HYPOTHESIS ON THE NEOTECTONIC CONDITIONING OF THE NATURAL DAMMING OF IVAÍ RIVER WATERS NEAR ITS MOUTH BASED IN ECHOBATHIMETRICAL DATA, PARANÁ AND MATO GROSSO DO SUL STATE LIMIT, SOUTHERN BRAZIL

Abstract

Echobathimetrical survey carried out at the confluence of the Ivaí and Paraná rivers showed asymmetrical channel morphology with natural dammed of the tributary water. At the confluence the Ivaí River is also deepest and with largest suspended load than the main river. The receptor channel of the Paraná River is shallow and presents coarse to fine sand bedload. Morphology and flow dynamics of the confluence and some considerations concerning the probably causes for asymmetrical confluence are also presented. Some hypotheses about neotectonics associated with paleoclimate are considerated in the origin of this peculiar confluence. Mouth evolution occurred in three phases: (A) Confluence occupied a flatted and extensive meandering alluvial plain under humid climate; (B) Normal neotectonic faults lowered the Paraná channel’s thalweg and consequently incision of the Ivaí River on Cretaceous bedrock and with the generation of an expressive terrace in the Paraná River’s right bank (Fazenda Boa Vista Geomorphological Unit); and (C) Paraná River received expressive sandy load up to the present time. Preliminary data indicate that last phase could have been initiated around 3.5 to 1.5 ka BP, under semi-arid climate conditions.