A diverse deep-sea trace fossil assemblage from the Adriatic Flysch Formation (middle Eocene – middle Miocene), Montenegro (central Mediterranean)

Abstract

Our study of ichnofossils in the Adriatic Flysch suggests that mid-fan environments, with 30 ichnospecies and 13 architectural designs, provided most productive and best preservation conditions in the fine-grained turbidite system of the Dinarides-Albanides-Helenides deep foreland basin. The Adriatic Flysch Formation at its type section in Crnjak Cove (Montenegro) represents a 300-m-thick turbidite succession deposited from the middle Eocene to the middle Miocene. The flysch is exposed in an asymmetrical syncline in a 750-m-long outcrop. The conglomerates, sandstones, and mudstones are arranged in seven distinct turbidite facies, which represent three superimposed submarine fans. The inner-, mid-, and outer-fan facies of the submarine fans contain sixteen ichnogenera and thirty three ichnospecies, which exhibit thirteen architectural designs. The inner fan conglomerates and coarse-grained sandstones contain a low diversity (Ophiomorpha rudis, Ophiomorpha annulata, and Halopoa imbricata), a low disparity (two architectural designs), and a low-density post-depositional trace fossil assemblages of the Ophiomorpha rudis ichnosubfacies of the Nereites ichnofacies. The mid-fan greywackes contain very diverse pre-depositional trace fossils (Belorhaphe zickzack, ?Cosmorhaphe isp., Desmograpton dertonensis, Gordia isp., Helminthopsis isp., Helminthorhaphe flexuosa, ?Megagrapton isp., Paleodictyon arvense, Paleodictyon hexagonum, Paleodictyon itallicum, Paleodictyon latum, Paleodictyon majus, Paleodictyon maximum, Paleodictyon strozzii, Paleodictyon isp., Scolicia strozzii, Squamodictyon isp.) having six architectural designs and abundant post-depositional trace fossils (Halopoa imbricata, Ophiomorpha annulata, Ophiomorpha nodosa, Ophiomorpha rudis, Planolites beverleyensis, Planolites montanus, Palaeophycus alternatus, Palaeophycus striatus, Palaeophycus tubularis, Rhizocorallium commune, Scolicia prisca, Scolicia strozzii, Zoophycos isp.) exhibiting seven architectural designs, which constitute the Paleodictyon-Ophiomorpha rudis ichnosubfacies of the Nereites icnhnofacies in the Adriatic Flysch. The waning turbidity currents with slower rates of sediment deposition and deficiency of nutrients favored grapholyptid colonization of the outer fans. Relatively low abundance, and great diversity of graphoglyptids (Helminthorhaphe flexuosa, Desmograpton dertonensis, ?Megagrapton isp., Paleodictyon arvense, Paleodictyon hexagonum, Paleodictyon majus, Paleodictyon minimum, Paleodictyon strozzii, Paleodictyon isp., Squamodictyon isp.), but low disparity (three architectural designs), represents the Paleodictyon ichnosubfacies of the Nereites ichnofacies in the outer fan facies of the Adriatic Flysch. The diversity of pre-depositional ichnofossils in the Adriatic Flysch exceeds post-depositional forms, reflecting a dominance of K-selected over r-selected population strategies in a stable oligotrophic environment.