RIVER KNICKPOINTS IN BASALTS OF THE PARANA BASIN, BRAZIL: LITHOLOGICAL AND TECTONIC CONTROLS

Knickpoints and knickzones are common features along rivers on the plateaus lying on the basalts of the Paraná Basin. According to current conceptual models, whereas knickpoints and knickzones would be formed at the massive zones of basaltic flows with a predominance of vertical joints, the vesicular-amygdaloidal zones and those with horizontal joints would tend towards the formation of low slope reaches due to their lower resistance to erosion. Since field surveys showed features with great complexities, the current analysis verified which controls produced knickpoints and knickzones in such a geological environment. The research was conducted in the Rio das Pedras, a 61-km-long mixed bedrock-alluvial channel. The longitudinal profile of the river was surveyed on a topographic map with 5 m contour intervals. Tectonic lineaments transversal to the channel and other lineaments in which the river lies longitudinally were identified from cartographical documents. A detailed field survey was also conducted to examine the lithologic characteristics of the riverbed. The results show that knickpoints and knickzones may be formed in any litho-structural zone of basaltic flows. On the other hand, low slope zones are predominantly sculptured in vesicular-amygdaloidal basalts. These zones are less resistant to erosion although this fact is not related to the joint density, which is practically the same as that of the massive basalts (4.26 and 4.42 m/m² respectively). The strength of vesicular-amygdaloidal basalts decreases according to the increase in jointing (6.2 m/m²) in reaches where the channel lies longitudinally in lineaments. The massive basalts do not present the same behavior. Regardless of the basalts’ litho-structural characteristics, most of the knickpoints and knickzones in the Rio das Pedras are directly or indirectly related to faults transversal to the channel. The indirect relationships are related to a flow that is inverse to the blocks’ dip or related to the differential erosion between the reaches that lie in the fracture zones (lineaments) and those that do not.