Chapter: Civil : Engineering Geology : Petrology

Briefly Explain About Sedimentary Rocks

Sedimentary rocks are also called secondary rocks.

BRIEFLY EXPLAIN ABOUT SEDIMENTARY ROCKS

� Sedimentary rocks are also called secondary rocks.

� This group includes a wide variety of rocks formed by accumulation, compaction and consolidation of sediments.

� The sediments may be defined as particles produced from the decay and

weathering of pre- existing rocks or may be derived from remains of dead sea or land animals in suitable environments.

� The accumulation and compaction of these sediments commonly takes place under water or at least in the presence of water.

. FORMATION

� The process of formation of sedimentary rocks is ever prevailing.

� The sediments so produced are transported to the settling basins such as sea floors where they are deposited, get compacted and consolidated and finally transformed into a cohesive solid mass.

That is a sedimentary rock.

� Some chemical processes especially evaporation and precipitation regularly operate on surface of water bodies containing dissolved salts and produce solids that settle down in those bodies.

� Sedimentary rocks are broadly grouped into three classes on the basis of their mode of formation: Mechanically formed or Clastic Rocks; Organically formed Rocks and Chemically formed Rocks

� The last two groups are considered as a single class and named as Non-Clastic Rocks.

Clastic ( Mechanically Formed) Rocks

� A series of well-defined steps are involved in the formation of clastic rocks.

Decay and Disintegration

� Rocks existing on the surface of the earth are exposed to decay and disintegration by the action of natural agencies like atmosphere, water and ice on them

� The original hard and coherent rock bodies are gradually broken down into smaller and still smaller fragments, grains and particles.

� The disintegrated, loosened material so formed and accumulated near the source is called detritus.

Hence, clastic rocks are often also called as detrital rocks

Transport of Sediments

� The detritus produced from the decay and disintegration of the pre-existing rocks forms the source of the sedimentary rocks but it has to be transported to a suitable place for transformation again into a rock mass.

� The wind, running water and ice in the form of glaciers are the very strong and

common agents of transport for carrying millions of tonnes of sediments and particles from one place to another including seas and oceans.

� The winds transport the sediments from ploughed fields, the deserts and dry lands in series of jumps (saltation) and in suspension modes.

� These loads of sediments are dropped down wherever intercepted by rains.

� The mightiest agents of transport of sediments are, of course, streams and rivers, all terminating into lakes or seas.

� The running water bodies transport the sediment load as bed-load, suspended-load and. dissolved load, all dumped at the settling basins.

� Ice in the form of huge moving bodies called glaciers also breaks the rocks along their bases and sides (in valley glaciers) and dumps the same at snow lines thereby making large volumes of the

clastic load available for further transport by other agencies. It is easy to imagine that millions of tonnes of land mass as scratched by these surface agencies is transported to seas and oceans every year and deposited there.

Dradual deposition

� The sediments as produced through weathering and erosion are transported to settling basins.

These basins may be located in different environments such as on the continents, along the seashores or in deep-sea environments.

� As such sedimentary rocks formed in different environments will show different inherent characters.

� In the continental environments may be included the glacial deposits, the fluvial

deposits, the glacio-fluvial deposits and the eolian deposits, each type giving rise to a definite type of

sediment accumulation.

� In the marine deposits, some sediments may be dropped just along the sea-shore, or at some shallow depth within the sea or miles away in the deep-sea environment.

Diagenesis

� The process of transformation of loose sediments deposited in the settlement basins to solid cohesive rock masses either under pressure or because of cementation is collectively known as

diagenesis.

� It may be achieved by either of the two methods: welding or cementation.

� Welding is the process of compaction of the sediments accumulated in lower layers of a basin due to the pressure exerted by the load of the overlying sediments.

� This results in squeezing out all or most of the water .from in between the sediments, thus bringing them closer and closer and consolidating them virtually in a solid rock mass.

� In fact the degree of packing of sediments in a sedimentary rock is broadly directly proportional to the load of the overlying sediments.

� Cementation is the process by which loose grains or sediments in a settlement basin get held together by a binding material.

The binding material may be derived from within the accumulated particles or the fluids that percolate through them and also evaporate or precipitate around those particles thus binding

them in a rock like mass.

. Chemically Formed (Non-clastic) Rocks

� Water from rains, springs, streams, rivers, lakes and underground water bodies dissolves many compounds from the rocks with which it comes into contact.

� In most cases all these dissolved salts are carried by the running water to its ultimate destination the sea.

� Hence the brackish or saltish taste of the sea water.

� In many other cases also, the local water-bodies may get saturated with one or other dissolved salt.

� In all cases, a stage maybe reached when the dissolved salts get crystallized out either through evaporation or through precipitation.

� Thus, limestone may be formed by precipitation from carbonated water due to loss of carbon dioxide.

� Rock salt may be formed from sodium-chloride rich seawater merely by the process of continued evaporation in bays and lagoons.

� Chemically formed rocks may be thus of two types: precipitates and evaporites. Examples are lime stones, rock salt, gypsum, and anhydite.

Organically Formed (Non-clastic} Rocks

� These extensive water bodies sustain a great variety of animal and plant life.

� The hard parts of many sea organisms are constituted chiefly of calcium and/or magnesium, carbonates.

� Death and decay of these organisms within the water bodies gradually results into huge accumulations of carbonate materials, which get compacted and consolidated in the same manner as the normal sediments.

� Lime stones are the best examples of organically formed sedimentary rocks

TEXTURES

(i) Origin of Grains

� A sedimentary rock may be partially or wholly composed of clastic (or allogenic) grains, or of chemically formed or organically contributed parts.

� Thus the rock may show a clastic texture or a non-clastic texture.

(ii) Size of Grains

� The grain size in the sedimentary rocks varies within wide limits.

� Individual grains of less than 0.002 mm and more than 250 mm may

form a part or whole of these rocks.

Three textures recognized on the basis of grain size are:

Coarse -grained rocks; average grain size> 5 mm

Medium grained rocks; average grain size between 5 and 1 mm.

Fine-grained rocks; average grain size < 1 mm

(iii)Shape of Grains

� The sediments making the rocks may be of various shapes: rounded, sub rounded, angular and sub angular.

� They may show spherecity to various degrees.

� Roundness and spherecity are the indications of varying degree of transport and abrasion suffered during that process.

� Thus, Breccias are made up mostly of rough and angular fragments indicating least transport and abrasion.

� Conglomerates are full of rounded and smooth-surfaced pebbles and gravels indicating lot of transport and rubbing action during their transport before getting deposited and consolidated into a

rock mass.

(iv) Packing of Grains.

� Sedimentary rocks may be open-packed or porous in textures or densely packed depending upon their environment of formation.

� The degree of packing is generally related to the load of the overlying sediments during the process of deposition.

(v) Fabric of Grains

� A given sedimentary rock may contain many elongate particles.

� Their orientation is studied and described in terms of orientation of their longer axes.

� If all or most of the elongated particles are arranged in such a way that their longer

axes lie in the same general direction, the rock is said to show a high degree of preferred orientation. This

direction is generally indicative of the direction of flow of the current during the period of deposition.

(vi) Crystallisation Trend

� In sedimentary rocks of chemical origin, the texture is generally defined by the degree and nature of crystallized grains.

Rocks may show perfectly interlocking grains giving rise to crystalline granular texture or they may be made up of non-crystalline, colloidal particles when they are termed as amorphous.

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