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Geology of Borjomi Gorge and trace fossils of paleogene sediments

Author: Levan Lebanidze

The oldest rocks in Borjomi Gorge are Paleocene-Lower Eocene flish deposits, known as “Borjomi suite” and are represented by sandstone-siltstone terrigenous-carbonaceous flish. It is characterized by rhythmic, graded bedding, cyclothemes and other features common for flish sediments. “Borjomi suite” is constructed by clastic limestones, marls, clays, argillites and marly and silty alevrolithic turbidites. This sediments are overlain by Middle Eocene thick volcanogenic-sedimentary series, subdivided into three suits. Borjomi suite is continuoued by Likani suite(1000m) of subalkaline basaltic bedded volcaniclastics. It is followed by Kvabiskhevi suite, represented by alternation of lava flows of different composition and bedded tuff (600m). The upper part of Middle Eocene is represented by Dviri suite, which is built up by low-Ti basaltic massive and coarse lava- aud tuffbreccias and lava flows (1500m). In thePaleocene part of “Borjomi suite”(Dabiskhevi subsuite) were for the first time defined representatives of ichnogenera Avetoichnus and Chondrities. Combination of Avetoichnus and Chondrites traces are characteristic for distal marly and silty turbidites of finegrained turbidite sustem(Italy, Poland, Spain). Detailed taphonomic analysis of specimens of Avetoichnus luisae has shown that a high degree of variation can be observed in their length, general shape, maximum diameter, maximum width of dots, shape of dots, dot distribution, spiral arrangement, central part(axis) and raised edges.These variations suggest that an unknown organism adjusted it’s activity (agrichnial or fodinichnial) using different strategies in accordance with changes in the turbiditic environment. Middle Eocene trace fossils are releited to upper part of Likani suite and are determined in the cross section of the river Mtkvari (Akhaldaba). In Middle Eocene ichnocomplexe we have determined complicated radial, spiral and meanric and combined traces, which belong to following ichnogenera: Cochlichnus, Glockerichnus, Nereites, Scolicia and Spirophicus. Such complicated traces are related to Nereites ichnofacies with biotops of stable environments and poor feeding supplies (deep sea bottom). All in all, the mention area of Paleocene- Lower Eocene basin of Borjomi district used to be a distal part of finegrained turbidite system and was characterized low-oxegene mode of sedimentation (indiator Chondrites). In Middle Eocene this area, based on gained ichnological data, was a bottom of the deep sea.

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