A HOLOCENE ASH-FALL RECONSTRUCTION FOR ANCHORAGE, ALASKA

Anchorage, Alaska is located 100 km east of the Aleutian arc. Because of its proximity to this volcanic island chain, over the last century Anchorage has experienced three major ash-fall events resulting from volcanic eruptions originating across Cook Inlet. Previous studies on Anchorage bluff exposures indicate eleven Holocene ash-fall events (Riehle, 1985). Our study attempts to evaluate the terrestrial record using the tephrostratigraphy obtained from small, closed lake basins. Lacustrine sediment cores provide a highly resolved and datable record from which the Holocene ash-fall history of Anchorage can be determined.

Eight lacustrine cores were recovered from two kettle lakes located 11 km apart in the Anchorage metropolitan area: Little Campbell, and Goose Lakes. Both basins are situated in closed, forested drainage basins of low relief (10-40m). Basal glacial clay and silt lamina obtained in a Goose Lake core suggest that a complete Holocene record was recovered. Basal 14C ages from Little Campbell, however, indicate that this record is limited to the last 7530 years B.P. A portion of the tephra units are identified visually as discrete, 0.5-1 cm gray fine-grained layers, though others are indistinct and can only be recognized using a high-resolution magnetic susceptibility (MS) meter. All tephra units are associated with a relatively high MS reading, and a low percent loss on ignition. Sixteen tephra samples have been inferred in Goose Lake, whereas seven have been identified in the Little Campbell cores. Magnetic susceptibility, loss on ignition, and visual stratigraphy records allow the preliminary correlation of tephrostratigraphy between basins, and suggest that Little Campbell contains an erosional hiatus, likely due to early Holocene lake level lowering. Terrestrial organic matter samples (for AMS 14C age dating) from strategic stratigraphic levels are being processed to constrain inter-basin and intra-basin core correlations and provide age control for specific ash units. Thinsections of each ash unit will be used to evaluate upper and lower contacts of the ash layers, and to assess the effects of possible reworking.

Riehle, J.R., 1985. A reconnaissance of the major Holocene tephra deposits in the upper Cook Inlet region, Alaska. Journal of Volcanology and Geothermal Research, v. 26, p. 37-74