Oligocene to recent evolution of the Calama Basin, northern Chile
The Calama and eastern Pampa del Tamarugal Basins are located between 22°S and 23°S within the forearc of northern Chile. They are filled by sediments deposited in alluvial braidplain, fluvial, playa sandflat, lacustrine and volcaniclastic environments under a semi-arid to hyper-arid climate. The nature of the alluvial braidplain depositional environment is unusual in that it combines elements of both alluvial fan and fluvial depositional systems, in contradiction to recently published models of alluvial fan sedimentation. Detailed sedimentary logging, magnetostratigraphy and dating of 14 volcanic interbeds by the 40Ar/39Ar laser fusion method has established a lithostratigraphic and chronostratigraphic framework for the 700 m thick basin-fill. Basin formation was investigated by regional subsidence during the Late Eocene or Early Oligocene, followed by widespread alluvial braidplain deposition during the Oligocene(?). A change to fluvial and playa sandflat deposition during the Early to Mid-Miocene is considered to be coincident with a decrease in active subsidence. Sedimentation ceased and thick (25 m) gypcrete deposits developed along the eastern margin of the basin during the Mid-Miocene as a response to an increasingly arid climate. Phases of minor lacustrine, fluvial and alluvial braidplain deposition during the Late Miocene-Early-Pliocene and the Late Pliocene(?) to Pleistocene were primarily controlled by small-scale fault movements and folding events, although climatic variations may have been important in some cases. A new lithostratigraphic division of the basin-fill is proposed here, which comprises 13 different formations. The previously defined El Loa Formation comprises a number of depositional units which are spatially and temporally discrete formations, and is therefore awarded group status.