Ichnology of a Macrotidal Tide-Dominated Deltaic Depositional System: Lajas Formation, Neuquén Province, Argentina

The Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, is interpreted as a 400–600 meter thick aggradational, macrotidal tide-dominated deltaic system. Tide-dominated systems are most commonly found in estuarine settings, and thus the biota found in such settings are generally influenced by lowered salinities. This paper documents the ichnology of the tide-dominated Lajas Formation, which includes a diverse ichnofauna, much of which is subject to tidal conditions, though not necessarily to reduced salinity. The ichnogenera recorded include: Asteriacites, Arenicolites, Asterosoma, Dactyloidites, Didymaulichnus, Diplocraterion, Chondrites, Cruziana, Helminthorhaphe, Macaronichnus, Ophiomorpha, Parahaentzschelinia, Palaeophycus, Planolites, Polykladichnus, Protovirgularia, Rhizocorallium, Rosselia, Schaubcylindrichnus, Scolicia, Siphonichnus, Taenidium, Teichichnus, and Thalassinoides. The range of tidal environments from which the trace fossils were collected encompasses tidal flats, tidal channels, and tide-dominated delta fronts, passing offshore into shelf mudstones of the Los Molles Formation and landward into the fluvial deposits of the Challaco Formation. The paleoenvironmental controls on the distribution of ichnotaxa are discussed in relation to Lajas Formation ichnology and sedimentology.