Diagenesis of the Sedimentary Rocks Enclosing Coaly 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 Gavatha area of northwestern Lesvos Island.Pure earbonates eonsist almost completely of calcite,the siliceous carbonate sediments of quartz,opal-CT and calcite,the shales of quartz,opal CT, K-feldspar,smecite-illite and ealcite,and the coaly layers of organic matter,quartz,opal-CT,feldspars and pyrite,Geochemical data indicate that smectite-illite,feldspars and associated elements(La,Zr,Y,Ba,Ce)are the products of alteration of volcanic rocks in a subtropical area A combination of sources in suggested for the formation of silica polymorphs:(a) biogenic or non-biogenic silica(opal-A) that was originally present in the form of diatiom frustules of in the form of inorganically prccipitated silica;(b)transformation o opall-A to opal-CT and quartz opal-C from alteration of volcanic glass of intercalated tuffites and overlying volcanics;and(c)opal-CT deposited primarily from hydrothermal solutions.

Diagenesis of the sedimentary rocks enclosing coaly 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 Gavatha area of northwestern Lesvos Island. Pure carbonates consist almost 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 elements (La, Zr, Y, Ba, Ce) are the products of alteration of volcanic rocks in a subtropical area. A combination of sources is suggested for the formation of silica polymorphs: (a) biogenic or non-biogenic silica (opal-A) that was originally present in the form of diatom frustules or in the form of inorganically precipitated silica; (b) transformation of opal-A to opal-CT and quartz opal-C from alteration of volcanic glass of intercalated tuffites and overlying volcanics; and (c) opal-CT deposited primarily from hydrothermal solutions.