Downstream evolution of turbiditic channel complexes in the Pab Range outcrops (Maastrichtian,Pakistan) |
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| Institution: | 1. Institut Français du Pétrole, 1,4 Avenue de Bois-Préau, 92852 Rueil-Malmaison, France;2. Premier Exploration Pakistan Limited, 4th floor Jang building, Blue area, Fazal-a-Haq Road, Islamabad, Pakistan;1. Institute of Chemical Sciences, University of Peshawar, 25120, Peshawar, Khyber Pakhtunkhwa, Pakistan;2. Department of Environmental Sciences, University of Peshawar, 25120, Peshawar, Khyber Pakhtunkhwa, Pakistan;1. Department of Earth Sciences, Quaid-i-Azam University, 45320, Islamabad, Pakistan;2. Department of Geology, Bacha Khan University Charsadda, Pakistan;3. Oil and Gas Development Company Limited (OGDCL), Pakistan;4. Akbar Associates (Private) Limited, Pakistan;1. Department of Petroleum Geosciences, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia;2. Department of Geology, Periyar University, Salem 636011, India;3. Institute of Oceanography and Environment, Universiti Malaysia Terengganu, 21030, Kuala Nerus, Terengganu, Malaysia;4. Department of Earth Sciences, University of Adelaide, SA 5005, Australia;5. School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China;6. Laboratoire Géosciences Océan, UMR CNRS 6538, Campus de Tohannic, 56000 Vannes, France |
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| Abstract: | The Pab Formation consists of deltaic and turbiditic sediments which were deposited during the Late Maastrichtian on the Indo-Pakistani passive margin. The margin geometry has been restored in the Pab Range from a regional transect 120 km long. Two superposed turbiditic systems onlap the slope carbonates and completely pinch-out southward. The lowest turbiditic system (Lower Pab) is a sand-rich basin floor fan, which consists of sand-rich channel complexes distally passing to lobes northward. This basin floor fan is overlain by a mud-rich slope fan formed during the subsequent sea-level rise, which drowned the shelf. The upper turbiditic system (Upper Pab) is a sand-rich slope fan, formed during the progradation of a deltaic system in the shelf setting. It consists of prograding tabular lobes passing upward to conglomeratic channels, and thins out northwards. The Lower Pab turbiditic system consists of three channel complexes (LP1, 2, 3) organised in a backstepping succession. Each channel complex has a multi-storey internal architecture, resulting from the amalgamation of several individual turbiditic channels. Five major facies associations have been determined in the LP3 channel complex. FA-1 corresponds to polygenic and monogenic debris-flows, FA-2 to high-density gravelly or sandy turbidites, FA-3 to by-pass deposits, FA-4 to thin-bedded turbidites (spill-over lobes and levees) and FA-5 to hemipelagites. The downstream evolution of the LP3 channel complex can be studied from canyon to mid-fan settings. Where it is confined in the canyon, the channel complex is 50 m thick and 1 km large, and shows a high sand/shale ratio. The development of overflow deposits is limited and occurs only at the top of the channel complex. At the canyon mouth, the channel complex is still deeply incised but overflow deposits start to expand laterally as a result of the decreased confinement. By-pass facies here are well-developed, and are related to hydraulic jump processes. In the mid-fan setting, the channel complex widens and the sand/shale ratio decreases. Erosion at the channel base is less developed, whereas internal and external levees are well-developed. Spill-over lobes form the last stage of the channel complex infill. The internal geometry of the channel complexes is a result of a complex interaction between lateral confinement, by-pass and lateral migration processes. |
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