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Thompson, C.H., 1996

Pre-late-Wisconsonian glacial history of the Naknek River valley, southwestern Alaska

Bibliographic Reference

Thompson, C.H., 1996, Pre-late-Wisconsonian glacial history of the Naknek River valley, southwestern Alaska: Logan, Utah, Utah State University, M.S. thesis, 90 p.

Abstract

The lower Naknek River in southwestern Alaska dissects thick (~20 m) exposures of Pleistocene glaciogenic sediments. The stratigraphy of the deposits and their physical, geochemical, mineralogical, and geochronological properties were studied to determine the number and timing of glacial advances represented. Multivariate data reduction methods (cluster and principal component analyses) were applied to the data to differentiate diamicton beds. The results show a clear separation of drift of the lower Naknek River valley from drift of northern Bristol Bay and from younger, moraine-comprising drift up valley. In the Naknek River valley, however, there is no stratigraphic trend to the clusterings. The similarity between diamicton units suggests that most of the drift in the Naknek River valley is from one advance, or that the clustering methods were not sensitive to detect multiple advances from overlapping source areas. At South Naknek beach, a marine-lag horizon separates two diamictons. Based on amino acid (D/L) ratios in fossil molluscs, this lag is correlated with the last interglacial (~125 ka). The underlying diamicton records an advance >125 ka. A thermoluminescence age estimate on a lava-baked diamicton at Telephone Point provides a minimum age on a lower diamicton of 250 +/- 20 ka. The age of the overlying, regionally extensive drift sheet is constrained by an optically stimulated luminescence age, amino acid ratios, and radiocarbon ages from drift at Halfmoon Bay. These data, together with sedimentologic evidence for glacial-estuarine conditions, suggest that ice advanced into a tidally influenced estuary during a time of high (about +12 m) relative sea level about 80 ka. Mak and Johnston hills, ridges previously mapped as moraines, are reinterpreted as ice-thrust ridges, and may not represent stable ice margins. The terms 'Mak Hill drift' and 'Johnston Hill drift' should be abandoned as discernible lithostratigraphic and climatological units, because they may not represent unique glacier advances, and because they appear to be lithostratigraphically indistinguishable.

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