PROVENANCE OF SUPRAGLACIAL CLASTS ON THE MATANUSKA GLACIER, ALASKA

The Matanuska Glacier in south-central Alaska flows north out of the Chugach Mountains, which are composed of rocks from the Chugach Terrane, the Penninsular Terrane, and rocks common to both. The most detailed geologic map of part of the Matanuska Glacier’s drainage basin is that of Burns et al. (1982): near the glacier’s terminus, Jurassic volcanic rocks, mostly tuff and tuff breccia, and plutonic rocks, including tonalite, diorite, and gabbro, of the Peninsular Terrane are exposed. Further up-valley, Tertiary felsite and clastic sedimentary rocks of fluvial and lacustrine origins common to both terranes are also exposed, but are less abundant than the Jurassic rocks. Cretaceous phyllite, argillite, and metagraywacke rocks of the Chugach Terrane occur even further up-valley. However, the geologic map of Burns et al. (1982) only extends from 61°40’N to 61°50’N; which excludes approximately 70% of the southernmost reaches of the glacier.

The glacier is actively eroding the mountains by abrasion and plucking processes, which entrain rock fragments that are then carried with the ice as it flows down valley and later exposed by ablation and deposited at the terminus. Supraglacial debris, which originates as rockfall, is transported as lateral and medial moraines. We have identified 23 different lithologies in supraglacial debris and ice-contact deposits, as well as several pebbles that are as-of-yet unidentified. Slate, phyllite, and metagraywacke are the most abundant lithologies. Also, the diversity of pebble lithologies is greatest along the southwestern side of the terminus. In order to determine the percent abundance of each lithology and their spatial distribution, pebble counts were taken at 18 different locations along the glacier’s terminus. A 1x1 meter grid was laid out, and 100 pebbles were collected from each site at 10 cm increments. Hand-sample and thin-section analyses were used to determine specific rock compositions. The goal of this research is to determine the provenance of these lithologies up-valley based on glacier flow lines. Our results suggest that the glacier’s source area is largely composed of Cretaceous rocks of the Chugach Terrane. The results of this study provide supplementary information to the existing geologic map of Burns et al. (1982), especially in the glacier’s source area.