Sedimentary Petrology

Sedimentary Petrology

SEDIMENTARY ROCKS

The decomposition and disintegration of rocks on the earth’s surface usually occur together, but one process is usually predominant.

Decomposition is more active in warm, moist, low-lying areas; disintegration occurs mainly in the drier, higher and colder regions of the earth’s surface.

DECOMPOSITION: The principal AGENTS of decomposition are water and air. The chief PROCESSES of decomposition are solution, oxidation, hydration and carbonation. Nearly all minerals are acted upon to some extent by water, especially when it contains certain dissolved substances. Some, however, are more susceptible than others and minerals may thus be divided into those which are relatively resistant, such as quartz, muscovite and zircon, and those which are altered with comparative ease, such as the feldspars and most of the ferromagnesian minerals.

The process of oxidation involves the alteration of minerals, with the production of oxides. It is especially active with iron-bearing minerals, forming the iron oxides haematite and limonite.

Hydration is a process by which minerals are altered into substances rich in combined water. Magnesium-bearing minerals, such as olivine, are thus altered into serpentine and talc.

(Mg,Fe)2SiO4 ð Mg6[Si4O10](OH)

In carbonation, the minerals are altered, with the formation of carbonates. It is especially effective with those minerals containing the alkali metals sodium and potassium, as well as calcium and magnesium.

The decomposition of a granite furnishes many different kinds of minerals:

Þ  Unaltered minerals: Includes quartz and zircon, which form sand grains. Muscovite produces mica grains.

Þ  Insoluble residues: Includes hydrous aluminium silicates, which are fundamental constituents of clays; and iron oxides, which are the colour matters of rocks.

Þ  Soluble substances: Includes salts of potassium, sodium, calcium, magnesium, iron, etc., and silica.

DISINTEGRATION: In high mountains, deserts and snow- or ice-covered regions, the process of decomposition is largely in abeyance and disintegration is the dominant mode of breaking down rocks. Disintegration may result from a variety of causes. The great diurnal variations of temperature in deserts and mountainous regions cause strains to be set up in the surface layers of rocks, by which fragments are scaled off. The process is known as exfoliation. The freezing of water in fissures tends to disrupt rocks into angular fragments and much of the weathering in high mountains takes place in this way, the summits being covered with a thick layer of rock debris. The abrasive action of sand carried by wind or water causes the disintegration of rocks in deserts, or in the channels of swift-running, sand-laden rivers. Glaciers my pluck and rend boulders from their beds; and by their slow restless movement grind the material they carry against the sides of the containing valleys, with the formation of sand and mud. Most streams issuing from glaciers are heavily laden with materials derived from this action. The pounding of waves may result in much disintegrative action, as is proved by extensive coast erosion. Finally, organic agents often have a marked mechanical effect upon rocks. The roots of plants prise open the fissures or rocks in their search for moisture and nourishment; burrowing animals turn over the soil and subsoil; and man himself, by tilling the ground, deforestation, tunnelling, quarrying, mining and in numerous other ways, helps to disintegrate the rocks.

By disintegration, a granite will break up into a coarse sand composed of fragments of quartz, feldspar and micas, mixed with pieces of rock not yet broken down into the component minerals. Many granite areas carry sands of this composition, which are called arkose sands, or, when accumulated, arkose. A basic rock broken up in the same way gives rise to a rock called wacke or greywacke, which is composed of plagioclase feldspar, ferromagnesian minerals and quartz. Disintegration may also produce rough, angular rubble consisting of any kind of rock, which may mantle a mountain-top or accumulate by the action of gravity at the foot of a slope. These accumulations are called talus or scree when unconsolidated, and breccia when consolidated or cemented into a coherent mass. The resultant of the twin processes of decomposition and disintegration is weathering, and the product thereof, in the first place, is the mantle of loose, broken and largely decomposed material, the regolith, which covers the surface of the earth. The finely broke upper layer of the regolith, well aerated and mixed with decayed organic matter, is the soil.

TRANSPORTATION: The soluble or insoluble material supplied by weathering is either accumulated in place or is transported and deposited elsewhere. The agents of transport are rivers, waves, oceans currents, wind, and glaciers. The loess of China is believed to be simply an extensive deposit of wind-blown dust derived from the Asian deserts. Wind is by far the most efficient agent of rounding, and grains which have suffered long transport by wind show almost perfect spherical forms. Ice transport, however, permits very little rounding. Wind, again, is the most efficient sorted of grains, and deposits carried by wind are often characterised by their homogeneity. In ice transport, however, there is little or no sorting of materials, and on melting or retreat of the ice, it is dumped down into an unassorted and heterogeneous mixture of rock flour, grains, pebbles and boulders of all sizes.

DEPOSITION: The ultimate destination of transported material, whether carried by water, wind or ice, is the sea, but it may be temporarily deposited on the land, and the deposits thus formed may persist for several geological periods before they resume their march to the sea. This leads to a distinction between continental and marine deposits. Deposition may be either mechanical or chemical, according to whether it affects the mechanically transported insoluble material or the substances carried in solution. The material carried in suspension or in other ways by water, wind and ice is deposited when the transporting medium is overloaded, when its velocity is checked, or when it suffers a chemical or physical change. Very extensive deposits of clay, silt and sand thus occur in the lower parts of river systems and also where rivers debauch into the sea(in deltas). The settlement of material entering the sea is aided not only by decrease in velocity of the river current, but also by admixture of salt water, which promotes a physical change (flocculation) favourable to the deposition of suspended material.

The soluble material derived from weathering may be deposited either on land or in water, directly by physicochemical processes such as precipitation, or indirectly by the agency of organisms.

Sedimentary Rocks

MINERALOGICAL, TEXTURAL AND STRUCTURAL CHARACTERS:

Þ  Mineral composition: The minerals of sedimentary rocks fall into two classes:

¨  Insoluble residues of rock decomposition: This includes the groups of:

à  Clay minerals such as kaolinite, halloysite, etc.

à  Micaceous minerals, including the hydromicas and chlorite.

à  Aluminium hydroxides like bauxite, gibbsite, etc.

à  Ferric oxides and hydroxides.

¨  Comparatively durable minerals from pre-existing rocks. These include:

à  Quartz (most abundant)

à  Accessory minerals like zircon, rutile, tourmaline, garnet, kyanite, magnetite, etc.

Mineral composition also depends upon the nature of the rocks forming the gathering round of the material. If the country rock consists mainly of some mineralogically uniform rock such as quartzite or a granite poor in ferromagnesian minerals and accessory minerals, the composition of the sediment resulting from its denudation will be simpler than that resulting from the waste of a lithologically or mineralogically heterogeneous region. The duration and nature of the transport is also a factor in determining the mineral composition. Long, continued drifting of particles separates them according to mass and surface area, and therefore according to composition. Wind is a particularly efficient sorter of sand grains. In deserts, mica flakes and dust are blown far away, and the remaining sands are sifted and redistributed until there is an approach to mineral uniformity. Long, continued transport in rivers or along shores may be almost equally effective in producing clean, graded and uniform deposits. Transportation tends to destroy the softer, more cleavable and brittle mineral grains, and thus produces greater mineral uniformity in the final material.

Those constituents, such as boulders, pebbles or mineral grains which have been formed elsewhere and have been brought into a sediment from outside are termed allogenic (originating elsewhere); those constituents which have been formed de novo/in situ are called authigenic (formed in place or on the spot).

TEXTURES OF SEDIMENTARY ROCKS:

Textures of sedimentary rocks are defined by at least six factors:

Þ  Origin Of Grains: A sedimentary rock may be partially or wholly composed of clastic(allogenic) grains, or chemically or organically evolved(authigenic) components, giving it contrasting textures. Thus, rocks rich in clastic grains of any size, shape and composition are said to show clastic textures and these form the two principle types of sedimentary textures.

Þ  Size OF Grains: The grain size in sedimentary rocks varies within wide limits. Individual grains of less than 0.002mm or more than 250mm may form a part or whole of these rocks. Accordingly, rocks are divided into fine grained (grain size < 1mm), medium grained (grain size between 1mm and 5mm) and course grained (grain size > 5mm). The type of weathering, the nature of the parent rock and the duration of transport are some of the factors that cause a variation in the grain size of the sediments.

HOLMES’ CLASSIFICATION OF SEDIMENTARY FRAGMENTS:

WENTWORTH-UDDEN SCALE (SI):

Þ  Shape Of Grains: Individual outlines of sediments are generally of considerable significance in defining the textural characteristics. These grains may be round, smooth and spherical, or angular or rough. Roundness and sphericity are the indications of a greater amount of abrasion and generally of a large amount of transportation in the clastic rocks.

Þ  Packing of the Grains: Sediments may be open-packed or close-packed. The density of the packing is generally related to the pressure, either from above (because of overlying strata) or from the sides (because of compressive forces originating during mountain-building periods).

Þ  Fabric of the Grains: A given sedimentary rock may contain many elongated particles. Their orientation, which is studied in terms of their long axis, is of great textural importance. If all or most of the elongated particles are arranged in such a way that their long axes lie in the same direction, the rock is said to show a high degree of preferred orientation. The direction of preferred orientation is commonly related to the direction of the current flow of the medium of transport.

Þ  Crystallisation Trends: In sedimentary rocks of chemical origin, textures are usually defined on the basis of degree and nature of crystallisation of the component grains. Rocks may show perfectly interlocking grains, giving rise to crystalline granular textures, or they may be composed of non-crystalline, colloidal particles, when textures are termed amorphous.

STRUCTURES OF SEDIMENTARY ROCKS:

The term structure includes some large-scale features of the sedimentary rocks that have been imposed on them during their formation. These can be best studied under three headings:

Þ  Mechanical Structures

Þ  Chemical Structures

Þ  Organic Structures

¨  Mechanical Structures: They include those structures that have developed because of some physical processes operating at the time of deposition of the sediments. These include:

Ø  Stratification: By stratification is understood a layered arrangement in a sedimentary rock and this may be very prominent or only mildly displayed. The different layers may be of similar or dissimilar colour, grain size and composition. These layers, also known as beds or strata if more than 1 cm thick, are separated from each other by planes of weakness - the bedding planes. When the bedding planes are very close to each other, or, in other words, the beds are very thin (generally <1cm thick), the term lamination is used instead of stratification, and the layers are known as laminae. Lamination is a characteristic feature of very fine-grained rocks like shale.

Ø  Cross Bedding: Changes in velocity and direction of the currents of the transporting agent result in an irregular type of stratification, variously called false bedding, current bedding or cross bedding. The cross-bedding is described as tabular when successive sets have essentially parallel top and bottom surfaces. It is termed as lenticular when layers show extreme irregularity in their shape and disposition; each layer or set of beds may be intersected by other lying at different angles. The cross-bedding is known as wedge-shaped when the structure is highly complex; the laminae of different beds dip in different direction and at different angles.