Abstract: Clay Mineralogy of Eastern Gulf of Alaska

Bruce F. Molnia, Paul T. Fuller

Analysis of the clay-mineral content of 87 bottom samples from the continental shelf of the northeastern Gulf of Alaska (Montague Island to Yakutat Bay) shows a remarkable uniformity in clay-mineral assemblage.

All samples are characterized by chlorite averaging 51%, illite averaging 37%, and kaolinite averaging 10%. Montmorillonite is present in only one-third of the samples analyzed and averages 2%. The rank of the major clay minerals differs from the western Gulf of Alaska where illite is in greater quantity than chlorite. Mineralogy was determined by X-ray diffraction of Mg⁺⁺ and ethylene glycol-saturated clay samples of less than 2µ. The quantity of each clay mineral present was determined by a peak-area technique.

The major source area for these sediments is the highly deformed and intruded metasedimentary and metavolcanic terrane of the Chugach, St. Elias, and Fairweather Mountains, where erosion is mainly mechanical, by glaciation. Most material enters the gulf as rock flour where it is distributed by surface currents from east to west, thereby achieving a nearly uniform distribution.

Samples from the Miocene to Pleistocene Yakataga Formation, which underlies the Holocene sediment offshore and which is the major onshore formation in the eastern part of the area, contain the same clay-mineral assemblages as the modern sediment, with percentages that fall within the same ranges. Apparently the bedrock of the Yakataga Formation, like the modern sediment, was eroded and deposited mechanically.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC