This document discusses unconformities, which are surfaces that separate younger rock strata from older rocks. Unconformities form due to erosion during periods of tectonic uplift or subsidence. There are several types of unconformities including angular unconformities, non-conformities, and disconformities. Unconformities are important geological features that can indicate past tectonic activity and paleogeography. They also represent gaps in the geological record where deposition did not occur.
2. An unconformity is a surface of erosion that
separates the younger strata from older
rocks.
It is one of the most common geological
feature found in rocks or in succession.
It is different then all other geological
structures viz. the fold, joints and faults
Unconformities are resulted due to tectonic
activity in form of uplift or subsidence of
land
It is referred to a period of non-deposition
4. 3. Difference in Structural Feature :-
4. Difference in Fossil Content :-
5. • Angular unconformity
• When the underlying (older) rocks and overlying
(younger) rock strata show some angle w.r.t one another =
Angular unconformity.
• Here both underlying and overlying rocks are of
sedimentary origin.
6. Types of Angular Unconformity :-
1. Amount of dip direction :-
2. Difference in Structural Feature :-
7. Non-conformity
When the underlying rocks are Igneous or Metamorphic
(i.e. unstratified) and the overlying younger rocks are
sedimentary (stratified) = Non-conformity
Essential concept – prolonged erosion must occur to
expose the intrusive before burial.
8. • Disconformity
• Also known as parallel unconformity.
• It is formed in areas of lesser magnitude of
diastrophism.
• When the underlying (older) and overlying (younger)
sedimentary rock strata are parallel and the contact
plane is an erosional surface = Disconformity
9. Also known as non-depositional unconformity.
It is similar to a disconformity but local in extent
and represents a short period of non-deposition.
The age difference between the overlying and
underlying beds is very less.
This is generally found in flood affected areas
representing short time gaps.
10. A surface of erosion, covered by residual
graded soil.
Lack of a sharp contact.
11. If two beds or two series of beds are non-
parallel on vertical section as exposed in a
hill or railway cutting, the surface of contact
represents unconformity.
The lowest bed above unconformity may
usually consist of conglomerate pebbles.
Unconformity may be indicated by sharp
contrast in the degree of induration.
12. Disconformity may be readily recognized in
outcrops, road cuts etc. if there is:
Sharp contrast in colour of the younger and older
bed.
Undulated appearance of the plane of
unconformity.
Radical difference in fossil contents.
Variation in lithologic peculiarities in between beds.
13. Unconformity represents a break or an interval
in deposition of beds and forms a record of
time gap.
Unconformity indicates a stratigraphical hiatus
and represents the period during which the
region was a land mass and eroded
subsequently.
In certain situations unconformity produces
potential oil traps and aquifers.
14. Unconformity helps visualizing
paleogeography of a region.
Unconformity is an important structural
feature that affects site foundations for
engineering works. It generally forms a
weak zone.