0.0 (0.0) Refer to Map 1D.1. Intersection of Franklin Avenue and U.S. Hwy 97 in Bend, OR. Drive west on Franklin Avenue. Remain on Franklin Avenue through the downtown business district.
0.2 (0.2) Underpass beneath the Burlington Northern-Santa Fe Railroad. Road cuts on either side of Franklin Avenue are in the Basalt of Bend (Sherrod et al., 2004).
0.6 (0.4) Intersection of Franklin Avenue and NW Bond Street. Continue west through two traffic lights; Franklin Avenue becomes NW Riverside Boulevard after the second traffic light. NW Riverside Boulevard curves gently to the south along the Deschutes River.
1.2 (0.6) Junction of NW Riverside Boulevard and NW Tumalo Avenue. Turn right (west) onto NW Tumalo Avenue.
1.3 (0.1) Cross the Deschutes River. NW Tumalo Avenue becomes NW Galveston Avenue after crossing the river here.
1.6 (0.3) Intersection of Galveston Avenue and NW 14th Street. Turn right (north) through the roundabout onto NW 14th Street.
2.0 (0.4) Intersection of NW 14th Street and Newport Avenue. Turn left through the roundabout onto Newport Avenue.
2.3 (0.3) Entrance road to Central Oregon Community College on the right. Newport Avenue becomes Shevlin Park – Market Rd. Remain on Shevlin Park – Market Rd. The road lies in a valley between two small, early Pleistocene basaltic shield volcanoes (Awbrey Butte to the north and Overturf Butte to the south) that rest on tuffaceous sediments and lava flows of the upper part of the Deschutes Formation (not exposed here). The valley acted as a conduit through which four major pyroclastic flows passed from west to east, erupted from the silicic Tumalo volcanic center (Taylor, 1978; Hill and Taylor, 1989; and Hill and Scott, 1990).
3.5 (1.2) The road curves to the right here, becoming parallel to a northwest trending normal fault with displacement down to the southwest (Map 1D.1). This fault is just one of many en echelon faults of the Tumalo Fault Zone examined in more detail on Field Trip 1C. Just ahead, the road passes over a narrow-lobed, basaltic lava flow from the High Cascades, capping pyroclastic deposits.
3.9 (0.4) Junction of Shevlin Park-Market Rd. and NW Crossing Rd. Pass through the roundabout and remain on Shevlin Park – Market Rd.
4.6 (0.7) Intersection of Shevlin Park-Market Rd. and Mt. Washington Drive. Pass through the roundabout and remain on Shevlin Park – Market Rd.
5.0 (0.4) Refer to Map 1D.2. The entrance to Shevlin Park is on the left side of the road here and at the left (west) end of a small parking area next to the road. Park here and hike the Shevlin Park Loop Trail for an excellent introduction to the pyroclastic rocks found in the Bend area (Figure 1.2), or if you would simply enjoy an extended hike through Shevlin Park while learning a little geology on the side. The trail can be found just inside the gate next to the trailhead sign on the left (see the Shevlin Park Loop Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike). For a less rigorous (and less time intensive) introduction, turn in here to explore Red Tuff Gulch. Drive through the entrance gate to the park, following the road for just over one-tenth of a mile; then park your vehicle in the lot on the left at the second gate. Walk up the road seven-tenths of a mile; you should see the sign for Red Gulch on your right.
Hike up Red Tuff Gulch to view several excellent exposures of volcaniclastic rocks. The short hike first provides access to a 3-meter-thick section of the Tumalo Tuff on the right displaying two pyroclastic surge flow lobes. Further up the gulch, take the right fork in the trail to you have been following onto a bench and look to the right for a road cut in dark, poorly indurated tuff containing lava bombs and some pumice lapilli. This unit is the Shevlin Park Tuff. Return to the park’s main road. In road cuts just to the southwest of Red Tuff Gulch (further up the road), the basal portion of the Bend Pumice is exposed.
Pause here to consider the nature and origins of the pyroclastic rocks you have examined. The foothills of the eastern central Oregon High Cascades west of Bend, Oregon contain an unusual sequence of five ash-flow tuffs and two pumice air-fall deposits, the products of explosive volcanism. From the base of the sequence to the top, the units are: 1) the Desert Spring Ash-flow Tuff, a black to gray, dacitic, poorly welded tuff containing large black pumice lapilli fragments and volcanic bombs; 2) the Bend Pumice, a rhyodacitic, vitric lapilli air-fall tuff; 3) the Tumalo Ash-flow Tuff, a pink or orange to tan colored, rhyodacitic, vitric, poorly welded tuff; 4) the Lava Island Ash-flow Tuff, a cream to buff colored, rhyodacitic, completely devitrified tuff; 5) the Pumice of Columbia Canyon, a dacitic, vitric lapilli air-fall tuff; 6) the Century Drive Ash-flow Tuff, a rhyodacitic to andesitic, vitric tuff; and 7) the Shevlin Park Ash-flow Tuff, a black, andesitic, moderately welded, and fairly well sorted tuff. The pyroclastic units are quite variable in thickness and extent, but are generally better preserved within paleostream valleys on the former landscape. The Desert Springs, Tumalo, and Shvlin Park Tuffs are well exposed in the rock outcrops along Tumalo Creek in Shevlin Park.
Westward increases in the thickness of the units, the size of mineral grains, the size of rock fragments and pumice lapilli, and the degree of welding indicates that these units were erupted from a major silicic vent complex, the Tumalo volcanic center, located east of Broken Top Volcano and west of Bend, Oregon (Taylor, 1978 and 1981; Hill and Taylor, 1989; and Hill and Scott, 1990). This complex is centered around a 25-km-long, arcuate belt of silicic domes and andesitic cinder cones from Three Creek Butte on the northwest to Edison Butte on the southeast.
The absence of a normally graded top to the Bend Pumice and a nonerosive basal contact of the Tumalo Tuff with the Bend Pumice indicate that these units were formed during the same volcanic eruption, the Tumalo Tuff having been produced by the collapse of the Bend Pumice eruption column. Similarity of composition between the Lava Island Tuff and Tumalo Tuff suggests that the Lava Island unit may be the product of a separate flow lobe during the same eruption having been derived from a deeper, gas-rich portion of the magma chamber. The Pumice of Columbia Canyon and Century Drive Tuff have limited distribution and are likely the result of small dome-forming eruptions in localized parts of the larger Tumalo volcanic center. Volcanic activity associated with this complex has been dated to between 650,000 and 300,000 years ago (Sarna-Wojcicki et al., 1989), preceding the construction of the Three Sisters and Broken Top stratovolcanoes.
Rock outcrops along the low cliff edges of Tumalo Creek’s valley within Shevlin Park display abundant exposures of Desert Springs Tuff (Figure 1D.1), Tumalo Tuff (Figure 1D.2), and of course, the Shevlin Park Tuff (Figure 1D.3), named for this locality. These pyroclastic rocks represent a suite of materials generated by progressive stages of explosive volcanism, typical of the extrusion of silicic magmas. Within these units are preserved evidence of: 1) nuee ardente, surges of hot, iridescent ash, pumice lapilli, and rock fragments boiling down slope away from the vent during the early stages of an eruption; 2) air-fall deposits, the product of ash and pumice lapilli, commonly referred to as tephra, injected high into the atmosphere that begin raining down on the landscape, normally after the passage of the pyroclastic surges; and 3) volcaniclastic sediments as well, the product of the reworking of formerly deposited pyroclastic material by mass wasting and running water.
Return to your vehicle and drive back through the park’s entrance gate.
Figure 1D.1. Desert Springs Tuff; notice the flattened, dark pumice lapilli scattered throughout.
Figure 1D.2. Tumalo Tuff; (A) shows the well-indurated, upper portion of the tuff unit in outcrop, while (B) shows a close up (arrow) of dark volcanic rock fragments and pinkish-buff, undeformed pumice lapilli in the tuff (dime for scale).
Figure 1D.3. Shevlin Park Tuff. Notice the generally dark groundmass containing gray to white, undeformed pumice lapilli and dark, scoriaceous rock fragments (dime for scale).
5.3 (0.3) Intersection of the Shevlin Park road and the Shevlin Park – Market Rd.; turn left (north).
5.7 (0.4) Top of the grade out of the valley of Tumalo Creek on the Shevlin Park – Market Rd. Here, Shevlin Park – Market Rd merges with and becomes Johnson Rd. The road shortly passes by Tumalo Butte at 3:00, a cinder cone older than the pyroclastic units you have been exploring. Road cuts on the cinder cone’s northwestern flank expose the fine-grained, marginal facies of the Shevlin Park Ash-flow Tuff. The level terrain north of Tumalo Butte is comprised of Pleistocene gravels sandwiched between the Tumalo Tuff and Shevlin Park Tuff, probably deposited by the ancestral Tumalo Creek and possibly related to an earlier glaciation of the Cascade Range.
7.9 (2.2) Refer to Map 1D.1. Johnson Rd makes a sharp right turn here and is passing over Pleistocene gravels overlying a narrow ribbon of High Cascades basaltic lava.
8.7 (0.8) The road makes a left-hand bend here and begins a downgrade into the valley housing Tumalo State Park. Road cuts on the right side of the road expose the lava flow overlying the Tumalo Ash-flow Tuff. The basal contact of the flow rests on a nice example of a paleosol, a red, oxidized soil once formed on the Tumalo Tuff and since buried by the lava.
9.9 (1.2) Refer to Map 1D.3. Junction of Johnson Rd and Tumalo Reservoir Rd. Turn left (west) onto Tumalo Reservoir Rd. The Desert Spring Tuff is exposed near this junction and is resting on the lavas of the upper Deschutes Formation.
10.2 (0.3) Pull onto the broad right shoulder of the road at this location and park. You will be walking along the road here, so watch for traffic. The road and stream bank cuts nearby provide a great opportunity to examine a nearly complete section of the Desert Spring Ash-flow Tuff (Taylor, 1981). The basal section from streambed to road level has a pink, devitrified, welded matrix containing partially collapsed black pumice fragments; the upper section from road level to the top of the unit (base of gravel) has an orange to yellow, hydrated, poorly welded matrix with abundant fresh, noncollapsed pumice fragments (Figure 1D.4). At the top of the road cut, alluvial gravels rest on an unknown tuff that is overlain by the basal portion of the Bend Pumice.
Figure 1D.4. The upper section of the Desert Springs Tuff and overlying fluvial sandy-gravels; note that in the inset photo that the ash-flow tuff has an orange to yellow, hydrated, poorly welded matrix with abundant fresh, noncollapsed pumice fragments (quarter for scale).
Bedded pumice deposits consisting of reworked pumice ash and lapilli recognized as the basal portion of the Bend Pumice are exposed at the right-hand curve in the roadway. In a small quarry upslope and to the right, the Bend Pumice is overlain by the rhyodacitic, poorly welded to nonwelded Tumalo Tuff containing a fine-grained, white, ashy intralayer at its base that has been interpreted as a pyroclastic ground surge deposit (Figure 1D.5). A normal fault associated with the Tumalo Fault Zone with about 12 feet of displacement on the northeast block is exposed in the road cut just to the left of the parking area where it places Desert Springs Tuff against fluvial deposits; and in the small quarry where it clearly offsets the ashy intralayer at the base of the Tumalo Tuff (Figure 1D.6).
Figure 1D.5. The Bend Pumice overlain by the rhyodacitic, poorly welded to nonwelded lower portion of the Tumalo Tuff containing a fine-grained, white, ashy intralayer at its base interpreted as a pyroclastic ground surge deposit.
Figure 1D.6. Normal fault associated with the Tumalo Fault Zone; downward displacement on the northeast block is about 12 feet (follow the white ashy intralayer from left to right on the image and note its offset relative to each side of the slumped area).
10.6 (0.4) Turn around; drive just beyond the junction of Tumalo Reservoir Rd and Johnson Rd to the junction of Tumalo Reservoir Rd and O.B. Riley Rd. Turn left (north) onto O.B. Riley Rd.
11.3 (0.7) Gravel pits near the road in this area are extracting aggregate gravels from late Pleistocene glacial outwash terraces of the Deschutes River.
11.7 (0.4) Intersection of O.B. Riley Rd and U.S. Highway 20. Turn right (south) onto Hwy 20 and head toward Bend, OR.
12.0 (0.3) The highway crosses the Deschutes River here.
15.2 (3.2) Refer to Map 1D.1. Highway interchange between U.S. Highway 20 and 97. Take exit 135A onto Hwy 97 (3rd Street) for Bend, OR.
17.9 (2.7) Intersection of U.S. Highway 97 (3rd Street) and Greenwood Avenue; continue straight ahead.
18.2 (0.3) Intersection of U.S. Highway 97 (3rd Street) and Franklin Avenue. This ends Field Trip 1D.
Road Route Maps
Map 1D.1. Color shaded-relief map of the Bend 7.5” Quadrangle containing segments of Field Trip 1A-1F and Field Trip 2A.
Map 1D.2. Color shaded-relief map of the Shevlin Park 7.5” Quadrangle containing segments of Field Trip 1A, 1D, and 1E, as well as Field Trip 2A.
Map 1D.3. Color shaded-relief map of the Tumalo 7.5” Quadrangle containing segments of Field Trip 1D and 1F.
Optional Hiking Trail
Shevlin Park Trail (Tr 1D.1)
Rock outcrops along the valley of Tumalo Creek within Shevlin Park provide excellent exposures of several pyroclastic rock units deposited during an extended episode of volcanism from a large silicic vent complex just west of Bend. These pyroclastic rocks represent a suite of materials generated by progressive stages of an explosive volcanic eruption, typical of the extrusion of silicic magmas. In part, these deposits represent nuee ardente, surges of hot, iridescent ash, pumice lapilli, and rock fragments boiling down slope away from the vent during early stages of an eruption, much like those infamously destructive events during the initial eruption of Mt. St. Helens in Washington state. Pyroclastic rocks here also represent air-fall deposits, the product of ash and pumice lapilli injected high into the atmosphere that begin raining down on the landscape after the passage of the pyroclastic surges; and volcaniclastic sediments as well, the product of the reworking of formerly deposited pyroclastic material by mass wasting and running water. As you walk the park’s main trail, try to image the forces at work here only a few hundred thousand years ago and compare it to the serene setting of today. Enjoy your hike here in this once cataclysmic landscape.
Begin your hike at the trailhead sign immediately to your left after passing through the park’s entrance gate (Map 1D.4). After walking a few hundred feet through a small aspen grove and entering a meadow, turn left at a trail fork. Several hundred additional feet carries you across a wooden foot bridge over Tumalo Creek. Ascend the steep slope that forms the southern edge of the modern Tumalo Creek valley and follow the signage for the Shevlin Park Loop Trail to your right (ignore two trails that join from the left at about 0.19 miles and 0.28 miles, respectively). The rounded cobbles along the trail and the natural bench-like topography at the top of the slope are remnants of a stream terrace left by the ancestral Tumalo Creek.
In roughly nine-tenths of a mile, you should reach another trail junction (Map 1D.4); stay on the signed Shevlin Park Loop Trail that follows the ridge. Before continuing, however, descend a short distance on the trail to your right (called the Tumalo Creek Trail) and examine the rock outcrops there. This is the Desert Springs Ash-flow Tuff (Figure 1D.1); notice the flattened black pumice lapilli. The trail merges with the bed of an old road for a short distance, then at one and two-tenths of a mile, it veers to the right, dropping part way into the canyon of Tumalo Creek and paralleling the edge of a low cliff comprised of Desert Springs Tuff. Good exposures of the tuff are passed at about one and a half miles. More rounded stream terrace cobbles are encountered at one and six-tenths miles.
At just under one and nine-tenths of a mile into your hike, another signed trail junction directs you downslope to the right and into a small tributary valley to Tumalo Creek (Map 1D.4). In a couple hundred yards, cross the tributary stream on a wooden foot bridge. Shortly, as the trail ascends out of the tributary valley, examine the rock outcrops to the right of the trail. These expose Desert Springs Tuff; notice again the flattened, black pumice lapilli characteristic of this pyroclastic unit. At the top of the slope, you see rounded stream terrace cobbles denoting a former level of the ancestral Tumalo Creek.
In a little more than four-tenths of a mile, the trail descends into the main valley of Tumalo Creek, passing outcrops of Desert Spring Ash-flow Tuff to your left (Map 1D.4); these are perhaps the nicest exposures of this tuffaceous unit in the park. At nearly two and a half miles into the hike, you cross over Tumalo Creek to the north bank on a wooden plank bridge. Shortly, you encounter a log post marked with an #8 and a trail merging from the left. Continue on the Shevlin Park Loop Trail that heads slightly upslope, but in a downvalley direction to your right. The large boulder here is a float block of Tumalo Tuff tumbled down from rock outcrops higher on the slope. Notice the much coarser rock fragments and lighter, unflattened pumice lapilli within the block.
A couple hundred feet beyond the post brings you to a complex, five-trail junction. The Shevlin Park Loop Trail has followed an old road up slope from the valley floor; do not continue on this road. Another old road, converted to the Tumalo Creek Trail, descends down slope from the left and crosses the Shevlin Park Loop Trail here to the right. Do not follow the Tumalo Creek Trail either. The proper trail ascends the slope above the old road you have been following (the far left trail heading upslope and down-valley) and begins following an old irrigation ditch in a couple hundred feet. As you walk, notice how the ditch contours the slope, barely dropping in elevation as you go. If you traced this ditch back up valley, eventually it would intersect with Tumalo Creek, from whence the irrigation water was once diverted. At just over a tenth of a mile from post #8 your reach another log post marked with a #17. The Tumalo Tuff outcrops here; notice the coarse rock fragments and undeformed pumice lapilli (Figure 1D.2b). This is the poorly welded lower portion of the Tumalo Tuff. Another, even better outcrop below the trail about 500 feet ahead exposes the welded central portion of the Tumalo Tuff (Figure 1D.2a). Here the pumice lapilli are somewhat flattened and both the lapilli and rock fragments have a preferred orientation in a down valley direction. Look for more isolated outcrops of Tumalo Tuff as you continue onward.
A “Y” fork in the trail greets you at two and eight-tenths of a mile into the hike (Map 1D.4). Ignore the log post with an arrow pointing to the downslope trail and continue on the left fork paralleling the ditch. As you walk, notice how the valley of Tumalo Creek is deepening to your right, while the trail continues to descend very gradually along the irrigation flume. This means you are progressing up-section within the pyroclastic rock units deposited in the area (the Tumalo Tuff overlies the Desert Springs Tuff). At three and nine-tenths miles, the Shevlin Park Loop Trail leaves the irrigation flume and begins to descend the slope (Map 1D.4). The exposure down slope to the right of the trail is a pyroclastic unit containing coarse mafic rock and obsidian fragments and is likely the uppermost portion of the Desert Springs Tuff. At four miles into the hike, you reach a “T” junction in the trail (Map 1D.4). The trail to your right descends to the Hixon Covered Bridge over Tumalo Creek. Continue straight on the Shevlin Park Loop Trail and just ahead you’ll find another three-way trail junction. The rightmost trail provides another route to the Hixon Covered Bridge. The trail straight ahead remains on the Shevlin Park Loop Trail. Walk down the abandoned road branching to your left beyond the piles of wood, old barrels, and other miscellaneous junk haphazardly stacked here to examine a road cut on the left. The road cut exposes a dark, andesitic, poorly-welded tuff containing abundant dark scoria and basaltic rock fragments, small lava bombs, and some white to gray pumice lapilli. This is the Shevlin Park Tuff (Figure 1D.3), named after the park where it is best exposed around the valley rim of Tumalo Creek; it is the uppermost pyroclastic unit exposed in the park.
In several hundred feet, the abandoned road temporarily rejoins the Shevlin Park Loop Trail coming in from the right, then shortly after, it veers off to the left again; stay on the trail. Shortly, the Shevlin Park Trail begins a gradual descent into the valley to the right. Another abandoned road descends more rapidly down slope to the right along Red Gulch (visited earlier at mile 5.3 of the road log). Leave the trail and follow the road through Red Gulch. If you didn’t visit Red Gulch previously, now is your chance; follow the old road for several hundred feet downslope to rock outcrops on the left just before a gate across the road. Examine the tuffaceous unit exposed here. It contains coarse fragments of mafic rock and black pumiceous lapilli and represents to upper, less well indurated portion of the Desert Springs Tuff. Walk beyond the gate to join the main park road and head left. Eventually, you will past the trailhead and through the park entrance gate to your waiting vehicle.
Hiking Trail Map
Map 1D.4. Color shaded-relief map of portions of the Bend and Shevlin Park 7.5” Quadrangles showing the Shevlin Park Loop Trail (Tr 1D.1).