Calderas, Maar Volcanoes, Ancient Lakes, and a Dash of Basin and Range Block Faulting

Overview

The Newberry Volcano and greater Fort Rock-Christmas Valley basin offer a veritable cornucopia of geological phenomena to explore, with some features completely unique to this field trip.  The landscape itself transitions from a more typical High Cascades climate and vegetation regime around Newberry Volcano to eastern Oregon’s High Desert in the Fort Rock-Christmas Valley basin, and thus, this field trip affords an opportunity to glimpse an ecological zone quite unlike other field trips featured in this guidebook.  Two field trip routes are provided (Figure 3.1); both are intended as multi-day excursions with plenty of sights to see, although the second option is the longer, more rustic (and potentially more interesting) of the two.  Field Trip 3A focuses on Newberry Volcano’s upper flanks and caldera, providing ample opportunities to observe numerous Pleistocene and Holocene volcanic landforms and deposits (Figure 3.1).  The trip highlights the formation and evolution of Newberry Volcano; its origins as a shield volcano, its caldera forming stage more classically associated with composite volcanoes, and post-caldera infilling by multiple, small, overlapping volcanic features of bimodal composition.  Many examples of cinder cones, lava domes, basaltic pahoehoe and aa lava flows, and silicic lava flows and pyroclastic deposits are visited and discussed.  Many campgrounds and a few private resorts with cabin rentals are available in the Newberry Volcano area, so accommodations are not a problem.  All main roads are paved, and secondary roads are surfaced with gravel/crushed rock; check accessibility through the caldera prior to departure in late fall or early spring.  Roads can be snow covered and blocked during winter (November through April).  The beginning and ending of the trip is the same location, the intersection of US Hwy 97 and 1st Street (Reed Rd) in La Pine, OR.

Field Trip 3B takes you further afield, visiting sites that highlight many aspects of the Quaternary volcanic and pluvial lake history in the vicinity of the compound Fort Rock-Christmas Valley-Silver Lake basin and the southeastern flank of Newberry Volcano (Figure 3.1).  Numerous features of Pleistocene and Holocene volcanism and pluvial lakes are explored.  Typical shield volcanoes, cinder cones, pahoehoe and aa lava flows, volcanic lava flow features and rock types, and pyroclastic deposits are observed aplenty; but two geological entities unique to this area are examined as well, including pluvial lake shoreline features of erosion and deposition, and maar volcanoes and their associated tuff rings.  The route for this field trip would likely take two or three days to complete in its entirety, with some backtracking involved.  Campgrounds along the route are primitive at best (no running water or toilets) and no motels are available except in La Pine and Christmas Valley.  All main roads are paved and secondary roads are surfaced with gravel/crushed rock.  Portions of the route require use of tertiary packed-dirt roads, but these are sound enough for passenger cars.  Seasonal snow and ice and wet conditions may prevent travel in late fall through early spring, so check road accessibility, especially on some Forest Service and BLM roads, prior to departure. Information is provided so that the route can readily be shorted or lengthened depending on your time available and level of interest.  The beginning of the field trip route is on the lower eastern flank of Newberry Volcano, and the route ends at Fort Rock, OR in the Fort Rock basin south of Newberry.

Newberry Volcano; The Life and Times of a Shield Volcano Complex (Field Trip 3A)

Beyond Newberry: Ancient Lakes, Maar Volcanoes, and a Dash of Basin and Range Block Faulting in the Fort Rock-Christmas Valley-Silver Lake Basin (Field Trip 3B)

Field Trip Route Lengths: 3A is 123.8 miles, and 3B is 132.4 miles.

Figure 3.3.1

Figure 3.1.  Field Trip routes 3A and 3B.

Geologic Summary

Newberry Volcano, named for John Strong Newberry, professor of geology and paleontology at Columbia University and member of several early explorations of the western United States, is a true giant as far as volcanoes go.  Cute in a way (that perhaps only a geologist could love) with all of its dimples, basaltic cinder cones and silicic domes (miniscule by comparison) dotting its surface; it is among the largest volcanoes formed during the Quaternary within the conterminous 48 states, and certainly the largest in Oregon.  It contains a great volume and diversity of volcanic rock types and landforms, and much of it is conveniently accessible.  Figure 3.2 displays a simplified geologic map of the volcano and its surroundings.  Much of Newberry Volcano’s “textbook” features are now part of Newberry National Volcanic Monument, set aside in 1990 for the geological enrichment of us all.  The monsterous volcanic edifice is roughly oval in its dimensions (longer north-south than east-west) and has a shield-like shape, although it is much too geologically complex to be defined simply as a shield volcano.  The volcano is centered about 20 miles southeast of Bend, OR, curiously offset around 35 miles east of the Cascade Crest; the products of its volcanic past cover an area in excess of 500 square miles.  At 7984 feet, the elongate Paulina Peak rhyodacite dome forms the highest point on the volcano, but hidden from all but a Birdseye view, lies its most interesting feature; Newberry’s summit preserves a spectacular example of a caldera, the result of several explosive eruptions in its storied past.

Figure 3.3.2

Figure 3.2. Geologic map of Newberry Volcano and adjacent areas (compiled from MacLeod et al., 1995, and Sherrod et al., 2004, and DOGAMI data, 2009).

Newberry Volcano formed at the western end of the High Lava Plains (Figure 20 in GEOLOGY OF THE CENTRAL OREGON CASCADES), a Miocene to Holocene volcanic province in central Oregon that extends slightly northwestward from the Harney Basin to the Cascade Range, about 50 miles wide and three times as long (Walker and Nolf, 1981).  The High Lava Plains are dominated by the west-northwest trending en echelon normal faults of the Brothers Fault Zone and a distinctive suite of abundant basaltic and rhyolitic igneous rocks expressing an unusual pattern of bimodal volcanism (Figure 23 and Figure 24 in GEOLOGY OF THE CENTRAL OREGON CASCADES).  Faulting in this geologic province is apparently contiguous with that of the northern portion of the Basin and Range province to the immediate south (Lawrence, 1976), where many of the north to northeast trending normal faults are known to bend westward and seem to merge with the Brothers Fault Zone.  Newberry Volcano itself is flanked by three fault zones (Figure 3.3): 1) the Tumalo Fault Zone disrupts older lavas on the volcano’s northwestern slopes and extends northwestward into the normal-faulted Green Ridge of the Cascade Range; 2) the west-northwest trending Brothers Fault Zone extends toward Newberry Volcano’s northeast flank, and although its faults do not cut lavas originating from the volcano, they probably blend into the Tumalo Fault Zone at depth; and 3) the Walker Fault Zone trends northeast onto Newberry’s southwestern slopes, cutting older lava flows.

Figure 3.3.3 - Fault Zones flanking Newberry

Figure 3.3.  Three fault zones flank Newberry Volcano’s massive form: the northwest trending Tumalo Fault Zone emerges from its northwest slopes; the west-northwest trending Brothers Fault Zone disappears beneath its northeast flank; and the northeast trending Walker Fault Zone trends treads onto Newberry’s southwestern slopes.

The High Lava Plains are punctuated by many substantial rhyolite domes and lava flows that show gradual younging westward along a broad front or several parallel belts that continue as far as Newberry (Walker, 1974; McKee, et al., 1976; MacLeod et al., 1981; MacLeod and Sherrod; 1988; and MacLeod, et al., 1995).  Silicic volcanism began about 10 million years ago east of the Harney Basin and progressed westward to Newberry Volcano by the late Holocene (Figure 24 in GEOLOGY OF THE CENTRAL OREGON CASCADES).  A spatially similar west-northwest trending belt of basaltic volcanism extends along the axis of the High Lava Plains, although it lacks a clearly defined age-progression; beginning well to the east at Jordan Craters near the Idaho border, through Diamond Craters near Burns, OR, and the Devils Garden flows on Newberry’s lower, southeast flank, and to the basaltic cinder cones and lavas of the Northwest Rift Zone on Newberry’s northwestern slopes.

Newberry Volcano’s north-south trending oval shape is primarily related to the distribution of its more voluminous basalt, basaltic andesite and andesite vents and lava flows, whereas its dacitic to rhyolitic volcanism has been more confined to the central summit and eastern slopes (Figure 3.2).  The longer, more gradual northern and southern flanks of the volcano are blanketed with a thick accumulation of basalt and basaltic andesite lava flows.  Pahoehoe lavas erupted from vents on Newberry’s lower, northern slopes are normally polarized, and thus, less than 780,000 years old (at least some flows younger than 400,000 years old based on their stratigraphic position above the Tumalo Tuff).  These lavas flowed  far to the north beyond Redmond, OR to flood the canyons of the Crooked and Deschutes Rivers.     Pyroclastic deposits as well as the more silicic domes and flows occur on the narrower eastern and western flanks.  Hundreds of cinder cones and fissure vents dot the volcano’s carapace and are roughly concentrated into three zones that parallel the Tumalo, Brothers, and Walker fault zones; although some vents and cones along the summit are generally aligned in arcs that are probably related to ring fratures formed during caldera eruptions (Jensen, 2006).  Many of the rounded hills of Newberry’s slopes are rhyolite domes and associated viscous flows of middle to late Pleistocene age.  Several voluminous, sheet-like deposits of pyroclastic material cover the east and west flanks of the volcano (and are probably buried beneath younger lavas on the north and side sides).  The oldest pyroclastic unit is found on Newberry’s eastern flank, the rhyolitic Tepee Draw Tuff, an ash-flow deposit that likely marks the earliest explosive eruption and caldera-forming event on the volcano.  This unit probably erupted about 300,000 years ago.  Two other major tephras, the Black Lapilli Tuff composed of basaltic andesite and the Andesite Tuff comprised of near vent agglutinate lavas and ash-flows, occupy the western flank of the volcano and indicate a second episode of caldera collapse that occurred approximately 80,000 years ago.  Mount Mazama erupted about 7,600 years ago to form the Crater Lake caldera in the southern Oregon Cascades, blanketing Newberry Volcano’s slopes with upwards of 3 feet of tephra, especially thick on the upwind (west and southwest) flanks.  The Mazama Ash represents an important stratigraphic time marker useful for distinguishing pre- and post-Mazama volcanic eruptions at Newberry and throughout central Oregon.  Notice that the many cinder cones plastering Newberry’s flanks are aligned into chains that appear to correspond with the lineament of the Tumalo and Walker Fault Zones (Figure 3.2 and Figure 3.3).

The summit of Newberry Volcano hosts a large caldera, partially filled with younger volcanic material and occupied by East Lake and Paulina Lake (Figure 3.2).  The caldera is a compound collapse structure formed of several nested walls or ring-fractures developed during two major periods of explosive volacanism that erupted first silicic, and then more intermediate magmas (unpublished data of Donnelly-Nolan reported in Jensen, 2006) as described above.  The caldera walls, exposed in only a few places, consist of a platey rhyolite base overlain by basaltic andesite lava flows, and localized palagonite tuff and welded tuff, silicic domes, flows, and air-fall deposits, as well as basaltic cinders and spatter.  The caldera contains as much as 1600 feet of volcanic, mass wasted, and lacustrine fill; most of the caldera floor (where not covered by water) is comprised of rhyolitic domes, flows, and pumiceous tephra, although more intermediate lava flows and palagonite tuff rings occur in a few places.  Rhyolitic rocks have probably been erupted nearly continuously since the last caldera- forming phase and include multiple rhyolite and obsidian domes and flows older than 7,600 years (covered by ash from the eruption of Mount Mazama), as well as post-Mazama age obsidian flows, pumice rings and cones,and pumiceous tephra deposits dated between 7,300 and 1,250 years.  Osborn and Bevis (2001) indicate the presence of protalus ramparts on the northeast face of the caldera rim below Paulina Peak, probably deposited during the last glacial maximum of the late Pleistocene.  Jensen (2006) reports that Donnelly-Nolan obtained evidence for glacial deposits and moraines of indeterminate age from the same area.

As indicated above, Newberry Volcano has not been quite in the recent past.  Late Pleistocene and Holocene eruptions are widespread, volcanic rocks include basaltic andesites on Newberry’s flanks, as well as rhyolitic domes, flows, and pyroclastic materials centered around the caldera.  Basaltic andesite eruption products occupy the east rim and upper north flank of the volcano that are dated at about 11,200 years old.  Young, basaltic andesite cinder cones and flows erupted as recently as about 7,000 years ago along the Northwest Rift Zone occupy the northwest flank of the volcano, burying older lavas and attesting to an inherent zone of weakness here that may be fault controlled.  This eruptive belt extends northwestward from the East Lake Fissure on the caldera’s north rim, on through the Lava Cast Forest Flow and its related fissure vent, and down to Lava Butte and its associated outpouring of lava flows that diverted the Deschutes River (see Field Trip 1A for details).  Rhyolitic volcanism began about 7,300 years ago with eruption of the East Lake Tephra, which covers much of the eastern half of the caldera, shortly followed by eruptions that formed the Central Pumice Cone and its associated obsidian flows, the Warm Springs Pumice Cone near the north caldera wall and smaller pumice cones near the south wall, and ending with eruption of the Interlake Obsidian Flow.  The two small East Lake Obsidian Flows south of East Lake erupted about 3,500 years ago.  The most recent and perhaps most spectacular silicic eruptions at Newberry Volcano occurred between about 1,450 and 1,250 years ago that culminated in formation of the Big Obsidian Flow (MacLeod, et al., 1981 and 1995). This volcanic cycle began with an early gaseous phase of explosive eruptions that produced a tall, narrow ash plume blown eastward by prevailing winds to deposit air-fall pumice and ash called the Newberry Pumice.  These pyroclastics form a long, thin band of material spread downwind along the southeastern part of the caldera and eastern flank of the volcano.  A second, less energetic phase in the eruptive cycle generated ground-hugging pyroclastic flows downslope and across the Paulina Lake basin deposited as the Paulina Lake Ashflow.  The final phase of volcanism was relatively quiet, erupting degassed silicic lavas to form the Big Obsidian Flow and its dome (asymmetrically placed near the southern edge of the flow).  Jensen (2006) provides evidence that can be used to infer an active rhyolitic magma chamber beneath Newberry Volcano, and thus, more silicic volcanism could occur in the near future.  He further suggests that the presence of this viscous, silicic magma plug may actually force denser, but hotter and less viscous mafic magma to the side, resulting in flanking eruptions of basaltic andesite as observed.

As one can see, Newberry Volcano has had a long and complicated volcanic history, exemplified by its massive size and the diversity of its geologic features.  Although it retains a relatively simple (shield-like), even passive, appearance from a distance; it like all of nature’s creations, has a complexity only revealed once you begin peeling back the layers.  Compositionally, the volcano displays a clear bimodal distribution of mafic and felsic eruptive products; basalt and basaltic andesite lava flows and cinder cones dominant on the north and south flanks, rhyolite domes, their stubby flows, and voluminous pyroclastic deposits concentrated east, west, and center.  Pahoehoe and aa lavas on Newberry’s northern slopes display its quiter, effusive volcanic moods, while the volcano’s summit portrays a much more violent past.  The occasional injection of silicic, gas-rich magmas, and resultant explosive eruptions have created a compound collapse caldera, more recently infilled with additional rhyolitic materials expressed as tuff rings, pumice cones, and obsidian flows.  Take the time for a lengthy visit, you won’t be disappointed.

Southeast of Newberry Volcano lies an unusual landscape of faulted, internally drained basins, dotted with unusual volcanic craters, tuff rings, and cones, and retaining evidence of former lakes of considerable size (Figure 3.1); welcome to a corner of central Oregon seldom seen, but not forgotten.  Figure 3.2 displays the geology of the greater Fort Rock-Christmas Valley-Silver Lake basin.  Upon entering the area, one is struck by the relative flatness of the terrain, a considerable contrast to the major volcanic edifices of the nearby Cascades.  However, just to the south are the impressive fault scarps of the Basin and Range province, a small taste of which you’ll get in the Silver Lake area (Figure 20 in GEOLOGY OF THE CENTRAL OREGON CASCADES).

The Fort Rock-Christmas Valley-Silver Lake watershed, an internally drained basin at the center, sits astride the transition zone between the High Lava Plains to the north and the Basin and Range to the south.  Newberry Volcano, a shield volcano with a complicated history if ever there was one, forms the northwestern slope into the drainage basin (Figure 3.1).  Newberry was constructed at the western end of the High Lava Plains, a Miocene to Holocene volcanic province in central Oregon that extends slightly northwestward from the Harney Basin to the Casacde Range, about 50 miles wide and three times as long (Walker and Nolf, 1981).  The High Lava Plains are dominated by the west-northwest trending en echelon normal faults of the Brothers Fault Zone and a distinctive suite of abundant basaltic and rhyolitic igneous rocks expressing an unique pattern of bimodal volcanism.  The High Lava Plains are punctuated by many substantial rhyolite domes and lava flows that show gradual younging westward along a broad front or several parallel belts that continue as far as Newberry (Walker, 1974; McKee, et al., 1976; MacLeod et al., 1981; MacLeod and Sherrod; 1988; and MacLeod, et al., 1995).  Silicic volcanism began about 10 million years ago east of the Harney Basin and progressed westward to Newberry Volcano by the late Holocene (Figure 24 in GEOLOGY OF THE CENTRAL OREGON CASCADES).  It is well expressed by a number of silicic domes and lava flows on Newberry’s eastern flank and caldera (Figure 3.2).  A spatially similar west-northwest trending belt of basaltic volcanism extends along the axis of the High Lava Plains, although it lacks a clearly defined age-progression.  A plethora of Holocene cinder cones and lava flows begin well to the east at Jordan Craters near the Idaho border, passing through Diamond Craters near Burns, OR, on to the Devils Garden and related flows at the northern periphery of the Fort Rock subbasin, and finally to the volcanics of the Northwest Rift Zone on Newberry’s northwestern slopes (Figure 3.2).  This combination of mafic and felsic volcanic features forms a topgraphic high and the drainage divide on the north and northeast sides of the Fort Rock-Christmas Valley-Silver Lake basin.

Faulting in the High Lava Plains is apparently contiguous with that of the northern portion of the Basin and Range province to the immediate south (Lawrence, 1976), where many of the north to northeast trending normal faults are known to bend westward and seem to merge with the Brothers Fault Zone (Figure 3.3).  The Fort Rock-Christmas Valley-Silver Lake basin is abutted on the southeast, south, and southwest sides by raised normal-fault blocks associated with the Basin and Range, of which the two normal faults bounding the northeast and southwest sides of the Silver Lake subbasin are the most prominent (Figure 3.1).  Basin and Range normal faults transition into the northeast trending Walker Fault Zone (Figure 3.3).  This zone of en echelon normal faults culminates on Newberry’s southwestern slopes and has generated a belt of topographic relief that serves as the western drainage divide of the Fort Rock-Christmas Valley-Silver Lake basin.

The basin occurs within the northwest portion of the Great Basin, a large region of internally drained basins generally occupying the region between California’s Sierra Nevada and Utah’s Wasatch Range.  The Great Basin contained numerous pluvial lakes during late Pleistocene glaciation, in part a product of higher precipitation as the axis of mid-latitude jet stream was pushed south into Nevada by growth of the Laurentide-Corilleran Ice Sheets.  Cooler air temperatures also considerably reduced evaporation, leaving more moisture on the ground, a factor that probably played an important role in the northern Great Basin pluvial period, as the jet stream was too far south to raise precipitation by much, if at all.  Eight pluvial lakes formed in Oregon’s share of the Great Basin, (Figure 27 in GEOLOGY OF THE CENTRAL OREGON CASCADES).  Allison (1979) mapped and named Fort Rock Lake, the northwesternmost lake in the Fort Rock-Christmas Valley-Silver Lake compound basin, on the basis of contiguous shoreline features, wave-cut cliffs and platforms, as well as beaches and spits, observed at an elevation of 4540 feet around the basin’s margins.  Jensen (2006), reporting on unpublished data from Sherrod, indicates that the most recent highstand of pluvial Fort Rock Lake occurred about 23,000 years ago, and that shallow lakes still existed as separate entities within the deeper section of each subbasin as late as 13,400 years ago.  The Silver Lake subbasin occasionally fills with an ephemeral lake during particularly wet years to this day.

High Lava Plains basaltic volcanism occurring in the northern and western portions of the Fort Rock-Christmas Valley-Silver Lake basin was regularly characterized by its association with groundwater saturated bedrock and sediments and/or lake water at the margin of the basin itself.  Magma encountering water quickly congealed, fractured, and exploded, causing violent release of gases and pyroclastics in the form of phreatic and/or phreatomagnatic eruptions.    Relatively mild eruptions generated basaltic tuff rings and cones, such as Fort Rock and Table Mountain, while more energetic steam eruptions created explosion craters such as Big Hole and Hole-in-the-Ground.  Craters occasionally fill with groundwater to the depth of the surrounding water table to form ponds or small lakes.  Collectively, these volcanic landforms are known as maar volcanoes. Pyroclastic deposits often immediately overlie lacustrine sediment, or are themselves eroded by wave action, suggesting the basin retained or was occupied by a sizable lake during or after these volcanic events.

The greater Fort Rock-Christmas Valley-Silver Lake basin, while not as lofty or stupendous as the nearby Cascade Range or Newberry Volcano, certainly holds it own with a goodly share of interesting geological phenomena.  Ancient lake features, maar volcanoes, and normal faults create a diversity of geologic processes and landforms to explore that is difficult to match elsewhere in central Oregon.