Trace fossil association related to the Upper Miocene transgression cycle in the Lower Chelif Basin, NW Algeria

The Tortonian transgression is expressed in the Lower Chelif Basin by the deposition of the Blue Marls Formation highlighted at the base by a deposition of sandstone and conglomerate, including ante-Neogene cobble-size clasts, in which the ichnological investigation shows moderately diversified invertebrate ichnofossils preserved mainly in the detrital-dominated deposits. The trace fossil record consists of 18 ichnospecies belonging to 13 ichnogenera, including Arenicolites carbonaria, Diplocraterion parallelum, Gastrochaenolites isp., Gyrolithes isp., Macanopsis isp., Macaronichnus cf. segregatis, Ophiomorpha nodosa, Palaeophycus tubularis, Phycodes circinnatum, Phycodes cf. curvipalmatum, Rhizocorallium isp., Rosselia socialis, Skolithos annulatus, Skolithos linearis, Skolithos verticalis, Thalassinoides horizontalis, Thalassinoides cf. suevicus, and Thalassinoides isp., produced mostly by deposit-, suspension-, and rarely detritus-feeder organisms. Among these traces, Gastrochaenolites isp. is the only specimen being recorded within the conglomerate deposits and represents, therefore, the evidence of the first colonization after the Tortonian marine ingression. The remaining trace fossils are exclusively reported from the sandstone deposits and are typical of shallow-marine settings composed of vertical and horizontal structures, largely dominated ethologically by domichnia, fodinichnia, and combined domichnia/fodinichnia behaviour groups. The lower interval of the sandstone deposits is dominated by mid- to deep-tier, vertical post-depositional dwelling (domichnia) forms (Arenicolites, Diplocraterion, Skolithos, Macanopsis, Ophiomorpha, and Gyrolithes) attributed to opportunistic suspension-feeding organisms, corresponding to the archetypal Skolithos ichnofacies that documents an upper-middle shoreface environment, whereas the upper interval is predominantly represented by shallowest- to shallow-tier horizontal deposit-feeders burrows [Macaronichnus, Ophiomorpha (horizontal components), Palaeophycus, Phycodes, Rhizocorallium, Rosselia (vertical components), and Thalassinoides] displaying intense degree of bioturbation, reflecting a perfect palaeoecological condition reigning in the water column and sea floor. Thus, the association commonly corresponds to the proximal trend of the Cruziana ichnofacies developed within the lower shoreface zone. Consequently, the ichnological investigation reveals that the detrital-dominated deposits show a slight deepening of the environment, shifting from a suspension feeder-dominated to a deposit feeder-dominated association, in which polychaete annelid and crustacean animals are the main benthonic epi- and infaunal trace makers of the recorded ichnofossils. The ichnotaxa Arenicolites carbonaria, Phycodes circinnatum, Phycodes cf. curvipalmatum, Rhizocorallium isp., Rosselia socialis, and Skolithos verticalis are reported herein for the first time from the Cenozoic of Algeria.