Trace fossil associations related to facies of an upper Ordovician low wave energy shoreface and shelf, Oslo-Asker district, Norway



Three trace fossil associations have been identified from facies interpreted as low wave energy upper, middle and lower shoreface deposits, transitional shoreface/shelf deposits and open epicontinental shelf deposits. These were developed in pre-regressive, regressive and transgressive phases associated with eustatic sea level changes caused by the upper Ordovician glaciation. The three associations are the Skolithos-Diplocraterion association, the Diplichnites-Phycodes association and the Thalassinoides association. The Skolithos-Diplocraterion association inhabited upper, middle and lower shoreface environments, the Diplichnites-Phycodes association a lowermost shoreface environment and the Thalassinoides association inhabited offshore epicontinental shelf environmcnts. The Skolithos-Diplocraferion and Diplichnites-Phycodes associations bear strong affinities with previously described Skolithos ichnofacies assemblages (indicating shoreline environments) and the Cruziana ichnofacies forms (indicating shallow water environments down to wave base) respectively. The Thalassinoides assemblage, however, closely compares with assemblages described from Mesozoic epicontinental shelf sequences. Trace fossil diversity achieves a maximum in the lowermost shoreface environment and reduces in deeper water and shallower water environments. This accords with diversity patterns identified in analogous Jurassic epicontinental palaeonvironments. The stratigraphic distribution of some trace fossil association boundaries accords with those of contemporary body fossil associations. However, several body fossil associations are included within the volume of distribution of a single trace fossil assemblage. □Trace fossil association, shoreface and shelf, Oslo district, upper Ordovician.