The parent material as the dominant factor in Holocene pedogenesis in the Uruguay River Basin

  • Martín Iriondo Consejo de Investigaciones Científicas y Técnicas – Facultad de Ingeniería y Ciencias Hídricas, Universidad Nacional del Litoral, C.C. 217, 3000 Santa Fe, Argentina.
  • Daniela Kröhling Consejo de Investigaciones Científicas y Técnicas – Facultad de Ingeniería y Ciencias Hídricas, Universidad Nacional del Litoral, C.C. 217, 3000 Santa Fe, Argentina.
Keywords: buried soil, tropical loess, Holocene, Uruguay River, South America.

Abstract

In the Uruguay River basin (South America; 365,000 km²), an episode of pedogenesis occurred during the Climatic Optimum of the Holocene, approximately between 8,500 and 3,500 yr BP. This period of 5,000 years was characterized by a humid and warm climate. In terms of factors of soil formation, an interesting pattern appears. The climate was relatively homogeneous over the whole basin. The age is the same in all cases and the influence of relief and vegetation was modest. Therefore those factors can be considered as basically uniform. On the contrary, the parent materials form a complex mosaic. The granulometric and mineralogical characteristics of the parent materials dominate the nature of the hypsithermal pedogenesis even at the higher level of the soil classification. Several cases of this regional pattern can be cited. Moderately to well developed B-horizons of five buried soils have been registered in the region. Each of them covers a significant area. A conspicuous Vertisol covers the southwestern of the river basin (in Entre Ríos province, Argentina). The parent material of this buried soil is a clayey playa unit (the Hernandarias Formation, lower Pleistocene in age). A buried soil belonging to the Ultisol order was developed on the Oberá Formation (the late Pleistocene tropical loess) in the north of the region (in Misiones province, Argentina and Rio Grande do Sul state, Brazil). On a late Pleistocene–lower Holocene paludal unit (the Tapebicuá Formation, in eastern Corrientes province) a distinct buried soil with a plinthic yellowish brown horizon was developed. A moderately developed Alfisol formed on the lower fluvial terrace of the Uruguay River (the Holocene Concordia Formation). On the late Pleistocene loess, at the southern end of the basin (in southeastern Entre Ríos province and western Uruguay), a well developed Alfisol occurs, which is similar to the typical soil of the Pampa region. Some profiles are pedocomplexes with clear evidence of soil formation interruption by the accumulation of Andean volcanic ashes.

The hypsithermal soil was truncated by eolian erosion in most of the region. Later, it was covered by a thin loess layer (the San Guillermo Formation, deposited during the Upper Holocene). Both, the remaining B and C horizons and the loess layer are affected by the current incipient pedogenesis under a humid subtropical/tropical climate which began 200 years ago. The only exception to this influence is represented by the Ultisol and the Alfisol (in the fluvial terrace), which are covered by 2–4 m of upper Holocene sediments, and consequently isolated from the present dynamic.

Published
2018-05-18
Section
Regular Papers