Spatially non-uniform landscape responses to late quaternary and present environmental changes in SE-Brazil (1)
|Location||International Geological Congress,oslo 2008|
|Author||Coelho-Netto, Ana Luiza; Avelar, André de Souza|
|Holding Date||03 September 2008|
Long term morphological, stratigraphic and processes studies have been conducted at several tributary basins of Paraiba do Sul river which is one of the regional collector in the Southeastern Brazilian Plateau. Structurally-controlled landforms prevail at both montainous and hilly lowland compartments. Chemical denudation and (pseudo)karst landforms together with shell-shaped and deep valley heads or topographic hollows are found in the mountain areas. On the left Paraiba valley side typical karst landforms are locally found in a heterogeneous and fine grain quartzite layer that overlie a folded metasedimentary sequence (quartzites, philites, schists and gneiss) trending NE-SW; it belongs to the stratigraphically lower Andrelandia tectonic compartment of a regional megasynform. In contrast, mechanical denudation has prevailed in the right valley side being mostly effective in the hilly lowlands. This area is inserted in the upper tectonic compartment and it includes the metasedimentary sequence of Paraíba do Sul Group (banded gneiss, schist and syn- and post-tectonic granites) trending NE-SW and dipping 35 degree NW in average. There is a clear spatial variability of process-operations and erosion rates at both Paraiba valley sides. To the right valley side the hilly lowlands (at Bananal, Barreiro de Baixo and Sesmarias river basins) were submitted to episodic and extremely high erosion rates during the Pleistocene-Holocene environmental transition and, more recently, in response to a regional deforestation due to the spread of coffe plantations in the mid-18th century, followed by cattle grazing in the early 20th century. A delayed response of their respective channel network development is still progressing throughout the unchanneled valleys due to seepage erosion by exfiltration of critical artesian flows within vertical joints. The regressive gully growth onto steep slopes can trigger landslides (slum type) providing large amounts of sediment yield during storm flow periods. To the left hilly valley side, however, the Turvo and Flores basins show no evidences of joint-artesian flows and seepage erosion; gullies did not develop and erosion rates are relatively lower. Process-operations in these two late basins are mostly associated to river bank scouring or roadcuts both of which can trigger shallow landslides. On the other hand, pseudo-karst landforms (doline-like) become more evident in the hilltops underlied by gneiss rocks, and minor hanging topographic hollows prevail nearby these hilltops. To the northern quartzose karst areas (early mentioned) sediment storage decreases despite the local occurrence of minor alluvial fans. The regional spatial variability of both chemical and physical denudation rates seems to be related to the underlying bedrock litho-structures in conjunction to macro-landforms parameters such as basin gradient. Rainfall regimes and land use only controls the amount of water entry and groundwater recharge.