Opaline Silica in Tertiary Sediments of Kerala

Abstract

Smooth and well rounded grains of opal are recorded in the Tertiary sediments exposed in Trivandrum and Quilon districts and from deeper levels in the subsurface sediments near Ambalapuzha, in Alleppey district of Kerala. The clear to translucent opal grains vary in size from 0.05mm to about 3mm. Opaline casts of fossil tests with characteristic shapes and organic structures on their surfaces suggest organic activity in selective precipitation of opal inside fossil tests. Silica infillings in partly broken shells of gastropods were also observed. Difference of pH level in porewaters and inside the fossil tests favoured by organic decay in them is considered to be the main controlling factor for their formation.