Neogene Continental Deposits of Bahia Blanca, Argentina. Palaeoenvironmental and Palaeoclimatic Inferences

Abstract

The present contribution involves deposits of continental Neogene age that outcrop in the Bahía Blanca region. The studies that were carried out in order to contribute to the paleoenvironmental and chronostratigraphical knowledge involve sedimentological, paleomagnetic and magnetic susceptibility analyses. One of the outcrops studied (Cueva de Los Leones) is mainly composed of facies of fine to thick sandstones, largely pedogenized and with important amounts of calcrete, whereas the other (Loma Sarmiento) has a predominance of loess-like silt facies, reddish brown in colour, an abundance of rhizoconcretions and evidence of pedogenic processes. Both outcrops culminate in an important layer of calcrete of regional scope, with particular morphologies at each of the sites. The paleomagnetic analysis leads to the conclusion that the sedimentation of the deposits underlying the calcareous crust at the two sites was not synchronous. Reverse polarity was recorded in the Cueva de Los Leones in contrast to Loma Sarmiento, where normal polarity is predominant. The paleomagnetic samples of calcareous crust on the top presented anomalous polarity directions in both sections, with very high values of magnetic intensity. The reverse directions in the Cueva de Los Leones can be assigned to the Gilbert Cron (6.0–3.6 Ma), without discarding younger ages in the Pliocene. In contrast, Loma Sarmiento seems to be older (Late Miocene). The susceptibility records also indicated differences between the two sites that would be linked to pedogenic development; they were higher in Loma Sarmiento and judging by the F% factor, in Cueva de los Leones the superparamagnetic particles contribution is higher. Petrographic, X-Ray difractometry, paleontological and soil micromorphology analyses, in some places at the studied sites, also contributed to the paleoenvironmental/paleoclimatic reconstruction carried out.