Revista de la Asociación Argentina de Sedimentología
version ISSN 1853-6360
MANASSERO, M; CAMILION, C and RONCO, A. Grain-size analysis of down-river bottom sediments within the «Pampa Ondulada», Buenos Aires Province, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2004, vol.11, n.2, pp. 57-68. ISSN 1853-6360.
The dynamics of bottom fluvial sediments play an important role in environmental studies, considering specially the fact that they may act as natural sinks of pollutants, and also as transporting agents. In this way, the transit of these finer sediments may result in a danger to the environmental quality of downcurrent areas of a basin. The size of the component particles is then, one of the fundamental textural characteristics of all fragmented deposits and their lithified equivalents (Passega, 1957, 1977, Day, 1965, Carver, 1971). In this contribution, bottom sediments sampled in equivalent distal positions of seven different fluvial basins of the Pampa Plains (Pampa Ondulada) of Argentina have been studied (Fig. 1). All the rivers flow from west to east within the Province of Buenos Aires reaching either the Paraná River or the Río de la Plata Estuary (Fidalgo and Martinez, 1983). Listed from north to south they are: del Medio, Ramallo, Tala, Arrecifes, Areco (northern sector) and Carnaval-Martín, del Gato and del Pescado (southern sector) (see also Fig. 2). Grain size determination of cores, taken at a water depth of 1 m in both margins, have been analysed (Fig. 3). The cores were divided in an upper layer (0-10 cm) and a lower layer (10-20 cm). Statistical parameters do not show large differences; the mean and the median are similar corresponding from coarse to very coarse silt (5-6 to 4-5 φ). In spite of this, most of the differences are shown in the percentile one (C) values (silts and sands), poor to moderate sorting (standard deviation), high kurtosis (concentration of peakness of the distribution) and positive skewness (Limarino and Martinez, 1992, Tripaldi, 2002) (Table 1). In this respect, the C-M diagram is specially usefull to discriminate different types of fluvial environments considering textural parameters and sedimentary procesess. From this diagram two types of rivers are recognized based on the characteristics of their bottom sediments (Figs. 4-6). River sandy bottom sediments with a tipically fluvial inprint, rolling-sliding, saltation and suspension populations are well defined (C different from M), suggesting important changes in the hydraulic regime. These rivers (del Medio, Arrecifes, Areco, Martín-Carnaval, and del Gato) are located within areas with stepper regional slopes and show complex provenance. In river silty bottom sediments that show finer textures, only well developed saltation and suspension populations are present (C similar to M), with moderate sorting, suggesting a steady flow and slow hidraulic regimes, where settling procesess dominate. These rivers (mainly del Tala, and some samples from Ramallo and del Pescado) are associated with areas with smaller local slopes and show generally a simple provenance. The provenance of these sediments is interpreted, in the former, as complex source produced by both water and wind erosion in an extense basins within the Pampa Plains, while in the latter, the source is poorly reworked eolian material (lime) (Ahlbrant, 1979, Pye, 1987, Bidart, 1992, 1996). All these sedimentary rocks are included within the depositional model of the Pampa Type Loess, where winds derived from the Patagonian ice field in the SE, during the Late Quaternary, formed a large sand sea and a loess belt behind it. The source rocks of these sediments are both the Andean Cordillera and the Pampean Range (Iriondo, 1987, Iriondo and Kröhling, 1996, Kröhling and Orfeo, 2002).
Keywords : Fluvial bottom sediments; Grain-size; C-M diagrams; Pampean area.