ABSTRACT
The aim of this paper is to present some characteristics of the Pleistocene-Holocene sequence outcropping on the middle valley of the Arroyo NapostA Grande, 27 km northwest of Bahia Blanca.
Fresh water gastropods, components of a Quaternary fauna of the Aqua Blanca (De Francesco, 1970) and Chacra La Blanqueada formations (Rabassa, 1989) has been studied. A statistical analysis of density distribution was done, as well as a quantitative study concerning Littoridina parchappii (D’Orbigny, 1835) individual length mean. In both cases the hypotheses tests yielded significative and periodic differences where, both the gastropod population densities and the L. parchappii average total length, occur in cycles. These cycles would indicate that the Holocene times can be divided in several phases, controlled by climatic changes as alternating drought-flood cycles, with a development closely related with temperature oscillations.
