Distribution of CaCO₃, Ca²⁺ & Mg²⁺ in Sediments of the Northern Half of Western Continental Shelf of India

Abstract

Surface sediment samples (90) collected from the continental shelf from the Indus river in the north to Ratnagiri in the south were analysed. The outer shelf sandy calcareous relict sediments were high in CaCO₃ (50 to 94%) than the innershelf silty clays or clayey silts (5 to 25%). Ca²⁺ was relatively high in the outer shelf sediments while Mg²⁺ was conspicuously low, especially in the area Gulf of Cambay to Ratnagiri. Mg²⁺ was relatively high in the sediments of the middle shelf between Ratnagiri and Gulf of Cambay, and middle and outer shelf off Kathiawar and was low in the nearshore terrigenous clays and in the outer shelf of the region north of the Gulf of Kutch which was largely supplied by the river Indus. Ca²⁺/Mg²⁺ ratio was high in the outer shelf sediments, where oolites were predominant, lower in the middle shelf, where the skeletal components were dominant in the sediment, and very low in terrigenous clays, which is because of higher magnesium content in the skeletal components than in the oolites. Distribution of CaCO₃ and Ca²⁺/Mg²⁺ ratio of the sediments of the shelf region reflected the different palaeoenvironmental conditions under which the recent inner shelf and the outer shelf relict sediments were deposited.