Abstract

TRIPALDI, Alfonsina et al. Sedimentología de arenas eólicas cuaternarias del área de Lomas de la Estancia, provincia de La Rioja (Argentina): un ejemplo de sedimentación en campos de dunas intermontanas. Rev. Asoc. Argent. Sedimentol. [online]. 1998, vol.5, n.1, pp.21-40. ISSN 1853-6360.

Sedimentological characteristics of an aeolian intermontane field located in the Lomas de la Estancia, La Rioja Province, northwest Argentina are analized (Figs. 1 and 2). The aeolian field includes five geomorphic units: 1) dunes, 2) interdunes, 3) blowouts, 4) aeolian sand sheets and 5) fluvial-reworked deposits (Table 1). Grain size distribution of each unit was achieved by dry sieving into ¼ phi fractions for sizes > 53 mm, and pipette analysis for sizes < 53 mm. Dune deposits correspond to transversal dunes, mostly composed of moderately well sorted fine and very fine sands (Table 3). These sands are positively skewed with scarce material finer than 56 mm (1,71% in average). At the present time, dunes are “frozen” and deflationary features have begun to develop in different parts of the dune profile. The blowout unit comprises two major deflationary features: saucer blowouts (here named "hoyos de voladura") and trough blowouts (here named "canaletas de deflación"). Both are formed by fine and medium sands with modal size of 1,85 f, exhibiting good to very good sorting with symmetrical distributions (SK1 = 0,08, Table 4). A small active aeolian sand sheet appears in the northeast of the studied area. It forms a flat to gently undulating topography that includes sand shadows, small gozes and different types of aeolian ripples. Mean grain size ranges from 1,97 f to 2,38 f and bulk samples are moderately sorted (Table 5). Interdunes form narrow muddy plains parallel to the toe of the dunes. They are much fine-grained than dunes and sand sheets accumulations, with a mean grain size of 4,22 f, and very poorly sorted (Table 6). Two small ephemeral fluvial channels appear in the north-northeast of the aeolian field. They show a grain size distribution similar to the dune and blowout deposits, with intermediate sorting and variable amounts of fine sediments (0,95% to 5,00 %, Table 7). The textural characteristics of the whole dune field show similar mean, mode, sorting and skewness than aeolian sand seas studied by different authors (Table 8 and Fig. 4). The main difference appears in the high proportion of fine material (finer than 53 mm) found in the aeolian sand sheet, which is interpreted as supplied from the closely associated alluvial plain. As far as the different deflationary forms (saucer and trough blowouts) are concerned, they are interpreted as originated by differential acceleration of the airflow along the windward slope (Fig. 7). In particular, trough blowouts would be produced by a flow acceleration corresponding to the area of maximum compression on the lower slope, while saucer blowouts would correspond to an increased erosion in the crestal region. In both cases, the very good sorting and symmetrical distributions must be interpreted as a consequence of the differential deflation of fine sediments (Fig. 6c). Environmental conditions favouring accumulation in the active sand sheet were not suitable for dune development. Three major causes are considered: 1) presence of vegetation that produce multidirectional surface airflow, 2) periodic flooding with reworking of dune sand, and 3) surface cementation in the alluvial plains that limits the quantity of loose sands available for dune building. Finally, an evolutionary model is proposed for the Lomas de La Estancia aeolian field (Fig. 10). It comprises three major stages: 1) during very arid conditions, dunes would move through the intermontane valley forming large aeolian fields dominated by transversal dunes (Late Pleistocene-Early Holocene?); 2) with a small climatic change, the sand sheet becomes the principal depositional area and dune development is inhibited (present conditions); and 3) if the climate changes to humid conditions, dune and sand sheet deposits would be “frozen” and they will probably be subjected to deflationary processes and fluvial erosion.

Keywords : Aeolian sedimentation; Dunes; Blowouts; Pleistocene.

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