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Exposure dating outwash gravels to determine the age of the greatest Patagonian glaciations

Andrew S. Hein, Tibor J. Dunai, Nicholas R. J. Hulton and Sheng Xu
Exposure dating outwash gravels to determine the age of the greatest Patagonian glaciations
Geology (Boulder) (February 2011) 39 (2): 103-106


The relict moraines in Argentine Patagonia archive major expansions of the Patagonian Ice Sheet throughout the Quaternary, and are one of the few terrestrial climate proxies in the middle latitudes of the Southern Hemisphere that extends beyond the last glacial cycle. Determining their individual ages has proved challenging but has important implications for our understanding of terrestrial climate change in southern South America over the duration of the Quaternary. Here, for the first time, we demonstrate that sediment on outwash terraces can be directly dated to determine the timing of early-middle Quaternary glacial advances in southern South America. Cosmogenic (super 10) Be and (super 26) Al surface exposure ages were obtained from outwash gravels associated with two of the oldest glacial sequences in the Lago Pueyrredon valley (47.5 degrees S), Argentina. The outermost Gorra de Poivre glacial sequence marks the greatest extent of the Patagonian Ice Sheet. A cobble from this surface gives (super 10) Be and (super 26) Al surface exposure ages of ca. 1.2 Ma, consistent with bracketing (super 40) Ar/ (super 39) Ar age constraints obtained elsewhere in Patagonia. This is the first time early Pleistocene glacial surfaces have been directly dated in Patagonia. Cobbles on a younger Canadon de Caracoles outwash terrace give exposure ages of ca. 600 ka, while 4 of 5 boulders on an associated moraine give exposure ages that are significantly younger. If the demonstrated stability of outwash terraces in the valley is common throughout the region, it will be possible to extend Patagonian glacial chronologies as far back as the early Pleistocene.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 39
Serial Issue: 2
Title: Exposure dating outwash gravels to determine the age of the greatest Patagonian glaciations
Affiliation: University of Edinburgh, School of Geosciences, Edinburgh, United Kingdom
Pages: 103-106
Published: 201102
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 28
Accession Number: 2011-019890
Categories: Quaternary geologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2011045
Illustration Description: illus. incl. sketch map
S47°40'00" - S47°00'00", W71°30'00" - W70°30'00"
Secondary Affiliation: Scottish Universities Environmental Research Centre, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201112
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