Environmental variability of Macaronichnus ichnofabrics in Eocene tidal-embayment deposits of southern Patagonia, Argentina

The Middle Eocene Man Aike Formation of southern Patagonia, Argentina, offers the opportunity to study the palaeoecological and palaeoenvironmental significance of Macaronichnus ichnofabrics in a tide-dominated, coastal embayment compound-dune complex. Four recurrent Macaronichnus ichnofabrics are recognized: Macaronichnus IF-1 characterized by Macaronichnus segregatis segregatis; Macaronichnus IF-2 with M. segregatis segregatis, Ophiomorpha nodosa, Skolithos linearis and rare Palaeophycus tubularis; Macaronichnus IF-3 typified by M. segregatis segregatis, O. nodosa, Asterosoma radiciforme and rare S. linearis; and Macaronichnus IF-4 consisting of M. segregatis segregatis, Nereites missouriensis, A. radiciforme, O. nodosa, Rosselia socialis, S. linearis, P. tubularis, Planolites isp. and Chondrites isp. Variations in these ichnofabrics are linked to changes in local hydraulic conditions in the upper and lower trough areas of the compound dunes. In high-energy, shallow and marginal marine environments, the Macaronichnus tracemaker is commonly the first to colonize the shifting, sandy substrates. Overprinting by other ichnoguilds occurs only after the physiochemical conditions became favourable and the colonization window length increases.