The ichnology of a submarine braided channel complex: the Whisky Bay Formation, Cretaceous of James Ross Island, Antarctica

Cretaceous rocks of the Whisky Bay Formation in northwest James Ross Island, Antarctica are described in terms of lithology, bed geometry, fabric, sedimentary structures and trace fossils. They are interpreted as a braided submarine channel complex in the inner part of a submarine fan.Nine lithofacies grouped in four lithofacies associations are defined. Major braided channel association is characterized by gravelly turbidity current and debris flow deposits. Terrace and minor braided channel association records the action of high density gravelly and sandy turbidity currents. Sandy plain association may represent deposition by “classical” turbidity currents. Interchannel area association evidences low density turbidity currents and suspension sedimentation, far away from channel processes.A brief description of trace fossils present in the unit is given and environmental implications of their distribution are emphasized. Arenicolites isp., Diplocraterion parallelum, Palaeophycus tubularis, Planolites cf. P. ophthalmoides, Skolithos linearis and Thalassinoides suevicus make up the ichnocoenosis A. It mainly represents dwelling-feeding structures (Domichnia) of suspension feeders in minor channels, marginal terraces and sandy proximal plains. The presence of these “shallow-water” traces is related to local conditions (high energy, sandy substrate, intermittent oxygenation and slow build up of organic detritus), rather than bathymetry. Cylindrichnus concentricus, Chondrites isp. and Zoophycos isp. (large and small form) make up the ichnocoenosis B. It is composed of deep deposit-feeders feeding structures (Fodinichnia) in interchannel areas. Ethologic group Agrichnia is completely absent. The ichnocoenosis B is anomalous in submarine fans. The dynamics associated with the coarse-grained inner channelized part of this submarine fan are a precluding factor in the development of highly specialized traces. The replacement of an Agrichnia and Pascichnia-dominated trace fossil assemblage (typical from submarine fans) by the Fodinichnia-dominated ichnocoenosis B is considered a result of the combination of different related factors. Probably contemporaneous volcanic activity originated frequent sediment gravity flows that introduced considerable amounts of organic material into the system. It is suggested that the analysed deposits were formed close to the source area at shallower depths than normal submarine fans. As a result of this particular depositional setting, an abnormal development of feeding opportunistic trace fossils took place. Conversely, the existence and evolution of more complicated and specialized graphoglyptid patterns of trace fossils was inhibited in these organic-rich sediments deposited in such an unstable setting.