Timing of alluvial fan development along the Chajnantor Plateau, Atacama Desert, northern Chile: Insights about climate variation

Abstract:

An extensive alluvial apron of coalescing gravel fans blankets the western flank of the Chajnantor Plateau in the Atacama Desert of northern Chile. Remnant alluvial surfaces, terraces, and intermittent debris flow deposits preserved in this bajada indicate multiple intervals of aggradation, incision and terrace abandonment, and deposition. The high preservation potential and sensitivity to climate shifts of the region provides a unique opportunity to elucidate the sedimentary response to climate variations at an extreme of Earth’s climate. We use cosmogenic ³⁶Cl exposure dating, aided by mapping, to establish a detailed chronology of the depositional history of the Chajnantor alluvial apron. Alluvial surfaces and gravel deposits yield cosmogenic exposure ages ranging from 20.7 ± 1.4 ka to 419.2 ± 39.6 ka. Debris flow boulders confined to modern and ancient channels yield cosmogenic exposure ages ranging from 9.3 ± 1.1 ka to 202.5 ± 19.6 ka. One localized (Qcf1) and two extensive (Qcf2 and Qcf3) abandoned alluvial fan surfaces yield cosmogenic exposure ages of ~145 ka, ~55 ka, and ~33 ka respectively. These abandonment ages coincide with periods of moraine stabilization and deglaciation on the adjacent Chajnantor Plateau. The cosmogenic exposure ages also reveal a transition from aggradation to incision during marine oxygen isotope stage II (MIS II), coincident with local deglaciation. We interpret that protracted periods of aggradation coincide with periods of increased precipitation and glacial occupation of the Chajnantor Plateau, and are punctuated by phases of incision and surface abandonment during interglacial periods. These results suggest that precipitation is the dominant mechanism driving alluvial fan formation and modulating sediment supply along the western margin of the Chajnantor Plateau.