STATE OF OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES Suite 965, 800 NE Oregon St., #28 Portland, Oregon 97232 OPEN-FILE REPORT O-02-03 Geologic Framework of the Clarno Unit, John Day Fossil Beds National Monument, Central Oregon By E.A. Bestland1, P.E. Hammond2, D.L.S. Blackwell1, M.A. Kays1, G.J. Retallack1, and J. Stimac1 2002 1 Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272 2 Department of Geology, Portland State University, Portland, OR 97207-0751 NOTICE The Oregon Department of Geology and Mineral Industries is publishing this paper be- cause the information furthers the mission of the Department. To facilitate timely distribu- tion of the information, this report is published as received from the authors and has not been edited to our usual standards. ii Oregon Department of Geology and Mineral Industries Open-File Report O-02-03 GEOLOGIC FRAMEWORK OF THE CLARNO UNIT, JOHN DAY FOSSIL BEDS NATIONAL MONUMENT, CENTRAL OREGON E.A. Bestland1, P.E. Hammond2, D.L.S. Blackwell1, M.A. Kays1, G.J. Retallack1, J. Stimac1 1Department of Geological Sciences, University of Oregon, Eugene OR 97403-1272 2Department of Geology, Portland State University, Portland OR 97207-07501 ABSTRACT Two major geologic events are recorded in the Above the conglomerates are thick but discontinu- Eocene-Oligocene volcaniclastic strata, volcanic flows, and ous red claystones (claystone of “Red Hill”), which record a paleosols of the Clarno Unit of the John Day Fossil Beds Na- long period of local volcanic quiescence, slow floodplain tional Monument. A major plate tectonic reorganization in aggradation, and long periods of soil formation. An abrupt the Pacific Northwest at about 40-42 Ma shifted volcanism climate change is inferred during accumulation of the red from the Clarno volcanic province, represented by Clarno beds because the lower sequence contains Ultisol-like pale- Formation andesitic flows and debris flows, to the Cascade osols, whereas the upper sequence is Alfisol-like paleosols. arc, represented by John Day Formation tuffaceous deposits The Ultisol-like paleosols and fossil plants from the “Nut and ash-flow tuffs. Evidence of the second major geologic Beds”, which directly underlie the red beds, are evidence of a event comes from paleosols and fossil remains of plants and climate that was subtropical and humid. The abrupt transition animals in these two formations which indicate a global pale- to Alfisols in the uppermost Clarno red beds may represent a oclimate change centered around the 34 Ma Eocene- decline in both temperature and rainfall to cooler subtropical Oligocene boundary when the earth changed from a tropical climate and correlates to a 42-43 Ma global paleoclimatic Eocene “hothouse” to a temperate Oligocene “icehouse.” cooling event during the late Eocene. Disconformably over- In the Clarno Unit area, the lower part of the Clarno lying the red beds are gray-brown siltstones and conglomer- Formation consists of structurally domed debris flow con- ates of the “Hancock Quarry” which have yielded a titanoth- glomerates, andesite flows (51.2 ± 0.5 Ma), and a dacite ere-dominated fossil fauna (Duchesnean North American dome (53.5 ± 0.3 Ma), both onlapped by less deformed debris Land Mammal Age). flow conglomerates, andesite flows (43.4 ± 0.4 Ma) and red The Clarno Formation is overlain abruptly by an beds. The onlapping conglomerates are composed of two ash-flow tuff of the basal John Day Formation, recently dated widespread units that are dominated by debris flows, separat- by single-crystal 40Ar/39Ar techniques at 39.2±0.03 Ma. A ed by red claystones (paleosols), and are each approximately major lithologic boundary occurs in the lower John Day For- 60 m thick. The lower unit, conglomerate of the “Palisades,” mation between kaolinite and iron-rich claystones (paleosols) consists of channel and floodplain debris-flow conglomerates of the late Eocene, lower Big Basin Member, and smectite and lahar runout deposits. The overlying conglomerate of and tuffaceous claystones of the early Oligocene middle Big “Hancock Canyon” also contain channel and floodplain de- Basin Member. An age determination of 38.2 ± 0.07 Ma for bris-flow conglomerates, but have in addition, fluvially re- a tuff in the lower Big Basin Member and a 33.6 ± 0.19 Ma worked conglomerates and pebbly sandstones, reworked tuff age determination for the “Slanting Leaf Beds” in the middle beds, a distinctive amygdaloidal basalt flow (43.8 ± 0.5 Ma) Big basin Member, support the interpretation that the bound- and the “Nut Beds” fossil site. Both units accumulated on a ary between these two members is close to the Eocene- floodplain between volcanic centers in response to volcanism Oligocene boundary. These fossil leaf beds are thus earliest (synvolcanic sedimentation) in an area of irregular topogra- Oligocene in age, similar in age to the type locality of the phy, including hills of a pre-existing dacite dome. Bridge Creek flora in the Painted Hills area. INTRODUCTION The scenic high desert of north-central Oregon con- matic change occurred across the Eocene-Oligocene transi- tains a colorful volcanic and alluvial sequence of Tertiary age tion during which conditions changed in central Oregon from (Fig. 1). Three units of the John Day Fossil Beds National subtropical humid to semi-arid temperate climate. The mag- Monument (Sheep Rock, Clarno, and Painted Hills) were es- nitude and timing of these paleoclimatic changes as well as tablished for the protection and appreciation of the geologic the stratigraphic position and age of fossil sites has been and paleontologic resources in this area. Strata exposed in worked out from detailed mapping and section-measuring in the National Monument record two important geologic the Clarno Unit area of paleosols (ancient soils), fossiliferous events: (1) The change from Eocene Clarno arc volcanism, beds, and radiometrically dated tuff beds. This mapping has represented by the Clarno Formation, to late Eocene Cascade also revealed a domal volcanic edifice of Clarno age that was arc volcanism and John Day Formation backarc deposition is emplaced early in the accumulation of the formation and recorded in these two formations. (2) A dramatic paleocli- which was subsequently onlapped by volcaniclastic deposits. Oregon Department of Geology and Mineral Industries Open-File Report O-02-03 1 The purpose of this paper is to provide a geologic Quarry” fossil sites. Stratigraphic data presented in this re- and paleoenvironmental summary of the Clarno and lower port were gathered largely by measuring and describing John Day Formations in the Clarno Unit area of the John Day stratigraphic sections of outcrops, with extensive trenching to Fossil Beds National Monument. This paper represents a exposed fresh rock beneath badlands mantled with soil, and synthesis of the combined efforts of three different groups mapping of units using low elevation color aerial pho- that have worked extensively in the Clarno area. A three year tographs (Fig. 3). New age determinations combined with study by Bestland and Retallack for the National Park Service local lithostratigraphic mapping facilitated correlation of fos- generated an extensive and detailed data base of mapping, sil sites in the Clarno Formation with the North American and volcanic and paleosol stratigraphy, new 40Ar/39Ar age deter- world-wide paleontologic and paleoclimatologic data base. minations, and discovery of new fossil sites (Bestland and The study of the paleosols in the area was also a major focus Retallack, 1994a). The University of Oregon Field Camp has of this project and is presented in two technical reports for the been mapping in this area since 1985 and is developing a re- National Park Service (Bestland and Retallack, 1994a, b). gional map of the Clarno and John Day Formations along the Most of the paleoenvironmental conclusions presented in this north side of the Blue Mountains uplift. Portland State Uni- paper are gleaned from this study. versity Geologic Field Methods students and staff have been The informal stratigraphic subdivisions of the mapping in this area since 1988 and have concentrated on de- Clarno and John Day Formations presented here are based on tailed lithostratigraphic mapping of the National Monument. rock type and stratigraphic position. New stratigraphic units During the study by Bestland and Retallack, special identified are all informal in accordance with rules about such emphasis was placed on stratigraphic and chronostratigraph- units in the North American Commission on Stratigraphic ic ordering of Clarno fossil sites (Fig. 2). Detailed strati- Nomenclature (1983). The new subdivisions will be denoted graphic work focused on the upper part of the Clarno Forma- by lower case such as “lower Big Basin Member.” tion in the interval containing the “Nut Beds” and “Hancock GEOLOGIC SETTING Clarno Formation convergent-margin andesitic compositions and textures, indi- The Clarno Formation is a thick section (up to 6,000 cate accumulation in and around andesitic volcanic cones ft./1800 m) of largely andesitic volcanic and volcaniclastic (Taylor, 1960; Noblett, 1981; Suayah and Rogers, 1991; rocks of Eocene age which crops out over a large area of White and Robinson, 1992; Bestland and others, 1995). The north-central Oregon. The formation was named by Merriam calc-alkaline volcanic rocks represent subduction-related an- (1901a, b) for exposures of volcanic rocks at Clarno’s ferry, desitic volcanism, probably on thin continental