PRELIMINARY ANALYSIS OF PLEISTOCENE BASE-LEVEL CHANGE IN GRANBY BASIN, COLORADO

Granby basin is a sub-basin of Middle Park basin in northern Colorado, ~35 km southwest of Rocky Mountain National Park. The basin is characterized by extensive Pleistocene glacial moraines, tills, and outwash deposits, as well as fluvial terraces deposited by the Colorado and Fraser Rivers. These rivers join and exit the basin at Windy Gap, a narrow bedrock notch (~150 - 500 m wide) in the Windy Gap Volcanic Member of the Middle Park Formation located ~6 km northwest of the town of Granby. Windy Gap is the only drainage outlet of the Colorado River from Granby basin.

As part of a recent Reclamation study, I analyzed Pleistocene base-level change in Granby basin from the extent of previously mapped glacial outwash deposits and a publicly available 2010 lidar dataset. This analysis is based on two primary assumptions: 1) that outwash deposits graded to base-level at the time of deposition, and 2) that preserved outwash deposits represent the maximum possible base-level elevation. I chose three relative age-correlated outwash deposits for this analysis: mid-pre Bull Lake (mpbo), late-pre Bull Lake (lpbo), and Early Bull Lake (ebo). The lowest elevation of each correlative deposit (mpbo = 2499 m, lpbo = 2437.5 m, ebo = 2407 m) represents the maximum base-level values for each relative unit of time, as compared to the modern base-level measured from the Colorado River at Windy Gap (2379 m).

Preliminary results from this analysis suggest the possibility of paleolakes in Granby basin. Although previous maps of glacial outwash deposits are limited to the area surrounding the Colorado River, apparent Quaternary surfaces within the basin identified using lidar and aerial imagery also grade to the contoured base-level elevations. Some of these surfaces were field-verified, although further work is needed to determine whether they are truly correlative to previously mapped outwash deposits. Quaternary surfaces at high elevations, up to 120 m greater than modern base-level, that grade to similar elevations, in conjunction with the small aperture size of Windy Gap suggest that this outlet may have been overwhelmed by the volume of glacial meltwater and outwash exiting the basin. Water and sediment may have backed up in the basin and potentially formed short-lived lakes.