Feições eólicas quaternárias e vulnerabilidades agrícolas em áreas de arenização no Pampa Brasileiro

Tania Gomes 1, *
* Corresponding author
Abstract : In southern Brazil, the southwest of Rio Grande do Sul exposes sand spots embedded in Brazilian Pampa landscape, a biome that represents 63% of the state's territory. Both stable and / or unstable wind deposits represent the inheritance of a drier climate than the current one and provide important paleoclimatic evidences for understanding this landscape. In contemporary climatic conditions, the region maintains hill fields (coxilhas, in the regional denomination), covered by herbaceous-shrub vegetation and sand spots, along testimonial slopes and valleys strands, disagreeing with the current wet climate typical landscape. These sand deposits are under wind action, in dry seasons, and subject to superficial and subsurface erosion, in rainy seasons, at once. These agents remobilize the non-consolidated eolic sediments, initiating both processes of arenização (sandisation) and sand sandy stretching. The objective of this thesis is to identify, inside the study area, associations between the current geomorphological features and the quaternary surface formations, as well as the erosive / depositional dynamics acting on these features, in the context of an area historically submitted to agricultural pressure. The spatial cutting area has about 144,000 ha and it is located along the RS-377 highway, between the municipalities of Alegrete and Manoel Viana. Methodological procedures included field, cabinet and laboratory work steps. Field observations led to the initial interpretations on the morphological evolution of stable and / or unstable eolian deposits, as well as to the survey of the four points of collection of sedimentary material, for both granulometric and morphological analyzes and for absolute dating, which was established by optically stimulated luminescence method (SAR protocol). In the cabinet stage, the mappings were carried out, in order to achieve the identification and classification of the eolic deposits and their attributes, the subdivision of the relief units and the mapping of land use and land cover. The sedimentology of the environment shows variability in its characteristics. The sediments are predominantly composed by quartz sands, of Fine sand modal class, varying to lesser fractions of Medium sand and Very fine sand. Both morphological and optical characteristics of the sediments clarify the eolic competence over the particles and their mobilization, demonstrating a certain degree of selection, which is evidenced by the decrease of the grain size in SE-NO direction, due to the reworking and the evolution of Medium sand to Fine sand. Both sub-rounded and rounded grains added percentages up to 70%, which may demonstrate a shorter rework time in an aqueous medium. Associating to roundness, sphericity and surface texture characteristics, it is estimated that both Medium sand and Fine sand grains may have been transported by the winds, displaced from the sector SE-S to N-NO, having been deposited on the region, according to the direction of the paleowind that was measured in field work. However, among grains with wind characteristics there may be grains with typical water action characteristics, such as sediments from Guará Formation or from recent fluvial environments. The geomorphological features distribution associated to the relief units shows that the eolic deposits occur substantially on the tops of hills, followed by dry valleys, reflecting on erosive processes occurrence percentage, which also occur on these compartments, majority (about 37% in hill tops and 23% in dry valleys). Likewise, the sand stretch feature occurs, on a higher percentage, at the tops of hills, followed by the dry valleys, confirming the dynamics of sand stretch activation and formation, which occur in eolic deposits, usually over hills, being triggered by erosive processes, mainly due to the action of water processes (rilling and gullying), which are typical in the current humid climate. In addition, the dunefields that were studied may be associated with broader regional dynamic contexts, present in both Argentinian and Uruguayan Pampas. In relation to land uses, which was mapped between 1982 and 2016, there is a significant increase in the area devoted to crops, in detriment to the field one. Forestry is only observed in 2016, as the incentive for exotic planting began in the late 1970s. Finally, the different uses of hills, associated to both stable and / or unstable wind deposits, determine the environmental dynamics and the economy in the Brazilian Pampa. The Pampas has livestock as its primary aptitude, but the conversion of land use, especially from pasture fields to crop, intensifies the pressure on water resources, on herbaceous vegetation and on sandy soils, which are particularly sensitive to erosion.
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Tania Gomes. Feições eólicas quaternárias e vulnerabilidades agrícolas em áreas de arenização no Pampa Brasileiro. Geography. Universidade Federal Do Rio Grande Do Sul; Le Mans Université, 2019. Portuguese. ⟨tel-02265926⟩

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