Study about the parent material genesis of a vertisol in “La Carolina” unit

Incidence of amphibolites in the proposed process

Abstract

The study of udolls with gilgai microrelief in the soil association “La Carolina” area type has allowed to demonstrate that soils can be developed from basic metamorphic rocks if a previous process of intense weathering took place. As much in soundings as in test pits, it was observed a more than 2 meters siltstone layer over amphibolitic rocks. This layer was produced under warmer and more humit paleoclimatic conditions than the present days ones, because silty rocks contain kaolinite. The age of weathering is about 1,7 millon years B.P. The silstone contains heavy sand minerals (mainly hornblende and epidote) and up to 3 kg blocks of pegmatite composition, showing a flow process after their formation. Over these siltstones – more or less eroded – were developed the udolls with an age of 6000±2000 years, conserved as paleosoils in the Entre Rios province (Argentine) by isostasic relative deeping. At the present time with isohygro regime sustained from 3000±500 years B.P., gilgai microrelief, vertical fractures and slicke slides are conserved, but the double perfil in some zones has disappeared; the A horizon has lost clay material and developed a Bt horizon. The most important geologic finding with a big agricole importance is that the siltstone over which is developed the analized udoll is the product of amphibolite weathering and not the result of aeolian dust accumulation. Climate evolution predictions in Uruguay make very feasible the hypothesis that these melanic soils will lose the vertic characteristics resembling brunosoles, maintening their present fertility and favoring its handling. This study allows at else to suggest why the association vertisoles-brunosoles without defined limits, is so common in Uruguay under isohygro climate conditions.