Geologic Map of the Mount Hood Wilderness, Clackamas and Hood River Counties, Oregon
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MISCELLANEOUS FIELD STUDIES DEPARTMENT OF THE INTERIOR MAP MF-1379-A UNITED STATES GEOLOOICAL SURVEY / '-• ' ,,_ '\ ! - --';--, 11);- I ·' -t· ,,' ' ' 'y'·- ,~- ' :}_\ y' '' ''· ';- ---------4- ''=-, lee-- ' ." --'1:_,1:~.: ', ''.' "'·' ." : _/ ' ' ' ./ I 45° 22'30" / ' '~ ,, cf , '-": f • T 1()-~S:A -- \_ /c,, /' 1' - --- ' / ', \ ,. - \' ' \ .,. ' ' - _,, ,1 :~ \- " ' \ ,·,, - _1...: ~-- ';~"",-_{;,,. '. "-c-" ~ ' ':,\'\.'..' ' \ ) ---- '1 '\ ·\\ :-:c:~_,..:_:::_;,~--:-:J.'.~-:;:+-a::_:;'_"__'_2__.:':_::___':+'z.::_~2-_c';'-2!._:__ _ _.:.,.::;~c;;_2:..'.._.;_._:.__:_.::£'.'....,:;,-..:__:/'..__:'.; __:;;:'.C.~-;;---'----.,...j.'.-,---- -":::-:-::--'---'--__::_:_-':_''_-_'__ _ ~'-.------,,::_.L.:--'-'"-~~~w:---c-""--','---'4-4c--\--------""~---~-~~,-,---~-----c ,; 45" 15' 45° 15' L---~'"'-~~.:_,.,__ _:_~:.c..~ - c__---"'-"''-'--"---__;__,--~-""¥, f' -· , 121" 37'30" ' 1212 t O JO' 122"00' 121°12'30" 121°45 Geo l ogy mapped bJ W.S . Wise, 1%8, 1969: Base from U.S . Geological Survey 1 :24,000 ) '-- ---~-, T.E.C. Kei t h', M.H Beeson, K. E. Bargor, Badger Lake, !lull Run Lake, Cathedral Ridge, SCA LE J 62 500 , • I 1979 , 1930; H.J. Meye r , J. Hook, M.W. Dog River, Government Cam p, Hickman Butte, Ganrl€t, 1980 Mount Hood Sou th, Rhododendron, 1962 2 5 M ILE$ ' ' ) OREGON \ e=occi'c"a~,,BUO~===='======~'====='c====~''====='5 Kl _u I' El E~s \.~---~ CONfOUR INTERVAL 40 FE ET AREA OF MAP NATIONAL GEODE r1c VERTICAL DATUM OF 1Y29 APPROX IMATE MEAN DECLINATION, 1982 CORRELATION OF MAP UNITS SEDIMENT ARY DEPOSITS VOLCANIC ROCKS Holocene 121" 45' 121°30' a, ,wrlh. The lower andesite flows southeast and east of Mount llood Studic!s Related to Wilderness 122° 15 122" 00' QUATERNARY --1•-------~- -----~--~ are chemicall y different from those on the 1<eat side and hal'e a The Wilforness Act (Public Law 88-577, September 3, 1964) and related Pleislocene a~c I Oap I Oba~ I a, vent area just southeast o f the map area at Gunsight Butte (Wlste, acts r equtre the L s. Ceological Survey and the TJ. S. Bureau of Mines ta I 1969). The flows are f,:actured and faulted, an<l low-temperat,ire Survey certain areas on Feder:11 Lands to detennine thetr, mineral re~ource potential. Results must be made available to the public and be submittted to Qsba iron oxidation .ctlong these breaks stained the roc1<.s reddish to orange brown. Faults in the Cast Creek and Short Creek areas on the Admioistration and the \"'.ortgress. These lllaps and reports present th" the north side of Zigzag Mountain offset both the lower andesite results o f a geologic and miner,,1 sur"ey of the Mount flood Wilder ness, Pliocene f lows and underlying Rhodndendron Fornt;1tlon and contain local Clackacras and Hood River Counties, Oregon. '"" quartz-p)'rite-limonite veins. The lower aodesi te flo"8 are aasignnd a L~te Miocene age on the h,qsis of three K-Ar age '" dete.rminatlons from J.li~e (1%9), and on the stratigraphic relatlon with the underlying Rhododendron Formation. On the e,,st ,, TER f lARY aide of Mount Hood, Wise (1969) dcternin,•d <, K-Ar ,ge of Miocene 7.0±0.8 m.y. on an an,lesite flow ,tt Polatlte Greek. From t he west side of Mount Hood, a K-Ar age of 5.8±0.8 m.y. was 45'30 "' determined from an andeaite fl<>w in the upper Clear Fork area, SUMMARY OF !HE GEOLOGY OJ,' THE fr and ,lll age of 5.S±Q.7 m.y. from an andesite flo1< on Zig~ag MQ[mT HOOD WILDERNESS, ORECON Mountain (Wise, 1969) . Volcanism 1<as pcobably sp(iradfr,,lly continuous throughout late Miocene ti,~e rh" Mount Hood Wilderness lles along the crest of the Cascade Ranee in Clackamab and !Iood River Counties, Oreg. Mount Hood is a Qu,it,,rnary INTRLJSIVR ROCKS (MIOGFNE)--~ledium-gray a.1desite plugs containing stratovol cano, still potentially active (Crand ell, 1980), which rises " phenocrysts of plagioclase, olivine, augite, and hypersthene in a c<Jnsplcuoosly to a<1 altitude of 11,234 ft (3,425 m) <,1,,,"e the Bllrrounding DESCRI"f'T[ON OF MAP UNITS Qbav ANDESITE AND ,BASALT OF VISTA RIDGE {PL!HSTOCENE)--Medium- to ci ark holocrystalline groundmass of plagioclase, augite, hematite, and ridges which have an average altitude of appro~lmate l y 4,000 ft (1,220 m). gray microporphyritic olivine andesite flows were erupted from magnetite. Propylitic alteration has pcoduced•epidote, chlorite, The glacier- and snow-covered volc,rno is ensily viAible snd accessible frmTl SED1'1ENTARY DEPOSITS severn1 vent~ along Vlsta Ridge. Minor arnollnta of dark-gray to and iron hydroxide replacement of primacy minerals and local Oregon's largest ,·tty o f Portlartd, SO mi (80 km) west, and is a year-round black aphanitic ba8alt occur locally as veat breccia. Red Hi11, zeolitic alteration at Cl)ntacts. Andesite is commonly red brown r ..creation ar<Ja for local and touring skiers, hikers, and camper.s. The Qnl ALLlJVIUM (HOLOCllNE)--Unconsolldated stream deposits of silt- to near the north end of this sertes of vents, is a basaltic cindar from alteration and iron 'mddatl<>n. The plugs intrude Wilderness covers 47,100 acres (19,061 ha) an<l includes the east, north, and boulder-size m"terial including glacial outwaah deposits. cone. Phenocrysts ln the andesite are plagioclase, olivine, and volcaniclastlc beds and lava flows of the Rhododendron Formatioe west flanks and the surnm lt area of t he voleano. Zigzag Mountain comprises the 45"15 Valleys of the Sandy, Ztg~ag, and White Rivers, and Jl.ast Fork of orthopyroxene . Groundmaas consists mostly of plagioclase, on t!w no r there and southern sides o f Zigzae Mountain and appe ,,r western part of the Wilder11ess an~ rises from an altitude of about 2,000 ft the Hood !liver are filled with reworked pyroclastic flow and olivine, a nd magnetite. Coarse-grained gahbroic xenolitha occur to have been feeder v ents for the chemically similar lower (610 m) to 4,971 ft (1,515 rn) at the highest point. The l< llderness Lies mudflow material from the most re,·ent volcanic eruptions of Mount locally in andesite flovs. The lavas ace chemically si,oil.ctr to andesite flows of late Miocene age (Wise, 1969) within parts of seven 7-1/2-minute quadrangles, !lull Run T,~ke, C,ithe,lral Hood, some as recently as 200 yc• ars ngo (Crandell, 1980) the andeaite flows f rom vents to the east nt The Pinnacle and at Ri dg", Ong River, Rhododendron, 'loun t Hood South, Government Camp, and J\adger Cloud Cap, but ate not genetfcally related to the lavas of Mount QUARTZ DIO'!UTE OF LAUREL l:I ILL (MIOCE~"E)--Da rk- to medium-gceenish Lake. o, GLACIERS (HOLOCENE)--Presently active ice ~odies that mautle much of Hood (Wise, 1969). The lavas erupted after the m.~in cone- gray mediuro-gr,:itned quart~ dtorito and qu.~rtz monzonite. Made up the summit of Mount Hood above an altitude of about 7 ,OOO ft building stage of Mount Hood but prior to the Fraser Glaciation largely of pla1\ioclase .,nd hornblende phenocrysts i n groundm.,~» All roc.:s in the Wilderness are of Tertiary and Quaternary age. The (2,134 m) (Crandell, 1980) of plagioclase, orthocla8e, qu~rtz, hornblende, and magnetite oldest exposed rocks are late Miocene volca11iclastfr rock~ and andesit,c lava (and ilmenite?). Pervasive prop~rlitic alteration has resulted in flows of the Rhododendrnn Form,.ti ort, which rest upon middle Miocene rocks of UNDIFFERENTIATED ANDllSITES J,'ROM SAI"ELLITIC VENTS (PLEISTOCENE) - Qgm GLACIAL MORAINES (HOLOCEm: AND PLEISTOCENE)--TJnsorted nnd poorl y Q• partial to locally complete replacement of feldspars and mafic the Columbia River Basalt Group exposed just outs i.dte the map ;,re,, and indurated glacial deposits. Mouinal deposits on Mount Hood Mediunr- to dark-gray microporphyriti.e oll"!ne nndesite flows mlnerals by actlnollte, cbl"rlte, epi date, hema tite, cl.1ys penetrated by geothermal drill holes ('Beeson and Moran, 1979; Covert and above an altitude of about 6,500 ft (2,000 m) are associated with erupted from vents on Ziezag nod Wst Chane<' Mountains. ,uiilerul s, and zeolites. The quactz diorite crops out on the Meyer, 1979). Volcanic activity has probably occurred sporadtcally from presently active glaciers. Moraines on Zigzag Mount~in and Phenocryats consist of plagioclase, olivine, and hypersthene; ~outh side of Zlg~ag Mountain Just south of the Wilderness Mi oc<!ne tlme to the present. 45°00' southeast of Mount Hood along Bluegrass Ridge vere deposited holocrystalline groundmass i~ composed of plagioclase, augite, bound,iry. Tvo geothermal dri ll holes on t he south sldc o f Zig~,~g Lit<' Miocene ande.site plngs intrude, and Flows overlie, the Miocene INDEX MAP during Fraser Glaciation betveo>.n 10,000 and 29,0110 years B. F. magnetite, and hematite. 1'.hese flows ,,re pe trographicallv Mountain have pene t rated the quartz dlorite; ln ane rlr111 hole, Rhodode"dron Formatio". The qusrtz dlorite of Laurel Hill probably was (Crandell, 1980). Patches of pr~Fraser and Fraser moraines, too similar to andesite flows from vents at Cloud Cap, The Pinnacle, the quartz diorite is overlain by a lower andesite flow (Covert cmplaeed during late Miocene tlme, and drill-hole data Indicate that It ls a small to be included at this map scale , hnvte b"en usecl by and Vista Ridge. They appear to he llt"r than the Pliocene and Meyer, 1979). Wise (1969) i ndicated that the 11.6±1.2 "'·Y • la~ger body than surface outcrops might indicate.