Salt Range Fm
Type Locality and Naming
Holotype section: Khewra Gorge, Eastern Salt Range (northern Indus Basin). Hypo: Well Marot-1, Punjab Platform. (Depth range 1,400-2,545 m). Author: Asrarullah, 1967. Reference section: Well Marot-1 drilled on Punjab Platform is being suggested by Nusrat K. Siddiqui, as full thickness of SRF has been drilled in this well.
Synonym: Khewrite Salt Range Fm
Lithology and Thickness
Evaporite. The Salt Range Fm (SRF) in the type locality and elsewhere essentially consists of an un-metamorphosed sequence of salt, marl, gypsum and dolomite with dominant red gypsiferous claystone without any apparent bedding. Bituminous shales, dolomites, and oil shale layers are common. The formation has been subdivided into a lower Billianwala Salt Member (>650 m), a middle Bhandar Kas Gypsum Member (>80 m) and an upper Sahiwal Marl Member (>40 m). The top of the formation is formed by a gypsum layer containing high-grade oil shale, or at places the uppermost unit is a highly altered volcanic rock known as Khewra Trap or Khewrite. The Khewra volcanic trap is a purple, reddish brown, orange to buff, rarely dark green rock, very fine-grained, porphyritic and vesicular to amygdaloidal; the phenocrysts are euhedral to skeletal spinifex and commonly grow radially.
Thickness: At the type locality, it is 830 m thick, while in well Marot-1 a maximum thickness of 1,145 m was encountered as the well bottomed in the basement. At Dhariala-1 well a large thickness of more than 2,100 m was encountered which seems to be due to salt flowage and hence, not a true thickness.
Relationships and Distribution
Lower contact
Underlain by metamorphic basement.
Upper contact
Conformably overlain by Khewra Sandstone Fm across a distinct contact.
Regional extent
The formation overlies the basement along the northwestern flank of the Indian Shield, and is widely exposed in the Salt Range in Pakistan, between Jogi Tilla in the east and Kalabagh in the west. Though the base is not exposed, but its full thickness has been encountered in a few wells like Karampur-1, Marot-1 etc. drilled on Punjab Platform (PP) where the Salt Range Fm overlies metamorphic Precambrian basement. It has also been encountered in wells drilled in Potwar region and further south of PP a well Marvi-1 drilled 363 m of this formation without reaching its base. Well Marot-1 on the PP drilled a maximum thickness of 1,145 m with TD in metamorphic basement.
GeoJSON
Fossils
The formation is devoid of fossils. The results of sulphur-isotpe measurements carried out on a gypsum sample from the top of Salt Range Fm indicate an age of 600 Ma (Kazmi & Jan, 1997).
Age
Depositional setting
The evaporatic sequence of Salt Range Fm was deposited in a restricted, shallow-marine environment, under arid conditions.
Additional Information
EMW: Salt and gypsum. Possible source rock for oil.