Climatic controls on alluvial-fan activity, Coastal Cordillera, northern Chile

Abstract

A description of the distribution, drainage basin characteristics, surface morphology, depositional process and age of 64 alluvial fan systems from both flanks of the hyper-arid Coastal Cordillera of northern Chile between 22°15′S and 23°40′S is presented. The coastal fans on the western flank of the Coastal Cordillera are dominated by debris-flow deposits fed from steep catchments. Two drainage basin types are recognized: type A drainage basins are small (10–30 km2) and do not cut back beyond the main coastal watershed; and type B drainage basins are large (up to 400 km2) and cut inland beyond the coastal watershed. The western Central Depression fans on the eastern flank of the Coastal Cordillera are characterized by sheetflood deposition fed from relatively shallow catchments in small drainage basins (10–50 km2). The surface morphology, sedimentation rates, a luminescence date and regional cosmogenic radionucleide data suggest that these fans have been inactive for at least the last 230 000 years and probably for much of the Neogene.The principal control on fan activity in the study area is climate. The Coastal Cordillera forms an orographic barrier to recent El Niño-related precipitation events that are restricted to the western flank of the Coastal Cordillera. These events did not penetrate into the Central Depression as indicated by the inactive nature of the western Central Depression fans located 25 km east of the active coastal-fan catchments. This scenario is considered to have prevailed for much of the Neogene. Climate also controls rates of weathering on alluvial-fan surfaces. The coastal fog results in rapid salt weathering of clasts on coastal fans resulting in the production of fines, but does not penetrate into the Central Depression. Fault activity is important in controlling drainage basin size. The larger (type B) drainage basins are commonly focused on active faults that cut the coastal watershed, facilitating drainage basin expansion. Source-area lithology is not important in controlling depositional processes. Fans on both sides of the cordillera have the same basaltic andesite and granodiorite source lithologies, yet coastal fans are dominated by debris-flow and western Central Depression fans by sheetflood deposition. A combination of chemical weathering and stream power related to gradient are considered to account for the differences in process.

Geomorphological evolution of the Tilcara alluvial fan (Jujuy Province, NW Argentina): Tectonic implications and palaeoenvironmental considerations

“…Under these arid conditions, the Tilcara fan sedimentary dynamics could be controlled by ephemeral stream flows, whose high effectiveness of sediment transport was due to large water discharges (Colombo et al 2000), and probably related to heavy rainfall associated with El Niño events occurring throughout the Late Pleistocene. At a regional scale in the Andes an evident relationship between historical alluvial fan activity and ENSO has been observed (Vargas et al, 2000;Keefer et al, 2003;Hartley et al, 2005;Colombo, 2005b).…”

“…Imbrication structures are locally observed. Clast size distributions and the geometry of the layers indicate flash floods, whereas the fining-upward sequences correspond to a loss of energy of the depositional currents (Wells and Harvey, 1987;Hartley et al, 2005). The presence of coarsening-upward sequences in the distal sectors of the fan corresponds to gravel lobes deposited at the channel toes.…”

Climatic and geomorphic interactions on alluvial fans in the Atacama Desert, Chile

“…Since the onset of hyper-aridity, steep mountainous bedrock surfaces erode more quickly than these fan surfaces due to the lack of precipitation to transport material (Nishiizumi et al, 2005). Hartley et al (2005) claim that fans along the Coastal Cordillera have not been active for ~230,000 years and probably since the Neogene based on cosmogenic and luminescence dating. Derived denudation rates (<0.1 m/My) for the Coastal 3 Cordillera suggest that the area is "inactive" and may be approaching a geomorphic steady state (Kober et al, 2007).…”

“…Historical precipitation data are sparse and incomplete. At present, annual precipitation averages ~5 mm and is delivered primarily by a coastal fog (camanchaca), with increasing precipitation to the east (Hartley, 2005).…”

Morphodynamics and facies architecture of streamflow-dominated, sand-rich alluvial fans, Pleistocene Upper Valdarno Basin, Italy

“…Bull 1977;Nemec & Postma 1993;Blair & McPherson 1994;Harvey et al 2005). The ongoing interest in alluvial fans demonstrates how their morphodynamics are still a matter for discussion, both in terms of autogenic v. allogenic dynamics (Schumm et al 1987;Field 2001;Ventra & Nichols 2014) and climate v. tectonic forcing (McGowen 1979;Allen & Densmore 2000;Blair 2000;Hartley et al 2005;Harvey 2005).…”