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DOGAMI Open-File Report O-92-10, Preliminary Geologic Map of The

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DOGAMI Open-File Report O-92-10, Preliminary Geologic Map of The OPEN-FILE REPORT 0-92-10 PRELIMINARY GEOLOGIC MAP OF THE MCCAIN CREEK QUADRANGLE MALHEUR COUNTY, OREGON By M. L. Ferns Oregon Department of Geology and Mineral Industries This unpublished Open-File Report has not been reviewed and may not meet all Oregon Department of Geology and Mineral Industries' standards. Field work conducted in 1991 Map Scale: 1:24,000 Funding Statement: Funded jointly by the Oregon Department of Geology and Mineral Industries, the Oregon State Lottery, and the U. S. Geological Survey COGEOMAP Program as part of a cooperative effort to map the west half of the lo by 2O Boise sheet, eastern Oregon. McCain Creek Low-silica rhyolite flows that comprise the summit of Mahogany Mountain are the oldest rocks exposed in the McCain Creek quadrangle. The sparsely phyric, flow-foliated flows make-up a flow-dome complex, which, according to Rytuba and others, erupted prior to the collapse of the Mahogany Mountain caldera. The high-standing escarpment on the north side of Mahogany Mountain is believed to mark the south wall of the caldera. The caldera is filled with a thick accumulation of tuffaceous surge deposits, non-welded ashflow tuff, and airfall tuff of units Ttlg and Ttsc. The tuff of Leslie Gulch (Ttlg) is the stratigraphically lowest unit exposed within the caldera. According to Vander Meulen and others (1987),the tuff of Leslie Gulch is made up mainly of orange and pale-yellow, crystal-poor, lithic ashflow and airfall tuffs that are generally non- to poorly- welded. Age is reportedly middle Miocene la 15.5 Ma). The irregular surface of the tuff of Leslie Gulch is overlain by a second, somewhat younger sequence of ashflow and airfall tuffs that makes up the tuff of Spring Creek (unit Ttsc). The tuffs are generally shades of yellow-gr-een and green in color and, according to Vander Meulen and others (1987) are generally crystal rich with as much as 20 to 25% sanidine, plagioclase, and quartz phenocrysts. According to Vander Meulen and others (1987),the tuff of Leslie Gulch is peralkaline (comendite) in composition while the tuff of Spring Creek is meta-aluminous. The tuff of Spring Creek presumably erupted during formation of the Three Fingers Caldera to the north; shortly after collapse of the Mahogany Mountain Caldera. The tuff of Spring Creek is not exposed south of the Mahogany Mountain escarpment. The low-silica, meta-aluminous rhyolitic tuff of Swisher Mountain (Ttsm) crops out south and west of Mahogany Mountain. Contact between the tuff of Swisher Mountain and the rhyolites on Mahogany Mountain is covered by a alluvial fan and gravel sheet (QTs) that formed prior to incising of the Owyhee River Canyon. MrCRIN CREEK QUADRANGLE t , Fluviatile and lacustrine deposits IHolecene and Pleistocene), Mainly uncansalidatsd deposits a+ stream gravels and silts deppsitad along the modern stream channals. I Landslides (Holocmw and Pleistocene?) Unstratified acc~mulat~ansof hasalt and rhyolite blacks along the north edge of the quobVangle, CharacOerizsd by hummocky tbtibgeaphy with sinrlll springs. Basalt of Jordan Craters (W~i~cene)Black iridescent vesicular alivfns basalt fl~ww'ith exceptionally we11 preserved tumulil paho,ehae surfa~aa,and collapse structures. Fresh flow surfares are exposed with ns soil cover. In thin section, consists af 2- 3mm olivine phenocrysts in a subophitic groundmass of plagiacIase, clinopyraxsne, and opaques. Chemically an alkali olivine basalt with a maximum age of 0.15 Ma, according ta Hart (1982) Minimum age of 2800 years indicated by radiocarbon date from organic debris in uppar Caw take {Mehringer, 1987). Cllluvial fan and gravel deposits (Holocene end pfgl Pleistocana) Uneansalidsfad atcumulatiuns af par%3Ally- to well-rounded boulders and cobbles of rhyalfke. 35ze o'f bloiks atid boulders decreases and dsgrete of rolund ing incr%a~e?ssmutheastward off of 'the flank of Mahogany Nounfain. 1 I Ol fvine basalt CPlrictocme? and Pliocenc) Bluish- gray t~ black, diktytaxitic olivine basalE,flaws expassed adjacent Upper Cow. Lake. lorally hrauily mantled by gravels WY Qfg. Finely vesicular, holacry~tallineflows with eubaphitic to ophitie clinopyraxanc, plagiorlasa, and intergranuiar olivine. Includes transitional and high alumina olivine basalts. Includes flaws dated at 3.84, 4.2 and 4.5 Ma (Hart,, 1982). Equ%vnlerrf to part af unit QTb of bJal.ker ( 1977). Tuffaceous siltstones (Pliocene) White and pale brown tuffs and tuffaceaus siltskcnes. Poorly exposed, c~nfarma6lyoverlies unit Ttm, Tuff of Swisher Mauntafn (Middle Pliocene1 Densely wclded, dark purple to reddish-purple, crystal- lithic ashflow tuff. Interior of ashflow is &vitrified. Flow top is locally marked by pumiceous carapace breccia5 containing blocks of black and banded red and black vitrophyre and reddish, vesicular, devitrified tuff. Contains about 15 - 20% broken plagiaclase crystals as much as 1 crn in length, light green pigeonite crystals, and as much as 5 X lithic fragments. Sanidine and orthapyroxenc occur as accessory minerals in some thin sections. Chemically, a law-s$lica mefa-alurninaus rhyolite (Analyses , Ta.ble 1). &sfiflaw is over 200 feet thick south of The Tuque and thickens southeastward into the Downey Canyon quadrang3&* Petrographically and chem.ical1y sf miZar to Wt&' tuff of Shish~rMountain as described by Ekren aSnd othiws (1982) and herein tansfdared to be a nar4hern exfension of the Swishar Mountain from the upper Ohyh&e anya an where mapped and dcocribgd by Evans (1990). The. $uff of Swisher Mountain is considered to be about 13.9 Ma in age (Ekren, 1?82). Tuffareaus sedimentary rocks (Miocene) Mainly white to light green tuffaceous epirlaskie silt- and fine- grained sandstones. Locally includes interbedded orange palaganitic tuffs and white sirfall tuffs. Uncanformahly overlles units Ttsc and Ttlg. Tuff of Spring Cre~k(Miocene) Mainly pale yellowish-green and green, nan-welded and partially welded crystal-lithrc ashflow and airfall tuffs. Locally includes an interbedded light-grayish- purple, densely wclded crystal-lithic ashflaw tuff ~hirhcontains flaftened pumice claste. Partially welded tuffs characteristically contain irregularly shaped masses of porphyritic vitrophyre, According tg Vander Meulen and athers 41987), the tuff of Spring Creek is a metaluminous rhyolite whi~his crystal-rich at the base, with 20 to 25% sanidine, plagioclase, and quartz phenacrysts. The tuff of Spring Creek also contains hornblende and biotite phenocrysts (Plumley, 1986). ? iBB % lib 1:; Set. T.:S,J R-(E.! Litholqy 93-136 Sk SU : 2' 44 Ktyhaiite tuff 03-157 fE SU 11 21 44 RPjoiitr BZB-153 NE 5E I 7 49 Rhynl~te BZB-13 Ni NW 35 ?? L4 Rhyalltetuff la turd"^^ 114 114.5~~. T, F. nap Bg A5 Au Ch ~g no PC Sa TI ZE B: CJ Ga Te B~ co cr F~ L; 8n ld!, Nusber g~it V@ pbe, p.!b ppr 3pa ppa epr pplt p era ppa 978 upr ppl pa :er ppa pp0 Dea ppa ppr 43-001 NE SE Ib ?75 44E Ttsc .I30 35.3 (i b,b3 .232 7,92 12.2 1.85 <.: 55.e 2 ,414 2.23 $1.3 -.r 2 1~15 $9 97 ,z h28-002 SE SE 16 275 44i Ttsc .Ib? 7.15 (1 4.3 !,I 3.81 2.75 f.05 '-5 10.9 !.25 i.1 i*.j t1.c) i.5 376 1 113 0.4? 51 75 -1 AZB-003 NW NU 22 275 44E Ttsc .I64 2e.7 :l 5.28 !.I 76.9 11.0 4.52 .;hi 37.; ',25 . 1.1 ,, 528 2 131 i.jl j? 151 e 428-604 jE Nu 1s E?S ;+5E TTBB ,167 39.9 4 3.12 -233 r.15 2.93 4,71 i.5 21.6 <.25 <.! <,5 ii,~~,j 71$ 1 52 3.75 b 57 3 AZB-CC5 N4 YE 36 2?5 45F Trsm .I55 12.9 2 ;,?? !.I 2,35 3.12 c.5 bQ.5 ;,25 :.f r.5 11.4 (.5 769 1 a? 1.2s s ?3 13 ,.-. .... E' .- .- -. t: t: k:: 1- e 17 , E .B . , ME I y-1 .c y i-e , U .H . , .+-* : 1 ! !e t t: , E .!-! , a i-.:c:j t"i ~4.i.- ,.; J.!.?..-; F:.F, 19@2!,CI~~-!I:I:~I:IS.~ st;~-atiqr-~._ ciif i,+e:s.i:eri? uwyhei~. 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