Guidebook for field trip to the Mount Bachelor-South Sister-Bend area, central Oregon High Cascades

Abstract

Three Sisters-Broken Top areaThe Three Sisters-Broken Top area is a long-lived center of basaltic to rhyolitic volcanism (Taylor, 1981; Hill and Taylor, this volume).The clustering of large composite cones sets the area apart from others in the High Cascades, although the Mount Mazama area, prior to the formation of Crater Lake caldera, was also a cluster of composite cones (Bacon, 1983).The ages of most volcanoes in the Three Sisters area are not precisely known.North Sister, a basaltic andesite pyroclastic and lava cone that rests on a shield volcano, is the oldest of the Three Sisters (Taylor, 1981) and postdates (Taylor, 1987) the 03-Ma (Sarna-Wojcicki and others, this volume) Shevlin Park Tuff of Taylor (1981).Middle Sister is intermediate in age between North and South Sister and, like South Sister, is compositionally diverse.Broken Top volcano is also younger than the Shevlin Park Tuff (Hill and Taylor, this volume) and is older than South Sister, but its age relation to Middle and North Sister is not known.The relative degree of erosion of Broken Top suggests an age probably equal to or greater than that of North Sister.Broken Top is a complex composite cone of dominantly basaltic andesite that intermittently erupted andesite, dacite, and rhyolite as lava flows, pyroclastic flows, and pyroclastic falls (Crowe and Nolf, 1977;Taylor, 1978).Cayuse Crater and two nearby vents on the southwest flank of Broken Top (figs.5, 17) erupted during earliest Holocene or latest Pleistocene time, but these events were probably unrelated to the long-inactive Broken Top system.South Sister is the youngest composite volcano of the Three Sisters-Broken Top center and has erupted lavas ranging from basaltic andesite through rhyolite (Taylor, 1981;Wozniak, 1982;Clark, 1983).Although not dated directly, most, if not all, of South Sister is probably of late Pleistocene age.This subjective judgement is based on the relatively little eroded profile of the volcano and the reasonably good preservation of lava-flow levees and other features, especially on the south and west flanks.The cone of basaltic andesite that forms the summit of South Sister is probably of latest Pleistocene age (Wozniak and Taylor, 1981;Scott, 1987); its crater is still closed and is filled with 60 m (Driedger and Kennard, 1986) of ice and snow.Le Conte Crater (fig.17), a basaltic andesite scoria cone on the south flank, is between about 15,000 and 6,850 yr old.The youngest eruptions recognized on the volcano occurred at a series of vents on the south and northeast flanks that erupted rhyolite tephra and lava flows and domes between about 2,200 and 2,000 yr B.