DIAGENESIS OF THE MARLY-GYPSUM FORMATIONS, IGOUMENITSA AREA, N.W. GREECE

Abstract

The Igoumenitsa area, in northwestern Greece, is composed of organic-rich clay evaporitic formations, of the Ionian zone. They have undergone diagenetic processes, which effect the mineralogical transformations. The purpose of this paper is the study of the mineralogical diagenesis and the possible implications to the geotechnical behavior of these organic-rich clay evaporitic sedimentary rocks. The diagenetic minerals reported in this study are: A: The authigenic swelling clay minerals, smectite and mixed layers of smectite. B. The two characteristic minerals phases of the sulfate diagenesis: I. Bassanite (semi hydrated gypsum-Ca04.l/2H20) and 2. Celestite (SrS04) that can be considered as an indicator of bacterial sulphate reduction, fluid flow and recycling ofevaporites. The two authigenic minerals have derived from the evaporites, mostly by the dehydration of gypsum and by the action of pore water that is charged with the ions derived from the dissolution of preexistent sulfates, halite and carbonates. C. The neoformation of the carbonate minerals aragonite and dolomite, due to probably bacterial sulfate reduction. When organic-clay rich sediments, are associated to evaporites, the diagenetic processes become more complex and interacted. The diagenetic alterations of the gypsum/anhydrite rocks are related to the geotechnical behavior of the formation and may lead to possible karstiflcation. That may cause problems in roads and tunnels construction on or through these evaporitic series