Flysch trace fossils from the Cretaceous Kodiak Formation of Alaska

Abstract

The Cretaceous Kodiak Formation at the northeastern end of Kodiak Island, Alaska, is interpreted to have been deposited in a distal deep-sea fan complex. Facies analysis of the sequence suggests deposition in channels (both major and distributary), lobe fringe (proximal and distal), and interchannel and interchannel/channel mouth environments. The strata contain a diverse and relatively abundant trace fossil assemblage consisting, at the ichnogeneric level, of twenty-six forms: Acanthorhaphe, Chondrites, Circulichnis, Cochlichnus, Cosmorhaphe, Desmograpton, Glockerichnus, Gordia, Helminthoida, Helminthopsis, Muensteria, Neonereites, Nereites, Paleodictyon, Palaeophycus, ?Phycosiphon, Planolites, Protopaleodictyon, Scolicia, Spirophycus, Spirorhaphe, Taenidium, Taphrhelminthopsis, Terebellina, Thalassinoides, and Yakutatia.Thirty-eight ichnospecies are described, three of which (Acanthorhaphe ichnosp., Cosmorhaphe helicoidea, and Desmograpton fuchsi) are recorded for the first time in North America. Final channel-fill (levee) and interchannel environments contain the most abundant and diverse ichnoassemblages, a reflection of favorable environmental parameters for inhabitation by benthic organisms, but more importantly the presence of diverse lithotypes conducive to the final preservation of their activity. Channel-fill sequences are devoid of trace fossils, a reflection of substrate mobility and lack of shale interbeds precluding toponomic preservation. Lobe fringe sequences contain only isolated trace fossils, a reflection of their poor exposure and largely monolithologic character.The ichnoassemblage is compared to 20 previously described and taxonomically well-documented deep-water flysch trace fossil studies from the Phanerozoic. Comparison at the ichnogeneric level is favorable with only Acanthorhaphe, Terebellina, and Yakutatia being relatively unique to the Kodiak sequence.