A Pluvial Age in the Puna de Atacama

Abstract

The puna de atacama, which Bowman has described admirably,1 is an area of basins, mountain knots and pied ont alluvial deposits at an elevation of about 10,000 feet, lying partly in Chile, Argentina and Bolivia, roughly between latitudes 260 and 180 S. The eastern marches of the Chilean Atacama Desert consist of a number of lofty depressions which are physiographically more akin to the Puna than to the low-lying coastal desert. Few geological features are easier to identify than old shorelines in a desert basin,* and they have been seen in the Puna by various workers3~6 who have ascribed them to high lake levels of a bygone pluvial period. In Bolivia, Moon7 filled his Ballivan (Titicaca-Poopo) and Minchin with meltwater released by a Pleistocene interglacial. Newell8 saw a gradual desiccation of the Titicaca region since a pluvial period in the late Pliocene and early Pleistocene. Briiggen9 said the same of northern Chile; and Ahlfeld10 extended one of his pluvial from Bolivia into the adjacent Chilean basin of Ascotan. Other writers11 have deduced from the archaeological record and certain natural phenomena, such as buried forests, a progressive desiccation of the Puna and the Desert of Atacama during the last few centuries. During August and September 1958 an expedition from Cambridge12 looked at eight of the lakes that figure prominently on maps of the Puna, in the hope of finding around them physiographic evidence of former high levels, and archaeological proof of recent climatic changes. The Chilean salares (salt-covered basin floors) of Ascotan, San and Atacama displayed clear shorelines into the usually soft volcanic or clayey rocks that surround them; the phrase parallel roads of San Martin will give an idea of the appearance of the shoreline stairway around that salar. Further, the Salar de Atacama was bounded along its north-western shore by an extensive delta-like terrace of gravel, cobbles and silt; and two streams that flow into the basin, the Rio Grande and the Vilama, exhibited strath and cut terraces along their courses, whose nature suggests that they formerly flowed into a deep Atacama lake that later underwent a rapid drop in level. The Bolivian lagunas closely resemble the salares in form and in the presence of various salt deposits within the basins. The presence of shallow lakes?shallow enough for flamingoes to wade across them?can be ascribed to the greater pre? cipitation which their elevation brings them. Shorelines have been into the soft ashes that surround the lagunas of Hedionda, Charcota and Inca Corral, and tufa has been deposited in horizontal beds; here too a rapid drop in level was suggested by the nature of the beaches.