The Paleogene marine sedimentary succession of northwestern Santa Cruz province is analyzed. A transgressive-regressive cycle with three sections deposited in a shallow marine siliciclastic shelf paleoenvironment is suggested. These successions contain important horizons with invertebrate fossils. Systematic ichnology, ethology, and paleoenvironmental aspects are considered. The trace fossil assemblage is grouped in two ichnocoenoses: Ichnocoenosis A: dwelling structures of suspension feeders in a sandy substrate of a shallow water platform (Mocaronichnus and Arenicolites); lchnocoenosis B: dwelling/feeding structures of deposit feeders in a muddy substrate of a distal platform (Rhizocorallium and Thalassinoides). The monoespecific association of Thalassinoides belongs to the Glossifungites ichnofacies. This association develops on a firmground related to an omission surface.KEY WORDS. Argentina. Santa Cruz. Eocene. El Chacay Formation. lchnology. Systematics. Paleoenvironment.