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

The 500 m thick Lower Triassic succession of western Spitsbergen comprises two shale-dominated formations, which both show upward-coarsening motifs. T hese reflect repeated coastal progradations into a basin dominated by low e nergy fine-elastic sediments. The trace fossils Rhizocorallium jenense and Skolithos are found in the coarser parts of these units and variations in s ize and orientation of R. jenense give important palaeoenvironmental inform ation. Rhizocorallium jenense occurs in storm-generated siltstones and sandstones, whose deposition interrupted prevailing intermediate energy levels. Size v ariations and trace fossil abundance suggest an optimal habitat in the shor eface zone, with poorer adaptation to both more offshore and shallower envi ronments. Age-equivalent marine sediments on northeastern Greenland also co ntain local abundant occurrences of Rhizocorallium. These Arctic occurrence s contrast with the same trace fossil's distribution in the Jurassic of Bri tain and France, where it characterizes shallower and higher energy environ ments; such sequences on Spitsbergen show an ichnofauna dominated by Skolit hos and bivalve escape shafts. Orientations shown by the R. jenense U-tubes show a generally, but not sole ly, unimodal distribution, with the curved distal ends usually oriented tow ard onshore. Presumed aperture lineations show strongly unimodal trends, pr obably related to longshore currents. Burrows in beds at the top of individ ual storm lobe units show more complex patterns, probably reflecting both c urrent and wave reworking following lobe abandonment. All finds suggest ear ly colonization by the burrowing organisms. These were not followed by othe r burrowers, either because of the nutrient-poor nature of the sediment or because of high sedimentation rates.