Deposition of Mount Mazama Tephra in a Landslide-Dammed Lake on the Upper Skagit River, Washington, USA

  1. James D. L. White4 and
  2. Nancy R. Riggs5
  1. J. L. Riedel1,
  2. P. T. Pringle2 and
  3. R. L. Schuster3

Published Online: 24 MAR 2009

DOI: 10.1002/9781444304251.ch14

Volcaniclastic Sedimentation in Lacustrine Settings

Volcaniclastic Sedimentation in Lacustrine Settings

How to Cite

Riedel, J. L., Pringle, P. T. and Schuster, R. L. (2001) Deposition of Mount Mazama Tephra in a Landslide-Dammed Lake on the Upper Skagit River, Washington, USA, in Volcaniclastic Sedimentation in Lacustrine Settings (eds J. D. L. White and N. R. Riggs), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304251.ch14

Editor Information

  1. 4

    Department of Geology, University of Otago, PO Box 56, Dunedin, 9015, New Zealand

  2. 5

    Geology Department, Northern Arizona University, Box 4099, Flagstaff, AZ 86011, USA

Author Information

  1. 1

    North Cascades National Park, 7280 Ranger Station Road, Marblemount, WA 98267, USA

  2. 2

    Washington DNR, Geology and Earth Resources Division, PO Box 47007, Olympia, WA 98504-7007, USA

  3. 3

    US Geological Survey, Box 25046, MS 966, Denver, CO 80225, USA

Publication History

  1. Published Online: 24 MAR 2009
  2. Published Print: 18 APR 2001

ISBN Information

Print ISBN: 9780632058471

Online ISBN: 9781444304251



  • deposition of Mount Mazama tephra in landslide-dammed lake on upper Skagit River, Washington;
  • bedrock in upper Skagit River watershed - geological setting;
  • deposits of Lake Ksnea;
  • textural analysis of the sediments - hydrometer method;
  • extent of landslide-dammed lake in upper Skagit River valley;
  • microprobe glass chemistry analyses of Mount Mazama ash;
  • texture of sediments from Lake Ksnea;
  • seven sedimentary units identified - units 1 to 7;
  • physical differences between the primary watersettled Mazama tephra and the tephra;
  • minimum age for damnation Creek landslide - 7040 14C yr BP


The cataclysmic eruption of Mount Mazama, Oregon, at c. 6730 14C yr BP, deposited tephra over 1.0 × 106 km2 of north-western North America. Primary tephra fall accumulated to a thickness of 2 cm in the upper Skagit River watershed, Washington. Mazama tephra eroded from this watershed was deposited in Lake Ksnea, of 14 km length and 40 m depth. This lake was created when a landslide blocked the Skagit River at 7040 14C yr BP.

Horizontally bedded, dark grey silt and clay were deposited slowly by suspension settling in Lake Ksnea before the eruption of Mount Mazama. The 2-cm-thick primary Mazama tephra layer abruptly caps 7 m of pre-eruption sediments, and is overlain by as much as 17 m of Mazama tephra deposited relatively rapidly on a delta at the mouth of Damnation Creek. Most of a 13-m-thick section is composed of lacustrine tephra containing rhythmic stratified beds deposited by suspension settling. Turbidity currents deposited centimetre-scale, cross-bedded silt and tephra at the top of some rhythmite beds. Lower in this section, tephra containing abundant fine-grained terrestrial sediments and other sedimentary structures interrupts the rhythmite beds. These structures include faulted and warped beds, flame structures and pendants created by soft-sediment deformation. Tephra deposits are overlain conformably with cross-bedded sands throughout most of a 200-m-long section. Coarse alluvial gravels and landslide deposits unconformably overlie the tephra and sand at several locations.

The deposits described are interpreted as an inversely graded, prograding delta sequence composed almost entirely of Mount Mazama tephra. Despite a lack of age control on the rate of tephra deposition, the sedimentology of this section indicates that the tephra delta was deposited within 1 yr or less.