Lea, P.D., 1990

Publication Details

  • Title:

    Quaternary environments and depositional systems of the Nushagak lowland, southwestern Alaska
  • Authors:

    Lea, P.D.
  • Publication Date:

    1990
  • Publisher:

    University of Colorado, Boulder 
  • Ordering Info:

    Not available
  • Quadrangle(s):

    Dillingham; Naknek; Nushagak Bay

Bibliographic Reference

Lea, P.D., 1990, Quaternary environments and depositional systems of the Nushagak lowland, southwestern Alaska: University of Colorado, Boulder, Ph.D. dissertation, 355 p., illust.

Abstract

Unconsolidated sediments in the southern Nushagak lowland provide a detailed record of Quaternary environmental change. The Nushagak Formation and Halfmoon Bay(?) drift are glacial and proglacial fluvial/estuarine deposits associated with an extensive advance of ice from the Ahklun Mountains and Alaska Peninsula. Both units have been deformed by advancing glaciers to create large ice-thrust moraines. Two overlying nonglacial sedimentary complexes have yielded pollen and beetle fossils. The Flounder Flat complex represents mainly deposits of colder-than-modern tundra lakes. The younger Etolin complex includes organic silt and associated inorganic sediments that accumulated in swales and on lower slopes >40,000 yr B.P. Lowermost peaty silt of the Etolin complex locally contains pollen spectra indicative of birch-shrub tundra, whereas higher samples suggest a colder-than-modern graminoid tundra. The Igushik Formation forms a ubiquitous blanket of periglacial eolian sediments deposited during the last glaciation. The distribution of eolian facies, including loess, sand-sheet deposits, and loess-sand intergrades, is controlled primarily by spatial and temporal variations in former outwash source zones, and secondarily by paleowind vectors. Initial aggradation of eolian sand was followed by deposition of the Tunuing Silt Bed, representing a long-lived slackening of eolian sedimentation about 22,000-17,000 yr B.P. The bulk of eolian sand-sheet deposits accumulated between about 17,000 and 12,500 yr B.P., after which climatic warming led to rapid ice retreat, a consequent sharp reduction in eolian sedimentation, and irreversible degradation of permafrost. Detailed facies analysis was undertaken of two depositional systems. Contemporaneous glacial-tectonism of proglacial intertidal and fluvial deposits of the Nushagak Formation produced a distally accreting wedge of deformed sediment that dictated the overall pattern of sediment dispersal and led to extensive gravity-driven resedimentation. Periglacial eolian sediments in southwestern Alaska are dominated by sand-sheet deposits and sand-loess intergrades. Similar sediments dating to the maximum of the last glaciation are probably widespread throughout Alaska, despite previous emphasis on end-member facies: sand-dune deposits and loess. Many surface dunes, heretofore ascribed to the glacial maximum, probably formed later by reworking of underlying sand-sheet deposits.

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