Sedimentation and erosion at Majuli Island, Assam, India: analysis of paleo-depositional environment

Abstract

ABSTRACT The World’s one of largest inhabited river-made island Majuli, developed in response to depositional processes of large sediment load carried by the Bramhaputra River and Subhansari River from the Himalayan hinterland. But, for decades it has been going through a significant rate of erosion, which is severely threatening the existence of the island and its rich cultural heritage. These erosional processes result in exposure to island depositional records and provide an opportunity to study these sedimentary deposits. This paleo depositional record is very well analysed by adopting the methodology of facies identification and their relationship diagram. A total of 18 exposed outcrop sections are studied with detailed sedimentological documentation, facies identification and symbols assigned to facies. Then calculations and tabulations are carried out for Facies Relationship Diagram (FRD) along with the construction of the sequence model, which provides a lateral and vertical lithofacies association developed in the entire island sequentially. On the basis of validation of obtained data through field survey and analysis of more vertical profiles, we study the correlation between existing bank erosion and dominated sedimentary facies at Majuli Island. This analysis results in two facies sequences: one mud-dominated and the other sand-dominated facies. The Massive Mud and Massive Sand beds are represented as the main facies between these two sequences. Therefore, mud-dominated sequence represents the Flood Plain deposit, whereas the sand-dominated sequence represents Bar and Channel deposits. This study also reveals that sedimentary sequences developed by the Bramhaputra River are mud dominated, while that of the Subhansiri River is both mud and sand sequences. This reflects the differential source to sink transport of sediments, as the Bramhaputra River carries sediment load for a longer distance than the Subhanshiri River. This study also reveals that areas of sand-dominated facies show higher erosion than mud-dominated facies. Therefore, the sedimentary record of the Subhansiri River is more vulnerable to erosion.