Unraveling the Peruvian Phase of the Central Andes: stratigraphy, sedimentology and geochronology of the Salar de Atacama Basin (22 degrees 30-23 degrees S), northern Chile

Abstract

The Salar de Atacama Basin holds important information regarding the tectonic activity, sedimentary environments and their variations in northern Chile during Cretaceous times. About 4000m of high-resolution stratigraphic columns of the Tonel, Purilactis and Barros Arana Formations reveal braided fluvial and alluvial facies, typical of arid to semi-arid environments, interrupted by scarce intervals with evaporitic, aeolian and lacustrine sedimentation, displaying an overall coarsening-upward trend. Clast-count and point-count data evidence the progressive erosion from Mesozoic volcanic rocks to Palaeozoic basement granitoids and deposits located around the Cordillera de Domeyko area, which is indicative of an unroofing process. The palaeocurrent data show that the source area was located to the west. The U/Pb detrital zircon geochronological data give maximum depositional ages of 149Ma for the base of the Tonel Formation (Agua Salada Member), and 107Ma for its middle member (La Escalera Member); 79Ma for the lower Purilactis Formation (Limon Verde Member), and 73Ma for the Barros Arana Formation. The sources of these zircons were located mainly to the west, and comprised from the Coastal Cordillera to the Precordillera. The ages and pulses record the tectonic activity during the Peruvian Phase, which can be split into two large events; an early phase, around 107Ma, showing uplift of the Coastal Cordillera area, and a late phase around 79Ma indicating an eastward jump of the deformation front to the Cordillera de Domeyko area. The lack of internal deformation and the thicknesses measured suggest that deposition of the units occurred in the foredeep zone of an eastward-verging basin. This sedimentation would have ended with the K-T phase, recognized in most of northern Chile.