ABSTRACT

Glaciation is the single most important process responsible for contributing sediment to the northeastern Gulf of Alaska continental shelf. Over 75 percent of the northeastern gulf's drainage basin is presently ice-covered or only recently has emerged from beneath Holocene glacial ice. Many glaciers terminate in rivers that drain into the gulf or terminate in tidewater bays. This study examines the mineralogy of the northeastern Gulf of Alaska continental shelf clay minerals in light of the glacially dominant nature of the area.

Analyses of 110 bottom samples from the Gulf of Alaska continental shelf between Icy Point and Prince William Sound show an average clay mineral assemblage of 61 percent kaolinite + chlorite, 37 percent illite, and 2 percent smectite. Over most of the shelf deviations from the average composition are generally <10 percent. One significant exception is an area south of the Copper River where illite content is as high as 65 percent and kaolinite + chlorite ranges from 35 to 48 percent.

Suspended-sediment and bottom samples collected in successive years from the Copper River, the largest sediment source to the gulf, have variations in clay mineralogy suggesting that different tributaries and hence different source areas control the composition of the river's mineralogy during different years. Upper river sources are rich in kaolinite + chlorite whereas illite-rich sediment is introduced in the lower 100 km. Transportation of sediment south of the Copper River, across the shelf, and into Prince William Sound is documented by a decrease in illite from a continental shelf point source south of the river's mouth.

In addition to modern fluvial input, a second source of the modern shelf sediment appears to be the glacially derived Miocene to Quaternary Yakataga Formation. Older formations such as the Poul Creek and Katalla Formations show fewer compositional similarities to nearby shelf sediment.