Field trip guide to the middle Eocene Wildcat Mountain Caldera, Ochoco National Forest, Crook County, Oregon by Jason D. McClaughry1, Caroline L. Gordon2, and Mark L. Ferns1 1Baker City Field Office, Oregon Department of Geology and Mineral Industries, Baker County Courthouse, 1995 3rd Street, Baker City, Oregon 97814 2Ochoco National Forest, 3160 NE 3rd St., Prineville, Oregon 97754 Overview: This field trip examines the stratigraphy of the middle Eocene Wildcat Mountain caldera exposed east of Prineville, in the Ochoco Mountains of north-central Oregon. Geologic factors that control landslide deposits and mineralization in the east part of the Lower Crooked Basin are discussed. This field trip is 152.7 km (94.9 mi). INTRODUCTION 124o 121o 118o KBML The Wildcat Mountain caldera, in the eastern part of the Lower Portland Crooked Basin, is a volcanic vent complex that collapsed and DCP filled with more than 90 km3 (21.5 mi3) of rhyolitic ash-flow BM 45o WV tuff during the middle Eocene. It is one of only a few caldera 3. Dale Baker City sources for Paleogene ash-flow tuff sheets identified in Oregon and is the first recognized to be Eocene (Figure 1) (Hladky and SFZ 1. 2. CR Unity Wiley, 1993; Hladky, 1996; Ferns and others, 2001; McClaughry Eugene Cougar and Ferns, 2007). Until recently, the lack of Paleogene calderas Prineville Rock WC HC in Oregon was a noteworthy anomaly considering that nu- HLP merous Paleogene calderas have been mapped elsewhere in BFZ OU the western United States. Several Paleogene calderas have 4. BR been identified in adjacent Idaho (McIntyre and others, 1982; KM KBML Leonard and Marvin, 1982; Moye and others, 1988), and many Hart Mountain more have been identified farther south in volcanic fields of the 42o 5. Great Basin (Steven and Lipman, 1976; Ludington and others, 1996). The Wildcat Mountain caldera is part of a broad sweep of voluminous ash-flow tuff magmatism, between ca. 41 and 25 Ma, preserved in volcanic and intrusive rocks distributed Mutton 3. Mountains across the axis of the Blue Mountains in central and eastern Or- Clarno EASTERN Dale egon (Walker and Robinson, 1990; Robinson and others, 1990), WESTERN Ashwood FACIES FACIES Painted Sheep the West Cascades in southwest Oregon (Retallack and others, Hills Rock Madras 2004), at Hart Mountain in south-central Oregon (Mathis, 1993) 2. (Figure 1), and near Potlatch in northern Idaho (Kauffman and Prineville 50 km others, 2003). Regionally, these igneous rocks may represent a 1. northward extension of the contemporaneous (ca. 43 and 23 SOUTHERN 50 mi Ma) “ignimbrite flare-up” in the Great Basin (Stewart and Carl- FACIES son, 1976; Best and others, 1989; Christiansen and Yeats, 1992; Ludington and others, 1996; Honn and Smith, 2007). In central and eastern Oregon this magmatic episode is recorded in the Figure 1. (top) Location of known Paleogene calderas in Oregon. Red-filled polygons are calderas: 1, Crooked River caldera; 2, Wildcat Eocene Clarno and middle Eocene to early Miocene John Day Mountain caldera; 3, Tower Mountain caldera; 4 and 5, unnamed, Formations, which cover more than 30,000 km2 (11,600 mi2) suggested calderas. Locations of known Paleogene volcanic centers (Figure 1; Swanson, 1969; Robinson, 1975; Robinson and others, at Cougar Rock and Hart Mountain are shown for reference. Orange 1984, 1990; Smith and others, 1998). shade represents approximate distribution of ash-flow tuffs in the Rocks of the middle Eocene Wildcat Mountain caldera record late Eocene to Oligocene John Day Formation. Brown polygons show the distribution of the Eocene Clarno Formation. Dashed black line the onset of widespread Paleogene ash-flow tuff eruptions in labeled KBML is the inferred trace of the Klamath-Blue Mountain north-central Oregon and overlap the regional transition from gravity-anomaly lineament. SFZ, Sisters Fault Zone; BFZ, Brothers dominantly calc-alkaline magmatism that characterizes the Fault Zone. Solid black lines demarcate physiographic provinces (after Clarno Formation to the bimodal basalt and rhyolite volcanic Walker, 1977): WV, Willamette Valley; CR, Coast Range; KM, Klamath assemblage associated with the John Day Formation. Investi- Mountains; WC, West Cascades; HC, High Cascades; DCP, Deschutes gators in central and eastern Oregon have long speculated on Columbia Plateau; BM, Blue Mountains; HLP, High Lava Plains; BR, Basin and Range; OU, Owyhee Uplands. (bottom) Enlarged central the sources of widespread tuffs that characterize the John Day part of area shown in top figure with the distribution of the “western,” Formation (Robinson and others, 1984, 1990), but only recent “southern,” and “eastern” facies after Robinson and others (1990) geologic mapping has revealed the locations of corresponding within the John Day Formation. Some geographic points are shown for reference. OREGON GEOLOGY, VOLUME 69, NUMBER 1, FALL 2009 5 vents. Regional workers have suggested that pyroclastic rocks (Coleman, 1949; Robinson and others, 1984, 1990; White and within the John Day Formation were vented from now-buried Robinson, 1992; Bestland and others, 1999). The location of the eruptive centers in or marginal to a nascent Cascade Range. Wildcat Mountain caldera well east of the previously postulated These presumed buried sources were used to build a tectonic source area documents a much more regionally extensive mag- model in which John Day volcanism documented a westward matic episode not related to a largely inferred ancestral Cascade jump of a subduction zone at the end of “Clarno arc” volcanism Range. 120.50O Tcau Rooster Tcau Rock Tcvs USFS Rd. 2730 Tjts Lucky Strike McKay Mine Saddle Tr. #832 USFS Rd. Trail 200 Tcal Ochoco Divide USFS Rd. Meadows Campground USFS Rd. 27 27 O Little McKay Creek Tcrh USFS Rd. Whistler Springs 2730 Tcrh 44.50 O 6 44.50 Hash Rock Tcdp 7 Twin Pillars Tcdp McKay Creek 26 Harvey Tcba Gap Tchp Qls USFS Rd. W. Fk Mill Creek Marks Creek Tcrh 4 33 Forked Horn Butte Tr. # 833a Tchp Tcag E. Fk Mill Creek Tr. # 832 Tjts Tr. # 833 Wildcat Tcib 5 Mountain Tcid Kidnap White Rock Springs USFS Rd. USFS Rd. 33 x 1 Tcrb 2 Springs 3350 Field trip stop and route Qls Tcrh Tr. #832 USFS trail Steins Pillar Boundary of Mill Creek Tcts White Fir Springs Tr. # 837 Wilderness Brennan Tcrb Palisades Caldera margin - dashed Tcba where inferred Rocky 3 Butte Trailhead Campground Prospect pit / collection site Tcal Mill Creek Mahogany Tcdp Butte Wildcat Mountain Tcid USFS Rd. 33 caldera 1 Wildcat 26 OREGON N Mountain 0 2.5 5 km Tcrh Ochoco Creek 0 2.5 5 mi caldera 120.50O Surficial deposits (Quaternary) Wildcat Mountain caldera (Eocene) Qa Alluvium Tcba Basaltic andesite lavas and intrusions Tcts Tuff of Steins Pillar (intracaldera facies) Qls Landslide deposits Tcag Dacite lavas and intrusions Tcau Andesite lavas Crooked River caldera (Oligocene) Tcib Andesite and rhyolite breccia dikes Tcid Andesite and dacite intrusions Tcal Andesite and dacite lavas Tjts Tuff of Smith Rock (outflow facies) Tchp Dacite intrusions Tcrb Rhyolite intrusions Tcvs Volcaniclastic sedimentary rocks and tuff Tcrh Ring-fracture rhyolite domes and lavas Tcdp Porphyritic andesite and dacite Figure 2. Geologic map of the Wildcat Mountain caldera and vicinity labeled with field trip route and associated stops 1 through 7. 6 OREGON GEOLOGY, VOLUME 69, NUMBER 1, FALL 2009 of both the Clarno and John Day Formations, lithologic distribu- tion, geochemistry, geochronology, and structure now indicate Mitchell Fault Wildcat these rocks form a single volcanic field. The original extent of Metolius River Mountain KBML o the Ochoco volcanic field is currently unknown, but it may be Green Ridge caldera 44.5 CSFZ preserved from the Maury Mountains on the south to the Horse Heaven mining district on the north, forming a belt of coinci- Squaw Creek Big Summit dent Eocene vents and quicksilver deposits (Figure 3; Waters Prairie Crooked and others, 1951; Brooks, 1963; Swanson, 1969). The Ochoco River caldera volcanic field is underlain by variably deformed Cretaceous and PRFZ older (?) sedimentary rocks exposed between Ochoco Divide SFZ Crooked River and Mitchell. It is flanked on the west, north, and east by Eocene Powell to Oligocene depocenters that are exposed in an arcuate belt Buttes between Gray Butte, Clarno, and the Painted Hills. The depocen- 44.0o ters preserve a regionally segmented and stratigraphically dis- continuous volcanic and sedimentary succession of the Clarno and John Day Formations (Walker, 1990; Robinson and others, Horse BFZ 1990; Smith and others, 1998; Retallack and others, 2000; Best- Butte Deschutes River land and others, 1999). The Horse Heaven mining district, char- acterized by a thick section of andesite, tuff, and a 42.1 ± 0.8 Ma Newberry Volcano mineralized (cinnabar) rhyolite flow and dome complex lies be- 0 10 20N km tween Clarno and Wildcat Mountain (Waters and others, 1951; 121.5o 121.0o 120.5o Swanson and Robinson, 1968; Fiebelkorn and others, 1982). The Oligocene Crooked River caldera forms an embayment along Figure 3. Fault lineament map of central Oregon with caldera structures the southwest part of the Ochoco Mountains, partially overlap- delineated in the Lower Crooked Basin. The Wildcat Mountain caldera ping the Ochoco volcanic field (Figure 3). is coincident with a relatively stable tectonic area near the junction of the northwest trending Brothers fault zone (BFZ) and the north- northwest trending Sisters fault zone (SFZ). The northern edge of this Regional Paleogene stratigraphy area is coincident with the Klamath-Blue Mountain gravity-anomaly lineament (KBML) (Riddihough and others, 1986). Deformed Paleozoic to Mesozoic accreted terranes and Cre- taceous marine rocks form the core of the Blue Mountains of central and eastern Oregon which is blanketed by a discontinu- Location and geologic setting ous succession of Paleogene volcanic and volcanogenic sedi- mentary rocks regionally referred to as the Clarno and John Day The Wildcat Mountain caldera is a ~16 km × ~11 km (10 mi × Formations (Figure 4).