UNCONFORMITIES.pptx

Unconformities and its types

1. UNCONFORMITIES

2. UNCONFORMITIES An unconformity is a break in stratigraphy, a surface of erosion or a period of non deposition usually the former that separates the younger strata from the older rocks. The younger of these rocks are nearly always of sedimentary origin and must have been deposited on the surface of the older rock. There are various kinds of unconformities depending upon the rocks involved and the tectonic history that is implied. Angular unconformity Disconformity Paraconformity Nonconformity Regional and local unconformity Blended unconformity

3. 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 angular unconformity. In this case, both the underlying and overlying rocks are of sedimentary origin, but the attitude of rock above and below the plane of discontinuity differs from each other.

4. Disconformity - An unconformity surface above and below which the bedding planes are essentially parallel and in which the contact between younger and older beds is marked by a visible, irregular, or uneven erosional surface is a disconformity. Disconformities are most easily recognized by this erosional surface. Disconformity surfaces, as well as angular unconformity surfaces, may be marked also by “fossil” soil zones (paleosols ).

5. Paraconformity- A paraconformity is an obscure unconformity characterized by beds above and below the unconformity contact that are parallel and in which no erosional surface or other physical evidence of unconformity is discernible. The unconformity contact may even appear to be a simple bedding plane. Paraconformities are not easily recognized and must be identified on the basis of a gap in the rock record (because of nondeposition or erosion) as determined from paleontologic evidence such as absence of faunal zones or abrupt faunal changes. In other words, rocks of a particular age are missing, as determined by fossils or other evidence.

6. Non conformity – when the underlying older formations are represented by igneous and metamorphic rocks and the overlying younger formations are sedimentary rocks, the unconformity is called non conformity.

7. Blended unconformity – unconformity which lack sharp contact, it is a surface of erosion, which may be covered by a thick residual soil that grades into underlying bed rock. Regional and local unconformities – when the unconformity extends over a large area, it is called regional unconformity and if it occurs over a relatively small area, it is called local unconformity. Significance of unconformities The presence of unconformities has considerable significance in sedimentological studies. Many stratigraphic successions are bounded by unconformities, indicating that these successions are incomplete records of past sedimentation. Not only do unconformities show that some part of the stratigraphic record is missing, but they also indicate that an important geologic event took place during the time period (hiatus) represented by the unconformity—an episode of uplift and erosion or, less likely, an extended period of nondeposition.

8. Recognition of unconformities It can be identified by proper geological investigations in the field, followed by mapping of area concerned. Some of the evidences are ; Difference in attitudes of two adjacent sets of beds. Remarkable difference in nature, age and type of fossils in adjacent sets of beds. Occurrence of conglomerate along the unconformity plane. Occurrence of residual soil/laterite/bauxite along the unconformity surface. Considerable difference in degree of metamorphism of two adjacents sets of beds.