EVOLUTION OF THE LANDFORMS ACROSS WHICH THE ATACAMA DESERT IS DRAPED, 20°-24°S LATITUDE

The extreme environment of the Atacama Desert of northern Chile is due to lack of precipitation and lack of water input by surface drainages. A major factor contributing to hyperaridity is the topographic barrier of the Andes, blocking moisture sourced from the east. An exception to the lack of surface water is the Loa River. We explore the Cenozoic evolution of the topography and drainage of the Atacama core.

The western Andean slope is the major landform feature of the Atacama Desert, which grades from the Central Depression (Tamarugal Basin) at ~1000 m elevation into the Western Cordillera at > 4000 m elevation. Long-wavelength rotation of strata reveals ~3000 m of relief generation across that slope since ~17 Ma, of which about ~1500 m occurred since 5 -10 Ma. A second major landform is the Pre-Andean Depression (Calama and Salar de Atacama basins), at ~2000 m elevation, interrupting the western Andean slope. A morphologic differentiation of the Calama and Salar de Atacama basins that was inherited from Cretaceous and Eocene tectonics was enhanced by Oligocene normal faulting, unusual regionally, but which acted as the initiation event of the anomalously large catchment of today's Loa River. The Calama basin overfilled with sediment and changed from endorheic to exorheic during the Miocene by integrating the Loa catchment, whereas the Salar de Atacama basin persisted in an endorheic state. Development of high stratovolcanoes to the east of all these basins during the middle Miocene contributed to capture of eastward-sourced moisture which thereafter fed minor streams as well as the Loa River. Because the Miocene Loa catchment predated thousands of meters of Andean uplift and the Plio-Quaternary volcanic chain, the paleo-environmental characteristics of the Miocene Calama basin/Loa deposits reveal the paleoclimate and catchment evolution of upland regions probably now located in the Altiplano plateau. Likewise, the Pliocene-Recent Calama basin/Loa deposits reflect hydrologic conditions of a unique drainage basin and are not representative of the Atacama Desert. In contrast, strata in the Atacama and Tamarugal basins record the environmental history representative of the Atacama Desert.