Geology of the High Cascades Volcanic Arc


Unintentionally, but perhaps appropriately, this author has “saved the best for last”.  The geological diversity of the Cascade Crest and its adjoining eastern and western slopes in the McKenzie Pass – Santiam Pass area verges on awe-inspiring; and when combined with its unparalleled scenery, investigating the geology here at times becomes a profound experience. This field trip includes four route descriptions (Figure 4.1); one much longer, primary route; and three shorter, subsidiary routes that offer opportunities to diverge from the main one.  Field Trip 4A, the primary route, could be completed in a single day as simply a driving tour, but would not do justice to the magnificent geology readily explored along the way.  The field trip focuses on the many superb examples of youthful, mafic volcanism along the Cascade Crest in this area.  Information is provided so that this route can be shorted or lengthened depending on time availability and level of interest.  It is best traveled as a multi-day excursion, with many fine day-hiking options to satisfy the curious and a host of good National Forest campgrounds to rest the weary.  One extended backpacking trip to visit the western flank of Middle and North Sister Volcanoes is described.  The short route of Field Trip 4B provides access to the Whychus Creek watershed draining the eastern slopes of the broad basin formed by Tam McArthur Rim, Broken Top, and the Three Sisters.  The route highlights the glacial geology of the basin and its relationship to the growth of South Sister Volcano.  It is intended to be completed in one day, although it could be expanded to afford hiking and overnight camping opportunities.  This route includes two spectacular hiking options, a day trip onto Tam McArthur Rim and the glacially sculpted eastern flank of Broken Top Volcano, and a multiday backpacking excursion to the Chambers Lakes area, alpine country at its finest nestled between Middle and South Sister Volcanoes.   The beginning and ending of both Field Trip 4A and 4B is the same location, the intersection of US Hwy 20 and FS Rd 16 in Sisters, Oregon.  Field Trip 4C extends the main route for this field trip into an area of the highly dissected, older volcanic terrain of the Western Oregon Cascades, and can also be completed in one day.  However, with several hiking options to Western Cascades summits available, it may be better to take an extra day or two.  Finally, Field Trip 4D offers a relatively short side trek into the upper watershed of the Metolius River and highlights its glacial history, a history that has been correlated with a broader history of glaciation in the Cascade Range and adjacent mountainous areas of the western U.S.  This latter route includes a day-hiking and short backpacking option for more in depth excursions into the geology and scenery of this fascinating area.

Field Trips

McKenzie Pass – Santiam Pass Loop: Youthful Volcanism in the High Cascades (Field Trip 4A)

Optional Hiking Trails for Field Trip 4A

The Whychus Creek Drainage; Its Volcanic and Glacial History Revealed (Field Trip 4B)

An Excursion Through the Western Cascades – High Cascades Transition (Field Trip 4C)

The Upper Metolius River Drainage: Defining the Glacial History of the Oregon Cascades (Field Trip 4D)

Field Trip Route Lengths: 4A is 84.8 miles, 4B is 40.6 miles, 4C is 56.2 miles, and 4D is 43.7 miles.

Figure 4.1.  Field trip routes 4A through 4D.

Geologic Summary

Traversing the spine of the Cascades, the distinctly asymmetric topography of the mountain range is readily observed as one passes from east to west (Figure 4.1).  Short, steep, higher elevation tributaries of the Deschutes River drain to the east from the flanks of the Three Sisters, Mt. Washington, Three Fingered Jack, and Mt. Jefferson, tumbling rapidly down to the Deschutes basin.  The upper McKenzie River drains more gradually downslope to the west from the High Cascades volcanic platform, which forms the main watershed divide in this area, and passes through the transition into the lower elevation Western Cascades geologic province.  The towering Mt. Jefferson and Three Sisters stratovolcanoes form the platform’s youthful northern and southern anchor points, while the central portion nearer the passes is comprised of a more subdued, but diverse topography of overlapping  shield volcanoes, cinder cones, and lava flows, both young and old.

Much of the western slope of the central Oregon Cascades receives substantially higher precipitation caused by orographic lifting of moist Pacific air-masses over the Cascade Crest.  An ice cap developed over this highland multiple times during the Quaternary, extending valley glaciers far down the McKenzie.  West of the High Cascades volcanic platform, the area is underlain by volcanic and volcaniclastic rocks of greater age, and thus, the affects of time have played their part as well the type of erosive forces.  As a consequence, its highly dissected topography and slopes thickly mantled with soil and lush vegetation show the affects of considerably greater rates of weathering and erosion.  The eastern slope of the Cascades lies in the rainshadow of the Cascade Crest, and is drier; its soils and vegetation thinner and more montane.  Glaciation affected this flank of the Cascades too, although valley glaciers where more restricted in their extent.

Figure 4.2 displays a simplified geologic map of the area covered in this field trip.  In general, that portion of the Cascade Range highlighted here represents a small portion of the transition zone between two geologic provinces, the High Cascades and Western Cascades (Figure 3 in GEOLOGY OF THE CENTRAL OREGON CASCADES), that span the development of the Cascade volcanic arc, formed by interactions between the Pacific plate, Farallon plate, and the North American plate during the last 35 million years (Priest, 1990; Taylor, 1990).  Figure 2 in GEOLOGY OF THE CENTRAL OREGON CASCADES displays the current geometry of these plate interactions, with the all-but-subducted Farallon plate now represented by the Explorer – Gorda – Juan de Fuca plate remnants.  The High Cascades province comprises the Cascade Crest, eastern slope of the Cascade Range, and uppermost portion of the western slope of the Cascade Range.  It is a belt of young, relatively unmodified, composite and shield volcanoes, cinder cones, lava domes, and their associated flows and pyroclastic deposits, which 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.  The Western Cascades forms the lower western portion of this area, and is comprised of the remnants of a highly eroded volcanic platform consisting of volcanic and volcaniclastic rocks and interbedded sedimentary rocks shed westward from an older volcanic arc now subsided under the High Cascades.

Figure 4.2.  A simplified geologic map of the McKenzie Pass – Santiam Pass area (compiled from Sherrod and Smith, 2000 and DOGAMI data, 2009).

The High Cascades and Western Cascades geologic provinces consist of two distinctive, parallel belts of volcanic rock (Sherrod and Smith, 2000).  As shown in Figure 3 in GEOLOGY OF THE CENTRAL OREGON CASCADES, the older volcanic arc rocks of the Western Cascades are dominantly composed of andesite, basaltic andesite, dacite, and associated pyroclastic material of intermediate composition; and the younger volcanic arc rocks of the High Cascades, although still dominantly composed of intermediate composition andesites and basaltic andesites, contain multiple isolated centers of more silicic dacite and rhyolite.  The topography of the Western Cascades is much lower, although of highly dissected relief, gradually sloping to the west into the Willamette basin.  Subdued high points generally occur just west of the central High Cascades graben.  The High Cascade volcanic arc includes 20 major volcanoes, among a total of over 4,000 separate volcanic vents including numerous stratovolcanoes, shield volcanoes, cinder cones, and lava domes.  Most of the present-day High Cascades volcanoes are less than 2,000,000 years old, and the highest peaks of the modern volcanic arc are less than 100,000 years old. Twelve stratovolcanoes in the High Cascades province are over 10,000 ft in elevation; Mount Jefferson and the Three Sisters (North, Middle and South) can be viewed in all their glory from various vantage points in the central Oregon Cascades and are a true focal point of Field Trip 4.

Onset of widespread arc-building volcanism is inferred from the central Western Cascades Upper Eocene Fisher Formation, a unit composed of interbedded intermediate volcaniclastic rock and marine sedimentary rock, and the rhyolitic Tuff of Bond Creek of the southern Western Cascades (Sherrod and Smith, 2000).  However, the best evidence comes from beyond the Western Cascades.  Marine sedimentary rocks in the Oregon Coast Range record an influx of volcanically derived lithic fragments and tuffaceous mudstones beginning as early as 45 million years ago, while the John Day Formation of the John Day River basin in northeast Oregon contains abundant layers of intermediate andesitic to dacitic air-fall and lapilli tuffs as old as 39 million years (Sherrod and Smith, 2000).  The Western Cascades formed primarily during two episodes of volcanism from about 35 to 17 Ma and about 14 to 8 Ma, separated by an interval of relative uplift and erosion with little volcanic activity (Priest, 1990; Sherrod and Smith, 2000).  Convergence rates were high during these time intervals, indicated by the formation of a fairly low and broad volcanic arc containing copious amounts of volcanic rocks produced by high rates of volcanism (as much as triple that of today’s rate).

Basalt, basaltic andesite, andesite, and dacite form most of the Western Cascades, where they are exposed in the western foothills and deeply incised central part of the province (Sherrod and Smith, 2000).  Eruptions of more mafic basalt and basaltic andesite were more widespread during the earlier part of the first interval of volcanism from 35 to 25 m.y. ago, than during the later part of the first interval from 25 to 17 m.y. ago.  Andesites form near-vent lava and tuff breccia, as well as thick sequences of volcaniclastic lapilli tuff that probably formed as lithic-rich pyroclastic flows and post-eruption debris flows.  Dacites commonly occur as small-volume, valley-filling ash-flow tuffs, although lava domes and flows are locally abundant. Rhyolites are limited to a few small dome complexes during this time period.  The main structural features in rocks ranging in age from 35 to 17 m.y. are northeast and northwest trending conjugate normal faults with relatively little offset (Sherrod and Smith, 2000).  These faults have been emphasized by differential erosion, creating the topographic pattern of northwest- and northeast-trending drainages found in the Western Cascades.

Based on the general paucity of volcanic rocks in the Western Cascades of southern Washington and southern Oregon, Sherrod and Smith (2000) suggest that volcanism was uncommon in these portions of the Cascade Range between about 17 and 7 m.y. ago.  In Oregon, Western Cascades arc-derived volcaniclastic interbeds are few and thin in the lower part of the flood basalts of the Columbia River Basalt Group, an important stratigraphic time marker emplaced between 17 and 14.5 Ma.  This further indicates that the Cascade Range in northern Oregon was relatively quiescent during this time interval.  However, the Western Cascades of central Oregon contain abundant basalt, basaltic andesite, and andesite erupted between 14 and 8 m.y. ago (Taylor, 1990; Sherrod and Smith, 2000).  Basalt and basaltic andesite form sequences of lava flows and breccia as much as a kilometer thick that are exposed in the south-central part of the range from about the latitude of Oakridge, OR south to Crater Lake and in the north-central part of the range near Detroit, OR.  Andesite forms lava flows and less abundant volcaniclastic strata in the south-central part of the range between Oakridge and Detroit; in contrast, andesitic tuff-breccias dominate to the north of Detroit.  Folding and local thrusting along anticlines occurred in northern Oregon from 17 to 11 m.y. ago, while the central and southern parts were only broadly warped between 17 and 7 m.y. ago (Sherrod and Smith, 2000).  Warping produced eastward tilting of strata by about 5° in much of the Western Cascades, accompanied by localized beveling by erosion prior to emplacement of younger material.

Convergence rates between the Farallon (Juan de Fuca) plate and the North American plate decreased significantly between about 9 and 7 Ma, accompanied by a steepening of the convergence angle and a shortening of the convergent margin as the Pacific plate – North American plate transform boundary migrated northward (Priest, 1990).  The combined result of these factors was the eastward migration and narrowing of the older volcanic arc and formation of the High Cascades volcanic arc, well expressed in Figure 4.2, as well as a reduction in magma production (Verplanck and Duncan, 1987).  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, normal fault bounded graben (Figure 4.2), 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).  Figure 4.3 depicts an idealized cross-sectional view through the High Cascades graben from west to east.

Figure 4.3.  A geologic cross-section drawn perpendicular to the Cascade Crest showing the nature of the High Cascades graben and its relationships to the adjacent Western Cascades and Deschutes Basin geologic provinces (modified from Smith, 1991).

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.  Andesite, dacite, and rhyolite of this age with Cascade volcanic arc chemistry 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, although similar volcanic centers of this age probably lie buried beneath younger rocks in the central graben of the Cascade Range (Taylor, 1981; Smith and Taylor, 1983).  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, uplift in the Western Cascades province occurred between 5 and 3.5 m.y. ago accompanied by a major reorientation of the regional stress regime; basalt and basaltic andesite dikes older than about 4 m.y. strike mostly northwest, whereas dikes younger than 4 m.y. strike mostly north (Avramenko, 1981; Sherrod and Pickthorn, 1989).  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) (Figure 4.2 and Figure 4.3).  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 normal faults alternately bound its west and east sides; however, a throughgoing, subsided central block is apparently lacking.

Quaternary volcanism in the Cascade Range has generally been limited to the High Cascades geologic province, although a few small-volume eruptions did occur in the Western Cascades (Sherrod and Smith, 2000).  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 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 copious 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, burying much of the High Cascades graben (Figure 4.2 and Figure 4.3).  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.  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, 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 the 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 volcanic arc.

Newberry Volcano, its volume much greater than that produced by volcanoes along an equivalent length of the High Cascades to the west, has been a major center of mafic to intermediate volcanism throughout the Quaternary, detailed in Field Trip 3A of the FIELD GUIDE TO NEWBERRY VOLCANO AND THE CHRISTMAS VALLEY-FORT ROCK AREA.  Although Quaternary volcanic rocks in the High Cascades are essentially undeformed, several normal faults do exist that trend northward 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 deformed the northwest flank of Newberry volcano in middle or late Pleistocene time (MacLeod and others, 1995).

As previously indicated, past glaciation has laid the tendrils of its icy grip on this volcanic landscape.  A mountain ice cap covered much of the higher elevations of the Oregon Cascade Range at least twice during the Quaternary (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 upper McKenzie River basin (Bevis et al., 2008), and the upper watersheds of Whychus Creek (Bevis et al.; 2011) and the Metolius River (Scott, 1977; Bevis et al., 2011) draining into the Deschutes River.  Scott (1977) identified and named three glacial episodes on the upper Metolius drainage.  The youngest he named the Cabot Creek glaciation, which climaxed in the Suttle Lake advance about 18,000 years B.P., but generated significant moraines higher in many northeast-facing stream valleys during a stillstand or slight resurgence of glaciers about 12,000 years B.P. that Scott called the Canyon Creek advance.  Bevis et al. (2011) correlate the Suttle Lake advance with the Last Glacial Maximum (LGM), recognized throughout the mountain ranges of the western U.S.  Scott inferred an older glacial period that culminated during late middle Pleistocene about 132,000 years ago that he named the Jack Creek glaciation, while deposits of an even older period of glaciation, the Abbot Butte glaciation, were considered to be of middle Pleistocene age.  Several of the taller peaks in or bordering the northern Great Basin of south-central Oregon (Newberry Volcano’s Paulina Peak and Yamsay Mountain to the south) also exhibit signs of late Pleistocene glacial activity (Osborn and Bevis, 2001).  Scott also recognized at least two periods of Holocene neoglaciation, a pre-Mazama eruption advance (older than 7,000 years ago) that he named the Jefferson Park glaciation after deposits observed on the north flank of Mount Jefferson, and a post-Mazama advance (likely including deposits of the Little Ice Age).