Depositional Environment of Clay Minerals from Northeast Gulf of Alaska: ABSTRACT

Abstract

Analyses of more than 100 recent bottom samples from the northeastern Gulf of Alaska continental shelf between Icy Point and Prince William Sound show an average clay mineral assemblage of 61% kaolinite and chlorite, 37% illite, and 2% smectite. Organic content generally is less than 2%. The clays being deposited today are predominantly glacially eroded, fluvially transported, and rapidly deposited. The present depositional environment is characterized by rapid mechanical erosion at the outcrop with little or no chemical weathering, rapid fluvial transport, and continental shelf sedimentation rates as high as 30 m/1,000 years. Of the 50 largest streams draining into the Gulf of Alaska, all are either glacially fed or drain a recently deglaciated area. The Yakataga Formation of Miocene through Holocene age which underlies much of the continental shelf in this area, has many mud-rich units similar to the modern shelf sediment in clay mineralogy and mode of origin. The Yakataga Formation averages 60% kaolinite and chlorite, 27% illite, and 13% smectite. Differences in the smectite and illite content of the Yakataga Formation and the modern shelf samples may represent post-depositional diagenesis of the Yakataga Formation. In climate, geography, and sedimentary regime, the northeastern Gulf of Alaska depositional environment has remained relatively constant since middle Miocene time. One question yet to be answered is whether the presently accumulating continental shelf sediment will prove to be a hydrocarbon source bed of the future. Drilling in the Yakataga Formation has failed to confirm its status as a major source unit. End_of_Article - Last_Page 753------------