WEATHERING

Sedimentology

Salah A. Hussain

THE SEDIMENTARY CYCLE

Sediments are what settle at the bottom of a liquid.

Sedimentology is the study of the sediments.

The sedimentary cycle consists of the phases of weathering, erosion,

transportation, deposition, lithifaction, uplift, and weathering again.

Weathering is the name given to the processes that break down rock at

the earth's surface to form discrete particles. Erosion is the name given to

the processes that remove newly formed sediment from bedrock.

Weathering is generally divided into biological, chemical, and physical

processes. Chemical weathering selectively oxidizes and dissolves the

constituent minerals of a rock. Physical processes of weathering are those

that bring about its actual mechanical disaggregation. Biological

weathering is caused by the chemical and physical effects of organic

processes on rock.

Erosion, the removal of new sediment, can be caused by four agents:

gravity, glacial action, running water, and wind. The force of gravity

causes the gradual creep of sediment particles and slabs of rock down

hillsides.

Glacial erosion occurs where glaciers and ice sheets scour and abrade the

face of the earth as they flow slowly downhill under the influence of

gravity. Moving water is a powerful agent of erosion in a wide spectrum

of geomorphological situations ranging from desert flash flood to

riverbank scouring and sea cliff undercutting. The erosive action of wind,

on its own, is probably infinitesimal. Wind, however, blowing over a dry

desert, quickly picks up clouds of sand and sandblasts everything in its

path for a height of a meter or so. Eolian sandblasting undercuts rock

faces, carving them into weird shapes, and expedites the erosion of cliffs

by gravity collapse and rainstorm.

WEATHERING

Weathering, as already defined, includes the processes that break down

rock at the earth's surface to produce discrete sediment particles.

Weathering may be classified into chemical, physical, and biological

processes. Chemical processes lead essentially to the destruction of rock

by solution. Physical processes cause mechanical fracture of the rock.

Biological weathering is due to organic processes. These include both

biochemical solution, brought about largely by the action of bacteria, and

acids derived from rotting organic matter, as well as physical fracturing

of rock such as may be caused by tree roots.

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Sedimentology

Salah A. Hussain

Biological Weathering and Soil Formation:

Soil is the product of biological weathering. It is that part of the

weathering profile which is the domain of biological processes. The study

of soils, termed pedology, is of interest to geologists insofar as it affects

rock weathering and sediment formation. Pedology is, however, of

particular importance to agriculture, forestry, and to correct land

utilization in general. Pedologists divide the vertical profile of a soil into

three zones, figure (1).

The upper part is termed the "A zone," or eluvial horizon. In this part of

the profile organic content is richest and chemical and biochemical

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weathering generally most active. Solutes are carried away by

groundwater.

The fine clay fraction percolates downward through the coarser fabric

supporting grains. Below the A zone is the "B zone," or illuvial horizon.

At this level downward percolating solutes are precipitated and entrap

clay particles filtering down from the A zone. Below the illuvial horizon

is the "C zone." This is essentially the zone where physical weathering

dominates over chemical and biological processes. It passes gradually

downward into unweathered bedrock. The thickness of a soil profile is

extremely variable and all three zones are not always present. Thus soil

thickness depends on the rate of erosion, climatic regime, and bedrock

composition.

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Sedimentology

Salah A. Hussain

As already seen, in areas of high relief, erosion can occur so fast that

weathering and soil formation cannot develop. By contrast, in humid

tropical climates granite can be weathered for nearly 100 m.

Physical Weathering

Four main types of physical weathering are generally recognized:

1-freeze-thaw, 2-insolation, 3-hydration and dehydration, 4- stress

release.

Freeze-thaw weathering occurs where water percolates along fissures

and between the grains and crystals of rock. When water freezes, the

force of ice crystallization is sufficient to fracture the rock. The two

halves of a fracture do not actually separate until the ice thaws and ceases

to bind the rock together. Freeze-thaw weathering is most active,

therefore, in polar climates and is most effective during the spring thaw.

Insolation weathering occurs by contrast in areas with large diurnal

temperature ranges. This is typical of hot arid climates. In the Sahara, for

example, the diurnal temperature range in winter may be 25~ Rocks

expand and contract in response to temperature. The diverse minerals of

rocks change size at different rates according to their variable physical

properties. This differential expansion and contraction sets up stresses

within rock. When this process occurs very quickly the stresses are

sufficient to cause the rock to fracture. This is why insolation weathering

is most effective in arid desert climates.

In climatic zones that experience alternate wet and dry seasons, a third

process of physical weathering occurs. Clays and lightly indurated shales

alternatively expand with water and develop shrinkage cracks as they

dehydrate. This breaks down the physical strength of the formation; the

shrinkage cracks increase permeability, thus aiding chemical weathering

from rainwater, while waterlogged clays may lead to landslides.

The fourth main physical process of weathering is caused by stress

release. Rocks have elastic properties and are compressed at depth by the

overburden above them. As rock is gradually weathered and eroded the

overburden pressure decreases. Rock thus expands and sometimes

fractures in so doing. Such fracturing is frequently aided by lateral

downslope creep. Once stress-release fractures are opened they are

susceptible to enlargement by solution from rainwater and other

processes.

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Sedimentology

Salah A. Hussain

Chemical Weathering:

The processes of chemical weathering rely almost entirely on the agency

of water. Few common rock-forming minerals react with pure water,

evaporites excepted. Groundwater, however, is commonly acidic. This is

due to the presence of dissolved carbon dioxide from the atmosphere

forming dilute carbonic acid. The pH is also lowered by the presence of

humic acids produced by biological processes in soil. The main chemical

reactions involved in weathering are oxidation and hydrolysis. Carbonic

acid dissolved in groundwater releases hydrogen ions thus:

H20 + CO 2 = H2CO 3 = HCO3- + H +.

The released hydrogen may then liberate alkali and alkali-earth elements

from complex minerals, such as potassium feldspar:

2KA1Si308 + 2H + + 9H20 = A12Si2O5 (OH)4 + 2K + + 4H2SIO4.

This reaction leads to the formation of kaolinite and silica. The

weathering reactions are extremely complex and still little understood.

The order in which minerals break down by weathering is essentially the

reverse of Bowen's reaction series for the crystallization of igneous

minerals from cooling magma, figure (2).

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Sedimentology

Salah A. Hussain

Chemical weathering separates rock into three main constituents: the

solutes, the newly formed minerals, and the residuum. The solute

includes the elements such as the alkali metals, principally sodium and

potassium, and the rare earths, magnesium, calcium, and strontium. These

tend to be flushed out of the weathering profile and ultimately find their

way into the sea to be precipitated as calcium carbonate, dolomites, and

evaporite minerals. The residuum is that part of the rock which, when

weathered, is not easily dissolved by groundwater.

Particles, Pores and permeability:

A sediment is, by definition, a collection of particles, loose or indurated.

Any sedimentological study commences with a description of the

physical properties of the deposit in question.

Physical properties of particles:

1- Surface Texture of Particles:

The surface texture of sediment particles has often been studied, and

attempts have been made to relate texture to depositional process.

Pebbles in arid eolian environments sometimes show a shiny surface,

termed "desert varnish." This is conventionally attributed to capillary

fluid movement within the pebbles and evaporation of the silica residue

on the pebble surface.

Electron microscopic studies show that there are several types of surface

texture on sand grains produced by glacial, eolian, and aqueous

processes. The surfaces of water-deposited sand grains are characterized

by V-shaped percussion pits and grooves. Glacial sands show conchoidal

fractures and irregular angled microtopography. Eolian sands show a

flaky surface pattern.

2- Particle Shape, Sphericity, and Roundness:

Numerous attempts have been made to define the shape of sediment

particles and study the controlling factors of grain shape. Pebble shapes

have conventionally been described according to a scheme devised by

Zingg (1935). Measurements of the ratios between length, breadth, and

thickness are used to define four classes: spherical (equant), oblate (disk

or tabular), blade, and prolate (roller). These four types are shown in

Figure (3).

The shape of pebbles is controlled both by their parent rock type and by

their subsequent history. Pebbles from slate and schistose rocks tend to

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