PLEISTOCENE GLACIATION OF THE GREATER YELLOWSTONE AREA

The Pleistocene Greater Yellowstone glacial system (GYGS) was large (~20,000 km²) and its center of mass shifted dramatically both during the last glaciation and between the last two glaciations. During the last (Pinedale) glaciation ice first advanced southward from the Beartooth Uplift into the Yellowstone Plateau but eventually built up on the plateau to heights sufficient to reverse direction and flow northward across the highest peak in central Yellowstone (Mt. Washburn). South of Yellowstone in Jackson Hole ice first advanced down a main drainage from the east (Buffalo Fork) and pushed northward up the main drainage from Yellowstone (Snake River), but later advanced southward from Yellowstone and pushed up the deglaciated lower Buffalo Fork valley damming a lake. In Jackson Hole, outwash as thick as 50 m of recycled quartzite cobbles buried or nearly buried moraines. Floods east, west, north, and south of the GYGS punctuated outwash deposition.

Cosmogenic ages around the periphery of the GYGS are oldest to the east (18.8 ± 0.9 ¹⁰Be ka), intermediate to the north (16.5 ± 1.4 ¹⁰Be ka), and youngest to the south (<14.6 ± 0.7 ¹⁰Be ka). The changes in flow direction and the pattern of cosmogenic ages are explained by progressive buildup of the GYGS toward the source of winter moisture coming up the Snake River Plain.

The northern outlet glacier of the GYGS receded 50 km to a stable position (Deckard Flats, 14.2 ± 1.2 ¹⁰Be ka), at which time ice again flowed southward into space vacated by downwasting of the Yellowstone Plateau icecap. Unloading by melting of 1,000 m of ice on the Yellowstone Plateau and caldera was not accompanied by volcanism as was common in other glaciated volcanic areas, indicating magmatic pressures beneath Yellowstone had not built high enough to erupt.

The penultimate (Bull Lake) glaciation extended well beyond the Pinedale on the south (48 km), southwest (23 km), and west (22 km) sides of the GYGS but on the north and east sides the Pinedale commonly exceeded the Bull Lake. Bull Lake moraines on the south of the GYGS date 151-157 ¹⁰Be ka and those to the west date ~150 ka by combined obsidian-hydration and K-Ar methods. The northeastward shift of the center of mass from Bull Lake to Pinedale may be explained by late Pleistocene uplift on the leading margin of the Yellowstone hotspot and subsidence on the trailing margin.