Ordovician Bathyal Trace Fossils From Metasiliciclastics in Central Norway and Their Sedimentological and Paleogeographical Implications

Abstract

Numerous trace fossils are recognized in metasiliciclastic rocks of the Lower-Middle Ordovician Lower Hovin Group, Upper Ordovician Ekne Group and on roofing slates from an unknown locality (probably Ordovician) from the Trondheim region, central Norway. The trace fossil assemblages are dominated by meandering pascichnial forms, i.e., Helminthoidichnites in the Lower Hovin Group and Helminthoidichnites and Nereites in the slates. Protovirgularia, Dictyodora, ?Planolites and ?Palaeophycus are less common. Alcyonidiopsis, ?Trichophycus, Chondrites, cf. Chondrites, Gordia, ?Phycodes, ?Helminthopsis, cf. Naviculichnium, Treptichnus, Saerichnites, Megagrapton and ?Paleodictyon (Squamodictyon) are rare. The trace fossils were collected or observed at 17 different localities and represent assemblages belonging to the Nereites ichnofacies (deep-sea flysch deposits with thinly bedded turbidites). They reflect a distal Nereites subfacies (fan-fringe or basin-plain deposits) transitional to a Nereites/Paleodictyon subfacies (proximal fan facies). The trace fossil assemblages indicate an opportunistic style of colonization related to fluctuations in food supply introduced by turbidity currents.

Keywords:

• trace fossils
• Nereites ichnofacies
• flysch
• Ordovician
• Norway

ACKNOWLEDGEMENTS

We are indebted to Torbjørn Tilrem (Brønnøysund) for drawing the attention of R. Binns to the trace fossils on the roofing slates and for providing facilities for their study. Without his initiative, this project would never have materialized. We thank Allan Krill (Norwegian University of Science and Technology, Trondheim) for field assistance and logistic help with the specimens. Jarle Johansen (Brønnøysund) photographed most of the material. Figures 1 and 2 were drafted by Jan-Petter Holm and Tove Midtun (University of Tromsø). We greatly appreciate the help given by David Roberts (Geological Survey of Norway) for informative discussion, informing us of unpublished information, and critically reading and commenting on the final draft of the manuscript. Fieldwork by all the authors was supported by the University of Tromsø, which also defrayed A. Uchman's travel expenses to Trondheim and most of the photography. A. Uchman prepared the descriptive part of the manuscript at the Institute of Palaeontology, Würzburg University, where he was supported by a scholarship from the Alexander von Humboldt Foundation. The paper benefited from the reviews by Patrick Orr (National University of Ireland, Galway) and Roland Goldring (University of Reading), and from further improvements by Ronald K. Pickerill (University of New Brunswick).

Notes

*Owing to the limited number of suitable exposures, the number of ichnotaxa present may be under-represented in the material collected.