Abstract

A 93 m core obtained at Portage, Alaska records four prograding cycles of estuarine deposition for the past 8230 ± 100 years. Analyses of texture, mineralogy, paleontology, and sedimentary structures enable definition of eight lithologic units. Mineralogic studies show that past and present sedimentation at Portage has been largely mud and sand from the Susitna River on the northwest side of Cook Inlet. Six radiocarbon dates from concentrated organic debris in the core and on the surface enable determination of sedimentation rates for four intervals and show rates to be higher and to vary more at depth than rates nearer the surface. The glacier-carved Turnagain Arm fiord has been receiving sediment for at least the past 14 000 years, as estimated by extrapolation of calculated sedimentation rates to the thickness of unconsolidated sediment drilled at Portage, which totals approximately 300 m. Presently, deposition is accomplished "instantly" as the result of tectonic subsidence and compaction of sediment caused by periodic earthquakes, in combination with a turbid estuarine system.

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