This section includes:
- What is an unconformity?
- What are different types of unconformity?
- How to recognize the unconformities?
When the sedimentary rock beds are found to have been deposited without interruption, they are said to be ”conformable”. Unlike conformable the unconformities are formed when there is a break in sedimentation. This creates a gap in the geological record. Major breaks in sedimentation are called ”Unconformities”. Thus an unconformity may be defined as an old erosion surface, which separates younger series of rocks from the older series. Unconformities represent an interval of time of unknown length, which is spent in the erosion of the underlying strata before the deposition of the overlying rocks.
In this figure, the beds/strata of the older rocks start forming. After a period of time, one side of the rock beds gets dipped and the other side uplifts as shown in figure B. According to figure C, the uplifted portion starts eroded by the process of weathering and erosion. Again as per figure; D, the accumulation of newer sediments starts, and younger beds/strata of the rocks forms above the older rock beds.
(2). Types of Unconformity:
Unconformities are of three types:
- angular unconformity
1. Angular Conformity: The rock beds on the opposite sides of an ”angular unconformity” are not parallel. The angular unconformities occur where the older series of beds have been tilted, deformed, and eroded before the deposition of younger beds.
2. Disconformity: The rock beds on opposite sides of a ”disconformity” are parallel. Disconformities occur where the strata of the older series have not been tilted or deformed in any way before the younger rock beds were deposited above them. They represent either a period of non-deposition or a period of erosion.
3. Nonconformity: When bedded sedimentary rocks overlie the non-bedded igneous mass, the structure is called the ”nonconformity”.
(3). How to recognize the Unconformities:
The factors, which help in recognizing the unconformities in the field, are as follow:
- The difference in structure: The group of rocks on either side of an unconformity may show structural discordance.
- The difference in Fossil records: Rock beds lying below an unconformity surface commonly contain faunas, which do not occur in the rock lying above (the younger rock). Meanwhile, the fossils in the younger series of the rock beds may be of higher evolutionary rank. This suggests that a gap exists in the succession of rock beds.
- Presence of Conglomerate: At an unconformity, a bed of conglomerate is commonly found at the base of the upper series of rocks. Such conglomerates contain fragments of the underlying rocks.
- Fossil Soil: Subaerial weathering profiles may be preserved at an unconformity as clay-rich reddened or ocherous horizons.
- The difference in Environment of Deposition: Association of such rocks beds, which were formed under contrasting conditions, indicates the presence of an unconformity. For example, nonmarine beds overlain by marine beds, or cross-bedded strata overlain by strata showing graded-bedding.
- Relations of intrusive material: Unconformities may be recognized by truncation of volcanic necks or other volcanic intrusions.
- The difference in the grade of metamorphism: The younger series of rocks are likely to be less metamorphized than the older rocks lying beneath the younger rock beds.