MODEL FOR LATE WISCONSINAN MASSIVE OUTBURST FLOODING, TOK RIVER VALLEY, NORTHEASTERN ALASKA RANGE

Physiographic and stratigraphic evidence indicates that huge floods emanated from the Tok River valley in the upper Tanana River drainage during the last major (Donnelly) glaciation. Our investigations support an earlier suggestion by Schmoll (1984) that the ultimate source of those flood waters was Glacial Lake Atna in the northeastern Copper River basin. Water from Glacial Lake Atna inundated the lower Slana River valley and entered the upper Tok River drainage through Mentasta Pass.

Vesicular volcanic clasts from the Wrangell Mountains are key components of flood gravels. Their presence in morainal deposits in the lower Slana River Valley, coupled with key physiographic features, provide evidence that ice from the Wrangell Mountains south of the northeastern Copper River basin blocked the Slana River drainage. Thick lacustrine sand containing scattered pebbles of both Wrangell Mountains and Alaska Range lithologies overlies till of Alaska Range origin in the Slana River drainage, documenting the existence of a meltwater lake dammed between Wrangell Mountains ice and a relatively small, local valley glacier in the upper Slana River- Mentasta Pass-Mineral Lake area. We suggest that rising waters of Glacial Lake Atna eventually destabilized the massive Slana glacier dam, subglacially releasing voluminous lake waters that overtopped the valley glacier at ~690 m elevation in the Mentasta Pass area, breached the Mineral Lake terminal moraine, and poured down the Tok River drainage. At peak flows, flood waters reached an elevation of ~730 m at the Mineral Lake moraine, destroying the northwestern half of the moraine. Depths of the flood flow through the 2.8-km-wide morainal breach could have been ~166 m

Vesicular volcanic clasts of Wrangell Mountains derivation have been traced 80 km northward through the Tok River drainage and 95 km westward down the Tanana River. Dilution by the influx of Alaska Range rock types decreased the abundance of vesicular volcanic clasts in flood gravels from ~54 percent where flood waters entered the Tok River drainage to ~10 percent at the toe of the Tok expansion fan.