History and dynamics of glaciation in the northern Yellowstone National Park area : A reconstruction and quantitive evaluation of the interacting ice masses that covered northern Yellowstone during the Pinedale Glaciation

Pierce, Kenneth

Large masses of ice formed in northern Yellowstone National Park during the Bull Lake and Pinedale Glaciations. Bull Lake terminal morains of the northern Yellowstone icemass occur only on the park's western margin, in the West Yellowstone area. The Bull Lake Glaciation here is about 150,000 years old as dated by combined obsidian hydration and K-Ar methods. Bull Lake ice advanced into the West Yellowstone basin largely through lowlands later filled by the West Yellowstone rhyolite flow. The moraines deposited by this advance are bulky, loess mantled, and have the muted but clearly glacial constructional morphology typical of the Bull Lake elsewhere in the Rocky Mountains. Bull Lake glaciers extended 20 kilometers beyond Pinedale glaciers west of the park; but north of the park, Pinedale glaciers overrode and obliterated Bull Lake terminal moraines. This difference is attributed to emplacement of large rhyolite flows in the western part of the park in the interval between the Bull Lake and the Pinedale maxima. These rhyolite flows have K-Ar ages between 70,000 and 120,000 years old. They did not affect the flow of Bull Lake ice but diverted Pinedale flow by blocking westward flow and shunting it to the north. At some time between the Pinedale and Bull Lake glacial maxima, ice from the Bear tooth uplift advanced southward beyond the Washburn Range and dammed the Yellowstone River, leaving erratics of Precambrian crystalline rocks. Obsidian hydration methods date Pinedale terminal moraines near West Yellowstone as about 30,000 years old. Pinedale glaciers probably remained near full-glacial size until 15,000 to 20,000 years ago, and deglaciation of the Pinedale icecap on the Yellowstone plateau was largely completed by 13,000 to 14,000 radiocarbon years ago. The mass balance of the reconstructed northern Yellowstone glacier is calculated using (1) the area-altitude distribution shown on plate 1A, (2) an equilibrium-line altitude of 2,835 m as approximated by the glaciation limit, and (3) curves of specific net balance from modern glaciers. These values result in an annual net balance in which both net accumulation and ablation are equal to about 3 km3 . Because the calculated values of accumulation and ablation are approximately equal, this glacial reconstruction can meet massbalance requirements. The mass-balance data when combined with very rapid rates of recession, or meltback, indicate that the increase in discharge of the Yellowstone River owing to meltback would be less than 9 percent of the discharge during the Pinedale glacial maximum. The paucity of both outwash and recessional moraines upvalley from the northern Yellowstone terminal moraines also indicates that extensive recession did not result in outwash deposition due to greatly increased discharge and sediment load. General models, devised to show the relations in space and time among the various source areas of the northern Yellowstone outlet glacier, demonstrate the differing size and efficiency of these source areas for generating ice under full- and waning-glacial conditions (figs. 50, 51). The waxing and waning of each source area did not occur in unison, and field relations documenting incursions of ice from some sources into areas previously occupied by ice from other sources demonstrate this lack of unison. This lack of simultaneous phase relations for the growth and recession of northern Yellowstone ice masses implies that end moraines deposited elsewhere for example, by large icecaps and small mountain-valley glaciers are not of the same age.

History and dynamics of glaciation in the northern Yellowstone National Park area : A reconstruction and quantitive evaluation of the interacting ice masses that covered northern Yellowstone during the Pinedale Glaciation

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