Rehanul Haq Siddiqui, Takahito Naka, Munir-ul- Haq, Imdad Ali Brohi


The  Ras  Koh 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. This arc is designated as frontal arc of the Chagai-Ras Koh arc system. The Late Cretaceous Kuchakki Volcanic Group is the most wide spread and previously considered the oldest unit of the the Ras Koh 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 Ras Koh arc. During the present  field  investigation  the  Rakhshani  formation  in  the  southeastern part  of  the  Ras  Koh  arc,  is  identified  as an accretionary complex, which is designated as Ras Koh accretionary complex. The Ras Koh accretionary comple is subdivided into  three  units: (a) Bunap  sedimentary  complex, (b)  Charkohan radiolarian chert, and (c) Ras Koh ophiolite melange. The Bunap sedimentary complex is farther divided into three tectonostratigraphic units viz., northern, middle and southern. Each unit is bounded by thrust faults, 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 is mainly comprised of allochthonous fragments and blocks of limestone, sandstone, mudstone and the volcanics in dark gray, greenish gray and bluish gray siliceous flaky shale. At  places  the  shale  is  metamorphosed into phyllite. This unit is thrust over the middle unit, which exhibits relatively a coherent stratigraphy, 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 which is generally moderately argillized. 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 is found in the southeastern part of the Ras Koh arc. This chert sequence occurs on the margins of a large exotic block (350m X 3 km) of volcaniclastic  rocks  of unknown origin, which makes an overturned syncline. This chert sequence is developed on its both limbs and has lower faulted contact with the Bunap sedimentary complex. 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 Ras Koh 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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