The ichnocoenosis of the bottom nepheloid layer (BNL) deposits : a case study from the Scaglia Toscana Formation (Paleogene, central Italy)

Some fine-grained sediments from the Eocene in age Scaglia Toscana Formation in the Northern Apennines (Trasimeno area), previously interpreted as mud-silt turbidites, have been reinterpreted herein as bottom nepheloid layers (BNL). They contain a rich ichnocoenosis dominated by endichnial forms, that formed progressively in line with the slow accumulation rate of mud transported by the oceanic thermohaline bottom currents. In a BNL idealized sequence, a slight upward increasing density of trace fossils, suggests some differences with typical muddy turbidites. Together with sedimentary structures, trace fossils form an ichno-sedimentary sequence through the bed, which is explained by a step by step colonization that accompanies deposition of the bottom nepheloid layer by continuous currents. It is possible that these ichnological features are recurrent and helpful in recognition of similar deposits within other geological contexts.