DEFORMATION OF PRO-GLACIAL SEDIMENT DUE TO AN ADVANCING ICE MARGIN AT THE MATANUSKA GLACIER, ALASKA

In recent years, the Matanuska Glacier, Alaska, has had an advancing ice margin that is propagating deformation into the proglacial zone by a thick-skinned style of glaciotectonics (Pyke et al., 2003). This study presents new survey data and field observations from the summer season of 2004 supporting this earlier observation. This movement was measured using a total station surveying instrument. The total station was set up over a stationary benchmark on a terminal moraine. Four rows of 19, 14, 10, and 13 survey flags were inserted into the proglacial sediment at either 5 or 10 meter increments, depending on accessibility. The distance between the total station and the flags was measured approximately every three days. To account for sources of error from meteorological conditions and human error, a second, stationary point on the moraine was also measured during each survey. Localized debris flows and slumping of pro-glacial sediments occurred during the survey period, destroying measurement sites and further restricting access. Sites affected by these events were disregarded in our analysis.

The average movement of the survey flags was between 0.2-0.4 m/d, with an average surveying error of 0.25 m/day. The survey flags located closer to the ice margin were, on average, the only survey points whose motion exceeded the margin of error. This result is likely indicating that the greatest magnitude of deformation occurs closest to the ice margin. Evidence of deformation was observed in old stream terraces that were up-thrust 1-3 m above the present streambed, forming large stair-step formations. There were also indications that deformation was occurring on a scale of less than 10 cm, where cracked sediment appeared to be thrust over the down-glacier sediment. These data and observations support the hypothesis of Pyke and others (2003) that the re-advancing ice margin is deforming proglacial sediments, with greater displacement closer to the active ice margin.