Revista de la Asociación Argentina de Sedimentología
version ISSN 1853-6360
SZELAGOWSKI, Manuel; ZARATE, Marcelo A and BLASI, Adriana M. Sedimentologic aspects of eolian sands from the Late Pleistocene-Holocene of the La Pampa Province. Rev. Asoc. Argent. Sedimentol. [online]. 2004, vol.11, n.2, pp. 69-83. ISSN 1853-6360.
The main goal of this paper is to analyze the provenience of sandy eolian deposits of the central eastern part of La Pampa Province. Considering present eolian models proposed by different authors, the hypothesis tested in this paper is that eolian sediments are the result of deflation of alluvial deposits from the Bermejo-Desaguadero-Salado-Curacó fluvial system. With this purpose in mind, three case studies were selected (La Pastoril-Santa Isabel, Toay and General Acha) (Fig. 1). The analysis was made following the concept of spatial scale hierarchy which considered the geological and general environmental setting of the studied areas along with fundamental geomorphological features of the analyzed dune systems. This was complemented by the stratigraphic survey of selected sections from which 20 samples were collected for sedimentological analysis (grain-size and mineralogy). Also, due to the lack of information, three samples were collected from the present Desaguadero fluvial system for general characterization. At the regional scale two main geoforms were identified: an extensive plain and the paleovalley of the Desaguadero fluvial system. The plain grades eastward and includes several longitudinal depressions (longitudinal valleys). A sandy silt mantle is covering the plain, whereas the main dune system is located along the longitudinal depressions and the Desaguadero paleovalley (Fig. 1). In the three studied areas two eolian units, informally named lower and upper, were identified. These units, separated by a buried soil, were generated by winds from the W-NW in Santa Isabel and from the S and SW in General Acha. Wind directions coincide with present wind systems. The upper part of the lower unit is modified by the development of a buried soil showing an A-C horizon sequence. This soil is only buried in areas deeply affected by anthropic activities. Away from these impacted settings, the soil is on the present groundsurface. In Toay, fossil remains of Megatherium americanum were exhumed from the lowermost part of the sampled section which together with regional evidences indicate a late Pleistocene-Holocene age to the lower eolian unit. The upper eolian unit is also restricted to areas deeply modified by human activities, where the vegetation cover was removed and the sediments exposed to eolian reactivation. The occurrence of cultural artifacts (pieces of wire fences, plastic bags) as well as written documents suggest that this eolian reactivation occurred during the XXth century. Based on the stratigraphic results, the sedimentolo-gical analysis was focused on the lower eolian unit. In all three areas the eolian sediments are medium to fine sands with unimodal distribution, transported in saltation and modified saltation or short term suspensions under relatively moderate to high wind shear velocities (Fig. 2; Tables 1 and 2). The most common light components are altered grains (24-56 %), quartz (14-36 %), K-feldspar (2-35 %), volcanic shards (3-23 %) and acid plagioclases (0.5-6 %) (Table 3). Microcline, complex pyroclast (crystals with glass edges) and lithics grains are present in low percentages (up to 3 %). A high percentage of heavy minerals (4.41-36.96 %), is present showing a high variable participation of each component (Table 4). Following the classification by Dott (1964) and modified by Pettijohn et al. (1972), the eolian sediments of Santa Isabel and General Acha may be considered as arkosic sands, those from Toay as lithic sands while the fluvial samples from Santa Isabel and Puelches can be considered as sublithic sands and lithic sands respectively (Fig. 3). However, due to the higher content of volcanic glass shards in most of the samples, the sediments were renamed considering this major component plus their alteration state. Consequently, the sedimentary material is classified as altered arkosic-vitric ash sands and altered lithic-vitric ash sand (Fig. 4). The mineral assemblages of the studied eolian sediments suggest mixed sources of materials, both from distal areas (the Bermejo-Desaguadero-Salado-Curacó fluvial system), and local areas (rock outcrops located in the central part of la Pampa Province). The high degree of roundness shown by the grains, indicates that the transported material was eroded and/or weathered prior to sedimentation, suggesting a policyclic process of reworking and subaereal exposure. Regionally, the areal distribution pattern of sandy eolian deposits with dominance of bedforms (dunes) along the depressions (longitudinal valleys) and paleovalley of the Desaguadero fluvial system suggest a topographic control in the spatial distribution of eolian facies. Likely, these topographic depressions, particularly the longitudinal valleys, acted as wind corridors. At the geographic scale analyzed, the working hypothesis tested oversimplified the complexity of the eolian system.
Keywords : Quaternary; La Pampa; Eolian deposits; Dunes.