THE PLEISTOCENE OF THE URUGUAY RIVER BASIN, SOUTH AMERICA

The Uruguay river basin (365,000 km2) comprises tropical and subtropical latitudes in eastern South America. The basin was shaped late in the Tertiary. The oldest Quaternary unit is the San Salvador Formation (Lower Pleistocene), which was generated by the union of the Uruguay and the Paraná rivers; the formation is composed of a very large meandering channel and associated flood-plain deposits. Subtropical humid conditions are inferred for that period. Around 1 Ma B.P., a dry and cold climate occurred in the continent (the Great South American Glaciation). The Uruguay river accumulated a large playa (Hernandarias Fm) composed of montmorillonite with segregations of gypsum; the river discharge at that time is calculated in only 13% of the present. Synchronically, the south of the basin underwent a Pampa-type loess accumulation (Punta Gorda Group).

The whole Middle Pleistocene is represented in the basin by a large hiatus. The Upper Pleistocene, on the contrary, is registered by a variety of sedimentary sequences. The OIS 5 is represented in the middle basin by El Palmar Formation, a fluvial unit (channel and associated flood-plain facies) sedimented under a tropical climate. A 10 m high marine terrace was generated at the Uruguay river mouth.

The OIS 3 is represented by a fluvial terrace in the tributaries of the basin (Arroyo Feliciano and Sopas Fm). Loess deposition occurred in the southwestern of the basin during OIS 2 (Tezanos Pinto Fm) under a dry and fresh climate. At that time the wind dynamic dominated also in the tropical upper basin, generating a red tropical loess (Oberá Fm). A generalized process of flash floods and mudflows under a dry climate affected the Oberá Formation, accumulating a diamict facies in the Uruguay valley (Yapeyú Fm). The Late Pleistocene is represented by a major channel shifting of the Paraná river, which invaded the present Uruguay basin and accumulated the Tapebicuá Formation in a wetland environment.

The Holocene events registered in the basin are similar with those published elsewhere for the North Pampa.