Loreto Formation (Spanish: Formación Loreto) is a sedimentary formation of Late Eocene age in the southernmost Magallanes Basin.

It overlies the Leña Dura Formation and the contact with an overlying formation is not observed.

[2][3][4] The formation contains fossil wood, leaf imprints and palynomorphs.

[5] Also fossil fish of Striatolamia macrota and Ischyodus dolloi and indeterminate Spheniscidae fossils have been found in the formation.

[6][7] The formation crops out in Brunswick Peninsula and Sierra Baguales among other places.