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  • The late Eocene Spencer Formation crops out in the low hills on thewestern edge of the central Willamette Valley
  • The late Eocene Spencer Formation crops out in the low hills on thewestern edge of the central Willamette Valley, Oregon. Surfaceexposures in eastern Benton and southeastern Polk Counties and oil by gaswell records and cuttings in Polk, Marion and Linn Counties werestudied to determine Spencer stratigraphy, regional lithologicvariations, and depositional environment. Methods used include: studyof outcrops, petrography, texture, and well cuttings, as well as thecorrelation of well logs and microfossil data of McKeel (1984, 1985).The distribution of the underlying early-late Eocene Yamhill Formationis also briefly considered.The Yamhill Formation consists of the Miller sandstone memberenclosed between mudstones. The lower and middle Yamhill recordshoaling from bathyal to marginal marine depths, and they are overlainby bathyal upper Yamhill mudstones. The Miller sandstone is lens-shaped,trends parallel to the Corvallis fault, and reaches a maximum thicknessof approximately 2,000 feet on the east side of the fault. The Millersandstone grades westward into bathyal mudstones, and eastward into volcanic tuffs and flows. Thinning of the Miller sandstone and upperYamhill mudstone along the Corvallis fault suggests movement duringearly late Eocene. The absence of Yamhill strata along the outcropbelt to the southwest may be related to this tectonic activity.Alternatively, Yamhill strata may have been misidentified as TyeeFormation or Spencer Formation.The Spencer Formation was deposited in a tectonically activeforearc basin during a transgression which was interrupted by severalshort-term regressional progradational events. The Spencer isstratigraphically divided (informally) into a lower sandstone-richmember and an upper mudstone member; it is also divided geographically(informally) into northwestern, east-central, and southern provinces.The lower member is 700 feet thick in the northwestern and southernareas, and thickens to 1400 feet in the east-central area. As comparedto the north and south areas, sandstones in the east-central area arecoarser (fine to medium versus very fine to fine), the sandstone tosiltstone ratio is higher, and volcanic interbeds are more common.Deposition is thought to have been at inner shelf and shoreface depths,grading eastward into nonmarine. In the northwestern area, abundanthummocky cross-bedding of arkosic to arkosic-lithic lower Spencersandstones suggests deposition on a storm wave-dominated shelf.Periods of shoaling to shoreface depths are indicated. In the south,sandstones are markedly more volcanic-rich (dominantly arkosiclitharenites), contain more fossils, and are more highly bioturbated.Shelf-storm deposits in the south are normally graded with a basal lagof coarse volcanic grains and fossils. Besides a more proximalvolcanic source, a shoal barrier within-the southern part of the basin may have caused the different sediment character. Deposition wasprobably at middle to inner shelf depths at the outcrop belt. It mayhave deepened slightly eastward before shoaling to nonmarine in theeasternmost part of the study area. Volcanism was active nearby on theeastern and southeastern margins of the basin. Small volcanic centerswithin the basin may have created highs and acted as localized volcanicsources.As transgression continued, upper Spencer mudstones were depositedat middle to upper bathyal depths. Volcanic activity increased on theeastern edge of the basin. Mudstones grade eastward and upward intotuffs and flows of the eastern Willamette volcanic facies.
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