Abstract

The lower Neogene unit of the Karlovassi basin, Samos island, Greece, was deposited in a continental basin during Miocene times and consists of four successions overlying a basal conglomerate. The four successions are, from the base upward, carbonates, tuffaceous rocks, claystones, and porcelanites.Volcanic glass was originally abundant in the tuffaceous succession but is now represented by its diagenetic products, including boron-bearing K-feldspar; the zeolites clinoptilolite, analcite, and mordenite; smectite; opal (disordered tridymite-cristobalite); cristobalite; and minor amounts of the zeolites erionite, phillipsite, and chabasite; and tridymite.Evaporite minerals such as gypsum, celestite, and thenardite are present along with authigenic silicates in tuffaceous intervals into the claystone succession. Authigenic silica polymorphs represented by opal and chalcedony were identified in the uppermost succession of porcelaneous limestones to chert.Five irregular concentric zones related to the presence and kind of zeolites and K-feldspar are recognized in the lower Neogene unit: (1) a marginal zone of carbonate and detrital silicate minerals (nonauthigenic silicates), (2) a clinoptilolite zone, (3) a mixed clinoptilolite-analcite zone, (4) an analcite zone, and (5) a central boron-bearing K-feldspar zone.The general pattern of zonation and distribution of authigenic silicates as well as their chemical characteristics indicates the presence of a saline-alkaline lake during Miocene times. This lake is analogous to those of the same age which existed in southern Yugoslavia and western Turkey, where borate, sulfate, and double carbonate deposits were formed along with authigenic silicates. It is possible that a large continental area in the eastern Mediterranean underwent large-scale dessication which took place during the Miocene (Messinian salinity crisis), resulting in the development of highly saline-alkaline lacustrine environments.

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