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

T.L. Péwé^1,*, J.A. Westgate^2,, S.J. Preece², P.M. Brown³ and S.W. Leavitt⁴

¹ Department of Geology, Arizona State University, Tempe, Arizona 85287, USA
² Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada
³ Rocky Mountain Tree-Ring Research, Inc., 2901 Moore Lane, Fort Collins, Colorado 80526, USA
⁴ Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721, USA

View larger version (60K): [in this window] [in a new window]   Figure 1. Extent of late Cenozoic glaciations (shaded) in Alaska and northwestern Canada and location of Fairbanks and Yukon-Tanana Upland. HH—site of Halfway House exposure, B—Birch Creek site, P—Palisades site, LC—Lost Chicken Mine. Figure was modified from Péwé (1975a) and Tarnocai and Schweger (1991).

View larger version (34K): [in this window] [in a new window]   Figure 2. Index map of the Fairbanks area, south-central Yukon-Tanana Upland, Alaska, showing Dawson Cut Forest Bed sites (*) and locations of new tephra studies. Heavy lines in inset map (B) show position and orientation of geological sections shown in the indicated figures. CA—Chatanika site; DC—Dawson Cut; WD—West Dawson; EN—Engineer Creek; GI—Geophysical Institute; U—University of Alaska campus; CH—Church site; BH—Birch Hill.

View larger version (63K): [in this window] [in a new window]   Figure 3. Extent of present-day boreal (taiga) forest in Alaska (after Viereck and Little, 1972). T—Tofty, P—Palisades site, K—Koyukuk River.

View larger version (21K): [in this window] [in a new window]   Figure 4. Schematic composite cross section of a creek valley near Fairbanks, Alaska, illustrating stratigraphic relations of late Cenozoic deposits. The unconformity between the Gold Hill Loess and overlying beds is emphasized by a heavier line.

View larger version (73K): [in this window] [in a new window]   Figure 5. Panoramic view of the late Cenozoic loess record at the eastern end of the Gold Hill Cut mining exposure, now known as the "Troy L. Péwé Climatic Change Permafrost Reserve." Maximum height of loess cliffs is 50 m. Inset map is plan view of Gold Hill Cut showing location of trenches 1, 2, 3, and 4. Location is shown in Figure 2, and detailed stratigraphy is shown in Figure 6.

View larger version (40K): [in this window] [in a new window]   Figure 6. This sketch of late Cenozoic loess stratigraphy at the east end of Gold Hill Cut illustrates the most complete and detailed record of the major climatic warmings and coolings in central Alaska over approximately the last 3 m.y. Here is the only known record of all three members of the Gold Hill Loess and five major unconformities in the loess, each indicating major topographical, climatic, and permafrost changes in the environment. The ages and names of these unconformities are as follows, in order of increasing age: Holocene-Wisconsin unconformity, 10 ka; the Eva Forest Bed Interglaciation unconformity, 125 ka; the Ester Interval unconformity, younger than 780–older than 610 ka; the Dawson Cut Forest Bed unconformity near the PA tephra, ca. 2 Ma; the unconformity between the lower Gold Hill Loess and Cripple Gravel, ca. 3 Ma. Generalized stratigraphy, loess polarity, and tephra beds are indicated in four vertical trenches in Gold Hill Loess: 1, 2, 3, 4 (see Fig. 5). Black bar—normal polarity; white bar—reversed polarity; diagonal lines—covered or no polarity recorded. Ages of polarity units are from standard geomagnetic time scales (Shackleton et al., 1990; Valet and Meynadier, 1993). xxxxxx—tephra bed, —ice-wedge cast, —concretions, tree stump. Details of stratigraphy, tephra, ice wedges, and forest beds have been omitted in Wisconsin and Holocene formations for simplicity. Basal date of ca. 3 Ma is from paleomagnetic studies (Westgate et al., 1990). Thermoluminescence (TL) dates on loess are by G.W. Berger (Berger and Péwé, 2001; Berger, 2003). Age of tephra beds is from glass fission-track methods (Preece et al., 1999). Old Crow tephra and Dome Ash Bed (DAB) were not recognized at this site but were identified at stratigraphic levels indicated on the figure during gold-mining operations when bluff was farther south. Tephra bed names as originally defined, except for: DAB—Dome Ash Bed, MG—Mosquito Gulch tephra, HH—Halfway House tephra, and VT—Variegated tephra.

View larger version (26K): [in this window] [in a new window]   Figure 7. Geologic cross section at the Ester Island mining exposure 2 km west of the confluence of Ester and Cripple Creeks, and 16 km west of Fairbanks, Alaska, showing distribution of sediments prior to removal of loess and dredging of the gravels by gold-mining operations during the period 1940–1965. The type section of the Ester Ash Bed is in the middle Gold Hill Loess, 1.5 m above the Cripple Gravel. Sheep Creek F tephra age by thermoluminescence (TL) dating on bracketing loess is from Berger et al. (1996). Other tephra ages are by glass fission-track methods (Preece et al., 1999). Holocene Engineer Loess and the Ready Bullion Formation, as well as ice wedges and stratigraphic details in the Goldstream Formation, are omitted for simplicity. MGHL—middle Gold Hill Loess. Surface topography, top of gravel, bedrock, and the fault are based on survey and drill-hole data from U.S. Smelting, Refining and Mining Company, Fairbanks, Alaska. This figure is modified after Péwé (1952). See Figure 2 for location of section.

View larger version (35K): [in this window] [in a new window]   Figure 8. (A) Dawson and Engineer placer mining cuts created in the 1940s and 1950s near the mouth of Engineer Creek, 13 km north of Fairbanks (Fig. 2). Cuts expose perennially frozen Pliocene-Pleistocene loess and auriferous stream gravel in creek valley bottoms and on adjacent rock terraces. Dawson Cut Forest Bed is preserved only in creek valley bottoms on younger Fox Gravel. (B) Composite sketch of stratigraphy at north wall of Engineer Creek. Exposure is type locality of EC tephra, Mining Camp tephra, and Dawson Cut Forest Bed. See supplementary information Table S1 (see text footnote 1) for data on wood of Dawson Cut Forest Bed.

View larger version (27K): [in this window] [in a new window]   Figure 9. Stratigraphic sketch of the lower part of the 60 m loess cliff at Ester Island, 16 km west of Fairbanks, Alaska, illustrating relationship of the Ester Interval unconformity and overlying conglomerate bed to the underlying Ester Ash Bed in middle Gold Hill Loess. Conglomerate is composed of pebbles and blocks of silt and rare schist fragments. Paleomagnetic ages on extreme right of figure: M—Matuyama reversed chron; J—Jaramillo normal subchron. Note difference in scale.

View larger version (158K): [in this window] [in a new window]   Figure 10. A 20-m-thick section of Pliocene-Pleistocene perennially frozen Gold Hill Loess at Gold Hill mining excavation overlain by late Pleistocene Goldstream Formation (loess). Upper part of the late Pleistocene and Holocene loess has been removed from the top of the section by placer gold-mining operations. The buried Eva Forest Bed represents the Sangamon Interglaciation. Lower part of section of Gold Hill Loess was temporarily covered by frozen and tumbled blocks of Gold Hill Loess, which were removed by jets of cold water under high pressure. North wall of Gold Mining Cut, 10 km west of Fairbanks, Alaska. Photograph PK 2159 by T.L. Péwé, 3 June 1952.

View larger version (43K): [in this window] [in a new window]   Figure 11. (A) Longitudinal geological section of late Cenozoic stratigraphy of perennially frozen sediments and forest beds exposed in the east wall of lower Eva Creek Cut mining exposure, 16 km west of Fairbanks, Alaska. Rectangle indicates area covered by Figure 15. (B) Geological cross section in upper part of Eva mining cut and the adjacent Eva Bench Cut. (C) Composite stratigraphic sketch of perennially frozen late Cenozoic loess, tephra, gravel, and forest beds on west wall of upper Eva Creek Cut mining exposure. Old Crow tephra age (131 ± 11 ka) is from this study; thermoluminescence (TL) age (190 ± 20 ka) of Sheep Creek F tephra on bracketing loess is from Berger et al. (1996). Location of sections is shown in Figure 2, and data on the pollen sample are given in Table 6. DAB—Dome Ash Bed.

View larger version (18K): [in this window] [in a new window]   Figure 12. Oxide bivariate plots showing correlatives of the Dome Ash Bed in the Fair-banks region (crosses) and the Birch Creek site (UT1835). UT745 is Dome Ash Bed at the reference site at Eva Creek (Figs. 2 and 11C).

View larger version (24K): [in this window] [in a new window]   Figure 13. Stratigraphic setting and identity of tephra beds in Gold Hill Loess in West Dawson area (see Fig. 2). (A) Three-dimensional sketch made by Péwé during placer gold-mining operations in 1956. Dome Ash Bed (DAB) is shown to be close to the unconformity separating the "Wisconsin Muck" from the "Illinoisan Silt" (now called Gold Hill Loess), but its precise stratigraphic position cannot be assessed on this diagram. (B–D) Other vertical exposures with tephra beds in upper part of Gold Hill Loess.

View larger version (49K): [in this window] [in a new window]   Figure 14. (A) Stratigraphic sketch of transverse section across the upper end of Sheep Creek Cut mining exposure at bend in mining cut, 15 km NW of Fairbanks, Alaska. Dawson Cut Forest Bed of iron-stained and cracked spruce logs and overturned stumps occurs at top of and in upper meter of Fox Gravel and underlies green middle Gold Hill Loess. Holocene silt overlying Goldstream Formation of Wisconsin age is generalized for simplicity. Unconformity beneath Goldstream Formation is indicated by heavy line. (B) Stratigraphy of west wall of Sheep Creek Cut. Dawson Cut Forest Bed consists of smashed, flattened, iron-stained spruce stumps, logs, and sticks in 1–3 m of organic-rich silt and in upper meter of Fox Gravel. All underlie green Gold Hill Loess. Goldstream Formation and Holocene loess are generalized for simplicity.

View larger version (180K): [in this window] [in a new window]   Figure 15. White spruce stump of Dawson Cut Forest Bed rooted in silt and gravel of the Fox Gravel at the base of the Gold Hill Loess at Eva Creek (see Fig. 11A). (Photograph 471 by Troy L. Péwé, 18 September 1949; previously published in Péwé, 1975).

View larger version (142K): [in this window] [in a new window]   Figure 16. Formerly prostrate fossil black spruce stump (Picea mariana) preserved near top of silty, coarse Fox Gravel with angular clasts at elevation of 250 m in Eva Creek valley bottom and excavated from level of Troy Péwé's feet. This stump is part of Dawson Cut Forest Bed (see sample no. 191, supplementary information, Table S1 [see text footnote 1]). Photograph PK29313 by T.L. Péwé, 14 June 1989.

View larger version (32K): [in this window] [in a new window]   Figure 17. Sketch of stratigraphy of perennially frozen late Cenozoic sediments of the basal part of Cripple Sump mining exposure (Fig. 7) exhibiting tephra and black silt of Dawson Cut Forest Bed; 16 km west of Fairbanks, Alaska. Tephra samples were collected by Péwé in 1949.

View larger version (29K): [in this window] [in a new window]   Figure 18. Stratigraphy of tephra beds close to the lower peat–forest bed at the Palisades site, Yukon River, Alaska (Fig. 1). Information was taken from Matheus et al. (2003). These tephra beds demonstrate that this lower peat–forest bed correlates with the Dawson Cut Forest Bed in the Fairbanks region.

View larger version (11K): [in this window] [in a new window]   Figure 19. 13C measured in cellulose component of modern and fossil wood samples from the Fairbanks area, Alaska. Holocene, Eva Forest Bed, and Dawson Cut Forest Bed samples are from perennially frozen loess and retransported loess. Numbered samples refer to specimens from Dawson Cut Forest Bed (see supplementary information, Table S1 [see text footnote 1]); unnumbered data points refer to samples in Figure 27 of Péwé et al. (1997).

View larger version (161K): [in this window] [in a new window]   Figure 20. Lost Chicken tephra above late Pliocene, preglacial peat at Lost Chicken Mine, east-central Alaska. This tephra bed has a glass fission-track age of 2.92 ± 0.15 Ma, the weighted mean and standard error of three age determinations. Scale is 10 cm long.