OPEN-FILE REPORT 0-92-09 PRELIMINARY GEOLOGIC MAP OF THE JORDAN CRATERS SOUTH QUADRANGLE MALHEUR COUNTY, OREGON By M. L. Ferns, and N. S. MacLeod Oregon Department of Geology and Mineral Industries This unpublished Open-File Report has not been reviewed and may not meet all Oregon Department of Geology and Mineral Industries' standards.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 1990/1991 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. Jordan Craters South A distinctive, densely-welded, high-silica rhyolite ashflow tuff (Ttlg?) comprises the oldest unit exposed in the Jordan Craters South quadrangle. The ashflow is characterized by high silica and low alumina abundance5 and is correlative with tuffs mapped by Plumley (1984) as Leslie Gulch Tuff in The Hole in the Ground quadrangle to the northwest. If his correlation is correct, Ttlg? is part of the outflow sheet erupted during formation of the Mahogany Mountain caldera tn the northeast. The ashflow is overlain by tuffaceous siltstones (Tsts) and aphyric platy andesite flows (Tmv). Tilted fault blocks comprised of all three units from steptoes around which younger basalts (Tbdb, Tbtm, QTb, Qbcb, Qbrb, and Qbjc) have flowed. The youngest of these (Qbjc) erupted at about 3,000 years ago from a small vent north of the quadrangle at Jordan Crater. The surface of the Qbjc flow is free of wind- blown silt and soil and contains many fragile surface features that are characteristic of very young basalt flows. JORDAN CRATERS SOUTH Colluvial deposits (Quaternary) Slope cnvering deposits of angular blocks of basalt from rim farming basal ts of units QTbbr, Tbab and fdb. I Lacustrine and eolian deposits (Quaternary). Unconsolidated lacustrine and eolian deposits of silt and fine sand atcurnula-ted in shallow pans peripheral to young basalt flows. Basalt of Jordan Craters (H~locene) Black iridescent vesicular 01-ivina basalt f ipw with exceptional ly well preserved tumqli, pahoahaa surface^, and collapsq ~~ryrturcs~Fresh flow surfaces are exposed with no !mil cover. Tr? Win section, consists of '2- 3mm ol$virwe phsnacry%+s in a subaphitic groundmass of plaqiacl,ase, slimpyroxene, and apaques. Chemically alkali olivine basalt with a maximum age of 0.-15 Ma, acsording Sa Hart ($982).Minimum age of 2GM) years indicated by radiocarbon data from organic debris i'n upper Cow Lake QMehringer, 1987). Basalt of Rocky Butte (Quaternary) Dark gray diktytaxitic olivine basalt flows, with well preserved primary volcanic structures such as tumuli, pahoehoe surfaces, and collapse structures. In thin mction, consists of olivine phenocrysts 3 mm In diameter and elongske plagjiaclase phenocrysts set in a subophitic groundmass pf cllnopyroxene, opaques, and glass. dccordinp to Hart t1982) the unit consists of alkali olivine basalt flows with a maximum age af 0.03 - 0.09 Ma. [- [- Basal t of Clarks Butte (Pleistocene) Grayish-black olivine basal flaws forming~lava field about Clark5 Butte. Well preserved tymuli, pahoehoe surfaces, and collapse structures are mantled by eolian and lacustrine silts, In thin sectinn, consists of phenocrytits of olivine and plagiacla~ewith glomerocrysts af plagioclaoe and olivine set in an intergranular groundma35 of plagi~clase,opaques, and clinopyfoxene, Chemically an alkali olivine basalt radiometrically daWed at 0.25 Ma (Hart, 1982 1 . Baealts of Three Mils Hill (Pliocene) Vesicular black diktytaxitic a,livine basalt flows, Includes halocrystalline flows wikh 3mm diameter plagioclase and olivine phenacrysts in an intergranular groundmass of plagioclase, olivine, clinopyroxene, and opaques. According to Hart (19W!) includes alkali olivine basalt flows radfafhetrirally dated at 1.9 Ma, BasaLts of Dear Butte IPliocenel Grayish- and bluish- black diktytaxitic 03ivfne basalt flows. Finely vesicular with a;uQaplhi5fc to #p:h,itic clinopyrorcene, plagiocl3se, and inbargranuior alfvin~. encludae both transitiaaal and high .&Ju&ina olivine tholeiBtes acca~darqto $hclsskiP73eatian of Hart (194315. PXiacenS~da$& 'based an K/Qf determinatldns of 4.1 and 4.5 Ha by Hart tiVal. Egui'valenk ta part af unit PTb of Walker (19772. hndasite and ba'saltic andesite IMiocene) Greenish- gray and reddish-gray, platy, aghyric basaltic ahdosite flaws and unconsolidafed dq'osifs of agglutinate and ci.nd.srs. Flows are pilotaxitic with plaqioclasc micro-phenacrysts. Tuffacenus siltstones (Miocene) White to yellowish- white tuffaceous siltstt3nes and diatomite. YelSawish tMfs grade downward inta rhyolite Flows of unbsrlylnq unit Trtf. Porphyrlfic rhyolite (P4iatene) Plainly densely welded, pale reddish-brown to gray, quartz-sanidine phyric, rhyoli*lc ashflaw tufQs. Includes ashflows with 2 - 10% sanid3ne and quartz phenocrysks as large as Smm in diameter. khemicslly a high silica peralkaline rhyolite (Analyses, Table 1). Extremely high silica and low alumina abundanres are characteristic af the Leslie Bulch Tuff where mapped by Plumley (17841 to the north. 1 i ; c',L : .* xi- .? ? 7182 8nG <a8 i20 *a20 P?C? I Eu ZT Q?. cr i -48 : i:* 7,;s~)',!E.! i:iholo~.y b?lt5:02 $1903 Fe?03 HqI Cr 5~ ,.- .., 3 ah .:. ;ec, - ee k b 4 !i % p?n ppa p)n pes pe# ?PR 9 opm ~ps ops r;> n:a ;~~-1j3 p# 5; 28 43 !llv]epbasa:t Tbdt L5.i . 1.12 !I.? 9.1 ii.? 8.35 0.35 ?.?8 ?.?? i2? 46 '?! 5?,3 67. !4 !I& 15 b? 29 ki4 b.2 RZB-if4 NE N@ ?C 28 43 Olivlle Da;alt 9Tb 47,9 15.9 i,13 1 l.? 1 1 8 1.34 6,:: C.20 275 57 140 ba.2 7'. I: 253 1s 3? 37 !p 5:+ Hart, W,!::, a17d Piel-:.:!nan; S,A,, 1983, [,a.te Cp,?~:izc;i.c \,::oj.r.3.j-?lil: st!-atj.graphy of 'i.hE! .J~'r.d~ti-~Valley areap ~c~[.~.the;i~i:;~i-!-i !:!i-.egon: Efregon Depai-tmc~n-!: i~<~i~].~gyai7cj r.li;.-ie!-.:3,:!. .- .,-. Ii-~cft!st:!-ies,Qregr;tn kiec~fc~qy,- Vi-11. 4.5, - . p. 15.--. 1. 9 = Jordan Craters South Quadrangle MAP SYMBOLS Contact -- approximately located - ---- Fault contact -- dashed where approximately located, dotted where concealed. Ball and bar on down throw side y Strike and dip of beds X Location of whole rock sample analyzed in Table 1 .