A STRATIGRAPHIC RECORD OF THE TIME TRANSGRESSIVE INDIA-ASIA COLLISION FROM NORTHWESTERN PAKISTAN

1. Map Legend RESULTS Kalat Region (Balochistan) Two major late Paleocene to early Eocene syntectonic fan conglomerate facies associated with the India-Asia collision have been recognized from the Kalat region. The lower conglomerate facies is about 500 meters thick and forms the uppermost part of late Paleocene Gidar Dhor Formation. Clast sizes of the conglomerate decrease eastward from cobble to pebble to coarse grained sandstone that intermingle with shallow marine shales. The lower contact is unconformable with Cretaceouse rocks and the upper contact is conformable with lower Ghazij shales. Quetta Region (Balochistan) The Quetta region is characterized by two phases of paralic to continental facies transitions. In each case, marine shale grades into paralic sandstone, siltstone, shale and coal which then grade into continental deposits of red mudstone, sandstone and conglomerate. The first phase was deposited at the very beginning of early Eocene time and the second occurs about 1200 meters above the first. These two sequences of syntectonic sedimentation are thus younger in age than the syntectonic sediments in the Kalat region. Bannu-Kohat Region (NWFP) The continental deposits here are well known as the Kuldana Fm. and they range from late early Eocene to middle Eocene in age. The formation is dominantly continental red mudstones. The lower ~30m (Gurguri Sandstone) consists of sandstone and shale with a few meter thick conglomerate facies, especially in the western part. Fence Diagram Legend Contact Information: Intizar H. Khan, Department of Earth Sciences,University of New Hampshire 56 College Rd., Durham, NH 03824, USA Geological Survey of Pakistan, P. O. Box 15, Sariab Rd., Quetta, PAKISTAN (ikhan@cisunix.unh.edu) William C. Clyde, Department of Earth Sciences, University of New Hampshire 56 College Rd., Durham, NH 03824, USA (will.clyde@unh.edu) A STRATIGRAPHIC RECORD OF THE TIME TRANSGRESSIVE INDIA-ASIA COLLISION FROM NORTHWESTERN PAKISTAN Intizar H. Khan, Department of Earth Sciences, University of New Hampshire, Durham, NH USA, and Geological Survey of Pakistan, Quetta, PAKISTAN William C. Clyde, Department of Earth Sciences, University of New Hampshire, Durham NH, USA INTRODUCTION The late Paleocene-early Eocene stratigraphic record exposed along the western margin of the Indian continent (northwestern part of Balochistan and NWFP provinces, Pakistan) has significant lateral and vertical facies variation that documents the time transgressive nature of the initial India-Asia collision. Initiation of compressional tectonics in this region during the Paleocene caused the creation of a series of SW-NE trending depositional centers (e.g. Sulaiman and Kohat-Potwar basins) adjacent to uplifted areas of the Indian shelf. The facies succession in each of these basins provides an independent record of local tectonism and thus can be used to gauge the timing and geometry of initial collision between India and Asia. The vertical facies distribution in the region south of Quetta near Kalat shows a progressive shallowing upward sequence that transitions from a Paleocene carbonate shelf limestone (Dungan Formation) to a shallow marine shale and sandstone facies (Gidar Dhor Fm.) to a thick late Paleocene-early Eocene conglomerate fan facies (Marap Fm.). Near Quetta, a late Paleocene carbonate shelf limestone transitions to an early Eocene shallow marine shale facies that progrades into paralic (sandstone, shale, coal) and continental (red mudstone and conglomerate) facies (Ghazij Fm.). Northeast of Quetta in the NWFP region, late Paleocene-early Eocene shallow marine shale, marl, and sandy limestone facies (Patala, Ponoba and Shekhan Fms.) transition to evaporite facies (Bahadar Khel Salt and Jata Gypsum) and late early Eocene continental red mudstone and sandstone facies (Kuldana Fm.). These stratigraphic records show that the main marine to continental facies transition associated with initial India-Asia collision is time transgressive from southwest to northeast.. Location of Study Area Eocene Ghazij and Kuldana Formations with Sites of Measured Stratigraphic Sections HOW TIME TRANSGRESSIVE WAS THE INITIAL INDIA-ASIA COLLISION? Figure 1: Index map showing location of study area in relation to tectonic setting of India and Asia. The collision between the Indian plate and the Asian plate represents the most recent continent-continent collision in geological history. This collision has generated the largest uplifted landmass on earth, the Tibetan Plateau and the Himalayan mountain belt, and is thought to have had significant effects on global climate, ocean circulation and biotic evolution. One outstanding question currently being debated is the degree to which the initial collision was time transgressive. This question has been difficult to address since much of the sedimentary record of the early stages of collision has been metamorphosed and heavily deformed by subsequent mountain building. The sedimentary record in western Pakistan, however, records the stratigraphic response to initial collision in relatively undeformed and unmetamorphosed condition making it an ideal place to evaluate the timing of initial collision (Figures 1&2). CONCLUSIONS These stratigraphic records show that the main marine to continental facies transition associated with initial India-Asia collision is time transgressive from southwest to northeast. This facies transition occurs in the late Paleocene in the area south of Quetta, near the Paleocene-Eocene boundary in the area around Quetta, and in the late early Eocene in the area northeast of Quetta. Although this stratigraphic pattern could be caused by differential erosion and/or consumption during post collision tectonism, it is most easily explained by a time-transgressive initial collision. Figure 2: Map of Eocene Ghazij and Kuldana Formations with sites of measured stratigraphic sections that are used in adjacent fence diagrams. Notice that the onset of syntectonic continental sedimentation is oldest in the southwest (Kalat Area), where it begins before the Paleocene-Eocene boundary, and is youngest in the northeast, where it occurs in the later part of the early Eocene.