Research about Portage, Alaska

USGS1983

Holocene history of the estuarine area surrounding Portage, Alaska, as recorded in a 93 m core

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.

Alaska↗

USGS1976

The Placer River Silt — An intertidal deposit caused by the 1964 Alaska earthquake

At the head of Turnagain Arm near Anchorage, the major lasting geologic consequence of the Alaska earthquake of 1964 was deposition of the Placer River Silt, an intertidal deposit covering an area of more than 18 km 2 and containing more than 20X10 6 m 3 of sediment. This formation, which was derived from erosion of intertidal bars in Turnagain Arm, averages 1.5 m thick seaward of the Seward Highway and 0.9 m thick landward. Its distribution is controlled both, by the arrangement of the major streams that enter tidewater and by manmade features such as the highway and railroad embankments. The Placer River Silt is still being deposited. The contemporary lowland sedimentation system of the Portage area includes a number of depositional settings: (1) In the Placer River Silt upper tidal flats, levees, and channelway fans, (2) elsewhere in the intertidal zone tidal stream channels, lower tidal flats, and saltwater marsh, and (3) in the supratidal zone gravel levees, freshwater marsh, bogs, and lakes. Since 1964, the critical environmental results of the earthquake-caused deposition have been abandonment of the settlement of Portage, formation of an unsightly blanket of silt, destruction of natural plant communities, localized erosion, and creation of quicksand hazards.

Alaska↗