An unconformity is a plane of discontinuity that separates two rocks, which differ notably in age

Introduction:

An unconformity is a plane of discontinuity that separates two rocks, which differ notably in age. The younger of these rocks are nearly always of sedimentary origin and must have been deposited on the surface of the older rock, which is a surface of erosion.

How it forms:

The formation of an unconformity may be attributed to three main processes like erosion, deposition and tectonic-activity. The stage of its development involves.

(i) The formation of older rocks.

(ii) Upliftment and subaerial erosion of the older rocks.

(iii) The formation of a younger succession of beds above the surface of erosion.

Thus at unconformity is a surface of erosion or non-deposi­tion that separates younger strata from older ones. It is accordingly regarded as a planar structure.

Description:

Usually the following characteristics are found to be associated with the unconformities:

(a) There is a difference in the lithological composition, thick­ness and order of superposition of the overlying strata with respect to underlying rocks.

(b) Difference in age as indicated by the fossil assemblages of the overlying and underlying beds.

(c) Sometimes the underlying beds possess different dip and strike value than that of the overlying beds.

(d) The presence, in most cases, of a conglomerate horizon at the bottom of the younger set of beds.

Classification:

Unconformities have been classified into various types on the basis of the factors like:

(i) The relationship existing between the underlying and above- lying rock beds, i.e., whether both are sedimentary or one of them is of igneous origin.

(ii) The attitude of the underlying and overlying beds.

The various types of unconformities may be enumerated as follows:

(a) Angular Unconformity.

(b) Disconformity.

(c) Local Unconformity.

(d) Non-conformity.

(e) Blended Unconformity.

(a) Angular unconformity:

If the beds beneath the erosion sur­face are folded or tilted so that there is an angular discordance between the younger and older beds, the contact is called an angular unconformity In this case, both the underlying and overlying rocks are of sedimentary origin, but the attitude of the rocks above and below the plane of discontinuity differs from each other.

(b) Diseonformity:

It is also known as ‘Parallel-unconformity’ in view of the fact that the bedding above and below the plane of discontinuity. The lower and upper series of beds dip at the same amount and in the same direction, thus this type or unconformity is formed when there is a lesser magnitude of diastrophism, or disturbance between the depositions of the two successions of strata.

(c) Local-unconformity:

It is also known as a ‘non-depositional unconformity. It is similar to diseonformity, but it is local in extent and hence the name. The time involved is also short. Thus it represents a short period of non-deposition. So the age difference between the overlying and underlying beds is very less. This is quite significant in the determination of top and bottom of folded-beds. Such an unconformity is also known as ‘Diastem’.

(d) Non-conformity:

It is commonly applied to structures in which the older formation made up essentially of plutonic Tocks, is overlain unconformably by sedimentary rocks or lava- flows. The essential concept being that prolonged erosion must have occurred to expose the intrusive before burial. It is not of any tectonic significance. According to some geologists, it should be termed as ‘Heterolithic unconformity.’

(e) Blended unconformity:

It is a surface of erosion, which may be covered by a thick residual soil that grades into the underlying bed rock. Younger sediments deposited above he surface may incorporate some of the residual-soil and a sharp con­tact may be lacking. Such a contact may be called as Blended- unconformity.

Besides the above important types of unconformity, various types of relationship of the underlying beds with that of the beds lying above the plane of discontinuity has been recognised, which are as follows :

1. Over-step:

It develops during marine transgression, so that the younger series rests progressively on older members of the underlying rocks.

2. Over-lap:

It is often found that younger bed completely covers up and advances much beyond the limits of the underlying, rocks. It is best observed along the gently sloping fringes of inland basins during marine transgression.

3. Off-lap:

It is reverse of overlap, i.e., lower beds of the upper series extends further than the younger ones due to marine regression.

4. On-lap:

It appears synonymous with overlap or sometimes, overlap overstep.

Recognition of unconformity in the field and their effects on out­crops.

Unconformities may be recognised in various ways, of which, observation in a single outcrop is the most satisfactory. Reliability in the identification of an unconformable relation, in general, increa­ses in proportion to:

(a) Time interval.

(b) Thickness of beds missing from the stratigraphic record.

(c) Structural discordance.

(d) Topographic relief.

(e) Evidence of weathering at the unconformable surface.

The kinds of evidence important in the recognition of uncon­formities are as follows:

1. Evidence of unrecorded interval:

(i) Gap in palaeontological record:

If rocks with upper triassic fossils is directly overlain by rocks with lower cretaceous fossils, it is said to be an unconformity.

(ii) Gap in stratigraphic record:

Good evidence of uncon­formity may be provided by the local absence of distinctive strata characterised by either lithologic or palaeontologic peculiarities or both. Abrupt changes in these characters may suggest the presence of an unconformity.

2. Structural-discordance:

The truncation of sedimentary layers at a stratigraphic contact is an unmistakable evidence of dis­continuity.

3. Topographic irregularity:

Disconformities preserve topo­graphy corresponding to all stages of the physiographic cycle ranging^ from undissected surfaces to near peneplains.

4. Evidence of old-land surface:

(1) Weathering may aid in the recognition of old land surface and the unconformities that mark their position in the stratigraphic section.

(2) Remnants of old-soil profile are more likely to be preserved in the accumulations of terrestrial sediments. Their presence within strata suggests the existence of an unconformity.

5. Sharp contrast in the degree of in duration between the underlying and above lying rocks.

6. Difference in the grade of regional metamorphism of the underlying and overlying rocks.

7. Significant difference in the intensity of folding in both- younger and older set of beds.

8. Presence of basal conglomerate horizon suggests the exis­tence of an unconformity.

In the outcrop, unconformities are usually found as a rugged and eroded surface.