Ichnology and Palaeoenvironment Significance of the Tortonian Blue Marls Formation in the Southern Margin of the Lower Chelif Basin, Algeria

ABSTRACT This study integrates ichnological and sedimentological data to interpret depositional environments in the siliciclastic deposits of the Blue Marls Formation exposed along the southern margin of the Lower Chelif Basin, northwestern Algeria. The Formation is subdivided into two main units; the lower unit consists of sandstone beds exhibiting abundant but low‐diversity trace fossils and various sedimentary structures, intercalated with marlstone intervals containing both planktonic and benthic foraminifera. In contrast, the upper unit comprises thick, massive, dark grey to green‐grey marl deposits that are rich in planktonic foraminifera but devoid of ichnological records. Micropalaeoentological analysis of the planktonic foraminifera indicates that this succession was deposited during the Tortonian marine transgression, a well‐documented event in the Lower Chelif Basin. The identification of the bioevent FO Neogloboquadrina acostaensis in the basal part of the formation enables its correlation with an age of approximately 10.55 Ma. Ichnological investigation of the detrital‐dominated lower unit reveals 17 ichnotaxa, including Arenicolites isp. A, Arenicolites isp. B, Conichnus conicus, Gyrolithes isp., Lockeia siliquaria, Macanopsis isp., Monocraterion tentaculatum, Ophiomorpha nodosa , Palaeophycus isp., Rosselia socialis, Rosselia isp., Skolithos annulatus , Sk . linearis , Sk . verticalis, Taenidium isp., Thalassinoides suevicus , and Thalassinoides isp. This trace‐fossil suite consists of mid‐ to deep‐tier, predominantly vertical post‐depositional burrows, ethologically dominated by domichnia (dwelling structures) produced by opportunistic suspension‐feeding organisms such as decapod crustaceans and polychaete worms. The assemblage is attributed to the Skolithos ichnofacies, with only rare occurrences of elements indicative of a proximal Cruziana ichnofacies. It indicates high‐energy conditions associated with storm events, good oxygenation, and abundant nutrient supply in both the water column and on the seafloor. Integrated sedimentological and ichnological analyses of the detrital‐dominated deposits suggest deposition in a lower shoreface to offshore‐transition setting, characterised by a wave‐dominated depositional setting governed by storm‐generated currents affecting macrobenthic communities and their lifestyle and surviving strategy.