MIOCENE TO RECENT HISTORY OF THE WESTERN ALTIPLANO IN NORTHERN CHILE REVEALED BY LACUSTRINE SEDIMENTS OF THE LAUCA BASIN (18-DEGREES-15'-18-DEGREES-40'S 69-DEGREES-30'-69-DEGREES-05'W)/

The intramontane Lauca Basin at the western margin of the northern Chi lean Altiplano lies to the west of and is topographically isolated fro m the well-known Plio-Pleistocene lake system of fluvio-lacustrine ori gin that covers the Bolivian Altiplano from Lake Titicaca to the north for more than 800 km to the Salar de Uyuni in the south. The Lauca Ba sin is filled by a sequence of some 120 m of mainly upper Miocene to P liocene elastic and volcaniclastic sediments of lacustrine and alluvia l origin. Volcanic rocks, partly pyroclastic, provide useful marker ho rizons. In the first period (6-4 Ma) of its evolution, the 'Lago Lauca ' was a shallow ephemeral lake. Evaporites indicate temporarily closed conditions. After 4 Ma the lake changed to a perennial water body sur rounded by alluvial plains. In the late Pleistocene and Holocene (2-0 Ma) there was only marginal deposition of alluvial and glacial sedimen ts. The basin formed as a half-graben or by pull-apart between 10 and 15 Ma (tectonic displacement of the basal ignimbrite sequence during t he 'Quechua Phase') and 6.2 Ma (maximum K/Ar ages of biotites of tuff horizons in the deepest part of the basin). Apart from this early basi n formation, there has been surprisingly little displacement during th e past 6 Ma close to the Western Cordillera of the Altiplano. Also, cl imate indicators (pollen, evaporites, sedimentary facies) suggest that an arid climate has existed for the past 6 Ma on the Altiplano. Toget her, these pieces of evidence indicate the absence of large scale bloc k-faulting, tilt and major uplift during the past 5-6 Ma in this area.