PROPIEDADES Y GÉNESIS DE LAS ACUMULACIONES DE CARBONATOS EN ARIDISOLES DEL CENTRO-ESTE DEL CHUBUT

BOUZA, P.J.; DEL VALLE, H. F.

Suelos con acumulaciones calcáreas y yesíficas de Argentina

Asociación Argentina de la Ciencia del Suelo

Lugar: La Plata; Año: 2014; p. 199 - 219

In the present contribution is made known the genesis of broadly distributed carbonate accumulations in soils of east-central Chubut through of morphological, physical, chemical, geochemical (stable isotopes), and mineralogical properties. The studied soils are associated with littoral terraces formed during the MIS 5e (Caleta Valdés Formation, CV) and with glacio-fluvial and pluvio-fluvial terraces of Plio-Pleistocene age (Rodados Patagonicos, RP). The carbonate content increases with the age of the geomorphic surfaces (terraces), constituting a tool to correlate soils and paleosols (chronological sequence). The accumulations of pedogenetic carbonates are revealed by the presence of typical morphological features, as: laminar structure, calcitic nodules and pendants. At microscopic and sub-microscopic scale, pellets, ooids and alpha (abiotic origin) and beta (biologic origin) microfabrics were observed. The alpha microfabric is composed by floating coarse mineral components, circumgranular cracks, matrix nodules and rhombohedral crystals (micritic groundmass). The beta microfabric is composed by calcified filaments, calcispheres and needle-fiber calcite (evidence of fungi activity). Palygorskite and sepiolite were identified in calcic and petrocalcic horizons developed on RP surfaces. Pedogenetic carbonate was qualified as low-Mg calcite, indicating that during its precipitation, the Mg2+ activity increased in the soil solution, favoring the smectite palygorskite transformation. The soil environment, favorable for this transformation, was the textural transition between the fine materials of sub-surface horizons and the coarsest deposit of RP, where temporary waterlogging occurred. The petrocalcic horizons, and their re-transported fragments, represent the oldest pedogenetic period, where sepiolite was the dominant clay mineral. During the calcretization processes, the sepiolite was precipitated from the soil solution following the formation of palygorskite. Fluorite was identified in the petrocalcic horizon, and its association with calcite and sepiolite indicated a successive precipitation of these minerals under alkaline conditions during evaporation processes. The analysis of the isotopic compositions of d13C and d18O showed an acceptable positive correlation (r2 = 0.5, p