VOL. 19, No. 7 A PUBLICATION OF THE GEOLOGICAL SOCIETY OF AMERICA JULY 2009 The impact of snowmelt on the late Cenozoic landscape of the southern Rocky Mountains, USA Inside: ▲ 2009 Medal and Award Recipients, p. 12 ▲ 2009 GSA Research Grant Recipients, p. 18 ▲ Groundwork: The state of interactions among tectonics, erosion, and climate: A polemic, p. 44 Not Just Software. RockWare. For Over 26 Years. 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Printed in the USA using pure soy inks. 43 Classified Advertising All of these tools (and more!) are now available in an annual license for academic use: unlimited installations within a department 44 Groundwork: The state of interactions among tectonics, for the same price as a single, standard license. erosion, and climate: A polemic $2,499 46 Journal Highlights Erratum: The 2009 Geologic Time Scale on p. 61 of the April-May GSA Today contained an error: Hirnantian was misspelled. A corrected time scale is posted at www.geosociety.org/science/timescale/. Since 1983 303.278.3534 • 800.775.6745 GSA celebrates our three-year association with the International Year of Planet Earth. RockWare.com GSATodayJuly09.indd 1 5/29/2009 12:40:49 PM The impact of snowmelt on the late Cenozoic landscape of the southern Rocky Mountains, USA Jon D. Pelletier, Dept. of Geosciences, University of Arizona, Shoshone River has incised at least 800 m into the Eocene Gould-Simpson Building, 1040 East Fourth Street, Tucson, Willwood Formation (near horizon) and lower units, exposing Arizona 85721-0077, USA; [email protected] sediments as old as Triassic. Shale maturation data suggest that Cenozoic erosion in this area could have been as great as 1.5 km (Heasler and Kharitonona, 1996), indicating that the ABSTRACT thickness of basin fill sediments could have been much greater The intramontane basins of the southern Rocky Mountains, than the 800 m exposed in Figure 1. Broad, low-relief Quater- USA, have undergone up to 1.5 km of erosion from the middle nary strath terraces illustrate that local downcutting took place Miocene to the present. Here I explore the hypothesis that this episodically, followed by periods of lateral erosion (Mackin, erosion could have been caused primarily by an increase in the 1937; Hancock and Anderson, 2002). Dated basalt flows within intensity of snowmelt flooding. In the middle Miocene, snow- basin fill remnants indicate that most of this erosion took place melt runoff was limited to the highest elevations (>3 km) and from the late Miocene to the present (McMillan et al., 2006). hence impacted only a small fraction of the regional landscape. The Bighorn Basin is not unique—significant magnitudes of As the global climate system cooled during the late Miocene late Cenozoic erosion took place in all of the intramontane and Plio-Quaternary periods, the fraction of total river dis- basins of the southern Rocky Mountains. McMillan et al. (2006) charge derived from snowmelt increased significantly in areas used GIS techniques and dated basin-fill remnants to map the between 1.5 and 3 km elevation, thereby increasing the magni- spatial distribution of minimum erosion in the region and tude of flooding during periods of snowmelt and the resulting found that up to 1.5 km of late Cenozoic erosion occurred in bedload sediment flux and erosion of rivers in that elevation the