The Ophiomorpha rudis ichnosubfacies of the Nereites ichnofacies: Characteristics and constraints

摘要

The Ophiomorpha rudis ichnosubfacies of the Nereites ichnofacies contains a low diversity (commonly 1-4 ichnotaxa) trace fossil assemblage that besides the eponymous ichnospecies commonly includes Ophiomorpha annulata and Scolicia, and rarely Nereites irregularis, Chondrites, graphoglyptids or other ichnotaxa. This ichnosubfacies occurs in channel and proximal depositional lobe facies of deep-sea fans or in thick-bedded facies of deep-sea clastic ramps. When considered with the Nereites and Paleodictyon ichnosubfacies, it can be used for determination of bathymetric trends. Specimens of O. rudis can also occur in overbank or fan-fringe facies in the Paleodictyon ichnosubfacies, in which ichnodiversity is distinctly higher. Ophiomorpha rudis ranges from the Tithonian but is rare in the Lower Cretaceous. It is distinctly abundant since the Turonian with a pronounced peak of abundance in the Eocene. After the Eocene, its abundance apparently declines. The occurrence of the O. rudis ichnosubfacies at present is stratigraphically limited to the Upper Cretaceous-Miocene. Ophiomorpha rudis is a very deep-feeding burrow system composed of oblique to vertical shafts and horizontal galleries. Probably. it was produced by shrimp-like crustaceans, which possibly fed by means of farmed microbes on decomposed phytodetritus close to or below the redox boundary. Tunnels of O. rudis were reinforced against collapsing, with irregular muddy sand granules. In more cohesive sediment, the burrows are smooth, without a wall, and in firmground sediment they are covered locally with casts of scratch ornament produced by the legs of the tracemaker.