Boundary Problems

Boundary problem refers to the conditions in which major geological changes has
taken place i.e appearance/disappearance of flora and fauna,major orogeny or
volcanic activity etc.Thus they are helpful in demarcating different eras.For
example Precambrian/Cambrian or K-T.

Stratigraphic boundaries are determined by one or more of geological events such

as volcanic activity, sedimentation, tectonism, paleo-environments & evolution of
life.

Faunal records have played major role in determining the boundaries of the
Phanerozoic units.

The other geological events are dated on the evidenceof fossil records.

PRECAMBRIAN-CAMBRIAN BOUNDARY
Paleozoic rock formation contain abundant fossil records which are in contrast to
largely unfossiliferous nature of Precambrian rocks.

In salt Range :- The oldest rocks exposed at the base of the southern cliffs are
known as “SALINE SERIES” Consisting of gypsum, marls & rock salt.

This succession is conformably overlain by Purple Sandstone, Neobolus Shales,

Magnesian Sandstone & Salt pseudomorph Beds in that order of succession.

Lower Cambrian trilobits fauna and well preserved trails and burrows are known
from Neobolus shales.
The Magnesian Sandstone, a succession of dolo-arenites, contains a horizon of
shales in the middle of the succession which has yielded fossils trails & burrows
resembling those of
Neobolus Shales.

The nature of contact between the Lower Cambrian fossiliferous succession & the
underlying “saline Series” of Salt Range is very often characterized by tectonic
disturbance that was explained in terms of disharmonic folding of competent rocks
of Purple Sandstone resting over highly incompetent salt & gypsum beds of the
“Saline Series” of Precambrian age.
Continuous successions of Precambrian-Cambrian rocks are exposed in various
parts of the tethyan Himalayan zone.

In many sections, however the Lower Cambrian rocks are unfossiliferous. Even in
the fossiliferous section, the fauna is poorly preserverd for specific & generic
identification.

In Kashmir :-
A rich assemblage of microbiota cryptarchs & algae of Late Precambrian (Late
Riphean Vendian) age have been reported from the Lower part of the Lolab
formation.

The upper parts of this formation has yielded trilobites & brachiopods of upper
Lower Cambrian affinity.

The Precambrian-Cambrian boundary is, thus situated somewhere in the middle of

the Lolab formation which has so far not yielded any fossils record.

In Spiti, the boundary is located in thee middle part of the

Kunzam la Formation. The fossil record of the Kunzam la formation is similar to
that of the Lolab Formation.

In Nandadevi, the fossiliferous Palaeozoic succession rests over about a 4 km

thick unfossiliferous sequesnce of argillaceous & calcareous fine grained rocks
know as Martoli
Formation.
This formation is overlain by about 100m thick Ralam Conglomerate that
constitutes the base of the fossiliferous Cambrian succession.

The Martoli Formation has been assigned a Late Precambrian age and the Ralam
Conglomerate presumably represents an unconformity demarcating the
Precambrian-Cambrian
Boundary

PERMIAN-TRIASSIC BOUNDARY
Similar observation have been recorded from Zanskar Range in the northwest &
Kumaun region in the southeast.
An almost uninterrupted succession of terrestrial sediments of Permian-Triassic
age is recorded in the Gondwana Sequence of the Indian Peninsula.
The Permian rocks of the Damuda Group (Lower Gondwana) are conformably
overlain by the Panchat Formation. The lower parts of the Panchat Formation
consisting of fine grained inter-channel deposits are genetically closer to the
underlying rocks of the Damuda Group.

The Upper Panchets are lithologically akin to coarse sediments of the Mahadeva
Formation of the Triassic age.

The Panchet rocks contain fossil records of particularly two reptiles, viz.,
Lystosaurus and Proterosuchus (Chasmatosaurus).

The macro-floral record of the Gondwana Sequence of India shows a major change
towards the end of the Permian Period that expressed by the withdrawal of a large
number of leaf
genera.

Spores & pollens have been regarded as the most reliable aid for dating the
Permian-Triassic boundary in the Gondwana Sequence of India. Thus, the
Permian-Triassic boundary in the
Gondwana Sequence of India lies somewhere in the middle of the Panchet
Formation.

CRETACEOUS-TERTIARY BOUNDARY The problem of demarcating the

boundary line between the Cretaceous & Tertiary successions has two aspects.

The one pertains to locating this boundary in a particular region on the evidence of
some fossil record & the other concern with the “fixing” of this boundary in the
internationally accepted
“Standard Stratigraphic Scale”.

Boundary between the Phanerozoic units are determined on the faunal evidence
with the presumption that the organic forms have periodically undergone profound
changes at certain periods of the earth history.

In general, this assumption has been substained by the fossil records. In many
parts of the world, stratigraphic breaks have been observed between the
Maastichtian & Montian beds.
 The post-Maastichtian &pre-Montian time interval is represented in other parts of
the world by “passage beds” which is known as Danian Stage.

Rocks above the “passage beds” contain fauna that have definite Tertiary aspects
while the rocks underlying them have a definite Cretaceous affinity.

There are three regions in the Indian subcontinent which have exposed such
“passage beds”.
 The Sind area in Pakistan.
 The Tiruchchirappalli area in south India.
 The Assam region in the north-eastern India.
 The Pab Sandstone which in mainly unfossiliferous contain Maastrichtian
fossils in some thin beds at the base & the top of the sandstone succession.

 Since the Cardita beaumonti beds overlie the uppermost Maastrichtian
succession, they have been assigned a Danian age & the Cardita Beaumonti
has been recognized as an index fossil of the Danian on the Indian
subcontinent.

 Of the three sub-division of the Ranikot Formation, the basal & lower sub-
division are composed of practically unfossiliferous sandstone of fluviatile
origin.

The upper sub-division of the Ranikot Formation contain abundant marine

Palaeocene fauna which indicates that the marine regression was short lived.

The Upper Cretaceous rocks of Assam region comprising the Mahadek Formation
is conformably overlain by a marina succession known as langpar Formation.

The Langpar Formation has yielded a Danian foraminiferal assemblage. The

marine rocks are overlain by a thick continental sequence of coal bearing
sandstone known as Therria Formation.

• Biswas (1962) has suggested that both the Langpar & the Therria formation were
deposited at the same timem the former being a marine facies equivalent of the
continental
Therria Formation.
Rama Rao grouped both the formation into the “passage beds” representind the
Cretaceous-Tertiary transition.

Sah & Singh have recorded the presence of a mic-floral break between the
Mahadek & Langpar formations representing the Cretaceous-Tertiary boundary in
Assam.

NEOGENE-QUATERNARY BOUNDARY

The Boundary in India has been investigated in the Continental deposits of the
Outer Himalayas & the Kashmir Valley & the marine deposits of Andaman-
Nicobar islands.

The basic criteria for the recognition of this boundary in the continental deposits
are the first appearance of certain group of mammals sach as Elephus, Equus &
disappearance of
Hipparion.

The continental succession contains evidence for the first climate deterioration
leading to the glacial phases at the commencement of the Quaternary deposits.

The boundary in the marine succession has been demarcated on the basis of the
appearance & extinction of certain planktonic foraminifera.