Indus Basin

The Indus Basin is the largest and more thoroughly studied basin of Pakistan. It trends NE-SW for over 1600 km along its axis and the width varies considerably with an average of 300 km. The basin contains sediments ranging from Precambrian to Tertiary with marked absence of Ordovician, Silurian, Devonian and Carboniferous. This basin is characterised by well-developed platform deposits of Jurassic throughout. The basin is divided into, the Upper Indus-Basin located north of 30th parallel and the Lower Indus Basin situated south of 30th parallel.Approximately, the Upper Indus Basin covers more than 50,000 km² area and in many ways the structural and stratigraphic conditions are different from those of Lower Indus Basin. The Lower Indus Basin covers an area of more than  250,000 km² where along with other rocks excellent exposure of Jurassic to Tertiary sedimentary section of about 13 to 16 km thickness, are present in its deeper part, it is
ideally suited for oil exploration. Distinctly except for some Tertiary continental deposits, almost the entire sequence of Lower Indus Basin is of marine origin.to Tertiary sedimentary section of about 13 to 16 km thickness, are present in its deeper part, it is ideally suited for oil exploration. Distinctly except for some Tertiary continental deposits, almost the entire sequence of Lower Indus Basin is of marine origin.

                The Indus Basin as a whole is characterized by syntaxes, irregular ridges or promontories of the Indian Shield; almost all of them trend NW. These promontories played an important role in controlling the sediments and acted as barriers in the shelf or neritic parts of the Indus Basin in different geological ages and thus subdivided the basin into two recognizable stratigraphic provinces: The Kohat-Potwar and the Sulaiman-Kirthar Provinces. Indus Basin is equated with Southern Basin Zone1 in the regional Tectonic frame work of Shahand Quennell(1980;Fig.2).

Axial Belt: Axial Belt is a long narrow sinuate-shape belt, which runs from Arabian Sea through
Bela, Khuzdar, Quetta, Zhob, and Waziristan. Further, the belt in its northern extension covers the
areas of Kurram Agency, Attock-Hazara folded belt (Fig. 1). 

It follows from this that the Axial Belt can be divided into two sectors, the Southern Sector with N-S alignment and Northern Sector with E-Walignmoat.
               The Southern Sector of the Axial Belt is an area of widespread allochthons. Littoral deposits as well as radiolarites and olistostromes occupy it. Ophiolites are associated with pelagic sediments, which contain gabbroic and soloaritic intrusions. Oligocenc-Mioccne molasses covers a considerable part of this sector in Zhob area. The Southern Secto rcan further be divided into western part, the Arenaceous Zone where the rocks are almost the same as those of eastern Balochistan Basin and the Calcareous Zone in the
eastern part, the rocks of which are identical to those of Indus Basin. This last two-fold division is
extremely convenient for correlation purpose.
                Shah and Quennell (1980) in their stratigraphic correlation of Turkey, Iran and Pakistan, equated this belt with “Southern Axial Zone II”, a component of their regional structural framework. They linked and described the Axial Belt of Pakistan as follows. "The Makran, from the Zendan Fault to the (southern) sector of the Axial Belt of Pakistan, occupies a break in the continuity of the Southern Axial Zone (and this break can now be joined via the Arabian Sea), which in Pakistan is resumed by the Axial Belt. The Axial Belt of Pakistan has a double sigmoidal plan and extends from the Arabian Sea to the Hazara region in the north"
           The structural evolution of the Axial Belt began in the Jurassic, culminated in the Eocene and
continued locally until the Pleistocene. The meridionally trending Southern Sector separates the Indus Basin from Balochistan Basin. There is westward gradation from the shelf type deposits of the Indus Basin to the oceanic andophiolite suite rocksof the belt, which include mafic-ultramafic rocks of Cretaceous-Paleocene age, complicated by Low-angle thrust faulting. The Northern Sector generally has a thrust fault (MBT) relationship with the Upper Indus Basin” and approximately the Panjal Thrust delimits it in the north. The area covered by the Northern Sector of the Axial Belt is almost the same, which has been delineated by La Fortune et al. (1992) as their “Hill ranges”. They defined this area as “where shelf sediments on the northern margin of the Indian continent were thrust southwards over the Potwar Plateau, along the Main Boundary Thrust (MBT)”, which is of Late Tertiaryage (Yeats and Hussain,1987).
            This belt is differentiated from Indus Basin on the abrupt change in the tectonics and orographic grounds as well as change of facies and diastrophic style, which are in marked contrast. Cambrian Precambrian and Mesozoic flysch-type sediments, carbonates and clastic rocks, and very low-grade metamorphic and small amountof igneous intrusives, make up the Northern Sector of the Axial Belt. Parautochthonous sediments of Attock - Cherat Range are different both from the northern and southern stratigraphic set up. In other words this area, structurally and stratigraphic different from Peshawar Basin as well as they are in marked contrast with the Indus Basin. The Northern Sectorof the Axial Belt has been separated by almost all workers of this region,some called it “sedimentary zoneoflesser Himalayas”, the others differentiatedit on gradeof metamorphism and still others on faults zone between MBT and Punjal Thrust. Stratigraphically, the sequence of rocks in this zone extends from platform type deposits to oceanic crust similar to the Southern Sector of the Axial Belt. Major and trace elements and Mg of the rocks indicate that the Panjal magma has been evolved as tholeiitic to slightly alkalic in composition with geochemical signatures similar to mid¬ ocean ridge basalts and ocean island tholeiites. The interbedded chert, limestone and associated turbidite deposits indicate shallow marine environment during the time of eruption of magma. The volcanic represent rifting of the northern margin of Indian continent and development of shallow
marine oceanic conditions during Upper Palaeozoic”
            In the Southern Sector of the Axial Belt, north-south running exposure (with in the Calcareous Zone of the Axial Belt) has similarities with the western part of Indus Basin. Similar to this situation, rocks of the Northern Sector of the Axial Belt extending laterally from west to east also have similarities with Kohat-Potwar Province on either side of MBT, except in the eastern part of the Potwar Plateau, where the absence of Mesozoic rocks have always been considered as a non depositional or erosional event, but are present on the northern side of the MBT.
           In recapitulation it can be said that the Axial Belt started developing on the margins of Indo Pakistan Shield during Jurassic (probably post Callovian) times with mild tectonic activities, but violent tectonic activities during post collisional times pronounced it into its present shape. Despite its position as a separate entity, the encroachment of Mesozoic stratigraphic sequence from the adjoining basins is frequent. Nevertheless, major structural episodes during post depositional times have made the Northern Sector of the belt as a lien to the adjoining basins.
           According to Mujtaba (2004), typical Jurassic and Early Cretaceous formations, i.e., Datta, Samana Suk, Chichali and Lumshiwal Formations are observed as Far East as Thandiani formation of theAbbottabad area. These rockunits are well exposed throughout the southern Hazara hills, Kala Chitta and then across the Indus River in the Samana Range. However, the situation across the Main Boundary Thrust, in the adjacent Potwar area, was found quite different as no formations of Jurassic and Cretaceous ages were encountered in wells, drilled in eastern and central Potwar region(Fig. 4).

In Mujtaba's opinion “the possible logic of this sharp contrast of presence and absence of the above noted formations, across the Main Boundary Thrust in the southern Hazara hills (including Margala hills) and the adjacent eastern and central portion of Potwar region, could be right-lateral strike-slip fault along the present Main Boundary Thrust. Thus resulting probably in the lateral displacement of Hazara block towards cast relative to the Potwar block.
          The Axial Belt of Pakistan is the folded zone between the Indian plate on the east and the Afghan and Iranian plates on the west, and thus has the same tectonic relationship as the Toros and Zagros segments have to the African and Turkish-Persian plates(Shah and Quennell, 1980).