THE GEOLOGY OF THE LOWER HALF OF THE POWDER RIVER CANYON BETWEEN THIEF VALLEY RESERVOIR AND THE LOWER POWDER VALLEY, BAKER COUNTY, OREGON
Ben Zublin
Science Department, Badgley Science Center, Eastern Oregon University, La Grande, OR 97850-2899
Abstract
The Powder River between Thief Valley dam and the Lower Powder Valley is the deepest (244 m; 800 ft) and the gradient of the river is steepest (~9 m/km; 48 ft/mi) where the river cuts across faults that mark the margins of uplifted and tilted fault blocks. These sections of the river are the site of three large landslides that formed because steep slopes created by rapid river down-cutting, structural failure of tuff beds underlying the thick olivine basalt flows in the area, and, possibly, earthquakes activity. Two large-scale meanders in the course of the river appear to be related to geologic structures in the metamorphic bedrock. If uplift rates in the Powder River Canyon area were similar to those to the north between Telocaset and the Grande Ronde Valley over the past 9 million years, then the Powder River began carving the canyon between Thief Valley dam and the Lower Powder River at ~2 million years ago, the same time as when Lake Idaho drained out through Hells Canyon. The high river gradients, the abundance of landslides, and the occurrence of a magnitude 3.6 earthquake in 1969 along one of the faults in the area adjacent to the river suggest that uplift is continuing in the area and indicate that the river is still adjusting to the rotation and tilting of the fault blocks in the area.
Introduction
The Powder River and its principal tributaries originate on the east and west flanks of the Elkhorn Mountains of Baker County, Oregon. The river enters the Baker Valley at Baker City, flows northward to a point a few miles east of the town of North Powder, and enters a canyon that is deeply incised into the Powder River Volcanic Field and the underlying Mesozoic volcanic arc sequence (Bailey, 1990). The river flows through the canyon southeastward approximately 32 km (20 miles) across a series of uplifted fault blocks that tilt toward the east before exiting into the broad Lower Powder Valley syncline (Brooks and others, 1976). Below Thief Valley Reservoir, the Powder River flows through a rugged gorge that is more than 244 m (800 ft) deep from rim to riverbed, yet often less than half a mile wide. The river elevation drops from 950 m (3100 ft) at the upstream end of this stretch to 850 m (2800 ft) at the outlet of the canyon. This part of the Powder River Canyon includes huge landslides, entrenched meanders, precipitous cliffs and rock outcrops, and other features of geologic interest (Fig. 1).
Figure 1. Satellite image of the Powder River Canyon between Thief Valley Reservoir and the Lower Powder Valley. Image modified from Google Earth.
This study focuses on a 19 km (12 mi) long stretch of the Powder River Canyon in northernmost Baker County between Thief Valley Reservoir and the Lower Powder River Valley, northwest of the community of Keating. The study area stretches across the Keating NW, Magpie Peak, and Telocaset 1:24,000 quadrangles and includes ~25 km2 (10 mi2) of land administered by the Bureau of Land Management in T7S, R40E and T7S, R41E. Initial work involved the study of maps and air photos to identify areas for subsequent field investigations. Fieldwork was done over the course of two months in Fall 2004. Due to the remote and rugged nature of the study area, these investigations were carried out on foot and by mountain bike. Geomorphic and geologic features were sketched and photographed in the field and mapped on the U.S. Geological Survey 1:24,000 Magpie Peak and Keating NW quadrangles. Emphasis was placed on describing landslides, meanders, and faults in the area. Lithologic contacts were mapped where not obscured by talus and other features and dips and strikes of rock units were measured with a Brunton compass. Samples of the rock units in the area were collected and examined in the laboratory.
Rock Units
The Powder River southeast of Thief Valley Reservoir has carved down through a sequence of normal-faulted Miocene volcanic flows and tuffs and into the underlying metamorphic rocks (Fig. 2). Gilluly (1937) assigned the basement rocks to the Permian-Triassic Clover Creek Greenstone Formation, which is widely exposed in the Wallowa Mountains to the east. These rocks crop out in the lowest elevations of the study area, where they exert control over the course of the inner canyon and riverbed. A number of widely scattered lenticular limestone units occur within this formation (Moore, 1937; Walker, 1979; Bailey, 1990) and were examined, but not mapped, by the author. The top of the Clover Creek sequence exhibits well-developed hill-and-valley paleotopography, indicating the presence of a substantial unconformity (Gilluly, 1937). This stratigraphic gap, which manifests itself as an irregular contact surface in the canyon walls, is overlain by a layer of heavily weathered tuff that occurs in widespread, but isolated, outcrops of limited exposure. The tuff was mapped by Brooks and others (1976) and Bailey (1990) in the northwest and southeast portions of the study area as "Tst", Tertiary sediments and tuff. It is not known if the outcrops in the study area are parts of one continuous formation or represent several distinct units. At least part of the tuff sequence may represent the distal portions of the Dooley Rhyolite Breccia. This unit includes ash-flow tuffs erupted ~14-16 m.y. ago from vents in the Dooley Mountain area southwest of Baker City, Oregon (Gilluly, 1937; Whitson, 1988).
Figure 2. General stratigraphic sequence in the study area. Age dates from Bailey, 1990.
The middle Miocene Columbia River Basalt Group, which dominates the geologic landscape in much of the surrounding region, is absent from the Powder River canyon area between Thief Valley dam and the Lower Powder River Valley. It is possible that the Columbia River basalts were emplaced here and since have been removed by erosion, but this scenario seems unlikely due to the relatively short time period this would have had to take place in before the eruption of the overlying tuffs and Powder River Volcanics. The irregularity of the upper contact of the Clover Creek Greenstone Formation suggests the paleotopography in this area was hilly at the time of eruption of the Columbia River basalts. It seems more likely that the pre-Tertiary paleotopography was pronounced enough in this area that the Columbia River basalts did not pool to a sufficient depth to bury the area (Mark Ferns, 2005, oral communication).
The tuff unit is overlain by a series of ~14-13 m.y. old (middle-late Miocene) olivine basalt, basaltic andesite, and dacite flows that are part of the Powder River Volcanic Series mapped by Bailey (1990). This volcanic sequence is over 120 m (400 ft) thick and is deeply dissected by the Powder River. Columbia River basalts, which are widespread in nearby areas, are not present in the study area. The olivine basalt unit consists of a number of flows, each averaging about 9 m (30 ft) thick, appear to have been erupted in relatively rapid succession, judging from the lack of paleosols and paleotopographic features on the tops of the flows. The olivine basalt unit occupies the middle elevations of the canyon walls, forming steep cliffs with broad aprons of talus that obscure the underlying metamorphic rocks. Capping these basalts are flows of basaltic andesites and dacites that form well defined scarps, typically 6 m (20 ft) high along the canyon rim. These rocks are easily identified by their closely jointed, platy appearance, which is most notable in weathered outcrops. This unit consists of one single flow in some areas and two fairly distinct flows in others.
Scattered deposits of moderately weathered tuffs mantle the lava flows in the study area and occur as interbeds in places within the Powder River volcanics. Mixed in with these tuffs in a few isolated locations are diatomites that contain benthic and shallow water planktic diatoms.
Structural Geology and Geomorphology
The Powder River canyon in the study area is centered on an uplifted fault block bounded by a system of northwest-trending normal faults that dip to the southwest (Gilluly, 1937; Brooks and others, 1976). The faulting is thought to have started developing at the same time as the eruptions of the olivine basalt unit (Bailey, 1990). The Powder River lava flows dip in an east-northeast direction at angles ranging from 5-10º (Fig. 3).
The river enters the canyon below Thief Valley dam at an elevation of ~945 m (3100 ft) and flows south, southwest, and south for the next 2.4 km (1.5 mi) through Clover Creek Greenstone (TrPu) that is capped on the east side of the canyon by dacites of the Powder River Volcanic Group (Td). The river drops 6 m (20 ft) in elevation, a gradient of 10 m/km (13 ft/mi). This section of the canyon is ~150 m (500 ft) deep and 0.8 km (0.5 mi) wide, with gradually sloping sides. The southwest rim of the canyon rises from ~1000m (3300 ft) to ~1040 m (3400 ft) and the elevation of the northeast rim of the canyon rises from ~1100 m (3600 ft) to ~1130 m (3700 ft).
For the next mile, the river flows east-southeast, then southeast, through Clover Creek Greenstone that is capped on the northeast side of the canyon by Powder River Volcanic Group olivine basalt flows (Tob) and dacites (Td) and on the southwest side by Powder River Volcanic Group olivine basalt flows (Tob). The elevation along this section of the river drops from ~940 m (3080 ft) to ~935 m (3070 ft), a gradient of 8 m/km (10 ft/mi). The canyon is ~180 m (600 ft) deep and 0.8-1.6 km (0.5-1 mi) wide in this section of the canyon, with gradually sloping sides that rise to steep cliffs at the rim. The southwest rim of the canyon rises to ~1130 m (3700 ft) elevation and the northeast rim reaches an elevation of ~1190 m (3900 ft). The slopes along the southwest rim of the canyon reflect the NNW strike and gentle ENE dip of the olivine basalt lava flows in this area. This section of the river parallels the NNW-trending faults to the east of the canyon, suggesting that the course of the river may be influenced by fractures in the bedrock.
Figure 3a. Geology of the Powder River Canyon from Thief Valley Dam to Mile 5.5.
Figure 3b. Geology of the Powder River Canyon from Mile 1.5 to Mile 9.
Figure 3c. Geology of the Powder River Canyon from Mile 7.5 to Lower Powder Valley.
The Powder River flows in an overall SSE direction for the next 5 km (3.1 mi), paralleling a system of NNW-trending faults on the northeast side of the river (Fig. 4). Uplift along these faults has created the asymmetrical trough through which the Powder River now flows. Large landslides cover much of the canyon floor in this area and olivine basalt flows (Tob) form cliffs on both sides of the river. The river drops along this stretch from an elevation of ~935 m (3070 ft) to an elevation of ~890 m (2920 ft), a gradient of 38 m/km (48 ft/mi). The canyon is ~240 m (800 ft) deep and 1.6 km (1 mi) wide, with hummocky and terraced landslide topography at the base of the slopes capped by steep cliffs of olivine basalt. Slopes along the southwest wall of the canyon in this area follow the ENE dip of the lava flows and the northeast wall is marked by steep NNW-SSE trending fault scarps that mark the western edge of the uplifted fault block. The southwest rim of the canyon drops from ~1130 m (3700 ft) elevation to ~1070 m (3500 ft) elevation and the northeast rim of the canyon drops from ~1190 m (3900 ft) to 1130 m (3700 ft). The straight SE course of the river in the lower part of this section may be the result of the river flowing along a fracture zone. Weathering along NW-SE and SW-NE trending joint systems has produced crenulated cliffs in the basalt units along the southwest wall of the canyon just north of the confluence of Magpie Creek. These cliffs are prominent in Sections 13, T7S, R40E. Below Magpie Creek the river flows south and then east through olivine basalts, crossing two fault zones that trend to the NNW and NE.
Figure 4. The Powder River Canyon (~Mile 5.5-6), looking east across the fault zones that cut across this area.
The next section of the river cuts across a fault block of uplifted Clover Creek Greenstone overlain by olivine basalt flows. This results in a steep-walled gorge that is 700 feet deep and half a mile wide, with prominent crenulated cliffs in Sections 18, 19, and 20, T7S, R41E. The Powder River drops ~30 m (100 ft) in ~5 km (3 mi) along this section of the river, a gradient of 26 m/km (33 ft/mi). The river flows southeastward around the toe of a large landslide and then turns sharply northward, forming a large-scale incised meander. Then the river flows eastward through a 1.6 km (1 mi) long section of the canyon that is roughly parallels the section of the Powder River to the northwest and the lower portion of Magpie Creek to the southeast. This suggests that this portion of the Powder River may be following an east-west trending fracture system in the Clover Creek Greenstone Formation that has been offset by later block faulting. The river flows past south-facing slopes in the southern half of Sect 17, T7S, R41E that display a peculiar convex form. The slopes cut into the Permian-Triassic greenstones in the river are steep and grow gentler toward the base of the overlying olivine basalts. Gilluly (1937) noticed this odd morphology and attributed it to the retreat of rim rock as the tuffs underlying the basalts weathered and eroded. A line of springs at the base of the basalt is a likely cause for this rapid erosion and retreat. Then the river heads south, flowing through a large incised double meander just north of the confluence with Big Creek. Overall, the southwest rim of the canyon drops from ~1070 m (3500 ft) to 1010 m (3300 ft) elevation along this section and the northeast rim drops from ~1130 m (3700 ft) to 950 m (3100 ft) elevation.
South of Big Creek, the river flows through olivine basalt flows in a SE direction through an ~150 m (500 ft) deep and 0.8 km (0.5 mi) wide canyon for ~2.5 km (1.5 mi), where it crosses a SW-NE trending fault zone. Faulting has jumbled the rock sequences exposed along the course of the river in this area. Just southeast of the confluence of the Powder River and Big Creek, Miocene-Pliocene tuffs and lacustrine deposits are found on a down-dropped block of olivine basalt next to Permian-Triassic metamorphic rocks on the uplifted block to the north. A series of erosional hogbacks and cuestas about 0.5 mi west of this fault system (Sect 21, 27 and 28, T7S, R41E) have been formed as the river and associated SW-trending gullies have cut into the ENE-dipping olivine basalt lava flows in this area. Crenulated cliffs in the basalt units along the north wall of the canyon are prominent in Sects 18,19, and 20, T7S, R41E. After crossing the fault zone, the river flows in an ESE direction through olivine basalt flows for ~0.6 l, (0.5 mi) and then flows for ~ 0.3 km (0.2 mi) through Miocene-Pliocene tuffs and lacustrine deposits before it exits to the Lower Powder River at an elevation of ~850 m (2800 ft). The river drops from ~860 m (2820 ft) to 850 m (2800 ft) along this 4 km (2.5 mi) long stretch of the Powder River Canyon, an average gradient of ~6 m/km (8 ft/mi). The southwest rim of the canyon drops from ~1010 m (3300 ft) to 850 m (2800 ft) elevation and the northeast rim drops from 950 m (3100 ft) to 850 m (2800 ft) elevation.
Overall, the Powder River drops ~90 m (300 ft) in elevation in ~19 km (12 mi) between Thief Valley dam and the Powder Valley (Table 1), an average gradient of ~20 m/km (25 ft/mi). The depth of the canyon and the gradient of the river are greatest in the uplifted portions of the canyon between Miles 2.7-9.4. This is the area where the large landslides are found.
TABLE 1. CHARACTERISTICS OF THE POWDER RIVER CANYON BETWEEN THIEF VALLEY DAM AND THE LOWER POWDER RIVER
Mile point |
Canyon Depth |
River Gradient |
Rock Unit |
Other Features |
0-1.5 mi |
150 m (500 ft) |
10 m/km (13 ft/mi) |
TrPu |
|
1.5-2.7 |
183 m (600 ft) |
6.3 m/km (8 ft/mi) |
TrPu |
|
2.7-6.2 |
244 m (800 ft) |
38 m/km (48 ft/mi) |
Tob (TrPu) |
Landslides, Faults |
6.2-9.4 |
213 m (700 ft) |
26 m/km (33 ft/mi) |
TrPu |
Landslides, meanders |
9.4-12.4 |
152 m (500 ft) |
6 m/km (8 ft/mi) |
Tob, Tst, Qal |
Faults |
Landslides
There are three large-scale landslide features in the central portion of the Powder River Canyon between Thief Valley dam and the Lower Powder River Valley. These features each exceed 0.25 mi2 in area. Each appears to have completely blocked the course of the river. The landslide complexes are marked by undulating terrain with numerous closed basins, pressure ridges, and chaotically tilted blocks of displaced rim-rock. These features are clearly visible in aerial photographs and easily identified on topographic maps of the canyon area (Fig. 5).
Figure 5. View southeast down the southern flank of the landslide on the south side of the Powder River in sections 11, 12, and 13 of T7S, R40E (Mile 2.6-4.7). The head scarp and hummocky deposits of a second landslide are visible at left-center across the river. The isolated pinnacle at river level is an outcrop of Clover Creek Greenstone that was mapped by Gilluly (1937).
The pair of landslides in sections 11, 12, and 13 of T7S, R40E both appear to be related to the down cutting of the Powder River, which produced the steep canyon walls. Structural failure of the tuff beds underlying the thick olivine basalt flows appears to have been an important factor precipitating the landslide activity in the Powder River canyon (Mark Ferns, oral communication, 2005), just as it has been in the Grande Ronde Valley to the north (Schlicker and Deacon, 1971; Fromwiller, 1997) and in the Columbia Gorge region. The slide on the west side of the Powder River in this area originated in basalt flows that dip at an angle of ~7º in an ENE direction toward the river, allowing the mass to slide into the canyon. The slide on the east side of the river originated in olivine basalt flows that dip into the canyon wall, away from the river at an angle that exceeds the dip values of rocks in nearby areas. This suggests that rotational motion has occurred, perhaps along a slip plane related to the NNW-SSE trending normal fault parallel to and approximately one-half mile to the east of the slide scarp (Fig. 6).
Figure 6. WSW-ENE geologic cross-section across the landslide area in sections 11, 12, and 13 of T7S, R40E.
A third landslide is located on the southern wall of the east-west trending portion of the Powder River Canyon in sections 18 and 19, T7S, R41E adjacent to the same fault zone (Fig. 7). This slide moved northward at an angle to the dip direction of the olivine basalt flows in the area. The landslide activity appears to have occurred in fault-fractured rocks in the area. Field investigation of the head scarp area failed to reveal any exposures of tuff deposits underlying the basalt flows in this area. Tuffaceous materials are exposed on the canyon walls directly across the river to the north, suggesting that similar units may be present in the head scarp area of the landslide, but are obscured by the thick deposits of talus that have fallen from the basalt cliffs above.
Figure 7. View to north of the landslide in sections 18 and 19, T7S, R41E (Mile 6.2-7), left center of photo just above large-scale meander. The low gradient slope below the retreating cliffs on the right is eroded into Clover Creek greenstones.
Meanders
The two large-scale meanders found on the west and east ends of the roughly east-west section of the Powder River in the NW¼ and SE¼ of T7S, R41E are noteworthy because of their extreme sinuosity. Both of these meanders appear to be related to geologic structures in the metamorphic bedrock.
The meander on the west (Mile 7-7.5) diverts the riverbed about 0.5 km (0.3 mi) to the south. This is apparently a result of changes in the resistance of the bedrock in the area. At the point of initiation of the meander, the east wall of the canyon is composed of prominent outcrops of resistant Clover Creek greenstone. A prominent mass of greenstone deflects the flow of the river to the southwest as it enters the tightest portion of the bend. This body of rock is truncated on its upstream side by a sheer rock face formed along a NNE-SSW trending fracture. This fracture continues above the riverbed to the southwest, where it is marked by a linear gully in the overlying olivine basalts.
The second meander, located approximately 0.4 km (0.25 mi) above the confluence of the Powder River and Magpie Creek (Mile 8.8-9.3), is a tight double curve (Fig. 8). Like the meander to the west, it appears to have formed because resistant masses of greenstone have interfered with the course of the down-cutting river. These outcrops are crosscut by a series of joints that are oriented in a northeast direction. One of these joints appears to have redirected the river's course about 60 m (200) ft to the northeast to form the meanders. This joint can be traced northeast into a linear gully above the river.
Figure 8. View northeast across the double meander (Mile 8.8-9.3). Prominent outcrops in this view are Clover Creek greenstones.
Geomorphic History
The Powder River meanders through the Upper Powder Valley northward to a point just south of Telocaset, where it makes an abrupt bend to the southeast. Overall, the course of the river mirrors a similar pattern shown by the Grande Ronde River in the Grande Ronde Valley to the north. Livingston (1928) proposed that the ancestral Powder River was part of an ancestral Snake River that flowed from Lake Idaho up through the Durkee and Baker Valleys and then continued through Pyles Canyon, northeast of the town of North Powder, into the Grande Ronde drainage and into the Columbia River system. As uplifting and faulting continued, according to Livingston's theory, the drainage of Lake Idaho was captured by a tributary of the Salmon River, shifting the course of the Snake River to Hells Canyon, and a fault block was tilted up across the path of the Powder River more rapidly than the stream was able to cut through it, causing the Powder River to shift to a southeast course and flow toward the Snake River. More recent research (see Van Tassell and others, 2001, for a review) suggests that Lake Idaho may have flooded up the Powder Valley rather than the draining through the Durkee Valley to the Baker Valley and indicates that Lake Idaho began draining through Hells Canyon approximately 2-3 million years ago. Fish and diatom fossils found in a water well in the area of Imbler, Oregon, indicate that Lake Idaho may have spilled into the Grande Ronde Valley over Telocaset Pass between ~3.6-2 million years ago (Van Tassell and others, 2001).
The idea that the present course of the Powder River flows along the margins of uplifted and tilted fault blocks, appears to be generally true for the section of the Powder River between Thief Valley dam and the Lower Powder Valley, but the river also flows eastward across the NNW-SSE trending faults mapped in this area by Gilluly (1937). This suggests that the path of the river has also been influenced by additional geologic structures in the area, perhaps of different ages, that have yet to be mapped.
An estimate of the time when the Powder River shifted to its new course is can be made by assuming that the down cutting of the Powder River Canyon has taken place at a rate similar to that of the faulting taking place between the summit of Telocaset Pass, to the north of the study area, and the bedrock floor of the Grande Ronde Valley over the past 9 million years. This rate (Van Tassell, unpublished data) is ~0.09 mm/yr (~0.0003 ft/yr). Dividing the maximum depth of the Powder River Canyon (244 m; 800 ft) by the uplift rate suggests that the Powder River began carving the canyon between Thief Valley dam and the Lower Powder River at ~2 million years ago, at approximately the same time as Lake Idaho drained out through Hells Canyon.
The high gradient of the river and the abundance of landslides where the Powder River cuts across one of the highest points on the topographic divide between the Baker Valley graben and the Lower Powder Valley syncline suggest that the river may be still adjusting to its new course. This may be because there hasn't been enough time for the river to adjust its grade or it may be an indication that the faulting in the area is keeping pace with down cutting by the river. The presence of active faulting in the area is suggested by a magnitude 3.6 earthquake that occurred ~0.7 km (2.4 mi) southeast of Thief Valley Reservoir (44°59"W, 117°45'W) on August 14, 1969 (Couch and Whitsett, 1969). This suggests that prehistoric earthquakes may have triggered the landslides along the river.
Conclusions
The Powder River has carved a rugged 19 km (12 mi) long gorge through faulted Mesozoic greenstones and Tertiary basalt flows and tuffs between Thief Valley dam and the Lower Powder Valley. The canyon is the deepest (244 m; 800 ft) and the gradient of the river is steepest (~38 m/km; 48 ft/mi) where the river cuts across faults that mark the margins of uplifted and tilted fault blocks. These sections of the river are the site of three large landslides that appear to be the result of steep slopes created by rapid river down-cutting and structural failure of tuff beds underlying the thick olivine basalt flows that rim the canyon walls. Earthquakes may also have helped trigger the landslides. Two large-scale meanders in the course of the river appear to be related to geologic structures in the metamorphic bedrock.
Assuming a rate of uplift in the area of ~0.09 mm/yr (0.0007 ft/yr) and that the river has cut down at the same rate in response to the uplift suggests that the Powder River began carving the canyon between Thief Valley dam and the Lower Powder River at ~2 million years ago. This is when Lake Idaho started draining through Hells Canyon as a tributary of the Salmon River eroded headward due to uplift in the region. The high gradient of the river, the abundance of landslides, and the occurrence of a magnitude 3.6 earthquake in 1969 along one of the faults in the area adjacent to the river suggest that uplift is continuing in the area and indicate that the river is still adjusting to its new course.
More detailed mapping of the faults and other geologic structures in the region is needed to better understand the geomorphology of the Powder River Canyon and when and how it began to form.
Acknowledgments
I am grateful to Mark Ferns of the Oregon Department of Geology and Mineral Industries in Baker City for sharing his knowledge of the geology of the Powder River area and helping to identify geologic rock units in photos of the area. Jay Van Tassell pointed out the changes in gradient of the river along its course, helped focus the discussion of the origin of the canyon, and helped edit the manuscript.
References Cited
Bailey, D.G., 1990, Geochemistry and petrogenesis of Miocene volcanic rocks in the Powder River volcanic field, northeastern Oregon: Ph.D. thesis, WashingtonState University, 341 p.
Brooks, H.C., McIntyre, J.R., and Walker, G.W., 1976, Geologic map of the Oregon part of the Baker 1° x 2° quadrangle: Oregon Department of Geology and Mineral Industries GMS-7, 1:250,000.
Couch, R., and Whitsett, R., 1969, The North Powder earthquake of August 14, 1969: The Ore Bin, v. 31, no. 12, p. 239-244.
Fromwiller, G., 1997, Grande Ronde Valley landslides: Eastern Oregon Science Journal,
v. 13, p. 27-29.
Gilluly, J., 1937, Geology and mineral resources of the Baker Quadrangle, Oregon: U.S. Geological Survey Bulletin 879, 119 p.
Livingston, D.C., 1928, Certain topographic features of north-eastern Oregon and their relation to faulting: Journal of Geology, v. 36, p. 694-708.
Moore, B.N., 1937, Nonmetallic mineral resources of eastern Oregon: U.S. Geological Survey Bulletin 879, 180 p.
Schlicker, H.C., and Deacon, R.J., 1971, Engineering geology of La Grande and vicinity: Oregon Department of Geology and Mineral Industries, 16 p.
Van Tassell, J., Ferns, M.L., McConnell, V., and Smith, G.R., 2001, The mid-Pliocene Imbler fish fossils, Grande Ronde Valley, Union County, Oregon: Oregon Geology, v. 63, no. 3, p. 77-84, 89-96.
Van Tassell, J., Ferns, M.L., McConnell, V., and Smith, G.R., 2002, Neogene history of the Grande Ronde Valley, NE Oregon: Geological Society of America Abstracts with Programs, v. 34, no. 5, p. A-39.
Walker, G.W., 1979, Reconnaissance geologic map of the Oregon part of the Grangeville quadrangle, Baker, Union, Umatilla, and Wallowa Counties, Oregon: U.S. Geological Survey Miscellaneous Investigations Map I-1116, 1:250,000.
Whitson, D.N., 1988, Geochemical stratigraphy of the Dooley Rhyolite Breccia and Tertiary basalts in the Dooley Mountain quadrangle, Oregon: M.S. thesis, Washington State University, 122 p.
POWDER RIVER LOG: THIEF VALLEY DAM TO LOWER POWDER VALLEY
Miles
00.0 Start at Thief Valley Dam. The dam is located just south of an E-W trending fault. For the next 1.5 miles, the Powder River flows through Clover Creek Greenstone (TrPu), which is capped by dacites of the Powder River Volcanic Group (Td) on the east side of the river. (1.5 mi)
01.5 The river flows through Clover Creek Greenstone that is overlain by olivine basalt flows of the Powder River Volcanic Group (Tob) on both sides of the river. Dacites cap the olivine basalt sequence on the east side of the river. (0.5 mi)
02.0 Leave Union County and enter Baker County. (0.7)
02.7 The river flows southeast through large landslides (Qls) on both sides of the river. Olivine basalt flows (Tob) form the cliffs on both sides of the river and are offset by NW-SE trending faults on the east side of the river. The straight section of the river between MP 4.8-5.3 may be the result of the river incising along a fracture or fault zone. (2.5 mi)
05.2 Magpie Creek. (0.1 mi)
05.3 The river flows south then east through cliffs of olivine basalt flows (Tob), which exhibit prominent crenulations due to weathering along NW-SE and SW-NE trending joint systems. The river crosses faults at MP 5.4, 5.7, and 6.1. (0.9 mi)
06.2 The Powder River enters an uplifted fault block and cuts through Clover Creek Greenstone (TrPu), which is capped by olivine basalt (Tob) on both sides of the river. There is a large landslide complex on the south side of the river between MP 6.2-6.9. The river flows through an entrenched (incised) meander between MP 6.9-7.6. (1.8 mi)
08.0 The south-facing slopes in this area display a peculiar convex form. The slopes cut into the Permian-Triassic greenstones in the river are steep and grow gentler toward the base of the overlying olivine basalts. Gilluly (1937) noticed this odd morphology and attributed it to the retreat of rim rock as the tuffs underlying the basalts weathered and eroded. A line of springs at the base of the basalt is a likely cause for this rapid erosion and retreat. There are several gold prospect pits in this area. (0.5)
08.5 The river turns south and flows through an incised double meander between MP 8.7-9.2. (0.9 mi)
09.4 Big Creek and diversion dam. From this point, the river flows south through olivine basalts (Tob). Tertiary sediments and tuffs (Tst) are exposed just over the ridge to the east, where they are in fault contact with olivine basalts exposed further to the east. The river crosses a fault at 11 miles and then flows in an eastward direction. (2.3 mi)
11.7 The river is bordered by outcrops of Tertiary sediments and tuffs (Tst) that are capped by olivine basalts (Tob) on both sides of the river. (0.3 mi)
12.0 The river enters the northwest end of the Lower Powder Valley, flowing through Quaternary alluvium (Qal) bordered by Tertiary sediments and tuffs (Tst), olivine basalt (Tob), and Clover Creek Greenstone (TrPu) on the north and olivine basalt (Tob) on the south. (0.2 mi)
12.2 Diversion dam. (0.2 mi)
12.4 Bridge across Highway 203.
End of River Log
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