Sedimentology of the Indus Group, Ladakh, northern India: implications forthe timing of initiation of the palaeo-Indus River

The upper reaches of the Indus River Form a longitudinal network that drain s a large portion of the western Himalaya. it Flays an important role as a sediment routing system, feeding the Indus Delta and submarine fan, and has played a controlling role in the denudational history of the western Himal aya. Given the rivers long-term significance, its timing of initiation rema ins poorly constrained. The facies, palaeocurrents and provenance of the po st-early Eocene Indus Group preserved along the upper reaches of the modern Indus reveal a history of fluvial/deltaic and deep-water sedimentation in an intermontane basin dominated by internal drainage. Hence, the Indus Grou p is not considered to represent the deposits of a palaeo-Indus River as pr eviously thought. Illite crystallinity values and apatite fission track dat ing of the Indus Group suggest that the succession was buried to temperatur es of 155-280 degreesC, and that unroofing started in early Miocene times a nd proceeded at 0.1-0.4 mm a(-1) to the present. The youngest sedimentary r ocks preserved along the Indus Basin are early Miocene in age from the tect onostratigraphical ly equivalent deposits around Kargil to the west of the study area. The transition from sediment accumulation to erosional unroofin g in early Miocene times coincides with accelerated regional unroofing of t he High Himalayas to the south, and the initiation of the Indus Delta/subma rine fan to the west. Differential uplift between the northward thrusting o f the Zanskar and Indus sedimentary succession against the undeformed Ladak h Batholith provides a mechanism for post-early Miocene initiation of a maj or longitudinal river. Hence, early Miocene times is believed to represent the earliest possible age of initiation of the palaeo-Indus river following this course.