Where Volcanoes and Glaciers Once Met
This field trip highlights many aspects of the Quaternary volcanic and glacial history in the vicinity of Broken Top, Mount Bachelor, and South Sister stratovolcanoes. Numerous features of Pleistocene and Holocene volcanism and glaciation; composite and shield volcanoes, cinder cones, pahoehoe and aa lava flows, volcanic lava flow features and rock types, pyroclastic volcanic rocks, and many features of glacial erosion and deposition. Two field trip routes (Figure 2.1) are provided in this section, the main route traverses the headwaters of the Deschutes River, an area generally referred to as the Cascade Lakes, and another route that extends from the southern end of the main route across the Cascade Crest and into an area that this author felt worthy of inclusion because it offers opportunities to see one of Oregon’s largest waterfalls and natural lakes. Field Trip 2A is intended to be a multi-day excursion. The beginning and ending of the trip is the same location, the intersection of US Hwy 97 and Franklin Avenue in Bend, Oregon. Many campgrounds and a few resorts with cabin rentals available are found in the Cascade Lakes area, so there is no shortage of places to rest. Information is provided so that the route can readily be shorted or lengthened, depending on your time available and level of interest. Field Trip 2B is provided as an extension of 2A, and describes the geologically interesting area around Willamette Pass; it can be completed in a single day or as an overnight excursion. The beginning and ending of this route is the intersection of Crescent Cutoff Rd and FS Rd 46 (Cascade Lakes Highway) at mile 67.4 of Field Trip 2A. These field trip routes travel almost entirely on paved roads, while the few secondary roads are surfaced with gravel/crushed rock; seasonal snow and ice may prevent travel in late fall and early spring, so check accessibility for the Cascade Lakes Highway and some Forest Service roads prior to departure.
Field Trip Lengths: 2A is 167.8 miles and 2B is 56.3 miles.
Figure 2.1. Field Trip routes 2A and 2B.
The Cascade Lakes and Willamette Pass areas generally occupy the volcanically diverse upper watershed of the Deschutes River on the eastern slope of the central Oregon High Cascades, renowned for its profusion of natural lakes, artificial reservoirs, and clear-water streams that provide fertile grounds for boating and fishing. Backed to the west and north by the high, precipitation inducing terrain of the main Cascade Crest and volcanic platform of the South Sister and Broken Top composite volcanoes, the Deschutes watershed has been heavily modified by glacial activity, the lakes for which the area is named principally formed by the damming effect of end moraines. Superimposed on this glacier- and stream-modified landscape are a host of youthful basaltic volcanoes, cinder cones, and lava flows related to the construction of Mount Bachelor and its smaller partners, as well as even younger silicic domes and flows on the southern slopes of South Sister Volcano. Figure 2.2 displays a simplified geologic map of this area.
The High Cascades are a north-south trending belt of upper Miocene to Quaternary shield and stratovolcanoes, cinder cones, lava flows and pyroclastic deposits erupted on the eastern margin of the older upper Eocene to Miocene Western Cascades volcanic platform (Taylor, 1981; Priest et al., 1983; Priest, 1990; Taylor, 1990; and Sherrod and Smith, 2000). As shown in Figure 2.2 in GEOLOGY OF THE CENTRAL OREGON CASCADES, the modern High Cascades geologic province represents the modern portion of the volcanic arc formed by subduction and partial melting of the Juan de Fuca plate under the North American plate at the Cascadia Subduction Zone (Priest, 1990; Taylor, 1990). The volcanic arc actually consists of two distinct parallel volcanic rock assemblages (Taylor, 1990; Sherrod and Smith, 2000) as shown in Figure 3 in GEOLOGY OF THE CENTRAL OREGON CASCADES: 1) the younger volcanic arc rocks of the High Cascades, such as those of the Cascade Lakes and Willamette Pass areas (Figure 2.2), which are dominantly composed of intermediate composition andesites and basaltic andesites, and isolated centers of more silicic dacite and rhyolite; and 2) older volcanic arc rocks of the Western Cascades which are dominantly composed of intermediate composition andesites, basaltic andesites, and dacites, and their associated pyroclastic deposits.
Figure 2.2. Geologic map of the Cascade Lakes and Willamette Pass areas (compiled from Sherrod and Smith, 2000 and DOGAMI data, 2009).
Formation of the High Cascades volcanic arc began as early as 9 million years ago. Convergence rates between the Farallon (Juan de Fuca) plate and the North American plate (Figure 2 in GEOLOGY OF THE CENTRAL OREGON CASCADES) decreased significantly between about 9 and 7 Ma, resulting in a steepening of the convergence angle, a shortening of the convergent margin, eastward migration and narrowing of the older volcanic arc, and a reduction in magma production, while the Pacific plate – North American plate transform boundary migrated northward (Verplanck and Duncan, 1987; Priest, 1990; Sherrod and Smith, 1990; and Taylor, 1990). Subsequent growth of the High Cascades from about 7 Ma to the present was accompanied by extension and thinning of the overlying North American plate and formation of a major N-S trending graben, subsequently buried by the eruptive products of the High Cascades volcanoes, a corresponding increase in mafic volcanism (although some silicic centers remained active), and further uplift of the Western Cascades (Priest, 1990).
The High Cascades province comprises the Cascade Crest and upper portion of the western slope, a belt of young, relatively unmodified, strato- and shield volcanoes, cinder cones, lava domes, and their associated flows and pyroclastics. This volcanic arc is comprised of more than 4,000 separate volcanic vents including numerous stratovolcanoes, shield volcanoes, cinder cones, and lava domes. The Cascade Lakes and Willamette Pass areas provide just a small sample of this geologically diverse region (Figure 2.2). Twenty major volcanoes occur, with twelve stratovolcanoes in the High Cascades province over 10,000 ft (3,000 m) in elevation, among them is South Sister Volcano at the northern margin of the Cascade Lakes area, measuring in as the second tallest of four volcanoes in Oregon of at least this height. Although volcanism associated with the Cascade Range began about 37 million years ago, most of the present-day High Cascades volcanoes are less than 2 million years old, and the highest peaks of the modern volcanic arc are less than 100,000 years old.
Basalt and basaltic andesite lava flows comprise more than half the volume of the volcanic rocks erupted during the interval 7 to 2 m.y. ago (Sherrod and Smith, 2000), especially abundant along the western edge of the High Cascades and Deschutes Basin, but andesite and dacite are locally abundant. More intermediate andesite, dacite, and rhyolite of this age with Cascade chemical affinities form lava flows, domes, and pyroclastic rocks along the east side of the Cascade Range near Tygh Ridge, in the Metolius River area of the Deschutes Basin, and in adjacent parts of the Basin and Range province. Taylor (1981) and Smith and Taylor (1983) suggest that some volcanic centers of this age probably lie buried beneath younger rocks in the Cascade Range. Concurrently, volcaniclastic and nonvolcanic sediment accumulated as alluvial fans, floodplain, and lacustrine deposits in several major depocenters, including the Deschutes basin where such material forms the Deschutes Formation. In the central Oregon Cascade region, a north-trending graben nearly 30 km wide and 50 km long developed in the High Cascades in the central part of the Cascade Range between about 5 and 4 m.y. ago (Smith and Taylor, 1983). The west side of the central block sank at least 600 m along the Horse Creek fault (Brown et al., 1980), and the east side sank as much as 1,200 m along the Green Ridge fault (Sherrod and Smith, 2000). Elsewhere in the High Cascades arc, major faults alternately bound its west and east sides; however, a throughgoing, subsided central block is lacking.
Quaternary volcanism in the Cascade Range has generally been limited to the High Cascades geologic province (Sherrod and Smith, 2000), although long-term eruption rates have been low in the central Oregon Cascades during this time (Verplanck and Duncan, 1987; Sherrod and Smith, 1990). A relatively low volcanic production rate of about 3 to 6 cubic kilometers per kilometer of arc length per million years along the crest of the Cascade Range from Crater Lake to Mount Jefferson has been the norm, although locally greater rates characterize the broad graben formed east and north of the Three Sisters (Priest, 1990; Hill and Priest, 1992). Short-term rates for volcanism during the past 25,000 years in the Crater Lake (Bacon, 1983) , Three Sisters at Mt. Bachelor (Scott and Gardner, 1992), and McKenzie Pass-Santiam Pass (Taylor, 1965 and 1981) areas have been somewhat higher than the average rate because of latest Pleistocene to Holocene volcanism.
A broad platform of basalt and basaltic andesite lavas was erupted from numerous small shield volcanoes and cinder cones during the Quaternary, extending from near Crater Lake north to near Mount Hood. Especially voluminous eruptions occurred between Mt. Jefferson and the Three Sisters (Taylor, 1965, 1968, 1981, and 1990) and near Mt. Bachelor (Scott and Gardner, 1992) in the Cascade Lakes area (Figure 2.2). During the same period, most eruptions of intermediate to felsic, andesite, dacite, and rhyolite were associated with the construction of four major composite volcanoes or volcanic complexes; from north to south these isolated volcanic centers are Mount Hood, Mount Jefferson, the Three Sisters-Broken Top area (Figure 2), and Crater Lake. The cluster of rhyolite domes near Broken Top deposited pumice-fall and ash-flow deposits near Bend, OR, a subject of Field Trip 1D in FIELD GUIDE TO THE BEND AREA. Other areas of dominantly intermediate Quaternary volcanism did occur, such as a locas of andesitic and dacitic vents near Mount Jefferson. These areas of more silicic volcanism have undergone considerable erosion and/or burial by basalt and basaltic andesite lava flows, relegating them to a less conspicuous topographic position within the High Cascades arc.
Although Quaternary volcanic rocks in the High Cascades are essentially undeformed, a few normal faults do exist that trend north and displace rocks as young as about 300,000 years by no more than 150m (Sherrod and Smith, 2000). Some younger, northwest trending, normal faults occur on the east flank of the Cascade Range near Bend (Sherrod et al., 2004). The Tumalo Fault (Taylor, 1981), a major fault within the Tumalo-Sisters Fault Zone extends about 30 km northwest of Bend and displaces Upper Pleistocene pumice and ash deposits by as much as a few meters. Northwest-trending normal faults related to the Walker Rim Fault Zone observed on this field trip (Figure 2.2) deformed the northwest flank of Newberry volcano in middle or late Pleistocene time (MacLeod and others, 1995).
Glaciation of the High Cascades (and upslope portion of the Western Cascades) has occurred throughout the Quaternary. A mountain ice cap covered much of the higher elevations of the Oregon Cascade Range at least twice during this time (Crandell, 1965), its maximum late Pleistocene extent shown in Figure 7 in GEOLOGY OF THE CENTRAL OREGON CASCADES. Evidence for several periods of glaciation has been reported for the higher portions of the Deschutes River basin (Scott, 1977; Scott and Gardner; 1990). Scott and Gardner (1990) provide a late Pleistocene ice cap reconstruction for Three Sisters-Broken Top area further to the southeast. Scott (1977) identified and named two glacial episodes on the upper Metolius, the younger Cabot Creek glaciation, which climaxed in the Suttle Lake advance about 24,000 to 18,000 years B.P., while the older Jack Creek glaciation culminated during late middle Pleistocene about 132,000 years ago. Bevis et al. (2008 and 2011) have mapped the age and distribution of glacial deposits in the watersheds of the upper McKenzie River, upper Metolius River, and Whychus Creek, interpreting drift units to represent a late Pleistocene LGM episode and an earlier late middle Pleistocene glacial period, and correlating these units with Scott’s Cabot Creek and Jack Creek glaciations. A regionally recognized stillstand or slight readvance of glacial ice occurred in the latest Pleistocene, depositing small, but recognizable moraines high in some, especially northeast facing valleys that is known locally as the Canyon Creek advance (Scott, 1977). Scott (1977) also recognized at least two periods of Holocene, pre- and post-Mazama eruption, neoglaciation. Scott (1990) describes the temporal relationships between volcanic eruptions in the Mt. Bachelor area of this field trip with fluctuations of late Pleistocene and Holocene glaciers.