Clay mineral distribution in the continental shelf sediments from Krishna to Ganges river mouth, East coast of India

Ninety six sediment samples (<2 μm fractions) of the eastern continental shelf of India between Ganges in the north and Krishna in the south have been studied by X-ray diffraction. On the basis of nature and abundance of different clay minerals and montmorillonite (M) to illite (I) ratio, 6 distinct clay mineral assemblages which reflect their sources are identified. Illite (80%) and chlorite-rich (20%) clay mineral suite, with less kaolinite (av. K/C=0.3) and traces of montmorillonite is found in the estuarine sediments of the Ganges. The Brahmani-Mahanadi province is characterised by the increase of montmorillonite (M/I = 0.4), kaolinite (av. K/C =1.5) and gibbsite. Mixed mineral assemblage in the offshore shelf regions off the Hooghly has resulted due to the mixing of clay mineral suite of Brahmani-Mahanadi with the Gangetic suite. A kaolinite-rich (up to 55%) assemblage with an average M/I ratio of 0.8 occurs in the eastern ghats coastal province. Variations in the abundance of clay minerals with sediment type have also been observed. The Godavari and Krishna assemblages are distinguished by highest concentrations of montmorillonite (up to 67%; M/I varies from 1 to 4.5) and kaolinite (28-36). Montmorillonite in this region is 12Å type in contrast to the 15Å montmorillonite in the remaining part of the shelf It is found that the Godavari derived sediments extend northwards and deposit up to Visakhapatnam under the influence of southwest monsoon currents