Resumo

TRIPALDI, Alfonsina. Sedimentology and evolution of Médanos Grandes dunefield (San Juan province, Argentina). Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.1, pp.65-82. ISSN 1853-6360.

Médanos Grandes dunefield, which is located near Vallecito city at the southeast of San Juan province (Argentina), is one of the largest eolian depositional systems in Argentina. It is formed by draas together with transversal and longitudinal dunes and large blowouts. Alluvial plain deposits related to San Juan and Bermejo rivers, and the Pie de Palo and Valle Fértil hills sourround the eolian system (Fig. 1). Previous works are very scarce (Groeber, 1937; Rodríguez, 1966), being Regairaz et al. (1987) the ones who described the dunefield as made up of complex dunes. The present study can be divided in three steps. Firstly, remote sensing images analysis were carried out to define and describe geomorphic units. Secondly, granulometric and compositional studies allowed characterizing the deposits that formed the eolian bedforms. Finally, a tentative model of the eolian system evolution is also proposed. The positive relief is formed by two mountain belts, the Pie de Palo range to the north and the Valle Fertil one to the east. The former is composed of Precambrian-Early Paleozoic anfibolites, maffic and ultramaffic rocks, migmatites and gneisses (Pie de Palo Complex) and the latter (Valle Fértil Complex) by crystalline rocks as well together with sedimentary rocks of Cretaceous and Triassic ages. Low areas are covered by Pleistocene to recent alluvial fan, alluvial plain and eolian sediments (Fig. 1). The eolian deposits include sandsheets and dunefields with several kinds of bedforms that predominate at the southwest of the valley. Médanos Grandes dunefield shows several patterns of dunes and draas of different types, size, spacing and alignment. Based on these configurations five geomorphic units have been defined and mapped (Figs. 3 and 4). Geomorphic unit 1 (UG1), which formed the north part of the dunefield is considered to be the oldest. It is made up of 55-meter tall and up to 2000-meter spacing, transversal draas with superimposed dunes (Fig. 5). The draas orientation, as well as that of the superimposed dunes, suggests southwestern paleowinds during their formation. Geomorphic unit 2 (UG2) shows similar transversal draas, better developed and with their slipface looking to the southwest. Herein there are also minor dunes over the megaforms, but with an oblique alignment in relation to the draas (Fig. 6). Therefore these smaller bedforms are considered as younger deposits. Geomorphic unit 3 (UG3, Fig. 7 and 8) has conspicuos erosional features characterized by large circular and triangular blowouts. Interference of crest lines belonging to draas of UG1 and UG2 probably produced enough wind acceleration to remove sand grains and develop the described blowouts. Geomorphic unit 4 (UG4) includes 40-meter tall and 150-meter spacing, longitudinal dunes with small transversal dunes in the interdune areas (Fig. 9). In this case dune alignment suggests paleowind from the south-southeast. Finally, geomorphic unit 5 (UG5), the youngest, is composed of transversal, barchanoid and longitudinal dunes (Fig. 10) and is related to present winds from the south-southeast. Grain-size characteristics of 78 samples corresponding to the eolian deposits were obtained by dry sieving, at 0.25 ø intervals. Mean and sorting were calculated following momentum method, whereas skewness, kurtosis, 1 % percentil and amount of material lower than 53 μ by Folk and Ward's (1957) graphical formulas. Textural parameters show that draas and dunes are made up of very good sorted (s=0,45 ø), fine to very fine (x=2,88 ø) sands, with skewness of nearly zero (SK 1 =0,05, Tables 1 to 4). They have less than 1 % of material lower that 53 μ and the average 1% percentil corresponds to medium sand (1,87 ø). These characteristics are very similar to those of other eolian sediments studied by several authors (Ahlbrandt, 1979; Chaudri y Khan, 1980; Brookfield, 1992; Limarino y Martínez, 1992; Tripaldi et al., 1998; among others). Despite the fact that UG1 and UG2 grain size distributions are similar, when compared in detail they show interesting differences. UG2 is coarser than UG1 and has better sorting and lower amount of clay and silt. Although, these differences could be the consequence of the different supply areas that have both units, a selective deflation of fine sediments during the UG2 formation must not be discharged. Modal analyses of the sand grain composition show that the eolian sediments are dominated by lithic fragments (in order of importance: volcanites, schists, anfibolites, chert, sedimentites and limestones clasts) together with low to moderate percentages of feldspar and scarce amount of quartz (Fig. 13). This composition suggests two major areas of provenance: 1) the Precordillera-Cordillera Frontal area to the west where volcanites and limestones are frequent and 2) the Sierras Pampeanas area that includes the above mentioned Pie de Palo and Valle Fertil mountains composed of crystalline rocks. Médanos Grandes dunefield had a complex evolution as pointed out by the presence of different generations of dunes, each of them represented by the above described geomorphic units, that were built up under a diverse paleowind regimen. The geomorphic units are here tentatively correlated with the climatic stages proposed by Iriondo y Kröhling (1996) in the pampean plain (Fig. 14). According to those authors, during the isotopic stage 4 (between 77.000 and 60.000 years B.P.) arid conditions were established related with glacial advance at the Andes. It is likely that Médanos Grandes dunefield, with draas and dunes migrating to the northeast (UG1), were given rise during this period favored not only by the aridness but also by a high sediment supply. A second generation of dunes could be related to a subsequently arid period that occured, according to Iriondo y Kröhling (1996), between 36.000 and 15.000 years B.P (isotopic stage 2). Due to the irregular relief in the area, reworking of previous draas to form new megaform migrating to the southwest (UG2) was not complete and therefore an interference zone, characterized by the triangular pattern and blowouts of UG3, was established. During a third state, probably associated with the arid period in the upper Holocene (between 3.500 and 1.400 years B.P, Iriondo y Kröhling, 1996) longitudinal dunes (UG4) and transversal, longitudinal and barchan dunes (UG5) were formed. Although at the begining the eolian system was likely to be more extensive, at present Médanos Grandes dunefield, as well as Médanos de la Chacras dunefield that occured to the north (Fig. 14), could be considered a remaining because of fluvial erosion.

Palavras-chave : Eolian sedimentation; Dunes; Draas; Pleistocene; San Juan province.

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