Abundant trace fossil Polykampton in Palaeogene deep-sea flysch deposits of the Lesser Caucasus in Georgia: Palaeoecological and palaeoenvironmental implications

The new ichnospecies of Polykampton georgianum occurs abundantly in a part of the Borjomi Flysch (Upper Paleocene – Lower Eocene) in Georgia, in particular in the 45.3 m thick Ardagani-1 section that is dominated by grey silt- and mudstones having sandstones intercalated. The trace fossil occurs in the middle and upper part of the section. The trace fossil is composed of a median tunnel and side lobes that are filled with material differing from the host sediment. It is interpreted as a sequestrichnion, wherein the tracemaker stored organic-rich mud in the lobes as a food resource for times of food deficiency. The today ferruginization of the median tunnel resulting from the former presence of pyrite suggests strong chemical gradient between its lumen and the surrounding. Chemichnial behaviour is not excluded as nutritional strategy of the worm-like tracemaker (a polychaete?). It occupies a middle tier and is commonly crosscut by Trichichnus, Chondrites and Scolicia. Rare or absent other trace fossils typical of deep-sea fine-grained deposits suggest slight dysoxia. Significant impoverishment of the tiering pattern (only Trichichnus present) and absence of trace fossils in the lower part of the section point to strong dysoxia or episodic anoxia. The trace fossil assemblage is atypical, but in general it resembles the Nereites ichnosubfacies of the Nereites ichnofacies, which characterizes distal parts of the turbiditic depositional systems. The contrast between the dark grey to grey silt- and mudstones deposited during the waning phase of gravitational flows and greenish pelagic mud suggests that the former originated from an organic-rich depositional setting, presumably within the oxygen minimum zone in the upper-slope to outer-shelf domain. Sequestration of the organic-rich mud was a successful adaptation of the Polykampton georgianum producers in this specific environmental setting.