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In order to understand the paleogeographic evolution of Viedma Lake (252 masl) sedimentary sequences with glaciolacustrine and lacustrine strata between 307 masl and 266 masl and bathymetric fluctuations between 27 ka. and 2.4 ka. were identified. The paleoenvironmental interpretation of the stratigraphic profiles was carried out based on the analysis of facies and their associations. The oldest age was recorded at 301 masl (27 ka) and the youngest corresponds to 266 masl (2.4 ka). The record of 27 ka was compared with another dated record of the Tar-San Martin Lakes basin showing that Viedma – Tar-San Martín Lakes were formed at similar ages. Taking into account that the deposits of Bahía Túnel are located at 266 masl and that the level of the lake would be above this height, it is proposed that this lake level would have restricted the movement of hunter-gatherer populations along the coast and to the west. From 2.4 ka on a new space started to be available for these populations broadening the wintering grounds (≤400 masl). The chronology of the archaeological record found in this new space fits the proposed paleogeographic evolution. This paper complements the research on the evolution of lacustrine systems in the Patagonian Mountain range and broadens the discussion of human mobility in the Viedma Lake basin during the Holocene. [ABSTRACT FROM AUTHOR]

Gastropods and ostracods are widely used in Quaternary paleoenvironmental reconstructions but the taphonomic studies that should precede them are not as common. In this paper we compare the taphonomic information obtained from assemblages recovered in Holocene fluvial sequences of the Salado River basin (Buenos Aires, Argentina) and analyze the changes or similarities between species, localities and stratigraphic units. Using taphonomic grades, gastropod shells were cataloged based on three attributes: fragmentation, discoloration and dissolution. For ostracods, discoloration and dissolution together with the population structure (adults-juveniles A/J, valves-carapace V/C and male-female ratios) were analyzed. For gastropods, Heleobia parchappii , Biomphalaria peregrina and Succinea meridionalis showed similar preservation patterns, and Miradiscops brasiliensis showed least evidence of alteration. In ostracods, these patterns varied among species. For V/C ratio, 7% of assemblages were formed only by valves, 82% were dominated by valves and 11% were dominated by carapaces. Fragmentation and population structure allowed autochthonous and allochthonous associations to be distinguished. Gastropod assemblages with low levels of fragmentation were recovered mainly from the uppermost levels of the stratigraphic profiles. For ostracods, the A/J ratio showed that 52% of the assemblages were dominated by adults with most ontogenetic stages represented and 29% retained their original population structure suggesting they would be autochthonous associations, whereas 19% of the assemblages were dominated by juvenile valves therefore assuming allochthonous associations. Our results indicate that taphonomic analysis is an effective tool that allows identifying autochthonous assemblages, which are the most reliable for robust paleoenvironmental reconstructions. • First taphonomic study combinating ostracods and gastropods in Argentina. • The most abundant species of gastropod showed similar preservation pattern. • In ostracods, the preservation states varied among the most abundant species. • Ostracod population structure allows recognizing autochthonuos or allochtonous assemblages. [ABSTRACT FROM AUTHOR]