Preliminary evaluation of emergent postglacial shorelines, Naknek and Iliamna Lakes, southwestern Alaska

Folowing late Wisconsin deglaciationof the Iliamna Lake and Naknek Lake basins, lowered lake levels created a flight of beach ridges and wave-cut terraces rimming the present-day lakes. Terraces at nearly identical altitudes above Iliamna Lake (at about 10, 25, 30 and 40 m above the present-day lake) and separated by over 70 km northeast to southwest indicate that there has been little if any tilting in this direction as a result of differential glacio-isostatic rebound or regional tectonism. Likewise, at Naknek Lake, the consistent altitudes of terraces at about 5, 15, and 30 m and separated by a maximum of 50 km southeast to northwest suggest that these shorelines are also horizontal, or nearly so. The most prominent terraces above both lakes lie about halfway between the highest terrace and the present-day lake level (24 m above Iliamna Lake and 15 m above Naknek Lake). If these terraces are correlative, then this correlation indicates some common control on lake-level fluctuations such as base level or climate. On the other hand, Iliamna Lake shows five terraces and Naknek Lake only three, suggesting that other factors, including different histories of outlet erosion, played a role in lake level changes at the two lakes. The 24-m terrace and higher terraces at Iliamna Lake and all three terraces at Naknek Lake were formed during latest Wisconsin and early Holocene time. The 40-m terrace at Iliamna Lake was probably cut during the Newhalen stade of the Brooks Lake glaciation. The 30-m terrace, and all higher terraces, are older than a prominent, pinkish-organe tephra, which is correlated by major element chemistry with the Lethe tephra, whose previously published ¹⁴C age is about 12.6 ka. Based on previously published ¹⁴C ages, the 24- and 17-m terraces at Iliamna Lake apparently formed at 8.5 and 5.5 ka, respectively. Lake level at Naknek Lake fell below about 9 m above present lake level before 7.4 ka and after the late-glacial Iliuk stade of the Brooks Lake glaciation.