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

A diverse ichnofossil assemblage characterizes the mixed siliciclastic-carb onate marginal marine succession of the upper Liard Formation (Middle Trias sic), Williston Lake, northeastern British Columbia. Sedimentary facies wit hin this succession consist of five recurring facies associations: FA1 (upp er shoreface/foreshore); FA2 (washover fan/lagoon); FA3 (intertidal flat); FA4 (supratidal sabkha) and FA5 (aeolian dune). Shoreface/foreshore sedimen ts (FA1) accumulated on a storm-dominated, prograding barrier island coast and are characterized by a low-diversity Skolithos assemblage (Diplocrateri on, Ophiomorpha, Palaeophycus, Planolites, Skolithos and Thalassinoides). W ashover fan/lagoonal sediments (FA2) are dominated by trophic generalists. (Cylindrichnus, Gyrochorte, Palaeophycus, Planolites, Skolithos, Trichichnu s and an unusual type of bivalve resting trace), consistent with deposition in a setting subject to periodic salinity and oxygenation stresses. Intert idal Aat deposits (FA3) an characterized by a diverse mixture of dwelling, feeding, and crawling forms (Arenicolites, Cylindrichinus, Diplocraterion, Laevicyclus, Lingulichnus, Lockeia, Palaeophycus, Planolites, Rhizocoralliu m, Siphonichnus, Skolithos, Teichichnus, Taenidium, and Thalassinoides, ref lecting the presence of adequate food resources in both the substrate and i n the water column. Vertical burrow-dominated trace fossil assemblages with in thin, sharp-based sand beds are interpreted as intertidal tempestites an d reflect post-event colonization of the intertidal zone by shoreface organ isms. Supratidal sabkha deposits (FA4) are characterized by an exceptionall y low-diversity trace fossil assemblage (Cylindrichnus, Monocraterion and r are diminutive Ophiomorpha). Solution collapse breccia and root traces over print many primary physical and biogenic sedimentary structures, reflecting numerous cycles of desiccation and flooding. Aeolian dune deposits (FA5) c onsist of unfossiliferous, exceptionally well-sorted sandstone beds. (C) 20 01 Elsevier Science B.V. All rights reserved.