Pre-late Wisconsin advances recorded in coastal bluff exposures

Our recent geochronologic and stratigraphic investigations of the southern Ahklun Mountains (Figs. 1 and 2) provide new information on the timing and extent of Pleistocene glacier advances. We have identified and estimated the ages of at least three, and as many as six, pre-late-Wisconsin glacial intervals (see summary Fig. 5).

 

FIG. 1. Southern Ahklun Mountains region showing the maximum extent of late Pleistocene glaciers (green line). Ice-free areas within glacial limit are not shown. Stars indicate newly discovered localities of the Old Crow tephra. Light shading depicts mountainous area >200 m; darker shading are elevations > 800 m; darkest shading are modern lakes.

 

 

FIG. 2. Location of geochronological sample sites discussed in text. Togiak Bay region; map location shown in Fig. 1. Light shading depicts mountainous area >100 m; darker shading are elevations >250 m. Inset shows shaded relief map of Togiak Tuya. Contour interval = 40 m with lowest contour at 40 m above sea level.

Middle Pleistocene Advances:

Hagemeister Island: The oldest advances are represented by glacial-marine sediment on Hagemeister Island (Fig. 3), where amino acid data provide the basis for subdividing the complexly convoluted marine sediment (Fig. 3b) into four aminozones. Based on an age equation and assumptions about the average postdepositional temperature, the mean age estimates of four aminozones range from ~500 to 280 ka. The mean age estimates for three of the four aminozones are centered over odd-numbered marine oxygen-isotope stages (13, 11, and 9), suggesting that they are associated with highstands of eustatic sea level. Given the large uncertainties associated with these age estimates, however, other correlations are possible. Furthermore, because the deformed stratigraphic sequences lack unequivocal evidence for multiple transgressive cycles, we cannot exclude the possibility that fewer high-sea-level intervals are represented. In the most conservative interpretation, the amino acid data suggest an overall average age of ~400 ± 100 ka for the glacial-marine drift on Hagemeister Island.

FIG. 3. Glacial-marine drift exposed on south coast of Hagemeister Island (general location shown in Fig.2). (a) Close up of massive stoney mud with fragmented and articulated molluscan shells. (b) Prominent white tephra bed traces the tight isoclinal folds in severely deformed section. Shovel is 50 cm long.

 

Togiak River valley: The next, more recent advance is evidenced by pillow lava of the Togiak tuya (location of tuya shown in Fig. 2; Fig. 4a), a glacially streamlined volcano composed of basaltic tuff and lava. A 5-m-thick bed of pillows is exposed along a steep slope on the west side of the tuya ~25 m below the summit (Fig. 4b), indicating that it formed subaqueously. To confine a water body at the mouth of the Togiak River valley that opens onto the continental shelf requires a glacier-ice dam. We argue that the pillow lava formed as the tuya erupted through glacier ice and thawed an intraglacial lake into which the lava flowed. The elevation of the pillows indicates that the lake surface was 300 m above the floor of the lower Togiak River valley. Glacier ice surrounding the lake was therefore at least 300 m thick. This advance that coincided with the eruption of the Togiak tuya was dated at 263 ± 22 ka by laser-fusion 40Ar/39Ar, and possibly correlates to the deposition of the youngest glacial-marine deposits on Hagemeister Island.

 

FIG. 4. Togiak tuya in the lower Togiak River valley (location shown in Fig. 2). (a) View to the southwest. (b) Pillow lava from ~25 m below the west-central rim of the tuya. Hammer is 35 cm long.

 

We presently lack evidence from anywhere in the Bristol Bay region for a glacier advance during marine oxygen-isotope stage 6. This absence suggests that glaciers in the Ahklun Mountains remained relatively small during stage 6. Stage 6 glacial deposits are apparently absent in the Yukon Cordillera as well, suggesting that they have been obliterated by subsequent advances of equal or greater extent.

Late Pleistocene Advances:

Togiak River Valley: In contrast, early in the last glacial cycle, during the early Wisconsin (sl), glaciers readvanced down the Togiak River valley and overran a basaltic lava flow (Fig. 2) dated by TL at 70 ± 10 ka. This is approximately the same age, or slightly younger, than the Nushagak Formation of eastern Bristol Bay. Unlike the Nushagak Formation of eastern Bristol Bay, we have not yet found definitive evidence for glacial-marine sediments associated with this advance in western Bristol Bay.

Glacier advances during the late Wisconsin were limited to the highest mountain valleys where glacier termini lay >40 km inland from the present-day coast.

 

 

FIG. 5. Summary diagram showing geochronological data, extent of glacier ice down the Togiak River valley (purple shapes), and the marine oxygen-isotope record (red line; numbers are oxygen-isotope stages). The marine oxygen-18 record is used as a proxy for eustatic sea level, which is probably above its average Quaternary position (-55 m) during intervals highlighted in pink. Ages and age uncertainties on volcanics are represented by the yellow hatched bars (Togiak tuya: 40Ar/39Ar; Old Crow tephra: fission track; Togiak Bay basalt: TL). Amino acid age estimates and uncertainties indicated by circles with vertical lines; associated glacier advances are indcated by the purple polygons. The amino acid data suggest that the youngest glacial-marine drift may have been deposited at the same time as the eruption of the Togiak tuya. Extent of ice prior to 100 ka is uncertain.

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