The Bajos Submeridionales basin comprises an extensive hydrological system in the eastern Chaco region of Argentina, which occupies the northern part of Santa Fe, the southern part of Chaco and eastern part of Santiago del Estero (Fig. 1). It is a large plain characterized by its low slope and the presence of fluvial, lacustrine and eolian sediments. Different source areas for the sediments have been proposed, as well as geomorphological processes controlled by regional climatic and tectonic dynamics. The aim of this study was to evaluate the influence of these processes in the genesis and distribution of parent materials and soils in the northwestern edge of the Bajos Submeridionales basin. For this purpose micromorphological studies in soil thin-sections, mineralogical analyzes by X-ray diffractometry (XRD) in bulk samples and in the clay-fraction samples, magnetic susceptibility (MS) measurements and soil physical and chemical analyzes (texture, organic matter, cation exchange capacity (CEC) and soil moisture equivalent) were performed. Four soil profiles developed in different landscape positions were studied: 1) Entic Haplustoll (C-50) in the gently undulating plain unit; 2) Typic Argiustoll (C-16) in the slope unit; 3) Typic Haplustert (C-53) in the low plane unit; and 4) Typic Endoacuert (C-27) in the drainage network (Fig. 2 and Table 1). Unlike the loess of Chaco region (Urundel Formation), microscopic studies show a high proportion of fresh acid volcanic glass in the coarser fractions. This composition relates it to the loess of Tezanos Pinto Formation, which covers the central and southern sector of the Bajos Submeridionales basin. Granulometric analyses show that the A horizon of the Haplustoll, Argiustoll and Haplustert soils have a finer texture than the subsurface B and C horizons. This could be related to the presence of lithological discontinuities in the parent materials. Effectively, both the high values of the estimated CEC for the clay-fraction and the soil moisture equivalent suggest an underestimation of the clay percentage in the granulometric analysis of the subsurface horizons (Table 2). In turn, micromorphological studies showed the presence of pseudo-sands and pseudo-silts and a high proportion of larger microaggregates of reworked soil material (i.e. Fig. 3b, Fig. 4c-d, Fig. 5b-c and Fig. 6b) in the subsurface horizons, in addition to inherited pedofeatures such as fragmented and deformed clay coatings (Fig. 3c-d). These features suggest that the soils parent material is constituted by sediments that has been eroded from pre-existing soils and transported to its current position. In turn, the angular- and subangular-blocky structure observed at the base of the Haplustoll (Fig. 4a) and the Haplustert (Fig. 6c) profiles, seems to constitute in situ soil relicts. The set of these characteristics of inherited microstructure and pedofeatures indicate the presence of a pedosedimentary paleosurface at the base of these profiles. For all profiles, the magnetic susceptibility values show two main sections separated by the paleosurface (Fig. 10). These first magnitude differences in the MS values are related to the characteristics and composition of the parent material. In Section I, the highest values are related to the volcaniclastic minerals of the regional loessic sediments and to the pedosedimentary components. This Section I is subdivided into three parts (Sub-section Ia, Ib and Ic) according to smaller magnitude variations, which are can be related to pedogenetic processes. In turn, the magnetic signal in Section II shows sharp and progressive decreases towards the base of the profiles. This variation in depth could be explained both by the increase in calcium carbonate and by the presence of sediments from local supply areas, such as the sandstones and the alluvial deposits of the Salado River. The XRD diagrams show the presence of illite, kaolinite and irregular interstratified of illite-smectite minerals (I-S) (Fig. 9). These minerals are found in different proportions along the profiles. The superficial A-horizons of the Haplustoll, Haplustert and Endoacuert show a dominance of illite, which can be related to the regional loessic sediments. Meanwhile, the pedosedimentary materials show a higher proportion of I-S minerals with smectite dominance. This be would related to both previous pedogenetic processes as well as the process of enrichment in smectites by transformation in the present soil, favored by the slightly alkaline chemical environment in the middle and basal sections of the profiles. The origin of the pedosediment could be linked to the erosion of a paleosurface, given by the tectonic dynamics and the climatic changes occurred during the Pleistocene. On the other hand, the mineralogical composition of the superficial A-horizons suggests that they could derive from the aeolian accumulation of sediments eroded by wind from the river channels, during an arid Holocene event. Then, the pedogenesis of these deposits would have occurred in a more humid recent event. These interpretations allow linking the pedological processes with the tectonic, sedimentary and climatic recent Quaternary cycles that occurred in this sector of the Bajos Submeridionales basin.
La cuenca de los Bajos Submeridionales consiste en un extenso sistema hidrológico para el cual han sido propuestas distintas áreas de aporte sedimentario, así como también, procesos geomorfológicos controlados por la dinámica climática y tectónica regional. Con la finalidad de profundizar en el origen y distribución de los materiales parentales y la génesis de los suelos en el sector noroccidental de la cuenca, se estudia la micromorfología, la composición mineralógica, se determina la susceptibilidad magnética y se discuten datos analíticos físicos y químicos de cuatro perfiles de suelo desarrollados en diferentes posiciones del paisaje. Las observaciones microscópicas revelaron la presencia de vidrio volcánico ácido fresco, a diferencia de lo señalado para el loess de la región chaqueña. Por su parte, los resultados analíticos muestran que en general los horizontes superficiales A son los de textura más fina. No obstante, los elevados valores de la capacidad de intercambio catiónico (CIC) de la fracción arcilla y la humedad equivalente sugieren una subestimación del porcentaje de las fracciones finas en el análisis granulométrico de los horizontes subsuperficiales (B y C). Los estudios micromorfológicos constataron la presencia de pseudo-arenas y pseudo-limos en estos últimos, además de pedorrasgos relícticos tales como fragmentos de revestimientos dispersos y abundantes microagregados de suelo retrabajados, revelando la presencia de una paleosuperficie pedosedimentaria en la base de los perfiles. Los horizontes B y C del suelo actual, desarrollados en el pedosedimento, presentan una mayor proporción de minerales interestratificados con dominancia de esmectita y un incremento de la susceptibilidad magnética respecto a los horizontes sobreyacentes. Su origen podría vincularse a la erosión de suelos preexistentes, proceso controlado por la dinámica tectónica y los cambios climáticos ocurridos en el Pleistoceno. Por otro lado, la similitud composicional determinada para los horizontes superficiales, sugiere que éstos podrían derivar de la acumulación de sedimentos provenientes de la erosión eólica de los cauces fluviales durante un ciclo árido del Holoceno. Luego, en un período reciente más húmedo habría tenido lugar la pedogénesis de estos depósitos. Estas interpretaciones permitirían vincular los ciclos climáticos, con los procesos tectónicos, sedimentarios y pedológicos ocurridos en el sector noroccidental de la cuenca.