Idaho Museum of Mining Field Trip – August 4, 2018

A preliminary Field Guide to the Geology along Highway 26 between Vale and John Day, OregonThere are no really good modern geologic maps covering the area between Vale and John Day. This field trip guide is a feeble attempt to bring together what has been learned about this region over the past 20 or 30 years. Mileage points are approximate. Highlights of the trip will be a look at a major ashflow tuff typical of the large rhyolite welded tuffs of the Yellowstone hotspot track and a look at the oldest basement rocks in the accreted terranes of northeast Oregon.

We begin in the southern part of the Miocene La Grande – Owyhee eruptive axis of Ferns and McClaughry (2013) and angle northwest, crossing out of both the eruptive axis and a major pre-Tertiary terrane boundary into a region underlain by older volcanic and plutonic rocks that were emplaced some 40 million years ago. This region has seen repeated episodes of mineralization associated with emplacement of a variety of volcanic rocks.

Field trip begins in Vale, Oregon. Vale Buttes immediately south of the town are erosional resistant arkosic and tuffaceous sandstones and shales that are part of the late Miocene – early Pliocene Idaho Group. The resistant sandstones are paleo aquifers that were silicified by hot spring fluids. The sandstones are equivalent in age to the Glenns Ferry Formation. Active hot springs now vent along the Malheur River along the northeast edge of the buttes.

Start in Vale at intersection of Hwy 26 and Hwy 20. Turn right onto Hwy 26 and proceed north toward John Day.

MP

1.3 Highway proceeds up Willow Creek . Thin terrace gravels on west (left)side of highway rest on eroded Idaho Group sediments. Willow Creek follows a wide, northwest-trending valley that is eroded into the Idaho Group. Geophysical data suggests that the valley is underlain by dense material, possibly mafic sills.

6.4 Low hills to right are part of the Idaho Group. Dark ledge former capping the ridge is a younger basalt flow that is intercalated with the Idaho Group. Mineralized hot springs sinter crops out at the base of the ridge, at Alkali Springs.

10.5 Sloping hill on the skyline to the left at 9:00 is Hope Butte. Hope Butte is held up by silicified sandstones and siltstones that contain anomalous amounts of mercury and gold. Hope Butte has been extensively drilled and is cut by chalcedonic quartz veins that mark paleo hot springs conduits. Long ridge to the north of Hope Butte is Cottonwood Mountain. Cottonwood is an uplifted fault block of older middle Miocene volcanic rocks that locally overlie pre-Tertiary rocks. Part of Cottonwood Mountain is capped by the rhyolite of Cottongwood, a very large rhyolite lava flows whose eruption marked the onset of large volume rhyolite volcanism in this area (Stage 3 of Ferns and McClaughry, 2013). The rhyolite overlies a sequence of Columbia River Basalt lava flows (Stage 1 of Ferns and McClaughry, 2013) that are interbedded with palognitc sediments and airfall silicic tuffs. Rounded shape to the slopes below the rhyolite do not resemble the typical ledge formers of Columbia River Basalt flows elsewhere in this region. The rounded topography makes the interpretation of air photos very challenging and require traverses to each and every little knob.

11.2 Old townsite of Willow Creek. Pointed hill north of Hope Butte is Sugarloaf, which has been interpreted by some as a younger mineralized rhyolite plug that was intruded along a fault system that extends from Neal Hot Springs to Hope Butte. Both the rhyolite and enclosing sandstones are silicified.

16.5 Dark ledge formers to the north on the right side of the highway are late Pliocene – Pleistocene lava flows that erupted following the high stand of old Lake Idaho. These comparatively young lavas erupted from small vents and are part of the Keeney sequence of Lees (1993) and the Stage 6 volcanics of Ferns and McClaughry (2013). The vent for this particular flow is marked by a small rounded hill that is a partially eroded cinder cone. Stage 6 lavas are usually alkalic and small in size, seldom flowing more than 2 miles from their vents. The younger lavas are alkalic and include a variety of lavas that erupted from different vents. Some of the Keeney sequence lavas have abundant olivine.

17.0 Old town of Jamison A series of faults run between Jamison and Cottonwood Mountain. One youthful Quaternary fault cuts alluvial fan and terrace deposits,

19.2 Lake Idaho lake sediments are exposed along the highway and in distance beneath the late Pliocene – Pleistocene lava flows.

22. Old town of Brogan

24.7 Outcrop former in the lowlands to the left is the Dinner Creek Ashflow Tuff which here rests on Columbia River Basalt lavas and related sediments. Top of the Columbia River Basalt (CRB) section here is marked by lavas that strongly resemble the Grand Ronde Basalt. Upper flows contain abundant iron and resemble the icelandites of (where else?) Iceland. The oldest of the Dinner Creek Ashflow Tuff units (DCT 1) is sandwiched between icelandite flows and has radiometric zircon ages of ~16.1 Ma for DCT (Streck and others, 2015; Ferns and others, 2017).Streck and others (2015) identify 4 iseparate and distinct rhyolite ashflow tuffs within what has been mapped as the Dinner Creek Ashflow Tuff. Oldest ashflow (DCT 1) erupted from a large caldera near Castle Rock. A second ashflow (DCT2) erupted from a smaller caldera centered at Ironside Mountain (Cruz, 2017). DCT3 is the smallest of the ashflows and appears to be largely confined to the area near Harper. DCT4 is the youngest of the ashflows and has been found over wide areas extending north of John Day and east of Burns.

26.6 Bald hills on both sides of highway are underlain by mafic lava flows and palagonitic sediments. All part of the CRB sequence

26.9 Mafic lava flows exposed in the roadcuts along the highway.

27.3 Non welded ashflow tuff exposed along highway here is the older Dinner Creek Ashflow Tuff #1

27.5 Highway road cuts expose Jurassic sandstones and siltstones of the Weatherby Formation. These sediments are part of the Olds Ferry-Izee terrane, which is made up of lavas and related sedimentary rocks that were once part of an ancient island arc fringing the western coastline of the North American Craton. 181 – 168 Ma zircon ages reported by Ware (2013) from similar sandstones west of here indicate that the sediments may be as young as middle Jurassic.

MP 27.6

STOP 1 Opportunity to look at a mafic dike cutting the older Weatherby Formation sandstones. Age of this dike is not known. We will try and determine whether it is a Columbia River Basalt dike or a younger feeder dike to the Keeney sequence. Presence of olivine crystals will be a clue.

29.4 Road cuts of Columbia River Basalt flows. Flows show evidence for interaction with water, including possible pillow structures. According to Lees, the CRB section includes an upper section resembling the Grand Ronde Basalt, a central section resembling the Imnaha Basalt, and a lower section resembling the Steens Basalt. Flows here form very poor outcrops, possibly due to extensive interaction with water during their emplacement.

30.5 Pleistocene lava flows the Stage 6 Keeney sequence of Lees (1994)cap the ridge to right. Lees (1994)reports a radiometric age of 1.7 Ma for this flow.

31.1 Brogan Hill summit. Road begins descent into Cow Valley. Cow Valley is a gravel plain underlain by mixed sediments and volcanics. The butte in the foreground to the right is Cow Valley Butte. The southeast end of Cow Valley Butte is a late Jurassic – early Jurassic composite intrusion that intrudes Weatherby sediments and is locally cut by antimony- and gold-bearing quartz veins. High mountains to the left of the highway include Cottonwood Mountain, Juniper Mountain and Ironside Mountain. Cottonwood Mountain and Juniper Mountain are capped by a sequence of CRB lavas and underlain by Mesozoic Weatherby sediments that locally overlay late Triassic volcanic and intrusive rocks that form the western most known exposures of the Huntington Formation.

33.0 Juniper Mountain to left.

35.0 Cow Valley Buttes to right.

39.6 Bold outcrop in the foreground to the right is an andesite flow that overlies Weatherby Formation sediments. The andesite is an erosional remnant of series of older late Eocene and Oligocene lavas and volcanic sediments that separate pre-Tertiary basement rocks from the Columbia River Basalt Group. At this point, not much is known about these rocks.

41.0 Small rounded hill on skyline to right is Baldy Mountain, a small late Pliocene – Pleistocene shield volcano composed of trachy basaltic andesite and trachyandesite lavas.

45.8 Town of Ironside. Lowland to south are composed of Dinner Creek Ashflow Tuff #1 and overlying Hunter Creek Basalt lavas. The Hunter Creek Basalt consists of a series of icelandite lavas that strongly resemble the Grand Ronde Basalt. Webb (2017) found Hunter Creek Basalt vent deposits that demonstrate simultaneous mafic (Columbia River Basalt) and large volume Yellowstone-like silicic volcanism at ~ 16.1 Ma.

48.0 View of Ironside Mountain in foreground. Ridge to north extends to Murray Hill. Ironside Mountain is believed to be a small caldera from which the DCT 2 ashflow erupted. Highlands extending north of Ironside Mountain are part of a large late Eocene-early Oligocene porphyry intrusive complex into Mesozoic Olds Ferry-Izee Terrane sediments. The subvolcanic intrusives are part of a late Eocene-early Olgiocene volcanic field that extends westward into the Clarno area of central Oregon.

52.1 Coarse gravels exposed along road to right. Bold outcrops to the left are a rheomorphic ash tuff that has been correlated with the ashflow that erupted from the Ironside Mountain caldera.

MP 52.2STOP 2 The poorly consolidated gravel deposit here may be a gravel that was deposited during the early development of the Western Snake River plain. Similar gravels west of here are interbedded with the late Miocene Rattlesnake Ashflow Tuff.

53.3 Bedded tuffaceous sediments are exposed beneath coarse gravels at the summit of Railroad Ridge grade. The famous Eldorado Ditch crosses the highway here.

54.6 Turn left, west onto Camp Creek Road Proceed west to STOP 3.

MP 64STOP 3 Thick outcrops of the Dinner Creek Tuff, unit 2 are exposed along the roade here. These are the best exposures of unit 2, which is believed to have erupted from a caldera at Ironside MountainReturn to Highway 26 The ashflow contains both black and white pumice and erupted about. ~ 15.5 Ma (Ferns and others, 2017). Fallout deposits from the eruption have been found north of the town of Elgin, in northeast Oregon. DCT 2 is a rhyolite ash-flow tuff believed to have been formed by partial melting of altered crust. Rhyolites are the most silica-rich volcanic rocks, typically containing as much as 78 wt per cent silica (Sio2)

74 Turn left back onto Highway 26.

75.6 Dinner Creek Ashflow Tuff overlies a series of andesite lavas along the highway.

77.6 Sequence of older volcanics comes into view beneath andesite lavas to the right. Area has not been mapped in detail. Older volcanics appear to be slightly older than earliest known CRB flows. This sequence of volcanics has been traced north to a vent complex at Unity Reservoir

79.4 Highway 26 crosses into Unity valley. The valley is underlain by tuffaceous sediments that are capped by coarse gravels similar to those exposed at Railroad Ridge.

83.1 Town of Unity. Bull Run Mountain and Mine Ridge lie to south. Dark area on east side of Bullrun Mountain is a slab of peridotite intruded by late Eocene porphyry intrusions. The porphyry system here is associated with gold mineralization. Most unusual gold deposit is the Record Mine where narrow pyroxenite veins in the porphyry contain magnetite, gold and molybdenite. The high grade gold pockets contain no quartz.

85.4 Turn right onto Highway 245.

87.8 Turn left into Unity Reservoir State Park

MP 88 STOP 4 Unity Reservoir State Park. Ridge on the east side of the reservoir is an uplifted fault block made up of rhyolite and dacite domes, intrusions, debris flows and ashflow tuff. Streck reports radiometric ages of 18 = 19 Ma for this volcanic complex. Researchers are currently exploring the idea that the Unity

Reservoir volcanic complex was a small-scale precursor to the later voluminous eruptions of the Columbia River Basalt. One idea is that these smaller andesite, dacite and rhyolite eruptive centers record early partial melting of accreted terrane crust generated by the emerging Yellowstone plume/hotspot.

88.2 Turn left onto Highway 245. Ironside Mountain – Bullrun Mountain and Monument Rock are visible to the to south. Monument Rock lies west of Monument Rock and is a middle – late Miocene dacite plug that marks a vent in the Strawberry Volcanic Field. The Strawberry Volcanic Field consists of a series of small calc-alkaline volcanoes that formed following the main phase of Columbia River Basalt tholeiitic volcanism. The later Strawberry Volcanics are believed to be partial melts of underlying accreted terrane rocks (Robyn

89.2 Highway 245 crosses a major northwest-trending fault onto an upthrown block of Unity Reservoir volcanic complex rocks

90 Thick outcrop former to left is a dacite lava flow interbedded with debris flows

90.6 Outcrops of coarse block dacite debris flows with prominent balanced rocks and hoodoo landforms to left. All of the blocks appear to be the same dacite and have been interpreted to be related to dome collapses.

91.5 Turn left on to gravel road leading to Whitney. Reverse course and return to Highway 245, heading for John Day. Ledge forming outcrops across the river are part of an older ashflow tuff that is believed to have erupted from the Unity Reservoir complex. 96.6 Turn right onto Highway 26 and proceed west toward John Day. View at intersection of Bullrun Mountain, Mine Ridge and Monument Rock to the south. Bullrun Mountain and Mine Ridge are pre-Tertiary peridotite and mélange rocks. Monument Rock is a younger Strawberry Volcanics dacite vent complex that formed about 14 Ma.

98 Outcrops of Dinner Creek Tuff to left. Not sure whether this is DCT 1 or DCT 2.

99.4 Younger Strawberry Volcanics flows on both side of highway. Flows overlie the DCT.

101. Bare ridge to right in foregrounds is a sequence of older late Eocene-Oligocene debris flows and lavas. Highway outcrops are younger middle Miocene Strawberry Volcanics.

103.3 Andesite outcrops along highway are Strawberry Volcanics.

105 Summit of Blue Mountain Pass

107.1 Dixie Butte comes into view. Dixie Butte is part of a pre-Tertiary complex composed of island-arc volcanic and subvolcanic rocks.

108.5 Greenhorn Mountains come into view to the right of Dixie Butte. The Greenhorns are older mélange rocks that have been intruded by a late Jurassic pluton. Both Dixie Butte and the Greenhorns are part of the Greenhorn subterrane of Ferns and Brooks (.

110 Highway outcrops have been mapped as part of the Strawberry volcanics. Basalt flows here overlie a middle to late Miocene sequence of diatomaceous sediments that in turn overlie the main sequence of Strawberry Volcanics. There is a possibility that these basalts are younger late Miocene/early Pleistocene flows that are considerably younger than the rest of the Strawberry Volcanics.

112.6 Austin Junction

115,2 Unconsolidated white ashy deposits along the highway are part of the airfall ash deposited by the eruption of Mt Mazama about 7,000 years ago, when Crater Lake formed.

117.1 Melange outcrops along highway to right.

118.6 Dixie Mountain pass. Columnar jointed basaltic andesite flows exposed along the highway to the right are part of the 40 Ma

120.4 Late Eocene – early Oligocene volcanic complex. Mix of dacite, dacite breccia cut by mafic dikes.

MP 120.9 STOP 5 Late Eocene – early Oligocene vent comple. Core of the complex is a dacite dome that grades into a dacite carapace breccia that is flanked by debris flows. The vent complex is cut by a series of mafic dikes of unknown age. The vent complex is considered to be part of the widespread Clarno Formation of central and eastern Oregon.

MP 121.5 STOP 6 John Day Overlook Strawberry Mountains to the south. Major lae middle Miocene composite volcano. Ridge to west to Canyon Mountain formed by Canyon Mountain Complex. Apparent "fans" are eroded alluvial gravel plain deposits which are interbedded with the ~7.4 Ma Rattlesnake Creek Ash-flow Tuff. The gravels may be correlative with the gravels at Railroad Pass.

123.3 Tilted section of olivine basalt flows over Dinner Creek Tuff. Tuff here is DCT 1

125.2 Prairie City

126.7 Coarse gravel deposits along the highway to the right are part of the gravel sheet that extends to the base of the Strawberry Volcanics.

127.7 Olivine basalt flows overlie the Dinner Creek Ashflow Tuff

130 Outcrops of Dinner Creek Ashflow Tuff (DCT 2) are exposed along the John Day River.

132 Dinner Creek Ashflow Tuff and overlying olivine basalt flows to right

132.4 Turn left onto Pine Creek Road

135 Thick exposures of Columbia River Basalt in rock pit to right

135.6 Turn left onto Baldy Road.

136 Cross fault onto serpentinite and serpentinized harzburgite at base of Canyon Mountain Complex (CMC)

136.6 Serpentinized harzburgite

MP 137STOP 7 Base of the Canyon Mountain Complex, the largest peridotite mass in northeast Oregon. The complex is an "ophiolite" that is defined by a lower ultramafic sequence of harzburgite, dunite and pyroxenite. Top of the complex is a gabbroic mass cut by dike complexes. The gabbroic and dike rocks in the upper part of the complex are considered to be partial melts that were derived from partial melting of the lower ultramafic sequence. The Canyon Mountain Complex is part of the upper mantle that has buoyantly risen through rift zones through the overlying crust. As the peridotite rises, partial melts are extracted from a residue that becomes more mafic forming the harzburgite such as exposed at the base of Canyon Mountain Complex. Harzburgites contain higher melting temperature minerals such as olivine, enstatite (orthophyroxene) and spinel and are among the most silica-poor igneous rocks, typically with less than 40 wt % SiO2). Harzburgites may contain pods of olivine (dunite) chromite (chrome-rich spinel) and pyroxenite (enstatite and diopside) that form during the partial melting process. Chromite pods such as those at Canyon Mountain are mined for chrome.

140.8 End of road at Baldy Creek trailhead. Restroom facility and overview of John Day ValleyReturn to Highway 26

149.0 The bluff to the north, across the John Day River is capped by a series of olivine basalt flows that overlie a massive sequence of Dinner Creek Ashflow Tuff unit 2.

149.1 Turn left onto Highway 26 and proceed toward John Day.

151.5 Sequence of basalt flows on north side of John Day River with intervening ledge-forming Dinner Creek Ash Flow Tuff (DCT1?). All of the basalts here have been mapped as Picture Gorge Basalt, one of the main units in the Columbia River Basalt. Further work is needed in this area.

153.8 Entering town of John Day. Ledge former in distance to west above the town is the Rattlesnake Ashflow Tuff which here rests on poorly consolidated gravels and sand.