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Rehanul Haq Siddiqui, Geoscience Advance Research Laboratories (Pakistan)
Takahito Naka, Ex-JICA expert in Geoscience Advance Research Laboratories (Pakistan)
Munir-ul Haq, Geological Survey of Pakistan (Pakistan)
Fazli Rabbi Khan, National Centre of Excellence in Geology (Pakistan)
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The Raskoh arc is about 250 km long, 40 km wide and trends in ENE direction. The arc is convex towards southeast and terminated by the Chaman transform fault zone towards east.
The Late Cretaceous Kuchakki Volcanic Group is the most wide spread and previously considered the oldest unite of the the Raskoh arc followed by sedimentary rock formations including Rakhshani Formation (Paleocene), Kharan Limestone (Early Eocene) and Nauroze Formation (Middle Eocene to Oligocene). Dalbandin formation (Miocene to Pleistocene), and semi-unconsolidated Subrecent and Recent deposits.
The Rakhshani Formation is the most wide spread and well-exposed unit of the Raskoh arc. During the present field investigation this formation in the southeastern part of the Raskoh arc is identified as an accretionary complex, which is designated as Raskoh accretionary complex. The Raskoh accretionary comple is subdivided into three units:
(a) Bunap sedimentary complex,
(b) Charkohan radiolarian chert, and (c) Raskoh ophiolite melange. The Bunap sedimentary complex is farther divided into three tectonostratigraphic units viz., northern, middle and southern.
Each unit is bounded by thrust fault, which is usually marked by sheared serpentinites, except northern unit, which has gradational and at places faulted contact with the Kuchakki Volcanic Group. The northern unit mainly comprised of allocthonous fragments and blocks of limestone, sandstone, mudstone and the volcanics in dark gray, greenish gray and bluish gray siliceous flaky shale. This unit is thrust over the middle unit, which is represented by greenish gray calcareous flaky shale with intercalation of thin beds and lenticular bodies of mudstone, sandstone and limestone.
The middle unit is again thrust over the southern unit, which is mainly composed of large exotic blocks of volcanic rocks, limestone, sandstone, mudstone and conglomerate embedded in dark gray, greenish gray and bluish gray siliceous flaky shale. The unit is thrust over the Kharan Limestone. During the present field investigation about 350 meter thick sequence of thin bedded maroon and green chert intercalated with the siliceous flaky shale of the same colour is discovered within this unit, which found in the southeastern part of the Raskoh arc.
Two samples collected from this chert sequence yielded radiolarian fauna, which include Parvicingula sp, Laxtorum sp., Parahsuum cf. simplum, Parahsuum sp., Nassellaria gen. et sp. indet., Hsuum cf. Matsuokai., Archaeospongoprunum sp., Nassellaria gen. et sp. indet. and Hagias gen. et sp. indet.Tricolocapsa sp. Hsuum sp. Ristola sp. Archaeospongoprunum sp. and Tritrabinate gen. et sp. indet.. This radiolarian chert sequence represents the late Early to Middle Jurassic pelagic sediment deposited in Ceno-Tethyan ocean floor; prior to the inception of volcanism in the Raskoh arc and accreted with the arc during Late Cretaceous to Eocene along with the Bunap sedimentary complex of Late Jurassic age.
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