Sedimentary Sequences of Krishna River Delta to Identify Paleo Depositional Environments

Abstract

Deltas play a crucial role in the economic and environmental development of the region, thus they gain much attention from the Government, as well as scientists, and researchers for different developmental activities. In this regard, the grain size studies are important to provide clues to the sediment provenance, transport history and depositional conditions. In addition to that, Foraminifera studies provide insights into the environmental changes of the deltaic regions, and they are used for environmental reconstructions. Thus, the present work focuses on textural and micropaleontological studies to identify depositional environments. Inturu core four sedimentary sequences were identified from bottom to top viz. river channel, estuarine channel, mud flats and flood plain. Nizampatnam core, seven sedimentary sequences were identified from bottom to top viz. river channel, estuarine channel, mud flats, flood plain, estuarine channel, mud flats and estuarine channel which represents sub environments of fluvial and estuarine systems. Coarse-grained and fine-grained sediments are related to the high energy, and low-energy conditions respectively. The presence of microfossils like Asterorotalia trispinosa, Asterorotalia subspionsa, Asterorotalia dentata, Asterorotalia gaimardi, ammonia tepida, nonionella and cibisides indicates an estuarine environment. Repetition or overlapping of environments indicates lateral shifting of the river course whereas, neotectonic activity or sedimentary basin subsidence or progradation of delta causes vertical shifting. The polymodal nature of river sediments indicates that these sediments might be contributed by various sources i.e. drainage basin, reworked sediments of the river bed load, river bank eroded sediments and addition of fine-grained material due to alteration of pre-existing minerals of the main sedimentary unit. The unimodal sediments suggested that the sediments were contributed by a single source or they had not undergone any post-depositional changes.