Abstract

MORRAS, Héctor J.M. Distribution and origin of surface loessic sediments of Northern Pampa based on sand mineralogy: Preliminary results. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.1, pp.54-64. ISSN 1853-6360.

Mineralogical information about the sand fraction (62- 246 µm), available at the Institute of Soils of INTA-Castelar, was analyzed by selection of 92 representative samples of deep (BC or C) horizons of soils from the Northern Pampa Region developed on loessic materials. On the basis of the observed quantitative variations and their common usage as sedimentological indicators, four mineralogical parameters were selected: quartz / feldspar + glass index; % heavy minerals; % pyroxenes; and % micas (Table 1). These parameters were then used to differentiate and cartographically group the data into sectors using prior information in the literature and statistical techniques (Figures 1 to 4). Finally a map was developed that integrates the information for the surficial sediments (Figure 5). Area A, located in the western portion of the region, occurs north of Buenos Aires, southwest of Santa Fe, and south east of Córdoba. This area is characterized by a high relative proportion of volcanic glass and feldspars [index (Q / F + V).100, less than 30], a high content of pyroxenes (more than 16 %), and a high percentage of total heavy minerals (more than 12 %). A strip towards the east that borders the Paraná River and the de la Plata River is characterized by higher proportions of quartz and lower proportions of pyroxenes and heavy minerals than the soils in area A. Furthermore the differences observed in the proportion of micas within this strip, which borders the fluvial axis, can be differentiated into two areas. In the first one northeast of Buenos Aires, area B, the percentages of mica are relatively low (less than 7 %) and similar to those in the area A. Conversely, an area southeast of Santa Fe and a small part NE of Buenos Aires, area C, has a markedly higher proportion of micas (more than 7 %) than area B. The mineralogical association dominant in area A coincides in general with classical Pampean loess that has been characterized by several authors. The high proportion of plagioclases, volcanic glass, and pyroxenes suggests a volcanic-pyroclastic association of Andean origin. Besides direct eolian contribution of ashes, recent work suggests diverse stages of fluvial and eolian transport of the materials until their sedimentation in the Pampean plain. In areas B and C near the Paraná - de la Plata Rivers, sand characteristics may be differentiated from those in area A by variation in sedimentary origin. The lower content of volcanoclastic minerals and total heavy minerals may be related to a relative diminution of the Andean contributions due to the great distance from the source and the occurrence of preferential selection during transport. Secondly due to the proximity of areas B and C to the Parana - de la Plata axis, a significant component of the sand fraction may be derived from the Paraná Basin. The quartz and heavy minerals contents are similar to the characteristics of the sediments of the alluvial plain of this fluvial system. Although the processes of supply of these materials to the surficial sediments of the Undulating Pampa are not obvious, one possibility is eolian re-distribution of the sediments of the Paraná's fluvial plain, particularly during the dry periods of the recent Quaternary. The relatively high mica content southeast of Santa Fe, area C, may be due to contributions from the local Pampean Sierras, possibly moved by fluvial transport along the Carcarañá River and redistribution by wind. Thus, the sediments in area C may well be a mixture of materials from three sources resulting in a compositional complexity that coincides with the complexity of the geomorphological processes active in this region. Nevertheless, there is a minority of samples and profiles in this strip near the fluvial axis (areas B and C) with a composition more similar to those of area A, and thus, to the classical Pampean loess of Andean provenance. The great compositional heterogeneity of this strip, in addition to the contrast in mica content between areas B and C, is clearly evidenced by statistically greater coefficients of variation for the characteristics of the sand fraction. Vertical and horizontal heterogeneities suggest the existence of distinct areas with varying contributions from the possible various sources. Our conclusions, derived from the mineralogical analysis of the sand fraction, substantiate a multiple origin for the sediments and suggest a great complexity of processes responsible for the transport and deposition of surficial loessic sediments in the Northern Pampa.

Keywords : Northern Pampa; Loess; Soils; Sand fraction; Mineralogy.

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