HR: 09:00h
AN: U31A-04 INVITED     [Abstracts]
TI: Late Oligocene to Recent Landscape Evolution in Atacama Desert: a Case for Episodic Pluvial Activity
AU: * Dunai, T J
EM: tibor.dunai@ed.ac.uk
AF: University of Edinburgh, School of GeoSciences, Institue of Geography, Drummond Steet, Edinburgh, EH8 9XP United Kingdom
AU: * Dunai, T J
EM: tibor.dunai@ed.ac.uk
AF: Vrije Universiteit, FALW, De Boelelaan 1085, Amsterdam, 1081 HV Netherlands
AB: Depositional surfaces of early Miocene sediments are preserved in the Coastal Cordillera, Atacama Desert, northern Chile (Dunai et al. 2005). Measurement of cosmogenic 21Ne in clasts from erosion-sensitive sediment surfaces show that these surfaces have been barely affected by erosion since 25 Ma. Predominantly hyperarid conditions since 25 Ma are required to create and preserve these oldest continuously exposed surfaces on Earth (ibid). The next oldest continuously exposed surfaces, in the Dry Valleys region, Antarctica, have about half this age (e.g. Sch„fer et al. 1999); van der Wateren et al. 1999). Eighty additional vein-quartz clasts were collected from the surface studied earlier (Dunai et al. 2005). These clasts were shed onto the surface from surrounding hill slopes after deposition of the main sedimentary body (22-25 Ma, references: ibid). The clasts were collected at the inlet of the first in a series of salt carst depressions at the bottom of a wide `valley' on the sediment surface(ibid). The 21Ne ages of these clasts show distinct clusters at 3.5-4 Ma, 8-10 Ma, 13-15 Ma, 17-18 Ma, ~24 Ma and 33-35 Ma. The ages that are younger than the deposition age give the timing of pluvial phases, in which the runoff was strong enough to incise hardrock channels into the surrounding hill slopes, and deposit material onto the sediment surface that had little to no pre-exposure to cosmic rays. Ages concordant to the deposition age probably represent laterally dislocated material of the original sediment surface. Ages greater than the deposition age indicate that portions of the local source region of the clasts were very slowly eroding/stable prior to the pluvial episode that eroded and transported them onto the sediment surface. The inferred pluvial phases are age-concordant with episodes of supergene enrichment in porphyry copper deposits of the Pre-Cordillera and the Andes to the east of the study area (e.g. Quang et el. 2005, Sillitoe and McKee 1996). Every Stage in the landform chronology of the region (Tosdal 1984; as revised by Quang et al. 2005) has its counterpart in the inferred pluvial phases in the Coastal Cordillera. The timing of all inferred pluvial phases is age-concordant with periods global and/or regional climate change (global cooling, regional wet-phases; Zachos 2001, Vasconcelos et al. 1994). It is therefore indicated that episodic pluvial activity, rather than episodic tectonic uplift (Quang et al. 2005), is responsible for the episodic changes in the Antacama section of the western flank of the Andes.
DE: 1130 Geomorphological geochronology
DE: 1155 Extinct radionuclide geochronology
DE: 1600 GLOBAL CHANGE
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1630 Impacts of global change (1225)
SC: Union [U]
MN: Fall Meeting 2005