Soil evolution of Late Quaternary eolian sequences of Pampa, Argentina

KRÖHLING, DANIELA

Cairns (Australia)

Congreso; XVII International INQUA Congress; 2007

International Union for Quaternary Research (INQUA)

The Pampean Eolian System (PES; Late Quaternary) is the most important morphosedimentary system of Pampa. It is formed by a sand sea in central Argentina (180,000 km²) and a loess belt downwind (2,000 km long and 300 km wide). Some paleopedologic studies on outcropping loess-paleosols sequences were carried out in the last few years. However, the major parts of the studies are not really integrated on a regional scale because such paleosols were differentiated along isolated sites, sometimes with strong influence of local environmental conditions. The paleoenvironmental reconstruction provided by studies of sequences of eolian formations interbedded with paleosols should be based on a coherent stratigraphic framework and a correct geomorphic analysis; nevertheless both have been frequently disregarded. This contribution seeks to reconstruct the spatial distribution of environmental changes by means of the integration of own data with the available results produced by other authors. The research focuses on widespread field data and is used for integrating the geomorphologic, stratigraphic and sedimentologic information with paleopedologic advances. Large geomorphologic environments were discriminated: the Pampean Ranges piedmont, the coastal belts and the inland plains. Significant differences registered between the sequences are explained by their geomorphologic position. The most important result deduced from the comparison of the large environments is the existence of two strong soil forming periods in the last interglacial - glacial cycle. Pedogenic processes as argillan/ferriargillan illuviation, horizon differentiation, pedoturbation, hydromorphism and carbonate mobilization were produced under such humid periods generating extensive paleosols of regional distribution. They were interrupted by erosive episodes and periods of eolian input. The soil developed during the Middle Holocene is a distinct pedostratigraphic marker identified in the main geomorphologic units of the PES. The soil appears on top of eolian formations correlated to MIS 2 and it is represented by a moderately developed brown Bt-horizon. In general, the soil was truncated by erosion and it is covered by a loessic formation (late Holocene in age). In a minor geomorphologic unit (fluvial valleys) that soil forms an accretionary pedocomplex of argillic horizons separated by the accumulation of Andean volcanic ash, concentrated by alluvial processes. The main differences between the buried soil and the paleosoil correlated to MIS 5a are the significant mobilization of iron and the degree of development (larger in this one). In all major environments, the older soil underwent more important erosion. The MIS 3 period has a weaker although regionally important pedogenic signal.