State of Washington DANIEL J. EVANS, Governor Department of Conservation H. MAURICE AHLQUIST, Director DIVISION OF MINES AND GEOLOGY MARSHALL T. HUNTTING, Supervisor Geologic Map GM-3 GEOLOGY OF THE SIMCOE MOUNTAINS VOLCANIC AREA, WASHINGTON By RICHARD A. SHEPPARD STATE PRINTING PLANT, OLYMPIA, WASH. 1967 For sale by Department of Conservation, Olympia, Washington. Price, $1.00 WASHINGTON DEPARTMENT OF CONSERVATION GEOLOGIC MAP DIVISION OF MINES AND GEOLOGY GM-3 - _, E R 6 E i:; ,. E EXPLANATION Oo T q / T 9 "'I H', 1 ·I Andesi te of Mount Adams ' lntroconyon flow , light- to dark-grey; vpper vesicular Contact port grades downward throuJh platy zone lo columnar Anticl ine jointed lower part; hyn lnpi liti~ nntJ rvotphy , ille; cc,n­ Dashed where approximate tai ns o,cillatory :z;onod plogioclose (An _ ), clino­ 50 28 Showing trace of axial plane. Dashed pyroxene (+2V,,,51°-49°), end hyper<thene (-21/=64° -60Q), ond rare oliv ine phenocry,t, in groundmo,s of where approximate; dotted where plogiodose (An _ gl , clinopyroxene (+2V"'13°-4°), 32 2 Li concealed. opaque minerol,, ond pink to p<lle - brown glo,s . Prob­ ~-,c~~~-~ .•••. ably e xtruded from the ;oul,,.,o,f flank of Mount Adorn, D Fault Dashed where approximate; dotted Ty Area mapped where concealed; U, upthrown Syn cl ine ~ side; D, downthrown side © Showing trace of axial plane and pl unge C l © T u Index map of Washington showing area Qb of axis, Dashed where approximate; 0 ~ I ' mopped Ol ivine basalt of King Mountain dotted where concealed /~ lnt-coconyon flow<, g,oy, g,aMro lly 10 lo 1S f,a,al th; ck; holocry,tolline, diktytoxitic, end porphyritic; contain Strike and dip of beds and flows plogioc lose (An _ ) ond olivine (Fo _ ) ph~no­ 58 50 78 60 crysts in groundmos1 of plog;oclose (An _ ), ophltic 50 46 to ,ubophitic cl inopyroxene (+2V:54°-52°), olivine {Fo ), end opoque minerol,; glomeroporphyritic 55 olivin e common. Extruded from King /V\ountain, o Horizontal• flows shield volcano southeast of Mount Adams in T. 7 N . R. 11 E. l QTd OTr Ty ~ Olivine basalt of the Simcoe Mountains O livine basalt of Lincoln Plateau Pyroxene-olivine basalt Andesite of Signal Peak Dacite Rhyoli te \ QT b, flows, groy, 3 to 40 fee t thick; inter­ Fl ow,, groy , generclly 10 to 20 feet thick; O Tp, Flows, grey, 20 to JO feet thick, holo• Flows , dork-groy, 10 to 30 feet thick; in ter~ Domes, light- lo da,k-g r<> y, poorly exposed; Flows on~ dome (?}; white to dork-grey, T ' granular to diktytowi lic, genera lly non­ holocry,tolline, intergn:mvl<>r to dikty­ crystalline, intergronulor to pilotaxitic, sertal ond porphyritic; contain oscillotay flow bonded, hyolapi li tlc, and p;;rphyri tic; poo r ly exp;;sed; flow bonded and spheru­ p;;rphy ritic but contoin os much as 15 and porphyritic; contain plogioclase zoned plog;oclo;e (An _ J, olivine con~in o,cillotory zoned plagioclase litic; genera I ly conh:Jin less than J per­ taxitic, ond porphyritic; conta in olivine 50 32 (An _ >, olivine (Fo _ ), clinc­ (Fo _ ), hypersthene (-2V=55°-50°), {An _ ), olivine (Fo }, cllnopyroxene cent plogioc lo,e (An _ ), sanidine, percent plagioclme (A n _ J and olivine {fo _ ) phenocryits in groundmcs, of 58 38 81 60 50 44 27 46 56 43 80 65 70 0 22 18 (Fo _ ) phenocrysts in nearly ho lo­ plogioclase (An _ ), clinop~roxene pyroxene (+2V~56°-49°), and rare ond clinopyroxene (+2V=50°) phenocrysh (+2V=Sl -48°), hypersthene (-2V=72° quart•, ond clinopyroxene (+2\1"'55°-53°) w 81 60 48 40 C cryotolline groundmos, of plogiocla,e (+2Va. S4° -50°), o livine (Fo ), opaque hypersthene pheoocrysts in groundmcs; in groundmoss of plagioclase (An3 _28)' - 53'), oxyhornblende, biotite, ond rare phenccrysts in glossy matrix containing © 60 6 u (An _ ), clinopyroxene (+2V=S6°- 45°), of pl agloclase (An _ ), clinopyroxene clinapyroxene (pigeoni1e and augi te), sonidi ne phenocrysts in groundmoss of obundont flow- a ligned cr)'Slol lites; 0 49 40 mineral,, end ap;;tjte; glorneroporphyritic 46 36 o livine (Fo _ ) , opaque mio,;,ml,, ,:,nd (+2V--54° - 49°), f,oceifo rou , olivine, and <>f><'<tUO minoro,ls, <>poli te , ond pola - b,own i,lo~ioclase {An _ ), c linopyraxene rcfradive ind~x <>f g lco» r~,-,ge , fr<>m a: 65 40 clot> of olivine pl,~,,u~ ry,t, common~ 30 26 V apatite; gloss content genera lly le;, than olivine phenocrysts commonly resorbed opoque mineral,; hypersthene resorbed gloss; gloss content generally 10 to 20 (pigeonite(?)), opoque mine ra l,, end l.501to 1 .4'12. Eruptedfrom local A pole-brown gloss; olivine ond hyperuhene 2 percent; olivine phenocrysts commonly and portly oltered lo iddingsite. Erupted and surrounded by granulor c linopyroxene. percent; reaction rims of cl;nopy roxene vents in central and eost-centrol ports - re,orbed and portly a ltered lo idd ingsitc; from o shield volcano on Lincoln Ploteou, Erupted from locol vent, surround olivine and hypersthene pheno­ rimmed by granular dinopyroxene; of m eo oxyhornblende and biotite rlmmed or rore quartz xenocry,t, ,urrounded by north of oreo QTp,, ,coda cones; mo;n ly red to dork•groy, cry,ts; glomeroporp~yritic plogioc lo;e common. Extruded from Slgnol Peak, entire ly reploced by opcGue minernls; clinopyroxene reoction rim; vesic le, poorly sorted generally uncon<0lidoted, o steep-sided volcono in sec . 36, T. 'l gloss content generally 30 lo 50 p,,,c.,nl •' locally lined with calcite ond (or) hyo­ stratified ,corio; contains p log i oclose, N. , R. 13 E. Qb lite. httuded from northwest- trending olivine, and dinopyroxene phenocr)·st,. wne of shield vokonoe, ond loco I vents May be os young o; Recent OTbs, ;coria cone,; generally 150 to T N T fj N JOO feet high; mainly red to dork-grey, UNCONFORMITY 1 poorly <0rted, gen,arcl ly umcnn«:, li doted, itratified scorio; ,ome ,pindle-shoped bom~. Mony in southern port of oreo SUMMARY I alignd olong northwest-trending Hnes, ,uggesting fou lt contro l. Moy ba: os young cs Recent EI lemburg Formation The o ldest formation exposed in the oreo is the Yo kimo aci,olt of Miocene end Pli<>cene age . This vo,t Pyroxene -oliv in e bo,o lt and rhyolite ore contemporoneous with the olivine booolt, but ondesite of Signol Moinly grovel ond mnd of fluviol origin, light-yellow ieries of plateau boso lt, covers o lorge portion of eo;tern Wo,hingtoo , northeo,tern Oregon, ond northwe,tern Peok and docile probably postdote oll of the bosolt except thot of the poro,itic scorio cones. The building of w C to ye llowi,h-brown, poorly sorted, poorly consolidated; Idaho. Fo l lowing th e extrusion of the Yokimo Boso It, quartzite-bearing grovel, of the EI lensburg Fo,motion numerous basa ltic ,cario cones marked the clos ing phase of volcon i,m. Many of the cones, po rticu lcrly those © u ,and, fine- to coorse-gr<>ined, micoceous, locally were deposited by southwestward flowing ,trecms. North of the mopped creo, the GUOrtzite - beoring grovel, in the southern eort of the oreo, show o lin ear arrangement, suggesting foult cont,ol. 0 crossbedded; grove l consist; mostly of rounded pebble, were s•,cceeded by o thick occumulotion of rhyodocit ic or onde,itic debris thot ,•,os ,tream-tro n,p;;rted from a: ond cobbles of GSortzjte in micoceou; sond ma trix; volcanic centers in the Co,code Mounta;ns (Wcters, 1961, p. 53-Sj). lntrocanyon flow, of olivine basa lt fram King Mountain , a shield volcano southeast of Mount Ad am,, grovel, in upper part of formation between Bowmen ond flowed down the Klickitat Conyon bi.it wer,a lote, eroded ~ut by th,a Klickitat River, A single intr~conyon Canyon Creeks locolly consist mostly of greenish-gray The east-we,t antic I ina I ridge, ,uch o, Peovine -Toppenish Ridge ond the Hone Heaven Hi 11, re,u lted ondesite flow from Mount Adorns then portia lly fi l led the new conyon. and light-purpl e porphyritic docite. Top of formation from o reg iono l deformation contemporaneous with deposition of the upper port of the Ellensburg Formation eroded, moximum expmed thickness 250 feet (Waters, 1955, p. 674-675). Eroiion ;ub,equently removed mo;! of the Ell ensburg Fornuli on from the onti­ There is no direct evidence t<> indicate that glacier< ever occupied any port of the Simcoe Mountains; cline, prior to the eruption of oliv in e bosalt of the Simcoe Mounta in s. however, both So tus Creek ond Devi ls Conyon head in omphitheoters. Possi bly, small alpine glaciers once occupied these amphitheaters. There appears to be little regular ity ;n the sequence of eruption of the vorioL1s ro ck type, in the Simcoe Mountoin, volconic oreo. Olivine bosoll wo, erupted th1ough0ut l~.e volcanic history . Much of the olivine REFEREN CE S 1 5 ~ T 5 N basalt wos erupted from o northwest-trending li ne of 1hield volcanoes between the westword extension of the Woters, A. C., 1955, Geomorphology of ,outh-centrol Wa,hington, i llu,troted by the Yokima Eo,t Guod rongle ; Horse Heoven Hil li ond Toppenish Ridge and become loco lized in t~e synclinol vol ley between the two r idges. Geol. Soc. Americo Bu ll., v. 66, p . 663-684. Yakima Basal t Olivinre bc,olt c l,o wa, ~rupted contemporaneously from o low ,hield volcano on the Lincoln Ploteou north of © the mapped area.