ICHNOLOGY OF THE SCHRADER BLUFF–PRINCE CREEK FORMATIONS AT SHIVUGAK BLUFFS, NORTH SLOPE OF ALASKA

The ichnology of shallow-marine, deltaic, and continental strata of the Upper Cretaceous (Santonian to late Campanian) Schrader Bluff and Prince Creek formations at Shivugak Bluffs along the Colville River of northern Alaska reflects autogenic and allogenic physicochemical conditions during the deposition of this high-paleolatitude, fluvial-deltaic succession. The Schrader Bluff Formation contains trace fossils attributed to the activity of marine organisms. Ichnotaxa representing their activity include Asterosoma (As), Chondrites (Ch), Gyrolithes (Gy), Helminthopsis (He), Ophiomorpha (Op), Palaeophycus (Pa), Phycosiphon (Ph), Planolites (Pl), Rhizocorallium (Rh), Schaubcylindrichnus (Sch), Skolithos (Sk), Teichichnus (Te), Teredolites (Tr), and Thalassinoides (Th). Fewer but significant traces include dinosaur tracks and rhizoliths (Ri)—rhizoliths (Rir), rhizohaloes (Rih), and rhizocretions (Ric)—that represent subaerial settings that experienced pedogenesis. Trace fossil associations in lithofacies interpreted as marine environments represent a mixed Skolithos and Cruziana ichnofacies suite, whereas those in deposits interpreted as continental environments represent the Scoyenia ichnofacies. The Skolithos Ichnofacies is dominated by vagile and suspension-feeding forms (Gy, Op, Pa, Pl, Sch, Sk) found in facies associations representing high-energy marine environments. The Cruziana Ichnofacies is dominated by sessile and deposit-feeding forms (As, Ch, Ph, Te, Th) found in facies associations representing low-energy marine environments. The Scoyenia Ichnofacies is dominated by locomotion animal traces and holdfast structures of plants found in facies associations representing subaerial settings. The Prince Creek Formation contains dinosaur tracks, Pl, and Rir, Rih, and Ric. Pedogenically modified deposits dominated by mud and heterolithics contain dinosaur tracks, rhizoliths, mottles, and such ped structures as blocky, subangular blocky, or platy structure. Sediments deposited in lacustrine settings contain few identifiable traces but likely include Pl. Marine and continental environments contain ichnofabric indices (ii) of 1 to 5. In marine settings, sediments accumulated under high sedimentation rates have lower ii; lower rates have higher ii.