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THE ICHNOFOSSIL HALIMEDIDES IN CRETACEOUS PELAGIC DEPOSITS FROM THE ALPS: ENVIRONMENTAL AND ETHOLOGICAL SIGNIFICANCE

¹ Université Claude Bernard Lyon 1, Centre National de la Recherche Scientifique (CNRS), Unité Mixte de Recherche (UMR) 5125, PaléoEnvironnements et PaléobioSphère, Université de Lyon, F-69622, Lyon, FranceChristian.Gaillard{at}univ-lyon1.fr

Numerous rhabdoglyphid trace fossils in Cretaceous pelagic limestones from the French and Swiss Alps are ascribed to the ichnogenus Halimedides Lorenz von Liburnau, 1902. The heart-shaped chambers, interpreted as storage chambers, are joined to tunnels in a linear pattern. The burrow system is classified as an agrichnium. Morphological characteristics of the burrows indicate that small infaunal crustaceans were the likely trace makers. Halimedides are deep-sea traces that indicate relatively firm media (substrates)—stiffground to firmground—based on paleoenvironmental and morphological evidence. The usual association of Halimedides with Rhizocorallium and Spongeliomorpha clearly place them in the Glossifungites ichnofacies. Halimedides may be an indicator of subtle gaps in sedimentation. Densely chambered Halimedides are interpreted to indicate lower oxygenation, whereas sparsely chambered burrows indicate higher oxygenation of bottom waters. Consequently, Halimedides may be a useful tool for interpreting water depth, media consistency, sedimentation rate, and seafloor oxygenation.