Dating fluvial terraces by230Th/U on pedogenic carbonate, Wind River Basin, Wyoming

Abstract Reliable and precise ages of Quaternary pedogenic carbonate can be obtained with 230 Th/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 232 Th, with median ( 230 Th/ 232 Th) = 270, making them well suited for 230 Th/U dating. Horizons as thin as 0.5 mm were sampled from polished slabs to reduce averaging of long (≥10 5 yr), and sometimes visibly discontinuous, depositional histories. Dense, translucent samples with finite 230 Th/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.