BULLETIN OF THE GEOLOGICAL. SOCIETY OF AMERICA VOL. 48. PP. 961-972. PUS. 77-78.1 FI6. JUNE 30.193S BASALTIC ROCKS IN THE UMPQUA FORMATION *f BY FRANCIS G. WELLS AND AARON C. WATERS (Accepted by the Committee on Publications, 1934) CONTENTS Page Introduction................................................................................................................... 961 Summary of geology..................................................................................................... 963 Effusive rocks................................................................................................................ 963 Amygdaloidal and ellipsoidal basalts................................................................. 963 Distribution.................................................................................................... 963 Petrography................................................................................................... 963 Structure and extrusive origin of the basalts............................................ 964 Palagonite tuff and breccia.................................................................................. 966 Intrusive rocks.............................................................................................................. 967 Olivine basalt dikes.............................................................................................. 967 Gabbro and norite................................................................................................. 968 Norite sills in the Umpqua formation.............................................................. 968 Distribution and character.......................................................................... 968 Petrography.................................................................................................. 968 Volcanic necks and dikes cutting the Calapooya formation.......................... 968 Distribution and character.......................................................................... 968 Petrography................................................................................................... 969 Hypersthene basalt dikes..................................................................................... 969 Chemical differences between Eocene and Miocene (?) igneous rocks................. 971 INTRODUCTION In 1930, during a study of the geology of an area in west-central Oregon within which occur the quicksilver deposits of Blackbutte, Elk- head, Bonanza, and Nonpareil, a series of extrusive and intrusive basal­ tic igneous rocks were encountered. Part of this area lies within the Roseburg quadrangle, mapped by Diller in 1898. Diller called all these rocks “diabase.” 1 It is the purpose of this paper to show that the basaltic rocks include amygdaloidal and ellipsoidal basalt flows, olivine basalt dikes, norite sills and necks, and hypersthene-augite * Manuscript received by the Secretary of the Society, September 10, 1934. t Published by permission of the Director, U. S. Geological Survey. 1J. S. Biller: Description of the Roseburg quadrangle [Oreg.], U. S. Geol. Surv., Geol. Atlas, Roseburg folio, no. 49 (1898) p. 3. (961) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 962 F. G. WELLS, A. C. WATERS----BASALTIC HOCKS IN UMPQUA FORMATION EXPLANATION BEDDED ROCKS <§¡1!i AlluviumE3 1( ESI ,p (Calapooya formation ^ UNCONFORMITY „[ Tev<QT,b Umpqua formation (sandstone and shale. Tev, and mterbeddedvp basalt flows.Teb) INTRUSIVE ROCKS tf E3 1 1 Noritic rocks E3 I I. Olivine diabase Strike and dip of beds F igure 1.— Geologic sketch map of Blackbutte-Elkhead-Nonpareil area, Oregon Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 INTRODUCTION 963 basalt dikes and that they represent more than one period of igneous activity. SUMMARY OF GEOLOGY A full discussion of the geology of this area is found in a recent bulle­ tin of the United States Geological Survey,2 it can be briefly sum­ marized as follows: The southwestern part of the area (Fig. 1) is underlain by the Umpqua formation, a series of interbedded sandstones and shales of Eocene age, that have been compressed into gentle folds with a north­ west trend. Intercalated in and contemporaneous with the sandstones are lenses of conglomerate, beds of palagonite tuff, and flows of amygda- loidal and ellipsoidal basalt. The Umpqua formation has also been intruded by norite sills and basalt dikes. The Calapooya formation, a series of volcanic conglomerates, andesitic tuffs, breccias, and flows, of Eocene (?) age, that dips at a low angle toward the east, rests un- conformably on the Umpqua formation in the eastern part of the area. The Calapooya formation is cut by basalt dikes and norite necks. EFFUSIVE ROCKS AMYGDALOIDAL AND ELLIPSOIDAL BASALT FLOWS Distribution.—Basalt flows are exposed in irregular, usually elongate outcrops, the long axes of which trend parallel to the strike of the Umpqua beds. These outcrops are most abundant in the southern part of the area, in Township 25 South, Range 4 West, and in a large elliptical area in Township 23 South, Ranges 4 and 5 West. Petrography.—Megascopically, specimens of the basalt are dark greenish gray to black and are usually aphanitic—though the amygda- loidal varieties are commonly porphyritic, the phenocrysts of plagio- clase being aggregated together in more or less conspicuous clots. A few small vesicles filled with zeolite minerals are present in most of the specimens. The amygdaloidal varieties are characterized by abundant white amygdules a centimeter or more in maximum diameter. The cavernous areas between the ellipsoids as well as the closely spaced radial joints have greatly facilitated weathering, and as a result the glass selvages are commonly stained. Under the microscope the lava is seen to be of normal basaltic char­ acter. In general the texture of the ellipsoidal flows is intersertal; of the amygdaloidal flows, intergranular. Augite and labradorite (An56) *F. G. Wells and A. C. Waters: Quicksilver deposits of sottthwestem Oregon, TJ. S. Geol. Surv., Bull. 850 (1934). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 964 F. G. WELLS, A. C. WATERS— BASALTIC ROCKS IN TJMPQUA FORMATION are the most abundant minerals and are present in approximately equal amounts, the augite with much opaque dark-brown glass occupying the spaces between the randomly arranged minute laths of labradorite. Augite and labradorite occur in two generations, and in each the pheno- crysts are aggregated together in glomeroporphyritic clots. Pseudo- morphs of a yellowish-brown alteration product after olivine are spar­ ingly present. Magnetite is a common accessory, and minute crystals of apatite are also found. In some specimens the groundmass has been partly altered to serpentine and a reddish-brown product. The amygdaloidal cavities commonly present an outer covering of chlorite and a central filling of fibrous chalcedony with negative elongation, or they may be filled with calcite and zeolites, of which thomsonite (PI. 77, fig. 1)—easily recognized by its low relief, positive elongation, and parallel extinctions—is the most abundant. Although glass is very abundant in the groundmass of the ellipsoidal basalts, the dark-brown opaque glass that forms the mesostasis be­ tween the adjacent minerals differs strikingly from the transparent glass formed by chilling in contact with water at the surface of the ellipsoids. This is well shown by a thin section of glass breccia exposed in a small quarry near the northeastern extension of the lava mass on Banks Creek. Under the microscope this rock is seen to be composed of large shards of perfectly clear light yellowish-brown glass in which are embedded a few tabular crystals of feldspar and grains of augite (PI. 78, fig. 1). The glass is perfectly isotropic. It is traversed by numerous cracks, along which it has been hydrated to a yellowish col- loidal-appearing substance showing very faint aggregate polarization and a relief less than that of canada balsam. This specimen prob­ ably represents an incipient stage in the alteration of the glass to palagonite. In the same thin section, fragments of exceptionally glassy lava may also be seen, but the glassy base of the lava is of the opaque variety. A chemical analysis of the flow rock (Table 1, col. 1) shows it to be a typical basalt, corresponding closely to the average of plateau basalts given by Daly (Table 1, col. 3). Structure and extrusive origin of the basalts.—Though most of the flows are massive, showing no structure, they may grade up­ ward into amygdaloidal or ellipsoidal material, or, more commonly, the whole flow may contain such material. In extreme examples, filled amygdules may constitute as much as 25 percent of the mass, and the ellipsoidal flows may consist of large individual pillows embedded in Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 BULL. GEOL. SOC. AM. VOL. 46, 1935, PL. 77 F i g u r e 1. BASALT FROM GASSY CREEK F i g u r e 2. DIABASE DIKE Showing a large amygdule filled with thomsonite. Exposed in the northwestern part of the Nonpareil- (Ordinary light, X 18) Note the low degree of crys- Bonanza area. Light gray (plagioclase),