The environmental significance of the trace fossil Rhizocorallium jenense in the Lower Triassic of western Spitsbergen

Abstract

The 500 m thick Lower Triassic succession of western comprises two shale-dominated formations, which both show upward-coarsening motifs. These reflect repeated coastal basin dominated by low energy fine-clastic sediments. The track fossils Rhizocorallium jenense and Skolithos are found in the coarser part of these units and variations in size and orientation of R. jenense give important palaeoenvironmental information. Rhizocorallium jenense occurs in storm-generated siltstones and stones, whose deposition interrupted prevailing intermediate energy levels. Size variations and trace fossil abundance suggest an optimal habitat in the shoreface zone, with poorer adaptation to both offshore and shallower environments. Age-equivalent marine sediments on north-eastern Greenland also contain local abundant occurrences Rhizocorallium. These Arctic occurrences contrast with the same trace fossil's distribution in the Jurassic of Britain and France, where it characterizes shallower and higher energy environments; such sequences on Spitsbergen show an ichnofauna dominated by Skolithos and bivalve escape shafts. Orientations shown by the R. jenense U-tubes show a generally, but not solely, unimodal distribution, with the curved distal entedusually oriented toward onshore. Presumed aperture lineations show strongly unimodal trends, probably related to longshore currents. Burrows in bed at the top of individual storm lobe units show more complex ably patterns probably reflecting both current and wave reworking following lobe abandonment. All finds suggest early colonization by the burrowing organisms. These were not followed by other burrowers, either because of the nutrient-poor nature of the sediment or because of high sedimentation rates.