Chapter: Civil : Engineering Geology : Petrology

Important Sedimentary Rocks

Breccia : It is a mechanically formed sedimentary rock classed as Rudite.

IMPORTANT SEDIMENTARY ROCKS

1. Breccia

� It is a mechanically formed sedimentary rock classed as Rudite.

� It consists of angular fragments of heterogeneous composition embedded in a fine matrix of clayey material.

� The fragments making breccia are greater than 2mm average diameter but some times these may be quite big in dimensions.

The angularity of the fragments indicates that these have suffered very little or even no transport after their disintegration from the parent rocks. On the basis of their source, following types of breccia are commonly recognized:

v Basal Breccia

This rock is formed by the sea waters advancing over a coastal region covered

with fragments of chert and other similar rocks

The advancing waters supply the fine mud, which is spread

over the rock fragments and acts as a binding material.

Once the seawater retreats, the loose chert fragments get cemented together as breccia rocks.

v Fault Breccia

This rock is also called crush-breccia. Such rocks are so named because they are made up of

angular fragments that have been produced during the process of faulting.

The fragments so produced due to crushing effect of the block movements subsequently get embedded in clay and other fine material (often also derived during the faulting process and called gouge) and ultimately form a cemented rock the crush-breccia

v Agglomeratic Breccia

It is a specific type of breccia containing angular and sub angular fragments derived from volcanic eruptions.

It may also contain some fused material that has been cemented together with the solid material broken and thrown out of the craters.

2. Conglomerates

Definition

� These are sedimentary rocks of clastic nature and also belong to rudaceous group.

� They consist mostly of rounded fragments of various sizes but generally above 2mm. cemented together in clayey or ferruginous or mixed matrix.

� The roundness of gravels making the rock is a useful characteristic to differentiate it from breccia in which the fragments are essentially angular.

� The roundness indicates that the constituent gravels have been transported to

considerable distances before their deposition and transformation into conglomerate rock.

Types

On the basis of the dominant grade of the constituent gravels in following three types:

Boulder-ConglomeratesCobble-Conglomerate Pebble-Conglomerate (gravels> 256mm) (gravels: 64-256 mm) (gravels: 2-64 mm)

On the basis of source of the gravels, as

(i) Basal-conglomerates Having gravels derived from advancing sea-waves over subsiding land masses;

(ii) Glacial-conglomerates In which gravel making the conglomerates are distinctly of glacial origin;

(iii) Volcanic-conglomerates In which gravels are of distinct volcanic origin but have subsequently been subjected to lot of transport resulting in their smoothening and polishing by river transport before their deposition and compaction or cementation.

On litho logical basis

(a) Oligomictic Simple in composition, these gravels are made up of quartz, chert and calcite;

(b) Polymictic. In these conglomerates the constituent gravels are derived from rocks of all sorts: igneous, sedimentary and metamorphic, all cemented together. The so-called Fanglomerates are conglomerates formed and found at the base of alluvial fans and cones.

3. Sandstones

� Sandstones are mechanically formed sedimentary rocks of Arenaceous Group.

� These are mostly composed of sand grade particles that have been compacted and consolidated

together in the form of beds in basins of sedimentation.

� The component grains of sandstones generally range in size between 2mm and 1/16

mm. Silica in the form of very resistant mineral QUARTZ is the dominant mineral constituent of most sandstones.

Composition.

� Quartz (Si02) is the most common mineral making the sandstones. In fact some varieties of sandstone are made up entirely of quartz.

� Besides quartz, minerals like felspars, micas, garnet and magnetite may also be

found in small proportions in many sand stones composition.

Texture.

� Sandstones are, in general, medium to fine-grained in texture.

� The component grains show a great variation in their size, shape and

arrangement in different varieties.

Thus, when the texture is determined on the basis of the grade of the component grains three types are recognized:

Type: Coarse-grain Medium-grain Fine -grain

Size-range: 2 mm-l/2 mm 1/2 mm-1/4 mm 1/4 mm-l/16 mm

Colour

� Sandstones naturally occur in a variety of colours: red, brown, grey and white being the most common colours.

� The colour of sandstone depends on its composition, especially nature of the cementing material.

For example, presence of iron oxide is responsible for the red, brown and yellow shades;

presence of glauconite gives a greenish shade to the sandstones.

Types

On the basis of their composition and the nature of the cementing material.

Siliceous Sandstones

� Silica (Si02) is the cementing material in these sandstones.

� Sometimes the quality of the siliceous cement is so dense and uniform that a massive compact and homogeneous rock is formed.

� This is named QUARTZITE. This type of sedimentary quartzite, when subjected to loading fractures across the grains showing clearly very dense nature and homogeneity of the

cementing silica with the main constituent silica of the rock.

Calcareous Sandstones. are those varieties of sandstones in which carbonates of calcium and magnesium are the. cementing materials.

Argillaceous Sandstones These are among the soft varieties of sandstone because the cementing material is clay that has not much inherent strength.

Ferruginous Sandstones As the name indicates, the cementing material is an iron oxide compound. On the basis of mineralogical composition

Arkose.

� This is a variety of sandstone that is exceptionally rich in felspar minerals besides the main constituent quartz.

It is believed that these rocks are formed due to relatively quick deposition of detritus derived from weathering and disintegration of crystalline igneous and metamorphic rocks like granites and gneisses

respe ctivel y.

� Arkose rock generally occurs in horizons that can be genetically related to some crystalline massif occurring in close neighbourhood.

Greywacke.

� These are broadly defined as grey coloured sandstones having a complex mineralogical composition.

They contain a fine-grained matrix. In this matrix, grains of quartz and some felspars are found embedded side by side with fragments of rocks like felsites, granites, shales etc.

� The exact composition of the matrix is so complex that it may not be easily determined in most cases.

Flagstone

� It is a variety of sandstone that is exceptionally rich in mica dispersed in parallel or sub parallel layers.

� The abundance as well as arrangement of mica, typically muscovite, renders the stone weak and easily splitting. Hence its use in load bearing situations is not recommended.

Freestone.

� It is a massive variety of sandstone that is rich in quartz and does not contain bedding planes or any mica. It is compact, dense, massive and a strong rock suitable for construction demanding high crushing strength.

� Ganister. It is another type of sandstone consisting of angular and sub

angular quartz grains and cement of secondary quartz with some kaolin.

Uses

� Sandstones of hard, massive and compact character are very useful natural resources.

� They are most commonly used as materials of construction: building stones, pavement stones, road stones and also as a source material for concrete.

� The Red Fort of India is made up of red sandstones.

Distribution.

� Next to shales, sandstones are the most abundant sedimentary rocks found in the upper 15 km of the crust and make an estimated 15 percent of total sedimentary rocks of the earth.

4. Shale

� Shale is a fine-grained sedimentary rock of argillaceous (clayey) composition.

� Shales are generally characterized with a distinct fissility (parting) parallel to the bedding planes and are made up of very fine particles of silt grade and to some extent of clay.

� Besides fissility, some shales show the laminated structure.

Compsition

� The exact mineralogical composition of shales is often difficult to ascertain because of the very fine size of the constituents.

� shales are very intimate mixtures of quartz, clay minerals and accessory minerals

like oxides of iron, carbonates, and organic matter.

� Silica and clay minerals together make more than seventy percent of shales in most cases.

Chemically speaking, shales exhibit still greater variation.

Average Chemical Composition of Shales

S.N Oxide % age S.No Oxide %age

1 - 58% 5 Ca 3%

2 Si02 15% 6 O 3%

3 Al20 6% 7 1%

4 3 2% 8 5%

Types

Shales have been classified variously. Three Classes On The Basis Of Their Origin:

on the basis of their mineralogical composition:

Quartz shales: rich in free quartz content.

Felspathic shales: in which felspars and clay minerals predominate; silica becomes a secondary constituent. Chloritic shales: in these shales, minerals of chlorite group and clay-group make the bulk of the shales. Micaceous shales: these are rich in muscovite mica and other flaky and play minerals.

Residual Shales:

These are formed from decay and decomposition of pr-existing rocks followed by compaction and consolidation of the particles in adjoining basins without much mixing;.

Transported Shales:

These are deposits of clastic materials of finer dimensions transportedover wide distances before final settlement in basins of deposition.

Hybird Shales

In such shales, materials derived both from clastic sources and non clasticespecially those from organic sources make up the rock.

on the basis of their mineralogical composition:

Quartz shales: rich in free quartz content.

Felspathic shales: in which felspars and clay minerals predominate; silica becomes a secondary constituent. Chloritic shales: in these shales, minerals of chlorite group and clay-group make the bulk of the shales. Micaceous shales: these are rich in muscovite mica and other flaky and play minerals.

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Civil : Engineering Geology : Petrology : Important Sedimentary Rocks |