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After reading this article you will learn about:- 1. Types of Unconformity 2. Field Evidences of Unconformities 3. Uses.
Types of Unconformity:
There are mainly three types of unconformity.
They are:
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1. Disconformity,
2. Angular unconformity, and
3. Nonconformity.
1. Disconformity:
The rock beds on opposite sides of a disconformity are parallel as shown in Fig. 4.36.
2. Angular Unconformity:
The rock beds on opposite side of an angular unconformity are not parallel.
3. Nonconformity:
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When the older rock formation is made up of plutonic igneous rocks and is overlain by unconformably sedimentary rocks, the structure is called ‘Nonconformity’.
Overlap:
In some cases the junction between two series of beds is smooth and represents a gently sloping old land surface, which slowly sank below sea level. As a result when series of rocks were deposited on the older beds, each new bed in turn as ‘C’, ‘B’ in the Fig. 4.39 encroached more and more on the land surface.
Field Evidences of Unconformities:
The following factors aid in recognizing unconformities in the field:
1. Marked difference in the fossils snowing gap in the succession of rock beds.
2. Presence of conglomerates at the base of upper series of rocks. Such conglomerates contain fragments of the underlying rock beds.
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3. Presence of structure discordance in the two series of rocks.
4. Presence of rock beds, which are formed under contrasting conditions such as non-marine beds overlain by marine beds cross-bedded strata overlain by strata showing graded bedding.
Uses of Unconformities:
James Hutton (1795) was the first to interpret the significance of unconformities:
1. Unconformity represents a gap, break or interval of the deposition of beds and forms a record of time gap.
2. In certain situation unconformity produces oil traps and aquifers.
3. Unconformity helps in visualizing paleogeography of a region.
4. Unconformity is an important structure that affects site conditions for engineering work. It generally forms a weak zone.