Deep-sea trace fossils of the Oligocene–Miocene Numidian Formation, northern Tunisia

Abstract

Twenty-two ichnogenera and thirty-one ichnospecies have been recorded in the Oligocene–Miocene Numidian Formation of northern Tunisia. Heterolithic successions of thin-bedded turbidite sandstones and interchannel mudstones contain the most diverse trace fossil assemblages. Thick- to very thick-bedded structureless sandstones and conglomerates representing the fill of channel complexes contain a low-diversity trace fossil assemblage. The ichnoassemblage in the lower part of the formation (Oligocene), which includes Paleodictyon isp., Scolicia strozzii, Spirorhaphe isp., ?Cosmorhaphe isp. and Halopoa isp., can be ascribed to the Paleodictyon ichnosubfacies of the Nereites ichnofacies. The ichnoassemblage in the upper part of the formation (Miocene: Aquitanian), including Diplocraterion cf. habichi, Scolicia vertebralis and Ophiomorpha isp., is interpreted as the shallower part of the Ophiomorpha rudis ichnosubfacies of the Nereites ichnofacies. The notable switch in ichnofauna between the Oligocene and lower Miocene reflects variation in environmental and depositional conditions. The common occurrence of trace fossils of the Ophiomorpha rudis ichnosubfacies, and Diplocraterion cf. habichi in the early Miocene, indicates an increase in energy level, greater environmental disturbance and probable shallowing. This is also confirmed by a corresponding decrease in the abundance and diversity of benthic foraminifera. The integration of ichnological, sedimentological and microfossil contents has allowed the distinction of two quite distinct geographical depositional settings within the Numidian Formation. The first domain includes the Numidian succession of the Tabarka, Cap-Negro, Cap-Serrat and Bouhertma areas, which are characterised by “distal” turbidites, showing the Paleodictyon ichnosubfacies that is compatible with a lower bathyal depth in the Oligocene and an upper slope depositional environment during the early Miocene.The second domain includes the NE part of Mogod Mountain (e.g. the Ras El Korane, Jebel Gattous–Zoukar), which exhibit more proximal characteristics compatible with a probable slope canyon interpretation. The southern margin of the Kroumirie (Balta and Zouza areas) and Sejnene area shows a distal setting compared with the Ras El Korane and Jebel Gattous–Zoukar areas. It is ascribed to a mid- to upper slope depositional environment during the Oligocene to early Miocene.