Timing and extent of early marine oxygen isotope stage 2 alpine glaciation in Skagit Valley, Washington

Abstract

Twenty-two new radiocarbon ages from Skagit valley provide a detailed chronology of alpine glaciation during the Evans Creek stade of the Fraser Glaciation (early marine oxygen isotope stage (MIS) 2) in the Cascade Range, Washington State. Sediments at sites near Concrete, Washington, record two advances of the Baker valley glacier between ca. 30.3 and 19.5 cal ka BP, with an intervening period of glacier recession about 24.9 cal ka BP. The Baker valley glacier dammed lower Skagit valley, creating glacial Lake Concrete, which discharged around the ice dam along Finney Creek, or south into the Sauk valley. Sediments along the shores of Ross Lake in upper Skagit valley accumulated in glacial Lake Skymo after ca. 28.7 cal ka BP behind a glacier flowing out of Big Beaver valley. Horizontally laminated silt and bedded sand and gravel up to 20 m thick record as much as 8000 yr of deposition in these glacially dammed lakes. The data indicate that alpine glaciers in Skagit valley were far less extensive than previously thought. Alpine glaciers remained in advanced positions for much of the Evans Creek stade, which may have ended as early as 20.8 cal ka BP.

Deglaciation of the North Cascade Range, Washington and British Columbia, from the Last Glacial Maximum to the Holocene

“…Paleoecological and stratigraphic evidence indicate that the climate at the LGM from about 25.0 to 21.4 ka was the coldest part of MIS 2 in this region (Barnosky et al, 1987;Grigg and Whitlock, 2002;Thackray, 2001Thackray, , 2008Riedel et al, 2010). Short, cool summers and long winters favored development of 30-40 km long alpine glaciers several hundred meters thick in valleys (Porter, 1976;Waitt, 1977;Heller, 1980;Porter et al, 1983;Riedel et al, 2010).…”

“…The Fraser Glaciation covers the same interval as MIS 2, and is also referred to as the late Wisconsin (Armstrong et al, 1965). Early MIS 2 included the Evans Creek stade alpine glacier advance, and ended at the Port Moody interstade (Riedel et al, 2010). Mid MIS 2 was marked by the advance and retreat of the CIS during the Vashon stade, and ended in the lowlands during a marine incursion into western valleys during the Everson interstade (Dethier et al, 1995).…”

“…Short, cool summers and long winters favored development of 30-40 km long alpine glaciers several hundred meters thick in valleys (Porter, 1976;Waitt, 1977;Heller, 1980;Porter et al, 1983;Riedel et al, 2010). The valley glaciers were limited in extent, however, and parts of some major mountain valleys were ice-free refugia during this period (Riedel, 2007).…”

“…It included both continental and alpine styles of glaciation, where ice advanced up some valleys and down others, only to reverse flow during retreat (Waitt and Thorson, 1983;Riedel et al, 2010). In lower Skagit valley Heller (1980) found evidence of three ice-flow directions in the last glaciation.…”

“…In the north, the alpine glaciers were relatively small by the time of ice sheet glaciation several thousand years later (Mackin, 1941;Riedel et al, 2010). In the south, beyond the area inundated by the ice sheet, the retreat of alpine valley glaciers after 21 ka occurred by backwasting of termini up mountain valleys (Porter, 1976;Porter et al, 1983;Porter and Swanson, 1998).…”