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Unpublished Paper
Quaternary Geology and Geochronology of the Uppermost Arkansas Valley, Colorado- Glaciers, Ice Dams, Landslides, and Floods
Crestone Science Center, Guidebook No. 6 (2010)
  • James P. McCalpin
Abstract
This field trip to the uppermost Arkansas Valley of central Colorado is based mainly on work performed since 2008, in an area with a long history of bedrock mapping by renowned geologists of the U.S. Geological Survey beginning in the 1880s (e.g. Emmons, 1886). The Quaternary features of the region were first described by Capps in 1909. Despite over 100 years of intermittent geologic studies here, there were still new discoveries to be made in the past few years. Day 1 of the trip emphasizes landslides (in the morning) and glacial deposits (in the afternoon). The “take-home messages” of the morning’s stops are: (1) that previously published bedrock maps of the area (e.g. Tweto, 1974a) severely underestimated the extent of landsliding in the region, and (2) there is a continuum of gravitational deformation between sackung, incipient landsliding, and full landsliding (which creates a rubbilized landslide deposit). Areas subjected to the first two stages appear to be intact bedrock at first glance, and have been mapped as such on almost all published geologic maps. This was done without regard for the rather obvious postglacial landforms (downslope-facing scarps, upslope-facing scarps, linear troughs, closed depressions) indicating extensional spreading/toppling at their heads. On the afternoon of Day 1 we look at the Quaternary geology around the town of Leadville, emphasizing the moraines and outwash of Pinedale, Bull Lake, and pre-Bull Lake episodes. The subsurface geology of unconsolidated deposits was described by Emmons (1886) and Emmons et al. (1927) based on mine shafts and tunnels, but it is difficult to relate what they saw to what we can see today at the surface. Day 2 concentrates on the glacial ice dams that dammed the upper Arkansas River during various ice advances, the lake that formed upstream from the dam (Three Glaciers Lake), and the flood gravels deposited downstream from the ice dams when they failed catastrophically. The evidence for the flood was first described by Scott (1975, 1984), but he only speculated on the location and height of the ice dams, and he never identified any features associated with the ice-dammed lake. His dating of the floods had to rely mainly on relative-age dating and correlation; neither luminescence dating nor cosmogenic surface-exposure dating had been developed at the time of studies. The “new” flood story was outlined in large part by Keenan Lee, working independently of the STATEMAP mappers (McCalpin and crew), the moraine mappers and hydrologic calculators (Eric Leonard and crew), and the cosmogenic daters, (Briner, Young and crew). USGS geologists Cal Ruleman and Ralph Shroba got all the parties together in 2008. We now have some more quantitative answers about the sequence and numerical ages of moraine deposition, river damming, lake formation, dam failure, and outburst flooding.
Keywords
  • glacial geology,
  • glacial lakes,
  • landslides,
  • outburst floods,
  • cosmogenic dating
Disciplines
Publication Date
2010
Citation Information
James P. McCalpin. "Quaternary Geology and Geochronology of the Uppermost Arkansas Valley, Colorado- Glaciers, Ice Dams, Landslides, and Floods" Crestone Science Center, Guidebook No. 6 (2010)
Available at: http://works.bepress.com/james_mccalpin/10/