Research paper Cosmogenic-nuclide burial ages for Pleistocene sedimentary fill in Unaweep Canyon, Colorado, USA Greg Balco a, * , Gerilyn S. Soreghan b , Dustin E. Sweet c , Kristen R. Marra b , Paul R. Bierman d a Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA b School of Geology and Geophysics, University of Oklahoma, 100 E Boyd St., Norman, OK 73019, USA c Department of Geosciences, Texas Tech University, Lubbock, TX 79409, USA d Geology Department, University of Vermont, 180 Colchester Ave., Burlington, VT 05405, USA article info Article history: Received 28 September 2012 Received in revised form 15 February 2013 Accepted 21 February 2013 Available online 14 March 2013 Keywords: Unaweep Canyon Pleistocene Cenozoic River incision River capture Gunnison River Colorado Plateau Aluminum-26 Beryllium-10 Burial dating Burial isochron dating abstract We applied both single-sample and isochron methods of cosmogenic-nuclide burial dating to determine the age of the sedimentary fill in Unaweep Canyon, western Colorado, USA. This stratigraphic sequence is of interest because it documents capture and diversion of the ancestral Gunnison River by the Colorado River during late Cenozoic incision of the Colorado Plateau. Seven 26 Ale 10 Be burial ages from sedi- mentary infill penetrated by a borehole in central Unaweep Canyon, as well as a 26 Ale 10 Be burial isochron age formed by multiple clasts and grain-size separates in a sample from the stratigraphically lower Gateway gravels, indicate that canyon blockage, initiation of lacustrine sediment accumulation, and presumed river capture, took place 1.41  0.19 Ma. Lacustrine sedimentation ceased 1.34  0.13 Ma. Ó 2013 Elsevier B.V. All rights reserved. 1. Unaweep Canyon Unaweep Canyon forms a 70-km-long wind gap through the Uncompaghre Plateau in western Colorado, exposing Proterozoic basement beneath the Mesozoic cover of the plateau. It stretches from near the confluence of the Colorado and Gunnison Rivers near its northeast end to the Dolores River near its southwest end (Fig. 1). The name “Unaweep” (“canyon with two mouths”) denotes the fact that it currently contains two small streams (East Creek and West Creek) flowing away from a topographic divide at 2150 m elevation in the center of the canyon (Figs. 1 and 2). Beginning with the Hayden Survey of the late 1800’s(Peale, 1877; Gannett, 1882), all observers have agreed that the drainage area of modern East and West Creeks is inadequate to account for canyon incision. Although the canyon has been hypothesized to have been formed by Quaternary glacial erosion (Cole and Young, 1983), its low elevation relative to other glaciated parts of the Rockies makes this unlikely. Most researchers agree that the canyon was formed by late Ceno- zoic fluvial incision due to past occupation by the Gunnison River, the Colorado River, or both (Peale, 1877; Gannett, 1882; Cater, 1966; Hunt, 1969; Sinnock, 1978; Lohman, 1981; Steven, 2002; Aslan et al., 2005; Soreghan et al., 2007 , 2008). Soreghan et al. (2007 , 2008) further argued that the canyon was originally formed in Proterozoic basement by Permo-Pennsylvanian glacial erosion, buried by late Paleozoic sedimentary fill, and re-exposed by Cenozoic incision. However, this hypothesis is not relevant to the Cenozoic incision history discussed in this paper. Most previous work proposed that the canyon was abandoned by the Gunnison and/or Colorado Rivers due to tectonic arching of the Uncompaghre Plateau (Lohman, 1961 , 1981; Cater, 1966; Hunt, 1969; Sinnock, 1981; Scott et al., 2001; Steven, 2002). In this hy- pothesis, the present arched long profile of the canyon would represent a deformed bedrock river profile. However, a geophysical survey by Oesleby (1978) and, subsequently, a drillcore collected by * Corresponding author. Tel.: þ1 510 644 9200; fax: þ1 510 644 9201. E-mail address: balcs@bgc.org (G. Balco). Contents lists available at SciVerse ScienceDirect Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo 1871-1014/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.quageo.2013.02.002 Quaternary Geochronology 18 (2013) 149e157