Polygenic vertisols and "Hidden" vertisols of the Paraná River Basin Argentina

Abstract

Vertisols and soils with vertic properties occupy considerable areas in the east of the Pampa Region. Many of thèse Vertisols have particular features that dif- ferentiate them from most of the Vertisols in the world. In the Mesopotamian Pampa, Vertisols have developed from a calcareous and gypsiferous loam, later covered by a thin loess mantle. Here it is considered that Vertisols would have been the domi- nant soils in the different segments of a hilly landscape. Later, under humid climate, and due to the buffering effect of the non-expansive silty loessic surface, clay illu- viation would have been a generalized process across the landscape. At the same time erosion processes took place. The conservation of the thin layer of loessic sedi- ments on the higher slopes resulted in the formation of Vertic Alfisols and few Mollisols, while its erosion in the backslopes caused the exhumation of buried Vertisols. On the other hand, in the High Undulating Pampa, there are no Vertisols but there are Vertic Argiudolls. Two superficial sedimentary levels have been distin- guished here. The lower one is a smectitic loessic sediment, covered by a relatively thick illitic loessic deposit. New studies undertaken in large pits have revealed the existence of diapiric structures in the lower sediment, which remained “hidden” up to the present. Therefore, it can now be considered that Vertisols were also the domi- nant soils here, later buried by the thick loess. In the humid periods, the summit of convex slopes would have been partly eroded, leaving the underlying sinectitic material closer to the surface. Consequently, current Mollisols on top of the land- scape developed vertic properties due to the mixing of inaterials and depending on the greater or lesser proximity to the paleosurface. Therefore, thèse pampean Vertisols and vertic Mollisols and Alfisols can be considered polygenic and related by different degrees of a same process. These advances in the understanding of the landscape and soils together with the quantitative analysis of soil profiles data, appear highly useful to distinguish vertic soils at the series level and improve sur- veying and mapping work.