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Revista de la Asociación Argentina de Sedimentología

versión impresa ISSN 1853-6360


IMBELLONE, Perla A  y  CUMBA, Andrea. A succession of superposed paleosols (Middle-Late Pleistocene and Holocene) in the south area of La Plata. Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.1, pp.03-21. ISSN 1853-6360.

A 11-m deep pedological sedimentary succession (Middle-Late Pleistocene and Holocene) has been studied in a quarry exposure located on a flat interfluve, in the vicinity of La Plata airport, Argentina (34º55´00´´S y 57º57´30´´W). The objective of this work is to characterize and analyze the sedimentary-pedological processes in order to a) obtain new evidences that allow to discuss the sedimentary-pedological model proposed by Teruggi and Imbellone (1987); b) record the correlation of the events on the perimeter of the quarry and c) correlate the presence of vitroclast-rich sediments with other profiles of La Plata zone. The pedologicalsedimentary units were described and correlated in the different cut-faces of the excavation; the units were also sampled in order to characterize particle size distribution and micromorphology. Six sedimentary pedologic units constituted by loessoid sediments and associated paleosols are identified. They may correspond to La Postrera Fm and Buenos Aires Fm, which are found at 2.10 and 4.45 m depth, respectively. The Ensenada Fm may be found immediately below. The thickness varies from 2.85 to 2.30 m in the bottom units and 1.15 and 2.10 m in the upper units. Lenses with very distinct hydromorphic features are observed between 4.00-4.50 m and 9.90- 10.90 m depth. Each unit is separated from the underlying one by erosive surfaces that can be: a) smoothly wavy and discontinuous, b) wavy and continuous, or c) planar and abrupt. The upper part of the succession, up to 4.45 m depth, shows subtler morphological changes than the lower part. resent soil, separated in one case by a smooth erosion surface and in the other by a temporal discontinuity insinuated only by changes in color and calcrete characteristics. From 4.45 m downwards, the succession reflects very defined and identifiable sedimentary-pedological events, clearly detectable in the field by morphological features and a well defined lateral extension. Three buried paleosols, separated from the overlying materials by clearly expressed erosive surfaces, have been identified. Two of them, at 4.45 and 9.60 m depth, are wavy, accompanied by carbonate concretions, while the other, at 7.30 m depth, is totally planar and is interrupted in its lateral continuity only by megakrotovines. The lateral variation of the paleosurfaces shows units of concordant, parallel and subparallel contacts with scarce variation in thickness among the sedimentary pedologic units. The original thickness of the bottom units was reduced by erosion, although it is not possible to ascertain the magnitude of the erosive event. The paleosols associated to each sedimentary deposit present similar illuviation features with minimum variations in their distinctness, thus allowing horizons to be identified easily. The most variable features among the different cut-faces of the quarry are those corresponding to the processes of calcification and hydromorphism, whose degree of expression changes from weak to strong. The sedimentary-pedologic cyclicity is best defined at the bottom of the succession. The textural classes of the materials always have a high quantity of silt, ranging usually from silty clay to silt loam, while the loam class is rarely found. Sedimentologically, the material corresponds to reworked loess. This is corroborated microscopically by the presence of clasts in the horizon matrix, pedologic textural features and laminate microstructure of the sediment. The modal interval in the whole succession is 62 - 31 mm (coarse silt). In the sands, the predominant interval is 62 - 125 mm (very fine sand) with scarce contribution of the coarser subfractions. Sand increased in all the subfractions in the lower half of the succession due to an increase of pseudo-particles. In general, it is possibly to separate two large sedimentary cycles: the upper one, up to 4.45 m depth, is finer, with sand content averaging 10 - 16% in the original sediment; the other cycle is coarser, with 15 - 43% sand (free-clay fraction). Micromorphology has been instrumental in the verification of: a) superimposed pedogenetic processes of illuviation, hydromorfism and calcification by the presence and distribution of the textural, amorphous and crystalline features; b) identification of biological features such as pedotubules and excremental fabric, which are very important in bioturbation processes and c) identification of laminar microfabric, sedimentary and pedologic relicts in the soil matrix , thus revealing the reworked character of the loess. In the deeper paleosols, the micromorphological evidences resemble the field observation in their degree of expression, mainly with regard to textural features. Macromorphologically, the superficial paleosols show illuviation evidences on ped faces. However, they are not observed micromorphologically, or at best are very scarce and thin; thus, they likely correspond to very fine friction surfaces; suggested by the best expressed void, grain striated and reticulate b-fabric. In the classic model of episodic sedimentation and pedogenesis, a period of landscape instability, with more or less quick loess sedimentation and another of relative quiescence for pedogenesis, sometimes followed by erosion episodes, are conceptually interpreted. This could be applied to the three deeper paleosols whose profiles are spatially separated with the identified loessic parent material and truncated by an erosive surface whose duration could not be determined. These paleosols would have evolved with a minimum contribution of loess and over a period of pedogenesis long enough to develop a mature soil, with characteristics similar to the present one. In the case of the upper paleosols, a slow continuous or discontinuous contribution of loess possibly occurred, thus allowing the formation of pedogenic features while the landscape built up accretionally; a superimposition of pedogenesis took place when the loess mantle was thin. The buried paleosols show pedological features similar to those of the present soil, suggesting that the paleoclimatic conditions during the time involved in the development of the succession have differed more in degree than in type. The study of this succession has provided new data on the sedimentary-pedological characteristics of the eastern region of the loessic Pampean plain. It also emphasizes the possibility of using paleosols as stratigraphic markers.

Palabras clave : Superposed paleosols; Micromorphology; Reworked loess; Middle - Late Pleistocene and Holocene; Buenos Aires Province; Argentina.

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