Sedimentological characteristics and geochemistry of lake waters of the Rio Grande basin, west of Bahia State (Brazil)

摘要

The lacustrine system of the Rio Grande basin is placed where it gathers singularities from both geological and climatic points of view. In order to understand the origin and dynamics of this system, a faciological description of the deposits from two lakes considers both textural and mineralogical elements via XRD analyses. This was followed by a chemical analysis of a dozen lakes' water, in order to standardize the evolution of the lowlands which form the lakes. The hydrochemical data revealed two trends: one group of lakes presented sodium and base, and the other group calcium and acid. Both these groups also presented some differences in their sediment characteristics, registering round pebbles, a higher percentage of sand and presence of mica in one of the lakes, contrasting with the muddy texture with predominance of quartz and kaolinite in the other lake. Such results allowed the proposition of two evolutionary models for the lakes of the Sao Francisco Lowlands, which share the same climate; evidence that the other landscape elements affect the system's dynamics. One of the models is represented by lakes that evolved from little permeable sedimentary rocks. These rocks, however, are more fractured, favouring the lakes area expansion and more water permanence with recent links to the river network. On the other proposed model, the lakes evolved from permeable but less fractured carbonic rocks with rapid vertical water infiltration and less water permanence, which reflects more atmospheric than lithological influence in their geochemical characteristics. This study provides important information for the comprehension of this landscape evolution.