Abstract

The < 2-µm equivalent spherical diameter (e.s.d.) fractions of 700 sediment samples from the major rivers and marginal seas of Alaska (constituting 74% of the continental shelf in the United States) were analyzed for clay mineralogy by X-ray diffraction. The clay-mineral assemblages in the Gulf of Alaska and south-central Bering Sea are consistent with the general latitudinal trends suggested for world oceans. In the north Bering, Beaufort, and east Chukchi Seas, however, kaolinite concentrations and kaolinite/chlorite ratios are similar to or higher than those commonly reported for mid-latitudes. The clay-mineral concentrations and dispersal patterns can be related to terrigenous sources and prevailing currents.

Three mineral suites were recognized in the Gulf of Alaska. The glaciomarine sediments of fjords and the eastern shelf consist typically of the primary phyllosilicates, micas, and chlorites, derived locally from argillite, graywacke, and metavolcanics under conditions of mild chemical weathering. More glycol-expandable clay minerals (GECM) in the north Gulf substantiate clay supply to that region primarily from the Yakataga and Poul Creek Formations, and to a lesser extent from the Copper River. The northern Gulf clay-mineral suite is transported to central Prince William Sound and northeast Kodiak Shelf by the Alaska coastal current. In the western Gulf, clay minerals adjacent to the Alaska Peninsula are derived principally from the Susitna River via Cook Inlet, whereas those of the outer shelf presumably have their provenance in Kodiak Island and the Katmai Ash submarine deposit.

In the southeast Bering Sea, a progressive northwestward decrease in GECM, from 65% to 11%, is caused by a gradually decreased sediment export from the Alaska Peninsula and the Aleutians, with a possible minor increase in illite from the Kuskokwin River. The Yukon River is the predominant clay source for the north Bering Sea, as signified by an abrupt increase in that area of GECM (20% to 35%) and of kaolinite/chlorite ratios. Clay mineralogy indicates that the central Chukchi Sea is the major repository of the Yukon clays, suggesting that clay-mineral stratigraphic studies have implications for interpretation of the Quaternary paleogeography of “Beringia.”

In the Beaufort Sea nearshore, influence of different fluvial inputs and their dispersals by currents are sharply defined. The lack of a definite pattern to the distribution of clay minerals in the outer continental shelf may be due to haphazard transport of clays by ice rafting and/or reworking and redistribution of sediments resulting from ice gouging of the sea substrate. Movement of sediment to the west in the Alaskan portion of the inner shelf of Beaufort Sea is evident. An apparent seaward increase in illite/GECM ratio in the Colville Delta is tentatively ascribed to regeneration of degraded illite-micas by K+ fixation in saline waters. An isolated area of sediment rich in expandable clay minerals in the central Beaufort Shelf presumably is relict.

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