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Silicic volcanic ash deposits investigated at 14 localities between 22 degrees and 25 degrees S in the Chilean Coastal Cordillera are found to be the distal ash fall from supereruptions in the Central Andean cordillera several hundreds of kilometers to the east. Depositional textures, modal composition and granulometry of the ashes and tuffs (the latter lithified by halite and gypsum under ultra-arid conditions) allow for a distinction between primary fallout/aeolian deposits (mean 4-5 Phi, sorting 1.5-2 Phi) and secondary deposits that formed by down wash from hill slopes during local rain fall. Primary volcanic components comprise two types of glass shards (with small stretched vesicles and coarse-walled with rounded to elliptic vesicles), and biotite. Previously published studies on ash deposits in the north Chilean Coastal Cordillera reported 14 Ar-40/Ar-39 and K/Ar ages on biotite or sanidine ranging between 6.66 +/- 0.13 and 0.6 +/- 0.4 Ma. In this project, three Ar-40/Ar-39 ages on biotite have been determined for samples from the Cuenca del Tiburon, the northern margin of Salar de Navidad and from the Quebrada de la Chimba (3.9 +/- 0.1 Ma, 4.1 +/- 0.1 Ma, 6.0 +/- 0.1 Ma, respectively). The range of the 17 ages coincides with the Late Miocene to Quaternary ages of the major ignimbrite-forming eruptions of the high Andes to the east such as the Altiplano Puna Volcanic Complex (APVC).

Electron microprobe data of glass and biotite of the Coastal Cordillera ashes have been compared with data from major ignimbrites of the APVC, of other major Central Andean volcanic fields, and of marine ashes (ODP Leg 201). Additional new biotite microprobe data from three APVC ignimbrites (Pastas Grandes, Pujsa and Guacha) have been included in the present study. Biotite composition of the investigated Coastal Cordillera ashes is similar to those of ignimbrites from the APVC. In particular, based in Fe, Mg, Mn and Ti, distal equivalents of the 3.96 +/- 0.08 Ma Atana and/or 4.09 +/- 0.02 Ma Puripicar and of the 5.6 +/- 0.2 Ma Pujsa and/or the 5.56 +/- 0.01 Ma Guacha eruptions can be identified. In addition, based only on age relations, distal ash units of the Pastos Grandes, Tatio and Purico eruptions may be present in the Coastal Cordillera. Composition of glass is comparable to APVC ignimbrite matrix glass and to marine glass, however, significant alkali depletion and SiO2 enrichment is attributed to in situ alteration.

The identification of these ashes demonstrates for the first time that the supereruptions in the southern Central Andes gave rise to voluminous ash clouds, most likely co-ignimbrite. The present outcrops represent ash dispersed by easterly winds, consistent with atmospheric models that show favorable westward-directed winds existing in the upper troposphere/stratosphere during the southern summer in the southern Central Andes. This requires that current volume estimates for the major eruptions to be considered minima with a significant augmentation likely. (C) 2013 Elsevier B.V. All rights reserved.