EVIDENCE FOR LATE WISCONSINAN OUTBURST FLOODS IN THE TOK-TANACROSS BASIN, UPPER TANANA RIVER VALLEY, EAST-CENTRAL ALASKA

The Tok-Tanacross basin is dominated by the Tok fan, which at the surface in the western half is Donnelly (oxygen-isotope stage 2) in age and is Holocene in age in the inset, eastern half. The Donnelly part of the Tok fan is primarily composed of massive, clast- and matrix-supported, pebble gravels with scattered small cobbles and interbedded medium to coarse pebbly sands. Gravels and pebbly sand beds are generally massive, ≤ l m thick, tabular with abrupt lower and upper contacts, and parallel the fan surface. In these coarse sediments, cross bedding is scarce, and lens-shaped channel fills of massive sand up to ~1 m thick are interlayered with the massive gravels.

Particularly instructive for understanding the development of the Tok fan during the last major glaciation is an exposure in the Donnelly fan remnant east of Tok River, where a 1.8-m diameter greenstone boulder was found in place near the top of clast-supported pebble gravel in the upper wall of a gravel pit. A nearby pile of extraordinarily large granite, quartz schist, greenstone, and basalt boulders, demonstrates that at least six other large boulders were encountered during pit excavation. Exceptionally large boulders of similar rock types were also encountered in the Tok fan only at two other, nearby localities directly in line with the Tok River valley and were previously interpreted as evidence of penultimate glaciation in the Tok-Tanacross basin.

Lack of glacial till in the Donnelly portion of the Tok fan, the absence of cut-and-fill structures and ripples, scarce cross bedding, and channel fills of massive sand indicate that the extraordinarily large boulders and enclosing sediments were not deposited by glacial ice or typical water floods. We propose 1) that the western half of the Tok fan is a large expansion fan deposited by massive, pulsating sheetflows during outburst floods emanating from the Tok River valley to the southwest and 2) that the extraordinarily large boulders were deposited as dropstones from icebergs and rapidly buried by hyperconcentrated flows during these periodic inundations. Evidence for outburst floods of Donnelly age was previously recognized in the upper Tok River valley by Schmoll (l984).