The Purilactis Group of Northern Chile: Boundary Between Arc and Backarc from Late Cretaceous to Eocene

Abstract

The approximately 2000- to 4000-m-thick detritic Purilactis Group exposed for ≈150 km along the western escarpment of the tectonic depression of the Salar de Atacama is subdivided from bottom to top into: (1) Tonel Fm. (≈1000 m of fine sandstones and gypsiferous mudstones), (2) Purilactis Fm. (750--3000 m of conglomerates with volcanic intercalations), and (3) Yesífera Superior Fm. (≈400 m of gypsiferous sandstones). The age of the Purilactis Group comprises the time interval from the latest Cretaceous to the Eocene-Oligocene boundary. Its depocentre bordered, to the west, probably along normal faults, on the relatively elevated area of a contemporaneously active magmatic arc in the Precordillera of Northern Chile (Cordillera Domeyko), and, hence, it is considered to be the westernmost part of an epicontinental backarc basin. To the east this basin was connected with the rift-induced depocentre of the Salta Group in northwestern Argentina. The vertical transition from the fine-grained sediments of the Tonel Fm. to the conglomeratic facies of the Purilactis Fm. at about 44 Ma coincides with a shift of magmatic activity towards the east. The Incaic orogenic phase at about 38 Ma led to general crustal shortening which affected the arc and especially the backarc area. Thrusts and folds developed at the former basin boundary which also was subjected to dextral strike-slip displacements as revealed by local vertical folds. Also, younger tectonics related to the development of the Salar de Atacama Depression affected the western escarpment, characterizing it as an important tectonic lineament.