HR: 0830h
AN: T31C-0857    [PDF]
TI: Investigations into the Numerical Ages of post-Miocene Fluvial Landforms in the Atacama Desert, Chile
AU: * Owen, J
EM: jowen@nature.berkeley.edu
AF: Division of Ecosystem Sciences, 151 Hilgard Hall University of California, Berkeley, CA 94720 United States
AU: Nishiizumi, K
EM: kuni@ssl.berkeley.edu
AF: Space Sciences Laboratory, 324 Space Science Addition University of California, Berkeley, CA 94720 United States
AU: Sharp, W
EM: wsharp@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Sutter, B
EM: bsutter@mail.arc.nasa.gov
AF: NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035 United States
AU: Ewing, S
EM: saewing@nature.berkeley.edu
AF: Division of Ecosystem Sciences, 151 Hilgard Hall University of California, Berkeley, CA 94720 United States
AU: Amundson, R
EM: earthy@nature.berkeley.edu
AF: Division of Ecosystem Sciences, 151 Hilgard Hall University of California, Berkeley, CA 94720 United States
AB: The hyper-arid Atacama Desert of northern Chile is virtually devoid of precipitation and vegetation, yet it is characterized by a succession of fluvial landforms (stream terraces and alluvial fans) that reflect periodic changes in climatic and/or tectonic regimes. The eastern regions of the desert, which rise toward the Andean foothills, possess a series of gravelly to bouldery geomorphic surfaces capped by the prominent Atacama Gravels. The Atacama Gravels have been dated via 40Ar-39Ar on ash and are well known to be Miocene age (15.3 to 10.5 Ma). Recently, Nishiizumi et al. have dated surficial boulders of Atacama Gravels via cosmogenic radionuclide (CRN) chemistry (10Be, 26Al, 21Ne), finding that the samples were at secular equilibrium with respect to 10Be and 26Al. However, with 21Ne, those authors were able to calculate CRN ages of 8.2-9.0 Ma, in accord with independent age estimates of ash 40Ar-39Ar ages. In addition, the CRN data indicate the lowest boulder erosion rates in the world outside of Antarctica, suggesting that younger surfaces should be amenable to CRN dating using 10Be and 26Al (if younger than secular equilibrium). We report on our initial exploration of the numerical ages of post-Atacama fluvial surfaces in three watersheds along a precipitation gradient of the Atacama Desert. All three watersheds contain what appear to be a similar sequence of well-defined post-Atacama Gravel geomorphic surfaces (K-Ar of biotite in ash in one location constrains the age of the surfaces to Pliocene or younger: 4.8 Ma). CRN analyses of surface boulders, combined with 40Ar-39Ar analyses of volcanic ash in related terraces, should provide an initial perspective into the sequence of ages of some of the oldest post-Atacama gravel surfaces in the region, and are the beginning of an effort to determine if there is a regional synchrony to the terrace and fans deposits of the central Atacama Desert.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1815 Erosion and sedimentation
DE: 5415 Erosion and weathering
SC: Tectonophysics [T]
MN: 2003 Fall Meeting