Unconformities and its types

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.