A TALE OF TWO RIDGES: RELIEF INVERSION ON DISTAL ALLUVIAL FANS IN THE PAMPA DEL TAMARUGAL REGION OF THE ATACAMA DESERT, CHILE

Alluvial fans of the Chilean Atacama Desert record a spectrum of flow events, from fluvial floods to sheet-like mudflows, which generate a complex network of active and abandoned channel segments. On the distal bajada, wind scour has resulted in landscape inversion of some channel segments. We characterize two inverted channel segments near Salar de Llamara as examples of the various factors producing similar-appearing ridges. Although both ridges mark former flow paths that incised into playa deposits and are in close proximity (within 150 m), these inverted channels differ markedly in traverse shape, erosion-resistant agent, grain size and composition. These examples highlight the challenges in reconstructing flow properties and development sequence solely from remotely sensed data, as is done for martian inverted channels.

Ridge 1 is a 0.5 km long segment, up to 3 m wide with a surface that is flat to concave. Discontinuous, thin gypsum plates (<15 cm) cap the surface and precipitated in standing water. An excavated trench on the eastern end reveals four indurated sandy layers (0.10-0.25 m thick) with varying amounts of mud associated with an aggrading fluvial system, and two lower layers of playa deposits that are regionally correlated with other outcrops. The vertical profile records episodic wet periods, as evidenced by root fragments and iron-stained plant molds, and dry intervals marked by salt hardpans at some layer boundaries. Short (<20 m) spurs suggests the ridge was part of a contributory drainage network.

Ridge 2 is an ~2 km long, gravel capped inverted channel segment with a pointed to rounded (up to 2 m wide) crest. The coarsest grain size (1-2 cm) are concentrated on the ridge crest, consistent with fluvial transport along this corridor. Threshold flows to transport the pebbles across the 0.5% regional slope would require ~30 cm deep flows with ~1 m/s average velocity. The ridge surface is generally unconsolidated, although there are isolated patches that are weakly indurated by efflorescent salt. Most ridge slopes are armored by a terrain-conformal halite crust that may also retard landform degradation. Beneath the gravel monolayer is a 0.5 m thick, massive, uncemented layer of fine sand with floating pebbles. Deflation of this fluvial deposit generated the uppermost gravel pavement that armors the ridge.