Paleomagnetic Chronology, Fluvial Processes, and Tectonic Implications of the Siwalik Deposits near Chinji Village, Pakistan

Abstract

A 2800-m section of Siwalik strata containing the stratotypes for both the Chinji and Nagri formations has been dated by magnetic polarity stratigraphy, and the observed polarity zonation has securely been correlated with the Chron 17-7 segment of the time scale. The base of the section is the base of the Kamlial formation, which occurs near the top of Chron 17 (18.3 m.y.). The Kamlial-Chinji formation boundary occurs in the middle of Chron 15 (14.3 m.y.), the Chinji-Nagri boundary near the bottom of Chron 10 (10.8 m.y.), and the Nagri-Dhok Pathan boundary at the Chron 8-9 boundary (8.5 m.y.). The Siwalik deposits near Chinji Village consist of four distinct classes of fluvial cycles, each with a characteristic periodicity: a first order of , and a fourth order of . The river system responsible for Siwalik sedimentation flowed from west to east and had properties much like the modern Ganges River system. Sedimentation rates in the Chinji Village area increased gradually through time, going from 0.12 mm/yr in the Lower Siwaliks to 0.30 mm/yr in the Middle Siwaliks. At about 11 m.y. blue-green hornblendes suddenly appear in abundance in channel sands. The appearance of these hornblendes, accompanied with accelerated sedimentation, is the effect of uplift in the source area, specifically the Nanga Parbat region. The modern Chinji Village area is no longer a depositional site, but instead is involved in overthrusting, uplift, and erosion. As the Indian Plate has drifted north during the past 18 m.y., the Chinji Village area has been gradually transformed from a subdued karst topography into a major depositional center for Himalayan sediments and finally back into a sediment source area to complete a sedimentary cycle that has spanned some 20 m.y.