10Be SURFACE-EXPOSURE CHRONOLOGY OF PINEDALE AND BULL LAKE MORAINES OF THE FREMONT LAKE REGION, WIND RIVER RANGE, WESTERN WYOMING, U.S.A.

¹⁰Be SURFACE-EXPOSURE CHRONOLOGY OF PINEDALE AND BULL LAKE MORAINES OF THE FREMONT LAKE REGION, WIND RIVER RANGE, WESTERN WYOMING, U.S.A.

By Kathryn Ladig

Thesis Advisor: Dr. George H. Denton

A Lay Abstract of the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Master of Science

(in Earth Sciences)

May, 2011

            First described by Blackwelder in 1915 in the Wind River Range of western Wyoming, the Bull Lake (outboard) and Pinedale (inboard) moraine sets, or their equivalents, have since been recognized in most formerly glaciated basins of the Rocky Mountains in the western United States.  Both moraine sets at Fremont Lake were deposited by an ice lobe that drained a high plateau ice cap of the Wind River Range and filled the lake basin at the mountain front.   ¹⁰Be cosmogenic dates from boulders rooted in moraine crests indicate Bull Lake-age moraine formation 148,500 ± 4,650 years ago (n = 4), as well as four stages of Pinedale-age moraine formation dating to 20,360 ± 500 years ago (n = 12), 17,580 ± 460 years ago (n = 8), 16,870±430 years ago (n = 3), and 16,070 ± 400 years ago (n = 6).  

The outer Pinedale moraines correspond in age with the outboard moraines deposited by the Laurentide Ice Sheet in the Great Lakes region and on Long Island.  This common signal implies that the Wind River moraine chronology in not an outlier for the middle latitudes of North America, at least for the last glacial maximum (LGM).  These dates also correspond to ages of moraine formation in the Southern Hemisphere and to a rise in atmostpheric CO­₂ concentration.  This indicates that the concentration of atmospheric CO₂, which affects temperatures on a global scale, is an important factor in the last deglaciation.  The moraine ages near Fremont Lake also point to slow glacial recession in the Northern Hemisphere, as opposed to an observed rapid recession in the Southern Hemisphere. This differing glacier behavior might point to a simultaneous southward shift of the westerly wind belts in both polar hemispheres as an important factor in initiating the last termination.