Trace-fossil assemblages and basin evolution of the Vardekloeft Formation (Middle Jurassic, central East Greenland)

Abstract

Middle Jurassic sandstones and shales (Vardekloft Formation) of Jameson Land and Scoresby Land are described in terms of lithology, grain size, primary sedimentary structures, sequential arrangement and trace fossils. Six different trace-fossil assemblages have been recognized, characterized by Diplocraterion habichi, Ophiomorpha, Curvolithus, Muensteria, Rhizocorallium and Phoebichnus, respectively. These ichnocoenoses are interpreted in the light oftrophic properties and it is shown that ichnocoenoses, grain size and sedimentary structures are strongly interrelated, reflecting the distribution of flow velocities over the bottom slope. On the basis of the spatial distribution of lithofacies and ichnocoenoses in relation to ammonite zonation, the depositional regime and the evolution of the Jameson Land basin are analyzed. Three main stages can be recognized (from below): 1) a regressive stage characterized by a gentle slope and four successive ichnocoenoses downslope (Curvolithus, Muensteria, Rhizocorallium, Phoebichnus); 2) a transgres- sive stage characterized by a near-shore bar regime with Ophiomorpha, followed seaward by Curvolithus, an off-shore bar regime of relict sand with Diplocraterion habichi, and finally Phoeb- ichnus; 3) a transgressive stage characterized by a relatively steeper slope with only three successive ichnocoenoses (Ophiomorpha, Curvolithus, Phoebichnus). Tectonic implications of the model are discussed.