COMPARATIVE ACCOUNTS OF ISOTOPIC AND PETROGRAPHIC PROVENANCE INDICATORS. CASE STUDY OF ASSAM PLAINS (KOKRAJHAR ALLUVIAL FAN).

Abstract

The southern extent of the Himalayan thrust and fold belt is characterized by a foreland basin which is called the Indo-Ganga-Brahmputra foreland basin. The basin was formed due to plate-plate collisions between India and Asia during the mid Miocene. The basin is filled by sediments brought by rivers draining the Himalayas with a small amount being supplied by cratonic rivers. The rivers draining the southern front of the Himalayas are subjected to the full spectrum of the Indian summer monsoon (ISM) and the tectonic-driven orography that varies in time and space. This variation is a result of an interplay between the ISM front that moves northwards into the Higher Himalaya during phases of strengthened monsoon and activities along major Himalayan thrusts. The alluvial fans stratigraphy that is built by the Himalayan rivers at the mountain front therefore can serve as an archive of this monsoon-Himalayan geology interplay. The proper estimation of sediment provenance of dated sedimentary units can inform on the hotspots of erosion and thereby the extent of ISM in the Himalayas. The literature review indicates the existence of three major proxies that fingerprint the sediment provenance (i) Sr-Nd isotopic systematics (ii) U-Pb chronology of detrital zircon (iii) sand petrography. However, there lack of proper understanding of the strengths of individual techniques. This dissertation work is based on the comparison between the Petrographic and Sr-Nd analysis techniques which gives us the provenance characterization. Towards this, a previous fan previously studied using Sr-Nd (Panda et al., 2023) was chosen. The relict fan surface under study, located approximately 40 meters above mean sea level, is incised, giving rise to a valley terrace designated as T1, which is characterized by meandering channel deposits. The Sr-Nd analysis of the alluvial fan tells us that the fan is composed of three distinct depositional phases that aggraded between 27 to 3 Ka. The petrography was carried out on the same samples that were previously characterized for the Sr-Nd. The results also from the petrography suggested that the possible provenance source for two out of three different lithofacies is mainly lesser Himalayan to Higher Himalayan crystalline and in the third phase the possible provenance source is outer and inner lesser Himalaya. The results corresponded well with the Sr-Nd data of Panda et al (2023). But in the meandering scroll, the sediments are mainly predominantly from the higher Himalayan crystalline and this possible source has a direct relationship with the changing monsoon condition.