Some Aspects of Lower Godavari River and Delta Sediments, India: ABSTRACT

Abstract

To study the progressive changes along a river and to delineate sedimentary environments based on variations in the litho- and chemo-facies, 400 recent sediments from the fluvio-marine environment of the Godavari were collected in the pre- and post-flood seasons. The present river morphology is a manifestation of the bed load material. Higher silt-clay ratios increase the degree of sinuosity and decrease the width/depth ratios of the channels. Decrease in the sinuosity of the river course in the last one hundred years is probably a result of the coarsening in the bed load. Mean size decreases and coefficient of sorting increases progressively along the river course; skewness changes from positive to negative while kurtosis remains constant. These changes are probably attributable to the decrease in the energy levels downstream. Heavy mineral percentages are directly proportional to the mean size of the sediments. Heavy minerals indicate a predominantly igneous (acidic) and high-grade metamorphic (khondalite, calc-granulite, and amphibolite) provenance. Few authigenic and rounded zircons are considered secondary. Downstream increase of pyroboles and sillimanite, and decrease of opaques and garnets, is apparently due to sorting based on shape and density. Delineation of sedimentary environments based on conventional size measures has been partially successful. Similarity in the backwater and marine shoal sediments north of the river confluence indicates that the former was a part of the open sea. Phosphate, uranium, and iron concentrations were determined in the clay fractions. Phosphate and iron are highest in the marshes. Iron concentration decreases in the backwater and is least in the river channels. Preliminary data show higher uranium content in the upper river. X-ray analyses show illite, chlorite, and Na-montmorillonite increasing and Ca-Na montmorillonite and kaolinite decreasing from the fluvial to the marine environment. Na-montmorillonite is more predominant in the swamps and illite in the backwater. The hydrographic data collected are being processed to understand the physico-chemical conditions of deposition. End_of_Article - Last_Page 354------------