Cenozoic geomorphological evolution of the Uruguay river basin, South America

IRIONDO, MARTIN; KROHLING, DANIELA

Florencia (Italia)

Congreso; 32nd International Geological Congress; 2004

International Union of Geological Sciences (IUGS)

The 1600 km long Uruguay river drains a basin of 365,000 km2 (26°30'-34°lat.S and 49-58°30'long.W; SE Brazil, NE Argentina and Uruguay). It is a complex territory, which links the tropics with temperate climates. The upper basin is developed in a three-level meset formed by Cretaceous basalt. Only relicts of the original surface remain in the headwaters in Brazil, in the Misiones province (Argentina) and in NW Uruguay (Masoller surface). A conspicuous feature of that old plain landscape, equivalent to King's Sul-American Surface, are extensive blocky fields.Most of the meset is formed by the intermediate surface (300-400 m below), round hills and mature valleys shaping a fluvial landscape of Middle Tertiary age (Velhas cycle). The original fluvial net is partially occupied by a different hydrographic system. The lower level (200 m.a.s.l) forms a peripherical area in Brazil and Argentina (Apóstoles Surface; Paraguaçú cycle). It developed in the Pliocene, destroying the intermediate surface by retreat and widening of valleys. A rim of ferricrete was formed by subsurface flow along the southwest limit. The river channel was shaped during a Tertiary humid climate before the Velhas cycle; it is formed at Present by large meanders incised about 120 m. In the middle basin the collector divides the basin in two systems: the western region is characterized by large paleochannels of the Paraná and Uruguay rivers, buried at different depths. The eastern region of the basin drains older and more diversified terrains, in which a well preserved net of very old paleochannels is the dominant feature in Rio Grande do Sul and NE Uruguay. The Quaternary had a few major scenarios. At the base of the Pleistocene, the intense mobility of river channels connected the Paraná and Uruguay in a single huge river. At the end of the Lower Pleistocene (Great South American Glaciation)developed a large playa-lake in the lower basin. The Middle Pleistocene have no significant geomorphological or sedimentary register. The Last Integlacial is represented by the upper fluvial terrace (OIS 5a). Other humid intervals occurred at OIS3 and Middle Holocene (lower terrace). The most important characteristic of the lower basin is the influence of the South Atlantic ocean on the dominant Quaternary sandy units. A geomorphic singularity of the Uruguay river channel is a narrow and deep trench excavated in the bottom along hundreds of kilometers at the Argentina-Brazil border.