The Santa Cruz Formation (late Early Miocene of Patagonia, Argentina) has been traditionally considered a continental unit and its contact with the underlying Early Miocene Monte León Formation as transitional or unconformable according to different authors. Invertebrates – mainly molluscs – present in the purportedly transitional beds were used as an argument to refer them to the marine Monte León Formation. This was generally accepted, but the lithological features shared with the continental Santa Cruz Formation indicate that they should be included within this unit instead. Analysis of the fauna revealed that the only in situ species in these beds is Crassostrea orbignyi (Ihering, 1897). This oyster belongs in a group adapted to inhabiting marginal marine settings with hard or soft bottom, and tolerating a wide range of salinity and subaereal exposure. The other accompanying members of the fauna are reworked marine molluscs referable to taxa included in the underlying Monte León Formation.