Blue Mountains Province
a. Obtain an overview of the Blue Mountains in order to compare terranes
b. Collect samples from the Pendelton Inliers in the Ukiah region to study age and relationship to the greater Blue Mountain area.
II. Road Log: (beginning from Cambridge, ID)
|Figure 1: The proposed route (from a google map).|
III. Specific Stops:
4.1 Hell’s Canyon Recreation Chamber Stop
|Figure 2: Hell’s canyon (Matt Gatewood, Photographer)|
i. Here we can see Columbia River Basalts, accreted terranes, and more. Hell’s Canyon is the result of down-cutting by the Snake River as it eroded through the local strata. At the base of the Canyon are rocks of the Wallowa terrane. After being accreted onto the North American Craton, this area was uplifted prior to the eruption of the Columbia River Flood Basalts. Erosion by the Snake River was aided by the episodic flooding from the now largely empty Lake Bonneville, the same floods which would create the Scab Lands further downstream. Due to weather concerns, the location of the stop had to be changed on the trip to the Hell’s Canyon region in Oxbow, OR. Here the Columbia River Basalts were spectacularly exposed. We discussed the basic terrane arrangement of the Blue Mountains Province and the heterogeneous nature of the Wallowa terrane as well as the stratigraphy of the CRBs and the corresponding, more rhyolitic, volcanism in the southern part of the region.
4.2 Highway 86 – Sparta Complex – Wallowa Terrane The Wallowa terrane is an island arc which has been accreted onto North America. Igneous rocks in the terrane crystallized between 264 and 225 Ma (Schwartz et al. 2010). To the south of the Wallowa terrane is the Baker terrane (composed of two subterranes; the Bourne and Greenhorn) and the contact between the two is characterized by an imbricate fault zone (Schwartz et al. 2010). Highway 86 roughly parallels the contact between the Wallowa and Baker terranes; it also travels through and stopping at the Sparta complex, a pseudo-ophiolite and part of the Wallowa terrane.
i. We will be seeing an exposure of the Sparta pseudo-Ophiolite
|Figure 3: Sparta Ophiolite (Matt Gatewood, Photographer)|
The Sparta pseudo-ophiolite is interpreted as the gabbro portion of an ophiolite, although its composition ranges into tonalite and trondjhemite. It is correspondingly more felsic than a classical ophiolite. The unit has two portions; one is 215 Ma and the other is 250 Ma. It is interpreted as having been emplaced by obduction, but due to its location it has been difficult to assign a terrane. It is most likely part of the Wallowa. In the outcrop we observed that the pseudo-ophiolite is composed of plagioclase feldspar, amphibole, biotite, and quartz, with the proportions varying from the eastern to western ends of the outcrop. Unfortunately this is probably not representative of the original composition of the unit as it is laced with felsic dikes and many of the smaller ones are mostly quartz. The entire “ophiolite” is composed as a number of randomly oriented fault slices. The larger faults can be greater than 20 feet wide zones of fractured, altered material. Secondary clinozoisite fills the fine thread fractures.
4.3 Baker City
i. A convenience stop as well as the start of the Umatilla National Forest. Here we met up with Mark Ferns, local geologist, who guided us to several interesting local features. Unfortunately, this led to time constraints which prevented us from making the drive from Baker City to John Day.
4.3a Virtue Flats
First we visited the contact between the Baker terrane and a local pluton. It was heavily altered and laden with dikes to the point of being almost unrecognizable. The location also gave us an excellent view of Sawtooth crater, a Miocene calc-alkaline shield volcano in the heart of the Imnaha flow eruptive axis. It’s not related to any subduction zone.
Nearby we saw a small volcanic feeder with a number of scoria bombs around it.
|Figure 5: A volcanic bomb, graduate student for scale.|
The final stop of the day is the Gray Eagle granitic intrusive. This is a very weathered unit which is full of dikes in some areas and exhibits spheroidal weathering in others. It has been dated at 150 Ma. In one outcrop we were able to see that it’s directly overlain by a rhyolitic ash deposit (dated as 15.2 Ma) with basalt flow directly emplaced on top of that. There is a bake zone at the contact, but no soil horizon at all. There are also places where the hot, flowing basalt bulldozed into the soft rhyolitic ash, creating an oddly hummocky contact. That night Mark Ferns kindly allowed us to stay in his home.
4.4 Elkhorn Ridge Argillite (near Sumpter, OR)
The Elkhorn Ridge Argillite is the primary constituent of the Bourne subterrane of the Baker terrane. Elkhorn Ridge is a mélange deposit composed largely of deep sea chert and argillite. Much of the Baker terrane has undergone metamorphism. This unit is known to contain placer gold deposits.
i. Seeing the Greenhorn and Bourne Sub-Terranes
4.5 Dixie Butte (Possible Stop)
i. Possible sampling location.
Dixie Butte is an outcropping of the Greenhorn subterrane, which along with the Bourne subterrane makes up the Baker Terrane. Dixie Butte is a serpentite matrix mélange as opposed to the chert argillite mélange of the Bourne subterrane.
4.6 John Day
i. Rest here for the night.
ii. Clyde Holliday Campsite (Hwy.26, 1 mile east of Mount Vernon, on US Highway 26, on the right, and 7 miles west of John Day on US Highway 26 on the left.)
Phone: 541-932-4453 or 1-800-452-5687 (Not taking reservations until April 14)
5.1 Mountain Home Metamorphic Complex
(From John Day, taking US 395, near Dale, OR )
The Mountain Home Metamorphic Complex (MHMC) is a largely unstudied series of plutons surrounded by unusually high grade (for the area) metamorphic rocks. The metamorphic rocks are not known to contain contact metamorphic textures so much as regional ones, but are intruded by the igneous rocks of the MHMC. Some of the emplaced plutons are also regionally deformed while others are reported to be largely undeformed. Many of the area’s igneous rocks (such as the gabbronorite of Carney Butte) contain zircons and some of the metamorphic rocks (such as the biotite schist of Yellow Jacket Road) contain garnet. Not much is known about the timescales of formation or the geochemistry of the Mountain Home Metamorphic Complex.
i. Possible sampling of Biotite schist, etc. on Yellow Jacket Rd
ii. Sample the Diorite of Alexander Creek
iii. Sample the detrital zircons of the metasediments along Yellowjacket road
The diorite of Alexander Creek contacts directly with the nearby Carney Butte Norite. It’s a non-cohesive and very easily weathered pluton which is not observed to be regionally deformed (Trauba 1975). In map view it may be interpreted as being intruded by the Carney Butte and its composition is reported to change with greater distance from the norite. The contacts between the two are only exposed to the north and south of Battle Mountain state park and south of Yellowjacket road. Trauba also reports it as being quite readily weathered to a depth of tens of meters.
We collected two samples of the Diorite of Alexander creek for thin section study.
|Figure 6: Photomicrograph of the Diorite of Alexander Creek in both Plain polarized light (left) and under crossed polarized light (right).|
Next we sought out the garnet-biotite schist of Yellowjacket road. This is mapped as extending to the east and is locally garnetiferous. It does not directly border the Carney Butte Norite anywhere; the diorite of Alexander creek is directly between them at every point. South of Battle Mountain State Park the schist is graphite bearing, but does not contain garnet except along the intrusive contact with the diorite of Alexander creek. The Yellowjacket road schist is interpreted as being metasedimentary in origin and possesses a strong schistosity and foliation. It also contains a number of lenses of slightly different material, as well as significant variation in garnet sizes across the length of the Yellowjacket road exposure. In some places garnets can be from 1 mm to nearly 1 cm in diameter; unfortunately it is unclear whether the larger ones only underwent a single phase of growth as the schist is cut by a number of dikes which may have resulted in later growth phases. There are also some features which may be the preserved remnants of boudinage. Several samples were taken for thin section work and possibly for geochronology and geochemistry as well.
|Figure 7: Deformation in the biotite schist of Yellowjacket road, roadcut scale.|
|Figure 8: Deformation in the biotite schist of Yellowjacket road, thin section scale (from location in figure 7).|
|Figure 9: Small crenulation folds in thin section scale, both under plain polarized light and under crossed polarized light.|
|Figure 10: A garnet showing textbook rotation as delineated by its pressure shadows. Garnet is shown in both plain polarized light and cross polarized light.|
IV. Regional Geology of the Area
a. The Blue Mountains are a series of terranes accreted onto the North American Craton. They have been hypothesized to be the remains of what was once an island arc which was significantly deformed before being accreted. An alternative hypothesis is that the Blue Mountains are the result of two separate island arcs which collided and deformed before accretion (Vallier, 1995). The major composite terranes are the Wallowa, Olds Ferry (both island arc derived), the Baker (subdivided into the Bourne subterrane, thought to be an altered accretionary prism, and the Greenhorn subterrane), and the Izee (a sedimentary basin). The Blue Mountains outcrop from beneath the Columbia River Flood Basalts and are bordered to the west by the Western Idaho Shear Zone. The Pendelton Inliers are plutonic system west of the Wallowa and containing several metamorphic rock units (the Mountain Home Metamorphic Complex).
Schwartz, J.J., Snoke, A.W., Frost, C.D., Barnes, C.G., Gromet, L.P., and Johnson, K., 2010, Analysis of the Wallowa-Baker terrane boundary: Implications for tectonic accretion in the Blue Mountains province, northeastern Oregon: Geological Society of America Bulletin, v. 122, no. ¾, pp. 517-536.
Trauba, W.C., 1975, Petrography of Pre-Tertiary rocks of the Blue Mountains, Umatilla, Northeast [Master’s Thesis]: Oregon State University, 171 p.
Vallier, T.L., 1995, Petrology of Pre-Tertiary Igneous Rocks in the Blue Mountains region of Oregon, Idaho, and Washington: Implications for the geologic evolution of a complex island arc system: U.S. Geological Survey Professional Paper 1438, pp. 125-209.
Umatilla National Forest
2517 SW Hailey Ave
Pendleton, OR 97801
Oregon Department of Geology and Mineral Industries