Late Pliocene to late Pleistocene environments preserved at the Palisades Site, central Yukon River, Alaska

Abstract

The Palisades Site is an extensive silt-loam bluff complex on the central Yukon River preserving a nearly continuous record of the last 2 myr. Volcanic ash deposits present include the Old Crow (OCt; 140,000 yr), Sheep Creek (SCt; 190,000 yr), PA (2.02 myr), EC (ca. 2 myr), and Mining Camp (ca. 2 myr) tephras. Two new tephras, PAL and PAU, are geochemically similar to the PA and EC tephras and appear to be comagmatic. The PA tephra occurs in ice-wedge casts and solifluction deposits, marking the oldest occurrence of permafrost in central Alaska. Three buried forest horizons are present in association with dated tephras. The uppermost forest bed occurs immediately above the OCt; the middle forest horizon occurs below the SCt. The lowest forest bed occurs between the EC and the PA tephras, and correlates with the Dawson Cut Forest Bed. Plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central Alaska today. Large mammal fossils are all from common late Pleistocene taxa. Those recovered in situ came from a single horizon radiocarbon dated to ca. 27,000 14C yr B.P. The incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments.

Late Pliocene Dawson Cut Forest Bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska

“…18). PA tephra occurs immediately below this forest bed (Matheus et al, 2003), which, therefore, must correlate with Dawson Cut Forest Bed at Fairbanks and have an age of ca. 2 Ma.…”

“…8) and is only 75 cm above PA tephra at the Palisades site on the Yukon River ( Fig. 1) (Matheus et al, 2003). This unconformity represents a time when huge amounts of loess were removed from the valley bottoms and lower slopes of the Fairbanks area.…”

“…Groundwater, with its dissolved solids, would have percolated through the thawed loess at this time, depositing an iron-oxyhydroxide on surfaces of clasts, and, over time, forming concretions along the more permeable surfaces, such as unconformities. Ice-wedge casts immediately below Dawson Cut Forest Bed at the Palisades site (Matheus et al, 2003) offer physical evidence for this warming and thawing of the permafrost and indicate that the mean annual air temperatures in central Alaska just prior to growth of the Dawson Cut Forest Bed was at most −3 °C, but probably closer to −7 °C (Matheus et al, 2003).…”

“…1). Information was taken fromMatheus et al (2003). These tephra beds demonstrate that this lower peatforest bed correlates with the Dawson Cut Forest Bed in the Fairbanks region.…”

Formation history and physical properties of sediments from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

“…Data sources include: (1) logging data gathered for the Mount Elbert well, (2) stratigraphic and geologic information from the Alaska North Slope (Reimnitz et al, 1972;Bird, 1981Bird, ,1999Collett et al, 1988;Valin and Collett, 1992;Frederiksen et al, 1998;Inks et al, 2008), and (3) information on permafrost and ground surface temperature (Wolfe, 1980;Wolfe and Upchurch, 1987;Wolfe, 1994;Brigham and Miller, 1983;Parrish et al, 1987;Spicer and Chapman, 1990;Elias and Matthews, 2002;Matheus et al, 2003;Kaufman et al, 2004;Bujak Research International, 2008). We make the following assumptions:…”