STRATIGRAPHY AND AGE OF THE LATE MIOCENE SEDIMENTS AND VOLCANIC DEPOSITS ALONG THE BAKER-COPPERFIELD HIGHWAY BETWEEN BANTA ROAD AND THE LOVE RANCH, KEATING, OREGON

Rob Ledgerwood and Jay Van Tassell

Science Department, Eastern Oregon University, One University Boulevard, La Grande, OR 97850; jvantass@eou.edu

Abstract

An ~8.6 m.y.-old ash in the sedimentary sequences exposed along the Baker-Copperfield highway between the Banta Road and Middle Bridge Road intersections in the Powder Valley east of Baker City and south of Keating, Oregon, confirms that these sediments are late Miocene in age. The sequences include stream, fan delta, and lake sediments that contain ash and diatoms, including one 30 cm-thick diatomite layer. The types of diatoms present in the sequences indicate an overall deepening of the lake prior to the deposition of the ash. After the ash was deposited, the water depths grew shallower in some areas and deeper in others.

The late Miocene Baker-Copperfield highway sequences contain diatom species similar to those in the Chalk Bluff/Bully Creek Formation of Malheur County, Oregon (Van Landingham, 1966, 1985) and the Chalk Hills Formation of western Idaho. This indicates that the Chalk Hills stage of Lake Idaho may have extended into the Powder Valley area during the late Miocene, but it is also possible that these late Miocene sequences were deposited in a separate lake basin that may have drained into the Lake Idaho system. The lack of an 8.6 Ma ash layer in the Grande Ronde Valley sedimentary sequence to the north suggests that the lake system in the Powder Valley area did not drain northward into the Columbia River system through the Grande Ronde Valley at this time. Since deposition, the late Miocene sequences exposed along the Baker-Copperfield highway have been tilted, eroded, covered with braided stream and alluvial fan gravels, and then tilted and eroded again.

Introduction

The geology of the Powder Valley area of northeast Oregon (Fig. 1) was first mapped by Lindgren (1901), who focused his attention on the adjacent gold mining areas. Gilluly (1937) recognized that the sedimentary and volcanic sequences in the Keating Valley overlie and are interbedded with the lava flows of the Columbia River Basalt Group. Prostka (1962) and Whitson (1988) have also mapped parts of the area. Brooks and others (1976) studied the area as part of a larger-scale mapping project and lumped the sediments and tuffs in the sequence into one unit. Bailey (1990) used geochemical analyses to subdivide the volcanic sequence into the Columbia River Basalt Group and the Powder River Volcanic Field, and confirmed that pyroclastic materials as well as fluvial and lacustrine sediments rich in volcaniclastic material that were lumped into the unit of Tertiary sediments and tuffs (Tst) by Brooks and others (1976) occur as: 1) thin

Figure 1. Locations of the sections along the Baker-Copperfield highway (OR 86) described in this paper. Section 1, located 0.2 miles east of the intersection with Banta Road , is UCMP locality 604371. Section 2, located 0.9 miles east of the intersection with Banta Road , is UCMP localities 604372 and 604373. Section 3, located near Love Ranch 0.4 miles east of the intersection with the eastern end of Middle Bridge Road , is UCMP locality 604375.

interbeds throughout the volcanic sequence, 2) a unit between the Grande Ronde Basalt unit of the Columbia River Basalts and the overlying olivine basalt that marks the lower boundary of the Powder River Volcanics, and 3) a unit above the Powder River volcanic sequence (Fig. 2). Gilluly (1937) and Brooks and others (1976) also recognized a prominent gravel layer that rests unconformably on top of the older rocks and sediments in the area.

One exciting early fossil find in the study area was the discovery of the lower jaw and other parts of the Miocene mastodon Gomphotherium that were found in 1924 by Albert Werner fifteen miles east of Baker City (Downs, 1952). Plant fossils were discovered approximately a decade later in the Late Tertiary diatomites interbedded with other sediments above the lavas of the Columbia River Basalt Group in the Keating area (Gilluly, 1937) and identified by paleobotanist Ralph Chaney as the leaves, fruits, nuts and seeds of maple, hornbeam, hickory, sweetgum, magnolia, tupelo gum, oak, redwood, swamp cypress, and water chestnut. Chaney (1959) described the Keating flora as "the swamp cypress component of the Baker area and suggested that the plant fossils came from poorly drained lowlands and well-drained slopes near the lowlands). Hoxie (1965) describes additional species in the Keating flora.

Figure 2. Generalized stratigraphy of the Tertiary sediments and volcanics of the study area.

Moore (1937) published an extensive study of the Keating diatomite deposits in preparation for possible mining of the deposits. Other diatom deposits less than a meter thick interbedded with volcanic ash and silt occur south of Keating along the Baker-Copperfield highway (Fig. 1). These diatomites were sampled and studied by Van Landingham (1966, 1985). The presence of the diatom Gomphonema cholnockyites, which has a stratigraphic range of upper Barstovian to middle Hemphillian, led Van Landingham (1985) to conclude that this sedimentary sequence is equivalent to the late Miocene Bully Creek or Chalk Bluff Formation near Bridge Creek in Malheur County, Oregon.. Bailey's (1990) geologic mapping suggests that the diatomites sampled by Van Landingham (1966, 1985) may overlie the Miocene olivine basalt flows in the area, which supports Van Landingham's conclusion that the diatomites and the sediments they are interbedded with are late Miocene in age, but faulting in the area leaves this interpretation open to question and the relationship of these diatomites to the ones that contain the Keating flora are not known.

       The objective of this research is to present the results of stratigraphic measurements and analysis of samples from the outcrops along the Baker-Copperfield highway studied by Van Landingham (1966, 1985), including a new Ar/Ar age date for a sample of a volcanic ash associated with the diatomite deposits in the sequence.

Methods

The three sections along the Baker-Copperfield highway studied by Van Landingham (1966, 1985) were measured by the authors on January 30, 2005 using the Jacob staff technique and samples were collected from each of the major sedimentary units present. The sections were located with a handheld GPS receiver. The sections (Fig. 1) include: 1) a section 0.2 miles east of the intersection with Banta Road (UCMP locality 604371) at 44° 50.099' N latitude, 117° 35.045' W longitude, 2) a section 0.9 miles east of the intersection with Banta Road (UCMP localities 604372/604373) at 44° 50.092'N latitude, 117° 34.316' W longitude (Fig. 2), and 3) a section at Love Ranch 0.4 mi west of the intersection with eastern end of Middle Branch Road (UCMP locality 604375) at 44° 49.609'N latitude, 117° 28.598' W longitude.

Jay Van Tassell returned to the westernmost outcrop (#1), measured the lower part of the sequence, and collected additional samples on November 11, 2005. The samples from the three outcrops were dried and described using a Leica Zoom 2000 binocular microscope. Smear slides were made by mixing small fractions of each sample with distilled water, dropping the water-sediment mixture on a petrographic slide, drying the slide on a hot plate, and then mounting a cover slip on top of the slide using Type A mounting medium. These slides were then examined at 400 power on a Leitz Laborlux Pol petrographic microscope and the percentages of different types of diatoms, ash, and other constituents were determined by point-counting.

Figure 2. Photograph of the section along the Baker-Copperfield highway 0.9 miles east of the Banta Road intersection. Note the bedded silt and ash layers overlain unconformably by a gravel unit.

A sample of a pure gray ash from near the top of the section along the Baker-Copperfield Highway 0.2 miles east of the Banta Road intersection (Section #1) was sent to the Geochronology Laboratory at the University of Alaska, Fairbanks, for ⁴⁰Ar/³⁹Ar step heat analysis.

Fossils

The fossils in the outcrops along the Baker-Copperfield highway studied by Van Landingham (1966, 1985) include diatoms, sponge spicules, and mammal bones. The only diatomite that was identified in the outcrops is a 30 cm-thick layer in the section along the Baker-Copperfield Highway 0.2 miles east of Banta Road, but diatoms are found in many of the layers in the sedimentary sequence. The diatoms include Anomeoneis, Asterionella, Aulacoseira (Melosira), Cocconeis, Cyclotella, Cymbella, Diatoma, Diploneis, Epithemia, Fragilaria, Gomphonema, Meridion, Opephora, Navicula, Nitzchia, Pinnularia, Stephanodiscus, Synedra, and Tetracyclus. The species of Opephora present in the sequence closely resembles O. glangeaudi illustrated by Van Landingham (1985).

Based on comparisons with diatoms in the Harper district of Oregon (Abbott, 1970), this assemblage of diatoms suggests clean freshwater with a pH of 7 and greater, both running and stagnant (calm) water, and lake environments that were eutrophic (shallow, rich in organic matter and nutrients with oxygen depletion in the hypolimnion during the summer). There are more benthic (littoral, epipelic, epilithic, and epiphytic) species than planktic diatom species, suggesting generally shallow water depths, but the planktic diatoms Aulacoseira and Stephanodiscus are abundant in some layers, suggesting deeper water depths. The sponge spicules, mostly Ephydatia fluviatilis, indicate shallow water depths, probably less than 1.5 m (Bright, 1982).

Overall, the diatoms and sponge spicules present suggest a range of water depths from shallow to deep. A bone was found in laminated silts near the top of the section along the Baker-Copperfield Highway 0.2 miles east of the intersection with Banta Road and identified by Laura Mahrt as the toe bone of a small mammal (Laura Mahrt, personal communication, February, 2005). This suggests the presence of nearby land.

Section Descriptions

The section along the Baker-Copperfield Highway 0.2 miles east of the intersection with Banta Road is the longest of the three sequences that were measured (Fig. 4). The bottom of the section appears to rest on top of the olivine basalt layer mapped by Bailey (1990). There are several faults cross-cutting the base of the sequence so the actual thickness of the sequence is not known. The base of the sequence has several intervals covered by vegetation and includes ~28 m of very pale brown and light gray silt with 0-2% diatoms, including Aulacoseira, Fragilaria, and Rhoicosphenia(?). This is overlain by 24 m of white silt with some ash and up to 30% diatoms, including Aulacoseira, Fragilaria, Cocconeis, and Opephora. The next 2.5 m is covered by sands and gravels slumped from overlying beds and is overlain by 7 m of light gray silt which contains some ash and 7% diatoms (mainly Fragilaria). This silt is overlain by a very pure 0.3 m-thick white diatomite layer that contains Anomoeneis, Asterionella, Aulacoseira, Cocconeis, Cymbella, Diatoma, Epithemia, Fragilaria, Gomphonema, Meridion, Opephora, Navicula, and Synedra. Planktic diatoms outnumber benthic diatoms in this diatomite. The diatomite is overlain by 0.25 m of light gray to white vitric ash that contains rare diatoms. This is the ash that was dated using ⁴⁰Ar/³⁹Ar techniques. The ash is overlain by 0.25 m of light grayish yellow to white silt with ash and 5% diatoms, including Aulacoseira, Cocconeis, Cyclotella, Diatoma, Epithemia, Fragilaria, Navicula, and Tetracyclus. The top of the section consists of 0.8 m of light tan laminated and cross-laminated silt that lacks diatoms. The toe bone of a small mammal was discovered in this unit. The cross-laminations suggest a paleocurrent flowing toward the NNW (352°). This silt is separated from the overlying brown jasper, metaquartzite, and diorite pebble and cobble conglomerate by an angular unconformity.

The section along the Baker-Copperfield Highway 0.9 miles east of the Banta Road intersection contains more indicators of current activity than the section 0.2 miles east of the Banta Road intersection (Fig. 5). It begins with 1.3 m of light brown silty sand and sandy silt that contains ash particles and 9% diatoms, including Cocconeis, Cyclotella, Cymbella, Meridion, Opephora, and Tetracyclus. This is overlain by 0.1 m of light brown silty sand with some ash particles. This unit contains 6% diatoms, including Aulacoseira, Cocconeis, Cymbella, Fragilaria, Navicula, Pinnularia, and Stephanodiscus. This is overlain by 0.1 m of light brown cross-bedded sand with ash and 1% diatoms, including Aulacoseira, Cymbella, Fragilaria, Pinnularia, Stephanodiscus, and Tetracyclus. The cross-beds suggest paleocurrents flowing to the northwest (305°). This unit is followed by 1.0 m of light brown laminated sandy silt that lacks diatoms. This unit includes some pebbles in the middle part and is overlain by 0.5 m of silt that contains ash and 2% diatoms (Fragilaria). This is overlain by 1.3 m of white to light gray ash that under the microscope closely resembles the vitric ash found near the top of the section 0.2 east of the Banta Road intersection. The ash contains rare diatoms, including Aulacoseira, Cyclotella, Cymbella, Epithemia, Fragilaria, Navicula, and Tetracyclus. It is cross-bedded, cross-laminated, and parallel-laminated in the upper part. The cross-bedding suggests paleocurrents flowing to the NW (340°). The ash sequence is overlain by 3.0 m of light brown to white silt which contains ash particles, sponge spicules, and ~2-10% diatoms, including Aulacoseira, Cymbella, Cocconeis, Fragilaria, Gomphonema, Navicula, Nitzchia, and Pinnularia. This silt is separated from the overlying light brown pebble and cobble gravel by an angular unconformity. Imbrication in the pebble and cobble gravel suggests paleocurrent directions flowing toward the NNE (16°).

The Love Ranch section is the shortest section measured (Fig. 5). It starts with 0.3 m of white silt with no diatoms. This silt is overlain by 0.3 m of light gray ash with no diatoms. Under the microscope, this ash resembles the ash near the top of the section along the Baker-Copperfield Highway 0.2 miles east of the Banta Road intersection. The ash is overlain by 0.2 m of light tan silt with some ash and 2% diatoms, including Aulacoseira, Fragilaria, Nitzchia(?), and Pinnularia. This silt is separated from the overlying pebble and cobble gravel by an angular unconformity. 

Facies and Environmental Changes

The three sections that were measured include a number of sedimentary facies. One of the most common facies consists of diatom-rich silts and sandy silts, plus diatomites, which were deposited in lake environments. These include: 1) sediments rich in benthic and planktic diatoms, including Stephanodiscus, that indicate deeper water settings, 2) sediments that are rich in benthic and planktic diatoms such as Aulacoseira, that were deposited in shallow, open water, and 3) sediments that contain sponge spicules and benthic diatoms that are indicative of shallow, marshy conditions. A second sedimentary facies that is prevalent in the section along the Baker-Copperfield Highway 0.9 miles east of the Banta Road intersection consists of massive, trough cross-bedded, and laminated ash-rich sands and sandy silts that have been reworked and deposited by currents (Fig. 6). Some of these beds were probably deposited in river channel environments, but others contain diatoms that suggest that they were washed into a lake by rivers and then deposited, perhaps in a fan delta environment. A third sedimentary facies in the sequences includes massive and parallel-laminated silts with no diatoms. The mammal toe bone was found in this facies, which may have been deposited in a floodplain environment. These sedimentary facies are overlain unconformably by imbricated pebble and cobble gravels that may have been deposited by braided streams, possibly in an alluvial fan environment.

Figure 4a. Lower part of the Late Miocene sequence exposed along the Baker-Copperfield highway 0.2 miles east of the intersection with Banta Road. Upper part of section is shown on the next page.

Figure 4b. Upper part of the late Miocene section exposed along the Baker-Copperfield highway 0.2 miles east of the intersection with Banta Road.

Figure 5. The late Miocene sections exposed along the Baker-Copperfield highway 0.9 miles east of the intersection with Banta Road and near the Love Ranch.

Figure 6. Photograph of cross-bedded and channeled facies along the Baker-Copperfield highway 0.9 miles east of the Banta Roadintersection. Note the large rip-up clast.

The section 0.2 miles east of Banta road indicates an overall upward change from floodplain to shallow, marshy environments to shallow open lake conditions and finally to deep water lake environments prior to the deposition of the ash layer above the diatomite layer in the sequence. This suggests deepening of the lake, perhaps related to subsidence in the area and/or climate change. Ash deposition began in the middle portion of the sequence. After the deposition of the ash bed above the diatomite layer in the upper part of the sequence, the water depth grew shallower, changing from deep water to shallow open water, and finally to floodplain conditions. This change may have been initiated by the input of sediment into the area related to volcanism and, perhaps, due to uplift and climate change in the area. After this the sediments were tilted, eroded, and then covered by the imbricated alluvial fan/braided stream sequence, which was later tilted and eroded.

       The section 0.9 miles east of Banta Road shows more evidence of reworking of sediments by currents. Deposition began with shallow, open lake silts followed by deposition of deeper lake sediments. The overall deepening of the lake water in the area was then interrupted by the deposition of massive and parallel-laminated silts and cross-bedded pebbly sands, which was followed by the deposition of shallow, open-lake. Next, a sequence of cross-bedded and parallel-laminated ash was deposited, perhaps as a fan delta prograded out into the lake. After the deposition of the ash slowed, the paleoenvironments changed from shallow and marshy lake margin environments to shallow open water sedimentation. This was followed by tilting, erosion, deposition of the imbricated gravel sequence, and more tilting and erosion.

The section exposed at Love Ranch started with the deposition of massive floodplain silts and ash, followed by a change to shallow open lake conditions. Like the other sections, tilting, erosion, deposition of the imbricated gravel sequence, and more tilting and erosion followed the deposition of the sediments at this locality.

Overall, the sequences record a change from deposition of floodplain and lake sediments that do not have a significant amount of ash to sediments that are rich in ash. This suggests an increase in volcanic activity in nearby areas. The differences between the sediments at the top of the section 0.2 miles east of the Banta Road intersection and the other two sections may reflect their relative locations at the time of deposition. At a time just prior to the deposition of the ash layer, the section 0.2 miles east of Banta Road was located in a relatively deep lake setting, while the section 0.9 miles east of Banta Road was located at a site where a fan delta prograded out into the lake, redepositing ash that had been eroded and washed in from upstream areas. At this time the section at Love Ranch appears to have been located on the subaerial portion of the fan delta or in a floodplain along the river that fed the fan delta (Fig. 7). Variations in the amount of sediment washed into each area, coupled with variations in subsidence related to faulting in the area, may explain the different patterns of deposition in the three areas.

Geologic Age of the Sequence and Origin of the Ash

The presence of the diatom Gomphonema cholnokyites in the Keating sequences suggests that they are upper Barstovian through middle Hemphillian in age (~12.5-6 m.y. old) and possibly equivalent to the late Miocene to Pliocene Bully Creek or Chalk Butte Formation near Bridge Gulch, Malheur County, Oregon (Van Landingham, 1985). In addition, we identified a species of Opephora which closely resembles O. glangeaudi. This diatom species, according to Van Landingham (1985), has a stratigraphic range of upper Barstovian through middle Hemphillian, and occurs in the lower Idaho Group in the Mann Creek deposits of  Washington County, Idaho (Chalk Hills Formation).

The ⁴⁰Ar/³⁹Ar step heat analysis of the ash sample from the upper part of the sequence along the Baker-Copperfield Highway 0.2 miles east of the intersection with Banta Road confirms Van Landingham's (1985) age for these sequences. Both analyses of the ash showed excess argon so that a traditional plateau age could not be obtained, which is not abnormal for ash samples. The analyses gave isochron ages of 8.61 ± 0.14 Ma and 8.56 ± 0.08, which are identical within error. The average of these two ages is 8.57 ± 0.07 (1 sigma error). This suggests that the sequences are late Miocene (Tortonian age) and were deposited during the late Clarendonian mammal age.

The age of the ash in the late Miocene Baker-Copperfield road sequences is the same, within experimental error, as the 8.48 ± 0.05 Ma age obtained by Deino and Grunder (unpublished; cited by Streck and Ferns, 2004) for the Prater Creek ash flow tuff in its type section on the walls of Poison Creek north of Burns, Oregon. This large ashflow sheet originated from an unknown vent somewhere in Harney County, Oregon, and has a known volume of over 10 cubic kilometers (http://users.bendnet.com /bjensen/ volcano/largerup.html). It seems likely that this is the source of the ash in the late Miocene Baker-Copperfield road sequences east of Baker City and south of Keating.

Figure 7. Paleoenvironments at ~8.7 Ma in the Keating area, modified from an illustration by Swirydczuk, K., Wilkinson, B.H., and Smith, G.R. (1979) for the depositional setting of the Pliocene Glenns Ferry Formation. The numbers indicate the approximate positions of the three sections described in this study. The paleoclimate record for the Grande Ronde Valley

(from Van Tassell and others, 2001) suggests that this was a wet period.

Comparison with Other Areas: A Lake Idaho Connection?

The late Miocene sediments in the sequences along the Baker-Copperfield highway east of Banta Road are very different from the diatomite sequences in the Keating area described by Moore (1937) and Gilluly (1937) and the late Pliocene Always Welcome Inn sequence near Baker City described by Davis and others (2005) because they contain significant amounts of ash. The late Miocene Baker-Copperfield road sequences also lack the thick diatomite layers and abundant plant and fish fossils found at these other localities.

As Van Landingham (1985) has pointed out, the diatoms in the late Miocene Keating sequences suggest a connection with the late Miocene to Pliocene Chalk Butte or Bully Creek Formation in Malheur County, Oregon. Kimmel (1982) pointed out that recent correlations based on fish fossils, volcanic ash chemistry, and fission dates suggest that the sediments placed in the Chalk Butte Formation include late Miocene Chalk Hills Formation sediments and sediments which are part of the late Pliocene Glenns Ferry Formation. As a result, Kimmel reassigned the sediments in the Adrian, Oregon, area that had been originally assigned to the Chalk Butte Formation and later to the middle Miocene Deer Butte Formation to the Chalk Hills Formation and suggested that they appear to have been deposited in the same basin as those of the Chalk Hills Formation. It is possible that the Chalk Butte or Bully Creek Formation in the Bridge Creek area that contains the same diatoms as those in the late Miocene Baker-Copperfield road sequences may also be equivalent to the Chalk Hills Formation. Our identification of Opephora glangeaudi in the late Miocene Keating sequences also strengthens the possibility of a connection with the Chalk Hills Formation of the western Snake River Plain.

The ash-rich sediments along the Baker-Copperfield Highway resemble the Chalk Hills Formation sediments in that both consist of sediments deposited in lake and stream environments. Middleton and others (1985) recognized horizontally stratified and cross-stratified sandy river deposits and cross-stratified silt, parallel-laminated mud, and diatomite lake facies in the Chalk Hills Formation that are similar to those in the late Miocene sequences along the Baker-Copperfield Highway.

Sediments and fossils of the late Miocene (~6-11 Ma) Chalk Hills Formation and the late Pliocene Glenns Ferry Formation preserve the record of Lake Idaho, the very large freshwater lake that occupied the western Snake River Plain episodically during the late Miocene and late Pliocene (Cope, 1870, 1883a,b; Russell, 1902; Lindgren and Drake, 1904; Malde and Powers, 1962; and many others). The similarities between the diatoms in the Chalk Hills Formation and the late Miocene sediments along the Baker-Copperfield highway east of Banta Road suggest the possibility that Lake Idaho extended into the Powder Valley area during the late Miocene. It is also possible that the lake in the Keating area was a smaller lake that may have drained into the late Miocene Chalk Hills lake system.

Kimmel (1982) suggested that damming of the outlet of Lake Idaho during the late Miocene by volcanic activity in eastern Oregon prompted progressive infilling and hypothesized that draining of the Chalk Hills Lake was the result of subsidence of the outlet area in late Miocene times, in contrast to the diversion of the Snake River into the Columbia River that resulted in the draining of the Glenns Ferry Lake around 2 million years ago (Wheeler and Cook, 1954). Both events resulted in unconformities on the tops of the lacustrine sequences. Similar events, combined with local tectonics and climate changes, may have produced the angular unconformity and gravel deposits on top of the late Miocene Keating sequences. The ages of the unconformity and gravel deposits need to be determined in order to better understand when and why they formed.

At the same time (~8.6 Ma) that river, fan delta, and lake sediments were being deposited in the late Miocene sequences along the Baker-Copperfield highway area, shallow open-water lake sediments were deposited in the Grande Ronde Valley, which was just beginning to form to the north (Van Tassell and others, 2001). The succession of environments preserved in the Grande Ronde Valley sediments suggests that this was a wet climatic period (Fig. 5). The suite of benthic and planktic diatom genera present in the Grande Ronde Valley sediments is very similar to the suite of diatoms in the Keating area, indicating a possible connection, but volcanic ash of the same age as the layers preserved in the late Miocene Keating sequences has not been found in the Grande Ronde Valley cores. This suggests that the two areas were not connected at ~8.6 Ma and indicates that Lake Idaho did not drain through the Grande Ronde Valley as suggested by Livingston (1928), Jenks and Bonnichsen (1989), and Van Tassell and others (2001) until later in the geologic history of the area.

Conclusions

An ⁴⁰Ar/³⁹Ar date of ~8.6 Ma in the ash- and diatom-rich sedimentary sequences exposed along the Baker-Copperfield highway between the Banta Road and Middle Bridge Road intersections confirms Van Landingham's (1966, 1985) suggestion that these sediments are late Miocene in age. The ash may be the Prater Creek tuff, a large silicic ash-flow tuff unit that was erupted from an unknown vent in Harney County, Oregon. Streams washed the ash downslope, across a fan delta, and into a lake, depositing ash on top of diatomite in the deeper part of the lake.

The types of diatoms present in the sequences indicate an overall deepening of the lake prior to the deposition of the ash. After the ash was deposited, the water depths grew shallower in some areas and deeper in others. This may reflect the relative balance between sediment deposition rates and rates of faulting in the lake basin.

The late Miocene Baker-Copperfield highway sequences contain diatom species similar to those in the Chalk Bluff/Bully Creek Formation of Malheur County, Oregon (Van Landingham, 1966, 1985) and the Chalk Hills Formation of western Idaho. This suggest that the Chalk Hills lake system, the late Miocene stage of Lake Idaho, may have extended into the Powder Valley area, but it is also possible that the late Miocene sequences in the Baker-Copperfield highway area were part of a separate lake basin that may have drained into Lake Idaho during the late Miocene.

Comparison with the sediments deposited in the Grande Ronde Valley to the north between 8-9 Ma suggests that the late Miocene sequences along the Baker-Copperfield highway were deposited during a wet climate interval when lake levels were high. The lack of an 8.6 Ma ash layer in the Grande Ronde Valley sequence suggests that the lake system in the Powder Valley area did not drain northward into the Columbia River system through the Powder and Grande Ronde Valleysas it did later during the late Pliocene.

Since they were deposited, the late Miocene sequences exposed along the Baker-Copperfield highway have been tilted, eroded, and covered with braided stream and alluvial fan gravels, which have since been tilted and eroded. Further research is needed to determine if the deposition of these gravels occurred after the draining of the late Miocene (Chalk Hills) or the late Pliocene (Glenns Ferry) stages of Lake Idaho and to better understand the relationship of these gravels to tectonics and climate change in the area.

Acknowledgments

The authors are grateful to Sarah Smith, who assisted with measuring the sections, and Mark Ferns, who first showed Jay Van Tassell the late Miocene section 0.9 miles east of the Banta Road intersection. Laura Mahrt identified the bone from the section 0.2 miles east of the Banta Road as the toe bone of a small mammal. Jason McClaughry provided astute comments in the field that were very helpful. Special thanks go to Paul Layer and Jeff Drake of the Geochronology Laboratory of the University of Alaska at Fairbanks for working over Thanksgiving break to provide us with the date for the ash in the section 0.2 miles east of the Banta Road intersection.

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