MAPPING AND PRELIMINARY PALEODISCHARGE ESTIMATES OF INVERTED FLUVIAL CHANNELS IN THE ATACAMA DESERT

Previous investigations in the Atacama Desert present evidence of more humid conditions in the geologically recent past. To advance this research, we examine a 40 km² area of sinuous ridges in the Pampa de Tamarugal (near 21°10’S 69°40’W). Initial in-situ observations show them to be low relief (1-2 m) landforms, topped with a thin pavement of fine gravel. On the basis of their positive relief, moderate sinuosity (avg. 1.1), networked appearance, and general down slope orientation, we interpret these features to be segments of partially preserved inverted meandering paleochannels.

Here, we present mapping of the area using WorldView-2 satellite visual imagery (0.5 m res.) to assess the stratigraphic context for paleochannel formation. Mapping yields 3 units differentiated by color and texture. A dark brown alluvium unit hosts the majority of the paleochannel segments (avg. length ~600 m and width ~20 m). Interspersed with mappable paleochannel segments, the unit displays a texture of convoluted curves and ridges, which may represent more degraded meandering or braided segments. The dark brown unit may be from a past depositional environment because no active channels appear to supply the area with new dark brown alluvium. We interpret a tan alluvium unit as the modern depositional unit, stratigraphically above the dark brown unit, because of its abundance to the east (upslope), where linear chains of tamarugo plants within tan channels suggest the most recent water flow. A red unit, interspersed with the dark brown and tan units, may represent localized weathering or oxidation, but its relative stratigraphic position is unclear.

The suggestion of active meandering during flow allows for the use of an empirical relationship between discharge and meander wavelength, derived from 31 rivers in the central U.S. (Calston, 1965; modified by Williams, 1984), to make preliminary estimates of paleodischarge. For segments with sinuosity > 1.1, a total of 63 applicable wavelengths (λ>145 m) from 19 inverted segments give an estimated mean annual paleodischarge at 1.5 m³/s (σ=0.6). This value is 5x greater than the reported modern perennial discharge of Tarapacá River (150 km north). Thus, the presence of these paleochannels and their estimated paleodischarge suggest a wetter paleoenvironment in this area of the Atacama Desert.