3. Introduction
Trough: Area or depression between the Hills formed by the rim of the
Tectonic Plates.
The Area under the study lies between the south Western corner of
Karachi trough between the Pub and the Kirthar Ranges.
The exposed rocks in the area are Nari and Gaj formations of Oligocene
and Miocene respectively. however Khirthar formation of Eocene is also
exposed in the north western part.
4. Formations are continuously exposed from western coast of Karachi
to northern Sindh. forming a series of asymmetrical folds and anticlines
in southwest.
Formations consists of shale, limestone and sandstones with some
weathered material from older surroundings.
5.
6. Stratigraphy and Geology of the area
The rocks exposed in the area are Sedimentry in the origin and were
laid down in shallow marine conditions. They range in age from Eocene
to recent.
Most area is occupied by the Oligocene and Miocene ages.
7. FORMATIONS:
Dada Formation : Pleistocene to recent (1.8 million years)
Manchar Formatiom: Pliocene (5.6 million years)
Gaj Formation: Miocene (23.8 million Years)
1) Lower part
a) Metan clay unit
b) Jhill limestone unit
c) Talawa limestone unit
2) Upper part
Nari Formation: Oligocene (33.7 million years)
Khirthar Formation: Eocene (54.8 million years)
8. Age of this formation is Eocene (54.8 million years)
This formation is dominantly consists of
limestone which is dark grey, golden brown, or light grey in color.
limestone is relatively weather erosion resistant unit and forms high
hills (162m) trending from north to northeast and south to southeast in
northwestern part of the area.
It is in the form of an anticline.
KHIRTHAR FORMATION
9. Age of this formation is Oligocene (33.7 million years)
NARI FORMATION
This formation
is divided into
two members.
Lower Nari
Member
Upper Nari
Member
10. comprises of limestone in lower part and sandstone
and shale in the upper part.
limestone is dark brown, hard, nodular, thin to
medium bedded, Fossiliferious and silicious.
Shale is greenish to grey, light brown, white with
flecked rust on the surface.
Lower Nari Formation
11. This formation is well exposed.
Upper Nari Formation
Comprised of four lithological units.
Pir mangho
limestone unit
Halkani
sandstone unit
Ghora laki
limestone
Orangi
sandstone unit
12. Comprised limestone which is dark brown or golden brown in color and
weathers into rusty brown or grayish brown.
Very hard thick bedded showing concoidal fracture.
Limestone is inter layered with sandstone and sandy shale.
This unit is 319m thick in pir mangho area.
Pir Mango limestone unit
13. consists of sanstone which is soft, friable,
dark grey or brown in color, medium to
coarse grained.
Sandstone is interlayered with shale which
is light green or grey.
It is 243m thick in pir mangho area.
Halkani sandstone unit
14. consists of limestone which is dark brown
or cream colored, hard, nodular to thin
bedded.
In pir mangho area it is 67m thick.
Ghora laki limestone unit
15. Consists of sandstone which is rusty
brown, pinkish, yellow grey coloured.
Soft, Friable medium to coarse grained.
Orangi sandstone unit
16. Age of this formation is miocene (23.8
million years)
Gaj formation is divided into two members.
1) Lower member.
2) Upper member.
GAJ FORMATION
17. 1) Lower member
comprises of clay in the lower part and
limestone in the upper part.
Lower member is comprised of three units.
a) Metan clay unit.
b) Jhill limestone unit.
c) Talawa limestone unit.
18. a) Metan clay unit:
consists of clay which is khaki, grey, grayish to brown in color, soft to
moderate hard and chunky.
it is 318m thick on the western limb of pir mangho anticline.
b) Jhill limestone unit:
consists of limestone which is very hard, thin
bedded, light brown to cream colored, massive and nodular.
It is 68m thick in Jhill hill.
19. c) Talawa limestone unit:
Consists of limestone which is brown to golden brown in color, hard,
thin to thick bedded.
It is 76m thick in talawa area.
20. 2) Upper member of gaj formation
consists of interbeded clays, limestones, and sandstones in variable
proportion in different areas.
clay is grey, brown, yellow, chocolate and red in color, silt, and medium
bedded.
Limestone is hard, dark brown, thin to medium bedded, nodullar and
fossiliferious.
Sandstone is grey to greenish brown, soft medium to thick bedded and
medium to coarse grained.
21. MANCHAR FORMATION
Age is Pliocene (5.3 million years)
Comprised of sandstone extending from to goth ibrahim hyderi to goth
rehri along the coast.
Sandstone is thick bedded and massive, grey and greenish grey, soft
friable, coarse grained, interconected with sandy clay.
Sandstone suggests shalow water environment.
Upper contact of manchar with dada formation is unconferrmable.
22. Age is pleistocene to recent (1.8 million years)
It is exposed along the beach from goth ibrahim hyderi to rehri and songal area.
This Formation mainly consists of gravels loosely cemented sand and silt
matrix.
Lower contact of dada formation with manchar is unconfirmable and upper
part is covered by sub recent to recent deposits such as rock fragments,
conglomerate,sand silt and clay.
DADA FORMATION
23. References
Fatima A N 1977 stratigraphy of Pakistan(
shah S.M.I) geological society of Pakistan,, memoir 12, 29-56
Kazmi A.H and Jan M.Q 1997 geology and tectonics of Pakistan.
