The large alluvial fan systems of the Central Depression in the hyper-arid core of the Atacama Desert of Northern Chile provide unique opportunity (through exceptional preservation) to examine landscape evolution processes over geologic time-scales (last 8Ma). We will specifically examine the role that bedrock landsliding plays within the catchment and fan evolution in terms of sediment flux and routing. Today the catchment area (1.7–4.3 km above SL) receives rainfall of ~50 mm pa whilst modern rainfall in the lower elevation (1-1.7km above SL) alluvial fan area is <1mm pa. Evidence from packrat middens suggests that even the wettest periods of the Quaternary were no more than 2 times wetter, thus maintaining a likely arid – hyperarid climate for the deposits and landforms examined in this study. Quebrada Arcas catchment has an area of 760km2 which incorporates 2.6km of vertical relief. Within the catchment are a series of deep-seated (100-200m) large bedrock landslides ranging in size from <1km2 to >22km2 in area. These can be broadly grouped into older landslides that pre-date a significant inset ignimbrite dated at 5.8 Ma and younger landslides that cross-cut and postdate this surface. The older landslides mostly populate an active NNE-SSW compressional fault structure (W of catchment), and the younger landslides are either associated with steeply tilted (38°) ignimbrite/sedimentary unit discontinuity surfaces (E of catchment) or over-steepened slopes associated with the main ephemeral river gorges. The younger and smaller (0.13km2) of these latter landslides is associated upstream with a 40m thick sequence of laminated lacustrine deposits (Salar de Carmen, 0.5km2 in area) which contains large (m-scale) soft-sediment deformation units indicative of discrete and repeated seismic activity over the life-time of the salar. Quebrada Arcas alluvial fan covers some 688km2 and comprises 3 main depositional areas based on relative age and sedimentology. 1) An older depositional lobe to the south which covers 281km2, has a probable age of 2-5 Ma and is dominated by clast-rich debris flows ; 2) A relatively younger lobe to the north which covers 244 km2 and is dominated by clast-rich debris flows. Cosmogenic dating using 21He suggests surface flows were deposited ~1Ma, and 3) An active clast-poor mudflow dominated lobe which covers 155 km2 below a fan head trench which extends 10 km from the fan apex. Lobes 1 and 2 demonstrate evidence for repeated large bi-modal debris-flow events. At the surface these are represented by linear chutes (some 1-2km long, 40m wide) lined with outsized (5m) granodiorite boulders, and associated downflow with large boulder-field splay lobes (some 0.3km2 each). Individual splay lobes represent debris flows of some 3000m3/sec. These are thought to represent landslide (flood) related deposits based on geomorphology, sedimentology and provenance.
|Publication status||Published - 2019|
|Event||INQUA XVII Congress - Cairns|
Duration: 1 Jan 2007 → …
|Conference||INQUA XVII Congress|
|Period||1/01/07 → …|