RATES OF LANDSCAPE MODIFICATION IN RESPONSE TO PALEOCLIMATE VARIABILITY SINCE ~12 MA, ATACAMA DESERT, PAMPA DEL TAMARUGAL, CHILE

The Atacama Desert landscape of northern Chile records ancient landscapes because the rate of change is extremely slow. How slow? Prior studies using cosmogenic nuclides show that middle Miocene landscapes are locally preserved, even though parts of the landscape are of Quaternary age. This study takes a holistic view of the entire landscape, erosional and depositional, in the southern part of the Pampa del Tamarugal (PdT) forearc sedimentary basin. This approach affords the opportunity to learn about the fraction of the landscape preserved from each of several historical stages. Throughout the Neogene, the PdT basin developed west of the progressively rising western slope of the Andes. The topographic rise of the catchment and rotation of the proximal basin led to overall westward offlap of the alluvial fans. However, a series of climate changes, from the dominant hyperarid state to short-lived arid conditions, produced five different stages of landform evolution. During times of hyperaridity, exposure surfaces evolved very slowly and Gypsisols developed as much as 2 m thickness of salts.

Dates of landscape stages are constrained by integrating the history of the sedimentary fill of the PdT basin with landforms. The strata are dated by conventional means -- the ages of volcanic ashes interbedded with alluvial deposits or, in the western zone, with lacustrine and salt-pan deposits. After the onset of hyperaridity ~12-11 Ma, the available data suggest that the times of arid climate and deposition of alluvial sediments are comparatively short lived, a million years or less, whereas the intervening times of hyperaridity are lengthy, typically 1-5 million years. During one interval of slightly wetter climate, in a relatively proximal part of the basin erosion caused broad degradation (<8 m) of the older alluvial fan. This translates to a denudation rate of ~2-4 m/myr during the wetter phase. During hyperarid phases, there was no net degradation, but rather aggradation of atmospheric-derived salts. If during >80% of the total time since 12 Ma the climate was hyperarid, a mean denudation rate weighted for the arid as well as the hyperarid times is ~0.5 m/my.

The recognition of five intervals of climate fluctuation from arid to hyperarid explains much of the controversy over the age of onset of hyperaridity – it happened multiple times.