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

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 shore-face 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, early Eocene shallow marine shale, marl, and sandy limestone facies (Patala, Ponoba and Shekhan Fms.) transition to evaporite facies (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. 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.