Cosmogenic Nucleide measurements in southern South America and implications for glacial chronology and geomorphology.

KAPLAN, M., DOUGLASS, D., SINGER, B., HULTON, N., CORONATO, A., STONE, J.

La Plata

Congreso; XVI Congreso Geológico Argentino; 2005

Asociación Geológica Argentina

In Patagonia, in situ cosmogenic nuclides have been used to constrain the ages of glacial deposits, quantify boulder erosion, and date lava flows. At Lago Buenos Aires (LBA), 46˚S, exposure ages suggest a major glaciation occurred at least once between 760,000 and 200,000 yrs ago, from ~190,000 to 109,000 yrs ago (best estimate is 150,000–140,000 yrs ago), and from ~23,000-16,000 yrs ago. Moraines corresponding to marine isotope stage 4 have not been found at LBA. In the Strait of Magellan, the last major glaciation occurred from ~25,000 to 17,000 yrs ago. On moraines >25,000 yrs old, the largest uncertainties in dating are the spatial and temporal variability in erosion and exhumation of boulders, whereas on younger moraines the largest uncertainty is due to present production rate uncertainties (up to 10%). Cosmogenic nuclides can be used also to quantify erosion rates. At LBA, boulder erosion rates are as low as 1-2.5 m/Myr (meters/million years), which explains how glacial landforms have survived in Patagonia since 1 Myr ago. In contrast, preliminary data indicate erosion rates and/or exhumation could be higher and episodic on Tierra del Fuego and in southern Santa Cruz, especially near the sea.