Regional geologic settingThe High Cascades of Oregon are a north-trending belt of upper Miocene to Quaternary volcanic rocks that were erupted on the east margin of the upper Eocene to Miocene Western Cascades volcanic province (Figure 2) (Taylor, 1981; Priest and others, 1983). Upper Pliocene and Quaternary rocks of the High Cascades form a broad platform of chiefly basalt and basaltic andesite volcanoes that fill a structurally subsided zone in the older rocks of the High Cascades (Taylor, 1981; Hughes and Taylor, 1986; Smith and others, 1987). Each of four major Quaternary volcanic centers along this platform (Mount Hood, Mount Jefferson, Three Sisters-Broken Top, and Crater Lake caldera [Mount Mazama]) have erupted lava flows and pyroclastic material that range in composition from basalt to dacite; except for Mount Hood, they have also erupted rhyolite. Newberry volcano, which lies east of the High Cascades, is also a compositionally diverse Quaternary volcanic center (MacLeod and others, 1981).
The Three Sisters-Broken Top area is a long-lived center of basaltic to rhyolitic volcanism (Taylor, 1981; Hill and Taylor, 1989). 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 approximately 0.3-million-year-old (MA) (Sarna-Wojcicki and others, 1989) 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 Shevlin Park Tuff (Hill and Taylor, 1989) 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 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, which is located between Broken Top and the Cascades Lakes Highway, and two nearby vents on the southwest flank of Broken Top (Figures 3 and 4, not included here) erupted during the 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 judgment 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 meters (Driedger and Kennard, 1986) of ice and snow. Le Conte Crater (Figure 4, not included here), a basaltic andesite scoria cone on the south flank, is between about 15,000 and 6,850 years 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 years before the present (yr B.P.) (Figures 4 and 5, not included here) (Taylor, 1978; Wozniak, 1982; Clark, 1983; Scott, 1987; Taylor and others, 1987).
Mount Bachelor volcanic chainThe Mount Bachelor volcanic chain provides one example of the type and scale of eruptive activity that has produced most of the High Cascades platform, which consists chiefly of scoria cones and lava flows, shield volcanoes, and a few steep-sided cones of basalt and basaltic andesite. The chain is 25 kilometers long; its lava flows cover 250 square kilometers and constitute a total volume of 30-50 cubic kilometers (Scott and Gardner, 1990).
Many vents in the field-trip area, including those of the Mount Bachelor
volcanic chain and the Holocene rhyolite lava flows and domes on South
Sister, define NNW-NNE-trending alignments
(Figure 6)
(Taylor and MacLeod,
written communication, 1981, in Bacon, 1985; Hughes and Taylor, 1986;
Scott, 1987). Normal-slip faults in the region, including one at the south end
of the Bachelor chain, also have this orientation
(Figures 2, 3, and 6)
(Venkatakrishnan and others, 1980; Kienle and others, 1981). These alignments
are oriented parallel to the north-south direction of maximum horizontal
compressive stress that affects the region (Zoback and Zoback, 1980).
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