Dating fluvial terraces by 230Th/U on pedogenic carbonate, Wind River Basin, Wyoming

AbstractReliable 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.

Download Full-text

Paraglacial origin for terraced river sediments in Bow Valley, Alberta

Canadian Journal of Earth Sciences ◽

10.1139/e82-196 ◽

1982 ◽

Vol 19 (12) ◽

pp. 2219-2231 ◽

Cited By ~ 60

Author(s):

Lionel E. Jackson Jr. ◽

Glen M. MacDonald ◽

Michael C. Wilson

Keyword(s):

Single Channel ◽

Sedimentary Environment ◽

River Sediments ◽

Early Period ◽

Rocky Mountain ◽

Radiocarbon Dates ◽

Fluvial Terraces ◽

Mountain Front ◽

Main Period ◽

Glacial Advance

Fluvial terraces flank the course of the Bow River for 100 km from the eastern margin of the Rocky Mountain Front Ranges to Calgary and beyond. The terraces are cut predominantly in gravel fill, which ranges in thickness from approximately 10 m in the Calgary area to 30 m near the mountain front. Sedimentary structures in the gravels indicate a braided stream sedimentary environment in contrast to the present quasi-stable, sinuous, single-channel form of the Bow River. Radiocarbon dates on ungulate remains from the gravels indicate the main period of fill occurred ca. 11 500–10 000 RCYBP (radiocarbon years before present). Previous workers have postulated that the gravels originated directly as outwash from a glacial advance to or beyond the mountain front. This explanation has been refuted by recent stratigraphic and palynological investigations. A complex nonglaciofluvial origin is proposed for these terraces and the sediments that form them. The last glacial advance to reach the mountain front was well into retreat by as early as ca. 13 400 RCYBP. Spruce and pine forest was established in the Bow River drainage by ca. 10 400–10 000 RCYBP and glaciers were restricted to high cirques. It is probable that the early period of fill deposition (ca. 11 500–10 000 RCYBP) was initiated when mountain tributary trunk streams of the Bow River were choked with debris-flow-delivered sediment during the construction of paraglacial debris fans and related phenomena. The debris flows were distinctive features of early nonglacial times, when landforms left unstable by ice retreat mass-wasted into the valleys. Paraglacial processes explain the early postglacial history of the Bow drainage and this example provides a model readily applicable to other drainages in formerly glaciated terrain.

Download Full-text