A depositional unit called DU2 identified for the period MIS 3 (ca—30,000–60,000 year B.P.) formed by only one sedimentary facies (F3) was found in the Luján and Salto-Arrecifes rivers basins. F3 is a fluvio–lacustrine unit that overlies in erosive unconformity over eolian sediments with ages of 56,400 ± 6500 and 50,400 ± 10,200 years B.P. and is unconformably covered by another eolian vitroclastic sandy loess deposit, dated as 32,000 ± 4000 years (Infrared Stimulated Luminescence , IRSL) (Blasi et al. 2009a). It represents the recurrence of ephemeral fluvial streamlets and the development of temporary pools by subsequent damming of channels. It corresponds lithologically to sandy muddy gravel, gravelly muddy sand, gravelly mud olive to pale olive, feldspar and quartz sands, bearing extinct mollusks such as Heleobia ameghini and Diplodon lujanensis. Radiocarbon chronologies obtained on monospecific samples of Cyprideis salebrosa hartmanni and Heleobia ameghini yielded ages of 37,710 ± 840 years 14C B.P. and >40,000 years 14C B.P., respectively. Furthermore, the age obtained through the IRSL technique was of 44,000 ± 6500 years. Based upon the analyzed bioproxies (malacological, phytoliths and diatomological content) F3 accumulated under variable climatic conditions, ranging from temperate to colder and from subhumid to drier. According to the exhaustive stratigraphic identification, it is proposed that in N-E Buenos Aires Province, the so-called Undulated Pampa region, the sediments that were accumulated during MIS3 occur only in the central portion of the studied fluvial basins. This prompted two hypotheses related to the existence of a particular drainage pattern for the Late Pleistocene, different from the present one, and subsequent tectonic controls that allowed the identification of DU2 sediments only in some of the analyzed sections.Facultad de Ciencias Naturales y Muse
