Ichnological analysis of the late Miocene shallow marine diatomaceous deposits of the Lower Chelif basin (northwestern Algeria): Paleoenvironmental insights and comparison with deep diatomites

The Messinian shallow marine diatomaceous deposits (Tripoli Diatomite Formation) of the Lower Chelif basin (northwestern Algeria) display a moderate diversity and high abundance of decapod crustacean trace fossils, preserved mainly as endichnia, epichnia and hypichnia. These deposits consist of diatomites intercalated by diatomaceous marls and rarely micritic limestones, containing nine ichnospecies from seven ichnogenera, dominated by domichnion trace fossils (Diplocraterion isp., Ophiomorpha isp., O. nodosa, Palaeophycus isp., Planolites isp., cf. Skolithos isp., Spongeliomorpha isp., Thalassinoides isp. and T. suevicus). Two ichnofabrics have been identified, represented by the Ophiomorpha ichnofabric which characterises the diatomaceous deposits of the Djebel Murdjadjo carbonate platform, and the Thalassinoides ichnofabric in the Tessala–Beni Chougrane carbonate platform. The ichnological, palaeontological and sedimentological data confirm that the trace fossil bearing diatomaceous beds were deposited in a nearshore shallow sea under moderate-to high-energy conditions constituting a part of the Skolithos ichnofacies in the Djebel Murdjadjo carbonate platform, and the Cruziana ichnofacies in the Tessala–Beni Chougrane carbonate platform. The comparison between shallow- and deep-marine diatomites indicates a regressive sequence in the deep diatomites, with abundant Chondrites and Zoophycos at the base of the succession grading downward into Thalassinoides-dominated trace fossil assemblage in the uppermost part, which is similar to the shallow-tier trace fossils of the two carbonate platforms, represented by Ophiomorpha-dominated ichnoassemblage in the Djebel Murdjadjo carbonate platform, and by Thalassinoides-dominated ichnoassemblage in the Tessala–Beni Chougrane carbonate platform.