Palaeoecological, taphonomic and biosedimentary analysis of Mid- Holocene lacustrine deposits from Ouargla area (Lower Sahara, SE Algeria)

The Northgrippian (Middle Holocene) lacustrine deposits of Hassi Mouillah (Ouargla, Algerian Sahara) are characterised by molluscan concentrations that yielded excellently preserved bivalves and gastropods. They hold athalasso- saline mollusc communities dominated by the two spe-cies Cerastoderma glaucum and Melanoides tuberculata associated with bioturbation and bioerosion traces. The study of this assemblage was made based on palaeoecological, taphonomic and biosed-imentological criteria. The taphofacies consists of bivalve and gastropod shells that were preserved in a random orientation, generally associated together in the same levels. The preservation style for C. glaucum is in the form of (i) single valves, (ii) closed valves, and (iii) butterfly position. Articulated valves rarely occur in a vertical or oblique position, and contain small specimens of M. tuberculata. The taphonomic analysis allows the recognition of three skeletal concentrations. Because the bivalve shells are bioeroded and many M. tuberculata specimens were found in the connected valves of C. glaucum, these gastropods are considered as predators of bivalves. However, M. tuberculata ex-ploited C. glaucum shells as domiciles and not for feeding. Three types of trace fossils were recorded: (i) burrows; (ii) endichnial ribbons; and (iii) Oichnus- like borings. C. glaucum has been considered as the potential tracemaker of the burrows, whereas the borings are regarded as produced by M. tuber-culata. Therefore, this fossil assemblage can be classified as sedimentological concentrations, which could be originated by storm events related to strong winds. The taphonomic characteristics observed in the fossil bivalves and gastropods are a consequence of transport, deposition and burial. The Hassi Mouillah's assemblage demonstrates the presence of at least two generations of bivalves, pre-and post-event.