Diagenesis of the Sedimentary Rocks EnclosingCoaly Layers in Gavatha Area, Lesvos Island,Based on Silica Polymorph's Transformations

A Tertiary non-marine stratigraphic sequence composed of carbonates (limestone),siliceous carbonates, coaly layers overlain by pyroclastic rocks and lavas, outcrops in the Ga-vatha area of northwestern Lesvos Island. Pure carbonates consist almoot completely of calcite,the siliceous carbonate sediments of quartz, opal-CT and calcite, the shales of quartz, opal-CT,K-feldspar, smectite-illite and calcite, and the coaly layers of organic matter, quartz, opal-CT,feldspars and pyrite. Geochemical data indicate that smectite-illite, feldspars and associated ele-ments (La, Zr, Y, Ba, Ce) are the preducts of alteration of volcanic rocks in a subtropicalarea.A combination of sources is suggested for the formation of silica polymorphs: (a) biogenicor non-biogenic silica (opal-A) that was originally present in the form of diatom frustules or inthe form of inorganically precipitated silica; (b) transformation of opal-A to opal-CT and quartzopal-C from alteration of volcanic glass of intercalated tuffites and overlying volcanics; and (c)opal-CT deposited primarily from hydrothermal solutions.