Southwestern Buenos Aires Province in centraleast Argentina is a favorable area to undertake a land-mammal biostratigraphic study of the Late Cenozoic, because sedimentary units crop out in a relatively small area (no more than 1250 km2; figure 1), bearing faunas representing at least the last 6 million years. Many of the exposures are composed of the so called ?Sedimentos Pampeanos? ( sensu Fidalgo et al ., 1975). These sediments are well known for their strong lithologic uniformity and broad distribution throughout the Pampean region. Consequently, the study of their paleontological contents is quite important for stratigraphic correlation.
Papers on vertebrate paleontology with stratigraphic approach of the Late Cenozoic of Argentina have been accomplished mostly in eastern Buenos Aires Province (Tonni and Fidalgo, 1979; Cione and Tonni, 1999 and references therein). In southwestern Buenos Aires Province the studies of vertebrate paleontology had been so far restricted to well known outcrops of the Atlantic coast (Tonni et al ., 1992 and references therein). Only isolated remains were known for the rest of the area until this study was accomplished. New collections in the area began in 1985 and yielded about 700 specimens with trustful information concerning stratigraphic provenance. Fossil prospecting was accompanied by detailed sedimentologic, palynologic, and microinvertebrates (ostracods) studies carried out by other researchers. In addition, absolute dating was used in correlation of Pleistocene deposits.
Zavala and Quattrocchio (2001) made a sequence stratigraphy analysis of the area characterizing the genesis of the deposits. Their proposal provided the geological frame for the biostratigraphic study and alerted about paleoenvironmental control and/or interpretation of the findings. The most significant profiles were selected to define biostratigraphic zones based on mammals, which were in turn chronostratigraphically correlated to each other and to other biozones of the Pampean area.
All the new data and materials were studied and the results are the base of the biostratigraphic and chronostratigraphic proposal, which includes the biozonation of the area. The paleoclimatic and paleoenvironmental trends observed through the vertebrate fauna, as well as some faunal descriptions and systematic revisions will be developed in future papers.