Late Pleistocene Glacial Chronology of the Western Ahklun Mountains, Southwestern Alaska
New glacial mapping and 35 cosmogenic 36Cl surface exposure ages, the first ever reported from Alaska, constrain the extent and timing of late Pleistocene glacial fluctuations in the western Ahklun Mountains, southwestern Alaska. Morphometric and soil relative-age data characterize two main drift un...
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DigitalCommons@USU
1998
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| Online Access: | https://digitalcommons.usu.edu/etd/6551 https://doi.org/10.26076/0037-ccf8 https://digitalcommons.usu.edu/context/etd/article/7777/viewcontent/1998_Briner_Jason.pdf |
| Summary: | New glacial mapping and 35 cosmogenic 36Cl surface exposure ages, the first ever reported from Alaska, constrain the extent and timing of late Pleistocene glacial fluctuations in the western Ahklun Mountains, southwestern Alaska. Morphometric and soil relative-age data characterize two main drift units deposited during the Arolik Lake and Klak Creek glaciations, named herein. During the Arolik Lake glaciation (early Wisconsin), outlet glaciers emanated from an ice cap over the central portion of the Ahklun Mountains and deposited moraines at or beyond the modern coast. These moraines have slope angles averaging about 11° and crests averaging about 35 m wide. Four moraine boulders deposited during this glaciation have a weighted mean surface exposure age of 53.6 ± 2.0 36Cl ka. During the Klak Creek glaciation (late Wisconsin), ice-cap outlet glaciers deposited moraines 20-80 km up-valley from Arolik Lake moraines. Valley glaciers expanded from high massifs that fringe the major river valleys in the western Ahklun Mountains and terminated independently from the relatively restricted ice-cap outlet glaciers. Moraines deposited during the Klak Creek glaciation have steeper slopes (mean = -18°) and sharper crests (mean= about 17 m) than do Arolik Lake moraines. Twenty-eight 36Cl ages were obtained from six Klak Creek moraines from three valleys and reveal two phases of glaciation during the late Wisconsin, one from about 25 to 23 36Cl ka, and another from 19 to 15 36Cl ka. An ice-cap outlet glacier moraine underlies a valley glacier terminal moraine, both of which have ages of 18-19 36Cl ka, and indicates that the ice-cap outlet glacier had retreated from its maximum position shortly before the valley glacier reached its maximum position. Equilibrium-line altitudes (ELAs) for reconstructed Klak Creek valley glaciers average about 400 m, which is only about 200 m below the estimated modem altitude. The restricted extent of Klak Creek glaciers might reflect a lack of available moisture as sea ice covered the Bering Sea ... |
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