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OREGON GEOLOGY Published by the Oregon Department of Geology and Mineral Industries OREGON GEOLOGY published by the Oregon Department of Geology and Mineral Industries VOLUME 43, NUMBER 7 JULY 1981 ORECON CiEOLOCiY DOGAMI adds new petroleum (ISSN 0164-3304) geologist to staff VOLUME 43, NUMBER 7 JULY 19B1 William L King, Petroleum Geologist, joined the profes­ sional staff of the Oregon Department of Geology and Mineral Published monthly by the State of Oregon Department of Geology and Mineral Industries (Volumes I through 40 were en­ Industries in March of this year. A native of Pennsylvania, tided The Ore Bill). King received his bachelor's and master's degrees in geology from the University of Pittsburgh. Governing Board John L. ~hwabe, Chairman ................... Portland C. Stanley Rasmussen .. .. ............... Baker Allen P. Stinchfield .. ...... .. .............. North Bend Sble Geologist. .. .. .. .. .. Donald A. Hull Deputy Statl GeaIogist .................... John D. Beaulieu Editor ... ......... .................. .. ... Beverly F. VOgl Main Office: 1005 State Office Building. Portland 97201, phone (S03) 229-5580. Biker Field Offa: 2033 First Street, Baker 97814, phone (503) 523-3133. Howard C. Brooks, Resident Geologist Grants Pus FIeld Offic.: 312 S.E. "H" Street, Grants Pass 97526, phone (503) 476-2496. Len Ramp, Resident Geologist Mined lind flamltiDn Program: 1129 S.E. Santiam Road, Albany 97321, phone (503) 967-2039. Paul F. Lawson, Supervisor William L. King Subscription rates: 1 year, $4.00; 3 years, SIO.OO. Single issuts, SAO at counter, S.7S mailed. Prior to coming to Oregon, King was a geologist with Available back issues of The Ore Bin; 5.2S at counter, 5.50 Peoples Natural Gas Company, PUre Oil Company, and Hum­ mailed. ble Oil and Refining Company (Exxon). His work in the Address subscription orders, renewals, and changes of address petroleum industry has taken him to Texas, Pennsylvania, 10 Oregon Geology. 1005 State Office Building, Ponland, OR California (onshore and offshore), Alaska, Mississippi, loui­ 97201. siana, Oklahoma, and the United Kingdom. King's responsibilities with DOGAMI include regulatory Send news, notices, meeting announcements, articles for publi­ work related to oil, gas, and geothermal exploration and cation, and editorial correspondence to the editor, Ponland of­ development in the State of Oregon. 0 fice. The Department encourages author-initiated peer review for technical articles prior to submission. Any review should be noted in the acknowledgments. Permission is granted to reprint information contained herein. Credit given to the Oregon Department of Geology and Mineral Industries for compiling this information will be appreciated. CONTENTS Subduction-related origin of the volcanic rocks of Second class postage paid at Portland, Oregon. the Eocene Clarno Formation near Postmaster: Send address changes to Oregon Geology, 1005 CherryCreek,Oregon............................ 91 State Office Building, Portland, OR 97201. Mount SI. Helens post-eruption map available. 99 Abstracts.. .... .... .... .... .... 100 BLM warns gold miners to beware of invalid claims. 101 COVER PHOTO Friends of Mineralogy to meet in September... .... .. 101 BLM sends $268,628 check to State of Oregon ....... .. 101 Grand Canyon of the Snake River, forming the boundary between Oregon and Idaho. This area is one Miningandmail .................................. 102 of many places in Oregon discussed in the third edition What comprises a barrel of crude oil? . ..... .... .... 102 of Geology o/Oregon. by Ewart M. Baldwin. A brief re­ New edition of Geelogyo/Oregon published. .... ... 102 view of this new book appears on page 102. (Photo courtesy Oregon State Highway Division) .. OREGON GEOLOGY, VOL. 43, NO.7, JULY t981 Subduction-related origin of the volcanic rocks of the Eocene Clarno Formation near Cherry Creek , Oregon by Jeffrey B. Nob/etl, Depaflment oj Geology, Colorado College, Colorado Springs, Colorado 8{)9{)3 ABSTRACT tions (Oles and Enlows, 1971) and marks the end of marine deposition in central Oreson. Overlying the Clarno Formation Terrestrial calc-alkaline volcanic rocks of the Eocene are the more silicic flu vial and lacustrine tuffs of the John Day Clarno Formation in nonh-central Oregon appear to be a Formation and basalt flows of the Columbia River Basa1t good example of subduction-zone magmatism. Mapping of an Group (Figure 2). area along the John Day River near Cherry Creek showed that Summaries of Clarno lithology (Steere, 1954; Beaulieu, the many individual nows could be placed into three 1972) and reconnaissance mapping by Swanson (1969) present petrographic types. The earliest lavas arc highly porphyritic a picture of late Eocene terrestrial calc-alkaline volcanism. two-pyroxene basaltic andesites. The key feature of the middle Petrochemical work (Rosers and Ragland. 1980) suggests that portion of the Clarno Formation is a group of nonporphyrilic, the Clarno Formation formed on thin continental crust and quanz-bearing basaltic andesites which have also been noted may be related to panial melting in the mantle. as single units in other stratigraphic columns throughout the Merriam (1901) first described Clarno rocks from typical Clarno. The nature of their origin suggests they may be corre­ exposures at Clarno's Ferry (present-day town of Clarno). He lau~d as a time horizon across the Clarno. An angular uncon­ estimated that there were over 400 ft of dominantly eruptive formity and thick saprolite separate the gently folded lower materials, particularly rhyolite and andesite, with Clarno beds from the much less voluminous upper Clarno characteristic ashy shale and other turraceous sediments. volcanic rocks. A dome of hornblende andesite is the most One of the thic kest sequences o f Clarno rocks was notable feature of these later flows. described by Waters and others (1951) in their investigations of Textural and chemical evidence suggests that the por­ the Horse Heaven mining district. situated alongside Cherry phyritic lavas formed as hydrous melts that rose from a sub­ Creek. They established four units comprising 5,800 ft o f sec­ ducted slab and interacted with the overlying mantle, whereas tion. Unit 1 consists of 600 ft of platy andesite interbedded the nonporphyritic lavas formed by partial melting of with clays. Unit 2 contains 1.350 ft of tuffs, volcanic mud· anhydrous quartz eclogite followed by rapid ascent. New age flows. and a few thin andesite flows and is partially equivalent data (Robinson, 1979, personal communication) suggest that to the lower sedimentary group of this study. Unit 3 has 1,750 the Clarno is older than 40-42 m.y. Subduction durins this ft of tuffaceous clay with a few andesite flows. Unit 4 is the time was fairly rapid and at about a 30° angle. Estimated 3.I00-rt rhyolitic tuff layer. It includes ISO ft of andesite temperatures on the surface of this plate match temperatures flows, one of which is equivalent to this study's nonporphyritic at which the Clarno andesites could have formed. A model of flows. A thick saprolite was developed on these units. and they masmatism arising from a shallowly dippinS hydrous portion were subsequently overlain Unconformably by a second se· of the subducted plate, with one brief increase in dip (about quence of lavas considered post-Clarno by Waters and others 5°) generatins the quartz lavas, can explain the seneral (1951) but since placed in the Clarno Formation by Swanson characteristics of Clarno volcanism. and Robinson (1968). Sporadic outcrops of Clarno-type rocks occur farther to INTRODUCTION the east, in the Canyon City quadrangle (Brown and Thayer, 1966). The northernmost Clarno exposure (Umatilla· Pilot The Eocene Clarno Formation is located in north-central Rock, Heppner district) is dominated by carbonaceous Oregon (Fisure I). It is a sequence of dominantly andesitic sediments and some andesite (Collier. 1914; Wagner. 1954; volcanic rocks that ranse in composition from basalt to Hogenson, 1964). rhyolite. Besides lava, the formation includes many intrusive feeders, volcanic breccias, mudflows, ash flows, and tuf­ AGE faceous sediments of fluvial and lacustrine origin. Many o f these units ue characterized by rapid lateral and vertical varia­ One of the more interesting problems of the Clarno is its tion. Previous workers on the Clarno Formation have concen­ ase. K·Ar ages range from 46 m.y. to 33 m.y. (see Walker and trated on localized descriptions, as no regionally extensive unit others. 1974, for a compilation). However. a date of 41 .0 ± 1.2 that can be used for correlation had been found. or 43 .0±0.6 was obtained on a rock that is from one of the One of the thickest sequences of volcanic flows, in­ youngest flows from the upper portion of the Clarno Forma­ trusives, and sediments occurs between Cherry Creek and the tion (Swanson and Robinson, 1968). Robinson carefull y mouth of Bridge Creek on the John Day River (Fisure I). The redated some of the "younger" Clarno rocks which appeared volcanic stratigraphy of this area was studied in detail by field to be lower Clarno lavas and discovered they were actually mappins, petrography, and petrochemical analysis. One of the about SO m.y. old (1979, personal communication). In no case mapped units. the nonporphyritic quartz-bearins andesite, has a sample been younger than 41 m.y. This Eocene age fits may prove to be a time horizon throusJtout the Clarno when well with the tectonic history as described in a later section. its probable origin on a subducted plate is considered. DESCRIPTION OF ROCK UNITS PREVIOUS WORK Near the confluence of Cherry Creek and the John Day The Clarno Formation lies unconformably on the River, the Clarno Formation is divisible into two groups of Cretaceous marine Hudspeth and deltaic Gable Creek Forma- rocks separated by a thick saprolite and an angular unconfor- OREGON GEOLOGY, VOL. 43, NO.1, J ULY 1981 " MAP SYMBOLS HiOhway All weather road 4 ~ NORTH ~ "I:I:J_-= __d O••• 5==:::::J I? MILES LOCATION MAP Figure 1. Map showing location oj study area in north-central Oregon. mity. The lower group, about 1 mi thick, comprises mudflows, sion of these lavas into two petrographic types is most useful andesitic lavas, and thick sedimentary clay and tuff sequences.
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