Tectonic Basins of Pakistan

Waqas Javaid, BS-Geology.

Bahria University Islamabad.

1. Abstract:

For the past 50 million years, very large scale forces have acted to shape this particular region of earth, the Indian-Eurasian collision was the last major event of this still continuous process. This collision created the greatest landmark in the form of HKH mountain ranges. Although a person is almost lost in the mesmerizing height of this mountain belt, the impact they have on this area cannot be ignored. One of which is the basins created on the foreland of these mountains. In fact, the geology of Pakistan is incomplete without considering the sedimentary basins of the region.

2. Introduction:

According to Geology, a large low relief area produced by geological processes which has a capability to accumulate sediments within itself is called a Basin. According to a more refined definition; sedimentary basins are regions where considerable thickness of sediments have accumulated (in places up to 20km), these are present offshore as well as onshore. (Basin Analysis: Principles and Applications, P Allen & J Allen.)

Plate tectonics is the major driving force behind the formation of these basins. Oceans are the basic example of sedimentary basins.

3. Classification of sedimentary basins:

Sedimentary basins are generally classified on the basis of their tectonic settings (divergent/convergent/transform boundaries and subduction zone) and their proximity to the source area.

Some types of sedimentary basins are as follows:

a) Rift valley:

Narrow and elongate Basin Formed on continents due to divergent plate boundary and normal faulting.

b) Trench:

A very deep basin formed at the subduction zone in ocean-ocean and ocean continent convergence.

c) Fore arc basin:

Formed at active continental margins near subduction zones, Contains marine sediments and are present in front of volcanic arc.

d) Back arc basin:

Consists of marine sediments and are formed at back of volcanic arc in ocean-ocean convergence.

e) Foreland basin:

Formed at the convergent plate boundary near collisional mountain belts, mostly contains alluvial deposits. Basic source of sediments are rivers and nearby tectonically uplifted areas.

f) Passive margin:

These are formed at passive continental margin. These basins are sourced by rivers and deltas and are the worlds largest in terms of accumulation of sediments (up to 20000m).

g) Intra-cratonic:

Less deep/shallow circular basins formed due to slow subsidence with in a continental plate.

h) Aulacogen:

These are the third arm of a triple junction (a three arm rift zone) which is failed and thus form a sedimentary basin.

4. Sedimentary basins of Pakistan:

Different geoscientists, in different times, have defined the sedimentary basins of Pakistan differently. But most agree that two basins; Indus Basin and Baluchistan Basin are the main sedimentary basins of Pakistan.

Both of them were different entities in past and they were joined together during cretaceous to Paleocene periods along with Ornachal and Chaman faults.

As mentioned earlier in this paper, the collision of Indian and Eurasian was the major event which changed the geological features of this region so the sedimentary basins also, could not be left unaffected. These two basins are then further and further sub divided according to their different characteristic properties.

The following chart shows the classification of Sedimentary basins of Pakistan.

4.1 Indus Basin:

Indus basin is present mainly in the present political provinces of Punjab and Sindh in Pakistan and to some lesser extent in Khyber Pakhtunkhwa.

Geologically it is south to the Main boundary thrust MBT, Indian shield is present in the east and Marginal zone of Indian plate at west and ends at offshore Indus near the Arabian Sea.

The Indus basin is regarded as the oldest sedimentary basin of Pakistan in terms that the Precambrian formations are exposed in this basin. Indus Basin is further sub divided into lower Indus basin and upper Indus basin.

4.1.1 Upper Indus Basin:

This part of Indus basin is present in the northern Pakistan. In the south of upper Indus basin, Kirana Hills (Sargodha Highs) act as a boundary between upper and lower Indus plane and separate these two. MBT marks the northern and eastern boundaries of Upper Indus Basin.

Oldest rocks of Precambrian age are present on outcrop at Salt range, present in Upper Indus Basin. The Basin is further divided into to two more basins named as Potwar Sub basin and Kohat Sub-Basin.

If we see the stratigraphy of these sub basins we come to know that Potwar sub basin contains exposed rock sequence ranging from Pre Cambrian to the most recent Pleistocene. Whereas the rock formations from Cambrian to Pliocene age are exposed in Kohat Sub-Basin.

The upper Indus basin is marked by an unconformity in rock sequence between Pre Cambrian and Permian; the Ordovician, Silurian, Devonian and carboniferous rocks formations are absent.

The important geological features present in Upper Indus Basin are: main boundary thrust, Potwar plateau, Kohat plateau, Indus River, Soan syncline, salt range and Himalayan frontal thrust.

4.1.2 Lower Indus Basin:

The lower Indus basin is located at south of Sargodha highs, at west of Indian shield and east of marginal zone. The oldest formation exposed in Lower Indus Basin is of Triassic age; Wulgai Formation. This basin is further divided into central Indus Basin and Southern Indus Basin. Kandhkot highs and Jacobabad highs act as a boundary between central Indus basin and southern Indus basin. The central and southern Indus basins further contains several units so in this paper we will explain them separately.

i. Central Indus Basin:

The central Indus basin is located between the Sargodha highs (present above) and Jacobabad highs (below). Sukkur rift zone is present in the south. The depth of basement is up to 15km in the depressed parts of the basin. As mentioned above, the oldest formation exposed is of Triassic age but formations up to Precambrian age (salt range formation) are found through drilling.

Some unconformities are also found in this basin due to Precambrian and non orogenic movements.

Central Indus basin us further divided into three units; Punjab platform, Sulaiman Depression, Sulaiman Fold Belt. (Raza et al, 1989).

a) Punjab Platform:

This unit is present at the east of Central Indus Basin. It is a wide monocline and dipping gently toward the Sulaiman depression (west) since Mesozoic age due to the Indian Eurasian collision. Sedimentary rocks are not exposed in this region. The depth of basement is from 1.8km to 4km in this unit. (Raza, 1989)

b) Sulaiman Depression:

In Central Indus Basin, the Sulaiman depression is a longitudinally oriented area of subsidence located in between Sulaiman fold belt (west) and Punjab platform (east). As compared to the Punjab platform, the stratigraphy sequence is more complete. In east, its strata pinches out and merge into Punjab platform. Some large anticlines are also buried in this unit e.g (Ramak).

Zindapir Inner fold zone is present in west, Mari Bugti Inner fold zone is present in south of Sulaiman Depression. (Kadri)

c) Sulaiman Fold Belt:

This is the most important region/unit in Central Indus Basin due to its unique tectonic features. A large number of disturbed anticlinal structures are present in this unit. This is the only unit in Central Indus basin where Triassic rocks are exposed on surface. In Paleocene and Eocene ages, most important litho-stratigraphic variations are present in the form of facies change from north to south and east to west. (kadri)

ii. Southern Indus Basin:

The southern Indus Basin is present in the south of Sukkur rift which at as a natural boundary between Central and southern Indus Basins. Both of these basins were undivided before the Jacobabad High in middle cretaceous period. This can be supported by the evidence from Similar Sand facies of Goru formation, similar lithologies of Chiltan and Sembar Limestone and data collected from Khairpur and Jhatpat wells. Triassic age rocks are the oldest formations found in this region. (kadri)

Southern Indus Basin is further divided into five units;

a) Thar Platform:

Thar platform is a gently dipping monocline located in the west of Indian shield, east of Kirthar and Karachi trough and south of Mari Bugti Inner Fold Zone. It can be distinguished from Punjab Platform on the basis that Thar Platform was developed by the recent counter clockwise movements of Indian plate.

One Important economic feature of Thar Platform is the presence of Goru Formation (cretaceous) sandstones which acts as reservoir rock for hydrocarbon.

b) Karachi trough:

It is a bay opening into the Arabian Sea. This unit contains the preserved rock formations from cretaceous periods. The one unique feature of Karachi trough is that it has been a trough throughout geological history. Another important feature which is not present anywhere else in Pakistan is the complete sequence of sediments from cretaceous to tertiary without a gap/unconformity at the end of cretaceous period. Several narrow chain anticlines are also present in Karachi through which are of economic importance as they contain gas fields e.g. Sari, Hundi and Kothar.

c) Kirthar Foredeep:

It is a north south depression with accumulated sediments thickness of 15km. although the Paleocene formations are well developed in this area, upper cretaceous sediments are missing as indicated by data obtained from Mari, Khairpur and Mazarani wells. There is also a great potential of maturation of source rocks in this area.

d) Kirthar fold Belt:

Kirthir Fold belt is located in east of Baluchistan Basin. It also trends towards the north-south and its structure and stratigraphy is almost identical to that of Sulaiman Fold belt. Pleistocene to Triassic age sediments are found in this region. Its western margin contains some economically important mineral deposits of Lead, Zinc, Barite, Manganese and Fluorite.

e) Offshore Indus:

This region comprises of passive continental Margin. Sedimentation in Offshore Indus has taken place in two main phases. One phase is from Cretaceous to Eocene age and second from Oligocene to Pleistocene age. Deltaic and submarine fan sediments were deposited in the latter. At 67E longitude, the offshore Indus is divided into a depression and platform along an almost parallel hinge line. This offshore platform is further divided into Karachi Trough and Thar platform deltaic area by a line which separates Karachi trough and Thar slope onshore.

4.2 Pishin Basin (Kakar Khorasan Basin):

The Pishin Basin is present between the Chaman Fault (North) and Ophiolitic margin (south). It was first thought that this basin is a part of Balochistan and Indus Basin, but recently it is recognized as a separate basin because of its own tectonic settings and different geological processes. Only the 50 percent of Pishin Basin is present in Pakistan, the rest 50% is present in Afghanistan where it is recognized as Kundar-Urgun Basin.

It is a very young basin containing tertiary sedimentary rocks formations and a thin continental crust or oceanic crust.

4.3 Peshawar Basin:

Peshawar Basin is located in Political province Khyber Pakhtunkhwa of Pakistan. It is present between Main Mantle Thrust MMT (west) and Main boundary Thrust MBT (east). It is an intermountain basin at the southern end of Himalayan Ranges. The basin is surrounded by Khyber mountain range, Lower swat mountain range and Attock Cherat mountain range. The most complete Paleozoic sedimentary rock sequence of Pakistan is found in Peshawar Basin. The Cambrian and Precambrian rock formations are also present, for example Ambar Formation and Tanawal Formation.

Tectonically, the Peshawar Basin is a transitional area between fold thrust belt (south) and metamorphic region. (North).

4.4 Siwalik Basins:

This name is given to all the depressions or trough areas in sub-continent which were formed due to the formation of Himalayan Mountain Belt. (kadri) They receive and accumulate the sediments coming from tectonically uplifted areas. It is further divided into lower and middle Siwalik. Siwalik is actually a 2000km Hill range extending from the Kohat plateau to the Indian syntax bend. The siwalik sediments seemed to have eroded from metamorphic rocks of Himalayan orogeny and are equivalent to Chinji and Nagri formations. (Aftikhar A. Abassi, 2009)

4.5 Balochistan basin:

This sedimentary basin is located at the western south end of Pakistan. It should not be confused with the political province Balochistan of Pakistan as these two have very distinguished and separate boundaries. Balochistan basin is present in west of Chaman fault and north of Arabian sea. The western part of Balochistan basin extends into Iran. Geologically this basin is present both offshore and onshore. Balochistan basin has a large area of three lac square kilometers but due to operational difficulties and geological complexities this basin is the least explored sedimentary basin in Pakistan.

a) Tectonic setting of Balochistan Basin:

In this basin, Arabian oceanic plate is subducting beneath the Eurasian plate, which give rise to number of geological structures which are present at subduction zone. If we start from Arabian Sea and come upwards, we encounter Makran trench, Makran coastal depression, Makran accretianary prism, Hamun-i-Mashakhel fore Arc basin, Ras Koh Arc, Mirjawa-Dalbandin Trough and the Chagai Arc. (Qadri)

If we see this whole picture in terms of sea floor spreading and plate tectonics, we can easily infer that the Arabian oceanic plate is subducting beneath the continental margin of Eurasian plate at Makran trench. Due to this subduction of oceanic plate under continental plate some lithospheric part from subducting slab of oceanic plate is scraped off and is currently present in the form of Makran accretionary prism. This subduction of oceanic plate also gave rise to Andes/Cordilleran type of volcanic mountains which are andesitic in nature. Chaghai Volcanic Arc and Ras Koh Arc are example of this. Mirjawa-Dalbandin Trough is a depression/gap between these two Arcs. Now if there is a subduction zone and a volcanic arc, then there must be a Fore arc basin in case of Ocean-continent convergence so it is present as Hamun-i-Mashakhel Fore Arc Basin. This is the widest arc trench system of world in which the distance between volcanic arc and trench is 500km. (Qadri).

b) Stratigraphy:

In Balochistan basin, the oldest rocks exposed on surface are of cretaceous age. This aging trend of rocks is from south to north i.e. older rocks are present on north and younger rocks are present in north.

Following rock formations are present in in Balochistan basin, containing volcanic rocks, volcanic conglomerates, shale, sandstones, fossiliferous and reef limestone.

Cretaceous: Sinjrani Formation, Humai Formation.

Paleocene: Rakhshani Formation, Ispikan Conglomerate.

Eocene: Saindak Formation.

Oligocene: Amalaf Formation, Siahan/Hoshab Formation.

Miocene: Panjgur Formation, Parkini Formation.

Pliocene: Talar Formation.

Pleistocene: Chatti Formation, Ormara Formation, Jiwani Formation.

c) Economic aspects:

Balochistan Basin is basically a trench arc system. Fore arc basins, like Hamun-i-Mashakhel are the best in terms of petroleum potential. Moreover the Makran Accretionary Prism compromise of Panjgur formation (sandstone) and khojak Formation (shale) which act as good reservoir and source rocks. All the drilling activity in past was done in Makran Accretionary Prism. But unfortunately all these wells had to abandon because of problems, both of operational and geological origin. This basin is also rich in ore minerals like zinc, copper, Sulfur and gold. Especially the Saindak area of Balochistan basin contains very valuable resources of gold and copper. It can be concluded that Balochistan basin have all the qualities needed for hydrocarbon and mineral accumulation but lack of infrastructure and less geological knowledge of area are the main factors which keep the oil and gas industries away from this area.

5. Comparison of Main Sedimentary Basins of Pakistan:

Indus Basin | Balochistan Basin | Pashin Basin

Size of Basin

Largest basin in terms of size if lower and upper Indus basins are joined.

Second largest basin of Pakistan.

Smaller as compared to Indus Basin and Balochistan Basin.

Oldest Rocks

Oldest rocks of Precambrian age are exposed.

Cretaceous age rocks are found as oldest in this Basin.

Oldest rocks are of tertiary age.

Tectonic Setting

Pre-collisional development which goes back to Precambrian age.

It is an arc-trench system near subduction zone of Arabian Sea plate.

Developed due to collisional interactions of Indian and Eurasian plate.

Hydrocarbon Potential

Most of the oil/ gas wells are drilled here. Specially lower Indus part is very perspective.

Contains good reserves of hydrocarbon and minerals but unexploited due to lack of infrastructure.

Not much explored due to operational difficulties.

6. References:

1. Petroleum Geology of Pakistan, Iqbal B. Kadri, 1995.

2. Imtiaz Ahmed, Paleozoic Stratigraphy of Peshawar Basin, 1991.

3. Aftikhar A.Abassi, Journal of Himalayan Earth Sciences 42 (2009) 61-85.

4. Azhar Farooq, Journal of Himalayan Earth Sciences 46(2) (2013) 65-77.

5. Geology and tectonics of Pakistan, Kazmi and Jan.

6. http://www.tcd.ie/Geology/assets/pdf/geology-for-engineers/L09_basins.pdf

7. http://www.oceanografia.ufba.br/ftp/Geologia_Marinha/Sed_basin_introd_Lippard.pdf

8. http://jcsp.org.pk/ArticleUpload/1159-5148-1-CE.pdf

9. http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-110-sedimentary-geology-spring-2007/lecture-notes/ch11.pdf

10. http://nceg.upesh.edu.pk/GeologicalBulletin/Vol-42-2009/Vol-42-2009-Paper6.pdf

11. http://www.ias.ac.in/jess/feb2012/73.pdf

The end.

Sedimentary basins of Pakistan

Indus Basin

Upper Indus Basin

Lower Indus Basin

Balochistan Basin

Pashin Basin

Kohat sub-basin

Potwar sub-basin

Central Indus Basin

Southern Indus Basin

Peshawar Basin

Siwalik Basins

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