OBJECTID|MapUnit|Name|FullName|Age|Description|HierarchyKey|ParagraphStyle|Label|Symbol|AreaFillRGB|AreaFillPatternDescription|DescriptionSourceID|GeoMaterial|GeoMaterialConfidence|DescriptionOfMapUnits_ID
|1||Upper Cenozoic surficial deposits||||01|DGMI H2|||||McClJD2024|||DMU01
2|Qf|modern fill and construction material|modern fill and construction material|upper Holocene|Artificial or constructed fill deposits of poorly sorted and crudely layered mixed gravel, sand, clay, and other engineered fill (Plates 1, 2, and 3). These deposits usually contain rounded to angular clasts ranging from small pebbles up to several meters across. The orientation of clasts is typically less uniform than is found in naturally occurring imbricated or bedding-parallel gravel. Older fills are likely uncompacted versus newer fills, which are likely engineered and reinforced using geotextiles or retaining structures. Deposits mapped as modern fill and construction material in the map area are generally associated with road embankments, causeways, culvert fills, and mined land (Plates 1, 2, and 3). The thickness of fill deposits may exceed 30 m (98 ft). Qf is assigned a late Holocene age (Figure 6 1).|01-01|DGMI Unit Label|Qf|Qf|255 254 254||McClJD2024|"Made" or human-engineered land|High|DMU02
3|Qa|alluvium|alluvium|Holocene and Upper Pleistocene|Unconsolidated, poorly to moderately stratified gravel and sand along streams (Plates 1, 2, and 3). Thickness of Qa is generally ≤5 to 7 m (16 to 23 ft); bedrock units may be locally exposed in the base of stream channels within areas mapped as Qa. Qa is assigned a Late Pleistocene and Holocene age on the basis of stratigraphic position and a lack of more precise age indications (Figure 6 1). Areas mapped as Qa are known to have been inundated by record floods during 1861, 1964, 1974, 1996, and 2006 (Plates 1, 2, and 3). They may include deposits containing human-made debris or artifacts, or deposits filling areas known to have been modified by humans such as excavations, roadways, or gravel pits.|01-02|DGMI Unit Label|Qa|Qa|235 214 184||McClJD2024|Alluvial sediment|High|DMU03
4|Qaf|fan deposits|fan deposits|Holocene and Upper Pleistocene|Unconsolidated, poorly sorted deposits of boulders, cobbles, pebbles, granules, sand, silt, and clay in upland drainages and as fan-shaped accumulations at the transition between low-gradient valley floodplains and steeper uplands (Plates 1, 2, and 3). Qaf may locally include debris flow deposits and talus from rockfall in steep drainages. Individual fans (generally cover ≤4 hectares (10 acres). The local thickness of alluvial fan deposits is variable but is probably ≤15 m (50 ft). Qaf includes fan deposits of Late Pleistocene and Holocene age (Figure 6 1).|01-03|DGMI Unit Label|Qaf|Qaf|194 156 113||McClJD2024|Alluvial sediment|High|DMU04
5|Qls|landslide deposits|landslide deposits|Holocene and Upper Pleistocene|Unconsolidated masses of rock and soil deposited by landslides (e.g., slumps, slides, debris flows, rock avalanches; Plates 1, 2, and 3). Qls in the map area typically consist of individual slide masses covering ≤45 hectares (111 acres) and more typically ≤6 hectares (≤15 acres). A majority of mapped Qls are simple slump or shallowly-seated earthflow-type features that occur along major drainages associated with Pliocene and younger intracanyon lava flows inset into sparsely vegetated, moderately to steep slopes underlain by weakly consolidated rocks of the Dalles Formation (Plates 1, 2, and 3). Qls locally includes rockfalls, large talus piles, shallow-seated landslides of colluvium, debris flow deposits, and more deeply-seated bedrock slides. Thickness of Qls is highly varied, but may be more than several tens of meters in larger deposits. Qls range in age from Late Pleistocene, relatively stable features, to those that have been recurrently active in relatively recent time (Figure 6 1). Qls are typically referred to as clay, boulders, rock, or rock and clay in water-well logs.|01-04|DGMI Unit Label|Qls|Qls|249 241 232||McClJD2024|Debris flows, landslides, and other localized mass-movement sediment|High|DMU05
6|Qc|colluvium|colluvium|Holocene and Upper Pleistocene|Unconsolidated poorly sorted deposits of rock and soil deposited along valley walls and as fans and aprons of debris at the foot of steep slopes (Plates 1, 2, and 3). Qc locally includes now completely vegetated talus deposits, debris flow deposits, and coarse-grained alluvium. Thickness of Qc is highly varied; maximum thickness is several meters. Qc is assigned a Late Pleistocene and Holocene age on the basis of stratigraphic position (Figure 6 1).|01-05|DGMI Unit Label|Qc|Qc|200 172 151||McClJD2024|Colluvium and other widespread mass-movement sediment|High|DMU06
7|Qmfs|Missoula flood deposits|Missoula flood deposits|Upper Pleistocene|Medium gray (N5) to grayish yellow (5Y 8/4), horizontally bedded, massive to interstratified, fine-grained sand, silt, and clay mapped unconformably over the Dalles Formation (Tmdl) along the valleys of Brown Creek, Chenoweth Creek, and Mill Creek (Figure 6 2a,b; Plate 3). Qmfs have a Columbia Basin provenance and are composed primarily of quartz, feldspar, ferromagnesian minerals, and mica, with varying amounts of clay. Deposits locally en¬close isolated pebbles, cobbles, and boulders of vol¬canic, granitic, and metamorphic lithologies exotic to the Mill Creek area that were dropped by melting icebergs as floodwaters receded (Allison, 1935; New¬comb, 1969; Gannett and Caldwell, 1998; O’Connor and others, 2001; Benito and O’Connor, 2003). Qmfs are as thick as 55 m (180 ft) in the map area. The maximum elevation of Qmfs is ~260 m (850 ft) along Brown Creek, 213 m (700 ft) along Mill Creek, and 183 m (600 ft) along Chenoweth Creek. Qmfs has been ex¬tensively eroded by receding floodwaters and inci¬sion of modern streams (Plate 3). Land associated with Qmfs is used for agriculture in the form of dryland wheat, vineyards, and orchards (Figure 6 2c). Qmfs are assigned a Late Pleistocene age on the basis of stratigraphic position and isotopic ages. Geochronology for Missoula Flood deposits established through radiocarbon dating, tephrochronology, and stratigraphic relations indicates passage of multiple cataclysmic floods through the Columbia River Gorge between ~20 ka and 14 ka (O’Connor and others, 2020).|01-06|DGMI Unit Label|Qmfs|Qmfs|155 122 96||McClJD2024|Alluvial sediment|Medium|DMU07
8|Qao|older alluvium|older alluvium|Holocene and Upper Pleistocene|Unconsolidated, well to poorly sorted, stratified gravel, sand, silt, and clay mapped along Fifteenmile Creek and smaller tributary streams. Thickness of Qao is ≤6 m (20 ft). Qao is assigned a Late Pleistocene and Holocene age on the basis of stratigraphic position (Figure 6 1).|01-07|DGMI Unit Label|Qao|Qao|216 183 158||McClJD2024|Alluvial sediment|High|DMU08
9|Qlo|loess|loess|Holocene and Pleistocene|Pale yellowish brown (10YR 6/2), micaceous, quartzo-feldspathic silt and minor amounts of very fine-grained sand and clay mantling uplands in the southeast part of the Brown Creek 7.5′ quadrangle (Plate 1). Qlo typically forms a massive, featureless deposit that has been extensively modified by dryland agriculture over the past century. Where the antecedent deposit was originally thin or has been further thinned by tilling, silt may be mixed with larger rock fragments from the underlying bedrock geology. Commonly, the edges of wheatfields constructed in Qlo are characterized by steep 1- to 2-m-high (3.3 to 6.5 ft) embankments created from circular, annual tilling of large fields. Long-term residents of the area report that annual tilling in the same circular direction has led to often thicker accumulations of Qlo along the outer edges of fields, in places removing the entire thickness of the unit near the center of fields. Thickness of Qlo ranges from thin veneers <1 m (3.3 ft) to as much as 6 m (20 ft), on the basis of information reported from well logs; roadcuts may locally show up to 5 m (16.4 ft) of loess in vertical section. The distribution of Qlo, as mapped here, has been determined on the basis of a combination of field observations, soils mapping by Green (1982), geologic mapping by Waters (1968), 1-m lidar DEMs, and 2014 and 2018 National Agriculture Imagery Program orthophotos (Plate 1). The distribution portrayed generally follows divisions in the soils map of Green (1982). Areas that Green (1982) described as dominantly silt or silty loam and that are of significant thickness to conceal the underlying bedrock geology have been mapped as Qlo; areas identified as mixtures of silt and rock (e.g., cobbly loam) have been mapped as part of the underlying bedrock geology. Most of the contacts portrayed represent human-modified boundaries, such as field edges, where silt-dominated deposits lie adjacent to or surround rock areas of mixed grain size. Thin and small Qlo deposits have not been mapped where the underlying geology can be reasonably inferred or is known to be present at the surface.  The distribution of Qlo across this part of the Columbia Plateau indicates episodic deposition by southwesterly winds (McDonald and others, 2012). Qlo deposits in the eastern Middle Columbia Basin are assigned a Pleistocene and Holocene age on the basis of stratigraphic position and may be as old as ~600 ka (Figure 6 1). Medley (2012) interprets loess deposits and Missoula Flood deposits exposed along U.S. Highway 197 between Dufur and The Dalles to lie above a series of older paleosols that contain pumice correlated to the ~600 ka Dibekulewe tuff from Nevada (Cordero, 1997). Pluhar and others (2014) reported paleomagnetically reversed paleosols at the base of U.S. Highway 197 section, so loess in that area may be older than 780 ka.|01-08|DGMI Unit Label|Qlo|Qlo|218 218 218||McClJD2024|Loess|High|DMU09
10|Qgot|older till|older till|Middle Pleistocene[?]|Unconsolidated, very poorly sorted glacial till preserved above an elevation of 1402 m (4,600 ft) in the cirque basin drained by upper Fifteenmile and Fret Creeks in the northwest part of the Flag Point 7.5′ quadrangle (Plate 3). Qgot contains very poorly sorted, angular to subrounded pebbles, cobbles, and boulders in a fine-grained silty sand matrix. Many clasts are faceted into crude pentagonal shapes and some are striated. Till may locally be complexly interbedded with poorly sorted silt, sand, and gravel. Qgot lies above and can obscure older bedrock. Well-weathered till deposits may display argillic B horizons tens of centimeters thick with weathering rinds 1 mm (0.04 in) or thicker on fine-grained clasts. Glacial deposits are typically poorly exposed and may include tills of several glaciations. Age constraints are few. Older till could be as young as marine oxygen isotope stage 6 (about 150 ka) (Scott and Gardner, 2017; Figure 6 1). Qgot till was chiefly deposited in ground or lateral moraines.|01-09|DGMI Unit Label|Qgot|Qgot|000 134 173||McClJD2024|Glacial till|High|DMU10
11||Upper Cenozoic volcanic and sedimentary rocks||||02|DGMI H2|||||McClJD2024|||DMU11
12||Quaternary and upper Pliocene volcanic and sedimentary rocks of the late High Cascades||||02-01|DGMI H3|||||McClJD2024|||DMU12
13||Products of regional Quaternary volcanoes||||02-01-01|DGMI H4|||||McClJD2024|||DMU13
14||Volcanics of Fir Mountain||||02-01-01-01|DGMI H5|||||McClJD2024|||DMU14
15|Qran|andesite of Neal Creek|andesite of Neal Creek of the Volcanics of Fir Mountain|lower Pleistocene|Andesite lava flow (SiO2 = 57.42 to 59.62 weight percent; K2O = 0.89 to 1.18 weight percent; n = 12 analyses [6 outside Mill Creek area]) mapped in the headwaters of Mosier and West Fork Neal Creek (Figure 6 3; Table 6 1; Plate 2; Appendix). Qran outcrops are characterized by broad blocky columns ≤3 m (9.8 ft) across and zones of platy jointing (Figure 6 3). Thickness of Qran is as much as 60 m (200 ft) in the headwaters of West Fork Neal Creek (Parkdale 7.5′ quadrangle; McClaughry and others, 2012). Typical hand samples are medium-to-light gray (N6) and aphyric to sparsely porphyritic, containing ≤1 percent (vol.) colorless, euhedral, prismatic, seriate plagioclase phenocrysts ≤3.5 mm (0.1 in) contained within a fine-grained hypocrystalline groundmass (Figure 6 4). Groundmass olivine occasionally displays areas or rims of dark yellowish orange (10YR 6/6) iddingsite (Figure 6 4b,c). Glomerocrysts are present but are rare. Thin bands of submillimeter, irregular vesicles define diktytaxitic intervals throughout the unit. Qran has reversed magnetic polarity and is assigned an early Pleistocene age (Figure 6 1; Plate 2; Appendix). Reversed polarity indicates an age older than 0.773 Ma. Qran was erupted from a vent that formed high topography between the headwaters of West Fork Neal Creek and Mosier Creek (Plate 2). The lava flow flowed northwest ~5 km (3.1 mi) from the vent, confined within the canyon of West Fork of Neal Creek (Plate 2; McClaughry and others, 2012).|02-01-01-01-01|DGMI Unit Label|Qran|Qran|148 203 120||McClJD2024|Intermediate-composition lava flows|High|DMU15
16|Qrbs|basaltic andesite of Beaver Spring|basaltic andesite of Beaver Spring of the Volcanics of Fir Mountain|lower Pleistocene|Basaltic andesite lava flow (SiO2 = 53.82 to 54.41 weight percent; K2O = 1.04 to 1.12 weight percent; n = 5 analyses) mapped as a broad plateau across the summit of Fir Mountain in the headwaters of Mosier Creek (Figure 6 5; Table 6 1; Plate 2; Appendix; McClaughry and others, 2012). Qrbs is also mapped discontinuously as an intracanyon lava flow for 10 km (6.2 mi) from Beaver Spring east down Mosier Creek to the intersection of Honeysuckle Creek in the northeast part of the Ketchum Reservoir 7.5′ quadrangle (Plate 2). Additional distal remnants of the lava flow are also mapped east of Honeysuckle Creek and south of the Chenoweth fault in the norwest corner of the Brown Creek 7.5′ quadrangle (Plate 1). These mapped Qrbs areas east of Honeysuckle Creek (Plate 1) lie at higher elevations than comparable distal outcrops to the west (Plate 2) and span an elevation range of ~590 ft (180 m). Notable differences in base elevation of the lava flow from west to east may be explained by complex faulting occurring near the junction of the Chenoweth and Maupin fault systems. The unit is very poorly exposed across Fir Mountain in a few roadbeds; more typically the unit is exposed as moderate orange-pink (5YR 8/4) to grayish orange (10YR 7/4) soils containing scattered loose blocks, 0.3 to >1 m (1 to 3.3 ft) in diameter (Figure 6 5). Thickness of the unit ranges from ~5 m (16.4 ft) to as much as 30 m (100 ft) across Fir Mountain; cliff-forming intracanyon parts of the unit mapped along Mosier Creek are as thick as 122 m (400 ft) (Plates 2 and 3). Typical hand samples are medium-light gray (N6) containing ≤1 percent (vol.) colorless, euhedral, prismatic, seriate plagioclase microphenocrysts and phenocrysts ≤2 mm (0.08 in) and 2 to 5 percent (vol.) fresh dusky green (5G 3/2) to dark greenish yellow (10Y 6/6), subhedral to anhedral, polygonal- to blocky-shaped, seriate olivine phenocrysts ≤2 mm (0.08 in), all contained within a fine-grained holocrystalline groundmass (Figure 6 8). Groundmass olivine occasionally displays areas or rims of dark yellowish orange (10YR 6/6) iddingsite (Figure 6 6b,c). Thin sections contain ~1 to 2 percent (vol.) opaque Fe-Ti oxides. Magnetic polarity of Qrbs is unknown due to a lack of in place outcrops. Qrbs is assigned an early Pleistocene age on the basis of stratigraphic relationships with Qran and Qrbr (Figure 6 1). Two topographically high points forming the summit of Fir Mountain, ~1 km (0.6 mi) south of Beaver Springs, are the extant vent complexes from which these lava flows erupted (Plate 2; McClaughry and others, 2012). Lava flows flowed away from the source vents to the northwest into the headwaters of Beaver Creek and to the northeast spreading out into the headwaters area of Indian and Mosier creeks (Plate 2).|02-01-01-01-02|DGMI Unit Label|Qrbs|Qrbs|252 082 098||McClJD2024|Intermediate-composition lava flows|High|DMU16
17|Qrbr|basaltic andesite of Round Prairie|glomeroporphyritic basaltic andesite of Round Prairie of the Volcanics of Fir Mountain|lower Pleistocene|Basaltic andesite lava flow (SiO2 = 53.35 to 54.85 weight percent; K2O = 1.05 to 1.14 weight percent; n = 6 analyses [3 outside Mill Creek area]) mapped west of Round Prairie and Ketchum Reservoir in the headwaters of Mosier Creek (Figure 6 7; Table 6 1; Plate 2; Appendix; McClaughry and others, 2012). The reader should note that distribution of the unit is significantly modified from that originally shown by McClaughry and others (2012) on the basis of additional field mapping and sample analysis in 2015 and 2016. Qrbr is deeply weathered, typically recognized as piles of meter-scale boulders and slabs floating in a matrix of moderate orange-pink (5YR 8/4) to grayish orange (10YR 7/4) soils. Coherent outcrops are only found in a few rock pits and smaller excavations (Figure 6 7). Thickness of the unit is ≤60 m (200ft). Typical hand samples are mottled yellowish gray (5Y 8/1) to medium-light gray (N6) containing ~1 percent (vol.) colorless, euhedral, prismatic, seriate plagioclase microphenocrysts ≤2 mm (0.08 in) long;  2 to 5 percent (vol.) relatively fresh-appearing dusky green (5G 3/2) to dark greenish yellow (10Y 6/6), subhedral to anhedral, polygonal- to blocky-shaped, seriate olivine ≤3 mm (0.08 in); ≥5 percent (vol.) glomerocrysts of plagioclase, olivine, ± pyroxene ≤5 mm (0.2 in); all contained within a fine-grained holocrystalline groundmass (Figure 6 6). Groundmass olivine occasionally displays areas or rims of dark yellowish orange (10YR 6/6) iddingsite (Figure 6 8b,c). Qrbr has reversed magnetic polarity and is assigned an early Pleistocene (Figure 6 1; Plate 2; Appendix). Reversed polarity indicates an age older than 0.773 Ma. The lava flow was likely erupted from an extant vent or series of vents that formed high topography, ~2 km (1.3 mi) south of Round Prairie and ~3.2 km (2 mi) west of Ketchum Reservoir (Plate 2). Qrbr lava flowed away from source vents to the northwest into the headwaters of the mainstem Neal Creek; it also spread to the northeast into the headwaters area of Mosier and Spring Creeks (Plate 2; McClaughry and others, 2012). Qrbr is compositionally nearly identical to the basaltic andesite of Beaver Springs (Qrbs), but is distinguished on the basis of conspicuous glomerocrysts and geomorphic expression of the lava flow field.|02-01-01-01-03|DGMI Unit Label|Qrbr|Qrbr|167 215 182||McClJD2024|Intermediate-composition lava flows|High|DMU17
18|Qgc|gravel|gravel|lower Pleistocene|Unconsolidated heterolithogic gravel exposed in low-lying hills on the lower southeast flank of Mill Creek Ridge (Plate 3). Gravel is assigned an early Pleistocene age on the basis of stratigraphic position directly above the basaltic andesite of Dog River (Qr5dr) (Figure 6 1). The gravel is interpreted to have been deposited in the early Pleistocene as the South Fork Mill Creek drainage was disrupted and locally redirected as the 1.87 Ma basaltic andesite of Dog River (Qr5dr) entered the channel.|02-01-01-02|DGMI Unit Label|Qgc|Qgc|169 163 161||McClJD2024|Sand and gravel of unspecified origin|High|DMU18
19|Qr5dr|basaltic andesite of Dog River|basaltic andesite of Dog River|lower Pleistocene|Intracanyon basaltic andesite lava flow (SiO2 = 55.88 to 57.56 weight percent; K2O = 1.08 to 1.24 weight percent; n = 23 analyses [17 outside Mill Creek area]) mapped along the South Fork Mill Creek (Figure 6 9, Figure 6 10; Table 6 1; Plates 1 and 3; Appendix). Outcrops of Qr5dr are characterized by a massive, rounded form, by broad blocky meter-scale columns, or by intervals of thick platy or slabby jointing (Figure 6 10). Qr5dr typically weathers to subrounded boulders up to 0.5 m (1.6 ft) across or thick, angular slabs up to 2 m (6.6 ft) across. Maximum thickness of Qr5dr along South Fork Mill Creek is 40 m (131 ft)(Plates 1 and 3). Typical hand samples of the basaltic andesite are medium gray (N5), containing 15 to 25 percent (vol.) chalky light gray (N7), distinctly resorbed, euhedral to subhedral, blocky to prismatic, seriate plagioclase microphenocrysts and phenocrysts ≤5 mm (0.2 in); ≤1 percent (vol.) subhedral olivine microphenocrysts ≤1 mm (0.04 in); ≤1 percent (vol.) dusky blue-green (5BG 3/2), subhedral to anhedral, blocky- to lath-shaped clinopyroxene microphenocrysts <1 mm (0.04 in); all distributed within a fine-grained hypocrystalline groundmass (Figure 6 11). Rare subangular to subrounded andesite xenoliths ≤1 cm (0.4 in) have also been found. Sieve textures in plagioclase, displayed by corroded and partially resorbed crystals, are common (Figure 6 11b,c). Clinopyroxene is common as inclusions in some larger plagioclase phenocrysts. Thin sections contain ~1 to 2 percent (vol.) Fe-Ti oxides. Qr5dr has normal magnetic polarity and is assigned an early Pleistocene age on the basis of stratigraphic position and an 40Ar/39Ar age of 1.87 ± 0.02 Ma (McClaughry and others, 2020a)(Figure 6 1; Appendix). A whole-rock K-Ar age of 1.7 ± 0.4 Ma (sample JA85023) was reported for this unit by Anderson (1987) for a sample collected along South Fork Mill Creek (Plate 1; Appendix). Qr5dr was erupted from an extant cinder cone-capped fissure located south of Mill Creek Buttes, along the Dog River-Mill Creek Divide at an altitude of 1,391 m (4,565 ft)(McClaughry and others, 2020a). South of Mill Creek Buttes, the lava flow descended into South Fork Mill Creek and flowed northeast as an intracanyon lava flow, ~25 km (15.5 mi) to Oak Flat, southwest of The Dalles (Figure 6 9; Plates 1 and 3). A Qr5dr lava flow lobe also flowed northwest into the Hood River graben and ancestral valley of the East Fork Hood River (McClaughry and others, 2020a).|02-01-01-03|DGMI Unit Label|Qr5dr|Qr5dr|255 209 220||McClJD2024|Intermediate-composition lava flows|High|DMU19
20|Qrbhp|basaltic andesite of High Prairie|basaltic andesite of High Prairie|lower Pleistocene|Basaltic andesite lava flows (SiO2 = 52.10 to 52.65 weight percent; K2O = 0.83 to 0.94 weight percent; Sr = 805 to 892 ppm; n = 9 analyses outside Mill Creek area) mapped between the summit of Lookout Mountain (Dog River 7.5′ quadrangle, McClaughry and others, 2020a), north into the uppermost reaches of Eightmile Creek and Dog River (Figure 6 12; Table 6 1; Plate 3; Appendix). The unit includes distal basaltic andesite lobes characterized by slightly higher silica (SiO2 = 53.96 to 54.88) and lesser strontium (Sr = 631 to 679)(n = 2 analyses) mapped north to Bottle Prairie and the upper parts of Puppy Creek (Dog River 7.5′ quadrangle, McClaughry and others, 2020a). Outcrops consist of multiple flow lobes of massive to subhorizontal platy- to tabular- and blocky-jointed lava, separated by intervening horizons of sintered, moderate red brown-oxidized (5YR3/4) flow breccia (Figure 6 12). On Lookout Mountain, flow cores are characterized by ~5 percent (vol.) conspicuous white-black speckled, subrounded plagioclase-pyroxene micro-gabbro inclusions (SiO2 = 46.02 weight percent; K2O = 0.17 weight percent; n = 1 analysis) ≤10 cm (4 in) across (Figure 6 12b; Plate 1). Maximum thickness of Qrbhp in the map area is ≤20 m (65 ft) (McClaughry and others, 2020a). Typical hand samples of the basaltic andesite are medium-light gray (N6) and medium gray (N5) to medium-bluish gray (5B 5/1), containing 3 to 8 percent (vol.) fresh clear to chalky white (N9), anhedral to subhedral, blocky, seriate plagioclase (commonly resorbed) microphenocrysts and phenocrysts ≤5 mm (0.2 in); 2 to 3 percent (vol.) waxy, iddingsitized dark yellowish brown (10YR 4/2) to fresh dusky green (5G 3/2), subhedral, blocky, seriate olivine microphenocrysts and phenocrysts 1 to 2 mm (0.04 to 0.08 in); 3 percent (vol.) grayish black (N2) subhedral blocky clinopyroxene phenocrysts 1 to 3 mm (0.04 to 0.1 in; locally megacrystic to 1 cm [2.5 in]); and 2 percent (vol.) plagioclase-pyroxene micro-gabbro inclusions; all contained within a mottled fine-grained holocrystalline groundmass (Figure 6 12b, Figure 6 13). Qrbhp has reversed magnetic polarity and is assigned an early Pleistocene age on the basis of stratigraphic position above the 2.35 Ma basalt of Bennett Pass Road (Qrbp) (Figure 6 1; Plate 3; Appendix). Along Puppy Creek, Qrbhp is characterized by an intracanyon geomorphic setting lying above the 2.06 Ma basaltic andesite of Blue Bucket Springs and underlying the 1.87 Ma basaltic andesite of Dog River (Qr5dr)(Dog River 7.5′ quadrangle, McClaughry and others, 2020a). Qrbhp erupted from an extant cinder cone-capped vent on the west slope of Lookout Mountain (Dog River 7.5′ quadrangle, McClaughry and others, 2020a).|02-01-01-04|DGMI Unit Label|Qrbhp|Qrbhp|128 179 128||McClJD2024|Intermediate-composition lava flows|High|DMU20
21|Qrba|basalt of Agnes Spring|basalt of Agnes Spring|lower Pleistocene|Basalt lava flow (SiO2 = 50.76 weight percent; K2O = 0.58 weight percent; n = 1 analysis) poorly exposed west of Perry Point (Table 6 1; Plate 3; Appendix). West of the map area, in the Dog River 7.5′ quadrangle, the basalt crops out in roadcuts between Brooks Meadow and Bottle Prairie (McClaughry and others, 2020a). Typical hand samples of Qrba are medium-light gray (N6) and aphyric to sparsely olivine phyric, containing 3 to 5 percent (vol.) fresh dusky green (5G 3/2) to dark greenish yellow (10Y 6/6), subhedral olivine microphenocrysts ≤1 mm (0.04 in), distributed within a mottled, locally diktytaxitic, fine-grained holocrystalline groundmass (Figure 6 14). Olivine are typically skeletal in form and partially resorbed, with rims altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 14b,c). Magnetic polarity Qrba is unknown due to a lack of in place outcrops. Qrba is assigned an early Pleistocene age on the basis of stratigraphic position above the 2.35 Ma basalt of Bennett Pass Road (Qrbp) (Figure 6 1; Plate 3; Dog River 7.5′ quadrangle, McClaughry and others, 2020a).|02-01-01-05|DGMI Unit Label|Qrba|Qrba|235 102 166||McClJD2024|Mafic-composition lava flows|High|DMU21
22|Qrbw|basaltic andesite of Ward Creek|basaltic andesite of Ward Creek|lower Pleistocene|Basaltic andesite (SiO2 = 52.29 to 53.24 weight percent; K2O = 0.99 to 1.09 weight percent; n = 8 analyses [3 outside Mill Creek area]) cropping out in roadcuts between Brooks Meadow and Bottle Prairie (Dog River 7.5′ quadrangle, McClaughry and others, 2020a) and in the upper parts of Middle and South Forks Fivemile creeks (Figure 6 15; Table 6 1; Plate 3; Appendix). Limited roadcut exposures are characterized by intervals of poorly developed blocky columnar jointing and platy to tabular jointing (Figure 6 15). Qrbw weathers to a landscape mantled by subrounded boulders up to 1 m (3.3 ft) across. Thickness of Qrbw is ~20 to 30 m (66 to 98 ft). Typical hand samples of the basaltic andesite are light gray (N7) to medium-light gray (N6) and aphyric to microporphyritic or very sparsely porphyritic, containing <1 percent (vol.) clear, euhedral prismatic, seriate trachytic plagioclase microphenocrysts and phenocrysts ≤2 mm (0.08 in)(rarely with phenocrysts up to 5 mm [0.2 in]) and <1 percent (vol.) fresh dusky green (5G 3/2) to dark greenish yellow (10Y 6/6), subhedral, seriate olivine microphenocrysts ≤1 mm (0.04 in) across, distributed within a mottled, locally diktytaxitic, fine-grained holocrystalline groundmass (Figure 6 16). Qrbw has normal magnetic polarity and is assigned an early Pleistocene age on the basis of stratigraphic position above the 2.35 Ma basalt of Bennett Pass Road (Qrbp) (Figure 6 1; Plate 3; Appendix). Qrbw lava flows vented from the upper part of Ward Creek (Dog River 7.5′ quadrangle, McClaughry and others, 2020a) and flowed at least 12 km (7.5 mi) downslope to the northeast, extending into upper reaches of both Middle and South Forks of Fivemile Creek (Plate 3).|02-01-01-06|DGMI Unit Label|Qrbw|Qrbw|128 179 204||McClJD2024|Intermediate-composition lava flows|High|DMU22
23|Qrbe|basalt of Hesslan Canyon|basalt of Hesslan Canyon|lower Pleistocene|Basalt lava flow (SiO2 = 48.89 to 50.23 weight percent; K2O = 0.99 to 1.07 weight percent; n = 4 analyses [1 outside Mill Creek area]) with high amounts of titanium (TiO2 = 2.12 to 2.40 weight percent) mapped over an ~10.5 km (6.5 mi) long outcrop belt between Bottle Prairie on the west and Hesslan Canyon on the east (Figure 6 17; Table 6 1; Plate 3; Appendix; McClaughry and others, 2021). Limited outcrops of Qrbe are characterized by broad blocky meter-scale columns or intervals of platy jointing (Figure 6 17). Qrbe generally weathers to subangular plates 0.25 to 1 m (0.8 to 3.3 ft) across. Thickness of the lava flow exceeds 32 m (105 ft) in the western part of the map area (Plate 1). Typical hand samples of the basalt are medium-light gray (N6) to medium gray (N5), containing 1 to 2 percent (vol.) clear, euhedral, prismatic, seriate plagioclase microphenocrysts and phenocrysts ≤2 mm (0.04 to 0.1 in) and ~3 to 5 percent (vol.) dark greenish yellow (10Y 6/6), subhedral to anhedral, polygonal to blocky shaped, seriate olivine microphenocrysts ≤1 mm (0.04 in) across, all distributed within a fine-grained hypocrystalline groundmass (Figure 6 18). Olivine are typically skeletal in form and partially resorbed, with rims altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 18b,c). Thin sections contain ≤1 percent (vol.) Fe-Ti oxides. Qrbe has reversed magnetic polarity and is assigned an early Pleistocene age on the basis of stratigraphic position above the 2.35 Ma basalt of Bennett Pass Road (Qrbp)(Figure 6 1; Plate 3; Appendix). No outcrops or vent area for the lava flow have been found west of Bottle Prairie.|02-01-01-07|DGMI Unit Label|Qrbe|Qrbe|125 091 094||McClJD2024|Mafic-composition lava flows|High|DMU23
24|Qrbj|basalt of Jacket Springs|basalt of Jacket Springs|lower Pleistocene|Basalt lava flow (SiO2 = 51.47 weight percent; K2O = 0.79 weight percent; n = 1 analysis) mapped between Eightmile Meadow and Jacket Springs (Figure 6 19; Table 6 1; Plate 3; Appendix). Qrbj is typically poorly exposed, weathering to a landscape mantled by subrounded boulders up to 1 m (3.3 ft) across. Limited roadcut exposures are characterized by thick intervals of platy to tabular jointing (Figure 6 19). Thickness of Qrbj is <10 m (33 ft). Typical hand samples of Qrbj are light gray (N7) to medium-dark gray (N4), aphyric to sparsely microporphyritic, containing 2 to 3 percent (vol.) dark greenish yellow (10Y 6/6), subhedral to euhedral to skeletal, seriate olivine microphenocrysts ≤1 mm (0.04 in) across, and ≤1 percent (vol.) clear to chalky white (N9), euhedral, prismatic, seriate plagioclase microphenocrysts ≤2 mm (0.08 in) long, distributed within a diktytaxitic, fine-grained holocrystalline groundmass (Figure 6 20). Olivine phenocrysts and microphenocrysts are typically skeletal in form, commonly occurring as corroded and partially resorbed crystals; olivine rims are typically altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 20b,c). Thin sections contain 1 to 3 percent (vol.) cubic- to needle-shaped Fe-Ti oxides. Qrbj has normal magnetic polarity and is assigned an early Pleistocene age on the basis of stratigraphic association and chemical similarity with the basalt of Bennett Pass Road (Qrbp)(Figure 6 1; Table 6 1; Plate 3; Appendix). The map pattern, as shown on Plate 3, indicates that the Qrbj lava flow likely vented from upslope areas in the vicinity of High Prairie (3.3 km [2.0 mi] southwest of the map area, Dog River 7.5′ quadrangle), although no evidence of the vent remains (McClaughry and others, 2020a).|02-01-01-08|DGMI Unit Label|Qrbj|Qrbj|240 226 247||McClJD2024|Mafic-composition lava flows|High|DMU24
25|Qrbp|basalt of Bennett Pass Road|basalt of Bennett Pass Road|lower Pleistocene|Basalt lava flow (SiO2 = 50.01 to 51.90 weight percent; K2O = 0.78 to 0.96 weight percent; n = 9 analyses [7 outside Mill Creek area]) mapped between Bottle Prairie on the west ~10.5 km (6.5 mi) east to the mouth of Hesslan Canyon (Figure 6 21; Table 6 1; Plate 3; Appendix; McClaughry and others, 2020a, 2021). The lava flow is mapped an additional 6 km (3.7 mi) southwest to High Prairie (McClaughry and others, 2020a). Limited outcrops of Qrbp are characterized by broad blocky meter-scale columns or intervals of platy jointing (Figure 6 21); weathered parts of the flow are characterized by subangular plates 0.25 to 1 m (0.8 to 3.3 ft) across. Thickness of Qrbp is ≥32 m (105 ft) in Hesslan Canyon. Typical hand samples of the basalt are light gray (N7) to medium-dark gray (N4), aphyric to sparsely microporphyritic, containing ≤1 percent (vol.) clear, subhedral to anhedral, prismatic, seriate plagioclase microphenocrysts ≤1 mm (0.04 in) and 2 to 3 percent (vol.) fresh dark greenish yellow (10Y 6/6), subhedral olivine microphenocrysts ≤1 mm (0.04 in), contained within a fine-grained holocrystalline groundmass (Figure 6 22). Very rare subhedral olivine phenocrysts occur up to 3 mm (0.1 in). Olivine microphenocrysts are typically skeletal in form and are partially to wholly altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 22b,c). Sieve textures in plagioclase crystals are common. Thin sections contain 1 to 3 percent (vol.) cubic-shaped Fe-Ti oxides. Qrbp has reversed magnetic polarity and is assigned an early Pleistocene age on the basis of stratigraphic position, intracanyon geomorphic setting, and a weighted mean whole-rock K-Ar age of 2.35 ± 0.03 Ma for an outcrop at Lookout Mountain (Conrey and others, 1996; McClaughry and others, 2020a)(Figure 6 1; Plate 1; Appendix). The map pattern, as shown on Plate 3, indicates that Qrbp lava flow likely vented from upslope areas in the vicinity of High Prairie (3.3 km [2.0 mi] southwest of the map area, Dog River 7.5′ quadrangle, McClaughry and others, 2020a). The lava flow descended into parts of Eightmile Creek and Hesslan Canyon, flowing east ~17 km (10.5 mi) downstream to the mouth of Hesslan Canyon (Plate 3; Wolf Run 7.5′ quadrangle; McClaughry and others, 2020a, 2021).|02-01-01-09|DGMI Unit Label|Qrbp|Qrbp|249 181 187||McClJD2024|Mafic-composition lava flows|High|DMU25
26|Qrbf|basaltic andesite of Flag Point|basaltic andesite of Flag Point|lower Pleistocene or upper Pliocene|Basaltic andesite lava flow (SiO2 = 54.79 to 56.61 weight percent; K2O = 0.98 to 1.38 weight percent; n = 14 analyses [9 outside Mill Creek area]) mapped along Cedar Creek and at Frailey Point in the northern part of the Flag Point 7.5′ quadrangle (Figure 6 23; Table 6 1; Plate 3; Appendix). Outcrops of Qrbf along Cedar Creek form the distal snout of an intracanyon lava flow discontinuously exposed ~5 km (3.1 mi) southwest to a vent area at Flag Point (Jason McClaughry unpublished geologic mapping, 2016). Roadcuts and outcrops of Qrbf are characterized by a massive, rounded form; by broad blocky meter-scale columns; or intervals of platy jointing (Figure 6 23). Qrbf generally weathers to rounded boulders up to 1 m (3.3 ft) across. Thickness of Qrbf is ~25 m (82 ft) in the southwest corner of the map area. Typical hand samples of the basaltic andesite are medium gray (N5) to medium-dark gray (N4) containing 3 to 5 percent (vol.) colorless, euhedral, prismatic, seriate plagioclase microphenocrysts and phenocrysts ≤2 mm (0.1 in) long (rarely with phenocrysts up to 7 mm [0.3 in] long) and <1 percent (vol.) dark greenish yellow (10Y 6/6), subhedral to anhedral, polygonal- to blocky-shaped, seriate olivine microphenocrysts <1 mm (0.04 in) across, all distributed within a locally diktytaxitic, fine-grained holocrystalline groundmass (Figure 6 24). Sieve textures in plagioclase crystals are common (Figure 6 24b,c). Olivine microphenocrysts are typically skeletal in form and have rare areas or rims altered to dark yellowish orange (10YR 6/6) iddingsite. Thin sections contain 1 to 3 percent (vol.) Fe-Ti oxides and include rare anhedral, embayed clear quartz xenocrysts rimmed by glass and an aggregate of granular pyroxene crystals, which separate it from the groundmass. Qrbf has reversed magnetic polarity and is assigned an late Pliocene or early Pleistocene age on the basis of stratigraphic position and an 40Ar/39Ar plateau age of 2.57 ± 0.02 Ma  for outcrops located on the north edge of Owl Hollow in the Friend 7.5′ quadrangle (Figure 6 1; Plate 3; Appendix; McClaughry and others, 2021). Texture, chemical composition, and map pattern indicates that Qrbf was erupted from an extant cinder cone-capped vent underlying the lookout at Flag Point (south of the Mill Creek area; Sherrod and Scott, 1995). Conrey and others (1996) reported a whole-rock K-Ar age of 1.92 ± 0.2 Ma for similar composition basalt underlying the Flag Point lookout.|02-01-01-10|DGMI Unit Label|Qrbf|Qrbf|219 196 211||McClJD2024|Intermediate-composition lava flows|High|DMU26
27|QTbk|basalt of Knebal Spring|glomeroporphyritic basalt of Knebal Spring|lower Pleistocene[?] or upper Pliocene[?]|Basalt lava flow (SiO2 = 50.38 to 50.88 weight percent; K2O = 0.58 to 0.64 weight percent; n = 5 analyses [2 outside Mill Creek area]) mapped above the trachydacite of Fivemile Creek (Tpdv) at Knebal Spring between South Fork Mill Creek and Middle Fork Fivemile Creek (Figure 6 25; Table 6 1; Plate 3; Appendix). QTbk is also mapped west, ~3.3 km (2.0 mi) to the vicinity of Brooks Meadow (Dog River 7.5′ quadrangle, McClaughry and others, 2020a). Good outcrops of QTbk are rare; lava flows are typically exposed as piles of meter-scale boulders and slabs across the forest floor. Limited exposures are characterized by intervals of platy to tabular jointing. The maximum thickness of QTbk in the map area is as much as 58 m (190 ft). Typical hand samples of the basalt are light gray (N7) to medium-light gray (N6) and abundantly plagioclase and olivine phyric, containing 5 to 10 percent (vol.) chalky white (N9) to clear, euhedral, prismatic, seriate plagioclase microphenocrysts and phenocrysts ≤8 mm (0.3 in); 5 to 10 percent (vol.) variably altered, fresh dark greenish yellow (10Y 6/6) to dark yellowish orange (10YR 6/6) iddingsitized, subhedral, blocky, seriate olivine microphenocrysts and phenocrysts ≤5 mm (0.2 in); and ≤5 percent (vol.) plagioclase-olivine glomerocrysts ≤5 mm (0.2 in); all distributed within a diktytaxitic, fine-grained holocrystalline groundmass (Figure 6 26). QTbk has reversed magnetic polarity and is assigned a late Pliocene(?) or early Pleistocene(?) age on the basis of stratigraphic position (Figure 6 1; Plate 3; Appendix). QTbk sits directly upon the 3.69 Ma trachydacite of Fivemile Creek (Tpdv) and predates the 1.87 Ma basaltic andesite of Dog River (Qr5dr)(Plate 3). Mapped distribution of QTbk suggests the vent area for the lava flow is likely to be the high ground immediately north of Brooks Meadow in the Dog River 7.5′ quadrangle (McClaughry and others, 2020a).|02-01-01-11|DGMI Unit Label|QTbk|QTbk|128 077 077||McClJD2024|Mafic-composition lava flows|High|DMU27
28||Pliocene volcanic and sedimentary rocks of the late High Cascades||||02-02|DGMI H3|||||McClJD2024|||DMU28
29|QTpg|sedimentary deposits|sedimentary deposits|lower Pleistocene and/or upper Pliocene|Unconsolidated to partially indurated cobble gravel and sand mapped as canyon-restricted lobes above the trachydacite of Fivemile Creek (Tpdv) in the northern part of the Fivemile Butte 7.5′ quadrangle.  The unit is also mapped further east into the Wolf Run 7.5′ quadrangle in the Dufur area (McClaughry and others, 2021; Figure 6 27; Plate 1). Typical thickness of QTpg along Fivemile Creek is ≤25 m (80 ft). QTpg is assigned a late Pliocene and/or early Pleistocene age on the basis of stratigraphic position above the 3.69 Ma trachydacite of Fivemile Creek (Tpdv) (Figure 6 1).|02-02-01|DGMI Unit Label|QTpg|QTpg|253 187 055||McClJD2024|Conglomerate|Medium|DMU29
30|Tpdf|dacite of Fifteenmile Creek|dacite of Fifteenmile Creek|upper Pliocene|Intracanyon dacite lava flow (SiO2 = 62.55 to 66.26 weight percent; K2O = 2.01 to 3.02 weight percent; n = 27 analyses [20 outside Mill Creek area]) capping a narrow ENE-trending, ~15 km (9.3 mi) long ridge along Fifteenmile Creek in the northern part of the Flag Point 7.5′ quadrangle and southern part of the Fivemile Butte 7.5′ quadrangle (Figure 6 28; Table 6 2; Plate 3; Appendix). Compositionally identical dacite lava flows and several prominent N15°E- to N25°E-striking dacite dikes (SiO2 = 62.55 to 66.26 weight percent; K2O = 2.01 to 3.01 weight percent; n = 19 analyses) mapped to the west in the upper parts of Culvert Creek in the Badger Lake 7.5′ quadrangle mark the vent area (McClaughry and others, 2020a). Tpdf crops out as precipitous cliff-forming exposures marked by meter-scale columnar jointing; columns are often characterized by internal, horizontal platy joint sets (Figure 6 28a,b; Figure 6 29a,b). Outcrops weather to meter-scale, subrounded boulders and angular slabs, forming extensive talus slopes beneath cliffs. Irregularly shaped gabbro-norite inclusions (SiO2 = 53.85 to 57.77 weight percent; K2O = 0.72 to 1.88 weight percent; n = 7 analyses outside Mill Creek area), ranging from centimeters up to several meters across, are a conspicuous and distinguishing feature of this unit throughout its outcrop distribution (Figure 6 29c; McClaughry and others, 2020a). Concentration of inclusions ranges from ~5 to ≤50 percent (vol.). The composite thickness of the unit in the map area is ~60 to 70 m (197 to 230 ft)(Plate 1). Typical hand samples of Tpdf are medium-light gray (N6), medium-dark gray (N4), light-bluish gray (5B 7/1), to medium-bluish gray (5B 5/1), containing 10 to 20 percent (vol.) clear to chalky white (N9), euhedral to subhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤1 cm (0.4 in); 2 to 3 percent (vol.) grayish black (N2), euhedral to subhedral, blocky to prismatic, seriate pyroxene microphenocrysts and phenocrysts ≤3 mm (0.1 in)(amount of orthopyroxene > clinopyroxene); and ~2 percent (vol.) plagioclase-pyroxene glomerocrysts ≤6 mm (0.2 in); all distributed within a very fine-grained holocrystalline to hypocrystalline groundmass (Figure 6 30). Gabbro-norite inclusions are medium-light gray (N6) to pale yellowish brown (10YR 6/2) and equigranular, with a mottled medium-grained holocrystalline groundmass of plagioclase and pyroxene (clinopyroxene + orthopyroxene) (Figure 6 29c). Tpdf has normal magnetic polarity and is assigned a late Pliocene age on the basis of stratigraphic position and a groundmass 40Ar/39Ar plateau age of 3.02 ± 0.02 Ma  obtained from outcrops between Fifteenmile and Larch Creeks in the Wolf Run 7.5′ quadrangle (Figure 6 1; Plate 3; Appendix). The 3.02 Ma age is similar to a previously obtained K-Ar analysis of 2.86 ± 0.06 Ma (plagioclase) reported by Sherrod and Scott (1995) and Gray and others (1996) for a sample near Cold Point.|02-02-02|DGMI Unit Label|Tpdf|Tpdf|187 118 042||McClJD2024|Felsic-composition lava flows|High|DMU30
31|Tpaf|andesite of Fret Creek|andesite of Fret Creek|upper Pliocene|Andesite lava flow (SiO2 = 59.83 to 60.79 weight percent; K2O = 1.73 to 1.94 weight percent; n = 7 analyses [5 outside Mill Creek area]) mapped between Oval Lake and Eightmile Meadow in the broad glacial cirque forming upper Fifteenmile and Fret Creeks in the northern part of the Flag Point 7.5′ quadrangle (Figure 6 31; Table 6 2; Plate 3; Appendix; McClaughry and others, 2020a). Tpaf lava flows are mapped eastward into the vicinity of Fifteenmile Campground and Cold Springs (Plate 1). Outcrops of Tpaf are characterized by thick intervals of platy to tabular jointing; more typically the unit is exposed as piles of meter-scale boulders and slabs scattered across the forest floor (Figure 6 31). The unit has an apparent thickness exceeding 300 m (984 ft) in upper Fifteenmile Creek (Plate 1). Typical hand samples of the andesite are medium-dark gray (N4) to medium-bluish gray (5B 5/1) and pale blue (5B 6/2), containing 5 to 10 percent (vol.) clear to chalky white (N9), euhedral to subhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤4 mm (0.1 in); ~2 percent (vol.) grayish black (N2), subhedral to euhedral, blocky to prismatic, seriate pyroxene microphenocrysts and phenocrysts ≤2 mm (0.08 in)(orthopyroxene > clinopyroxene); and ≤1 percent (vol.) plagioclase-pyroxene glomerocrysts ≤1 cm (0.4 in), all contained within a fine-grained holocrystalline groundmass (Figure 6 32). Tpaf has normal magnetic polarity and is assigned a late Pliocene age on the basis of stratigraphic position below the ~3.02 Ma dacite of Fifteenmile Creek (Tpdf) and the 3.14 Ma andesite and dacite of Senecal Spring (Figure 6 1; Plate 3; Appendix; McClaughry and others, 2020a). Tpaf overlies the 3.77 Ma porphyritic trachydacite of Culvert Creek between the East Fork Hood River and Brooks Meadow Road in the Dog River 7.5′ quadrangle (and others, 2020a).|02-02-03|DGMI Unit Label|Tpaf|Tpaf|237 193 053||McClJD2024|Intermediate-composition lava flows|High|DMU31
32|Tpbh|basalt of Hood River|basalt of Hood River|upper Pliocene|Basalt lava flows (SiO2 = 47.67 to 49.75 weight percent; FeOTotal = 9.10 to 13.07 weight percent; 6.14 to 8.31 weight percent MgO; K2O = 0.19 to 0.78 weight percent; n = 7 analyses outside Mill Creek area) mapped above the Dalles Formation (Tmdl) in the northwest part of the Ketchum Reservoir 7.5′ quadrangle (Table 6 2; Plate 2). Tpbh in the map area is equivalent to abundantly plagioclase-porphyritic basalt that underlies the city of Hood River, ~10 km (6.2 mi) to the northwest (McClaughry and others, 2012). At Hood River, the basalt typically outcrops as interstratified, blocky-jointed, vesicular pahoehoe lobes and tumuli ranging from 2 m to 8 m (6.6 ft to 26 ft) thick. The composite thickness of the lava flow succession in the Hood River area is ~30 m (98 ft) (McClaughry and others, 2012). Typical hand samples of the basalt are medium light gray (N6) to medium-bluish gray (5B 5/1) and distinctly plagioclase-porphyritic, containing ≤50 percent (vol.), colorless, euhedral, prismatic, seriate plagioclase (labradorite) microphenocrysts and phenocrysts ≤8 mm (0.3 in); 2 to 3 percent (vol.) greenish black (5GY2/1), subhedral, blocky clinopyroxene ≤3 mm (0.1 in); and ~5 to 10 percent (vol.) greenish black (5GY2/1) to dusky red (5R 3/4), subhedral to anhedral, polygonal to blocky shaped, seriate iddingsitized olivine microphenocrysts and phenocrysts ≤4 mm (0.2 in); all contained within a diktytaxitic, trachytic, intersertal to ophimottled, coarse-grained hypocrystalline groundmass (Figure 6 33). Thin sections show subophitic clinopyroxene with numerous embedded plagioclase laths and contain 1 to 3 percent (vol.) Fe-Ti oxides (Figure 6 33b,c). Tpbh has normal magnetic polarity and is assigned a late Pliocene age on the basis of stratigraphic position between the 3.05 Ma basalt of Post Canyon and the 3.67 Ma basalt of Ruthton Point near Hood River (McClaughry and others, 2012; Figure 6 1).|02-02-04|DGMI Unit Label|Tpbh|Tpbh|202 159 179||McClJD2024|Mafic-composition lava flows|High|DMU32
33|Tpdd1|tuff breccia of Engineers Creek, lower unit|tuff breccia of Engineers Creek, lower unit|lower Pliocene|Tuff breccia and pumice tuff mapped below the dacite of Fifteenmile Creek (Tpdf) in a NE-oriented outcrop belt along Fifteenmile, Cedar, and Larch creeks in the Fivemile Butte and Flag Point 7.5′ quadrangles (Plate 1). Tpdd1 is mapped ~5 km (3 mi) west of Fifteenmile Campground into the headwaters of Engineers Creek, along the eastern escarpment of the Hood River graben (McClaughry and others, 2020a), and is also mapped northeast of the headwaters of Larch Creek for ~40 km (25 mi) through Dufur to Rice (Figure 6 34; McClaughry and others, 2021). In the Dufur area to the east, Tpdd1 is interbedded with a compositionally similar sequence of ash-flow tuff and additional tuff breccia units (units Tpdd2, Tptf1, and Tptf2 of McClaughry and others, 2021). Tpdd1 is massively bedded to faintly stratified, matrix- to clast-supported, and very poorly sorted. Tpdd1 characteristically contains highly angular, texturally similar-appearing clasts of dense to highly vesiculated, grayish black (N2) plagioclase-phyric glassy andesite to rhyolite. Most clasts are ≤10 cm (4 in) across, but deposits occasionally contain outsized blocks ranging from 10 cm (4 in) to several meters across. Tpdd1 has a dusky red (5R 3/4) oxidized to grayish yellow (5Y 8/4), yellowish gray (5Y 7/2), and grayish orange pink (5YR 7/2), poorly sorted matrix tuffaceous sandstone matrix. Tpdd1 contains a texturally similar, but chemically diverse assemblage of dense to vesiculated and fractured, angular to subangular clasts of medium-dark gray (N4) to black (N1) porphyritic andesite, trachyandesite, dacite, and trachydacite (SiO2 = 58.91 to 68.72 weight percent; K2O = 2.00 to 3.42 weight percent; Nb = 14.2 to 29.9 ppm; Zr = 194 to 428 ppm; n = 26 analyses outside the map area)(Table 6 2; McClaughry and others, 2020a, 2021). The breccia also includes very sparse angular clasts of light brownish gray (5YR 6/1) to very pale orange (10YR 8/2), flow-banded rhyolite (SiO2 = 69.34 to 75.31 weight percent; K2O = 3.09 to 4.38 weight percent; Nb = 22.2 to 29.9 ppm; Zr = 150 to 430 ppm; n = 3 analyses outside the map area), and black (N1) perlitic obsidian nodules (McClaughry and others, 2020a, 2021). Tpdd1 is assigned an early Pliocene age on the basis of stratigraphic position beneath the 3.02 Ma dacite of Fifteenmile Creek (Tpdf) and 3.68 Ma tuff of Friend (Figure 6 1; Plate 3; McClaughry and others, 2021). A sample of the non-welded pumice tuff, defining the base of Tpdd1 near Dufur, has an 40Ar/39Ar plateau age from plagioclase of 3.83 ± 0.01 Ma (McClaughry and others, 2021). A single clast also obtained from Tpdd1 near Dufur has an 40Ar/39Ar plateau age on plagioclase of 3.71 ± 0.02 Ma (McClaughry and others, 2021). Collectively, Tpdd1 mapped between Engineers Creek and Rice, define a ~50-km-long (31-mi-long) early Pliocene paleochannel (Figure 6 34; McClaughry and others, 2020a, 2021). The source area for Tpdd1 is presumed to lie within the Hood River graben.|02-02-05|DGMI Unit Label|Tpdd1|Tpdd1|248 158 101||McClJD2024|Intermediate-composition pyroclastic flows|High|DMU33
34|Tpdv|trachydacite of Fivemile Creek|trachydacite of Fivemile Creek|lower Pliocene|Trachydacite lava flow (SiO2 = 64.09 to 66.67 weight percent; K2O = 2.76 to 3.36 weight percent; Nb = 24.8 to 31 ppm; Zr = 356 to 421 ppm; n = 20 analyses [18 outside Mill Creek area]) mapped as an intracanyon lava flow along Fivemile Creek (Figure 6 35; Table 6 2; Plates 1 and 3; Appendix). Mapping over the larger Middle Columbia Basin indicates that Tpdv is present from western exposures on the East Fork Hood River and south of Mill Creek Buttes (McClaughry and others, 2020a), heading northeast ~30 km (18.6 mi) downstream to outcrops along Fivemile Creek at Pleasant Ridge Road in The Dalles South 7.5′ quadrangle (Figure 1 2). Tpdv crops out as prominent cliffs and benches marked by massive to meter-scale columnar-joints and irregular zones of horizontal platy jointing (Figure 6 35). The common bench and rise topography associated with outcrops along Fivemile Creek and intervening vesicular horizons may indicate that two chemically identical and age-equivalent intracanyon lava flows are present within the unit (Figure 6 35). Outcrops weather to meter-scale, angular subrounded boulders and angular plates forming extensive talus slopes beneath cliffs. Composite thickness of unit Tpdv is ≥90 m (296 ft) in the canyon of Fivemile Creek (Plates 1 and 3). Tpdv is locally capped by sedimentary deposits of QTpg (Figure 6 27). Typical hand samples of the Tpdv are dark gray (N3) containing 1 to 2 percent (vol.) clear, subhedral to anhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤3 mm (0.1 in); ~1 percent (vol.) grayish black (N2), euhedral to subhedral, prismatic- to lath-shaped pyroxene microphenocrysts ≤1 mm (0.04 in)(orthopyroxene ≥ clinopyroxene); and ≤1 percent (vol.) plagioclase-pyroxene microglomerocrysts ≤1 mm (0.1 in); all distributed within a very fine-grained holocrystalline to hypocrystalline groundmass (Figure 6 36). Tpdv has reversed magnetic polarity and is assigned an early Pliocene age on the basis of stratigraphic position and an 40Ar/39Ar groundmass plateau age of 3.69 ± 0.02 Ma obtained from the outcrops in Fivemile Canyon (Figure 6 1; Appendix; McClaughry and others, 2021).|02-02-06|DGMI Unit Label|Tpdv|Tpdv|191 077 000||McClJD2024|Felsic-composition lava flows|High|DMU34
35|Tpbrc|basalt of Rock Creek|glomeroporphyritic basalt of Rock Creek|lower Pliocene|Low-potassium tholeiitic basalt lava flow (SiO2 = 49.3 to 49.54 weight percent; FeOTotal = 11.25 to 11.54 weight percent; 7.49 to 7.59 weight percent MgO; K2O = 0.18 to 0.23 weight percent; n = 3 analyses) mapped in a NNE-trending outcrop belt between Rock Creek and Snyder Canyon in the northwest part of the Ketchum Reservoir 7.5′ quadrangle (Figure 6 37; Table 6 2; Plate 2; Appendix). Tpbrc is deeply weathered, typically recognized as piles of meter-scale boulders and slabs floating in a matrix of moderate orange-pink (5YR 8/4) to grayish orange (10YR 7/4) soils (Figure 6 37). Coherent outcrops are only found in a few rock pits and smaller excavations. Thickness of the unit is as much as 20 m (66 ft). Typical hand samples are medium-light gray (N6), containing 2 to 5 percent (vol.) clear to white (N9), euhedral, prismatic, seriate plagioclase microphenocrysts and phenocrysts ≤7 mm (0.3 in); ≤1 percent (vol.) grayish black (N2) to dusky green (5G 3/2), subhedral, blocky, clinopyroxene ≤3 mm (0.1 in); and 2 to 5 percent (vol.), dark greenish yellow (10Y 6/6), euhedral to anhedral, polygonal to blocky, seriate olivine microphenocrysts and phenocrysts ≤3 mm (0.1 in) across, and 5 to 10 percent (vol.) glomerocrysts of plagioclase, olivine, ±pyroxene ≤1 cm (0.4 in); all contained within an intersertal to ophimottled, fine-grained holocrystalline groundmass (Figure 6 38). Thin sections show subophitic clinopyroxene with numerous embedded plagioclase laths (Figure 6 38b,c). Olivine are typically skeletal in form, corroded and partially resorbed, with rims altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 38b,c). Thin sections contain 1 to 3 percent (vol.) Fe-Ti oxides. Tpbrc has reversed magnetic polarity and is assigned an early Pliocene age on the basis of stratigraphic position and geochemical composition (Figure 6 1; Plate 2; Appendix). Tpbrc is geochemically similar to the underlying basalt of Snakehead Creek (Tpbs); it is distinguished on the basis of conspicuous glomerocrysts and geomorphic expression (Figure 6 38a,b). Tpbrc was erupted from an extant vent that forms high topography between the headwaters of Rock Creek and Snakehead Creek in the west-central part of the Ketchum Reservoir 7.5′ quadrangle (Plate 2). The lava flow flowed north away from source vents into the headwaters of Rock Creek and south toward Beaver Creek.|02-02-07|DGMI Unit Label|Tpbrc|Tpbrc|171 100 134||McClJD2024|Mafic-composition lava flows|High|DMU35
36|Tpbs|basalt of Snakehead Creek|basalt of Snakehead Creek|lower Pliocene|Low-potassium tholeiitic basalt lava flows (SiO2 = 48.53 to 50.35 weight percent; FeOTotal = 10.62 to 12.37 weight percent; 6.49 to 8.42 weight percent MgO; K2O = 0.16 to 0.39 weight percent; n = 19 analyses [6 outside Mill Creek area]) mapped in a NNE-trending outcrop belt between Neal Creek and Snyder Canyon in the northwest part of the Ketchum Reservoir 7.5′ quadrangle (Figure 6 39; Table 6 2; Plate 2; Appendix). Precipitous cliffs between Neal Creek and Snakehead Creek are composed of a stacked flow-on-flow succession of thin (1 to 5 m [3.3 to 16.4 ft]) blocky- to columnar-jointed pahoehoe lava flows (Figure 6 39a). Individual pahoehoe flow lobes are separated by intervening layers of ‘A’ā flow breccia. Singular vesicles or vesicle cylinders are common within individual lava flow layers (Figure 6 39b). Composite thickness of Tpbs is as much as 110 m (360 ft). Typical hand samples are medium-light gray (N6), containing ≤1 percent (vol.) clear to white (N9), euhedral, prismatic, seriate plagioclase microphenocrysts ≤1 mm (0.04 in); ≤1 percent (vol.) fresh grayish black (N2) to dusky green (5G 3/2), subhedral, blocky, clinopyroxene ≤3 mm (0.1 in); 2 to 3 percent (vol.) fresh dark greenish yellow (10Y 6/6), euhedral to anhedral, polygonal- to blocky-shaped, seriate olivine microphenocrysts and phenocrysts ≤3 mm (0.1 in) across; and ≤1 percent (vol.) glomerocrysts of plagioclase, olivine, ±pyroxene ≤5 mm (0.2 in); all contained within an intersertal to ophimottled, fine-grained hypocrystalline groundmass (Figure 6 40). Thin sections show subophitic clinopyroxene with numerous embedded plagioclase laths. Olivine are typically skeletal in form, corroded and partially resorbed, with rims altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 40b,c). Thin sections contain 1 to 3 percent (vol.) Fe-Ti oxides. The relatively equal distribution of groundmass plagioclase and intergranular olivine results in a typical speckled appearance for the rock. The lowest lava flow in Tpbs has normal magnetic polarity, while upper lava flows have reversed magnetic polarity (Appendix). Tpbs is assigned a lower Pliocene age as the unit shares a similar stratigraphic position and geochemical composition with the 4.19 Ma basalt and basaltic andesite of Rimrock Creek (McClaughry and others, 2020a) on Surveyors Ridge (Figure 6 1; Plate 2; Appendix). Magnetic transition from normal polarity lower lava flows to upper reversed polarity lava flows is also consistent with eruption of Tpbs during the normal to reversed polarity transition at 4.2 Ma in the upper part of the Gilbert reversed polarity chron.|02-02-08|DGMI Unit Label|Tpbs|Tpbs|158 127 162||McClJD2024|Mafic-composition lava flows|High|DMU36
37||Lower Pliocene and upper Miocene volcanic and sedimentary rocks of the early High Cascades||||02-03|DGMI H3|||||McClJD2024|||DMU37
38||Dalles Formation||||02-03-01|DGMI H5|||||McClJD2024|||DMU38
39|Tmdl|Dalles Formation, undivided|Dalles Formation, undivided|lower Pliocene[?] and upper Miocene|Moderately to well indurated, pale yellowish brown (10YR 6/2), medium gray (N5) to very light gray (N8) boulder breccia, fluvial conglomerate and sandstone, tuffaceous siltstone, and pumice-, ash-, and lithic-tuff, broadly exposed above the CRBG in the Mill Creek area (Figure 6 41, Figure 6 42, Figure 6 43; Plates 1, 2, and 3). A majority of Tmdl beds are often thin, discontinuous, texturally indistinct, and occur at a scale too small to map separately and are therefore grouped here. Thickness of undivided Tmdl is ≤487 m (1,600 ft) in the Brown Creek 7.5' quadrangle (Plate 1), ≤460 m (1510 ft) in the Ketchum Reservoir 7.5' quadrangle (Plate 2), and ≤495 m (1,625 ft) in the Fivemile Butte 7.5' quadrangle (Plate 3). Breccia beds are typically well exposed, forming erosionally resistant ledges along canyon walls (Figure 6 41e). Where breccia beds are deeply weathered, surfaces are mantled by meter-scale boulders, intermixed with coarse-grained sandy soils (Figure 6 41f, Figure 6 42a). Breccia is typically matrix supported with localized clast-supported areas (Figure 6 41a, Figure 6 42b, Figure 6 43b,c). The deposits range from monolithologic to heterolithologic, dominated by a variety of medium-light gray (N6) to pale blue (5B 6/2) and pale purple (5P 6/2), subrounded to subangular clasts of plagioclase and two-pyroxene (± hornblende) andesite and dacite (SiO2 = 59.14 to 65.14 weight percent; K2O = 0.73 to 1.92 weight percent; Nb = 4.6 to 9.2 ppm; Zr = 113 to 169 ppm; n = 17 analyses [13 outside Mill Creek area]) (Table 6 3; Appendix: McClaughry and others, 2020a, 2021). Clasts are typically enclosed in a matrix composed of volcanic lithics, pumice, partly altered volcanic glass, and plagioclase and pyroxene crystals. Locally, breccia is interbedded with massive to planar stratified sandstone and pebble conglomerate deposited by hyperconcentrated flood-flows, tuff deposited by pyroclastic flows and falls, and lenticular bedded, massive to trough- and planar-cross bedded, pebble- to cobble-rich conglomerate, pebbly sandstone, and sandstone and massive to laminated tuffaceous siltstone deposited in fluvial environments (Figure 6 41b-d). Fluvial conglomerate in the map area is locally exposed consisting of 3- to 15-m-thick (9.9- to 49-ft) typically lenticular beds, containing moderately to well-sorted, well-rounded cobbles and pebbles derived from Cascadian volcanic sources and the underlying CRBG. Exotic metamorphic clasts or non-Cascadian clasts were not found in fluvial deposits in the map area. Northeast of the map area, roughly along Dalles-Umatilla syncline of Newcomb (1967, 1969). Tmdl as presently mapped, includes fluvial facies with an inferred Deschutes River provenance. These strata contain clasts thought to be derived from older Tertiary volcanic sources unique to parts of Central Oregon (e.g., John Day and Clarno Formations; Cannon and O'Connor, 2019; O'Connor and others, 2021). Tmdl is assigned a late Miocene and early Pliocene(?) age, ranging between ~8.8 and 5 Ma, on the basis of stratigraphic position, numerous isotopic ages in the Middle Columbia Basin, and vertebrate and leaf fossils (Figure 6 1; Plates 1,2, and 3; Appendix; Buwalda and Moore, 1929; Chaney, 1944; McClaughry and others, 2020a, 2021).|02-03-01-01|DGMI Unit Label|Tmdl|Tmdl|217 237 220||McClJD2024|Sedimentary and extrusive igneous material|High|DMU39
40|Tmdj|dacite of Jordan Butte|dacite of Jordan Butte of the Dalles Formation|lower Pliocene or upper Miocene|Andesite and dacite lava flows (SiO2 = 62.26 to 64.69 weight percent; K2O = 1.02 to 1.28 weight percent; n = 4 analyses [3 outside Mill Creek area]) mapped on the north side of Owl Hollow along USFS Road 2730 in the northeast part of the Flag Point 7.5′ quadrangle (Figure 6 44; Table 6 3; Plate 3; Appendix). Outcrops here are correlative with chemically and lithologically similar lava flows that underlie Jordan Butte, ~4 km (2.5 mi) south of the map area (Sherrod and Scott, 1995). Roadcuts and outcrops of Tmdj are characterized by blocky meter-scale columns or intervals of platy jointing (Figure 6 44); Tmdj generally weathers to rounded boulders up to 1 m (3.3 ft) across. Thickness of Tmdj in the map area is ≥25 m (82 ft). Typical hand samples of the andesite and dacite are light gray (N7) to medium-light gray (N4), containing ~3 to 5 percent (vol.) clear to chalky light gray (N7), prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts up to 1 cm (0.4 in) long; ~1 percent (vol.) fresh black (N1), subhedral to anhedral, prismatic- to lath-shaped, seriate orthopyroxene and clinopyroxene microphenocrysts and phenocrysts ≤2 mm (0.08 in)(orthopyroxene > clinopyroxene), and sparse microglomerocrysts of plagioclase, pyroxene, glass, and Fe-Ti oxides ≤1 mm (0.1 in); all contained within a hypohyaline groundmass (Figure 6 45). Tmdj has normal magnetic polarity and is assigned a late Miocene age on the basis of stratigraphic position (Figure 6 1; Plate 3; Appendix). Bunker and others (1982) reported a whole rock K-Ar age of 5.28 ± 0.5 Ma for andesite lava flows cropping out along USFS Road 27 and Jordan Creek (south of the map area, 5.3 km [3.3 mi] east of Jordan Butte). Sherrod and Scott (1995) interpreted this outcrop to be correlative with lava flows at nearby Jordan Butte. However, the isotopically dated andesite lava flows along Jordan Creek are chemically dissimilar from the lava flows that form the main mass of Jordan Butte and those cropping out north of Owl Hollow (Appendix). Geochemical sampling and mapping along USFS Road 27 north of Jordan Creek indicates that Tmdj overlies the 5.28 Ma andesite. Therefore, Tmdj, as defined here, is younger than 5.28 Ma.|02-03-01-02|DGMI Unit Label|Tmdj|Tmdj|186 200 198||McClJD2024|Felsic-composition lava flows|High|DMU40
41|Tmde|andesite and dacite of East Fork|andesite and dacite of East Fork of the Dalles Formation|upper Miocene|Andesite and dacite lava flows (SiO2 = 59.45 to 65.68 weight percent, avg = 62.86 weight percent; K2O = 0.99 to 1.65, avg = 1.27 weight percent; n = 53 analyses [50 outside Mill Creek area]) cropping out in an east-west trending ridge on the north flank of Mill Creek Buttes in the Fivemile Butte 7.5′ quadrangle and in a NNE-trending ridge in the southwest corner of the Ketchum Reservoir 7.5′ quadrangle (Figure 6 46; Table 6 3; Plates 2 and3; Appendix; McClaughry and others, 2020a). Tmde is also mapped extensively west of the map area in the Dog River 7.5′ quadrangle between Mill Creek Buttes and Gibson Prairie and along the East Fork Hood River (McClaughry and others, 2020a). In the Dog River 7.5′ quadrangle Tmde includes multiple porphyritic lava flow sequences, interfingering with Dalles Formation volcaniclastic rocks (Tmdl) at variable stratigraphic intervals. Outcrops of Tmde in the map area are commonly characterized by meter-scale, poorly developed columnar joint sets or intervals of platy jointing (Figure 6 46). Weathered outcrops break down to meter scale, subrounded boulders, forming extensive talus slopes beneath ridge-capping outcrops. In some locations, Tmde lava flows have elongated, irregular diktytaxitic vesicles. Dissolution of plagioclase on weathered surfaces leaves vesicle-like pits. Thickness of Tmde in the map area is ≤70 m (230 ft). Typical hand samples of Tmde are grayish blue (5PB 5/2) to medium-bluish gray (5B 5/1), containing 10 to 20 percent (vol.) clear to chalky white (N9), subhedral to anhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤6 mm (0.2 in); and 1 to 2 percent (vol.) grayish black (N2) euhedral to subhedral, prismatic to blocky seriate pyroxene microphenocrysts and phenocrysts ≤4 mm (0.1 in; orthopyroxene ≥ clinopyroxene); all contained within in a very fine-grained holocrystalline to hypocrystalline groundmass (Figure 6 47). Rare andesitic or dacitic rock fragments <2 mm (0.08 in) across are also present. The abundant conspicuous plagioclase phenocrysts distinguish Tmde from adjacent units (Figure 6 47). Tmde has both normal and reversed magnetic polarity flows. It is assigned a late Miocene age on the basis of stratigraphic position and whole rock K-Ar ages of 7.15 ± 0.8 Ma and 8.18 ± 0.06 Ma in the Dog River 7.5′ quadrangle, west of the map area (Figure 6 1; Plates 1 and 2; Appendix; Wise, 1969; Keith and others, 1985; McClaughry and others, 2020a).|02-03-01-03|DGMI Unit Label|Tmde|Tmde|128 153 102||McClJD2024|Felsic-composition lava flows|High|DMU41
42||Mill Creek Buttes|||Hornblende-phyric dacite and microdiorite exposed across Mill Creek Buttes in the headwaters of the South Fork of Mill Creek (Plate 3; Dog River 7.5′ quadrangle, McClaughry and others, 2020a). Relative to other units in the Dalles Formation, rocks at Mill Creek Buttes have lower potassium (K2O = 0.75 to 1.08 weight percent) and higher strontium (Sr = 748 to 1047 ppm) (Appendix; McClaughry and others, 2020a). Isotopic ages obtained on Mill Creek Buttes indicate that rocks range in age from ~7.5 to 6 Ma (McClaughry and others, 2020a). The absence of vesicular zones between units, great apparent thickness, and high-standing topography of Mill Creek Buttes likely indicates emplacement as a late Miocene dome complex.|02-03-01-04|DGMI H6|||||McClJD2024|||DMU42
43|Tmdh|hornblende-porphyritic microdiorite of Mill Creek Buttes|hornblende-porphyritic microdiorite of Mill Creek Buttes of the Dalles Formation|upper Miocene|Microdiorite (SiO2 = 63.64 to 65.13 weight percent; K2O = 0.75 to 0.95, Sr = 778 to 1047 ppm; n = 9 analyses [7 outside Mill Creek area]) forming most of the eastern and southern flanks of Mill Creek Buttes (Figure 6 48; Table 6 3; Appendix). Outcrops of Tmdh are characterized by intervals of tabular to vertical platy jointing; the weathered unit forms land surfaces covered with angular plates or subangular blocks <0.5 m (1.6 ft) across (Figure 6 48). Thickness of Tmdh is ≤130 m (600 ft). Typical hand samples of the microdiorite are medium-light gray (N6), containing 3 to 10 percent (vol.) black (N1) to brownish black (5YR 2/1) iridescent, fresh to corroded, subhedral to anhedral, prismatic- to lath-shaped, seriate hornblende phenocrysts ≤5 mm up to 1.5 cm (0.2 to 0.6 in); <1 percent (vol.) clear, prismatic to blocky, seriate plagioclase microphenocrysts ≤1 mm (0.04 in), and <1 percent (vol.) plagioclase-hornblende glomerocrysts ≤1 mm (0.04 in); all distributed within a fine- to medium-grained holocrystalline groundmass (Figure 6 49). Tmdh has reversed magnetic polarity and is assigned a late Miocene age on the basis of stratigraphic position below the 6.83 Ma microdiorite of Mill Creek Buttes in the Dog River 7.5′ quadrangle, west of the map area (Figure 6 1; Plate 3; Appendix; McClaughry and others, 2020a)|02-03-01-04-01|DGMI Unit Label|Tmdh|Tmdh|128 179 077||McClJD2024|Fine-grained, felsic-composition intrusive igneous rock|High|DMU43
44|Tmdf|dacite of Fivemile Butte, upper flow|dacite of Fivemile Butte, upper flow of the Dalles Formation|upper Miocene|Dacite lava flow (SiO2 = 63.28 to 63.72 weight percent; K2O = 1.07 to 1.09, n = 5 analyses, [2 outside Mill Creek area]) mapped forming a narrow ridge capping Fivemile Butte and continuing east for ~8.4 km (5.2 mi) to Joes Point (Figure 6 50; Table 6 3; Plate 3; Appendix). Roadcuts, quarry exposures, and outcrops of Tmdf are characterized by intervals of vertical to subvertical tabular to platy jointing. Tmdf weathers to subangular fragments ≤0.5 m (1.6 ft) across. Thickness of Tmdf in the map area is ≤40 m (130 ft). Typical hand samples of the dacite are light gray (N7) to medium-bluish gray (5B 5/1), containing 2 to 3 percent (vol.) clear to chalky white (N9), euhedral, prismatic, seriate plagioclase microphenocrysts and phenocrysts 1 to 4 mm (0.04 to 0.2 in) across and up to 1 cm (0.4 in) long; ≤1 percent (vol.) dark gray (N2), rod-like orthopyroxene phenocrysts ≤4 mm (0.2 in); and ~1 percent (vol.) plagioclase-pyroxene glomerocrysts ≤1 cm (0.4 in); all distributed within a fine-grained holocrystalline groundmass (Figure 6 51). Tmdf has both normal and reversed magnetic polarity outcrops.  Tmdf is assigned a late Miocene age on the basis of stratigraphic position directly above the 7.71 Ma lower flows and domes of the dacite of Fivemile Butte (Tmdv)(Figure 6 1; Plate 3; Appendix).|02-03-01-05|DGMI Unit Label|Tmdf|Tmdf|204 255 204||McClJD2024|Felsic-composition lava flows|High|DMU44
45|Tmdv|dacite of Fivemile Butte, lower flows and domes|dacite of Fivemile Butte, lower flows and domes of the Dalles Formation|upper Miocene|Dacite lava flows and/or domes (SiO2 = 62.79 to 64.51 weight percent; 1.17 to 2.12 weight percent K2O, n = 18 analyses, [9 outside Mill Creek area]) underlying the headwaters of Dog River, Fivemile Butte and Perry Point (Figure 6 52; Table 6 3; Plate 3; Appendix; McClaughry and others, 2020a). East of Perry Point Tmdv is mapped for ~8 km (5 mi) as an intracanyon lava flow across the top of the east-west-trending ridge between Eightmile Creek and Hesslan Canyon. Roadcuts and outcrops are either massive or characterized by broad blocky meter-scale columns or intervals of vertical platy jointing (Figure 6 52). Tmdv weathers to meter-scale, subrounded boulders and angular plates, forming extensive talus slopes beneath cliffs. Thickness of Tmdv in the map area is ~40 m (131 ft)(Plate 1). Typical hand samples of the dacite are medium gray (N5) to medium-dark gray (N4), containing ~3 to 5 percent (vol.), clear to chalky light gray (N7), euhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤1 cm (0.4 in) long and ≤1 percent (vol.) fresh grayish black (N2) pyroxene microphenocrysts and phenocrysts ≤2 mm (0.08 in)(orthopyroxene ≥ clinopyroxene), all distributed within a very fine-grained holocrystalline to hypocrystalline groundmass (Figure 6 53). Tmdv has both normal and reversed magnetic polarity outcrops. Tmdv is assigned a late Miocene age on the basis of stratigraphic position and a plagioclase K-Ar age of 7.71 ± 0.17 Ma obtained by Gray and others (1996) for an outcrop in the saddle between Perry Point and Fivemile Butte (Figure 6 1; Plate 3; Appendix). Tmdv is interpreted to be younger than the dacite of Wolf Run (Tmdw) on the basis of isotopic ages; no outcrops exposing the contact between the two units are known in the map area or adjacent Dog River 7.5′ quadrangle (McClaughry and others, 2020a).|02-03-01-06|DGMI Unit Label|Tmdv|Tmdv|061 183 136||McClJD2024|Felsic-composition lava flows|High|DMU45
46|Tmds|dacite of South Fork Fivemile Creek|dacite of South Fork Fivemile Creek of the Dalles Formation|upper Miocene|Dacite lava flow (SiO2 = 63.57 to 64.2 weight percent; 1.14 to 1.22 weight percent K2O, n = 7 analyses) mapped in a narrow outcrop belt between Fivemile Butte on the west and Pollywog Butte on the east (Figure 6 54; Table 6 3; Plate 3; Appendix). Tmds is characterized by intervals of vertical platy jointing intermixed with zones of blocky meter-scale columns (Figure 6 54). When weathered, Tmds leaves a landscape covered in angular plates or subangular blocks ≤0.5 m (1.6 ft). Thickness of Tmds is ≤55 m (182 ft; Plate 1). Typical hand samples of the dacite are light bluish gray (5B 7/1) to medium-bluish gray (5B 5/1), containing ~3 to 5 percent (vol.) clear to chalky light gray (N7), subhedral to anhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤4 mm (0.2 in); ≤3 percent (vol.) fresh black (N1) pyroxene microphenocrysts and phenocrysts ≤2 mm (0.08 in; orthopyroxene ≥ clinopyroxene); and ~1 percent (vol.) glomerocrysts of plagioclase-pyroxene ≤5 mm (0.2 in); all distributed within a fine-grained hypocrystalline groundmass (Figure 6 55). Thin sections contain 1 to 2 percent (vol.) opaque Fe-Ti oxides. Tmds has normal magnetic polarity and is assigned a late Miocene age on the basis of stratigraphic position (Figure 6 1; Plate 3; Appendix). Tmds directly underlies the 3.69 Ma dacite of Fivemile Creek (Tpdv) between Skyline Quarry and Pollywog Spring (Plate 3); the lava flow also appears to underlie 7.71 Ma Tmdv, so is therefore also older than that unit.|02-03-01-07|DGMI Unit Label|Tmds|Tmds|141 178 171||McClJD2024|Felsic-composition lava flows|High|DMU46
47|Tmdw|dacite of Wolf Run|dacite of Wolf Run of the Dalles Formation|upper Miocene|Dacite lava flow (SiO2 = 63.49 to 65.11 weight percent; K2O = 2.76 to 2.96, n = 13 analyses [7 outside Mill Creek area]) mapped capping a NE-trending ridge between Eightmile Meadow and Hesslan Canyon in the Fivemile Butte 7.5′ quadrangle (Figure 6 56; Table 6 3; Plate 3; Appendix; McClaughry and others, 2020a, 2021). Tmdw has a unique chemical composition for a lava flow of Dalles Formation age with relatively higher potassium (K2O >2.5 weight percent) and higher amounts of niobium (Nb = 15.7 to 17.6 ppm) and zirconium (Zr = 319 to 341 ppm) ( Table 6 3; Appendix). The chemically distinct Tmdw forms an important stratigraphic marker in the Middle Columbia Basin, reaching ~24 km (15 mi) eastward from the headwaters of Eightmile Creek in the eastern escarpment of the Hood River graben to the intersection of Eightmile Creek and Wolf Run as a topographically inverted intracanyon lava flow overlying volcaniclastic rocks in the Dalles Formation (Tmdl) (Plate 1; McClaughry and others, 2020a, 2021). A vent area for the lava flow has not been determined and no outcrops of the unit are known west of the single mapped exposure in the East Fork Hood River area (McClaughry and others, 2020a). Tmdw forms prominent cliff-forming exposures marked by meter-scale columnar jointing (Figure 6 56); outcrops weather to subrounded boulders, forming extensive talus slopes beneath cliff-forming outcrops. Vesicles are commonly lined by masses of white to pink zeolite. Tmdw is as thick as 105 m (345 ft) in the upper reach of Deadman Gulch (Plate 3). Typical hand samples of the dacite are medium gray (N5) to medium-dark gray (N4), containing 20 to 30 percent (vol.) clear to chalky white (N9), subhedral to anhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤5 mm (0.2 in); ≤1 percent (vol.) grayish black (N2), euhedral to subhedral, prismatic pyroxene microphenocrysts and phenocrysts ≤2 mm (0.08 in; orthopyroxene ≥ clinopyroxene); and ≤1 percent (vol.) plagioclase-pyroxene glomerocrysts ≤3 mm (0.1 in); all contained within a very fine-grained holocrystalline to hypocrystalline groundmass (Figure 6 57). Tmdw has normal magnetic polarity and is assigned a late Miocene age on the basis of stratigraphic position and an 40Ar/39Ar plateau age for plagioclase of 7.91 ± 0.08 Ma  for a sample collected from outcrops at the intersection of Wolf Run and Eightmile Creek in the Dufur West 7.5′ quadrangle, east of the map area (Figure 6 1; Plate 3; Appendix; McClaughry and others, 2021). Farooqui and others (1981), Bunker and others (1982), and Bela (1982) reported a comparable, but less precise whole rock K-Ar age of 8.2 ± 0.8 Ma collected from this unit a short distance southwest of Camp Baldwin (Plate 3; Appendix).|02-03-01-08|DGMI Unit Label|Tmdw|Tmdw|056 115 065||McClJD2024|Felsic-composition lava flows|High|DMU47
48|Tmdb|dacite of Bulo Point|dacite of Bulo Point of the Dalles Formation|upper Miocene|Dacite lava flows (SiO2 = 63.31 to 64.6 weight percent; K2O = 1.18 to 1.35, n = 8 analyses) mapped between Bulo Point and Cedar Creek in the northern part of the Flag Point 7.5′ quadrangle (Figure 6 58; Table 6 3; Plate 3; Appendix). Tmdb forms precipitous cliff-forming exposures, typically marked by horizontal to subvertical platy jointing (Figure 6 58). Weathered parts of Tmdb form landscapes mantled by meter-scale, subrounded boulders. Extensive talus slopes are common beneath cliff-forming outcrops. Maximum thickness of Tmdb is ≤70 m (230 ft) (Plate 3). Typical hand samples of Tmdb are light bluish gray (5B 7/1) to medium-bluish gray (5B 5/1), containing 3 to 5 percent (vol.) clear to chalky white (N9), subhedral to anhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤1 cm (0.4 in); 1 to 2 percent (vol.) grayish black (N2), euhedral to subhedral, prismatic to lath-shaped pyroxene phenocrysts and microphenocrysts ≤2 mm (0.08 in); and ≤1 percent (vol.) light greenish gray (5GY 8/1), euhedral to subhedral, prismatic to lath-shaped hornblende microphenocrysts ≤1 mm (0.04 mm); all distributed within a fine-grained strongly trachytic holocrystalline groundmass (Figure 6 59). Thin sections contain 1 to 2 percent (vol.) cubic-shaped opaque Fe-Ti oxides. Tmdb has normal magnetic polarity and is assigned a late Miocene age on the basis of stratigraphic position beneath the 7.91 Ma dacite of Wolf Run (Tmdw)(Figure 6 1; Plate 3; Appendix).|02-03-01-09|DGMI Unit Label|Tmdb|Tmdb|136 188 167||McClJD2024|Felsic-composition lava flows|High|DMU48
49|Tmdd|tuff breccia of Fifteenmile Creek|tuff breccia of Fifteenmile Creek of the Dalles Formation|upper Miocene|Matrix- to clast-supported, cliff-forming boulder breccia mapped along Fifteenmile Creek (Figure 6 60; Plate 3). Tmdd is mapped eastward into the Dufur area (McClaughry and others, 2021). Breccia beds are typically massively bedded, very poorly sorted, and matrix supported, with localized clast-supported areas (Figure 6 60). Clasts are typically angular to subrounded, dense to pumiceous, accounting for 50 to 60 percent (vol.) of the deposit. Maximum observed clast size is ~2.0 m (6.6 ft); average size of the largest observed clasts at all outcrops is ~1.2 m (3.9 ft). Tmdd typically contains a texturally similar, but chemically diverse assemblage of unaltered, very light gray (N8) to medium-dark gray (N4) and grayish orange-pink (5YR 7/2) hornblende-porphyritic andesite and dacite (SiO2 = 61.72 to 67.35 weight percent; K2O = 1.39 to 2.22 weight percent; Nb = 8.3 to 10.5 ppm; Zr = 137 to 211 ppm; n = 12 analyses outside Mill Creek area)(Table 6 3; McClaughry and others, 2021). Typical hand samples of Tmdd clasts contain ~10 to 25 percent (vol.) clear euhedral to anhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts up to 6 mm (0.2 in) in length; ~5 to 10 percent (vol.) dark reddish brown (10R 3/4) euhedral to anhedral, prismatic- to lath-shaped, seriate hornblende microphenocrysts and phenocrysts up to 7 mm (0.3 in); and 1 to 3 percent (vol.) dark gray (N2) pyroxene microphenocrysts and phenocrysts up to 2 mm (0.08 in)(orthopyroxene > clinopyroxene); all distributed within a fine- to medium-grained hypocrystalline groundmass (McClaughry and others, 2021). Clasts are typically enclosed by a very light gray (N8) to grayish orange-pink (5YR 7/2), poorly sorted, coarse-grained tuffaceous sandstone matrix composed of angular to subrounded, very light gray (N8) to medium-dark gray (N4), and grayish orange-pink (5YR 7/2) porphyritic dacite lithics and plagioclase grains. The matrix component typically accounts for 40 to 50 percent (vol) of the deposit. Tmdd is assigned a late Miocene age on the basis of stratigraphic position and an 40Ar/39Ar hornblende plateau age of 8.75 ± 0.05 Ma determined for a sample from a single boulder clast collected from outcrops east of the map area at the Henderson Pioneer Cemetery in the Dufur West 7.5′ quadrangle (Figure 6 1; McClaughry and others, 2021). Along Fifteenmile Creek, in the Wolf Run 7.5′ quadrangle, Tmdd contains a clast with a reported K-Ar age for plagioclase of 7.73 ± 0.16 Ma (Gray and others, 1996; McClaughry and others, 2021).|02-03-01-10|DGMI Unit Label|Tmdd|Tmdd|108 210 182||McClJD2024|Intermediate-composition pyroclastic flows|High|DMU49
50||Middle and lower Miocene volcanic and sedimentary rocks||||02-04|DGMI H3|||||McClJD2024|||DMU50
51||Columbia River Basalt Group||||02-04-01|DGMI H4|||||McClJD2024|||DMU51
52||Saddle Mountains Basalt|||The Saddle Mountains Basalt is represented by only the Pomona Member in the Mill Creek area (Plates 1 and 2). Effusion of 33 lava flows composing the Saddle Mountains Basalt produced ~2424 km3 (581 mi3) of lava that covered an area of ~34,800 km2 (13,436 mi2) in the Pacific Northwest (Reidel and others, 2013a). Saddle Mountains Basalt accounts for ~1.1 percent of the total volume erupted in the CRBG flood basalt province. Thickness of the Saddle Mountains Basalt in the map area is ≤37 m (120 ft)(Plates 1 and 2).|02-04-01-01|DGMI H5|||||McClJD2024|||DMU52
53|Tsp|Pomona Member|Pomona Member of the Saddle Mountains Basalt of the Columbia River Basalt Group|middle Miocene|High-magnesium basalt lava flow (SiO2 = 51.42 to 53.61 weight percent; TiO2 = 1.61 to 1.71 weight percent; FeOTotal = 9.97 to 11.05; MgO = 5.42 to 7.09 weight percent; n = 17 analyses [16 outside Mill Creek area]) mapped along Mosier Creek in the northeast corner of the Ketchum Reservoir 7.5′ quadrangle and northwest corner of the Brown Creek 7.5′ quadrangle (Figure 6 61; Table 6 4; Plates 1 and 2; Appendix). Tsp is characterized by entablature-style jointing with elongate, irregular, 5- to 20-cm-wide (2- to 8-in-wide) columns (Figure 6 61). Locally exposed upper and lower colonnades consist of variably vesicular, massive meter-scale blocky columns. Deeply weathered outcrops are flanked by talus slopes consisting predominantly of angular fragments ≤20 (8 in). Thickness of Tsp in the map area is ≤37 m (120 ft), thinning to the south (Plates 1 and 2). Typical hand samples of Tsp are medium-dark gray (N4), containing 2 to 5 percent colorless to white (N9), euhedral to subhedral, prismatic to blocky plagioclase phenocrysts and glomerocrysts ranging from 5 mm (0.04 in) to ≤1 cm (0.4 in); and ≤1 percent (vol.), dusky green (5G 3/2) to dark greenish yellow (10Y 6/6), euhedral to anhedral, polygonal to blocky shaped, seriate olivine microphenocrysts and phenocrysts ≤3 mm (0.1 in); all distributed within a fine-grained, equigranular, hypocrystalline groundmass of plagioclase, intergranular clinopyroxene and olivine, minor Fe-Ti oxides, and intersertal glass (Figure 6 62). Olivine are typically skeletal in form, corroded and partially resorbed, with rims altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 62b,c). Tsp has reversed magnetic polarity (Choiniere and Swanson, 1979) and is assigned a middle Miocene age (Figure 5 7, Figure 6 1; Plates 1 and 2; Appendix). McKee and others, (1977) reported a K-Ar age of 12 Ma for the Pomona Basalt (Tsp), while Barry and others (2013) provided a whole rock 40Ar/39Ar age of 11.21 ± 0.42 Ma. Cogliatti and others (2021) used 40Ar39Ar experiments to date silicic glass interbeds above and below the Pomona Basalt, constraining the age of the lava flow between 11.34 ± 0.17 and 10.70 ± 0.18 Ma. Tsp is equivalent to the Pomona chemical type of Wright and others (1973) and Swanson and others (1979b). Effusion of Tsp produced over 600 km3 (143 mi3) of lava covering an area >20,550 km2 (7,934 mi2) (Tolan and others, 1989). Tsp was erupted from vents and fissures in the Clearwater Embayment of western Idaho and flowed westward to the Pasco Basin along an ancestral tributary to the Columbia River (Camp, 1981; Anderson and Vogt, 1987). This lava flow covered large parts of the central Columbia Plateau as a conformable unit; west of Pasco, Washington, Tsp was largely confined as an intracanyon lava flow along an ancestral Columbia River channel (Anderson and Vogt, 1987). The longitudinal distance from the vent area to distal part of Tsp at the Pacific Coast is ~600 km (373 mi).|02-04-01-01-01|DGMI Unit Label|Tsp|Tsp|250 190 167||McClJD2024|Mafic-composition lava flows|High|DMU53
54||Wanapum Basalt|||The Wanapum Basalt in the Mill Creek area consists of lava flows assigned to the Priest Rapids and Frenchman Springs members. Effusion of ~68 lava flows composing the Wanapum Basalt produced more than 12,175 km3 (2,920 mi3) of lava that covered an area exceeding 87,400 km2 (33,745 mi2) in the Pacific Northwest (Reidel and others, 2013a). Composite thickness of the Wanapum Basalt in the Mill Creek area is ≤220 m (720 ft) (Plates 1, 2, and 3).|02-04-01-02|DGMI H5|||||McClJD2024|||DMU54
55||Priest Rapids Member|||The Priest Rapids Member forms the upper part of the Wanapum Basalt in the Mill Creek area, unconformably overlying the Frenchman Springs Member (Plates 1 and 2). The Priest Rapids Member consists of two lava flows, including the Basalt of Lolo (Twpl) and the Basalt of Rosalia (Twpr)(Wright and others, 1973; Camp, 1981; Anderson, 1987; Anderson and Vogt, 1987). Priest Rapids lava flows are distinguished from other CRBG units by their relatively high concentrations of magnesium, titanium (TiO2 >3.0 weight percent), iron, and phosphorus (Appendix). Lava flows in the Priest Rapids Member also have reversed magnetic polarity, which serves to distinguish them from adjacent units (Swanson and others, 1979b). Lava flows of the Priest Rapids Member were erupted at 15.9 Ma (Kasbohm and Schoene, 2018; Kasbohm and others, 2023) from linear dike systems in the Clearwater Embayment of western Idaho and flowed west (Camp, 1981), covering most of the north and central parts of the Columbia Plateau. Twpl are mapped into the Mosier and Mill Creek areas east of the Cascade crest, while more extensive Twpr intracanyon lava flows traversed the area of the present-day Cascade Range (Anderson and Vogt, 1987; McClaughry and others, 2012) (Figure 5 6). Effusion of the Priest Rapids Member produced more than 2,800 km3 (672 mi3) of lava that covered an area exceeding 57,300 km2 (22,123 mi2) (Tolan and others, 1989; Reidel and others, 2013a). The composite thickness of the Priest Rapids Member in the Mill Creek area is >30 m (100 ft) and may exceed 150 m (492 ft) where lava flows filled paleocanyons (Plates 1 and 2). The Priest Rapids Member is divided in the Mill Creek area into:|02-04-01-02-01|DGMI H6|||||McClJD2024|||DMU55
56|Twpl|Basalt of Lolo|Basalt of Lolo of the Priest Rapids Member of the Wanapum Basalt of the Columbia River Basalt Group|middle Miocene|High-titanium-phosphorus basalt lava flow (SiO2 = 48.6 to 51.97 weight percent; TiO2 = 3.11 to 3.65 weight percent; P2O5 = 0.73 to 0.78 weight percent; n = 19 analyses [10 outside Mill Creek area]) mapped beneath the Pomona Member (Tsp) in the northern part of the Brown Creek 7.5′ quadrangle (Figure 6 63; Table 6 4; Plate 1; Appendix). Twpl is also mapped along North Fork Mill Creek between Thompson Point and Willow Spring on the south limb of the Mill Creek Ridge anticline (Plates 1 and 2). Higher chromium concentrations (Cr = 87 to 122 ppm) differentiate Twpl from the underlying Basalt of Rosalia (Twpr; Hooper, 2000). Outcrops of Twpl are characterized by a well-developed colonnade, with sections of vertical platy jointing. A ≤5-m-thick (16 ft) interval of pillow palagonite locally occurs near the base of Twpl (Figure 6 63). In the Mosier area, Twpl is irregularly separated from the overlying Pomona Member by the Selah Member of the Ellensburg Formation; Twpl is locally underlain by a thin (<2 m [7 ft]) clay layer that is correlative to the Byron Member of the Ellensburg Formation (McClaughry and others, 2012; Lite, 2013). Thickness of Twpl in the map area is ~10 to 17 m (33 to 56 ft; Plate 1). Typical hand samples are medium-dark gray (N4), containing ≤5 percent colorless to white (N9), euhedral to subhedral, prismatic to blocky plagioclase phenocrysts and glomerocrysts <7 mm (0.3 in) to ≤1 cm (0.4 in) and ≤1 percent (vol.), dusky green (5G 3/2) to dark greenish yellow (10Y 6/6), euhedral to anhedral, blocky shaped, seriate olivine microphenocrysts and phenocrysts ≤3 mm (0.1 in), all distributed within a fine-grained, hypocrystalline groundmass of plagioclase, intergranular clinopyroxene and olivine, minor Fe-Ti oxides, and intersertal glass (Figure 6 64). Twpl has reversed magnetic polarity and is assigned a middle Miocene age. The lava flow overlies a sedimentary interbed outside the Mill Creek area with a U/Pb age of 15.923 ± 0.030 Ma (Kasbohm and Schoene, 2018; Kasbohm and others, 2023). Watkins and Baksi (1974) reported a K-Ar age of 14.5 Ma for Twpl outside the Mill Creek area (Figure 5 7, Figure 6 1; Plates 1 and 2; Appendix). Twpl is equivalent to the Lolo chemical type of Bond (1963) and Wright and others (1979).|02-04-01-02-01-01|DGMI Unit Label|Twpl|Twpl|171 160 000||McClJD2024|Mafic-composition lava flows|High|DMU56
57|Twpr|Basalt of Rosalia|Basalt of Rosalia of the Priest Rapids Member of the Wanapum Basalt of the Columbia River Basalt Group|middle Miocene|High-titanium-phosphorus basalt lava flow (SiO2 = 49.49 to 51.89 weight percent; TiO2 = 3.47 to 4.13 weight percent; P2O5 = 0.69 to 0.91 weight percent; n = 33 analyses [20 outside Mill Creek area]) mapped beneath the Basalt of Lolo (Twpl) in the northern part of the Ketchum Reservoir and Brown Creek 7.5′ quadrangles (Figure 6 65, Figure 6 66; Table 6 4; Plates 1 and 2; Appendix). Twpr is also mapped along North Fork Mill Creek between Thompson Point and Willow Spring on the south limb of the Mill Creek Ridge anticline (Plates 1 and 2). Lower chromium concentrations (Cr = 2 to 10 ppm) differentiate Twpr from the overlying Basalt of Lolo (Twpl) (Hooper, 2000). Twpr is distinguished from older lava flows in the Frenchman Springs Member on the basis of reversed magnetic polarity and relatively higher amounts of titanium (avg TiO2 = 3.74 weight percent) and phosphorus (avg P2O5 = 0.81 weight percent) and lesser amounts of chromium (avg Cr = 5 ppm)(n = 33 analyses in Middle Columbia Basin; Figure 5 10; Madin and McClaughry, 2019; McClaughry and others, 2012, 2021). Twpr crops out as indistinct ledges characterized by a hackly-jointed entablature or basal colonnade with blocky columns ≤2 m (6.5 ft) across and vertical platy joint sets (Figure 6 65, Figure 6 66). Pillow lava and palagonite are notably absent over much of the Ketchum Reservoir and Brown Creek 7.5′ quadrangles, except in local rock-pit exposures in the north. This observation contrasts with intracanyon exposures that are prevalent to the north in the Hood River area (Anderson and Vogt, 1987; McClaughry and others, 2012).  Where pillow lavas are present in the Mill Creek areas, pillows are often surrounded by sediment and display little or no vesiculation (Plates 1 and 2). Thickness of Twpr over much of the Ketchum Reservoir and Brown Creek 7.5′ quadrangles is ~40 m (130 ft) (Plates 1 and 2). The unit thins to the south toward the Mill Creek Ridge anticline and thickens to the north toward Hood River Mountain, where composite lava flow thickness locally ranges between ~150 and 200 m (492 and 656 ft)(McClaughry and others, 2012). Typical hand samples of the Twpr are dark gray (N3) and aphyric to sparsely porphyritic to abundantly microporphyritic, with 3 to 5 percent (vol.) dark yellowish orange (10YR 6/6), euhedral to subhedral, prismatic to blocky, seriate plagioclase microphenocrysts, phenocrysts, and glomerocrysts ranging from 1 mm to 1 cm (0.04 to 0.4 in); 1 to 2 percent (vol.) pale greenish yellow (10Y 8/2), euhedral to subhedral, prismatic to blocky clinopyroxene microphenocrysts ≤2 mm (0.08 in); and ≤1 percent (vol.) moderate olive-brown (5Y 4/4), euhedral to anhedral, polygonal to blocky olivine microphenocrysts ≤1 mm (0.04 in); all contained within a hypohyalline groundmass of plagioclase, intergranular clinopyroxene ± olivine, minor Fe-Ti oxides, and intersertal glass (Figure 6 67). Olivine are typically skeletal in form, corroded and partially resorbed, with rims altered to dark yellowish orange (10YR 6/6) iddingsite (Figure 6 67b,c). Twpr has reversed magnetic polarity and is assigned a middle Miocene age (Figure 5 7, Figure 6 1; Plates 1 and 2; Appendix). U/Pb dating of an ash between the Basalt of Rosalia and the overlying Basalt of Lolo outside the Mill Creek area provided an age of 15.923 ± 0.030 Ma (Kasbohm and Schoene, 2018; Kasbohm and others, 2023). The Basalt of Rosalia overlies the ~16.1 Ma Frenchman Springs Member. Twpr is equivalent to the Rosalia chemical type of Swanson and Wright (1978) and Swanson and others (1979b).|02-04-01-02-01-02|DGMI Unit Label|Twpr|Twpr|211 218 146||McClJD2024|Mafic-composition lava flows|High|DMU57
58||Frenchman Springs Member|||The Frenchman Springs Member forms the lower part of the Wanapum Basalt in the Mill Creek area, consisting of 3 to 5 individual lava flows. On the basis of texture, phenocryst distribution, and geochemistry, these lava flows are divided from oldest to youngest into the Ginkgo (Twfg), Sand Hollow (Twfh), and Sentinel Gap (Twfs) basalt (Figure 6 1; Table 6 4; Plates 1, 2, and 3; Beeson and others, 1985; Martin and others, 2013). Frenchman Springs lava flows are distinguished other CRBG units by large, widely scattered to locally abundant plagioclase phenocrysts and high titanium contents (TiO2 = ~2.9 to 3.06 weight percent) (Figure 5 10; Beeson and others, 1985, 1989; Appendix). Upper lava flows (Twfs, Twfh) in the Frenchman Springs Member have normal magnetic polarity, while those in the lower part of the member (Twfg) have excursional polarity. U/Pb ages obtained by Kasbohm and Schoene (2018) and Kasbohm and others (2023) from silicic tuff interbeds between lava flows on the Columbia Plateau constrain eruption of the Frenchman Springs Member between 15.9 and 16.1 Ma. 40Ar/39Ar dating experiments by Baksi (2022) further refine eruption ages of the entirety of the Frenchman Springs Member to ~16.1 Ma. In the Mill Creek area, Frenchman Springs lava flows overlie the Sentinel Bluffs Member (Tgsb) at the top of the Grande Ronde Basalt; they are overlain by reversed-polarity lava flows of the 15.9 Ma Priest Rapids Member, forming the upper part of the Wanapum Basalt (Twpr)(Figure 5 7; Plates 1, 2, and 3). Frenchman Springs Member lava flows were erupted from a northerly-striking vent system in eastern Washington and northern Oregon (Kuehn, 1995) and flowed west down an ancestral paleoslope (Figure 5 6). Effusion of the Frenchman Springs Member produced more than 7628 km3 (1830 mi3) of lava that covered an area exceeding 72,595 km2 (28,029 mi2)(Martin and others, 2013). Composite thickness of the Frenchman Springs Member in the Mill Creek area is ~200 m (660 ft) (Plates 1, 2, and 3). The Frenchman Springs Member is subdivided in the Mill Creek area into:|02-04-01-02-02|DGMI H6|||||McClJD2024|||DMU58
59|Twfs|Basalt of Sentinel Gap|Basalt of Sentinel Gap of the Frenchman Springs Member of the Wanapum Basalt of the Columbia River Basalt Group|lower Miocene|High-titanium basalt and basaltic andesite lava flows (SiO2 = 49.69 to 52.88 weight percent; TiO2 =2.92 to 3.24 weight percent; n = 71 analyses [46 outside Mill Creek area]) mapped beneath the Dalles Formation (Tmdl) and above the Basalt of Sand Hollow (Twfh) along the Mill Creek Ridge anticline in the central part of the map area and along the northern parts of the Ketchum Reservoir and Brown Creek 7.5′ quadrangles (Figure 5 7, Figure 6 68; Table 6 4; Plates 1, 2, and 3; Appendix). Twfs is distinguished from the underlying Basalt of Sand Hollow (Twfh) on the basis of relatively higher amounts of titanium (avg TiO2 = 3.09 weight percent) and phosphorus (avg P2O5 = 0.63 weight percent) and lesser amounts of chromium (avg Cr = 14 ppm)(n = 71 analyses in Middle Columbia Basin; Figure 5 10; Table 6 4; Appendix; Madin and McClaughry, 2019; McClaughry and others, 2012, 2020a, 2021). Twfs is differentiated from the overlying Priest Rapids Member on the basis of geochemistry and normal magnetic polarity (Appendix). Twfs outcrops are typically characterized by well-defined colonnade jointing and/or platy jointing (Figure 6 68). Where exposed, the upper part of Twfs is characterized by 0.2- to 2-m-thick (0.7- to 6.6-ft-thick) vesicular flow lobes. Thickness of Twfs in the map area is ~ 40 m (130 ft)(Plates 1, 2, and 3). Typical hand samples of the basalt are medium-dark gray (N4) to dark gray (N2) and aphyric to very sparsely porphyritic with ≤1 percent (vol.) clear to pale yellowish orange (10YR 8/6), euhedral to subhedral, prismatic to blocky, seriate plagioclase microphenocrysts and phenocrysts ≤7 mm (0.3 in) with ~1 phenocryst/m2; ≤2 percent (vol.) grayish black (N2) clinopyroxene microphenocrysts ≤1 mm (0.04 in); and rare olivine microphenocrysts ≤1 mm (0.04 in); all enclosed within a fine-grained holocrystalline to hypocrystalline groundmass of plagioclase, intergranular clinopyroxene, Fe-Ti oxides, and intersertal glass (Figure 6 69). Twfs has normal magnetic polarity and an early Miocene age (Figure 5 7, Figure 6 1; Plates 1, 2 and 3; Appendix). U/Pb dates obtained by Kasbohm and Schoene (2018) and Kasbohm and others (2023) on ash interbedded between lava flows outside the Mill Creek area bracket the eruption of the Basalt of Sentinel Gap between 16.1 and 15.9 Ma. Baksi (2022) reported 40Ar39Ar ages that constrain eruption of the Basalt of Sentinel Gap to 16.1 Ma. Twfs is equivalent to the Sentinel Gap flows of Mackin (1961), basalt of Sentinel Gap of Beeson and others (1985), and the flows of Union Gap of Powell (1982).|02-04-01-02-02-01|DGMI Unit Label|Twfs|Twfs|195 187 069||McClJD2024|Mafic-composition lava flows|High|DMU59
60|Twfh|Basalt of Sand Hollow|Basalt of Sand Hollow of the Frenchman Springs Member of the Wanapum Basalt of the Columbia River Basalt Group|lower Miocene|High-titanium basalt and basaltic andesite lava flows (SiO2 = 49.14 to 53.72 weight percent; TiO2 = 2.85 to 3.27 weight percent; n = 156 analyses [129 outside Mill Creek area]) mapped beneath the Basalt of Sentinel Gap (Twfs) and above the Basalt of Ginkgo (Twfg) and Sentinel Bluffs Member (Tgsb) along the Mill Creek Ridge anticline in the central part of the map area and along the northern parts of the Ketchum Reservoir and Brown Creek 7.5′ quadrangles (Figure 5 7, Figure 6 70; Table 6 4; Plates 1, 2, and 3; Appendix). Twfh is distinguished from other Frenchman Springs units (Twfs, Twfg) on the basis of sparsely distributed plagioclase phenocrysts and relatively lower amounts of titanium (avg TiO2 = 2.97 weight percent) and phosphorus (avg P2O5 = 0.56 weight percent) and higher amounts of chromium (avg Cr = 38 ppm)(n = 156 analyses in Middle Columbia Basin; Table 6 4; Appendix; Madin and McClaughry, 2019; McClaughry and others, 2012, 2020a, 2021). Twfh consists of two flow units in the Mill Creek area characterized by well-developed platy to blocky colonnade jointing (Figure 6 70). Where Twfh is deeply weathered, adjacent slopes are mantled by subrounded boulders, generally <1.5 m (4.9 ft) in diameter (Figure 6 70a). Thickness of Twfh in the map area ranges between ~90 and 120 m (300 and 400 ft) (Plates 1, 2, and 3). The lower part of Twfh in the Middle Columbia Basin is aphyric to very sparsely porphyritic. Typical hand samples from the lower part of Twfh are pale blue (5B 6/2) to medium-dark gray (N4) and aphyric to microporphyritic and very sparsely porphyritic (~1 phenocryst/m2)(Figure 6 71a). The basalt contains 1 to 2 percent (vol.) yellow brown (5YR 5/6) to very pale orange (10YR 8/2), euhedral to subhedral, prismatic- to lath-shaped and blocky, plagioclase phenocrysts up to 7 mm (0.3 in) in length; 1 to 2 percent (vol.) conspicuous blocky to lath-shaped plagioclase microphenocrysts ≤1 mm (0.04 in); and ≤1 percent (vol.) clinopyroxene microphenocrysts ≤1 mm (0.04 in); all enclosed within a fine-grained equigranular hypocrystalline groundmass of plagioclase, intergranular clinopyroxene, minor opaques, and intersertal to hyalophitic glass (Figure 6 71a,c,d). Very rare olivine microphenocrysts ≤0.5 mm (0.02 in) are also present. The upper part of Twfh in the Middle Columbia Basin is distinctly plagioclase porphyritic, and a distinct flow-top zone, ≤25 m (82 ft) thick is commonly present. Typical hand samples from the upper part of Twfh are medium-dark gray (N4) to dark gray (N2) and abundantly porphyritic and microporphyritic (Figure 6 71b). The basalt contains 3 to 5 percent (vol.) yellow brown (5YR 5/6) to very pale orange (10YR 8/2), euhedral to subhedral, prismatic to lath-shaped, seriate plagioclase phenocrysts and glomerocrysts ≤2 cm (0.8 in); 2 to 3 percent (vol.) conspicuous blocky to lath-shaped plagioclase microphenocrysts ≤1 mm (0.04 in); and ≤1 percent (vol.) clinopyroxene microphenocrysts ≤1 mm (0.04 in); all enclosed within a diktytaxitic, fine-grained holocrystalline groundmass of plagioclase, intergranular clinopyroxene, and minor Fe-Ti oxides (Figure 6 71b,e,f). Twfh has normal magnetic polarity and an early Miocene age (Figure 5 7, Figure 6 1; Plates 1, 2 and 3; Appendix). U/Pb dates obtained by Kasbohm and Schoene (2018) and Kasbohm and others (2023) on ash interbedded between lava flows outside the Mill Creek area bracket the eruption of the Basalt of Sand Hollow between 16.1 and 15.9 Ma. Baksi (2022) reported 40Ar39Ar ages that constrain eruption of the Basalt of Sand Hollow to 16.1 Ma. Twfh is equivalent to the Sand Hollow flows of Mackin (1961), basalt of Sand Hollow of Beeson and others (1985), and the Kelly Hollow flow of Powell (1982).|02-04-01-02-02-02|DGMI Unit Label|Twfh|Twfh|181 194 073||McClJD2024|Mafic-composition lava flows|High|DMU60
61|Twfg|Basalt of Ginkgo|Basalt of Ginkgo of the Frenchman Springs Member of the Wanapum Basalt of the Columbia River Basalt Group|lower Miocene|High-titanium basalt lava flows (SiO2 = 50.67 to 52.76 weight percent; TiO2 = 3.04 to 3.36 weight percent; n = 62 analyses [49 outside Mill Creek area]) mapped beneath the Basalt of Sand Hollow (Twfh) and above the Sentinel Bluffs Member (Tgsb) north of the Chenoweth fault in the Ketchum Reservoir and Brown Creek 7.5′ quadrangles (Figure 5 7, Figure 6 72; Table 6 4; Plates 1 and 2; Appendix). Twfg is distinguished from other Frenchman Springs units (Twfs, Twfh) on the basis of abundant (>5 percent [vol.]) plagioclase phenocrysts and glomerocrysts and relatively high amounts of titanium (avg TiO2 = 3.18 weight percent) and phosphorus (avg P2O5 = 0.62 weight percent) and lower amounts of chromium (avg Cr = 14 ppm)(n = 62 samples in Middle Columbia Basin; Table 6 4; Appendix; Madin and McClaughry, 2019; McClaughry and others, 2012, 2020a, 2021). Twfg is typically not well exposed in the map area, tending to form talus-covered or boulder-armored slopes with rounded boulders 0.5- to 1-m (1.6 to 3.3 ft) across. Outcrops of Twfg are characterized by meter-scale blocky columnar jointing (Figure 6 72). Thickness of Twfg in the map area is typically ≤40 m (130 ft) (Plate 1 and 2). Typical hand samples of Twfg are medium-dark gray (N4) to dark gray (N2) and abundantly porphyritic and microporphyritic containing ~3 to 5 percent (vol.) yellow brown (5YR 5/6) to very pale orange (10YR 8/2), euhedral to subhedral, prismatic to lath-shaped, plagioclase phenocrysts and glomerocrysts ≤3 cm (1.2 in); and conspicuous blocky plagioclase microphenocrysts (Figure 6 73). Phenocrysts and microphenocrysts are distributed within an inequigranular holocrystalline groundmass of plagioclase, intergranular clinopyroxene, and minor Fe-Ti oxides (Figure 6 73b,c). Twfg has excursional magnetic polarity and an early Miocene age (Figure 5 7, Figure 6 1; Plates 1 and 2; Appendix). U/Pb dates obtained by Kasbohm and Schoene (2018) and Kasbohm and others (2023) on ash interbedded between lava flows outside the Mill Creek area bracket the eruption of the Basalt of Ginkgo between 16.1 and 15.9 Ma. Baksi (2022) reported and 40Ar/39Ar plateau age of 16.12 ± 0.05 Ma for the Basalt of Ginkgo (Twfg). Twfg is equivalent to the Ginkgo flows of Mackin (1961) and basalt of Ginkgo of Beeson and others (1985).|02-04-01-02-02-03|DGMI Unit Label|Twfg|Twfg|231 239 182||McClJD2024|Mafic-composition lava flows|High|DMU61
62||Grande Ronde Basalt|||The Grande Ronde Basalt is the thickest, most voluminous formation of the CRBG representing 72 percent of total volume erupted (Figure 5 7; Reidel and Tolan, 2013). Effusion of the Grand Ronde Basalt from vents and fissures in the eastern and southeastern parts of the Columbia Plateau produced more than 150,400 km3 (36,082 mi3) of lava flows that covered an area exceeding 169,600 km2 (65,482 mi2) (Figure 5 6; Tolan and others, 1989; Reidel and others, 1989; Barry and others, 2010; Reidel and Tolan, 2013). The Grande Ronde Basalt spans at least four paleomagnetic zones and has been stratigraphically subdivided into the R1, N1, R2, N2 magnetostratigraphic units by Swanson and others (1979b) (Figure 5 7). The Grande Ronde Basalt mapped in the Mill Creek area consists of lava flows assigned to the Sentinel Bluffs (Tgsb), Winter Water (Tgww) and Ortley (Tgo) members of the N2 magnetostratigraphic unit (Figure 5 7). These lava flows directly overlie the Grouse Creek member (Tggc) of the R2 magnetostratigraphic unit, the stratigraphically lowest mapped geologic unit in the Mill Creek area. Composite thickness of the Grande Ronde Basalt in the Mill Creek area is ≥400 m (1,312 ft) (Plates 1, 2, and 3). Grande Ronde Basalt is generally monotonously fine-grained, aphyric, and petrographically non-distinctive, but can be distinguished on the basis of chemical composition combined with careful geologic mapping (Table 6 4). Grande Ronde lava flows are distinguished from the overlying Wanapum Basalt by the conspicuous absence of large plagioclase phenocrysts and glomerocrysts, and significantly lesser titanium contents (Figure 5 10a-b; Table 6 4; Appendix; Hooper, 2000; Tolan and others, 2009b).|02-04-01-03|DGMI H5|||||McClJD2024|||DMU62
63||Normal-polarity (N2) magnetostratigraphic unit|||The N2 magnetostratigraphic unit is the youngest Grande Ronde Basalt magnetostratigraphic unit, made up of chemically distinctive lava flow packages that cover an area of ~114,500 km2 (44,208 mi2) (Figure 5 7). The estimated volume of the N2 magnetostratigraphic unit is ~35,300 km3 (8,469 mi3) (Reidel and Tolan, 2013). Ages of N2 Grande Ronde units are bracketed by U/Pb dates outside the Mill Creek area of 16.134 ± 0.03 Ma obtained on ash in the lower part of the overlying Vantage Member and 16.260 ± 0.03 Ma for ash between the Wapshilla Ridge and Meyer Ridge members of the underlying R2 magnetostratigraphic unit (Kasbohm and Schoene, 2018; Kasbohm and others, 2023; Figure 5 7, Figure 6 1; Plates 1, 2, and 3; Appendix). The N2 magnetostratigraphic unit is subdivided into the following units:|02-04-01-03-01|DGMI Unit Label|||||McClJD2024|||DMU63
64|Tgsb|Sentinel Bluffs Member|Sentinel Bluffs Member of the Grande Ronde Basalt of the Columbia River Basalt Group|lower Miocene|Basaltic andesite to basaltic trachyandesite lava flows (SiO2 = 53.30 to 55.19 weight percent; TiO2 =1.74 to 2.06 weight percent; n = 139 analyses [111 outside Mill Creek area]) mapped beneath the Frenchman Springs Member (Twfg, Twfh) and above the Winter Water Member (Tgww). Tgsb forms the core of the Mill Creek Ridge anticline in the Fivemile Butte and Ketchum Reservoir 7.5′ quadrangles, crops out in fault blocks in the Hood River fault zone in the northwest part of the Ketchum Reservoir 7.5′ quadrangle, and is exposed in fault-bounded blocks north of the Chenoweth fault in the Brown Creek 7.5′ quadrangle (Figure 5 7, Figure 6 74; Table 6 4; Plates 1, 2, and 3; Appendix). Tgsb is distinguished from other members of the Grande Ronde Basalt on the basis of common diktytaxitic texture and higher contents of magnesium (avg MgO = 4.99 weight percent; 139 analyses in Middle Columbia Basin; Figure 5 10a-b; Table 6 4; Madin and McClaughry, 2019; McClaughry and others, 2012, 2020a, 2021). Tgsb is composed of thin lava flow lobes, generally <5 to 20m-thick (17 to 66 ft), that are characterized by well-developed vesicular flow tops and basal flow breccia. Flow interiors are typically characterized by massive meter-scale colonnade jointing with lesser hackly entablatures (Figure 6 74). Tgsb is generally more resistant to erosion than the overlying Basalt of Ginkgo (Twfg), tending to form cliffs and steep slopes (Figure 6 74). Composite thickness of Tgsb in the Mill Creek area is as much as 170 m (560 ft) (Plates 1, 2, and 3). Typical hand samples of Tgsb are medium-light gray (N6) to medium gray (N5) and aphyric to very sparsely microporphyritic, containing ≤2 percent (vol.) clear, euhedral to subhedral, prismatic to blocky, plagioclase microphenocrysts distributed within an equigranular, fine-grained, diktytaxitic hypocrystalline groundmass of plagioclase, intergranular clinopyroxene, minor Fe-Ti oxides, and intersertal glass (Figure 6 75). Minor amounts of olivine microphenocrysts are also present in some lava flows. Tgsb has normal magnetic polarity and an early Miocene age (Figure 5 7, Figure 6 1; Plates 1 and 2; Appendix). U/Pb dates obtained by Kasbohm and Schoene (2018) and Kasbohm and others (2023) on ash interbedded between lava flows outside the Mill Creek area bracket the eruption of the Sentinel Bluffs Member between 16.3 and 16.1 Ma. Kasbohm and others (2023) reported an 40Ar/39Ar groundmass age of 16.135 ± 0.04 Ma for the Basalt of Museum within the Sentinel Bluffs Member. Baksi (2022) reported a similar 40Ar/39Ar plateau age of 16.15 ± 0.07 Ma for the Basalt of Museum. Tgsb was erupted from a northerly trending vent system in eastern Washington and northern Oregon and flowed west down an ancestral paleoslope (Figure 5 6; Tolan and others, 2009b; Reidel and Tolan, 2013). Effusion of Tgsb produced ~10,150 km3 (2,435 mi3) of lava flows that covered an area exceeding 167,700 km2 (64,749 mi2)(Reidel and Tolan, 2013). Tgsb is equivalent to the high-MgO Yakima flows of Wright and others (1973), the Sentinel Bluffs unit of Reidel and others (1989), and the Sentinel Bluffs Member of Reidel (2005) and Reidel and Tolan (2013).|02-04-01-03-01-01|DGMI Unit Label|Tgsb|Tgsb|216 223 255||McClJD2024|Mafic-composition lava flows|High|DMU64
65|Tgww|Winter Water Member|Winter Water Member of the Grande Ronde Basalt of the Columbia River Basalt Group|lower Miocene|Basaltic andesite lava flows (SiO2 = 54.44 to 57.56 weight percent; TiO2 =2.01 to 2.25 weight percent; n = 41 analyses [24 outside Mill Creek area]) mapped below the Sentinel Bluffs Member (Tgsb) and above the Ortley Member (Tgo). Tgww is mapped in core of the Mill Creek Ridge anticline in the Fivemile Butte and Ketchum Reservoir 7.5′ quadrangles and crops out in fault blocks in the Hood River fault zone in the northwest part of the Ketchum Reservoir 7.5′ quadrangle (Figure 5 7, Figure 6 76; Table 6 4; Plates 2 and 3; Appendix). Tgww is chemically distinguished from the underlying Ortley Member (Tgo) by higher amounts of titanium (avg TiO2 = 2.10 weight percent) and from the overlying Sentinel Bluffs Member (Tgsb) by lesser amounts of magnesium (avg MgO = 3.43 weight percent)(n = 41 analyses in the Middle Columbia Basin; Table 6 4; Appendix; Madin and McClaughry, 2019; McClaughry and others, 2012, 2020a, 2021). Outcrops of Tgww in the Mill Creek area are typically hackly jointed and are composed of irregular, narrow (5 to 15 cm [2 to 5.9 in] wide) columns that extend across horizontal, vesicle-rich bands (Figure 6 76). Talus slopes beneath outcrops are typically composed of angular, equidimensional fragments up to 10 cm (3.9 in) in diameter. Thickness of the Tgww in the Mill Creek area ranges between 46 to 61 m (150 to 200 ft) (Plates 2 and 3). Typical hand samples of the Winter Water Member (Tgww) are medium-dark gray (N4) to dark gray (N2), aphyric to very sparsely microporphyritic and glomeroporphyritic, containing 1 to 2 percent (vol.) clear, euhedral, prismatic plagioclase microphenocrysts and v-shaped or radial, spoked glomerocrysts ≤2 mm (0.08 in), all contained within an inequigranular hypocrystalline groundmass of plagioclase, intergranular clinopyroxene, intersertal glass, and Fe-Ti oxides (Figure 6 77). Conspicuous plagioclase phenocrysts and glomerocrysts are diagnostic field characteristics which distinguishes this unit from other lava flows in the N2 magnetostratigraphic unit (Reidel and others, 1989; Wells, personal commun., 2011). Glomerocrysts and phenocrysts can be less abundant in the basal colonnade relative to the entablature portion of the flow (Tolan , 1982; Reidel and Tolan, 2013a). Tgww has normal magnetic polarity and is assigned an early Miocene age bracketed by isotopic ages from outside the Mill Creek area. The upper possible age for the Winter Water Member is constrained by 40Ar/39Ar ages of 16.13 and 16.15 Ma obtained for the overlying Sentinel Bluffs Member; the lower age of the unit is constrained by a U/Pb date of 16.260 ± 0.03 Ma obtained from ash between the Wapshilla Ridge and Meyer Ridge members of the underlying R2 magnetostratigraphic unit (Kasbohm and Schoene, 2018; Baksi, 2022; Kasbohm and others, 2023; Figure 5 7, Figure 6 1; Plates 2 and 3; Appendix). Tgww is equivalent to the Winter Water flow of Powell (1978), the Winter Water unit of Reidel and others (1989), and the Winter Water Member of Reidel and Tolan (2013).|02-04-01-03-01-02|DGMI Unit Label|Tgww|Tgww|147 155 249||McClJD2024|Mafic-composition lava flows|High|DMU65
66|Tgo|Ortley member|Ortley member of the Grande Ronde Basalt of the Columbia River Basalt Group|lower Miocene|Basaltic andesite lava flows (SiO2 = 54.65 to 57.6 weight percent; TiO2 =1.90 to 2.15 weight percent; n = 30 analyses [19 outside Mill Creek area]) mapped below the Winter Water Member (Tgww) and above the Grouse Creek member (Tggc). Tgo is mapped to form the core of the Mill Creek Ridge anticline in the Fivemile Butte and Ketchum Reservoir 7.5′ quadrangles and crops out in fault blocks in the Hood River fault zone in the in the northwest part of the Ketchum Reservoir 7.5′ quadrangle (Figure 5 7, Figure 6 78; Table 6 4; Plates2 and 3; Appendix; McClaughry and others, 2012). Tgo is chemically and texturally similar to lava flows in the underlying reversed-polarity Grouse Creek member (Tggc), but is distinguished on the basis of normal magnetic polarity. Tgo lava flows are differentiated from the overlying Winter Water Member (Tgww) on the basis of overall absence of plagioclase phenocrysts and lesser amounts of titanium (avg TiO2 = 1.98 weight percent)(n = 30 analyses in the Middle Columbia Basin; Appendix; McClaughry and others, 2012). Tgo outcrops are characterized by hackly-jointed entablatures alternating with more well-defined columnar-jointed intervals (Figure 6 78). The composite thickness of Tgo in the Mill Creek area is ~130 m (430 ft) (Plate 1). Typical hand samples of the Ortley Member (Tgo) are medium-dark gray (N4) to medium-bluish gray (5B 5/1) and aphyric to very sparsely microporphyritic, containing 1 to 2 percent (vol.) clear, euhedral to subhedral, prismatic to blocky, plagioclase micophenocrysts ≤1 mm (0.04 in) contained within a fine-grained, equigranular, hypocrystalline groundmass of plagioclase, intergranular clinopyroxene, Fe-Ti oxides, and intersertal glass (Figure 6 79). Tgo has normal magnetic polarity and is assigned an early Miocene age bracketed by isotopic ages from outside the Mill Creek area. The upper possible age for the Ortley member is constrained by 40Ar/39Ar ages of 16.13 and 16.15 Ma obtained for the overlying Sentinel Bluffs Member; the lower age of the unit is constrained by a U/Pb date of 16.260 ± 0.03 Ma obtained from ash between the Wapshilla Ridge and Meyer Ridge members of the underlying R2 magnetostratigraphic unit (Kasbohm and Schoene, 2018; Baksi, 2022; Kasbohm and others, 2023; Figure 5 7, Figure 6 1; Plates 2 and 3; Appendix). Tgo is equivalent to the low-MgO Yakima flows of Wright and others (1973), the Ortley unit of Reidel and others (1989), and the Ortley member of Reidel and Tolan (2013).|02-04-01-03-01-03|DGMI Unit Label|Tgo|Tgo|110 096 170||McClJD2024|Mafic-composition lava flows|High|DMU66
67||Reversed-polarity (R2) magnetostratigraphic unit|||The R2 magnetostratigraphic unit is the most areally extensive Grande Ronde Basalt magnetostratigraphic unit, made up of chemically distinctive lava flow packages that cover an area of ~117,730 km2 (45,456 mi2). The estimated volume of the R2 magnetostratigraphic unit is ~56,000 km3 (13,435 mi3) (Reidel and Tolan, 2013). Kasbohm and others (2023) indicated that a majority of the R2 unit ranges in age between 16.260 ± 0.036 Ma and 16.327 ± 0.040 Ma (Figure 5 7). The R2 magnetostratigraphic unit in the map area includes the following map unit:|02-04-01-03-02|DGMI H6|||||McClJD2024|||DMU67
68|Tggc|Grouse Creek member|Grouse Creek member of the Grande Ronde Basalt of the Columbia River Basalt Group|lower Miocene|Basaltic andesite lava flows (SiO2 = 54.30 to 56.90 weight percent; TiO2 =1.91 to 2.0 weight percent; n = 11 analyses [10 outside Mill Creek area]) mapped below the Ortley member (Tgo), forming the core of the Mill Creek Ridge anticline in the Ketchum Reservoir 7.5′ quadrangle (Figure 5 7, Figure 6 80; Table 6 4; Plate 2; Appendix; McClaughry and others, 2012). Tggc is the oldest stratigraphic unit exposed in the Mill Creek area. Tggc is chemically and texturally similar to the overlying normal-polarity Tgobut is distinguished on the basis of reversed magnetic polarity (Table 6 4). Outcrops of Tggc are characterized by hackly-jointed entablatures alternating with more well-defined columnar-jointed intervals. Locally, lava flows have pillowed bases. The composite thickness of Tggc in the map area may exceed 60 m (200 ft); the base of the unit not exposed in the map area, so is therefore queried on geologic cross sections (Plates 1, 2, and 3, cross sections). Typical hand samples of Tggc are medium-dark gray (N4) to medium-bluish gray (5B 5/1) and aphyric to very sparsely microporphyritic, containing 2 percent (vol.) clear, euhedral to subhedral, prismatic to blocky, plagioclase micophenocrysts ≤2 mm (0.08 in); ~2 percent (vol.) greenish black (5G 2/1), euhedral, prismatic- to blocky clinopyroxene microphenocrysts ≤2 mm (0.08 in); and ~1 to 2 percent (vol.) glomerocrysts of plagioclase and clinopyroxene ≤2 mm (0.08 in); all contained within an equigranular, hypocrystalline groundmass of plagioclase, intergranular clinopyroxene, rhombohedral Fe-Ti oxides, and intersertal glass (Figure 6 81). Tggc has transitional reversed magnetic polarity (Wells and others, 2009; Appendix). Tggc is assigned an early Miocene age bracketed by isotopic ages from outside the Mill Creek area. The age for the Grouse Creek member is constrained by U/Pb dates of 16.260 ± 0.036 Ma from red clay (bole) beneath the overlying Meyers Ridge Member and 16.327 ± 0.040 Ma obtained from red clay (bole) at the base of the older Wapshilla Ridge Member (Kasbohm and Schoene, 2018; Baksi, 2022; Kasbohm and others, 2023; Figure 5 7, Figure 6 1; Plate 2; Appendix). Tggc is equivalent to the Grouse Creek flows of Ross (1978), the Grouse Creek unit of Reidel and others (1989), and the Grouse Creek Member of Reidel and Tolan (2013).|02-04-01-03-02-01|DGMI Unit Label|Tggc|Tggc|160 127 144||McClJD2024|Mafic-composition lava flows|High|DMU68
69|Tgu|Grande Ronde Basalt, undivided|Grande Ronde Basalt of the Columbia River Basalt Group|lower Miocene|Older lava flows of the Grande Ronde Basalt that lie stratigraphically below the Grouse Creek Member (Tggc) but do not crop out in the map area. The base of Tgu is therefore queried on geologic cross sections (Figure 6 1; Plates 1, 2, and 3, cross sections).|02-04-01-03-02-02|DGMI Unit Label|Tgu|Tgu|202 159 179||McClJD2024|Mafic-composition lava flows|High|DMU69
70||Other rocks||||02-05|DGMI H2|||||McClJD2024|||DMU70
71|QTfb|fault breccia|Fault breccia|upper Pleistocene[?] to middle Miocene|Brecciated rock mapped along and between fault strands in the North Fork Mill Creek fault zone, Hood River fault zone, and Chenoweth Creek fault zone (Figure 6 82, Figure 6 83; Plates 1 and 2). Other smaller areas of fault breccia locally crop out along many mapped fault strands in the area, but are of too small an extent to show separately. QTfb in the map area  typically crops out in roadcuts and quarries or in high-standing erosionally resistant hoodoos (Figure 6 82, Figure 6 83, Figure 6 84). QTfb outcrops are flanked by loosely consolidated talus composed of mixed angular rock fragments (approximately ¾″ minus gravel fraction) and soils. Typical hand samples obtained from QTfb are intensely brecciated, being cross cut by a number of irregularly oriented fractures leading to a poorly defined, but pervasive fissility (Figure 6 82b, Figure 6 83b, Figure 6 85, Figure 6 86) The intersection of numerous foliation planes forms a pencil-like cleavage, where the rock easily breaks into irregularly faceted and smoothly curved interfering surfaces (Figure 6 85, Figure 6 86). Thin section analysis of QTfb shows rock pervasively cross cut by numerous anastomosing and intersecting microfractures, forming branch-like networks (Figure 6 85c,d, Figure 6 86b,c). Curvilinear to stair-stepped microfractures generally range from ≤0.1 to 0.5 mm (0.004 to 0.02 in) across. Microfractures either bound and crosscut ~2- to 3-mm-wide (0.08- to 0.1-in-wide) zones of intensely brecciated and disconnected rock fragments and crystals floating in a pulverized matrix of yellowish gray (5Y 7/2) to dark-yellowish orange (10YR 6/6) isotropic clay and zeolite or cross cut areas of apparently more coherent rock. At fracture intersections, rock fragments or larger microphenocrysts (~1 mm [0.04 in]) display incipient (crackle) to complete (jigsaw) brecciation, where once-fitted angular crystal or rock fragments are now separated by thin zones of finer matrix material. Fractures are filled by a fracture-parallel, foliated, dark yellowish brown (10YR 4/2) matrix of isotropic clay and zeolite. In places, this intrafracture foliation is well defined by blebs or feathered streaks of isotropic, moderate brown (5YR 3/4) to black (n1), finely milled clay or material resembling pseudotachylite(?).  QTfb thin sections do not show evidence of secondary mineral replacement along veins by either silica or calcite cement; brecciated rock fragments or crystals have no visible evidence of secondary alteration and are altogether identical to fresh samples obtained from outcrops outside the fault zone (Figure 6 85c,d, Figure 6 86b,c). QTfb is assigned a middle Miocene to Upper Pleistocene age on the basis of displacement of units with variable stratigraphic age (Figure 6 1; Plates 1 and 2, cross sections). The North Fork Mill Creek fault zone and Chenoweth fault zone displace units as young as the late Miocene and early Pliocene-aged Dalles Formation (Tmdl), while the Hood River fault zone displaces units as young as late Pliocene (Tpbs, Tpbrc)(Plates 1, 2, and 3). Offset along some fault strands is likely to have occurred into the Pleistocene.|02-05-01|DGMI Unit Label|QTfb|QTfb|102 077 102||McClJD2024|Igneous rock|Medium|DMU71
