Varied Geology within Easy Reach of Central Oregon’s Largest Urban Setting
This field guide describes an even half-dozen short field trip routes that highlight the origins of many of the more obvious and accessible geological phenomena near the city of Bend, Oregon (Figure 1.1). The beginning and ending point for each trip is the intersection of US Hwy 97 (3rd Street) and Franklin Avenue in Bend. The field trips described here explore numerous features of Pleistocene and Holocene volcanism, faulting, and glaciation, and their influence on landscape evolution in the Bend area. Significant features and their formative processes include; cinder cones and their associated lava flows, lava flow features, lava tubes, extrusive igneous rock types, pyroclastic volcanic rocks and deposits, the faults of the Tumalo Fault Zone, stream drainage disruption and associated lake formation, and the formation of waterfalls.
Field Trip lengths: 1A is 51.6 miles; 1B is 44.4 miles; 1C is 64.1 miles; 1D is 18.2 miles; 1E is 28.3 miles; and 1F is 57.1 miles.
Figure 1.1. Field trip routes 1A through 1F.
Bend, OR, playground of year-round recreational enthusiasts, and home of the Deschutes Brewery (the best micro-brewery in the country as far as this author is concerned) is literally brimming with evidence of the volcanic activity that created the marvelous landscape that is central Oregon. Figure 1.2 displays a simplified geologic map of the Bend area. Bend is located just north and slightly west of Newberry Volcano, on a wide expense of basaltic pahoehoe lava flows originating from fissure vents on the shield volcano’s lower north flank that were erupted no more than 780,000 years ago, based on their normal magnetic polarity (Sherrod, et al., 2004). These fluid lavas poured to the north beyond Redmond into the canyons of the Crooked and Deschutes Rivers, and form intracanyon basalts such as those observed at Cove Palisades State Park. The source vents for these lavas were buried by Holocene lava flows associated with the Northwest Rift Zone. The Newberry basalts have been differentiated in the Bend area into two units; the Basalt of Bend (Jensen, 2006) and the Basalt of the Badlands (Sherrod et al., 2004). The Basalt of Bend underlies much of Bend east of the Deschutes River and extends to the southern edge of Redmond. This is the same flow that extended down Newberry’s western flank, forcing a rerouting of the Deschutes River into its modern channel, and the same flow which contains the lava tube of Lava River Cave (visited on Field Trip 1C). The western lobe of the Basalt of the Badlands enters Bend from the northeast and is part of a larger field of lavas erupted from a chain of spatter cones on Newberry’s lower northeast slope. These spatter cones may be a shield volcano on Newberry’s lower flank, or more likely, they represent rootless vents overlying lava tubes draining from an upslope eruptive source higher on Newberry itself. Pilot Butte is the prominent cinder cone on Bend’s eastern edge (visited on Field Trip 1C). This small, middle late Pleistocene volcano and its associated andesitic lava flow have been dated at less than 780,000 years old based on its normal magnetic polarity, but must be older than the Basalt of Bend because it forms a kipuka surrounded by these younger lavas.
Figure 1.2. A geologic map of the Bend area (compiled from Sherrod et at., 2004 and DOGAMI data, 2009).
Ash-flow and air-fall tuffs erupted from the Tumalo volcanic center (Taylor, 1978; Hill and Taylor, 1989; and Hill and Scott, 1990) and now partially buried by the eastern edge of Broken Top’s basaltic andesite platform are exposed along the western outskirts of Bend (Figure 1.2). These pyroclastic units are interlayered with basaltic lava flows originating from vents in the High Cascades further west, and are onlapped from the east by the Basalt of Bend, but are younger than underlying lavas and volcaniclastic deposits of the Deschutes Formation erupted from sources further to the north. Pyroclastics on Bend’s western edge have been differentiated into the uppermost Shevlin Park Ash-flow Tuff erupted less than 170,000 years ago, the underlying Tumalo Ash-flow Tuff and associated Bend Air-fall Pumice erupted between 300,000 and 400,000 years ago, and the basal Desert Springs Ash-flow Tuff erupted between 600,000 and 700,000 years ago. The Bend Pumice has been geochemically correlated with the Loleta ash bed (Sarna-Wojcicki et al., 1987), whose age is probably about 400,000 to 300,000 years old, while several K-Ar age determinations with the same age range have been obtained from the Tumalo Tuff and epiclastic strata that immediately underlie the Tumalo Tuff (Sarna-Wojcicki et al., 1989). These pyroclastic units are observed in detail on Field Trip 1D. Basalt of Bend overlies the Tumalo Tuff and therefore must actually be younger than 400,000 years (Sherrod et al., 2004). The composition, thickness, and extent of these pyroclastic materials indicate a long-active, silicic source of explosive volcanism immediately upslope and upwind of Bend, fortunately now long extinct. Eruption of felsic magmas was apparently concurrent with mafic volcanism in the High Cascades to the west.
On Bend’s proximal northwestern skyline lies Awbery Butte, a basaltic cinder cone of uncertain age and affinity (Sherrod et al., 2004). Awbery Butte erupted in the late Miocene or Pliocene and is probably related to similar volcanism occurring further north in the Deschutes basin. As a local topographic high, it remained above and was surrounded by the younger pyroclastic units of the Tumalo volcanic center and basaltic lavas of the High Cascades that poured in from the west.
Lava flows and pyroclastic materials in the Bend area are disrupted by faulting related to the southern end of the Tumalo Fault Zone (Figure 1.2), a northwest trending belt of en echelon normal faults (Jensen, 2006). This zone of faulting extends southeastward from near Sisters, OR, where Sherrod et al. (2004) name it the Sisters Fault Zone, only to disappear under younger lava flows on the upper northwest flank of Newberry Volcano. The northern end of the Tumalo Fault Zone appears to merge with normal faults associated with crustal extension and graben formation under the High Cascades at Green Ridge, while the southern end may bend eastward to merge with the western end of the Brothers Fault Zone of the High Lava Plains. Fault scarps within Bend’s city limits related to the Tumalo Fault Zone are examined in Field Trip 1C; however, it should be noted that currently, no evidence of faulting in the Holocene has been unearthed. Cinder cones on the northern flank of Newberry Volcano, some within Bend’s southern limits, are generally aligned along this fault zone, the faults serving as conduits for venting of lava (Jensen, 2006). Lava Butte, other cinder cones and their associated lava flows, the youthful volcanics of the Northwest Rift Zone (Jensen, 2006), may be related to the same fault trend (visited on Field Trips 1A and 1B).
Jensen (2006) reports two dates for the Basalt of Bend (78,000 years) and Basalt of the Badlands (75,000 years) that are considerably younger than the eruptive dates I suggested earlier. While these ages do not disagree with the general age relationships of the geologic units in the Bend area discussed above, they do disagree with ages reported in Sherrod et al. (2004) and they suggest that at least some of the basaltic lavas occupying Newberry Volcano’s lower northern flank erupted much more recently than previously thought. These more youthful ages also suggest that the faulting which has disrupted the lava flows must also be relatively recent, at least as recent as the late Pleistocene.
Glaciers certainly never reached their icy tendrils into Bend itself, but they did occupy stream valleys draining the eastern slopes of the Cascade Range just to the west. Field Trip 1E takes you to the valley of Tumalo Creek; where the action happened. Tumalo Canyon has plenty of geomorphology; in addition to the valley’s ample features of Pleistocene age glacial scouring and deposition, the stream itself displays a spectacular example of ongoing fluvial erosion in the form of Tumalo Falls.
North of Bend, OR, lies Smith Rock State Park. Perhaps better known for is spectacular rock climbing opportunities and fly fishing in the Crooked River, it nevertheless plays host to an unique array of volcanic rocks that tell a geologic story not to be missed. The park is bisected by the Crooked River. Lying on the river’s southwest side is the margin of the Basalt of Bend (a vast, mafic lava flow originating miles to the south at Newberry Volcano), while on the northeast side are the substantially older, more felsic volcanics of the Gray Butte-Smith Rock complex, forming a late Oligocene age bimodal volcanic center (Bishop, 1989) thought to correspond to an eruptive event forming the Crooked River Caldera (McClaughry and Ferns, 2006). Mafic olivine basalts and basaltic andesite form the northwestern flanks of Gray Butte, while its southeastern flanks consist of the silicic Gray Butte Rhyolite. Caldera-filling rhyolitic ash-flow tuff forms the volcanic rock of Smith Rock, subsequently intruded by rhyolite. The volcanic rocks of the Gray Butte-Smith Rock complex have been assigned their age based on stratigraphic relationships (Smith, 1986a) and fossil content (Ashwill, 1983; McFadden, 1986), corroborated by radiometric ages ranging from 30.8 to 25.8 m.y. B.P. (McClaughry and Ferns, 2006) on the Smith Rock Tuff and surrounding rhyolite domes and lava flows correlated with caldera formation.
Imagine then, hot fountains of lava building cinder cones in your backyard, or fiery streams of lava pouring into Bend from the surrounding slopes. Or, if you will, glowing clouds of volcanic ash and gases washing over the landscape, smashing, burying, and/or setting fire to all human constructs in their path. Perhaps the climate would just get real cold as glaciers grew once more in the Cascades. Yes, Bend, OR is an amazing pearl in a geologically fascinating landscape, but there are monsters lurking under the bed; better feed them and keep them happy (I hear the ale in town is good), or they just might get a little harsh on us all.