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Dissertations and Theses Dissertations and Theses
1980 The structure and stratigraphy of the Columbia River basalt in the Chehalem Mountains, Oregon
Abdul-Rahman Mohammed Al-Eisa Portland State University
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Recommended Citation Al-Eisa, Abdul-Rahman Mohammed, "The structure and stratigraphy of the Columbia River basalt in the Chehalem Mountains, Oregon" (1980). Dissertations and Theses. Paper 3200. https://doi.org/10.15760/etd.3191
This Thesis is brought to you for free and open access. It has been accepted for inclusion in Dissertations and Theses by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. AN ABSTRACT OF THE THESIS OF Abdul-Rahman Mohanuned Al-Eisa for the Master of Science in Geology presented November 26,
1980.
Title: The Structure and Stratigraphy of the Columbia
River Basalt in the Chehalem Mountains, Oregon.
APPROVED BY MEMBERS OF THE THESIS COMMITTEE:
The Chehalem Mountains area, encompassing 70 square kilometers, is at the western extent of the Columbia River
Basalt Group as mapped in western Oregon. The flows in the study area were identified as belonging to subdivisions of the Columbia River Basalt Group on the basis of physical characteristics and trace element geochemistry. The basalt
flows are poorly exposed in the area and weathering is deep
and extensive where the flows have been exposed. Where 2 erosion has exposed the underlying marine sedimentary rocks, the basalt has failed in landslides.
The stratigraphy of the Columbia River basalt is useful in determining the path of the basalt flows into western Oregon, in mapping the structure, and in reconstruc ting the tectonic development of the region.
Two formal units of the Yakima Basalt subgroup are exposed in the Chehalem Mountains; the Grande Ronde Basalt and the Frenchman Springs Member of the Wanapum Basalt are distinguishable in the 140 meters accumulation. The sequence of the flows present is as follows: three to four flows of the low MgO chemical type of the Grande Ronde
Basalt, two flows of the high MgO chemical type of the
Grande Ronde Basalt, and two flows of the Frenchman Springs
Member of the Wanapum Basalt. A sedimentary interbed separ ates the Frenchman Springs flows from the basalt flows beneath. All of the basalt flows in the area possess normal polarity.
The flows dip east and northeast, defining a northwest trending structural feature that probably developed origi nally as a broad anticline. The west front of the Chehalem
Mountains may represent a normal fold limb, or it may repre sent a fault. Another major fault separates the Chehalem
Mountains from Parrett Mountain that trends northeast southwest. THE STRUCTURE AND STRATIGRAPHY OF THE
COLCMBIA RIVER BASALT IN THE
CHEHALEM MOUNTAINS, OREGON
by Abdul-Rahman Mohanuned Al-Eisa
A thesis submitted in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE in GEOLOGY
Portland State University
1981 TO THE OFFICE OF GRADUATE STUDIES AND RESEARCH: The members of the Committee approve the thesis of Abdul-Rahman Mohammed Al-Eisa presented November 26, 1980.
'.# • • t. • ... IN THE NAME OF ALLAH THE MERCIFUL THE COMPASSIONATE ACKNOWLEDGMENTS
I would like to extend my deepest gratitude to Dr. Marvin Beeson, who was the advisor for this thesis. He suggested the problem and provided assistance during both the field and laboratory phases of this study. Appreciation is also expressed to the members of my thesis cormnittee, Dr. Leonard Palmer and Dr. Robert Van Atta, Earth Science Department, Portland State University, for their critical review of the manuscript. Gratitude is expressed to the faculty of the Earth Science Department at Portland State University. TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS • iii
LIST OF TABLES v
LIST OF FIGURES • . vi
INTRODUCTION 1
DESCRIPTION OF THE AREA . • . 4
PREVIOUS WORK . . • . • • . 8
METHOD OF COLLECTION AND ANALYSIS OF DATA • 10
REGIONAL GEOLOGY 15
Grande Ronde Basalt 16
Vantage Member • 16
Wanapum Basalt 17
STRATIGRAPHY • • . 20
Grande Ronde Basalt • • . 20
Vantage Member • • . . . . • 39
Frenchman Springs Member of Wanapum Basalt • 40
STRUCTURE . • . • . . . • • . . • • 52
GEOLOGIC HISTORY S'Z
CONCLUSION 59
REFERENCES CITED 61 LIST OF TABLES
Table Page I Trace Element Geochemical Analysis (Including
Fe, K, and Na) .• 12
II Comparison of Chehalem Mountains and Columbia Plateau Major Oxide Concentrations for the
High MgO/Low MgO Geochemical Types 44
III Frenchman Springs Member - Geochemical
Comparison of Flows at Chehalem Mountains
Area with Flows from the Type Localities
and the Clackamas River Area • • . • • 49
IV Grande Ronde Basalt: High MgO and Low MgO
Geochemical Types - Geochemical Comparison
of Flows at Chehalem Mountains Area with
Those from Other Areas • • • • 50 LIST OF FIGURES
Figure Page
1. Regional Distribution of Columbia River
Basalt • 2 2. Index Map of Thesis Area 5 3. Stratigraphy of the Columbia River Basalt
Group • • • . • . . • • • • • 21 4. Animal Well Farm Stratigraphic Section in Chehalem Mountains Area ...... 27. 5. Cedar Well Stratigraphic Section in Chehalem Mountains Area ...... 32 6a. Irregular Blocks at the Top of Low MgO Flow at Jaquith Road ...... 33 6b. Low MgO Flow Exposed Along Bald Peak Road Near Bald Peak Park ...... 33 7. Low MgO Flow Showing Cubic (a) and Columnar
(b) Jointing, Bald Peak Quarry, at Bald
Peak Road 35
Sa. Platy Swirling Pattern Within the Columns'
Bottom of a Low MgO Flow • • 36'
Sb. General View Looking East Toward Cooper
Mountain 36 vii
Figure Page
9a. Deeply Weathered Surface of High MgO Flow Over-
lain by Silt 38 9b. Vantage Horizon Nearby Highway 219 • • 38
10. Deeply Weathered Surface of Frenchman Springs
Flow Overlain by Silt 41
11. Covariance Diagram for La-Sm 46
12. Distribution of Samples into Chemical Groups on Na La the Basis of 48 Sm vs. Sc ...... 13. Geologic Map ...... (Map Pocket) 14. Geologic Section c - c ...... (Map Pocket)
15. Geologic Sections A - A and B - B . . . (Map Pocket) 16. Geologic Sections D - D and E - E . . . (Map Pocket) INTRODUCTION
The Columbia River Basalt Group comprises a series of
Miocene tholeiitic flood basalts that erupted from vents in eastern Oregon, eastern Washington, and western Idaho
(Waters, 1961; Taubeneck, 1970; Swanson, Wright, and Helz,
1975), covering extensive parts of the Columbia Plateau
(Figure 1). These flows were erupted between sixteen and six million years ago (Watkins and Baksi, 1974; McKee,
Swanson, and Wright, 1977), and thus constitute the youngest sub-aerial flood basalt accumulation on earth. They form a huge plano-convex lens--the Columbia Plateau--more than
1500 meters thick, whose upper surface slopes gently inward except where modified along the western margin by eastward trending folds.
Swanson et al. (1979) refer to the Imnaha, Picture
Gorge, and Yakima Basin subgroups within the Columbia
River Basalt Group. There are three formations defined within the Yakima basalts, including the Grande Ronde
Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt.
These correspond to the Lower, Middle, and Upper Yakima basalts of Wright, Grolier, and Swanson (1973).
The present project involves the use of geologic, geochemical, and geophysical methods to investigate the bedrock geology and geologic history in the area. Emphasis • -· - • ·121•- • - • -· ..... , .._. - • • 111• - • - ...... (llllf Ill flll: IAIEll, 1111) i. i. I I =: I ...... l. i I. .I .. '·l.
47•
...... -¥ u -.... ··:-u
N •••
CJ
0 ~ 25 50 100 ., ...
Figure 1. Regional distribution of Columbia River Basalt. "' 3 is placed on defining the detailed stratigraphy and struc ture of Columbia River basalt in the Tualatin Valley Chehalem Mountains area.
The Tualatin Valley is geographically located in the northwestern part of the Willamette Valley of Oregon.
It is characterized by broad valley plain and prominent uplands which are controlled by the folding and faulting of the underlying bedrock. Columbia River basalt underlies the entire Tualatin Valley at depths ranging from a few meters to about 450 meters. Not enough petrographic and chemical work has been done on the basalts in the Tualatin
Valley area to determine what formation(s) in the Columbia
River Basalt Group are present in this region.
Advances in theory and technology have made it pos
sible to distinguish between different groups of flows of Columbia River basalt using geochemical and geophysical methods and have facilitated the determination of the
basalt stratigraphy. Recent work (Osawa and Goles, 1970;
Nathan and Fruchter, 1974; and others) has established
that much of the Miocene Columbia River basalt in central
and eastern Oregon and Washington can be differentiated
by comparing concentrations of selected elements using
Instrumental Neutron Activation Analysis, but emphasized
that the more reliable identification tools are petrography
and major element composition. DESCRIPTION OF THE AREA
The Tualatin Valley is the northwestern part of the Willamette Valley of Oregon, southwest of Portland,
Oregon. It consists of broad, flat lands, the lower slopes of the Tualatin River drainage basin, and prominent uplands.
The lowland includes the Tualatin Valley, the Chehalem
Wapato Lake Valleys, Newberg Valley, and Wilsonville Valley.
The prominent uplands include the Tualatin Mountains,
Cooper and Bull Mountains, Parrett Mountain, the Chehalem
Mountains, David Hill, and the Red Hills of Dundee.
The area under study is basically the Chehalem Moun tains (Figure 2). The Chehalem Mountains include approxi mately 70 square kilometers. The area covers portions of Washington and Yamhill County, and is bounded by the
Tualatin Valley and Cooper Mountain north and northeast,
Wilsonville Valley and Parrett Mountain southeast, Newberg
Valley, Chehalem Creek and the Red Hills south and south west, and Wapato Lake and David Hill northwest.
The Chehalem Mountains trend northwesterly from the southeast corner of the project area to the northwest corner and mark the boundary of the Tualatin lowland.
It reaches a maximum elevation of about 490 meters at
Bald Peak. 5
lo I 0 lo Km·
Figure 2. Index map of thesis area. 6
Landslides are conunon in this region of heavy rain fall. On the west front of the Chehalem Mountains erosion has exposed the underlying Oligocene marine sediments.
The basalt has failed in landslides along this front.
The heavy rainfall and mild climate promote a lush vegetation, which is a deterrent to field work. The common trees are ash, cottonwood, vinemaple, and Oregon white oak. Small plants include bracken, poison oak, and native grasses.
The top of the basaltic lava, deeply weathered and moderately eroded, forms the slopes in the Chehalem Moun tains. The basalt of the upland area is weathered to residual lateritic soil. This deeply weathered material forms the "red land" soils of the area. On high ridges the soil cover is occasionally very thin with hard rock only a few meters beneath the surface. In flatter areas the basalt is deeply weathered and buried by a thick cover of silt and clay. 'rhe weathered flows may show a columnar or close-cubic ("brick-bat") jointing, but joints have been opened and their surf aces contain thick coatings of iron and manganese oxide materials (Schlicker and Deacon,
1967). Natural outcrops, however, are generally deeply weathered. The weathered zone, up to 30 meters thick, consists of red to brown clay with fragments of decomposed basalt. Unweathered basalt surfaces are brownish-gray to dark blue-gray, dense, and finely crystalline with 7 well-developed colwnnar jointing.
Columnar jointing separates the flows into rudely hexagonal columns which extend perpendicular to the cooling surfaces, generally at the top and bottom of the flow.
Rectangular jointing separates some flows into rudely cubic blocks. The colwnns commonly are transected by cross joints and locally may be shattered by zones of platy parting. The lower part of the columns normally is somewhat vesicular.
The top of these columns is marked by a slaggy vesicular zone.
The basalt exposures in the study area are limited, occurring only on structural highs and in the steepest cliffs or stream bluffs, and in artificial exposures, such as quarries and roadcuts. The basalt exposed in the quarries is much less weathered than the basalt exposed on the roadcuts or the stream bluffs. The basalt dips gently northeast into the Tualatin Valley and the erosional scarp faces southwest with steep slopes. This basalt has been extensively dissected by streams with local stream canyons as deep as 30 to 40 meters. The downwarped parts of the basalt are overlain by deposits of semiconsolidated silt, clay, and sand. Most of the area is crisscrossed with roads or cut trails that, if not navigable by car, are an aid to access by foot. PREVIOUS WORK
The Tualatin Valley is a small part of the area covered in a report by Piper (1942). Regional studies
and mapping covering the project area have been previously conducted by Treasher (1942), by Warren, Norbisrath, and
Grivetti (1945), and by Trimble (1963). Bretz (1925,
1928), Allison (1933, 1935, 1936), and Glenn (1965) dis
cussed the origin of the Willamette silt deposits on the
valley floors. Lowry and Baldwin (1952), Theisen (1958),
and Trimble (1963) discussed the genesis of the upland
silt. The U.S. Department of Agriculture Soil Conservation
Service also published the following county soil maps:
Washington County (Watson et al., 1923), Yamhill County
(Kocher et al., 1920), Marion County (Torgerson and Glassey,
1927), and Clackamas County (Kocher et al., 1926). A
geologic map of Oregon west of the 12lst meridian, mapped
by Wells and Peck (1961) at a scale of 1:500,000, also
covers the area. These works are either interpretive
and often lack field corroboration or are of a general
reconnaissance nature. The most notable deficiency is
the lack of field verified structural information.
The most complete geologic mappings of the area
have been conducted by Hart and Newcomb (1965) and Schlicker
and Deacon (1967). Discrepancies existing between these 9
two reports are primarily the result of a finer subdivision of units used by Schlicker and Deacon. Although detailed petrographic studies were not made, the basalt is identified
as Columbia River basalt on the basis of stratigraphic position and gross lithologic similarity to Columbia River basalt in the Columbia Gorge. METHOD OF COLLECTION AND
ANALYSIS OF DATA
This project was approached in four phases:
1) An initial reconnaissance phase:
The reconnaissance phase was an important step in assessing the best means by which to accomplish data collec tion. During this phase, the upper and lower contacts of the Columbia River basalt were sought along with preliminary identification of field recognizable units with the aid of the geologic map of the Tualatin Valley region, Oregon, by Schlicker and Deacon (1967) and the small-scale geology map of western Oregon by Peck (1961).
2) A detailed data collection phase:
During this phase stratigraphic sections with three or more flows were examined and described in detail. These
two stratigraphic sections are from the University of
Oregon Medical School Animal Farm Well, located in the
northeastern corner of the project area, within the Laurel wood quadrangle (see Figure 2, p. 5), and from Cedar Well, which is located in the southern part of the project area,
within Newberg quadrangle (see Figure 2, p. 5). Ten sam
ples were selected from drill cuttings of each well for
chemical analysis purposes. Other flows were also analyzed, 11 such as flows exposed in the roadcuts or in quarries as single flows or two flows in contact.
Remnant magnetic polarity (RMP) was checked in the field with a fluxgate magnetometer. At least three samples from each flow were checked for RMP. Where possible, the samples for RMP were taken from flow tops adjacent to contacts.
3) An analytical phase:
Field recognition of the individual flows is hindered by similarities in hand-specimen appearance and jointing, by lack of extensive marker interbeds, and by rapid thick ness changes owing to the irregular underlying topography.
All of the flows, however, were identified and confirmed by the laboratory work which included detailed trace element geochemical analysis on 80 samples (Table I) employing
Instrumental Neutron Activation Analysis, and each has been assigned a stratigraphic position.
Trace element analysis allows determinations of the stratigraphic relationship of particular groups of flows to known chemical boundaries consistent throughout the expanse of Columbia River basalt. Other important features for mapping stratigraphy, especially for horizontal correlation, were topographic expression and vegetation.
Topographic benches formed at contacts were used as topo graphic aids in extending the boundaries horizontally.
Major oxide analyses were conducted by Dr. Peter Hooper, 12
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BRl 5.44 0.18 2.30 0.35 2.23 0.01 27.12 0.79 31. 00 0.30 6.05 0.07 4.99 1. 82 1. 15 1. 65 51. 58 10.3 10.0 11. 7 549.35 92.0 N. O. PB4 8.13 0.21 N.O. 2.13 0.02 24.86 0.61 27.35 0.70 6.11 0.08 4.87 0.74 1.19 0.15 50.66 7.64 9. l 1.10 462.35 67.2 N. O. PBS 9.23 0.24 N.O. 2.15 0.02 26.26 1. 57 31.71 0.36 6.45 0.09 5 . 78 0.79 1. 44 0.17 51.12 8.27 I 10.3 6.28 H4. 80 71. 6 N. O. PB6 8.91 0.20 N.O. 2. 34 0.03 44.81 2.57 36.19 0.86 9.86 0.12 5.33 0 . 82 2.20 0. 22 64.02 9.08 7.8 1. 22 688.26 77. 8 N.O. PB7 9.62 0.29 N.O. 2.37 0.03 24.39 1. 70 35.55 0.91 6. 14 0 .10 5 .17 0.81 N .0. 51.12 9.67 8.8 1.18 524.09 74.9 N.O. N.O. OR 8.32 0. 14 N.O. 2.07 0.03 22. 34 1. 26 30.68 0. 83 5.35 0.09 4.40 0.70 0.678 0.38 46.98 7.65 6.9 1. 02 437.09 61. 8 :1Rl 7.35 0.30 N.O. 2 . 21 0 . 03 25.26 l . 41 33.12 0.90 5.61 0.09 4. 77 0.68 0.48 0. 40 39.38 7.12 6 . 7 1. 02 502 . 34 63.2 N.O. N.O. HO 9.30 0. l 8 N.O. 2.10 0 . 03 30.31 2.25 37.35 0.31 6. 4 6 0. 10 4.76 0.75 1. 54 0.66 48.82 7.52 7.8 1. 10 444.ll 67. 4 N. D. \o\"R 7.41 0.26 N.O . 1. 97 0.04 26.19 1. 41 40.04 ]. 52 7. l 8 0 . 12 6.93 0. 86 l.11 0 . 54 48 . 36 8.94 10.7 1. 27 684 . 01 78.5 N.O. i• ~ l 7.61 0 . 26 N.O. ]. 88 0.01 21. 08 0.73 27.61 0.39 4.71 0.06 3. 41 0. 90 0. 96 0. 16 32.38 8.7 8.4 2.13 N.D. HC5 8.06 0.29 N.D . 1. 91 0.01 l7. 22 0.73 32 . 67 0.79 5 . 08 0.06 2.84 . 94 0.80 0.37 32 . 79 9.6 N.O. N.D . 65.8 13.9 HC2 8.26 0.19 l. 69 0.35 2.16 0.01 34.70 0.96 32.80 0.31 7.90 1. 08 4.39 1. 70 1. 20 0.59 44.90 10.4 8.4 2.6 N.D . N.O. HC 6.20 0.16 1.12 0.28 1. 43 0.01 19.41 0.36 32.73 1. 05 5.60 0.06 N.D. 1.22 0. 15 31.27 8.96 I\. 0. N. D. 54.2 12.7 14 . 7 1'1H 8 . 00 0.28 N.D. 1. 93 0.01 18. 48 0.75 36.19 0.63 5.33 0.07 3. 72 0 . 97 0.71 0.37 34. 72 10.5 7.6 1. 37 N.D. 63.7 HH9 8.00 0.33 2.00 0.44 2.12 0.01 21. 05 1. 20 34 . 72 0.90 6 . 86 0. 08 5. 43 1. 91 1. 47 1. 15 49.28 9.85 7 . 6 4. 97 472.17 94 . 3 N.O. HH4 8.00 0.18 N.O. 2.11 0.01 19. 41 0. 72 35.30 0.77 5.26 0.07 2.37 0. 91 1. 07 .53 29.57 9.36 6 . 8 1. 30 N. O. 59 . 8 13 . 7 HH2 8. 45 0 . 19 2.08 0.45 2.14 0.01 22.58 1. 27 33.63 0.29 7.25 0.08 5.84 2.03 1. 23 0.59 49.28 7.45 9.1 3.18 594 . 95 84.9 N. D. 14 . 8 'Tgh UN 7.67 0.26 N.D. l. 80 0.01 29. 78 1. 83 43.50 0.79 8.03 0.09 4. 16 1. 03 1. 64 1. 4 7 55.27 10 . 5 8.5 1. 88 N. D. 61. 1 I PRl 8 .13 0.21 2.80 0. 45 1. 99 0.01 20.49 0.62 38.89 0. 4 7 5. 7 5 0.07 2.57 1. 15 1. 05 0.52 38. 7 3 9 . 57 7.6 2.81 N.D. 67.3 12.8 PR2 9.95 0.30 N.D. 1. 4 4 0.01 54. 58 2.60 46 . 70 0.94 12.08 0. 10 6.81 1. 09 2.65 0.21 64.48 12 . 0 11. 4 4. 16 799.81 96.l N.O. N.D. GC 8.00 0.23 N. O. 2.38 0.01 26.52 1. 05 32.97 l .11 8. 15 0.08 5.01 0.99 0 . 87 0.41 51 . 15 9.62 9 . 9 2.74 N.D . 50.7 G& 7.15 0 . 14 N.D. 1. 34 0.02 24.55 1. 76 38.95 0. 79 9.60 0.11 3.06 0. 71 1. 57 0.61 59.87 8.12 7.7 1.10 406 . 22 76.3 10.8 Gb l 9.36 0.22 N.D. 2.26 0.03 21.40 1. 56 38.31 1. 03 5. 74 0.09 3.38 0 . 68 1.30 0.55 37.35 7.46 7.3 1. 06 339.57 70.3 78 . 7 10.2 Gn: 7.54 0.18 N.O. 1. 93 0.03 42.28 2 . 77 38.76 0.67 9.08 0.11 3.41 0.77 3.53 0.41 66.78 8.3 4 7.8 1. 09 404.82 7. 91 77.9 10 . 7 GLB 7.54 0.15 N.D. 1. 91 0.03 26.33 1. 44 39.21 0.96 6.46 0. l 0 5.69 0.77 0.51 0.41 40.07 7.50 8.1 1.13 877 . 0 68 . 7 FRO 10.66 0.35 2.42 0.39 2.05 0.01 23.86 0.87 34. 91 0.55 6 . 73 0. 07 2. 94 1. 26 0.66 0.46 49.28 9.53 8.6 1. 12 N.O. N. D. N.O. BK 9. 75 0.35 N.D. 1. 95 0.01 22.80 1. 19 33.50 0 .30 6.42 0.07 2.97 0.95 1. 01 0.50 37.17 10 . 0 N.D. N.D. J Twfs JRl 10.40 0.19 N.D. 2.24 0.01 25.86 1 . 41 37.35 1. 06 7.25 0.08 3.81 0.92 1. 07 0.52 54.81 11. 3 9.9 1. 67 N.D. N.D. EXPL.:..~ATION: First column is conc en tration in Parts Per Million (Fe, K, and Na in %) Second colu.-nn is R,..l a tive Error (s ame u~ its) Unit sy:nbols f or Col u..,-.bia Rive:r bc.salt are the same as tl1os e us e d on geologic map (Figure 13) 14 Washington State University, on one complete Animal Farm Well section in the Grande Ronde formation, and on samples from the area near Rex Quarry and the area near Bald Peak Quarry. Thin sections were made on selected samples. Compari son of samples based on this information proved difficult because of a narrow range in mineralogic and textural parameters, and because minor differences were not readily discernible from the detailed descriptions. 4) A Post-analytical Field Mapping Phase: Geologic mapping, based largely on geochemical identi fication of many samples and analysis of aerial photographs, has yielded a geologic map on which a number of faults are indicated. All identified units were mapped, corre lated, and subjected to detailed structural study. REGIONAL GEOLOGY The oldest known rocks in and underlying the Tualatin Valley region are the late Eocene to early Miocene altered volcanic rocks and marine sedimentary formations. The marine sedimentary rocks are exposed mainly in a broad arc-shaped patch which originates in the Chehalem Mountains and curves about the eastern and northern margin of the Tualatin Valley. They consist largely of sandstone and siltstone, and include undifferentiated Eocene volcanics and sediments, the upper Eocene Yamhill and Spencer forma tions, undifferentiated Oligocene marine sediments mentioned by Schlicker and Deacon (1967), and the Oligocene Scappoose formations (Trimble, 1963). The latter two, undifferentia ted Oligocene sediments and the Oligocene Scappoose formations, are probably correlative (Beaulieu, 1971). The Skamania volcanic series (late Eocene to early Miocene) were named by Felts (1939b) and described by Trimble (1963). The series may be correlative in part to Schlicker and Deacon's (1967) undifferentiated Eocene volcanics and sediments. Over much of the Tualatin Valley area these older rocks are overlain unconformably by the Columbia River basalt (Miocene in age). The Columbia River basalt is widely distributed west of the Willamette and Columbia 16 rivers. It underlies the entire Tualatin Valley in a closed basin (Hart and Newcomb, 1965), the Tualatin Moun tains, Cooper and Blue mountains, Parrett Mountain, the Chehalem Mountains, David Hills, and the Red Hills of Dundee. The Yakima Basalt Subgroup is the primary interest in this study, since the Imnaha and Picture Gorge basalts were apparently excluded from western Oregon and Washington. In the project area, the Grande Ronde Basalt and the Wanapum Basalt have been recognized. The Grande Ronde Basalt The Grande Ronde Basalt is the most voluminous, laterally extensive, and oldest formation within the Yakima basalts. It has been estimated that this formation consti tutes as much as 80 percent by volume of the Yakima Basalt (Swanson and Wright, 1978) with local thickness in the Central Plateau of up to 1200 meters (Bentley, 1977). Thickness in peripheral areas generally ranges from 150 to 500 meters, with other Yakima basalts sometimes totally absent. As many as 23 individual flows have been recog nized within the Grande Ronde Basalt in some localities. Vantage Member The Vantage Member was deposited during a period of time between flood basalt episodes that was perhaps a million years in duration. The Vantage Member is 17 represented by a weathered hiatus and sometimes presents an interbed of variable sedimentary composition. It is the most distinctive stratigraphic marker in the Western Columbia Plateau. Wanapum Basalt Four members within the Wanapum formation have been named including, from oldest to youngest, the Eckler Moun tain, Frenchman Springs, Roz·a, and Priest Rapids members (Swanson et al., 1979). Thus far, only the Frenchman Springs Member has been clearly identified west of the Cascade Mountain Range (Beeson, Moran, and Olson, 1976a) and preceded major uplift of this range since later Wanapum members were excluded except as intracanyon flows. It is the most voluminous of the Wanapum basalts in the Colum bia Plateau. Flows number as many as ten in the central Plateau (Bentley, 1977) and as many as six west of the Cascades (Beeson et al., 1976a). Large-scale deformation of the newly formed basalt surface occurred during the uplift of the Cascade Range and the subsidence of the Willamette Valley region. Local tectonism in the Portland area also produced major topo graphical features. These include the Tualatin, Chehalem, Cooper and Bull Mountain anticlinal structures, and the Tualatin Basin (Schlicker and Deacon, 1967). Beeson, Johnson, and Moran (1975) report a complexly folded and 18 faulted zone extending southeastward from, and aligned with, the Portland Hills. Major faulting of this zone trends N 35°W and includes the Portland Hills Fault, a major right lateral fault located along the eastern base of the Portland Hills. Beeson and his colleagues describe the Portland Hills as a complex anticlinal structure con sisting of two subparallel folds. As deformation of the Columbia River basalt continued, its upper surface was deeply weathered and laterized Trimble, 1963). Unconformably overlying the Columbia River basalt and filling the Willamette and Tualatin struc tural basins are thick lacustrine and fluvial sediments of lower Pliocene age (Trimble, 1963). These include the Troutdale Formation (Trimble, 1963) and the Sandy River Mudstone (Trimble, 1963). A gray basaltic rock containing nearly microscopic phenocrysts of olivine has been named by Treasher (1942) as the Boring Lava. It crops out along the western slope of the Tualatin Mountains and in the Boring Hills. Boring volcanics generally over lay the weathered Troutdale and older formations. Boring Lava and the weathered and eroded Troutdale Formation are overlain by the Portland Hills Silt. The Portland Hills Silt is largely distributed south and west of the valley of the lower Columbia River, on the uplands of northwestern Oregon, and thins away from the valley (Trimble, 1963). The Portland Hills Silt is generally 19 overlain by the Willamette Silt or younger stream allu vium. Late Quaternary geology in the region was marked by widespread deposition of lacustrine deposits. These deposits (Willamette Silt of Allison and younger alluvium) occurred in the Willamette, Tualatin, and lower Columbia River valleys (Trimble, 1963). STRATIGRAPHY The flows mapped for this study are all members of the Wanapum and underlying Grande Ronde formations of the Yakima Basalt Subgroup (Figure 3). The Grande Ronde sequence can be subdivided into two subtypes based upon both geochemistry and paleomagnetics, including in descending stratigraphic order a high MgO sequence with normal polarity and a low MgO sequence with normal polarity. The Wanapum Basalt is part of the Frenchman Springs Member with two subunits apparent within the Chehalem Mountains area. Grande Ronde Basalt The Grande Ronde formation is separated from the Wanapum formation on the basis of chemistry ( major and trace elements); the presence of an interbed between the two formations; and petrography, mainly grain size (Mackin, 1961; Swanson, 1973). Also, based on trace element content and magnetic polarity, two geochemical types within the Grande Ronde Basalt have been identified in the Chehalem Mountains area. These include the low MgO with normal polarity and the overlying high MgO with normal polarity of the Grande Ronde Basalt, such as those documented else where in the western part of the Columbia Plateau (Wright 21 $Ull ••A1 A(Of MAGNI T1( llAtlS FO.-llHHOft MUolllll 0- rLOW °"°"" GIQ.)1111 ..... ,.. IOl.A••T f LOWER MO~UMENTAL MEMBER 6 N cw l::ROSIONAL U~CONFORMITY % ICE HARBOR MEMBER Ill BASALT OF GOOS!-.: ISLAi•D 8.5 N 8..... BASALT OF MAHTlNDALE 8. 5 R ~ BASALT OF BASU CITY 8.5 N Ill "0. EROSIONAL UNCONFOPMITY 0.. ::> SADDLE BUFORD :-1E~BER R MOU.'1TAINS ELEPHANT ~OUNTAlN MEMBER 10. ~ ~~IT - BASALT EROSIO~AL UNCONFOR~ITY !'1ATTA~'A FLOW . N POMONA :-1EMBER 12 EROSIONAL UNCONFORMITY ESQUATZEL ~EMBER s EROSIO~:AL UNCONFOR."'11TY WEISSEtffELS RIDGE MEMBER :~ 0.. ~~C:al.'T' (Ii:" C:T lPPFl - .~; GR/\NDE RONDE BASALT w R: <...:> a:; 0 Lil <...:> E-- z taJ ~ Lil :x. < :-.;. u :::> U) 0..... f- < u CQ ~ ...... a:; a. tiJ " ~ ..J R . lMNAlfA T BASALT t N. ~····----- f(,, Figure 3. Stratigraphy of the Columbia River Basalt Group After Swanson, 1978. 22 et al., 1973; Nathan and Fruchter, 1974; Atlantic Richfield Hanford Company, 1976; Taylor, 1976; Beeson, Bentley, and Moran, 1976b; Beeson et al., 1976a). These two groups are megascopically as well as chemically distinct, but hand sample identification is not always reliable due to variations in appearance. A positive stratigraphic assignment can be made with trace element analysis, mainly by comparing La, Sc, and Sm. Minor elements Ce, Eu, Th, La, and Sm all tend to have concentrations that are notice ably lower in the high MgO basalts. The RMP also has been used for assigning stratigraphic position within the Grande Ronde flows. All the flows in the project area have normal polarity, including flows of both low and high MgO Grande Ronde basalt as well as the entire Wanapum section. The Grande Ronde flows in general have a well-developed lower colonnade and a thick upper entablature (Swanson, 1967). The entablature is often the dominant feature, comprising up to approximately 80 percent of the thickness of an individual flow (Diery, 1967). Low MgO Basalt The low MgO flows are the lowest unit exposed in the Chehalem Mountains area. This unit, like most of the low MgO flows, is very fine-grained to fine-grained, black to gray-black, occasionally containing scattered, clear phenocrysts of plagioclase about 1.0 to 3.0 23 millimeters in size. Low MgO flows were encountered in two wells that have been drilled in the northern and southern part of the project area. Also, low MgO flows are exposed in artificial exposures such as roadcuts and quarries. Two subsurface stratigraphic sections have been compiled from drill cuttings of Animal Farm Well and Cedar Well. Animal Farm Well Stratigraphic Section. A strati graphic section containing five flows of Grande Ronde basalt was examined. The stratigraphic section has been compiled using the drill cutting sequences of the basalt. The well site is located within the Laurelwood quadrangle (Sec. 23, T.ls, R.3W) in the southwest Tualatin Valley (Figure 2, p. 5). The well site excavation is approximately 57 meters. The total depth of the well is 135 meters. The boring penetrated through the basalt. The basalt has been weathered to a depth of several meters resulting in a soft, clayey sandy silt. The unweathered basalt was encountered below 18 meters. A summary of composite sample descriptions is as follows (Paul Hughes, Personal communication) : 0-12 m clay and sand 12-18 m weathered basalt 18-113 m basalt 113-135 m sandstone and shale The study had been done on the basalt (not including the 24 weathered basalt) with a total thickness of 95 meters. The basalt samples were collected every 3 meters. After cleaning the samples and separating the large chips (pebble size) from the small chips (granule size), the large chips were used for lithologic description and thin section purposes, whereas some of the small chips were used for the chemical analysis. Based on lithologic, petrographic, and geochemical data, five flows have been distinguished within the Animal Farm Well stratigraphic section. Generally, all five flows are similar in lithologic and petrographic characters. The fresh basalt chips that were obtained from the drill cuttings are very hard, can be broken off only with heavy blows of the geology hammer, and are dark to gray in color. All are nearly non-porphyritic but a few have tiny pheno crysts of small, elongate plagioclase 0.5 to 2.0 millimeters in length, visible with a hand lens. Abundant vesicles occur near the top or bottom of the flows, but slightly vesicular zones may occur in between. Some of the vesicles have glassy rims. The upper flows of this stratigraphic section tend to be lighter colored basalt. Also, they are slightly coarser in grain size and less dense than flows beneath. Their texture is slightly open, while the lower flows tend to be more dense and darker in color, maybe due to a higher glass content. The weathered basalt chips, 25 particularly at the flows' boundaries, are medium soft to very hard, green to blue-gray and cannot be crumbled between the fingers, but can easily be crushed with light blows of the geology hammer. In many of the samples, a thick coating of iron oxide mineral stained interstices, vesicles and diktytaxitic cavities, and replaced glass. All flows, as mentioned above, are similar in mineral composition. Plagioclase is the dominant mineral in nearly every flow, comprising about 45 percent of most rocks and about 50 percent of all crystals. Microphenocrysts of plagioclase occur sporadically in many thin sections. The flows also contain about 35 percent clinopyroxene, 10 percent opaque minerals (principally magnetite and minor amounts of ilmenite), and 5 percent mineraloids and clay minerals. The ground.mass is composed of fine grained plagioclase, augite, opaque minerals, and glass or its clay alteration products. The glass contains micro lites of plagioclase and magnetite. It grades from nearly opaque into clear, light to medium-brown glass. Texture varies with the glass content. Samples showed subophitic or hyalophitic texture, but generally they showed intersertal-intergranular texture. Some of the thin sections contain pools of glass as much as 1 millimeter in diameter scattered irregularly throughout intersertal-intergranular texture. Because the Animal Farm Well section has been compiled from drill cuttings beneath the surface, the section lacks 26 observable geometric form of its flaw's jointing system. Instrumental Neutron Activation Analysis was performed on ten selected samples from the Animal Farm Well strati graphic section to confirm the expected number of flows and their boundaries within the stratigraphic section, and to determine the basalt type. The ten samples used for the analysis are listed below in order of decreasing elevation. Basalt Samples Selected at Elevation Intervals 17 AFWL (22.5)-(19.5) m. 16 AFWL (16.5)-(13.5) m. 18 AFWL (10.5)-( 7.5) m. 19 AFWL ( 4.5)-( 1. 5) m. 20 AFWL (-1. 5)- (-4. 5) m. 21 AFWL (-7.5)-(-10.5) m. 22 AFWL (-19.5)-(-22.5) m. 23 AFWL (-34.5)-(-37.5) m. 24 AFWL (-46.5)-(-49.5) m. 25 AFWL (-49.5)-(-52.5) m. Based on trace element concentration, particularly the Sm, Sc, and La, the basalt type was distinguished as Grande Ronde basalt. The number of flows are five, including two flows of high MgO on the top of the stratigraphic section and three flows of low MgO below. Their boundaries have been confirmed as well (Figure 4). These basalt flows will be referred to as number (-1) to (-5) from top to bottom. 27 I:·=-= .¥: ... +J HYDROLOGICt Jl ,!i ·'"4 LITHOLOGY UNIT DESCRIPTION ,.....J:l :::>c DATA I I I 1 ~ RECENT j Sand, clay, and silt I ! J ..... i i ~.=.=:·:: I I -· - HIGH MgO CHEMICAL TYPE ' ! I 04 -l :::> I 21 ~· ~ \ i l? 1 i I I ~ t CJ) ...... Two aphyric flows, aquifer -~ iI 8 ...... ~ .•~· ~ =.. . ~ gray to dark gray, dense 10 qpm l ~ H C:i slightly open texture i <~ cs ! >t L I 1 isl I : t I I aquifer I ~ I 04 I I !" ::· ;.,-: ~F'i I 5 gpm ! 8 J a; ~ I ~: l? 0 z 18 I fi 8 ~ -~ LOW Mgo CHEMICAL TYPE i I ~ I ' I 8H CJ) X! ' I i I ..... I 21 -s •, ' ? ? MARINE SEDIMENTARY bvd I ROCKS Figure 4. Animal Farm Well stratigraphic section in Chehalem MoWl tains area. 28 Locally the lowermost low MgO flow (-5) on the strati graphic section is at least 21 meters in thickness. It may be separated from the low MgO flow above it by an aquifer. (Hydrologic data provided by Paul Hughes in a personal communication.) Thickness of the second flow up (-4) is about 24 meters. This flow is similar to the flow above it (18 meters) in lithology, but despite the obvious similarities of one flow to the next and the demon strable lateral variations in many of the flow features, certain flows (-3 and -4) possess distinctive features that are useful to regional correlation. These flows contain rare, clear plagioclase phenocrysts less than 5 millimeters in length and tend to be very fine-grained, while the low MgO flow (-5) below contains tiny micro phenocrysts less than 2.0 millimeters in size and tends to be fine-grained. Cedar Well Stratigraphic Section. A subsurface stratigraphic section has been compiled using the drill cutting sequences of the basalt in Cedar Well. The well site is located within the Newberg 7~ Quadrangle (Sec. 11, T.3S, R.2W) at the southern part of the Chehalem Mountains (Figure 2, p. 5). The well site elevation is approximately 108 meters. The total depth of the well is 150 meters. The boring penetrated into the basalt. The basalt has been weathered to a depth of about 24 meters, resulting in a soft clayey sandy silt. The unweathered basalt was 29 encountered below 24 meters. The following is a summary of composite sample descriptions: 0-15 m. clay and sand 15-24.5 m. weathered basalt 24.5-150 m. unweathered basalt The weathered basalt was encountered from 15 meters to 24 meters, in which relict basalt texture may be observed. However, the weathered basalt has been excluded from the study because of difficulties in recognizing its lithologic and petrographic characters. The study has been conducted on the unweathered basalt encountered below 24 meters with a total thickness of 126 meters. The basalt chips obtained from Cedar Well at 3 meter intervals are smaller than those obtained from Animal Farm Well. However, these small basalt chips were cleaned, in a similar way to that of Animal Farm Well's chips. The smaller chips were used for the chemical analysis purposes; thin sections were made of the larger chips. The upper 3 to 4.5 meters of the stratigraphic section are highly weathered. The chips are medium soft and easy to break. Their weathered color is pale reddish to pale yellowish; some are pinkish-gray or brownish-black. The less weathered chips are generally gray in color. Vesicles are stained with a thick coating of iron oxide mineral. This weathered zone exists also at each flow boundary. Vesicles are not necessarily confined to the 30 top of the flows, but occur within the columnar zone. Nichols (1936) interpreted layers of similar lava, separated by vesicular zones without sedimentary interbeds or weathering features, as successive outpourings from the front of a partly consolidated flow; he termed them flow units. The fresh basalt chips are very hard, dense, very fine-grained to fine-grained, and grayish-black to black. A few basalt chips in the upper flows have small phenocrysts or plagioclase about 5 millimeters in size. The lower flows have tiny microphenocrysts less than 2 millimeters in size. Some of the fresh basalt chips contain vesicles with glassy rims. These glassy rims were seemingly either chilled during adiabetic expansion of escaping gases or froze owing to increased viscosity upon loss of volatiles (Swanson, 1967). These are the lithologic characteristics that can be obtained from the drill cuttings. Other physi cal characters, such as joint systems, which serve many purposes in identifying the basalt type, are obscured because the section is a subsurface section. Eighteen thin sections of basalt from the Animal Farm Well stratigraphic section and Cedar Well stratigraphic section were studied and showed that the petrographic characteristics are nearly similar. These results were in substantial agreement with conclusions reached by Waters (1961) and Swanson (1967). Plagioclase close to An , 50 31 optically positive clinopyroxene, glass, and ilmenite magnetite are the major constituents of most of the basalt samples. Instrumental Neutron Activation Analysis was performed on ten selected samples from the Cedar Well stratigraphic section (Figure 5) to determine the basalt type and to distinguish the flows' boundaries within the section. Based on the results of the chemical analysis, the basalt type was distinguished as low MgO of Grande Ronde basalt and the number of flows is four. The lowermost of the two low MgO flows are at least 63 meters in thickness. These flows contain tiny microphenocrysts about 1 to 2 millimeters in size and tend to be fine-grained. Similarity exists between these flows and the low MgO flow (-5) in the Animal Farm Well stratigraphic section, particularly in hand specimen appearance. Also, similarity in hand specimen appearance exists between the upper two flows (69 meters) in Cedar Well stratigraphic section and flows (-3 and -4) in the Animal Farm Well stratigraphic section. Low MgO flows similar to the flows within the strati graphic sections are commonly well-displayed in quarries and roadcuts exclusively along the western edge of the Chehalem Mountains (Figure 6), forming cliffs in some places. Most of these flows are not exposed in contact nor are they seen completely from top to bottom, due to erosion and cover. These flows are not seen in sequence, 32 ~ 0 u ..... fl LITHOLOGY UNIT DESCRIPTION ~ ~ -'V) RECENT Sand, clay, and silt t I i ., - I f, I I LOW MgO CHEMICAL TYPE I ' \ -.. - I ,_. - I ; 30 Two aphyric flows with small phenocrysts very fine grained j ~ I j I txl lz - I I (ii ·...J 1!~ c c H :E Two aphyric flows with tiny microphenocrysts fine grained 17. 5 ... ? ..... ? -- ?-...... Figure 5. Cedar Well stratigraphic section in Chehalem Mountains area. (b) Figure 6. (a) Irregular blocks at the top of low MgO flow at Jaquith Road; (b) Low MgO flow exposed along Bald Peak Road near Bald Peak Park . 34 therefore the actual number of flows is unknown. The Yakima basalt flows in the Chehalem Mountains area show some of the primary structural features described in flood basalts elsewhere. It is not my intent to specu late about the origin of these features--interpretations may be found in papers by James (1920), Fuller (1931a), Tomkeieff (1940), Waters (1941, 1960), Mackin (1961), Spry (1962), and Swanson (1967). My intention is rather to describe them and indicate their usefulness in flow correlation. All the low MgO flows exposed in this area possessed normal magnetic polarity. About 10 meters of a flow is ex posed in Bald Peak Quarry (Figure 7) which is located in the Laurelwood Quadrangle (Sec. 26, T.2S, R.3W). The lower contact is obscured. The flow has a relatively thin, but well-formed colonnade of quite variable thickness. The columns are stained with a reddish-brown weathering color. The bottoms of the columns have a platy swirling pattern in places (Figure 8), "suggesting that directional stresses were set up in still viscous lava by a surge of movement after partial consolidation of the colonnade" (Atlantic Richfield Hanford Company, 1976). The entablature is non-existent. If there was an entablature, it has eroded away or been buried; the lack of entablature is due to erosion and lack of exposure rather than a flow 35 (a) (b) Figure 7. Low MgO flow showing cubic (a) and columnar (b) jointing, Bald Peak Quarry, at Bald Peak Road. 36 (a) (b) Figure 8. (a) Platy swirling pattern within the columns ' bottom of a low MgO flow; (b) General view looking east ~oward Cooper Mountain. 37 characteristic. Blocky jointing (about 1 meter) overlies the colonnade with a vesicular zone marking their tops (Figure 6a). This flow is covered by vegetation and talus from above. The high MgO basalt is similar in hand specimen appearance to the low MgO flows except that it is usually coarser grained. Therefore, it can be confused with low MgO flows if stratigraphic context is not considered. The grain size of the high MgO basalt is about 0.5 milli meter. Grains are dark gray, weathering to a speckled orange and light gray. In the Animal Farm Well stratigraphic section (Figure 4, p. 27) the upper two flows, above the low MgO flows, were identified as high MgO flows. The lower flow (-2) is about 18 meters thick. This flow is generally coarser textured than the low MgO flow below. Although it may be locally fine-grained, a thick vesicular zone marks its top. The thickness of the flow is approximately 21 meters. This flow is separated from the flow below (-2) by an aquifer, at least locally. The high MgO flows in the study area are poorly exposed (Figure 9a). They possess a normal polarity. These flows display a recognizably different jointing sequence from that seen in the underlying flows. The upper flow (-2) exposed in contact with the Vantage horizon near Highway 219, Sec. 32, T.2S, R.2W. The colonnade has 38 (a) (b) Figure 9 . (a) Deeply weathered surface of high MgO flow overlain by silt; (b) Vantage horizon nearby Highway 219 . 39 irregular, widely spaced blocky joints resembling that of Frenchman Springs more closely than it does that of the low MgO Grande Ronde. The high MgO flows are widely distributed in the area and are usually distinguishable by one or more of the techniques employed in this study. Generally speaking, the distribution of Grande Ronde subunits is remarkably uniform across the project area. Vantage Member Separating the Grande Ronde flows from the Frenchman Springs flows is the laterized pebbly sand and clay sedi mentary interbed defined in this project as being correla tive to the Vantage Sandstone of central Washington. This interbed represents a period of non-volcanic activity throughout the area covered by the Columbia River basalt and period of deposition in the Chehalem Mountain area. This important stratigraphic horizon is indicated on the geologic map only as the contact between the high MgO flows and the Frenchman Springs flows. Exposed poorly in the area, this unit has a maximum thickness of about one meter (Figure 9b). It is not always clearly detectable and is sometimes missing entirely. Aside from being a good stratigraphic marker, it is impor tant to the local area as a slipface over which the overlying units (Frenchman Springs Member) have long been subjected to downslope movement. 40 Frenchman Springs Member of Wanapum Basalt Frenchman Springs flows are poorly exposed in most of the Chehalem Mountains area (Figure 10). These flows possess normal paleomagnetic polarity as determined by a portable fluxgate magnetometer. Although Frenchman Springs flows are often described as megascopically distinct from the high MgO Grande Ronde flows due to the phyric nature of the former, some Frenchman Springs flows are aphyric or rarely phyric. However,.aphyric Frenchman Springs flows were not found in the study area. The phyric flows of the Frenchman Springs basalt represent the only flows in the area that can be mapped with certainty in the field without corroborative laboratory analysis, due to their distinctive lithologic, petrographic, and physical characteristics. All are coarse-grained, from 0.4 to 2.0 millimeters, and often have large pheno crysts and glomerocrysts of plagioclase in open and light matrix that is generally coarser than underlying Grande Ronde flows, and they are commonly blocky. The phyric flows of the Wanapum Basalt were observed in numerous localities in the study area. All contacts except the Vantage horizon (in some places) at the base of the member are buried. The total number of the Frenchman Springs flows and their thickness was not found. Those flows that were found lacked geometric perfection because of the limited 41 Figure 10. Deeply weathered surface of Frenchman Springs flow overlain by silt. 42 exposures. These flows tend to form a rounded hill line behind the prominent bench. Erosion has reached to a depth near or below Vantage in many localities. This erosion has resulted in the complete or partial removal of the Frenchman Springs unit in the area. Frenchman Springs flows are not geochemically distinc tive on a flow-by-flow basis. Two distinctive Frenchman Springs flows of the Frenchman Springs Member of the Wanapum Basalt can be identified in the study area, overlying the Grande Ronde and Vantage. From the standpoint of the proposed plateau termino logy (Bentley, 1977), the lower flow type (+l) resembles the Ginkgo type, having relatively abundant phenocrysts and microphenocrysts of plagioclase. In areas of abundant plagioclase, larger phenocrysts could be found in every sample broken off the outcrop and hand-sized samples often had two or more phenocrysts. The texture is coarse and the color is very dark gray. This unit has poorly developed basal columnar joints forming blocks of about one meter maximum in diameter. Thickness of the flow is unknown. The lower flow type is easily distinguished from the Grande Ronde flows since it has abundant large plagio clase phenocrysts in a glomero porphyritic texture as well as microphenocrysts of plagioclase. There are rare local spots where phenocrysts are hard to find, making 43 identification of the outcrop more difficult. The dark gray matrix is generally coarser than Grande Ronde basalt but there are local spots where it is fine-grained, making correlation more difficult than if the unit had consistent characteristics. The upper flow type (+2) resembles the Kelly Hollow type, lacking the very abundant large phenocrysts of the Ginkgo. The lower flow type is not as widely distributed as the Kelly Hollow type and is not mapped as a single flow on the geologic map, but is mapped with the Kelly Hollow type as Frenchman Springs basalt where both are present. All Frenchman Springs flows are easily distinguished from the Grande Ronde flows with trace element analysis. In the Frenchman Springs flows Fe content is greater than 10 percent and the Na/Sm ratio is less than 0.33. A geochemical horizon marked primarily by a sharp increase in the concentration of MgO was recognized in the upper part of the Grande Ronde Basalt sequence by Wright, Grolier, and Swanson (1973). This geochemical horizon has been recognized over much of the western Columbia Plateau. The chemical break within the Grande Ronde basalts of the Chehalem Mountains area has been shown by both trace element (Table I) and major oxide (Table II) concentrations. Increases occur in MgO, Al o , and Cao while Sio , 2 3 2 FeO, Na o, K o, Ti0 , and P o decreases. MnO is the 2 2 2 2 5 44 TABLE II COMPARISON OF CHEHALEM MOUNTAINS MAJOR OXIDE CONCENTRATIONS WITH OTHER AREAS FOR HIGH MgO/LOW MgO GEOCHEMICAL TYPES Chehalem Mountains*** C.R. B. Averages* C.R.B.Averages** H. Mg~/ L. Mgolf H. Mg:O L. Mg:O H. MgO L. MgO Si0 53.89 55.15 54.09 54.98 53.78 55.94 2 Al o 14.96 14.82 14.33 13.76 14.45 14.04 2 3 FeO 11.06 11. 55 11. 42 12.30 11. 35 11.77 MgO 4.49 3.56 4.90 3.38 5.25 3.36 cao 8.48 7.05 8.72 7.11 9.07 6.88 Na o 2.59 2.84 2.84 3.27 2.83 3.14 2 K o 1. 33 1.70 1.17 1. 78 1. 99 2 1. 05 Ti0 1.99 2.06 1.78 2.20 2 1. 78 2.27 P205 0.31 .32 .31 .39 .28 .43 MnO 0.21 .20 .20 .21 .19 .19 'Fll Number of flows averaged * Wright et al. (1974) ** Swanson et al. (1979) *** Peter Hooper, 1980, Wash. St. Univ. Analyst 45 only major oxide among the ten major oxides determined that shows little or no change across this geochemical break. The significance of these changes is that the average values of all low MgO flows and all high MgO flows of the Chehalem Mountains area and the regional averages of Wright, Grolier, and Swanson (1973) and Swanson et al. (1979) (Table II) are closely comparable and the ele mental variation between chemical types is similar in every case. In many instances it was only possible to obtain weathered samples. Since weathering affects the trace element composition, correlations were more difficult. However, except for the highly weathered samples, corre lation was usually possible with less confidence. A compositional distinction between Frenchman Springs flows of the Wanapum Formation, high MgO type and low MgO type of the Grande Ronde Formation, is shown in a covariance diagram (Figure 11) of the element pair La-Sm. Several general observations can be made regarding the data: (1) clustering of analyses from dominant chemical types of basalt of the Yakima Basalt Subgroup is common, (2) there is a clear distinction between flow groups, and (3) not all of the elements were found useful in dis tinguishing between flow groups (Table I, p. 12). The values of some elements such as Na overlapped (poor distinction) in the flow groups. 46 13 1 11 10 e 9 e Sm x 8 x • EXPLANATION: • • FRENCHMAN SP. -A • A HIGH- Mg - x 7 • A LOW-M9 - . 4 x WEATH.SAMPLES -0 • 6 • •• x •• x • • x • • x • x 5 • 14 18 22 6 3 3 38 A2 A6 So 54 58 La Figure 11. Covariance diagram for La-Sm. 47 For this work, Sm and Fe had higher values for French man Springs flows, while the values of the same elements for the low MgO flows and high MgO flows overlapped. High MgO flows could be separated out as low in Sm, La, Th, and Ce, and high in Sc and Cr. Of these elements, Th, La, Sm, and Sc proved to be most significant considering relative error. Low MgO flows were found to be depleted in Sc and enriched in Th relative to the other two groups. The highly weathered samples were found to be enriched in rare earth elements and minor elements as well. The composition of these samples is as expected from normal surface weathering. For example, Na was depleted and Fe was enriched. Other elements such as Sc had no signif i cant change in their concentrations, in both fresh and weathered samples. Plots of quantities of reliable elements and ratios of elements show grouping of samples that agree with previ ously obtained results for these groups in other areas (Figure 12, and Tables III and IV, West Linn, Clackamas River, and Chehalem Mountains). Tables III and IV contain a list of averaged concentrations for trace elements that are most diagnostic of the Frenchman Springs geochemical type and the high MgO/low MgO geochemical horizon. There seems to be a systematic difference between the data from the project area and data from previous ones. For example, La values for high MgO are close to 4 HIGH-MG LOW-MG • CD370• 400 GR1 CD;uo 0 • • HC5 Ff14 Mj1 Na MH • • OHGj • Sm • CR~oo • PR1 I e 35o • I PB5 _ _I_ • JRI e••fRQ9 • J( LEGEND: + • - Chehalem Mt. 0 e -Clackamas R. •-West Linn 250 •- Sentinal Gap x - Sand Hollow FRENCHMAN SPRINGS • +-Ginkgo • ___ L__ .450 .500 -550 -600 .sso 100 750 .aoo ·850 .!.!. Sc Figure 12. Distribution of samples into chemical groups on the basis of Na La Sm vs. sc· Values are from Tables III and IV. .a::. Q) 49 TABLE III FRENCHMAN SPRINGS MEMBER - GEOCHEMICAL COMPARISON OF FLOWS AT CHEHALEM MOUNTAINS AREA WITH FLOWS FROM THE TYPE LOCALITIES AND THE CLACKAMAS RIVER AREA Clackamas Sentinal Sand River* Gap** Hollow** Ginkgo** Chehalem (Three Lynx (type (type (type Mountains section) locality) locality) locality) (section) Na 2.03 1. 93 1. 99 1. 93 2.08 La 25.6 28.2 26.7 28.6 24.2 Sm 7.4 7.9 6.4 6.6 6.8 Fe 10.8 11. 0 11.1 11.1 10.27 Ce 53.0 N.D. N.D. N.D . 46.5 Lu . 64 N.D. N.D. N.D. • 91 Th 5.5 4.0 4.5 3.7 3.3 Sc 37.2 34.8 36.0 34.4 35.25 "!}_/ ii .!/ .!/ .!./ ]._/ * Anderson (1978) ** Data from A.R.H.c.o. (1976) n/ Number of flows or flow units averaged N.D. Not determined TABLE IV GRANDE RONDE BASALT: HIGH-MG AND LOW-MG GEOCHEMICAL TYPES - GEOCHEMICAL COMPARISON OF FLOWS AT CHEHALEM MOUNTAINS WITH THOSE FROM OTHER AREAS Clackamas River*** West Linn** Chehalem (Three Lynx Section) (Portland) Tygh Ridge* Mountains H.-Mg L.-Mg H.-Mg L.-Mg H.-Mg L.-Mg H.-MS{ L.-Mg Sc 38 34 37 33 37.3 31.0 40 34 Th 3.8 7.8 2.9 4.8 3.35 5.6 3.9 4.8 Ba 800 900 N.D. N.D. 533 607 660 700 La 18.5 26.5 20.3 25.4 21. 4 27.2 21. 9 26. 9 Sm 5.4 6.6 5.2 6.1 6.5 7.5 5.6 6.5 Ce 40 50 35 48 N.D. N.D. 40 46 Na 2.07 2.35 2.19 2.27 N.D. N.D. 2.10 2.38 n/ '!:_/ !/ '!:_/ ]j §_/ J_I '!:_/ }_/ * Nathan and Fruchter (1974) values from BCR-1 ** Beeson et al. (1976) *** Anderson ( 197 8) Ul !!_/ Number of flows or flow units averaged 0 N.D. Not determined 51 22 ppm whereas the PEG project (Beeson, Johnson, and Moran, 1975) reported values around 20 ppm. For Frenchman Springs, La values are around 24 ppm, compared with 25 ppm in the PEG report. In Figure 12, the Frenchman Springs type localities in eastern Oregon (Table III, Ginkgo, Sentinal Gap, Sand Hollow) have values that do not agree with those from Frenchman Springs in western Oregon. STRUCTURE Detailed study of the structure of the Columbia River basalt in the Chehalem Mountains area .is hampered by lack of readily measurable bedding attitudes. Exposures of flow sur faces and interbeds are relatively scarce. Exposures in some areas are obscured. Previous structure studies of the area include Wells and Peck (1961), Bromery and Snavely (1964), Hart and Newcomb (1965), and Schlicker and Deacon (1967). In the present study, the overall structure of the Columbia River basalt in the Chehalem Mountains area was interpreted by measuring attitudes and by determining stratigraphic position of exposed flows. Columnar joints are conunonly well-displayed in artificial exposures such as quarries and in roadcuts. As these joint columns are primary structures formed perpendicular to cooling surfaces, planes normal to column axes may be taken as geometric approximations of flow surfaces. The Columbia River basalt flows spread widely over fairly gentle slopes, and flow surfaces were presumably nearly horizontal at least locally. Undulatory contacts made attitudes difficult to measure, especially where 0 dips were less than s • The problem of irregular flow surfaces is avoided by considering only continuous exposures of reasonable length and uniform column orientations. 53 Exposed faults also have been studied, and air photos and topographic maps observed for lineations and topographic expression that may indicate structures, particularly where the evidence for such structures seems at least equivocal. Strike and dip symbols on the geologic map (Figure 13) represent outcrops. These outcrops reflect post-depositional deformation more than the topography that existed at the time of deposition. The Chehalem Mountains, which stand about 430 meters higher than the Tualatin Valley to the east, are the physio graphic expression of a complex northwest trending anticline, but that has been modified by subsequent faulting and erosion so that its present principal structure is obscure. The east limb and crest of the structure are adequately defined, but the west flank may represent a normal fold limb that has been eroded and covered by sediments or it may represent a fault as it has been mapped previously by Peck (1961). The alternative hypotheses may be evaluated on the bases of structural data, geomorphic features, and seismic records. The latter is not included within the investigation methods here. Several physiographic features indicative of faulting along the western edge of the Chehalem Mountains are appar ent on the Laurelwood, Dundee, and Newberg 7~' topographic quadrangle maps and on aerial photographs. The western part of the Chehalem Mountains is markedly linear. A 54 straight edge can be placed along the break in slope on the map for as much as eight kilometers. Segments of the line everywhere trend within 7° of N.42W. The break in slope lies within about 90 meters of a line drawn from the intersection of Mountain Home Road and Bell Road, to the Bald Peak Road at Bald Peak Park. Spur ridges extending northwest exhibit notable uniformity. This uniformity may be accounted for in part by the uniform lithology and resistance to erosion of the Columbia River basalt which forms them. These physiographic features suggest that the western front of Chehalem Mountains is a fault-line escarpment. The possibility that the western limb of the fold has been eroded and buried in the Gaston area cannot be eliminated, owing to the lack of attitudes in the Columbia River basalt under younger cover and owing to landslides that occurred locally in areas of steep topography underlain by marine sedimentary rocks. Preference must be given to faulting as the explanation, however, on the basis of structural and geomorphic evidence. This faulting probably took place sometime after the deposition of the Columbia River basalt in the project area. The primary structural framework in the Chehalem Mountains is expressed by the northwest trending anticline ridge dipping generally eastward into the Tualatin Valley. Dips near the margins are low, commonly a few degrees, 55 although dips as high as 12° have been measured along the east flank of the Chehalem Mountains. The decrease in dip downslope, however, was probably progressive and undoubtedly the true dip is less than 6°. The beds dip into the valley in most instances, and locally flows dip steeply southwest into the Newberg Valley. To the east and west of the Chehalem Mountains are the TUalatin basin syncline and Newberg and Wilsonville valleys, respectively. These synclines are the result of broad regional downwarping that has produced synclinal structures. Newberg and Wilsonville valleys plunge south erly into the structural basin of the Willamette Valley. The major known fault, with a possible 80 kilometers northeast-southwest trend, separates Parrett Mountain from the east end of the Chehalem Mountains (Schlicker and Deacon, 1967). Another major known fault displacement includes the northwest trending fault followed by Gales Creek. This fault is located outside the project area, therefore it has not been shown in the geologic map. The relation between Gales Creek Fault and the fault bordering the west edge of the Chehalem Mountains is still unclear because of the lack of conclusive physical evidence. Effort has failed to relate these faults to each other. Most of the major bedrock structures of tectonic 4Jrigin in the project area are due to folding, though some of the adjacent structures are largely due to 56 displacement, such as Parrett Mountain. The earth stresses that produced the major structures undoubtedly produced many minor displacements that are not detectible. GEOLOGIC HISTORY During the Oligocene, the study area was submerged beneath a sea. Volcanic sediments were being transported and deposited. The region was elevated long enough for considerable erosion to occur before deposition again took place (Warren and Norbisrath, 1946). In middle and late Miocene time, floods of basaltic lava poured from fissure vents into northwestern and north-central Oregon. Lavas of this stage of volcanism flowed into the map area, reaching a thickness of up to 140 meters and covering an erosional topography that had developed on the Oligocene marine sedimentary rocks. The low MgO Grande Ronde sequence was the first to enter the area, coming from the northeast and burying the deformed marine sediments. The rest of the Grande Ronde section is represented in the area by two flows of high MgO. A change in chemistry of these flows strongly suggested a change in the source producing the high MgO flows that next invaded the region. After the high MgO flows transgressed the area, a period of non-deposition of up to one million years occurred (Beeson, Personal communication). During this period, the top of the uppermost high MgO Grande Ronde was weathered extensively. Soils formed and trees grew. 58 This period of deposition is known as Vantage horizon and is estimated as occurring about 15.5 to 14.5 mybp (Beeson, Personal communication}. The Vantage Member represents what is probably the longest single interf low period within the Grande Ronde and Wanapum basalts (Mackin, 1961). The first of the Frenchman Springs flows flowed in over the topography created druing Vantage time. After the last flow of Frenchman Springs member a period of erosion occurred. The drainage pattern was reestablished as numerous steep-sided canyons were cut into the basalt and a deep lateritic soil was deposited. On canyon walls erosion stripped the overlying silt and weathered basalt, exposing hard unweathered. The units were folded in a northwest trending struc ture developed originally as broad anticline that has been modified by faulting and erosion (Schlicker and Deacon, 1967). Most of the structural relief, however, probably developed after the deposition of the Columbia River basalt in this area. A large normal fault that trends in a northwesterly direction borders the west edge of the Chehalem Mountains (Wells and Peck, 1961; Bromery and Snavely, 1964; Schlicker and Deacon, 1967). This fault may have occurred while folding was going on. CONCLUSION The mapping of the Chehalem Mountains area was accom plished by means of lithology, physical characteristics, paleomagnetism, trace elements, and major elements. In most cases geochemical analysis provides positive identi fication of the Columbia River basalt flows present in the project area, and was used to identify the Grande Ronde flows and the Frenchman Springs Member of the Wanapum formation. These two major units represent the Yakima Basalt Subgroup in the thesis area. Within the Grande Ronde Basalt, there are three to four low MgO flows, the uppermost having small phenocrysts. Two high MgO flows are poorly exposed. The most complete sec tion in the study area was compiled from drill cuttings containing five flows of Grande Ronde basalt. Within the Wanapum Basalt two flows of the Frenchman Springs Member are exposed. However, exposures of these flows are rela tively scarce. The lower flow resembles the Ginkgo type; the upper flow is more closely similar to the Kelly Hollow type. The flows of the Grande Ronde Basalt are widely dis tributed and uniform across the project area. The flows of the Frenchman Springs Basalt are not widely distributed, are not uniform, and are rarely seen in contact. There is no significant chemical difference between the flows in the 60 Portland and Clackamas River areas and the Chehalem Moun tains area. In many instances it was only possible to obtain weathered samples and, since weathering affects the trace element composition, correlation was more difficult. How ever, except for the highly weathered samples, correlation was usually possible with less confidence. The area must have been a topographic low when the Yakima Basalt accumulated. The structural relief probably developed after the deposition of the Columbia River basalt in the project area. The structural character of the area is dominated by a broad anticline that may be cut by a large normal fault which trends in a northwesterly direction bordering the west front of the Chehalem Mountains. Lack of certainty could be explained by erosion and burial by younger cover. This fault would probably connect with the fault exposed along Gales Creek, to form a break approximately 45 kilo meters long. Another major fault is a cross fault separating the Chehalem Mountains from Parrett Mountain. The general dip is to the northeast, where the east front of the Chehalem Mountains dips gently. A local steep dip has been measured on the southwest flank of the fold, dipping into the Newberg Valley. REFERENCES CITED Allen, J. E., 1975, Volcanoes of the Portland area, Oregon: Ore. Bin, v. 37, no. 9, p. 145-157. Allison, I. s., 1933, New version of the Spokane flood: Geol. Soc. America Bull., v. 44, no. 4, p. 675-722. 1935, Glacial erratics in the Willamette Valley: Geol. Soc. America Bull., v. 46, no. 4, p. 615-632. 1936, Pleistocene alluvial stages in northeastern Oregon: Science, v. 83, no. 2158, p. 441-443. Anderson, J. L., 1978, The structure and stratigraphy of the Columbia River Basin in the Clackamas River drainage: Portland State University, Oregon, M.S. thesis, 136 p. Atlantic Richfield Hanford Company, 1976, Preliminary Feasibility Study of Storage of Radioactive Wastes in Columbia River Basalts: ERDA Rept. ARH-ST-137, v. 1, (text, 168 p.) and v. 2 (appendices, 264 p.). Beaulieu, J. D., 1971, Geologic formations of western Oregon: Oregon State Dept. Geol. Min. Inds. Bull., v. 70, 71 p. Beeson, M. H., Bentley, R. D., and Moran, M. R., 1976b, Preliminary correlation of Lower Yakima Basalt flows in western Oregon with the type area in central Washington: Geol. Soc. America Abs. with programs, v. 8, no. 3, p. 355. , Johnson, A. G., and Moran, M. R., 1975, Portland ---Environmental Geology--Fault Identification: Portland State University, Oregon, 107 p. , and Moran, M. R., 1979, Columbia River Basalt ---Group stratigraphy in western Oregon: Oregon Geol., v. 41, no. 1, p. 11-14. , Moran, M. R., and Olson, F. L., 1976a, Geochemical ---data on Columbia River basalt stratigraphy in western Oregon: Geol. Socl. America Abs. with programs, v. 8, no. 3, p. 354. 62 Bentley, R. D., 1977, Stratigraphy of the Yakima basalts and structural evolution of the Yakima Ridges in the western Columbia Plateau, in Brown, E. H., and Ellis, R. C., Geologic excursions in the Pacific Northwest: Bellingham Dept. of Geol., W. Wash. Univ., p. 334-389. Bingham, J. w., and Walters, K. L., 1965, Stratigraphy of the upper part of the Yakima Basalt in Whitman and eastern Franklin counties, Washington: U.S. Geol. Survey Prof. Paper 525-C, p. C87-C90. Bretz, J. H., 1925, The Spokane flood beyond the channeled scablands: Jour. Geology, v. 33, no. 2, p. 97-115 and 236-259. 1928, Alternate hypothesis for channeled scabland: Jour. Geology, v. 36, no. 3, p. 193-223 and 312-341. Bromery, R. w., and Snavely, P. D., Jr., 1964, Geological interpretation of reconnaissance gravity and aero magnetic surveys in northwest Oregon: U.S. Geol. Survey Bull. 1181-N. Campbell, c. D., and Runcorn, s. K., 1956, Magnetization of the Columbia River basalts in Washington and northern Oregon: Jour. Geophys. Res., v. 61, p. 449-458. Diery, H. D., 1967, Stratigraphy and structure of Yakima Canyon between Roza gap and Kittitas Valleys, Central Washington: University of Washington, Seattle, Ph.D. thesis, 116 p. ~~~' and McKee, B., 1969, Stratigraphy of the Yakima Basalt in the type area: Northwest Science, v. 43, no. 2 , p. 4 7-6 4. Doell, R. R., and Cox, A., 1962, Determination of the magnetic polarity of rock samples in the field: U.S. Geo!. Survey Prof. Paper 450-D, p. 105-108. Felts, W. M., 1939b, Keechelus undesitic lava flows of Washington in southward extension: Pan-Am. Geologist, v. 71, no. 4, p. 294-296. Fuller, R. E., 1931a, The aqueous chilling of basalting lava on the Columbia River plateau: Am. Jour. Sci., v. 21, p. 281-300. 1950, Structural features in the ColunIDia River Basalt: Northwest Sci., v. 24, no. 2, p. 65-73. 63 Gast, P. W., 1968, Trace element fractionation and the origin of tholeiitic and alkaline magmatypes: Geochim. Cosmochim. Acta, v. 32, p. 1057-1086. Glenn, J. L., 1965, Late Quaternary sedimentation and geologic history of the north Willamette Valley, Oregon: Oregon State University, Ph~D. thesis, 231 p. Gordon, G. E., Randle, K., Gales, G. G., Corliss, J.B., Beeson, M. H., and Oxley, s. S., 1968, Instrumental activation analysis of standard rocks with high resolution x-ray detectors: Geochim. Cosmochim. Acta, v. 32, p. 369-396. Gray, Jane, and Kittleman, L. R., 1967, Geochronometry of the Columbia River Basalt and associated floras of eastern Washington and western Idaho: Am. Jour. Sci., v. 265, p. 257-291. Green, D. H., and Ringwood, A. E., 1967, The genesis of basalt magmas: Contr. Mineral. Petrol., v. 15, p. 103-190. Hart, D. H., and Newcomb, R. c., 1965, Geology and ground water of the Tualatin Valley, Oregon: U.S. Geol. Survey Water-supply Paper 1697. James, A. V. G., 1920, Factors producing columnar structure in lavas and its occurrence near Melbourne, Australia: Jour. Geology, v. 28, p. 458-469. Kienle, C. F., Jr., 1971, The Yakima Basalt in western Oregon and Washington: University of California, Santa Barbara, Ph.D. thesis, 171 p. Kocher, A. E., and others, 1920, Soil survey of Yamhill County, Ore.: U.S. Dept. Agric., Bur. Soils. 1926, Soil survey of Clackamas County, Ore.: U.S. Dept. Agric., Bur. Soils. Laval, W. N., 1957, Primary structures of the Columbia River basalt flows, south-central Washington (Abstract): Geol. Soc. America Bull., v. 68, p. 1867. Lentz, R. T., 1977, The petrology and stratigraphy of the Portland Hills silt: Portland State University, 144 p. 64 Lowry, w. D., and Baldwin, E. M., 1952, Late Cenozoic geo logy of the Lower Columbia River Valley, Oregon and Washington: Geol. Soc. America Bull., v. 63, no. 1, p. 1-24. Mackin, J. H., 1961, A stratigraphic section in the Yakima Basalt and the Ellensburg Formation in south-central Washington: Washington Div. Mines and Geology, Rept. Inv. 19, 45 p. McDougall, I., 1976, Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington: Geol. Soc. America Bull., v. 87, p. 777-792. McKee, E. H., Swanson, D. A., and Wright, J. L., 1977, Duration and volume of Columbia River basalt volcanism, Washington, Oregon and Idaho: Geol. Soc. America Abs. with Programs, v. 9, no. 4, p. 463-464. Nathan, s., and Fruchter, J. s., 1974, Geochemical and paleo magnetic stratigraphy of the Picture Gorge and Yakima Basalts (Columbia River Group) in central Oregon: Geol. Soc. America Bull., v. 85, p. 63-76. Newton, V. c., Jr., 1969, Subsurface geology of the lower Columbia and Willamette Basins, Oregon: Oregon Dept. of Geol. and Mineral Industries Oil and Gas Investi gation, no. 2, 121 p. Nichols, R. L., 1936, Flow units in basalt, J. Geol., v. 44, p. 617-630 .. Osawa, M .. , and Gales, G. G.. , 1970, Trace element abundances in Columbia River basalts: Proc. 2nd Columbia River Basalt Sym., Eastern Washington State College, Cheney, Wash. 1961, p. 55-71. Peck, D. L., Griggs, A. B., Schlicker, H. G., Wells, F. G., and Dole, H. M., 1964, Geology of the central and northern parts of the western Cascade Range in Oregon: U.S. Geol. Survey Prof. Paper 449, 56 p. Piper, A. M., 1942, Ground-water resources of the Willamette Valley, Ore.: U.S. Geol. Survey Water-supply Paper, 89 p. Schlicker, G. H., and Deacon, J.R., 1967, Engineering geo logy of the Tualatin Valley region, Oregon: Oregon State Dept. Geol. Min. Inds. Bull., v. 60, p. 103. 65 Schmincke, H. U., 1967, Stratigraphy and petrography of four upper Yakima Basalt flows in south-central Washington: Geol. Soc. America Bull., v. 78, p. 1385-1422. Snavely, P. D., Jr., and Wagner, H. c., 1963, Tertiary geo logic history of western Oregon and Washington: Washington Div. Mines and Geology Rept. Inv. 22, 25 p. , and Macleod, N. S., 1974, Yachats Basalt - an upper ---Eocene differentiated volcanic sequence in the Oregon Coast Range: Jour. Research U.S. Geol. Survey, v. 2, no. 4, p. 395-403. , Macleod, N. s., and Wagner, H. c., 1973, Miocene ---tholeiitic basalts of coastal Oregon and Washington and their relations to coeval basalts of the Columbia plateau, Geol. Soc. America Bull., v. 84, p. 387-424. Spry, Alan, 1962, The origin of columnar jointing, particu larly in basalt flows: Geol. Soc. Australia Jour., v. 8, pt. 2, p. 191-216. Swanson, D. A., 1965, Tieton dike swarm--Yakima Basalt rela tionships, Tieton River area, Washington (Abstract): Geol. Soc. America Spec. Paper 82, p. 283. 1966, Tieton volcano, a Miocene eruptive center in the southern Cascade Mountains, Washington: Geol. Soc. America Bull., v. 77, p. 1293-1314. 1967, Yakima Basalt of the Tieton River area, south central Washington: Geol. Soc. America Bull., v. 78, no. 9, p. 1077-1100. --~' and Wright, T. L., 1978, Some important facts and inferences concerning the Columbia River Basalt Group: Tectonics and seismicity of the Columbia Plateau workshop, Seattle, Washington. ___ ,Wright, T. L., Grolier, M. J., 1973, Chemical vari ation related to the stratigraphy of the Columbia River basalt: Geol. Soc. America Bull., v. 84, p. 371- 386. , Wright, T. L., and Helz, R. T., 1975, Linear vent ---systems and estimated rates of magma production and eruption for the Yakima Basalt on the Columbia Plateau: Am. Jour. Sci., v. 275, p. 877-905. ~--'Wright, T. L., Hooper, P.R., and Bentley, R. D., 1979, Revisions in stratigraphic nomenclature of the Columbia River Basalt Group: U.S. Geol. Survey Bull. 1457G. 66 Taubeneck, W. H., 1970, Dikes of Columbia River Basin in northeastern Oregon, western Idaho, and southeastern Washington, in Gilmour, E. H., and Stradling, D., eds., Proc. 2nd Columbia River Basalt Sym., Eastern Washington State College, Cheney, Wash., 1961, p. 73-96. Taylor, T. L., 1976, The basalt stratigraphy and structure of the Saddle Mountains of south-central Washington: Washington State Univ., Pullman, M.S. thesis, 116 p. Theisen, A. A., 1958, Distribution and character of loess like soil in northwestern Oregon: Oregon State Univ., M. s. thesis, 5 p. Timm, s., 1979, The structure and stratigraphy of the Columbia River Basin in the Hood River Valley, Port land State University, M.S. thesis, 52 p. Tomkeieff, S. I., 1940, The basalt lavas of the Giant's Causeway district of northern Ireland: Bull. Vulcanologique, v. 6, p. 90-143. Torgerson, E. F., and Glassey, T. W., 1927, Soil survey of Marion County, Ore.: U.S. Dept. Agric. Bur. Soils. Treasher, R. c., 1942, Geologic history of the Portland area: Oregon Dept. of Geol. and Mineral Industries Short Paper 7. Trimble, D. E., 1963, Geology of Portland, Oregon and adjacent areas: U.S. Geol. Surv. Bull., v. 1119, 119 p. Uyeda, s., 1958, Thermo-remanent magnetism as a medium of paleomagnetism, with special reference to reverse thermo-remanent magnetism: Japanese Jour. Geophys., v. 1, p. 1-123. Verhoogen, J., 1956, Ionic ordering and self-reversal of magnetism in impure magnetites: Jour. Geophys. Res., v. 61, p. 201-209. Warren, W. c., Norbisrath, H., and Grivetti, R. M., 1945, Geology of northwestern Oregon west of the Willamette River and north of lat. 45 15': U.S. Geol. Survey Oil & Gas Inv. Map OM42. Waters, A. C., 1961, Stratigraphic and lithologic variations in the Columbia River basalt: Am. Jour. Sci., v. 259, p. 583-611. 67 1941, Collapsed vesicles, alternation banding, and platy jointing of basalt: Geol. Soc. America Bull., Abs., v. 52, p. 1958-1959. 1960, Determining direction of flow in basalts: Am. Jour. Sci., v. 258-A, Bradley Volume, p. 350-366. 1961, Stratigraphic and lithologic variations in the Columbia River basalt: Am. Jour. Sci., v. 259, p. 583-611. 1962, Basalt magma types and their tectonic associa tions: Pacific Northwest of the United States: Am. Geophys. Union, Geophys. Mon. 6, p. 158-170. Watkins, N. D., 1965, Paleomagnetism of the Columbia Plateaus: Jour. Geophys. Res., v. 70, p. 1370-1406. , and Baksi, A. K., 1968, The age of the Picture Gorge ~~~ Basalt at the type section and at Imnaha, and the maximum duration of a Miocene geomagnetic epoch: Geo!. Soc. America, Abs. with Programs (Ann. Mtg.), Mexico City, p. 312-313. , and Baksi, A. K., 1974, Magnetostratigraphy and ~~~ oroclinal folding of the Columbia River, Steers and Owyhee Basalts in Oregon, Washington and Idaho: Am. Jour. Sci., v. 274, p. 148-189. Watson, E. B. and others, 1923, Soil survey of Washington County: U.S. Dept. Agric., Bur. Soils. Wells, F. G., and Peck, D. L., 1961, Geologic map of Oregon west of the 12lst meridian: U.S. Geol. Survey Misc. Inv. Map I-325, scale 1:500,000. Wright, T. L., Grolier, M. J., and Swanson, D. A., 1973, Chemical variation related to the stratigraphy of the Columbia River Basalt: Geo!. Soc. America Bull., v. 84, p. 371-386.