Trace fossil assemblages in a Middle Triassic mixed siliciclastic-carbonate marginal marine depositional system, British Columbia

A diverse ichnofossil assemblage characterizes the mixed siliciclastic-carbonate marginal marine succession of the upper Liard Formation (Middle Triassic), Williston Lake, northeastern British Columbia. Sedimentary facies within this succession consist of five recurring facies associations: FA1 (upper shoreface/foreshore); FA2 (washover fan/lagoon); FA3 (intertidal flat); FA4 (supratidal sabkha) and FA5 (aeolian dune). Shoreface/foreshore sediments (FA1) accumulated on a storm-dominated, prograding barrier island coast and are characterized by a low-diversity Skolithos assemblage (Diplocraterion, Ophiomorpha, Palaeophycus, Planolites, Skolithos and Thalassinoides). Washover fan/lagoonal sediments (FA2) are dominated by trophic generalists. (Cylindrichnus, Gyrochorte, Palaeophycus, Planolites, Skolithos, Trichichnus and an unusual type of bivalve resting trace), consistent with deposition in a setting subject to periodic salinity and oxygenation stresses. Intertidal flat deposits (FA3) are characterized by a diverse mixture of dwelling, feeding, and crawling forms (Arenicolites, Cylindrichnus, Diplocraterion, Laevicyclus, Lingulichnus, Lockeia, Palaeophycus, Planolites, Rhizocorallium, Siphonichnus, Skolithos, Teichichnus, Taenidium, and Thalassinoides, reflecting the presence of adequate food resources in both the substrate and in the water column. Vertical burrow-dominated trace fossil assemblages within thin, sharp-based sand beds are interpreted as intertidal tempestites and reflect post-event colonization of the intertidal zone by shoreface organisms. Supratidal sabkha deposits (FA4) are characterized by an exceptionally low-diversity trace fossil assemblage (Cylindrichnus, Monocraterion and rare diminutive Ophiomorpha). Solution collapse breccia and root traces overprint many primary physical and biogenic sedimentary structures, reflecting numerous cycles of desiccation and flooding. Aeolian dune deposits (FA5) consist of unfossiliferous, exceptionally well-sorted sandstone beds.