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Section
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Blue Mountains (21)
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Canada
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Western Canada
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British Columbia
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Queen Charlotte Islands (1)
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Vancouver Island
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Cascade Range (1)
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Cascadia subduction zone (2)
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East Pacific Ocean Islands
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Europe
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Alps
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Limestone Alps
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Northern Limestone Alps (1)
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Central Europe
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Austria
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Northern Limestone Alps (1)
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Polynesia
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Jackson County Oregon (1)
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Olympic Mountains (1)
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commodities
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Be-10 (1)
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Pb-206/Pb-204 (2)
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Pb-207/Pb-204 (1)
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Pb-208/Pb-204 (2)
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Rb-87/Sr-86 (1)
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Sm-147/Nd-144 (1)
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stable isotopes
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C-13/C-12 (1)
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Hf-177/Hf-176 (2)
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Nd-144/Nd-143 (3)
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O-18/O-16 (2)
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Pb-206/Pb-204 (2)
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Pb-207/Pb-204 (1)
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Pb-208/Pb-204 (2)
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Pb-208/Pb-206 (1)
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Rb-87/Sr-86 (1)
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Sm-147/Nd-144 (1)
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Sr-87/Sr-86 (7)
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metals
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alkali metals
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rubidium
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Rb-87/Sr-86 (1)
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alkaline earth metals
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beryllium
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Be-10 (1)
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strontium
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Rb-87/Sr-86 (1)
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Sr-87/Sr-86 (7)
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hafnium
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Hf-177/Hf-176 (2)
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lead
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Pb-206/Pb-204 (2)
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Pb-207/Pb-204 (1)
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Pb-208/Pb-204 (2)
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Pb-208/Pb-206 (1)
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rare earths
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neodymium
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Nd-144/Nd-143 (3)
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Sm-147/Nd-144 (1)
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samarium
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Sm-147/Nd-144 (1)
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yttrium (1)
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oxygen
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O-18/O-16 (2)
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fossils
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Chordata
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Vertebrata
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Tetrapoda
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Mammalia
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Theria
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Eutheria
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Artiodactyla
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Ruminantia
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Carnivora
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Fissipeda
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Canidae (1)
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Perissodactyla
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Invertebrata
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Insecta
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Hymenoptera (2)
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Brachiopoda
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Articulata
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Rhynchonellida
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Cnidaria
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Scleractinia (1)
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Mollusca
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Bivalvia
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Pectinacea
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Halobia (1)
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Cephalopoda
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Ammonoidea (3)
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Porifera
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Calcarea
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Demospongea (1)
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Protista
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Miliolina (1)
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Radiolaria (4)
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Plantae
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Rhodophyta (1)
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Filicopsida (1)
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Spermatophyta
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Angiospermae
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Dicotyledoneae
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Quercus (1)
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Gymnospermae
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Coniferales (1)
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thallophytes (2)
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geochronology methods
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K/Ar (2)
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tephrochronology (2)
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thermochronology (1)
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U/Pb (11)
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geologic age
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Cenozoic
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Quaternary
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Cordilleran ice sheet (1)
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Holocene
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upper Holocene (1)
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Pleistocene
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Lake Missoula (1)
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upper Pleistocene
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Weichselian
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upper Weichselian
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Younger Dryas (1)
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upper Quaternary
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Pinedale Glaciation (1)
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Tertiary
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Arikareean (1)
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Challis Volcanics (1)
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John Day Formation (2)
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Neogene
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Hemphillian (1)
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Miocene
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Columbia River Basalt Group (12)
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Grande Ronde Basalt (2)
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lower Miocene (1)
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middle Miocene (2)
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Picture Gorge Basalt (2)
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Saddle Mountains Basalt (1)
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upper Miocene (1)
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Pliocene (1)
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Paleogene
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Eocene (1)
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Oligocene
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lower Oligocene (1)
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upper Cenozoic (1)
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Laurentide ice sheet (1)
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Mesozoic
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Cretaceous
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Lower Cretaceous
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Albian (1)
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Middle Cretaceous (1)
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Upper Cretaceous
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Cenomanian (2)
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Great Valley Sequence (1)
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Jurassic
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Lower Jurassic (1)
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Middle Jurassic (2)
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Upper Jurassic (6)
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lower Mesozoic (1)
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Triassic
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Lower Triassic (1)
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Upper Triassic
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Norian (7)
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Paleozoic
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Carboniferous
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Mississippian (1)
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Devonian (1)
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Permian
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Precambrian
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Archean (1)
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igneous rocks
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igneous rocks
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plutonic rocks
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diorites
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tonalite (5)
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trondhjemite (1)
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gabbros
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norite (1)
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granites (3)
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granodiorites (3)
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ultramafics (1)
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volcanic rocks
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flood basalts (5)
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olivine basalt (1)
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tholeiite (1)
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dacites (1)
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keratophyre
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pyroclastics
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rhyolites (3)
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ophiolite (1)
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metamorphic rocks
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metaigneous rocks
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metaplutonic rocks (1)
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ophiolite (1)
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silicates
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pyroxene group
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framework silicates
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feldspar group
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barium feldspar
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offretite (1)
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orthosilicates
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nesosilicates
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zircon group
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zircon (8)
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sheet silicates
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clay minerals
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nontronite (1)
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smectite (1)
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mica group
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biotite (1)
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Primary terms
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absolute age (15)
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biogeography (9)
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Canada
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Western Canada
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British Columbia
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Queen Charlotte Islands (1)
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Vancouver Island
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Victoria British Columbia (1)
-
-
-
-
-
carbon
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C-13/C-12 (1)
-
-
Cenozoic
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Quaternary
-
Cordilleran ice sheet (1)
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Holocene
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upper Holocene (1)
-
-
Pleistocene
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Lake Missoula (1)
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upper Pleistocene
-
Weichselian
-
upper Weichselian
-
Younger Dryas (1)
-
-
-
-
-
upper Quaternary
-
Pinedale Glaciation (1)
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-
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Tertiary
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Arikareean (1)
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Challis Volcanics (1)
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John Day Formation (2)
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Neogene
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Hemphillian (1)
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Miocene
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Columbia River Basalt Group (12)
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Grande Ronde Basalt (2)
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lower Miocene (1)
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middle Miocene (2)
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Picture Gorge Basalt (2)
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Saddle Mountains Basalt (1)
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upper Miocene (1)
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Wanapum Basalt (2)
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Yakima Basalt (1)
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Pliocene (1)
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Paleogene
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Eocene (1)
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Oligocene
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lower Oligocene (1)
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upper Cenozoic (1)
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chemical analysis (2)
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Chordata
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Vertebrata
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Tetrapoda
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Mammalia
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Theria
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Eutheria
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Artiodactyla
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Ruminantia
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Tylopoda
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Camelidae (1)
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-
-
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Carnivora
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Fissipeda
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Canidae (1)
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-
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Perissodactyla
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Hippomorpha
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Equidae (1)
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Europe
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Alps
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Limestone Alps
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Central Europe
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plutonic rocks
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tonalite (5)
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trondhjemite (1)
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gabbros
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norite (1)
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granites (3)
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granodiorites (3)
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ultramafics (1)
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volcanic rocks
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andesites (2)
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basalts
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alkali basalts
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spilite (1)
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flood basalts (5)
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olivine basalt (1)
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tholeiite (1)
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-
dacites (1)
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keratophyre
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pyroclastics
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rhyolites (3)
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intrusions (13)
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Invertebrata
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Arthropoda
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Insecta
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Neoptera
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Endopterygota
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Hymenoptera (2)
-
-
-
-
-
-
-
Brachiopoda
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Articulata
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Rhynchonellida
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Rhynchonellidae (1)
-
-
-
-
Cnidaria
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Anthozoa
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Zoantharia
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Scleractinia (1)
-
-
-
-
Mollusca
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Bivalvia
-
Pterioida
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Pteriina
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Pectinacea
-
Halobia (1)
-
-
-
-
-
Cephalopoda
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Ammonoidea (3)
-
-
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Porifera
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Calcarea
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Sphinctozoa (1)
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Demospongea (1)
-
-
Protista
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Foraminifera
-
Fusulinina
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Fusulinidae (1)
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Miliolina (1)
-
-
Radiolaria (4)
-
-
-
isotopes
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radioactive isotopes
-
Be-10 (1)
-
Pb-206/Pb-204 (2)
-
Pb-207/Pb-204 (1)
-
Pb-208/Pb-204 (2)
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Rb-87/Sr-86 (1)
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Sm-147/Nd-144 (1)
-
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stable isotopes
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C-13/C-12 (1)
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Hf-177/Hf-176 (2)
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Nd-144/Nd-143 (3)
-
O-18/O-16 (2)
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Pb-206/Pb-204 (2)
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Pb-207/Pb-204 (1)
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Pb-208/Pb-204 (2)
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Pb-208/Pb-206 (1)
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lava (4)
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mantle (2)
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Mesozoic
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Cretaceous
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Lower Cretaceous
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Albian (1)
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Middle Cretaceous (1)
-
Upper Cretaceous
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Cenomanian (2)
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Coniacian (1)
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Hornbrook Formation (2)
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Santonian (1)
-
-
-
Great Valley Sequence (1)
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Jurassic
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Lower Jurassic (1)
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Middle Jurassic (2)
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Upper Jurassic (6)
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lower Mesozoic (1)
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Triassic
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Lower Triassic (1)
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Upper Triassic
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Carnian (3)
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Norian (7)
-
-
-
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metals
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alkali metals
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rubidium
-
Rb-87/Sr-86 (1)
-
-
-
alkaline earth metals
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beryllium
-
Be-10 (1)
-
-
strontium
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Rb-87/Sr-86 (1)
-
Sr-87/Sr-86 (7)
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hafnium
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Hf-177/Hf-176 (2)
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lead
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Pb-206/Pb-204 (2)
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Pb-207/Pb-204 (1)
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Pb-208/Pb-204 (2)
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Pb-208/Pb-206 (1)
-
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rare earths
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neodymium
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Nd-144/Nd-143 (3)
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Sm-147/Nd-144 (1)
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samarium
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Sm-147/Nd-144 (1)
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yttrium (1)
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metamorphic rocks
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metaigneous rocks
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metabasalt (1)
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metaplutonic rocks (1)
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metamorphism (4)
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mineralogy (2)
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North America
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Basin and Range Province
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Methow Basin (1)
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North American Cordillera (7)
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North American Craton (2)
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Oceania
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O-18/O-16 (2)
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Paleozoic
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Devonian (1)
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Permian
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Middle Permian (1)
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palynomorphs (1)
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petrology (7)
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Plantae
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algae
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Rhodophyta (1)
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Pteridophyta
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Filicopsida (1)
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Spermatophyta
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Angiospermae
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Dicotyledoneae
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Quercus (1)
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Gymnospermae
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Coniferales (1)
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plate tectonics (13)
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Precambrian
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Archean (1)
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reefs (1)
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sedimentary petrology (2)
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micrite (1)
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packstone (1)
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GeoRef Categories
Era and Period
Epoch and Age
Book Series
Date
Availability
New occurrences of mammals from McKay Reservoir (Hemphillian, Oregon) Open Access
Geochronology and geochemistry of the Huntington Formation, Olds Ferry terrane, Blue Mountains province, northern U.S. Cordillera: Implications for accreted terrane correlation and assembly Available to Purchase
The Chief Joseph dike swarm of the Columbia River flood basalts, and the legacy data set of William H. Taubeneck Open Access
THE PALEOBOTANY AND PALEOECOLOGY OF THE EOCENE HERREN BEDS OF NORTH-CENTRAL OREGON, USA Available to Purchase
Voluminous and compositionally diverse, middle Miocene Strawberry Volcanics of NE Oregon: Magmatism cogenetic with flood basalts of the Columbia River Basalt Group Available to Purchase
ABSTRACT The mid-Miocene Strawberry volcanic field of northeastern Oregon is an example of intracontinental flood volcanism that produced lavas of both tholeiitic and calcalkaline compositions derived by open-system processes. Until now, these dominantly calc-alkaline lavas have not been considered to have a petrogenetic origin similar to that of the flood basalts of the Pacific Northwest because of their calc-alkaline composition. These lavas are situated in between and co-erupted with the dominant volcanic field of the Columbia River Basalt Group (CRBG). Due to the timing, location, and diversity of these erupted units, the Strawberry Volcanics may hold valuable information about the role of crustal modification during large magmatic events such as hotspot volcanism. The earliest eruptions of the Strawberry Volcanics began at 16.2 Ma and appear continuous to 15.3 Ma, characterized by low-silica rhyolite. High-silica, A-type rhyolite eruptions followed at 15.3 Ma. The silicic eruptions continued until 14.6 Ma, with an estimated total volume up to ~100 km 3 . The first eruptions of the intermediate lava flows occurred at 15.6 Ma and continued with both tholeiitic and calc-alkaline, and transitional, lavas until 12.5 Ma. Volume estimates of the intermediate lavas are ~1100 km 3 . The mafic lavas are sparse (~2% of total volume) and are distributed throughout the upper sequences, and they appear to be near last to arrive at the surface. Herein, we show that the Strawberry Volcanics are not only related in time and space to the Columbia River Basalt, but they also share some chemical traits, specifically to the Steens Basalt. Evidence of this similarity includes: overlapping normalized incompatible trace-element patterns, selected trace-element ratios, and radiogenic isotopes. Furthermore, we compared the Strawberry rhyolites to the other mid-Miocene rhyolites of eastern Oregon associated with the inception of the Yellowstone–Snake River Plain hotspot and found overlapping eruption ages, trace and rare earth element compositions, and “A-type” rhyolite characteristics. This research concludes that the Strawberry Volcanics were part of the regional basalt to rhyolite magmatism of the Yellowstone–Snake River Plain hotspot.
Provenance analysis of the Ochoco basin, central Oregon: A window into the Late Cretaceous paleogeography of the northern U.S. Cordillera Available to Purchase
ABSTRACT Cretaceous forearc strata of the Ochoco basin in central Oregon may preserve a record of regional transpression, magmatism, and mountain building within the Late Cretaceous Cordillera. Given the volume of material that must have been eroded from the Sierra Nevada and Idaho batholith to result in modern exposures of mid-and deep-crustal rocks, Cretaceous forearc basins have the potential to preserve a record of arc magmatism no longer preserved within the arc, if forearc sediment can be confidently linked to sources. Paleogeographic models for mid-Cretaceous time indicate that the Blue Mountains and the Ochoco sedimentary overlap succession experienced postdepositional, coast-parallel, dextral translation of less than 400 km or as much as 1700 km. Our detailed provenance study of the Ochoco basin and comparison of Ochoco basin provenance with that of the Hornbrook Formation, Great Valley Group, and Methow basin test paleogeographic models and the potential extent of Cretaceous forearc deposition. Deposition of Ochoco strata was largely Late Cretaceous, from Albian through at least Santonian time (ca. 113–86 Ma and younger), rather than Albian–Cenomanian (ca. 113–94 Ma). Provenance characteristics of the Ochoco basin are consistent with northern U.S. Cordilleran sources, and Ochoco strata may represent the destination of much of the mid- to Late Cretaceous Idaho arc that was intruded and eroded during and following rapid transpression along the western Idaho shear zone. Our provenance results suggest that the Hornbrook Formation and Ochoco basin formed two sides of the same depositional system, which may have been linked to the Great Valley Group to the south by Coniacian time, but was not connected to the Methow basin. These results limit northward displacement of the Ochoco basin to less than 400 km relative to the North American craton, and suggest that the anomalously shallow paleomagnetic inclinations may result from significant inclination error, rather than deposition at low latitudes. Our results demonstrate that detailed provenance analysis of forearc strata complements the incomplete record of arc magmatism and tectonics preserved in bedrock exposures, and permits improved understanding of Late Cretaceous Cordilleran paleogeography.
Paleomagnetism and rotation history of the Blue Mountains, Oregon, USA Open Access
ABSTRACT An important element in reconstructions of the Cordilleran margin of North America includes longstanding debate regarding the timing and amount of rotation of the Blue Mountains in eastern Oregon, and the origin of geometric features such as the Columbia Embayment, which was a subject of some of Bill Dickinson’s early research. Suppositions of significant clockwise rotation of the Blue Mountains derived from Dickinson’s work were confirmed in the 1980s by paleomagnetic results from Late Jurassic–Early Cretaceous plutonic rocks, and secondary directions from Permian–Triassic units of the Wallowa–Seven Devils arc that indicate ~60° clockwise rotation of the Blue Mountains. This study reports new paleomagnetic data from additional locations of these Late Jurassic–Early Cretaceous plutonic rocks, as well as Jurassic sedimentary rocks of the Suplee-Izee area. Samples from three sites from the Bald Mountain Batholith, two sites from small intrusive bodies near Ritter, Oregon, and six sites from the Wallowa Batholith have well-defined magnetization components essentially identical to those found by previous workers. The combined mean direction of both sets of data from these Late Jurassic to Early Cretaceous intrusive rocks is D = 30, I = 63, α 95 = 6°. Samples from Jurassic sedimentary rocks in the Suplee-Izee area include four sites of the Lonesome Formation, three sites of andesitic volcanics in the Snowshoe Formation, and three sites from the Trowbridge Formation. The Lonesome and Trowbridge samples all had very well-defined, two component magnetizations. The in-situ mean of the combined Lonesome and Trowbridge Formations is D = 28, I = 63, α 95 = 15°. Upon tilt-correction, the site means of these units scatter and fail the paleomagnetic fold test in spectacular fashion. The similarity between the directions obtained from the remagnetized Jurassic rocks, and from the Late Jurassic to Early Cretaceous plutonic rocks suggests that a widespread remagnetization accompanied emplacement of the intrusives. Similar overprints are found in Permian and Triassic rocks of the Blue Mountains. Directions from 64 sites of these rocks yields a mean of D = 33°, I = 64°, k = 26, α 95 = 3.7°. Comparing the directions with North America reference poles, a clockwise rotation of 60° ± 9° with translation of 1000 ± 500 km is found. Together with data from Cretaceous and Eocene rocks, clockwise rotation of the Blue Mountains has occurred throughout the past ca. 130 Ma, with long-term rotation rates of 0.4 to 1 °/Ma. Approximately 1000 km of northward translation also occurred during some of this time.
Intensities, Aftershock Sequences, and the Location of the 1936 Milton‐Freewater Earthquake near the Oregon–Washington Border, U.S.A. Available to Purchase
Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue Open Access
Isotopic compositions of intrusive rocks from the Wallowa and Olds Ferry arc terranes of northeastern Oregon and western Idaho: Implications for Cordilleran evolution, lithospheric structure, and Miocene magmatism Open Access
Intrusive and depositional constraints on the Cretaceous tectonic history of the southern Blue Mountains, eastern Oregon Open Access
A strong contrast in crustal architecture from accreted terranes to craton, constrained by controlled-source seismic data in Idaho and eastern Oregon Open Access
Magnetic fabrics of arc plutons reveal a significant Late Jurassic to Early Cretaceous change in the relative plate motions of the Pacific Ocean basin and North America Open Access
Formation of the ferruginous smectite SWa-1 by alteration of soil clays Available to Purchase
Geology of the Wallowa terrane, Blue Mountains province, in the northern part of Hells Canyon, Idaho, Washington, and Oregon Available to Purchase
Abstract The Wallowa terrane is one of five pre-Cenozoic terranes in the Blue Mountains province of Oregon, Idaho, and Washington. The other four terranes are Baker, Grindstone, Olds Ferry, and Izee. The Wallowa terrane includes plutonic, volcanic, and sedimentary rocks that are as old as Middle Permian and as young as late Early Cretaceous. They evolved during six distinct time segments or phases: (1) a Middle Permian to Early Triassic(?) island-arc phase; (2) a second island-arc phase of Middle and Late Triassic age; (3) a Late Triassic and Early Jurassic phase of carbonate platform growth, subsidence, and siliciclastic sediment deposition; (4) an Early Jurassic subaerial volcanic and sedimentary phase; (5) a Late Jurassic sedimentary phase that formed a thin subaerial and thick marine overlap sequence; and (6) a Late Jurassic and Early Cretaceous phase of plutonism. Rocks in the Wallowa terrane are separated into formally named units. The Permian and Triassic Seven Devils Group encompasses the Middle and Late(?) Permian Windy Ridge and Hunsaker Creek Formations and the Middle and Late Triassic Wild Sheep Creek and Doyle Creek Formations. Some Permian and Triassic plutonic rocks, which crystallized beneath the partly contemporaneous volcanic and sedimentary rocks of the Seven Devils Group, represent magma chambers that fed the volcanic rocks. The Permian and Triassic plutonic rocks form the Cougar Creek and Oxbow “basement complexes,” the Triassic Imnaha plutons, and the more isolated Permian and Triassic plutons, such as those in the Sheep Creek to Marks Creek chain and in the southern Seven Devils Mountains near Cuprum, Idaho. The Seven Devils Group, and its associated plutons, are capped by the Martin Bridge Formation, a Late Triassic platform and reef carbonate unit, with associated shelf and upper-slope facies, and overlying and partly contemporaneous siliciclastic, limestone, and calcareous phyllitic rocks of the Late Triassic and Early Jurassic Hurwal Formation. Younger rocks are a subaerial Early Jurassic volcanic and sedimentary rock unit of the informally named Hammer Creek assemblage, and a Late Jurassic overlap sedimentary unit, the Coon Hollow Formation. Late Jurassic and Early Cretaceous plutons intrude the older rocks. Lava flows of the Miocene Columbia River Basalt Group overlie the pre-Cenozoic rocks. Late Pleistocene and Holocene sedimentation left discontinuous deposits throughout the canyon. Most impressive are deposits left by the Bonneville flood. The latest interpretations for the origin of terranes in the Blue Mountains province show that the Wallowa terrane is the only terrane that, during its Permian and Triassic evolution, had an intra-oceanic (not close to a continental landmass) island-arc origin. On this field trip, we travel through the northern segment of the Wallowa terrane in Hells Canyon of the Snake River, where representative rocks and structures of the Wallowa terrane are well exposed. Thick sections of lava flows of the Columbia River Basalt Group cap the older rocks, and reach river levels in two places.
Composite Sunrise Butte pluton: Insights into Jurassic–Cretaceous collisional tectonics and magmatism in the Blue Mountains Province, northeastern Oregon Available to Purchase
The composite Sunrise Butte pluton, in the central part of the Blue Mountains Province, northeastern Oregon, preserves a record of subduction-related magmatism, arc-arc collision, crustal thickening, and deep-crustal anatexis. The earliest phase of the pluton (Desolation Creek unit) was generated in a subduction zone environment, as the oceanic lithosphere between the Wallowa and Olds Ferry island arcs was consumed. Zircons from this unit yielded a 206 Pb/ 238 U age of 160.2 ± 2.1 Ma. A magmatic lull ensued during arc-arc collision, after which partial melting at the base of the thickened Wallowa arc crust produced siliceous magma that was emplaced into metasedimentary rocks and serpentinite of the overthrust forearc complex. This magma crystallized to form the bulk of the Sunrise Butte composite pluton (the Sunrise Butte unit; 145.8 ± 2.2 Ma). The heat necessary for crustal anatexis was supplied by coeval mantle-derived magma (the Onion Gulch unit; 147.9 ± 1.8 Ma). The lull in magmatic activity between 160 and 148 Ma encompasses the timing of arc-arc collision (159–154 Ma), and it is similar to those lulls observed in adjacent areas of the Blue Mountains Province related to the same shortening event. Previous researchers have proposed a tectonic link between the Blue Mountains Province and the Klamath Mountains and northern Sierra Nevada Provinces farther to the south; however, timing of Late Jurassic deformation in the Blue Mountains Province predates the timing of the so-called Nevadan orogeny in the Klamath Mountains. In both the Blue Mountains Province and Klamath Mountains, the onset of deep-crustal partial melting initiated at ca. 148 Ma, suggesting a possible geodynamic link. One possibility is that the Late Jurassic shortening event recorded in the Blue Mountains Province may be a northerly extension of the Nevadan orogeny. Differences in the timing of these events in the Blue Mountains Province and the Klamath–Sierra Nevada Provinces suggest that shortening and deformation were diachronous, progressing from north to south. We envision that Late Jurassic deformation may have collapsed a Gulf of California–style oceanic extensional basin that extended from the Klamath Mountains (e.g., Josephine ophiolite) to the central Blue Mountains Province, and possibly as far north as the North Cascades (i.e., the coeval Ingalls ophiolite).
The Yellowstone “hot spot” track results from migrating basin-range extension Available to Purchase
Whether the volcanism of the Columbia River Plateau, eastern Snake River Plain, and Yellowstone (western U.S.) is related to a mantle plume or to plate tectonic processes is a long-standing controversy. There are many geological mismatches with the basic plume model as well as logical flaws, such as citing data postulated to require a deep-mantle origin in support of an “upper-mantle plume” model. USArray has recently yielded abundant new seismological results, but despite this, seismic analyses have still not resolved the disparity of opinion. This suggests that seismology may be unable to resolve the plume question for Yellowstone, and perhaps elsewhere. USArray data have inspired many new models that relate western U.S. volcanism to shallow mantle convection associated with subduction zone processes. Many of these models assume that the principal requirement for surface volcanism is melt in the mantle and that the lithosphere is essentially passive. In this paper we propose a pure plate model in which melt is commonplace in the mantle, and its inherent buoyancy is not what causes surface eruptions. Instead, it is extension of the lithosphere that permits melt to escape to the surface and eruptions to occur—the mere presence of underlying melt is not a sufficient condition. The time-progressive chain of rhyolitic calderas in the eastern Snake River Plain–Yellowstone zone that has formed since basin-range extension began at ca. 17 Ma results from laterally migrating lithospheric extension and thinning that has permitted basaltic magma to rise from the upper mantle and melt the lower crust. We propose that this migration formed part of the systematic eastward migration of the axis of most intense basin-range extension. The bimodal rhyolite-basalt volcanism followed migration of the locus of most rapid extension, not vice versa. This model does not depend on seismology to test it but instead on surface geological observations.