The document discusses unconformities, which are breaks or gaps in the geological record where one rock formation overlies another with evidence of erosion. There are three types of unconformities: parallel, angular, and nonconformity. Unconformities can be identified using structural features like angular relationships between tilted/folded rocks, sedimentary features like basal conglomerates, and paleontological features like changes in fossil assemblages. Precambrian unconformities can be difficult to identify due to deformation and metamorphism obscuring original relationships.

1. Dr. Mohammed Aleem Pasha

2.  When stratified rock formations are
deposited regularly and continuously one
above the other without any disturbance
or break in the succession presenting a
series of parallel beds, the sequence is
called conformable beds or series and the
structure is called conformity.
 If, on the other hand one set of strata is
deposited on the eroded surface of an
older series that is not the next in the
succession the two series are said to be
unconformable and the erosion surface is
between that separate the two series is
called unconformity.
 Unconformity indicates discontinuity,
disruption or breaks the deposition and
therefore a major time gap or a major
break. The shorter gaps are known as
Diastems. The relief of the erosion surface
between the older and the new or younger
series may be smooth or irregular.

3.  Unconformity is of three kinds.
 Parallel unconformity
 Angular unconformity
 Non-conformity

4.  An erosion surface with an uneven relief between two
parallel (conformable) series.


5.  An unconformity in which a younger parallel series
deposited on an erosion surface of a lower deformed
(tilted, folded and or faulted) older series with an
angular discordance

6.  An unconformity between two series of rock of
different origins like an upper younger stratified
formation and an older non-stratified or massive
igneous or metamorphic rock.

7.  There are three broad classes of criteria-structural,
sedimentary and paleontological.
 Structural criteria
 The most commonly encountered structural criteria are the
angular unconformity and the disconformity represented
by an undulatory surface which is produced by erosion and
emergence and which cuts across the bedding planes of the
underlying formation. The truncation of one group of beds
by another may result from both faulting and the
develelopment of unconformity. An angular unconformity
can therefore be established only if the field observations
rule out the possibility of occurrence of a fault along the
surface of discontinuity.

8.  Sedimentary criteria
 Among the sedimentary criteria, the most important is
the presence of a basal conglomerate in the unit lying
above the surface of discontinuity. A sub-aerial
discontinuity is also indicated by the presence of
residual chert and buried soil profiles. Submarine
disconformities may be respresented by zones of
glauconite, phosphatized pebbles or by manganiferous
zones.

9.  Paleontological criteria
 Paleontologic criteria, the most important of which are
the sudden changes in faunal assemblages and the
presence of a significant gap in the evolutionary
development of the fauna.


10.  An unconformity can be easily recognized when flat-lying, undeformed or
weakly deformed sediments of a younger age overlie an older group of
schists and gneisses. The older crystalline rocks in such a situation are
generally described as a basement while the overlying sedimentary strata
are described as a cover. Because of later meta-sedimentary strata are
described as a cover. Because of later metamorphism and deformation, it is
often extremely difficult to determine the exact nature of the basement
cover relationship in may Precambrian terrains.

 An unconformity may even record a worldwide stratigraphic event. Thus,
Suess (1906) suggested that the unconformities associated with Late
Cretaceous marine transgression are related with eustatic or worldwide
changes in the sea level. Similarly, the basal Cambrian unconformity
(matthews & cowie, 1979) is of virtually global extent. So are the
unconformities associated with the major Ordovician transgression
(McKerrow 1979, Vail et.al, 1977) and the post-cretaceous regression
(Hallam, 1963). It should be noted that eustatic changes of sea level do not
explain all the features associated with the major global-scale
unconformities. The angular unconformities must have been associated
with some tectonic activities.

11.  The main processes which modify the initial angular
unconformity between a basement and its
sedimentary cover and to enumerate the problems of
identifying the basement in deformed and
metamorphic terrains.
 When a crystalline basement covered with sediments,
and with a nonconformity or an angular unconformity
between the two groups of rocks, is deformed in a later
orogenic cycle, there may be different degrees of
folding of the cover (plis de couverture of Argand,
1922) while the basement remains more or less passive.

12.  For example, the cover rocks of Mesozoic age are detached
from the basement of crystalline Palaeozoic rocks are
folded independently. This process of detachment, known
as decollement, usually takes place along extremely
incompetent beds of anhydrite or salt-bearing clays. Inspite
of the occurrence of the plane of detachment or
dislocation, it is not difficult in this case to have an idea of
the character of the original interface between the cover
and the basement. Indeed, in Jura and elsewhere in
Western Europe a thin layer of the cover often remains
attached to the basement while detachment has taken
place on a slightly higher horizon.

13.  The nature of the original interface can also be identified when the
basement is weakly deformed to produce large warps on the
interface or when it is broken up by a number of faults, while the
cover passively adjusts itself by folding. The interface, however,
becomes greatly modified in a type of deformation in which slices of
the basement are thrust into the cover as basement wedges or, as in
the extreme case of the Eastern Alps, a slice of the basement is
dragged over a low-angle thrust over rocks which form the
sedimentary cover of the Western Alps. The basement along with its
sedimentary cover may be deformed together in a ductile manner
and may give rise to what has been described by Eskola (1949) as a
mantled gneiss dome. A remarkable ductility of the basement is
shown by the Monte Rosa type of Nappse in the Western alps and
elsewhere where the basement forms the cores of huge recumbent
fold.
 In such strongly deformed terrains as in the Alps, the basement can
be identified because of the presence of a good stratigraphic control
so that a cover bed of known age can be identified in different
regions and sometimes can be traced from a weakly deformed to a
strongly deformed terrain.

14.  Identification of the basement becomes extremely
difficult in many Precambrian terrains where the
stratigraphic control is poor and where the cover and
the basement have undergone multiple deformations
and polyphase metamorphism. Many Precambrian
terrains have large expanses of granite gneiss and
migmatites and are bordered by schist belts of
metasediments and metavolcanics. The problem in
these regions is to decide from field evidence whether
the gneissic terrain represents a basement or whether
the gneisses are younger than the schist belt

15.  The occurrence of a conglomerate bed along the
schist-gneiss contact no doubt strongly suggests that
the interface represents an unconformity surface.
However, a conglomerate may not be present in all
such terrains. While identifying a surface of erosion
from the presence of a conglomerate bed in such
strongly deformed terrains, one should be careful to
distinguish between a true sedimentary conglomerate
and an autoclastic conglomerate. An autoclastic
conglomerate is generally produced by intense
deformation and fragmentation of a layer and may
look very similar to a true conglomerate.

16.  While establishing the presence of an angular unconformity in
such strongly deformed terrains, one should always remember
that the angular relation must be between the depositional
surfaces of the two groups of rocks. Thus, an angular
unconformity is not established when the bedding or foliataion
of the schist belt or the banding and the foliation of the gneisses
terminate against the schist-gneiss boundary, because this
boundary does not necessarily represent a depositional surface.
For the same reason an angular relation between the bedding in
the schists and the foliation in gneisses does not indicate an
unconformity.
 If we find that the gneissic complex contains a foliation or a set
of folds which is earlier than all the structures of the schist belt
and if there is no sign of faulting between the two groups of
rocks, then it is reasonable to suggest that the schist-gneiss
interface represents an unconformity.

17.  Moreover, it has been demonstrated that migmatization in the
B.G.C. does not have a separate structural entity and all the
generations of folds and foliations in it also occur in the Aravalli
Group of rocks. These relations apparently indicate that the
Aravalli metasediments cannot be younger than the B.G.C. On
the other hand, the B.G.C. in southern Rajasthan contains rocks
as old as 3500 Ma.

 These relations are in agreement with the conclusion that the
B.G.C. represents an ancient basement on which the Aravalli
sediments were unconformably deposited.

 The foliation and axial planes of folds in these enclaves often
occur at an angle to the synmigmatitic foliation of the newly
formed host gneiss.

18.  In the Precambrian gneissic complex of the Schirmacher Hills in E.
Antarctica there is a series of such enclaves of an older basement
deformed to different degrees during a later event of synkinematic
migmatization which produced the host gneiss.

 The dykes cut across the migmatitic foliation of the Scourian gneisses.
During the Laxfordian phase the dykes were folded, metamorphosed
and migmatized. The Scourian gneiss was also rejuvenated in
Laxfordian times. Where this process was intense, the distinctive
characters of the earlier gneiss were blotted out.

19.  Unconformity in the field is recognized by some of the following
criteria:
 Direct observation in hill sides, valleys slopes, cliff, quarries and
excavations
 One formation resting on the tilted or folded and eroded edges of
several beds
 Presence of a bed of conglomerate called basal conglomerate consisting
of pebbles of underlying older beds on an erosion surface.
 Contrast in the trends dip, strike and folding, faulting, fossils etc of the
two adjacent or successive series.
 Termination of dikes and other intrusive igneous bodies and fault in
lower series at the junction at the two series
 Presence of residual soil in between tow series
 Rock formations of different origin like volcanic or sedimentary rock
resting upon the eroded surface of a igneous or metamorphic rock
formation