RECONSTRUCTING THE GLACIAL HISTORY AND PALEOCLIMATE RECORD NEAR THE PLEISTOCENE-HOLOCENE TRANSITION AT THREE SISTERS VOLCANOES, OREGON, UNITED STATES

Recent research on glacial activity during the last deglaciation has identified several glacial advances occurring within the last 14,000 years in Washington, Idaho, California, and northeastern Oregon. While these studies have improved our understanding of the glacial history in the western U.S., the glacial record is still incomplete and a geographic hole in the form of undated glacial deposits remains to be filled in the Oregon Cascades. Without detailed information about the glacial advances in Oregon a paleoclimate scenario of western North America cannot be completed.

At least four glacial advances marked by end and lateral moraines occurred on the eastern flanks of the Three Sisters Volcanoes and the northern flanks of Broken Top Mountain in the Central Oregon Cascades. The youngest of these advances was the LIA glaciation, which took place 200-150 yrs. B.P. and is well defined by the large moraines adjacent to the modern glaciers. Less than 100 m downslope from these moraines, a second set of sparsely vegetated lateral moraines mark the Late-Neoglacial advance of the glaciers. These Late-Neoglacial moraines predate the local rhyolitic eruptions from the Devil's Hill and Rock Mesa (DHRM) volcanic chain on South Sister (2.1 ± 0.4 ka B.P.) and post date the Mount Mazama eruption (~7.7 ka B.P.). A third set of Early-Neoglacial end moraines are 100-500 meters downslope from the LIA deposits. These moraines also pre- and postdate the DHRM and Mazama eruptions. From a sediment core extracted from Camp Lake between Middle and North Sister, we infer that the moraines date between 8 – 5 ka B.P. A fourth set of lateral and end moraines, 300-800 meters downslope of the LIA set are overlain by a Mazama ash deposit. These moraines are thought to be latest Pleistocene or early Holocene in age because of their weathering characteristics, flat crests, vegetation cover, and soil development.

While more precise minimum and maximum dates of these moraines are required, preliminary data suggest that the latest Pleistocene or early Holocene moraines at the Three Sister Wilderness are correlative with similar deposits found in Washington (Heine, 1998) and northeastern Oregon (Licciardi, 2004). This suggests a common climate influence affecting glaciation throughout the Pacific Northwest approximately 11 – 9 ka B.P.