| Ceno-Tethyan Ammonite Fauna from the Raskoh Arc,
Balochistan, Pakistan |
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| 关键词:Ammonite fauna Raskoh arc tectonostratigraphic significance |
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| Ceno-Tethyan Ammonite Fauna from the Raskoh Arc,Balochistan, Pakistan |
| 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 widespread and previously considered the oldest unit of the Raskoh arc followed by sedimentary rock formations including Rakhshani Formation (Paleocene), Kharan Limestone (Early Eocene), Nauroze Formation (Middle Eocene to Oligocene), Dalbandin Formation (Miocene to Pleistocene), and semi-unconsolidated Subrecent and Recent deposits. The Rakhshani Formation is the most widespread and well-exposed unit of the Raskoh arc. During the present field investigation the Rakhshani Forma-tion in the southeastern part of the Raskoh arc is identified as an accretionary complex, which is designated as Raskoh accretionary complex. The Raskoh accretionary complex is subdivided into three units: (a) Bunap sedi-mentary complex, (b) Charkohan radiolarian chert, and (c) Raskoh ophiolite mélange. 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 grada-tional and at places faulted contact with the Kuchakki Volcanic Group. The northern unit mainly comprises al-lochthonous 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, lime-stone, 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 the Bunap sedimentary complex. These fossils are identified as Pachysphinctes cf. P. africanus a Lower Kimmeridgian, Torquatisphinctes cf. P alterniplicatus, an Upper Kim-meridgian and Parodontoceras cf. Blandfordiceras wallichi: a Lower Tithonian ammonite. The Bunap sedimentary complex was probably deposited on the ocean floor of the Ceno-Tethys that once occurred between the newly dis-lodged collage of Cimmerian continent (Central Iran, Afghan blocks, Lhasa and West Burma) and the northern passive margin of Gondwana. |
| Rehanul Haq SIDDIQUI,Munir ul HAQ,Fazli RABBI.2009.Ceno-Tethyan Ammonite Fauna from the Raskoh Arc,Balochistan, Pakistan[J].Acta Geoscientica Sinica,30(S1):61-69. |
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