Late Quaternary Glacial Geology, Shoreline Morphology, and Tephrochronology of the lliamna/Naknek/Brooks Lake Area, Southwestern Alaska

• Main Author: Stilwell, Karen B.
• Format: Text
• Language: unknown
• Published: Utah State University 1995
• Subjects: Mak; glacier; Alaska
• Online Access: https://dx.doi.org/10.26076/2067-f8b5; https://digitalcommons.usu.edu/etd/6595

This study focuses on the late-Wisconsin Brooks Lake glaciation, lake-level fluctuations, and volcanic deposits in the lliarnna/Naknek/Brooks Lake area on the northern Alaska Peninsula, southwestern Alaska. The Brooks Lake glaciation consists of five stades, from youngest to oldest: Kvichak, Uiarnna, Newhalen, Iliuk, and Ukak. This thesis reassigns the type Mak Hill moraine to a pre-late-Wisconsin glaciation, and considers the moraine enclosing Naknek Lake an early-late-Wisconsin deposit correlative to either the Kvichak stade, lliarnna stade, or both. The presence in the Iliamna Lake valley, and the absence in the Naknek Lake valley of a two-fold earliest-late-Wisconsin Kvichak/Iliamna glacial sequence suggest that the two glacial systems responded differently to climate change, or glacier/bed dynamics due to differing ice sources and glacier configurations. Plant macrofossils dated at 26,155 ± 285 I4C yr BP afford a new maximum-limiting age on the type Kvichak moraine. Slope angles on the type Kvichak and Iliamna moraines are less steep (~11-15°) than on younger Newhalen, lliuk, and Ukak moraines (~18-20° ), indicating that a considerable length of time separated the Iliamna and Newhalen stades. Correlation of this time-stratigraphic marker with other better dated Alaskan glacial sequences suggests that the interstadial occurred -13-14 ka. Following late-Wisconsin de glaciation of the Iliamna and Naknek lake basins, lake levels lowered, creating a flight of wave-cut terraces. Horizontal terraces, formed during latest-Wisconsin/early-Holocene time, at ~40 m above Iliamna Lake, and ~15 and ~30 m above Naknek Lake, suggest that these shorelines are not tilted as a result of glacial isostatic rebound or regional tectonism. The most prominent terraces above both lakes lay about halfway between the highest terrace and present-day lake level. If these terraces are correlative, then this indicates some climate control on lake-level fluctuations. Electron-microprobe analysis of six late-Pleistocene tephra samples allows five samples to be correlated with latest-Pleistocene Lethe tephra, and extends the Lethe ash plume ~125 km westward, and ~150 km northwestward of its presumed source area. Analysis of four early-Holocene black tephras fails to support any correlations, suggesting that there are multiple black tephras in the area. Ash C, a tricolored ash, consists of more than one chemically distinct tephra, and there is little consistency between color zones of the ash at different sites.