Geologic studies of the Columbia Plateau: Part I. Late Cenozoic ...

Geologic studies of the Columbia Plateau: Part I. Late Cenozoic evolution of the southeast part of the Columbia River Basalt Province V. E. C A M P do Directorate P. R. H O O P E R Department

General of Mineral Resources, P.O. Box 5219, Jeddah, Saudi Arabia of Geology, Washington State University, Pullman, Washington 99164

ABSTRACT Creation and evolution of the Columbia River Basalt Province occurred in late Cenozoic time, during an 11-m.y. interval in which most of the volcanism was centered in the east and southeast part of the province. Tectonism contemporaneous with volcanism resulted in: (1) progressive r : se of the eastern margin of the Columbia Plateau and creation of a westerly dipping regional paleoslope; (2) development of uplifted plateau surfaces (Nez Perce plateau and Joseph plains) isolated from further (younger) volcanism; (3) initiation of the Lewiston, Troy, and Stites structural basins; and (4) initiation of the Blue M o u n t a i n s uplift of southeast Washington and northeast Oregon. Although deformation was continuous throughout the eruptive episode, it is most evident during its later, waning stages. Deformation co itinued after cessation of volcanism a b o u t 6.0 m.y. B.P. A combination of epeirogeny and a stress regime, having a horizontal NNW-SSE axis of maximum compression and a horizontal W S W - E N E axis of minimum compression (tension), imposed on basalt overlying an older structural grain can account for most structural elements in the southeast part of the province.

excluding the Grande Ronde (La Grande) graben and uplifted blocks of basalt locally capping the Wallowa Mountains. In Idaho, the area includes the Clearwater and Salmon River drainages (the Clearwater embayment of Bond, 1963) but excludes the Weiser embayment to the southeast. T h r o u g h o u t most of the plateau, only the upper stratigraphic units are exposed and regional correlation of older units can be difficult. However, the southeast part of the plateau surface has been uplifted to nearly 2,000 m resulting in deep canyon erosion by the Snake, Salmon, Imnaha, Grande Ronde, and Clearwater River systems (Fig. 1). Erosion exposes the base of the Columbia River Basalt G r o u p and provides a series of excellent cross sections, up to

INTRODUCTION The Columbia Plateau was formed from 17.0 to 6.0 m.y. B.P. by eruption of basalt from n o r t h - n o r t h w e s t - t r e n d i n g fissures between the actively rising calc-alkaline volcanic mountains of the Cascade Range to the west and the still-rising mountains of Late Cretaceous to Tertiary rocks of the Idaho batholith and Challis volcanics to the east (Axelrod, 1968). From the north and northeast, the present plateau surface slopes gently d o w n toward the Pasco Basin in eastcentral Washington; the western plateau is marked by many eastwest—trending monoclines and anticlines. T o the southeast, gentle northwesterly slopes prevail up to the crest of the Blue M o u n t a i n s anticline. This paper is Part 1 of a report concerned with the geology of the southeast part of the Columbia Plateau; it deals with the late Cenozoic tectonic evolution of the area. Part II deals with the upper Miocene flow distribution in the Clearwater Embayment of Idaho (Bond, 1963). The southeast part of the plateau is defined as that part that lies southeast and east of the Blue Mountains uplift in northeastern Orgeon and west-central Idaho (Fig. 1). In Oregon, the area described is restricted to that part south and east of the Grande Ronde River,

Figure 1. Location and physiographic map of the southeast part of the Columbia River Basalt province.

Geological Society of America Bulletin, Part 1, v. 9 2 , p. 6 5 9 - 6 6 8 , 7 figs., S e p t e m b e r 1 9 8 1 .

659

CAMP A N D

660

nearly 1,200 m thick, through the earliest known flows of the Columbia River Basalt Group. The area has been mapped at a scale of 1:250,000 as part of a larger project encompassing the entire Columbia River Basalt Province. Preliminary maps of southeast Washington (Swanson and others, 1980) and part of the Clearwater embayment (Camp, in Swanson and others, 1979a) are available, and remaining areas in northeast Oregon and Idaho are expected to be available by 1981. The mapped units, individual flows or groups of flows, have been identified by field appearance, petrography, major element analyses (XRF), and magnetic polarity. The distribution of mappable units and their variation in thickness across the area provides a record of the eruptive and tectonic evolution of this part of the province. FLOW DISTRIBUTION A N D DEFORMATION DURING BASALT EXTRUSION Waters (1961) and Bond (1963) recognized an older porphyritic basalt unit (the picture Gorge Basalt of Waters) and a younger

Formation

Member

Magnetic Polarity

Lower Monumental Ice Harbor

N,R

Buford Saddle

Elephant Mountain

K, T

Pomona Mountains Esquatzel Basalt

Weissenfels Rid/re Asotin Wilbur Creek Umatilla Priest Rapids

Wanapum

Roza

Basalt

Frenchman Springs

R3,T

Eckler Mountain Grande Ronde Basalt Picture Gorge Basalt

HOOPER

aphyric Yakima Basalt unit in the southeast part of the Columbia River Basalt Province. Bond (1963) mapped the boundary between the two units in the Clearwater embayment and traced a few individual flows for short distances in the field. A revitalization of interest in the Columbia River Basalt began in the 1970's. Detailed studies, using precise chemical analyses and magnetic polarity, have helped to establish a basalt stratigraphy workable over much of the southeast par: of the basalt province (Holden, 1974; Hooper, 1974; Price, 1974, 1977; Camp, 1976; Holden and Hooper, 1976; Kleck, 1976; Reidel, 1978; Ross, 1978; Shubat, 1979; Swanson and others, 1980). Parts of this work have been summarized in Hooper and others (1976, 1979), Vallier and Hooper (1976), and C a m p and others (1978). Swanson and others (1979c) have revised and formalized the stratigraphic nomenclature for the Columbia River Basalt Group, dividing it into four formations and several members (Fig. 2). All of these units are present in the southeast part of the province except the Picture Gorge Basalt, which is confined to north-central Oregon, and the Esquatzel and Ice Harbor Members of the Saddle Mountains Basalt, which are restricted to the central part of the province. Field work in the southeast part of the plateau has established the presence of several new map units belonging to the W a n a p u m and Saddle Mountains Basalts. It is beyond the scope of this paper to describe the field and chemical characteristics and the distribution of these units individually; the reader is referred to Ross (1978), C a m p (in Swanson and others, 1979a; and 1981, this issue), Shubat (1979), and Hooper (unpub. map and data). Feeder dikes for the basalt are remarkable for their consistent north to north-northwest trend. Waters (1961) emphasized the concentration of dikes in limited areas: the M o n u m e n t dike swarm, feeding only the Picture Gorge Basalt (Fruchter and Baldwin, 1975); the Grande Ronde dike swarm in the lowest reaches of the Grande Ronde River on the eastern extremity of the OregonWashington state line; and the Cornucopia dike swarm on the southeast side of the Wallowa Mountains. Taubeneck (1970) emphasized the preseence of dikes over a much larger lozenge-shaped area from Almota or. the lower Snake River in Washington, south to Huntington on the upper Snake River on the Oregon-Idaho border. This area encompasses both the Grande Ronde and Cornucopia dike swarms and a large part of the area included in the present study. More detailed studies of the Grande Ronde dike swarm (Gibson, 1969; Price, 1974, 1977) have traced individual dikes and correlated them with flows. Swanson and others (1975) described eruptive sources marked by eroded cinder cones, and local accumulations of welded spatter. These are also aligned in a north-northwest direction and often are on trend with dikes of similar lithology. Dikes occur as far west as the Pasco Basin and it is probable that more are buried beneath the central parts of the Columbia Plateau, where subsidence has prevented deep erosion. The following sections describe the distribution of the basalt units in the southeast part of the plateau. Imnaha Basalt

Imnaha Basalt

R0 ?

Figure 2. Stratigraphy of the Columbia River Basalt Group, Modified after Swanson and others (1979c). Asterisk denotes basalt unit, which does not occur in the southeast part of the Columbia Plateau.

The coarse plagioclase and olivine-phyric flows of the Imnaha Basalt represent the earliest known eruptions of the Columbia River Basalt Group. Thin sequences of generally one or two flows are exposed in the ' ; Lewiston structure" northwest of Clarkston, Washington (Camp, 1975, 1976), throughout the Clearwater embayment (Bond, 1963; Camp, in Swanson and others, 1979a), on the northwest flanks of the Wallowa Mountains (Shubat, 1979),

GEOLOGIC STUDIES OF COLUMBIA PLATEAU: PART 1

and possibly in the headwaters of the Grande Ronde River, west of La Grande (R. D. Bentley and P. R. H o o p e r , unpub. data, 1977). The exposed thickness of Imnaha Basalt increases southeastward into the Weiser embayment of Idaho (J. Fitzgerald, 1979, personal commun.). In the canyons of the lower Salmon and Imnaha Rivers, Imnaha Basalt rests on the pre-basalt surface and has a thickness of between 3 0 0 and 5 0 0 m. Some 20 flows can be distinguished, with a m a x i m u m of 14 flows present in the thickest sections. M o s t Imnaha Basalt Hows have normal magnetic polarity (Hooper and others, 1976, 1979). However, the upper two flows in the Imnaha area and farther south have transitional polarity. Fluxgate field magnetometer readings of the youngest flows in the Clearwater embayment (V. E. C a m p , unpub. data, 1979) and along the M i n a m River, east of La Grande, Oregon (W. H . Taubeneck, D. A. Swanson, and D. O . Nelson, unpub. data) are reversely polarized. Although no Grande Ronde Basalt has, as yet, been found interbedded with Imnaha Basalt, extrusion of flows belonging to both formations may have occurred during the same reversed magnetic interval. However, it is notable that no flow at the top of the Imnaha Basalt sequence has yet shown a reverse polarity on laboratory demagnetized samples (Hooper and others, 1979). Four dikes of Imnaha Basalt have been mapped in the Imnaha and Snake River canyons (Keck, 1976; this study). Dikes of Imnaha Basalt appearance and m a j o r element chemistry occur as far east as the m o u t h of Peasley Creek along the South Fork of the Clearwater River (about 25 km southeast of Grangeville, Idaho), and many more have been collected by W. Taubeneck and analyzed by Wright and others (1979) from the Copperfield, Mineral, Sturgill Peak, Eagle Cap, and Cornucopia quadrangles in areas adjacent to the southern tip of the area mapped. Despite the generally poor exposures under a thick cover of younger basalt flows, it is a p p a r e n t that the thickest sequences of Imnaha Basalt occur in the extreme southeast part of the basalt province, and that they were fed from dikes in the same area, or further south-southeast. The thickness varies greatly, since Imnaha Basalt flows filled deep valleys of pre-basalt topography on the order of 9 0 0 m. It is difficult to define the northwest continuation of these flows. Imnaha Basalt apparently thickens to the southeast from the present axis of the Blue M o u n t a i n s and into the Weiser embayment. There is no indication that Imnaha Basalt occurs northwest of the present Blue M o u n t a i n s axis where lowest G r a n d e Ronde Basalt flows lie directly on pre-basalt basement, suggesting that an older, pre-basalt ridge was present at the site of the Blue M o u n t a i n s anticline during Imnaha extrusion. The Imnaha in the Clearwater embayment is generally restricted to one or two flows that probably erupted late in the sequence and flowed into the embayment, either by the breaching of a relatively low col in the Cott o n w o o d Butte to Blue M o u n t a i n s divide, or by extruding from the extension of one or more feeder dikes north-northwestward into the Clearwater—lower Snake River drainage. Grande Ronde Basalt In the southeast part of the plateau, Grande Ronde Basalt lies conformably on Imnaha Basalt with no discernable time gap greater than that between flows above and below. T h e formation comprises over 90 percent of the estimated volume of the Columbia River Basalt G r o u p (Swanson and others, 1979c). At its type section, near the m o u t h of the Grande Ronde River (Fig. 1), the formation contains 3 5 basalt flows with an aggregate thickness of 800 m (Camp and others, 1978). T h e flows are generally fine-grained

661

and aphyric, and they are readily distinguished from flows of the Imnaha Basalt by texture, mineralogy, and m a j o r element composition. Although individual flows within the G r a n d e Ronde Basalt have only minor chemical and petrographic differences, which are typically gradational, Reidel (1978) has demonstrated the possibility of identifying and tracing these flows by detailed sampling and correlation of trace and m a j o r element analyses. However, this approach is impractical when mapping at reconnaissance scale. The Grande Ronde Basalt is conveniently divided into four magnetostratigraphic units (Swanson and others, 1979c, Fig. 2): R,, N , , R 2 , and N 2 , where R is reversed polarity and N is normal polarity (Fig. 2). Recent mapping has shown that each of these units contains flows that, as a group, are mappable across the whole province (Swanson and others, 1979a). Dikes of Grande Ronde chemistry (equivalent to Yakima chemical type of Wright and others, 1973) occur over much of the mapped area but are scarce east of the Snake River in the Clearwater embayment. They are a b u n d a n t in the Grande Ronde and Cornucopia dike swarms (Taubeneck, 1970) and at the head of M i n a m River on the northwest edge of the Wallowa M o u n t a i n s (Shubat, 1979). Their apparent concentration is due, in part, to depth of exposure, but in the Imnaha drainage, in line with the Grande Ronde dike swarm and where depth of erosion is even greater than in the Grande Ronde River, relatively few dikes are found. H o o p e r and others (1979) have also shown that there are many more thin flows of R, in the lower Grande Ronde area than to the south or east, suggesting that during early Grande Ronde time, at least, this area near the juncture of the three states was the center of volcanic activity. Figure 3 shows the original distribution and isopach thicknesses for each G r a n d e Ronde magnetostratigraphic unit in the southeast part of the province. The G r a n d e Ronde Basalt and each of its magnetostratigraphic units thin to the east from the Grande Ronde dike swarm, pinching o u t in the Clearwater e m b a y m e n t against the Rocky M o u n t a i n s highlands of west-central Idaho. Flows of the R, magnetostratigraphic unit were extruded upon Imnaha Basalt during a reverse magnetic interval. The thickest sections of R, occur in the Imnaha River drainage (about 3 8 0 m). The unit thins to the northeast into the Clearwater e m b a y m e n t (Fig. 3, A). T o the southeast, a single road section suggests that the unit thins to 250 m over the Imnaha—Pine Creek divide (Figs. 1 and 3, A), and then increases in thickness to 3 5 0 m into the Weiser embayment. Figure 3, B shows that N , , like R l 5 decreases in thickness to the east into the Clearwater embayment. However, N, does not go as far up the N o r t h Fork of the Clearwater River as R,. Similarly, N, breaches the Imnaha-Pine Creek divide to the south, but pinches o u t quickly rather than thickening southward as does R,. Deformation during N, is evident because of the abrupt thickening of N, flows near the present confluence of the Middle and South Forks of the Clearwater River (Fig. 3, B).The ponding of N, flows in this area represents the initiation of the Stites basin during early Grande Ronde time. The basining may have resulted from w a r p i n g of older flows, but we believe it is more likely the result of vertical movement along reactivated n o r t h - s o u t h - t r e n d i n g pre-basalt faults, much like those in the Riggins area to the south (Hamilton, 1972). Flows of the R2 magnetostratigraphic unit thin abruptly to the east and pinch out about midway across the Clearwater embayment (Fig. 3, C). The unit thins to the south as well, pinching out immediately north of the Imnaha—Pine Creek divide. A thin island of R2 occurs in the Stites basin a b o u t 35 km east of where the main

662

CAMP A N D

HOOPER

Figure 3. Isopach maps for Grande Ronde magnetostratigraphic units (A) R,, (B) N , , (C) Il 2 , (D) N 2 . Outcrop pattern is original distribution of Imnaha and Grande Ronde Basalts (streams are added for location convenience). Line with T-pattern marks original distribution of unit in question. Contour interval = 2 5 0 ft. (76m).

body of R> pinches out. An R2 tlow may have poured into the Stites basin from the west along a n a r r o w valley, but a more likely explanation is that it was fed f r o m a local dike and ponded in the basin. Whatever the mechanism, the Stites basin continued to evolve through R2 time. N 2 flows occur only along the G r a n d e Ronde drainage in the western part of the m a p area (Fig. 3, D). T o the north, in Washington, the unit pinches o u t along a NNW-SSE—trending line (Swanson and others, 1980). In northeast Oregon N 2 pinches o u t along a

NE-SW—trending line and thickens rapidly to the northwest. W e believe (and concur with S. Reidel, 1980, personal commun.) that this is the result of deformation during late R2 time, before extrusion of N 2 flows. T h e d e f o r m a t i o n is marked by initiation of movement along the Limekiln fault (Fig. 3, D). Vertical uplift along the fault produced a monocline along its southwest projection. T h e monocline created a slope upon which N 2 flows thinned and pinched out. The plateau surface southeast of the Limekiln fault and monocline w a s uplifted at least to some extent during this time.


The uplifted areas include the present-day Nez Perce plateau in Idaho and the Joseph plains in Oregon (Fig. 1). A series of volcanic cones rise above the R2 plateau surface of the Joseph plains (Fig. 3, D). These are the Joseph volcanics of Kleck (1976). The cones are composed of small glassy basalt flows and spatter of Grande Ronde chemical type; they have normal magnetic polarity, lie along the projected strike of the G r a n d e Ronde dike swarm, and are clearly older than a valley-filling vent flow of the Roza M e m b e r (Fig. 2). The evidence suggests that they are Grande Ronde vents of N 2 age. Failure of the vents to feed significant lava flows is probably related to uplift of the Joseph plains along the Limekiln fault-monocline complex (Camp and Hooper, 1980). N 2 dikes of the Grande Ronde swarm presumably broke the surface northwest of the Limekiln fault-monocline complex, extruding lava that flowed mainly westward, pinching out to the east against he monoclinal paleoslope. Southeast of the Limekiln fault and monocline, in the Joseph plains, N , magma did not have enough fluid pressure to overcome the higher plateau elevation and only minor venting of N 2 lava occurred. Figure 4 illustrates the m a p view of the southeast part of the plateau immediately after extrusion of the G r a n d e Ronde Basalt. The cross section (A-A") drawn from the G r a n d e Ronde River to

L i m e k i In f a u l t - monocline complex A'

Grande

Ronde

N R mnaha Prebasalt

Figure 4. M a p view and cross section of the southeast part of the Columbia River Basalt province immediately after extrusion of G r a n d e Ronde Basalt.

663

the N o r t h Fork of the Clearwater River shows that each Grande Ronde magnetostratigraphic unit thins to the east. Moreover, each successive unit pinches o u t farther to the west than the preceding unit, indicating a continuous westward tilting during Grande Ronde extrusion. The only exceptions to this regional tilting are (1) the unusually thick pile of N, and the occurrence of R2 in the Stites basin, which implies continued development of the basin through N, and R2 time (Fig. 3, B and C) and (2) the thickening of R, toward the southeast into the Weiser e m b a y m e n t of Idaho. Westward tilting in this part of the plateau is consistent, on a regional scale, with evidence that a westward-dipping paleoslope extended across the whole plateau during eruption of late (N 2 ) Grande Ronde Basalt (Swanson and others, 1979b). Uplift of the Joseph plains and the Nez Perce plateau along the Limekiln fault-monocline complex during G r a n d e Ronde extrusion had a significant impact on the distribution of N 2 and all younger basalt flows of the W a n a p u m and Saddle M o u n t a i n s Basalt. The Limekiln fault marks the southeastern boundary of the present Lewiston basin, and the southwest monoclinal extension of the fault is essentially the southeast boundary of the present Troy basin (Figs. 1 and 4). Thus, by the end of Grande R o n d e time, the Stites, Lewiston, and Troy basins were defined, probably as broad topo-

664

CAMP A N D

graphic features but lacking the faults associated with the structures of the present basins. M o v e m e n t along the Limekiln fault effectively isolated the Joseph plains and most of the Nez Perce plateau from further Columbia River volcanism. T h e uplifted plateau surface in these areas has been exposed to surface weathering and erosion for the past 14 m.y. T h e area south of Grangeville, Idaho, and east of the Salmon River (Fig. 1) has been exposed over the same period. This may be the result of uplift along the M t . Idaho fault (Fig. 1) soon after extrusion of G r a n d e Ronde Basalt, effectively isolating the area from younger W a n a p u m and Saddle M o u n t a i n s flows which poured into the Stites basin to the north. Wanapum Basalt Figure 5A shows that the distribution of members of the W a n a p u m Basalt is restricted to the Troy, Lewiston, and Stites basins and the northern part of the Clearwater embayment. The distribution of each W a n a p u m Basalt member is not discussed in detail, but this information is available elsewhere (Camp, 1976; 1981, this issue; Ross, 1978; Hooper, unpub. data). Of the W a n a p u m members in Figure 2, the Troy basin contains the Eckler

HOOPER

M o u n t a i n , Frenchman Springs, and Roza Members, whereas the Clearwater embayment: contains only the Priest Rapids Member. M a n y of the W a n a p u m members in the Troy basin were extruded from local dikes (Swanson and others, 1975; Ross, 1978), as was the Priest Rapids M e m b e r of the Clearwater e m b a y m e n t (Camp, 1981, this issue). Figure 5A demonstrates that deformation before W a n a p u m time was a controlling factor in the distribution of W a n a p u m Basalt members in the southeast part of the plateau. W a n a p u m Basalt ponded in the Stites basin and was unable to flow out to the west because of the uphfted Nez Perce plateau. It flowed a r o u n d the lower northern part of the Nez Perce plateau and into the Lewiston basin. It was restricted from flowing o u t of the basin to the south by the Limekiln fault. W a n a p u m Basalt in the Troy basin was similarly restricted from flowing to the south and east o n t o the Joseph plains because of the southwest monoclinal extension of the Limekiln fault. Saddle Mountains Basalt M e m b e r s of the Saddle M o u n t a i n s Basalt were extruded periodically between about 13.5 and 6.0 m.y. B.P. and tend to lie uncon-

Figure 5. Distribution maps for (A) Wanapum and (B) Saddle Mountains Basalts in southeast part of Columbia River Basalt Province. Outcrop pattern marks original distribution of Columbia River Basalt (streams are added for location convenience).


formably on each other and older flows (Swanson and others, 1979c). T h e formation is composed of many flows, most of which have their source in the southeast part of the plateau (Camp, 1976; 1981, this issue; Ross, 1978; Swanson and others, 1979c). Like the W a n a p u m , the distribution of these flows is controlled by uplift of the Nez Perce plateau and the Joseph plains along the Limekiln fault-monocline complex, and by active rise of the Blue Mountains. Figure 5B shows that they occur in the Troy, Lewiston, and Stites basins, and over a large part of the Clearwater embayment. Some Saddle M o u n t a i n s Basalt units occur on the present lower, northeastern part of the Nez Perce plateau, but it is believed that they are present because their dikes broke the plateau surface rather than flowing o n t o the plateau surface from a more distant source (Camp, 1981, this issue). STRUCTURE AND

TECTONICS

T h e geologic m a p of Idaho (Bond, 1978) reveals a NW-SE and NE-SW fault trend pattern in the pre-basalt rocks of western Idaho. H o o p e r and C a m p (1981) argue that if a stress regime having a horizontal NNW-SSE axis of m a x i m u m compression and the east to west regional tilting is imposed on basalt overlying this older structural grain, then most of the structural elements in this part of the plateau can be explained. These structural elements include (1) east-west folds with associated reverse faults; (2) faults displaying a predominantly vertical displacement with NW-SE, NE-SW, and, to a lesser extent, N-S trends; and (3) the vertical basalt feeder dikes, which are remarkable for their consistent NNW-SSE trend. T h e offlap of progressively younger basalt units (Imnaha through G r a n d e Ronde N 2 ) from pre-basalt topographic highs is illustrated in Figure 4. Progressive westward tilting away from the pre-basalt margin of the plateau is most likely the result of relative uplift of lighter pre-basalt rocks due to isostatic adjustment (Hooper and

Figure 6. Structure-contour m a p of the t o p of the Grande R o n d e Basalt. C o n t o u r interval = 5 0 0 ft (152 m) except for the 4 7 5 0 ft contour shown as a broken line on the Joseph plains. Structures mentioned in text are: (1) Lewiston structure, (2) Lewiston syncline, (3) Limekiln fault, (4) Stites fault, (5) M t . Idaho fault, (6) White Bird fault, and (7) Grouse Flat syncline.

C a m p , 1981). Figure 4 illustrates that this uplift continued at least through G r a n d e Ronde time. By the end of the extrusive period, an undetermined a m o u n t of vertical movement had occurred along the Limekiln and M t . Idaho faults, and the Lewiston, Stites, and Troy basins were developed to some extent. However, most tectonic features in the southeast part of the province developed after Columbia River Basalt extrusion. Figure 6 is a structure c o n t o u r m a p of the top of the Grande Ronde Basalt. It adequately illustrates the present structural features occurring in the southeast part of the plateau. These structures are discussed below.

665

Structural Basins Ross (1978) showed that gentle warping along east-west fold axes occurred during late G r a n d e Ronde time in the Troy basin (Fig. 1). This is consistent with the timing of uplift along the Limekiln fault and its monoclinal extension forming the southeast boundary of the Troy basin. M o v e m e n t on most observed faults associated with the basin was shown to be later than basalt extrusion (Ross, 1978). All structural features in the Troy basin were interpreted by Ross (1978) to be the result of a primary compressive stress oriented in a NNW-SSE direction, the same stress regime proposed by H o o p e r and C a m p (1981) to explain the structural elements found t h r o u g h o u t the southeast part of the plateau. C a m p (1976) showed that the Lewiston basin developed between R2 and late W a n a p u m time. Since no Grande Ronde N 2 flows occur in the Lewiston basin, it is not k n o w n whether the basin was developed by N 2 time. However, evidence given above shows that the

666

CAMP A N D

Limekiln fault was activated prior to Grande Ronde N 2 time and the fault marks the southeast border of the basin. Figure 7 is a schematic diagram of the structural features of the Lewiston basin. The structural reflection of the basin is the Lewiston syncline (Figs. 6 and 7), a broad, shallow fold whose southeast limb dips 3° away from, and is truncated by, the Limekiln fault. The basin is bounded to the north by the Lewiston structure (Camp, 1975, 1976), simplified in Figure 7. The structure is basically an asymmetric anticlinal horst block with a reverse fault truncating its northern limb. T o the east, the structure grades into a southerly dipping monocline that in essence becomes the northern limb of the Lewiston Basin syncline (the southern limb being the northeast-trending monoclinal extension of the Limekiln fault). Like the structures described by Ross (1978) in the Troy basin, the Lewiston structure is best explained by north-south compression producing an east-west trending anticline broken by reverse faulting (Fig. 7). The vertical fault marking the south wall of the anticlinal horst offsets the youngest member of the Columbia River Basalt G r o u p (the Lower Monumental Member), dated at 6.0 m.y. B.P. (McKee and others, 1977). M a j o r movement, and probably development of the Lewiston structure, is thus later than basalt extrusion. This is further supported by small, sympathetic reverse faults recorded in adjacent gravels younger than the Lower Monumental Member (Kuhns, 1980). The age and origin of the Stites basin is somewhat different from that of the Troy and Lewiston basins discussed above. The unusually thick sequence of Grande Ronde N, flows in the Stites basin (Fig. 3, B) indicates that it developed earlier than either of the other two basins. Faults associated with the Stites basin have north-south trends and vertical displacements (Fig. 6). The Stites fault (Fig. 6) is

HOOPER

the largest of these, offsetting Saddle Mountains flows by more than 150 m. The Stites basin lies along the extreme eastern edge of the Columbia Plateau and is therefore particularly susceptible to renewed movement along underlying pre-basalt structures. Several large north-south trending faults are mapped in the pre-basalt rocks south of the Stites basin (Hamilton, 1962). This same trend appears to continue beneath the Columbia River Basalt flows, and reactivation of these pre-basalt structures has caused vertical block faulting and basining to occur in the Stites area. This movement began in N, time and probably continued through the extrusive period, but major movement along the vertical faults came after basalt extrusion. P. Meyers (1980, personal commun.) has shown that in some cases the displacement along these reactivated faults is in opposite direction from their original sense of displacement. In contrast to those structures associated with the Troy and Lewiston basins, the structures of the Stites basin are not the product of horizontal NNW-SSE directed stress, although the north-south vertical faults would not be incompatible with such a regime by movement along pre-basalt zones of weakness. Faults The dominant structural pattern in the southeast part of the basalt province is reflected by faults with vertical displacements trending NW-SE, NE-SW, and, to a lesser extent, N-S (Fig. 6). This pattern is similar to that in the Idaho batholith and adjacent older rocks that underlie the Columbia River Basalt (Bond, 1978). A horizontal component of movement along these faults has been established in only a few cases. Hooper and C a m p (1981) suggest

LEWISTON SYNCLINE

BASIN

Figure 7. Schematic diagram of Lewiston basin and associated structures.

SADDLE MOUNTAINS -WANAPUM

GRANDE RONDE

IMNAHA PREBASALT


that the structural pattern of underlying pre-basalt rocks was used to accommodate Miocene to Pleistocene stress related to (1) vertical isostatic forces resulting in regional w e s t w a r d tilting and (2) m a x i m u m horizontal compression in a NNW-SSE direction. Faults trending NW-SE are most c o m m o n but less severe, generally displacing units less than 60 m vertically. Faults trending NE-SW and N-S generally have greater vertical offsets. These include (Fig. 6): the Limekiln fault, with over 4 5 0 m of vertical displacement (Camp, 1976; Reidel, 1978); the Stites fault (over 150 m); the M t . Idaho fault (over 4 5 0 m); and the White Bird fault (over 600 m). The present structural pattern of the southeast part of the Columbia Plateau may reflect an older structural grain; the faults trending NE-SW and N-S may represent m a j o r structural features of that grain, whereas those trending NW-SE may represent more c o m m o n , but less significant, pre-basalt structural elements.

667

Mountains flows f r o m occurring on the uplifted areas. They therefore accumulated in the three structural basins and the low-lying areas of the Clearwater embayment. Basining in the Stites area began before or during N , time, possibly due to movement along the Stites fault (Fig. 6). However, there is n o direct evidence to suggest that any other faults were active before R 2 time. T h e Limekiln and M t . Idaho faults were active in late Grande Ronde time, and many more smaller faults were active during W a n a p u m and Saddle M o u n t a i n s time, but movement along the majority of faults is later than the most recent basalt flow present. It has been argued elsewhere (Hooper and C a m p , 1981), that deformation continued over the duration of Columbia River Basalt eruption, but that earlier structures are hidden by later flows and by the very rapid rate of eruption during G r a n d e Ronde time. ACKNOWLEDGMENTS

C O N C L U S I O N A N D DISCUSSION Since over half of the area mapped is covered by flows no younger than R2 (e.g., Nez Perce plateau and Joseph plains), it is difficult to establish the precise timing of all tectonic events during the 11 m.y. interval of basalt extrusion. However, many areas (e.g., Troy, Lewiston, and Stites basins) contain a succession of flows that were extruded, at least intermittently, over most of this time interval, enabling a generalized tectonic scenario to be d r a w n concerning the evolution of this part of the Columbia Plateau. The increased thickness of Imnaha basalt to the south-southeast into the Weiser embayment probably reflects the lower elevation of that area during the initial phases of basalt extrusion. The I m n a h a - P i n e Creek divide was an effective topographic barrier during this time; it was eventually breached by G r a n d e Ronde R, flows, which thin over the divide but also thicken into the Weiser embayment. By the end of R, time, the topographic lows had been filled and the divides largely buried. Rise and general westward tilting of the plateau continued through G r a n d e R o n d e time and caused Grande Ronde magnetostratigraphic units to thin and progressively pinch o u t away from pre-basalt highs along the eastern margin of the plateau. The source for most members of the W a n a p u m and Saddle M o u n t a i n s Basalts lies near the eastern margin of their outcrop areas (Swanson and others, 1975; C a m p , 1981, this issue), indicating that the distribution of flows was governed in part by a westward-dipping paleoslope. Paleobotanical evidence (Axelrod, 1968) shows that uplift of the northern Rocky M o u n t a i n s began in the Oligocene; information presented here implies that uplift continued into the Miocene resulting in rise of the eastern margin of the Columbia Plateau t h r o u g h o u t the period of basalt eruption. Although evidence suggests that uplift and westward tilting was continuous, dating of specific structural elements is more difficult. By late W a n a p u m time the Troy, Lewiston, and Stites basins were developed or enhanced by uplift of the Nez Perce plateau and Joseph plains along the Limekiln fault-monocline complex. This deformation occurred at a b o u t the same time as initiation of the Blue M o u n t a i n s uplift of southeast Washington. Both events are dated by the Priest Rapids M e m b e r (late W a n a p u m ; Fig. 2), which does not occur south of the Blue M o u n t a i n s of northeast Oregon (Swanson and others, 1979c, 1980) nor south of the Limekiln fault on the Nez Perce plateau (Camp, 1976; 1981, this issue). Movement along the Limekiln fault may have occurred in response to regional uplift of the Blue M o u n t a i n s in early to middle W a n a p u m time. D e f o r m a t i o n during this time restricted W a n a p u m and Saddle

The authors wish to thank D. A. Swanson, W. H . Taubeneck, and T. L. Wright for critically reviewing the manuscript and offering several helpful suggestions. Geologic mapping was performed under U.S.G.S.—Department of Energy interagency agreement EY78-1-06-1078. The mapping is part of the regional geologic studies effort of the basalt waste isolation project by Rockwell H a n f o r d Operations for the U.S. Department of Energy.

REFERENCES CITED Axelrod, D. I., 1968, Tertiary floras and topographic history of the Snake River Basin, Idaho: Geological Society of America Bulletin, v. 79, p. 713-734. Bond, J. G., 1963, Geology of the Clearwater e m b a y m e n t : Idaho Bureau of Mines and Geology Pamphlet 128, 83 p. 1978, Geologic m a p of Idaho: Idaho Bureau of Mines and Geology, M o s c o w , Idaho. C a m p , V. E., 1975, Structure and basalt stratigraphy of the " L e w i s t o n D o w n w a r p " , I d a h o - W a s h i n g t o n : Geological Society of America Abstracts with Programs, v. 7, no. 3, p. 300. 1976, Petrochemical stratigraphy and structure of the C o l u m b i a River basalt, Lewiston basin area, Idaho-Washington [Ph.D. dissert.]: Pullman, Washington State University, 201 p. 1981, Geologic studies on the Columbia Plateau: II, Upper Miocene basalt distribution reflecting source locations, tectonism, and drainage history in the Clearwater e m b a y m e n t , Idaho: Geological Society of America Bulletin, v. 92, p. 6 6 9 - 6 7 8 (this issue). C a m p , V. E., and H o o p e r , P. R., 1980, Tectonic evolution of the southeast part of the C o l u m b i a River Basalt Plateau: Geological Society of America Abstracts with Programs, v. 12, no. 3, p. 100—101. C a m p , V. E., Price, S. M . , and Reidel, S. P., 1978, Descriptive s u m m a r y of the G r a n d e Ronde Basalt type section, Columbia River Basalt G r o u p : Rockwell H a n f o r d O p e r a t i o n s D o c u m e n t R H O - B W I - L D - 1 5 , Richland, Washington, 26 p. Fruchter, J. S., and Baldwin, S. F., 1975, Correlations between dikes of the M o n u m e n t swarm, central O r e g o n , and Picture Gorge Basalt flows: Geological Society of America Bulletin, v. 86, p. 514—516. Gibson, I. L., 1969, A comparative account of flood basalt volcanism of the C o l u m b i a River Plateau and eastern Iceland: Bulletin Volcanologique, v. 33, p. 4 1 9 - 4 3 7 . Hamilton, W., 1962, Late Cenozoic structure of west-central Idaho: Geological Society of America Bulletin, v. 73, p. 5 1 1 - 5 1 6 . Holden, G. S., 1974, Chemical and petrographic stratigraphy of the Columbia River basalt in the lower Salmon River C a n y o n , Idaho IM.S. dissert.]: Pullman, W a s h i n g t o n , W a s h i n g t o n State University. Holden, G. S., and H o o p e r , P. R., 1976, Petrology and chemistry of a Columbia River basalt section, Rocky C a n y o n , west-central Idaho: Geological Society of America Bulletin, v. 87, p. 2 1 5 - 2 5 5 . H o o p e r , P. R., 1974, Petrology and chemistry of the Rock Creek flow, C o lumbia River basalt, Idaho: Geological Society of America Bulletin, v.

668

CAMP A N D

85, p. 1 5 - 2 6 . H o o p e r , P. R., and C a m p , V. E., 1981, D e f o r m a t i o n of the southeast part of the C o l u m b i a Plateau: Geology, v. 9, no. 7, p. 3 2 3 - 3 2 8 . H o o p e r , P. R., C a m p , V. E., Kleck, W . D., Reidel, S. P., and Sundstrom, C. E., 1976, Magnetic polarity and stratigraphy of the southeastern part of the C o l u m b i a River basalt plateau: Geological Society of America Abstracts with Programs, v. 8, no. 3, p. 383. H o o p e r , P. R., Knowles, C. R., and W a t k i n s , N . D., 1979, Magnetostratigr a p h y o f ' t h e Imnaha and G r a n d e Ronde Basalts in the southeast part of the C o l u m b i a Plateau: American J o u r n a l of Science, v. 279, p. 737-754. Kleck, W . D., 1976, Chemistry, petrography, and stratigraphy of the Columbia River G r o u p in the Imnaha River Valley region, eastern Oregon and western Idaho [Ph.D. dissert]: Pullman, W a s h i n g t o n State University, 2 0 8 p. Kuhns, M . J., 1980, Late Cenozoic deposits of the lower C l e a r w a t e r Valley, [M.S. dissert.]: Pullman, W a s h i n g t o n , W a s h i n g t o n State University. McKee, E. H., Swanson, D. A., and Wright, T. L., 1977, D u r a t i o n and volume of C o l u m b i a River Basalt volcanics, W a s h i n g t o n , O r e g o n , and Idaho: Geological Society of America Abstracts with Programs, v. 9, p. 463. Price, S. M . , 1974, A geochemical classification of dikes of the G r a n d e R o n d e s w a r m , C o l u m b i a River basalt: U. S. Atomic Energy Commission R e p o r t ARH-SA-202, Atlantic Richfield H a n f o r d Co., 29 p. 1977, An evaluation of dike-flow correlations indicated by geochemistry, Chief J o s e p h swarm, C o l u m b i a River basalt [Ph.D. dissert.]: M o s c o w , University of Idaho, 3 2 0 p. Reidel, S. P., 1978, T h e stratigraphy and petrogenesis of the G r a n d e Ronde Basalt in the lower Salmon and adjacent Snake River C a n y o n s [Ph.D. dissert.]: Pullman, W a s h i n g t o n State University, 4 1 5 p. Ross, M . E., 1978, Stratigraphy, structure and petrology of the C o l u m b i a River Basalt in a p o r t i o n of the G r a n d e Ronde—Blue M o u n t a i n s area of O r e g o n and W a s h i n g t o n [Ph.D. dissert.]: M o s c o w , Idaho, University of Idaho, 4 0 7 p. Shubat, M . A., 1979, Stratigraphy, petrochemistry, petrography and structural geology of the C o l u m b i a River Basalt in the M i n a m - W a l l o w a River area, n o r t h e a s t O r e g o n [.M.S. dissert.]: Pullman, Washington,, W a s h i n g t o n State University, 156 p. Swanson, D. A., Anderson, J., Bentley, R. D., Byerly, G. R., C a m p , V. E., G a r d n e r , J. N., and Wright, T . L., 1979a, Reconnaissance geologic m a p of the C o l u m b i a River Basalt G r o u p in eastern W a s h i n g t o n and

HOOPER northern Idaho: U.S. Geological Survey Open-file Report 7 9 - 1 3 6 3 , scale 1 : 2 5 0 , 0 0 0 . Swanson, D. A., Bentley, R. D., Byerly, G. R., G a r d n e r , J. N., and Wright, T . L., 1979b, Preliminary reconnaissance geologic m a p s of the Columbia River Basalt G r o u p in parts of eastern W a s h i n g t o n and n o r t h ern Idaho: U.S. Geological Survey Open-file Report 79-534, scale 1:250,000. Swanson, D. A., Wright, T . L., C a m p , V. E., G a r d n e r , J. N . , Helz, R. T., Price, S. M . , Reidel, S. P., and Ross, M . E., 1980, Reconnaissance geologic m a p of the C o l u m b i a River Basalt G r o u p , Pullman and Walla Walla quadrangles, southeast W a s h i n g t o n and a d j a c e n t Idaho: U.S. Geological Survey Miscellaneous Geologic Investigations M a p 1-1139, scale 1 : 2 5 0 , 0 0 0 . Swanson, D. A., Wright, T. L., and Helz, R. T., 1975, Linear vent systems and estimated rates of m a g m a production and e r u p t i o n for the Y a k i m a Basalt o n the C o l u m b i a Plateau: American J o u r n a l of Science, v. 275, p. 8 7 7 - 9 0 5 . Swanson, D. A., Wright, T. L., H o o p e r , P. R., and Bentley, R. D., 1979c, Revisions in stratigraphic n o m e n c l a t u r e of the C o l u m b i a River Basalt G r o u p : U.S. Geological Survey Bulletin, 1457-G, 5 4 p. Taubeneck, W . H., 1970, Dikes of the C o l u m b i a River Basalt in northeastern O r e g o n , western Idaho and southeastern W a s h i n g t o n , in Gilmour, E. H., and Stradling, D., eds., Proceedings of the second C o l u m b i a River Basalt Symposium: Cheney, Eastern W a s h i n g t o n State College Press, p. 7 3 - 9 6 . Vallier, T. L., and H o o p e r , P. R., 1 9 7 6 , Geologic guide to Hells C a n y o n , Snake River: Geological Society of America, Cordilleran Section Meeting, Pullman, W a s h i n g t o n , 1976, Field Guide no. 5, 3 8 p. Waters, A. C , 1961, Stratigraphic and lithologic variations in the C o l u m b i a River Basalt: American J o u r n a l of Science, v. 259, p. 5 8 3 - 6 1 1 . Wright, T. L., Grolier, M . J., and S w a n s o n , D. A., 1973, Chemical variation related to the stratigraphy of the C o l u m b i a River basalt: Geological Society of America Bulletin, v. 84, p. 371—386. Wright, T. L., Swanson, D. A., Helz, R. T., and Byerly, G. R., 1979, M a j o r oxide, trace element, and glass chemistry of C o l u m b i a River basalt samples collected between 1971 and 1977: U.S. Geological Survey Open-File R e p o r t 79-711, 13 p. M A N U S C R I P T R E C E I V E D BY T H E S O C I E T Y A U G U S T 2 1 , REVISED MANUSCRIPT RECEIVED APRIL 9, MANUSCRIPT ACCEPTED APRIL 2 2 ,

1980

1981

1981

Printed in U.S.A.