Mineralogical and geochemical aspects of the marine sediments off Goa

Abstract

The state of Goa has a 105 km long ria-type coastline and the morphometric analysis of major rivers indicate submergence of land mass in relation to the offshore region. In the coastal region, mineral assemblage of Morgim - Arambol and Tila-Mati beaches and off Mandovi and Zuari Rivers indicates predominant metamorphic source rocks. In the nearshore areas the concentrations of organic carbon and phosphorus have been controlled by detrital supply and organic productivity and that of carbonate to its modern skeletal production. In the offshore regions i.e. shelf and slope, clays and carbonate determine the sediment texture. Clays chiefly contain varying proportion of smectite and illite. The carbonate sands mainly consist of shells and skeletal fragment of various organisms whose age indicates their formation approx. 10,000 yrs before present and hence termed as 'relict'. Two main minerals which compose the carbonates are calcite and aragonite. Calcium carbonate content of these sediments varies from less than 10% to more than 75%. Although organic carbon content of the sediment ranges from 0.04 to approx. 4.0%, its higher content is dominantly encountered on the slope sediments which is attributed to its preservation due to reducing environment caused by impingement of 'oxygen minimum' waters on the floor of the slope between water depths of 120 to 1200m. Phosphate concentrations in sediments are as such not very appreciable; its occurrence as concretions in the core collected from the top of bathymetric high has very high concentration of approx. 32% and is very significant. Its formation is also related to the reducing environment of deposition and high organic content of sediments which eventually release phosphate for formation of phosphorites authigenically by biological mediation. Higher concentration of A1 sub(2) 0 sub(3) and Ti02 in the inner shelf sediments compared to outer shelf and slope point out to their higher contribution by terrigenous influx. Titanium is mainly contributed by the heavy minerals present in the sediments. The occurrence of higher Fe and other trace metals (Mn, Cu, Ni, and Zn) in the inner shelf sediments results due to their contribution mainly by terrigenous component followed by authigenic component whereas some contribution of Cu, Ni and Zn could be biogenic in slope sediments. These studies in general warrant future exploration for heavy minerals in nearshore regions, for older phosphorite and hydrocarbon deposits on the slope and methane gas in the adjoining deep sea