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Rehanul Haq Siddiqui, 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, which occurs in the western part of Pakistan, is about 250 km long, 40 km wide and trends in ENE direction. This arc is designated as frontal arc of Chagai-Raskoh arc system. Arc is convex towards southeast and is terminated by the Chaman transform fault zone towards east. The Raskoh arc is a fossil oceanic island arc which was formed due to the intra-oceanic convergence in the Ceno-Tethys.
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.
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 a 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 several poorly preserved ammonite fossils were collected from the dark green to black mudstones of the middle unit of of the Raskoh accretionary complex. These fossils are identified as Pachisphinctes cf. P. africanus a Lower Kimmeridgian, Torquatisphinctes cf. P alterniplicatus, an Upper Kimmeridgian and Parodontoceras cf. Blandfordiceras wallichi. a Lower Tithonian ammonite.
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