Six biozones ranging from the uppermost Miocene to historical times were recognized for southwest of the Buenos Aires Province, and correlated within a chronostratigraphic chart.
A Late Miocene age (Late Huayquerian) was assigned to the ?Pampean Sediments? exposed at Grünbein and Barrancas de Sarmiento, and Early Pliocene Age (Montehermosan) to those exposed at Las Obscuras, Dique Paso Piedras and Balneario Saldungaray localities. La Delta Sequence was assigned to the end of the Early Pliocene (Chapadmalalan)-Late Pliocene (Marplatan), the San José Sequence to the Middle Pleistocene (Early Bonaerian), the Agua Blanca Sequence to the Late Pleistocene (Lujanian)-Holocene (Platan), and the Chacra La Blanqueada Formation and Matadero Saldungaray Formations partially to the Holocene (Platan) and partially to the Historical Times-present.
The aim of the study was the biostratigraphy of the southwestern Buenos Aires Province, but this entailed the systematic and chronological revision of the fauna on which the biozonation could be accurately determined. As a synthesis it may be said that two new taxa of the genus Chloephaga were found (see Appendices 1 and 2); the oldest records of the fish genera Pimelodella ( P . aff. laticeps ), Callichthys ( C. callichthys ) and Percichthys , the Muridae Oxymycterus , Lundomys and Phyllotis , and the Tayassuidae Tayassu are those of Bajo San José; the first record of Berthawyleria in Argentina is that of Grünbein. The genus Actenomys was preliminary reviewed. Some of the known biochrons of the taxa changed with the findings in this area. Some others were corrected reanalyzing the original papers in light of new studies of the geology of their type localities (see Appendix 2 and Comments).
The author is greatly indebted to M.E. Quattrocchio, D.H. Verzi, M.G. Vucetich, and A. Riccardi for their valuable suggestions; to S. Bargo and M. Reguero (MLP), and A. Kramarz (MACN) for access to material under their care. Reviews of J. Rabassa and A. Candela, and comments of A. Cione and E.P. Tonni contributed to improve the manuscript. This study was partially funded by CONICET (Grant PIP 2099 to M.G. Vucetich) and SECyT (Universidad Nacional del Sur to M.E. Quattrocchio).