Review of the Bulgarian Geological Society vol:69 issue:1-3 pages:61-68
In the Pobiti Kamani area, up to 10 m high and meter-diameter tubular concretions (so-called columns) are
exposed within Lower Eocene sands and sandstones (Dikili Tash Formation) about 20 km west of Varna (NE Bulgaria).
These calcite-cemented sandstone concretions have been a subject of many geological studies addressing their formation.
In the present contribution a short review is presented as result of a recent (and still ongoing) study about the origin of these
structures in relation to past methane seepage. The systematic mapping of the morphology and 2D-spatial distribution of
the tubular concretions indicated that: a) the Paleogene structural framework likely played an important role in directing
fluid movement to the paleo-seafloor and b) the morphology of different types of tubular concretions was controlled by the
subvertical path of ascending gas-bearing fluids through the unconsolidated host sediments as well as by the characteristics
of the host lithology and lateral differences in seepage conditions. Based on a detailed petrographical, geochemical and
lipid biomarker study, it was furthermore shown that interparticular low-magnesian calcite cementation of the unconsolidated
host sediments around the rising methane-bearing fluid plume, occurred at shallow depth below the seafloor and was
triggered by the microbial mediated anaerobic oxidation of methane.