Paleoenvironmental setting and description of an estuarine oyster reef in the Eocene of Patagonia, southern Argentina

•We described the earliest oyster reef in estuarine facies from southern hemisphere.•The Eocene reef grew in an outer estuary environment subject to salinity changes.•The reef would have been located in a tidal channel convergence area.•The estuaries were refuges for Crassostrea many times throughout the Cenozoic.

A middle Eocene Crassostrea sp. reef near Río Turbio, southwestern Patagonia (Argentina), represents the earliest record of an oyster reef associated with estuarine facies in the southern hemisphere, and also one of the few known worldwide occurring in Paleogene rocks. The reef grew in an outer estuary environment subject to periodic changes in salinity and may have reached a maturing phase. The Río Turbio reef - by its dimensions, geometry, and substrate lithology- would have been located in a tidal channel convergence area. This reef provides new evidence suggesting that estuaries served as refuges for Crassostrea populations allowing them to disperse into fully marine environments many times throughout the Cenozoic.