Ichnology of prodelta deposits of the Mezardere Formation (late Eocene – early Oligocene) in the Gökçeada Island, western Turkey

On Gökçeada Island, the Mezardere Formation is dominated by the prodelta facies composed mostly of light grey, thinly bedded, fissile calcareous mudstones and siltstones, which mostly display parallel lamination or rarely ripple lamination. These sediments are intercalated with isolated thin beds of very fine- or fine-grained slightly muscovitic sandstones, which display parallel lamination in the lower part and ripple lamination in the upper part, or only ripple lamination. Thicker sandstone beds, referred to as mouth bars, are present in places. The prodelta sediments are poorly bioturbated (ii = 0–1), but moderately diverse trace fossils occur on some bedding surfaces, including 22 ichnotaxa referred to 13 ichnogenera and three trace fossils left in open nomenclature. The impoverished trace fossil assemblages are typical of deltaic sediments, but paucity of the trace fossils and poor bioturbation of the prodelta sediments in the Mezardere Formation is striking. The typically marine ichnoataxa are rare or absent in most outcrops. This trace fossil assemblage, resembling the Mermia ichnofacies, suggests a strong brackishness of the palaeoenvironment; however, the occurrence of Bichordites kuzunensis in one outcrop suggests local and temporal normal marine conditions referred to as salinity fluctuations.