Analysis of the influence of tectonics and climatic forcing on the landscape evolution of the Uruguay River basin

KRÖHLING, DANIELA

Natal

Congreso; XIV Congresso da Associaçao Brasileira de Estudos do Quaternário, ABEQUA; 2013

Associaçao Brasileira de Estudos do Quaternário, ABEQUA

The Uruguay Basin (365,000 km2) connects tropical humid areas of S Brazil and NE Argentina with temperate areas of E Argentina and W Uruguay, an advantage in Quaternary palaeoclimatic reconstructions and regional scale correlations. Morphometrical analyses of the upper basin indicated the dominance of erosional processes on the Paraná Basaltic Plateau (PBP), a stepped system of Cenozoic planation surfaces. The base level for erosion in the upper basin was provided by the Uruguay River (UR). A characteristic of the sinuous channel is a narrow, deeply incised channel into the bedrock river channel. The canyon is continuous and permanently submerged, except in short segments, as in the Moconá area. There, the UR crosses an elevated tectonic block producing the emergence of the deep canyon forming a scenic system of longitudinal waterfalls. The canyon occupies 1/3 to 1/10 of the channel width and it is incised between 6 to 32 m. Plunge pools up to 47 m depth were also detected by the method of Phase Measurement Bathymetric Sonar System. The canyon has a Quaternary history represented by high incision rates. Downstream of the PBP, the UR displays two fluvial terraces, associated to a single mixed bedrock alluvial channel. The Pliocene/Early Pleistocene fluvial record of the argentine middle basin is represented by a meandering palaeochannel and associated floodplain deposits, very probably representing a connection between the UR and the Paraná River. Playa-lake deposits and loess accumulation were generated in the middle basin during cold and dry conditions in the Early Pleistocene. The Middle Pleistocene is represented by a large hiatus and generalized erosion. The Late Pleistocene upper fluvial terrace of the UR (OIS5a) is formed by a braided depositional system dominated by gravelly to sandy channel facies accumulated in a warm and humid climate; the Holocene lower terrace is characterized by floodplain facies and interbedded soils. A fluvial terrace in the tributary valleys of the basin was formed in the EIO3. Loess accumulation occurred at the SW area of the basin during the EIO2; particularly in the Last Glacial Maximum the wind dynamic dominated also in the upper basin, generating a red loessic mantle on the PBP. A wetland environment developed at the Pleistocene-Holocene transition because of the migration of the Paraná to the middle UR basin. A regional pedogenesis characterized the Optimum Climaticum of the Holocene. During the Late Holocene dry phase, the basin was affected by aeolian action that truncated the soil and that formed a loessic mantle and dune fields in the middle/lower basin. Littoral dynamic controlled the lowermost reaches of the alluvial channel of the UR. A delta and associated landforms were generated during the Middle Holocene highstand. Interactions between upstream climatic controls and tectonic uplift, and downstream sea-level variations in the basin are analyzed.