CONICET | Buscador de Institutos y Recursos Humanos

The Koluel Kaike Formation (KKF) is a thin (40 m) continental unit from Central Patagonia. It is constituted by silicified pyroclastic mudstones and bentonites, mostly modified to ochre or reddish paleosols. Caolinite is dominant in the lower section, and decreases in the upper one along with smectite increase (Raigemborn, 2004). The paleosols are preliminary analyzed in three Chubut localities (Las Flores, Km 170, ¨Pique de Manganeso¨) for a better comprehension of the climatic and environmental conditions. They present defined horizons, with blocky or platy peds or massive structure. The main structural elements are mottles and both; nodules and concretions. All of these elements are included in a whitish grey to light green, fine pyroclastic matrix. Such features define three recurrent pedogenic patterns, which frequently grade lateral and vertically. The mottles are abundant (> 20 %), middle to coarse, orange to red, with sharp boundaries. The Pattern I shows, in vertical section, a reticulated mottling, which remains to bricks of tens cm by side. In plane, it shows polygonal shapes. The Pattern II presents vertically elongated mottles, 2-8 cm wide and up to 1,10 m high, with occasionally short ramifications downward. Pattern II has been only recorded in the lower section. The concretions and nodules range from yellowish orange to red. They are constituted by Fe sesquioxides, forming hardened horizons. In less proportion, black concretions constituted by Mn oxides, also occur. The presence of abundant ferric nodules and concretions defines the Pattern III horizon. Frequent argillans were observed in thin-sections, suggesting that significant clay illuviation (Bt horizon) took place. Alumina/bases ratio ranges from 9 to 16 in the lower section, and decreases to less than 4 in the upper one. Ba/Sr ratio is always high. It decreases up in the succession, suggesting that development time, leaching and drainage were higher in the lower section paleosols. The following reconstructed profile is proposed for the KKF paleosols: Upper horizon: 0,7 m with nodular Pattern III; Upper Middle horizon:1, 6 m with Pattern I mottling; Lower Middle horizon: 2,56 m with Pattern II mottling; Lower Horizon: 0,9 m constituted by a massive, whitish pyroclastic mudstones. This profile is recognized in the three studied localities, but not always in a complete form. The described pedofeatures suggest the presence of dominant hydromorphic processes. Both, mottles and concretions generate by Fe reorganization due to moisture changes. Fe and Mn reduce and mobilize when the soil is water saturated, and oxidize and immobilize when the soil is dry, depositing in opened places to air circulation. In this sense, Patterns I and II would represent poral spaces associated to root systems which define "rooting patterns". The analyzed sequence would correspond to long-weathering time soils, formed in a warm humid and seasonal climate. Comparisons to lateritic profiles (Tardy 1992), allow recognize an upper glaebular zone (Pattern I/III); a lower glaebular zone (Pattern II); and a basal (?) alteration zone (massive pyroclastic mudstone). Molecular weathering ratios also demonstrate that the KKF paleosols underwent strong leaching. Changes in clay composition suggest a rainfall decrease during deposition of the upper section. The structural, geochemical and clay-mineral evidences would indicate the presence of strongly developed soils in the KKF: Ultisols-Oxisols in the lower section, and Ultisols in the upper one. These soils generally support vegetation ranging from coniferous or hardwood to rain forest. It is probably that the Pattern II mottling; in the lower section; represent paleosols with deeply penetrating roots of trees. ; Lower Horizon: 0,9 m constituted by a massive, whitish pyroclastic mudstones. This profile is recognized in the three studied localities, but not always in a complete form. The described pedofeatures suggest the presence of dominant hydromorphic processes. Both, mottles and concretions generate by Fe reorganization due to moisture changes. Fe and Mn reduce and mobilize when the soil is water saturated, and oxidize and immobilize when the soil is dry, depositing in opened places to air circulation. In this sense, Patterns I and II would represent poral spaces associated to root systems which define "rooting patterns". The analyzed sequence would correspond to long-weathering time soils, formed in a warm humid and seasonal climate. Comparisons to lateritic profiles (Tardy 1992), allow recognize an upper glaebular zone (Pattern I/III); a lower glaebular zone (Pattern II); and a basal (?) alteration zone (massive pyroclastic mudstone). Molecular weathering ratios also demonstrate that the KKF paleosols underwent strong leaching. Changes in clay composition suggest a rainfall decrease during deposition of the upper section. The structural, geochemical and clay-mineral evidences would indicate the presence of strongly developed soils in the KKF: Ultisols-Oxisols in the lower section, and Ultisols in the upper one. These soils generally support vegetation ranging from coniferous or hardwood to rain forest. It is probably that the Pattern II mottling; in the lower section; represent paleosols with deeply penetrating roots of trees.

enviar mensaje