FLOW DIRECTION OF LATE MIOCENE BASALT AND METAQUARTZITE RIVER DEPOSITS IN THE STARKEY AREA, NE OREGON
Drew Sherman
Science Department - Badgley Hall
Eastern Oregon University, La Grande, OR 97850-2899

Abstract

       The imbrication of cobbles in a gravel layer underlying the 13-14.5-Ma Powder River Volcanic Field olivine basalt 4.7 km northeast of Starkey, Oregon, indicates that direction the stream that deposited the gravel flowed in a northwest direction.  This finding matches the orientation of lava flows in the area.
 

Introduction

       The geological history of northeast Oregon is very complex.  Studies have revealed that the area experienced extensive volcanic activity during the middle Miocene.  Massive outpourings of Columbia River Basalt lavas buried most of northeast Oregon within a 1-2 m.y. time span.  Relatively smaller eruptions continued in northeast Oregon into the late Miocene.  These eruptions produced several well-defined volcanic fields, including the Powder River Volcanics.  Most of the lava flows in the area consist of iron-rich basalts and andesites.  Magnetic polarity and chemistry of the flows are commonly used to map these layers, but very little is known about the paleodrainage patterns during the eruptions of this period (Ferns and McConnell, 2000).

       A well-defined gravel layer underlies a 13-14.5 Ma layer of Powder River olivine basalt near the Starkey store west of La Grande, Oregon.  The gravels are located along the Grande Ronde River ~4.7 km upstream from the Starkey junction off Oregon Highway 244 (Fig. 1).

Figure 1.  Location of the gravel outcrop near the community of Starkey (NE 1/4, SW 1/4, T3S, R35E).

       The outcrop can be reached by heading west from La Grande and taking the Starkey exit off Interstate 84 onto Highway 244.  Follow highway 244 approximately 12.1 miles to the Starkey junction.  Turn left, cross the Grande Ronde River, and follow the signs toward the community of Starkey,  Proceed 5 miles to the outcrop in the quarry located on the southeast side of the highway.

       The gravel underlies a well-exposed Powder River olivine basalt flow in the quarry.  This basalt has not been radiometrically dated, but similar flows have yielded dates between 13-14.5 Ma (Ferns and McConnell, 2000).  The gravel consists of an ~1m thick sequence of poorly sorted basalt and metaquartzite cobbles in a brown, coarse-grained, sandy matrix that contains quartz, muscovite, biotite, amphibole and traces of jasper, chert and rhyolite.  The gravel layer fines upward and the cobbles show a strong imbrication (Fig. 2).

   Figure 2.  Photograph of the poorly sorted basalt and metaquartzite gravel outcrop underlying the Powder River olivine basalt in the vicinity of Starkey,

Oregon.

The purpose of this paper is to present evidence for the drainage direction of the stream that deposited this gravel during the eruption of the Powder River Volcanics based on the measurement of dip directions and the angle of imbrication of the cobbles within the gravel layer.
 

Methods

       110 random samples were collected from the outcrop and measured.  The maximum grain size and composition of each sample were determined.  Graphs of the gravel size and composition were then constructed to aid in the determination of the depositional environment.  The dip direction and angle of imbrication of the cobbles in the gravel deposit were measured at 12.5 cm intervals along the outcrop.  Measurements were then plotted on a rose diagram.  The imbrication measurements were not corrected for the regional dip of the rock layers in this area since these dips are only a few degrees and do not significantly affect the imbrication measurements presented here.
 

Results

       The average maximum size of the poorly sorted basalt and metaquartzite cobbles in the Starkey gravel outcrop is 7.32 cm.  This indicates that this deposit is a cobble gravel (Fig. 3).

   Figure 3.  Distribution of particle sizes (maximum grain size) in the Starkey gravel samples.

       The composition of the gravel is ~5% basalt and ~25% metaquartzite particles.  The coarse grain sizes, well-rounded shapes, poor sorting, strong imbrication and fining-upward character of the deposit suggests that it is a stream deposit.

   Figure 4.  Average composition of the Starkey cobble gravel outcrop.

The imbrication measurements reveal that the stream that deposited the gravels flowed from in a northwest direction, orienting the majority of the cobbles with an imbrication toward the southeast.  A few of the cobbles are oriented in an ENE-WSW direction, transverse to the main flow.

   Figure 5.  Rose diagram showing the direction of imbrication of the cobbles in the Starkey gravel outcrop.  The dominant southeast imbrication direction indicates a flow direction toward the northwest.

Conclusions

       The Starkey cobble gravel outcrop consists of poorly sorted stream gravels.  The dominant southeast imbrication suggests a flow direction from southeast to northwest prior to the eruption of the Powder River olivine basalt flow above it at ~13-14.5 Ma.  This flow direction is similar to the flow directions of lava flows of the same age on the Columbia River Plateau of Oregon and Washington presented by Fecht and others (1987).

       The presence of metaquartzite cobbles in the outcrop suggests that the source of the gravel may have been from central or Eastern Idaho, although it is also possible that these gravels were reworked from Cretaceous to Paleocene fan-delta deposits, fragments of which are still present in the Elkhorns and the Wallowas (Allen, 1991; Vallier, 1991; Cisneros, 1999; Trafton, 1999).

       Further analysis of the Starkey gravel outcrop is needed to determine whether the stream was braided or meandering.  Calculations could also be made to estimate the stream flow velocity and gradient.  Studies of other gravel outcrops in the area between Starkey and Pendleton might help give more of a regional overview of the paleodrainage pattern during the eruptions of the Powder River Volcanics.
 

Acknowledgments

       Special thanks go to Glen Fromwiller, who assisted with the field analysis and writing of this paper.  Jay Van Tassell provided input and helped edit the paper.
 

References Cited

Allen, J.E., 1991, The case of the inverted auriferous paleotorrent? exotic quartzite gravels on Wallowa Mountain peaks:  Oregon Geology, v. 53, no. 5, p. 104-107.

Cisneros, G., 1999, Reconstruction of a Late Cretaceous-Paleocene river system? Geometry of an Early Tertiary nonconformity in the Blue Mountains, northeastern Oregon:  Twelfth Keck Research Symposium in Geology proceedings, p. 291-294.

Fecht, K.R., Tallman, A.M., and Reidel, S.P., 1987, Paleodrainage of the Columbia River system on the Columbia Plateau of Washington State: A summary:  in  Schuster, J.E., ed., Selected papers on the geology of Washington:  Washington Division of Geology and Mineral Resources Bulletin 77, p. 219-248.

Ferns, M. and McConnell, V., 2000, Field trip guide to the Tower Mountain caldera, Eastern Oregon:  unpublished field guide, Baker City, Oregon, Oregon Department of Geology and Mineral Industries.

Trafton, K.S., 1999, Paleogeographic implications of Late Cretaceous-Early Tertiary quartzite-bearing fluvial sediments, Elkhorn Mountains, northeast Oregon: Twelfth Keck Research Symposium in Geology proceedings, 287-290.

Vallier, T., 1991, Correspondence:  Oregon Geology, v. 53, no. 6, p. 129.

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