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Sembar Formation
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Sembar Fm base reconstruction

Sembar Fm

Period: 
Jurassic, Cretaceous

Age Interval: 
Early Cretaceous (mainly Neocomian), may extend into the Late Jurassic


Province: 
Pakistan Indus Basin

Type Locality and Naming

Mona Jhai Gr – lower formation. Holotype section: Two km SE of Sembar Pass, SP. Author: M.D. Williams, 1959. Reference section: None.


Lithology and Thickness

Claystone. In the type section, it consists of black, silty shale with interbeds of black siltstone and nodular rusty weathering argillaceous limestone beds or concretions. Glauconite is commonly present which gives the greenish hue to the weathering color. In the basal part, pyritic and phosphatic nodules and sandy shales are developed locally. The sand content increases towards the SE corner of Pakistan, close to the border with India, where interbedded sand beds may form a combination of source, reservoir and the cap rock.

Thickness: A few meters to 262 m in outcrops, while more than 800 m thick in subsurface as encountered in one of the oil wells in Badin area. The thickness in type section is 133 m but it thickens to 262 m in Moghal Kot section of the Sulaiman Range. In the vicinity of Quetta and Ziarat the thickness is reduced to a few meters. In subsurface, however, the thickest Sembar has been encountered in extreme SE of Pakistan in one of the oil wells.


Lithology Pattern: 
Claystone


Relationships and Distribution

Lower contact

Disconformably underlain by Mazar Drik Member of Takatu Fm and also the Shirinab Fm in Axial Belt, while in Sulaiman Province, and in the oil wells drilled in KrP, it is underlain by Takatu Fm as the Mazar Drik Member is missing.

Upper contact

Conformably overlain by Goru Fm (gradational).

Regional extent

It is absent in parts of Axial Belt and the Kirthar Province, where the Goru Fm directly overlies Jurassic limestone. However, the formation occurs virtually throughout the Indus Basin. In Karachi Embayment, it is over 760 m thick. The wells drilled in Badin area invariably encountered this formation, which is considered as the source rock for the hydrocarbons being produced from the Lower Goru sands of Lower Cretaceous age.


GeoJSON

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Fossils

Most common are belemnites Hibolithes pistilliformis, H. subfusiformis and Duvalia sp.


Age 

Early Cretaceous (mainly Neocomian), may extend into the Late Jurassic.

Age Span: 

    Beginning stage: 
Berriasian

    Fraction up in beginning stage: 
0.0

    Beginning date (Ma): 
143.10

    Ending stage: 
Valanginian

    Fraction up in the ending stage: 
1.0

    Ending date (Ma):  
132.60

Depositional setting

It is believed to have been deposited on a broad shelf, sloping west ward off the Indian Peninsula.


Depositional pattern:  


Additional Information

EMW: Source rock for oil and gas in Pakistan.


Compiler:  

Nusrat K. Siddiqui