

waves and these can be large clasts.

Traction: material dragged along by the sea in contact with the beach (like river bed load).

Therefore distinctive coastal landforms are produced i.e. wave cut platforms and notches.

Transport is by wave and current action i.e. "long shore drift".

Effect of Erosion on Sediment The processes involved

in transport such as attrition and abrasion have the following effects on clasts:

1. The clasts get smaller2. The clasts become more

rounded3. Become better sorted. In a river therefore at the

source the clasts will be larger, poorly sorted and angular.

At the mouth they will be fine, well sorted and rounded.

Ice erodes by: Abrasion: ice carries

embedded clasts that act like sandpaper that scrapes away at the rock below. This produces angular fragments and rock “flour”.

Striations indicate the direction of ice movement.

Plucking: Pieces of rock are pulled from the underlying outcrop.

Plucking is most common where the underlying rock has lines of weakness i.e. jointing or has been weathered.

Ice freezes onto the rock and as the ice moves forward it pulls the rock with it.

Broken pieces of rock can fall onto the glacier i.e. from freeze thaw.

Material carried by glaciers may be deposited as "moraine".

The clasts vary in size from rock flour to very large boulders "erratics".

Therefore poorly sorted and tend to be angular.

Wind is normally only energetic enough to transport sand sized clasts.

Most sand is moved by saltation.

During storms the sand may go into suspension.

Larger clasts may creep along the floor during storms or when hit by sand.

The resulting sand grains are: Well sorted Well rounded Frosted “Millet seed” texture

The larger clasts may have 3 flattened sides (dreikanter) as shown in the diagram.

The Hjulstrom Diagram