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Dating fluvial terraces by Th/U on pedogenic carbonate, Wind River Basin, Wyoming |
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| Authors: | Warren D Sharp Kenneth R LudwigOliver A Chadwick Ronald AmundsonLaura L Glaser |
| Institution: | a Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA b Department of Geography, University of California, Santa Barbara, CA 93106, USA c Division of Ecosystem Sciences, University of California, Berkeley, CA 94720, USA d Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA |
| Abstract: | Reliable and precise ages of Quaternary pedogenic carbonate can be obtained with 230Th/U dating by thermal ionization mass spectrometry applied to carefully selected milligram-size samples. Datable carbonate can form within a few thousand years of surface stabilization allowing ages of Quaternary deposits and surfaces to be closely estimated. Pedogenic carbonate clast-rinds from gravels of glacio-fluvial terraces in the Wind River Basin have median concentrations of 14 ppm U and 0.07 ppm 232Th, with median (230Th/232Th) = 270, making them well suited for 230Th/U dating. Horizons as thin as 0.5 mm were sampled from polished slabs to reduce averaging of long (≥105 yr), and sometimes visibly discontinuous, depositional histories. Dense, translucent samples with finite 230Th/U ages preserve within-rind stratigraphic order in all cases. Ages for terraces WR4 (167,000 ± 6,400 yr) and WR2 (55,000 ± 8600 yr) indicate a mean incision rate of 0.26 ± 0.05 m per thousand years for the Wind River over the past glacial cycle, slower than inferred from cosmogenic-nuclide dating. Terrace WR3, which formed penecontemporaneously with the final maximum glacial advance of the penultimate Rocky Mountain (Bull Lake) glaciation, has an age of 150,000 ± 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum. |
| Keywords: | U-series Geochronology Pedogenic carbonate Wind River Bull Lake glaciation Fluvial terrace Incision rate Alpine glaciation |
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