HR: 0800h
AN: C41A-0047    [Abstracts]
TI: Ice-rafted Landslide Deposits Versus `Regular` Supraglacial Debris --- an Issue for Paleoclimatic Reconstruction?
AU: * Shugar, D H
EM: dshugar@sfu.ca
AF: Centre for Natural Hazard Research, Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A1S6, Canada
AU: Clague, J J
EM: jclague@sfu.ca
AF: Centre for Natural Hazard Research, Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A1S6, Canada
AB: Parts of many glaciers in high mountains are covered by debris. The debris may originate from recurrent rock falls or from less common, but much larger landslides. Although relatively rare, rock avalanches rapidly distribute thin sheets of blocky debris over large portions of a glacier`'s surface. The debris sheets affect glacier regimen by decreasing ablation and increasing glacier velocity. In some instances, they may trigger surges. A supraglacial debris sheet is modified as it is carried downvalley by the glacier and ultimately is redeposited at the glacier`'s lateral margins and snout as a moraine. Although these moraines differ in lithology and sedimentology from those built by climatically driven glacier advances, they have been misidentified as such in many mountain ranges, including the Karakoram, Italian Alps and Southern Alps of New Zealand, leading to erroneous conclusions about Holocene climatic fluctuations. Work in the Karakoram and elsewhere has improved our ability to differentiate Quaternary moraines and landforms produced by rock avalanches. Few workers however, have attempted to distinguish moraines consisting of catastrophic landslide debris transported, en masse, by glaciers, from moraines formed during, or at the culminations of, glacier advances. To provide a better basis for interpreting moraines derived from landslides, we collected sedimentologic data from the debris sheet of an earthquake-triggered rock avalanche that fell onto Black Rapids Glacier, Alaska, in November 2002. Macrofabric and matrix texture distinguish the rock avalanche deposit from supraglacial debris deposited by rock fall, sampled in the vicinity of the landslide. On a landscape scale, the rock avalanche deposit has a coarse boulder rim, crude inverse grading with a blocky surface cap, a matrix of silt and sand, and conspicuous striping, with alternating bands of finer and coarser monolithologic material oriented parallel to the direction of debris transport. Similar features have been reported from other rock avalanches. Clasts are generally larger, more poorly sorted, and more angular than rock fall clasts transported longer distances at the surface of a medial moraine. Contoured fabric diagrams indicate a strong preferential orientation of elongate boulders parallel to fine and coarse stripes at the surface of the proximal part of the debris sheet. Clast macrofabrics from distal parts of the landslide deposit are weaker, and no transverse fabrics were measured. The debris sheet is being transported downvalley by Black Rapids Glacier at a rate of about 30-35 m per year, and in the five years since its emplacement has become elevated ~15-20 m above adjacent debris-free glacier ice. It remains unclear how the characteristic physical properties of the debris sheet will be modified prior to its emplacement as lateral and end moraines later in this century and in the next.
DE: 0720 Glaciers
DE: 1621 Cryospheric change (0776)
DE: 1810 Debris flow and landslides
SC: Cryosphere [C]
MN: 2007 Fall Meeting