Abundant Zoophycos and Chondrites from the Messinian (Upper Miocene) of northwestern Algeria

The Messinian diatomaceous deposits are widely exposed in the Lower Chelif Basin (NW-Algeria). The upper part of the Diatomaceous Formation is intercalated by four diatomaceous beds rich in Zoophycos and Chondrites. Both these traces were produced by the members of endo-benthonic fauna adapted to nutrient exploitation within sediments. In the Diatomaceous Formation, larger dark-coloured Chondrites targionii cross-cuts smaller C. intricatus which could indicate ecological succession perhaps due to conditions getting less oxygenated and causing the need for larger burrows. Size increase of Zoophycos upwards in the section may indicate that environment became less stressed, shallower and likely had a higher nutrient content or alternatively there was a change in the trace-maker biological affinity. The depositional environment shows some evidence of low-oxygenated to dysoxic conditions, because abundant Chondrites occurs without any other trace fossils with the exception of Zoophycos. Chondrites trace-makers likely maintained a connection to oxygenated water but penetrate into anoxic sediments rich in sulfides or ammonium that are required for microbe-breeding and feeding. Most likely studied Zoophycos-Chondrites and pure Zoophycos associations of the Upper Member of the Diatomaceous Formation are of deeper shelf origin. The diatomaceous volcanic ash beds of the upper part of the formation are intensively bioturbated by a horizontal network of large-size Thalassinoides, which represents domichnial and fodinichnial structures, and suggesting an enough oxygenated environment with sufficient nutrients. Thalassinoides level likely represents shallower water conditions (shallow shelf) than the underlying levels characterised with Zoophycos-Chondrites and Zoophycos associations.