Upper Indus Basin (Kohat-Potwar Province) cambrian age

Upper Indus Basin (Kohat-Potwar Province)      
The area discussed in this basin covers the Kohat-Potwar Province (Fig. 8), which includes Salt Range and Khisor Range. The Cambrian formations recognized in these regions arc as follows; (serially numbered,oldest No.1)
5. Khisor Formation
4.Baghan wala Formation
3.Jutana Formation
2.Kussak Formation
1.KhewraSandstone
The Cambrian rocks of the Khisor and Salt ranges consist of sandstone, shale, and dolomite with glauconitic interbeds, which were essentially deposited in shallow water, except for the lower most and upper most formations, which represent transgressive and regressive facies respectively.

1.Khewra Sandstone:
The name “Khewra group" was originally proposed by Noetling (1894). Prior to that, Wynne (1878) called the formation “Purple sandstone series”. The latter name continued until recently, when the name of the formation was formalized as “Khewra Sandstone” by the Stratigraphic Committee of Pakistan, Fatmi (1973). The type locality is in Khewra Gorge near Khewra Town,Salt Range.
                 The formation consists predominantly of purple to brown, yellowish-brown, finegrained sandstone. The lowermost part of the formation is red, flaggy shale. The sandstone is mostly thickbedded to massive. Sedimentary features like ripple marks; mud cracks etc. are common in the formation.  the subject of controversy, which arose because of age dispute of the Salt Range Formation. Sahni (1947) and his school of thought regarded the age of Salt Range Formation as Tertiary and postulated a thrust at the contact between Salt Range Formation and the Khewra Sandstone. Gee (1945, 1981) mapped the entire Salt Range and concluded that the contact between the two formations is sedimentary. The formation is predominantly
sandstone, which is taken to mark the base of the overlying Kussak Formation. The Khewra Sandstone is  In the western Salt Range,it is 200 m and the exposed thickness in the Khisor Range is about  as trilobite trails by Schinde wolf and Seilacher (1955) and are not indicative of any particular age. However, as the overlying Kussak Formation is not older than late Early Cambrian, the age of the Khewra Sandstone is almost certainly Early Cambrian.

                  

Baqri et al(1991) carried out a detailed study of the Khewra Sandstone and they were of the opinion that Khewra Sandstone represents a deltaic sequence with distinct bottom-set, fore-set and top-set deposits of the ancient Cambrian delta. Khewra Sandstone can be texturally divided into sandstones, silty sand, sandy silt and siltstones, and consequently, may be termed as Khewra Siltstonc. The sandstone appears to be a good reservoir for oil, gas and water and may be exploited for its hydrocarbon potential in the subsurface.

2.Kussak Formation:

Wynne (1878)applied the name'Obolus beds' or 'Siphonotrata beds' to a predominantly greenish-grey, glauconitic, micaceous sandstoneandsiltstone. Waagen and Wynne (1895) used the name “Neobolus beds” for the same unit. Noetling (1894) proposed  the name of the unit as Kussak Formation (Fatmi, 1973). The type locality lies near Kussak Fort in the eastern part of the Salt Range.

                At the type locality, the formation is composed of greenish-grey, glauconitic micaceous sandstone, greenish-grey siltstonc, interbedded with light grey dolomite. Pink gypsum lenses are present near the top. The formation contains 5 to 25 cm long thin lenses of fossil asphalt (gilsonitc) as observed by the present author. The general lithology throughout the Salt and Khisor ranges is uniform. However, thicknesses vary at different places. The formation is widely distributed throughout theSaltRange with its bestexposures in theeastern part.Thicknessat the type locality is 70 m but varies from 6 to 53 m at other places. In the Khisor Range the formation is 55 m thick.

                 The formation is fossiliferous and has yielded the following fauna: Neobolus warthi, Botsfordia granulata, Lingulella wanniecki, L. fixchsi, Hyolithes wynnei, Redlichia noetlingi. Schindewolf and Seilacher (1955) regarded the age as Early Cambrian. However, Teichert (1964) has shown that R. noetlingi is allied to or perhaps even identical with R. forresti that occurs in Australia in beds of early Middle Cambrian age. The age of the formation is, therefore, either late early or early Middle Cambrian.

3.Jutana Formation: 

Fleming (1853) named this unit “Magnesian sandstone”. Noetling (1894) described it as Jutana stage. The Stratigraphic Committee of Pakistan formalized the name as JutanaFormation. The type locality lies near Jutana Village in the eastern Salt Range.

                At the type locality, the lower part of the formation consists of light green, hard, massive partly sandy dolomite, while the upper part is composed of light green to dirty white massive dolomite. In the upper part, brecciated dolomite is also present with matrix and fragments of the same rock.it is overlain by the Baghanwala Formation. In the western-most Salt Range, it is not present. However,(“Khisor gypsiferous beds”), which is equivalent to the Baghanwala Formation of the eastern Salt Range.

               It thins westward. Its exposures in the Khisor Range have a thickness of 50 m. The formation is fossiliferous. Schindcwolf and Seilacher (in Teichert 1964) collected and described some fossils from the shale unit in the middle part of the formation exposed in the eastern Salt Range including Lingulella fuchsi, Botsfordia granulata, Redlichia noetlingi and also a gastropod identified as Pseudotheca cf. subrugosa. The age of the formation is described as late Early Cambrian to early Middle Cambrian.

4.Baghanwala Formation:

The name Baghanwala Formation is now given to the rocks of the “Pseudomorph Salt Crystal Zone” of Wynne (1878) and the “Baghanwala Group” of Noetling (1894), which overlies the Jutana Formation. Holland (1926) called these beds “Salt Pseudomorph beds” and Pascoe (1959) named them “Baghanwala Stage”. The type section is located near Baghanwala Village in the eastern Salt Range.

               The formation is composed of red shale and clay with alternate beds of flaggy sandstone. The flaggy sandstone exhibits several colours including pink grey or blue green, especially in the lower half of the formation. Sedimentary structures such as ripple marks and mud cracks are common . Numerous pseudomorphic casts of salt crystals,which are found along the bedding planes, are the diagnostic feature of this formation.

             Presence of casts of salt pseudomorphs coupled with the absence of fossils indicates lagoonal environmentand arid climaticconditionsforthe deposition of the formation.Theformation is mainly developed in the eastern Salt Range. The outcrops disappear a few kilometres west of Makrach but reappear at Chittidil and Ghhidru localitiesin the centralSalt Range.The formation has been encountered in the sub surface at Karampur village. Near Baghanwala, its thickness ranges from 100 to 116 m. The same thickness has been reported from northeast of Khewra, but in the Khewra Gorge, it is reduced by erosion to only 40 m. The upper contact of the Baghanwala Formationwith the overlyingTobraFormation is unconformable,whereas the lower contact with the Jutana Formation is conformable.

             The formation contains only trace fossils. Since the Baghanwala Formation rests conformably on the Jutana Formation, which is considered as early Middle Cambrian in age, the same agemaybe assigned to the Baghanwala Formation.

5.Khisor Formation:

Gee (1945) described a unit of rocks composed of gypsiferous dolomitic shale and dolomite lying between the Jutana Formation and the Tobra Formation in the Khisor Range as “Gypsiferous Series”. Hussain (1960) undertook a detailed study of the unit and named it “Khisor gypsiferous beds”. The Stratigraphic Committee of Pakistan approved the name Khisor Formation. The Khisor Formation is limited only to the Khisor Range, where it is exposed only at one place near Saiyiduwali. This formation is not present in the Salt Range. However, its stratigraphic equivalent, the Baghanwala Formation,is fully developed in the easternSaltRange. At the type locality Hussain (1960) divided the rock unit into two parts. The lower part consists of massive white gypsum, with few thin beds of fine, grey, crystalline dolomite. At the base of the
formation,a meter thick band of dolomite and dolomitic shale occurs.

                      The upper part is composed of light coloured, bedded shale, with finely crystalline dolomite and white gypsum. The shale is light grey in colour and is thin- to medium-bedded. The dolomite is

medium to thick-bedded and the gypsum is massive.

                   The formation has conformable contact with the underlying Jutana Formation, whereas the upper part of the formation is terminated by disconformity with the Tobra Formation.

                    The formation is devoid of mega-fossils. However, the rocks have not been examined for micro fauna. Since the formation occupies the same stratigraphic position as the Baghanwala Formation of the Salt Range, it is assumed that it represents early Middle Cambrian age.