Short essay on the Structures of Sedimentary Rocks

Sedimentary rocks are the secondary rocks, in which the presence of different layers, beds or strata distinguishes (hem from the igneous and metamorphic rocks. Sedimentary structures are both organic and inorganic in origin. Depending on the mechanism of formation, the inorganic structures are classified into.

I. Primary.

II. Secondary structures

I. Primary structures:

The primary structures are due to mechanical action of current and show the following characteristics:

(a) these structures show palaeo-current condition,

(b) rate of supply of sediments,

(c) mode of transportation,

(d) environment of deposition, and

(e) top and bottom of beds etc.

The primary sedimentary structures include the following:

(i) Bedding or stratification:

Insoluble mechanically transported material is deposited in layers on the surface of accumulation which may be horizontal or inclined. Stratification may be the result of variations in composition of different layers, colour of layers, textures of the layers and porosity of the layers. These are most conspicuous particularly in the rocks formed under water. A glacial, chemical and biological deposit lacks stratification. Aeolian deposits seldom show bedding.

If the individual layers are extremely thin the structure is known as lamination and the layers as laminae. There are two types of laminations:

1. Dimictlc lamination:

Where the contact between two laminae is sharp.

2. Symictic lamination:

Here the contact between the laminae is rough.

The plane of contact ii known as laminating plane, e.g., varve.

When the bedding planes are disposed approximately parallel to one another, they are known as concordant. If bedding planes are inclined to the major lines of stratification, they are said to be discordant.

(ii) Cross-stratification:

Here the beds are found to lie slightly oblique to the major bedding planes and bound by layers of concordant bedding. Mostly found in arenaceous rocks.

It is formed due to change in the velocity and direction of flow of streams. The oblique lines of a cross-bedded layer always meet the upper concordant bedding at a higher angle and lower portion tangentially.

In wind formed current beddings, the laminations are curved and of larger magnitude. The cross-stratification is also known as current bedding or false bedding etc. When the top and bottom beds converge at a point, it is said to be wedge-cross stratification.

(iii) Torrential bedding:

It shows an alternation of coarse current bedded material and finer horizontal laminae. Here the forest beds are straight and they characteristically develop in alluvial fans.

(iv) Graded bedding:

In this case there is a gradation of grain size from coarser at the bottom to finer at the top. It is having a sharp contact with the underlying strata. This in consolidated form is known as turbidites. Graded beddings are seen in ‘Graywackes.’

(v) Ripple marks:

These are minute undulations formed due to current or wave action, developed on arenaceous rocks. These are of two types:

1. Oscillation or wave ripple

2. Current ripple.

1 Oscillation ripples:

These are symmetrical with sharp crests and broad rounded troughs. These are shallow water features and are indicative of a stagnant body of water frequently agitated by waves.

2. Current ripples:

These are asymmetric in nature, having parallel, long, more or less equidistant ridges trending in straight lines at right angles to the direction of current. Here both the crests and troughs are rounded.

Aqueous ripples contain finer materials at the crest and coarser material at the troughs but in the case of aeolian ripples coarser materials are found at crests. Aeolian ripples are invariably asymmetric in nature.

(iv) Mod-cracks:

These cracks typically develop in clayey sediments due to prolonged exposure to the atmosphere. These are also known as shrinkage cracks or sun cracks. They are wider at the top but tapers towards the bottom.

(vii) Rain-prints:

These are shallow depressions surrounded by a low-ridge formed by the impact of the drop, hailstones, bubbles etc.

(viii) Imbricate structure:

In conglomerate and pebble beds fragments having a noticeable elongation are sometimes deposited with the long axes of the pebbles lying more or less parallel to one 1 another, leaning in the direction of current flow.

(ix) Tracks and trails:

These are markings indicative of the some animal over soft sediments. Trails are the winding passages through which long bodied and short bodied animals moved.

(x) Convolute bedding:

Here, the sedimentary lamina is contorted into a series of anticlines separated by broad synclines. Distortion increases upwards but it is confined to one bed and is often abruptly truncated by overlying sediments.

II. Secondary structures:

These are the products of chemical action contemporaneous with sedimentation or shortly thereafter and includes:

(a) Concretions:

They are spherical to elliptical bodies, usually small and of diverse chemical nature than the rocks in which they occur. They include nodules, oolites, pisolites, geodes etc.

(b) Solution structures:

Irregular openings commonly in calcareous rocks and are produced due to ground water action e.g. Vugs.

Organic structures:

Fossils impressions, as well as petrified remains of animals or plants are the common organic structures found in sedimentary rocks.

Sole structures:

These are the structures preserved on the base of a bed which is sharply differentiated lithologically from the bed below, e.g., sandstone overlying shale. They also include flute cast (which are grooves eroded by turbulent flow and later filled with coarse sediments) and groove casts formed on the surface of sandstone layers, tracks and trails, prod marks, etc.