Sedimentologic and geomorphic studies on the Quaternary alluvial fan and terrace deposits along the Ganga exit

摘要

Sedimentologic and geomorphologic records of a section of the Ganga Valley in the tectonically active eastern part of the Dehra Dun were studied to understand the interaction of tectonics and climate during Late Pleistocene-Holocene. The 50 km long stretch of the valley examined encompasses the mountain front from north of the Main Boundary Thrust (MBT) to south of the Himalayan Frontal Thrust (HFT). The landscape comprises two to four levels of fluvial terraces, both strath and fills, and alluvial fans. The distribution of these terraces reveal that the two topmost terraces (T4 and T3) are abruptly terminated along the HFT but the two successively lower terraces (T2 and Tl) are developed across the MBT and HFT. Facies distribution coupled with sediment composition permits division of the fan and fluvial terraces of the study region into two distinct types: 1) those dominated by rounded to well rounded polymict clasts derived from the Higher and the Lesser Himalaya with palaeoflow similar to present day Ganga River, referred to as Ganga river terraces (CRT), and 2) those enriched in angular to sub-rounded argillites and quartzite with sandstone clasts deposited by local ephemeral stream and slope failures, interpreted as local river terraces (LRT). Sedimentologic analysis coupled with optical stimulated luminescence (OSL) dating indicates that the nature of aggradation and incision phases vary across the mountain front. North of the HFT, aggradation continued until the development of T4, i.e. 11 ka, followed by incision, coinciding with a climatic change from drier to wetter conditions. This incision phase was punctuated by intermittent aggradation at 9.7 and 6.9 ka in the form of the T3 and T2 terraces respectively. On the other hand, extended aggradation, south of the HFT, until 6 ka suggests continuous creation of accommodation space. Abrupt termination of T4 and T3 along HFT suggests that at least two phases of tectonic activity along the HFT occurred after 11 and 9.7 ka but before 6.9 ka. The uninterrupted occurrence of T2 and T1 terraces across the MBT and the HFT suggest that tectonic activity on these faults was not pronounced after 6 ka, or that it might be unidentified due to subdued scarps and erosion by lateral migration of the Ganga River.