THE GEOLOGY OF THE LOWER
HALF OF THE
Ben Zublin
Science
Department,
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
Figure 1. Satellite image of the
This study
focuses on a 19 km (12 mi) long stretch of the
Rock Units
The
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
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
Scattered
deposits of moderately weathered tuffs mantle the lava flows in the study area
and occur as interbeds in places within the
Structural Geology and Geomorphology
The
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
Figure 3b. Geology of the
Figure 3c. Geology of the
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
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
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
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
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
Figure 5. View southeast down the southern flank of the
landslide on the south side of the
The pair of landslides in sections
11, 12, and 13 of T7S, R40E both appear to be related to the down cutting of
the
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
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
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
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
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
Assuming a rate of uplift i 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
Acknowledgments
I am
grateful to Mark Ferns of the Oregon Department of Geology and Mineral
Industries in
References Cited
Bailey, D.G., 1990, Geochemistry and petrogenesis of Miocene
volcanic rocks in the Powder River volcanic field, northeastern
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,
Gilluly,
J., 1937, Geology and mineral resources of the Baker Quadrangle,
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
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
POWDER RIVER LOG:
THIEF VALLEY DAM TO
Miles
00.0 Start at
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
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
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