The John Day Formation In The Picture Gorge Quadrangle Oregon

Author : Willis Harkness White
Publisher :
ISBN 13 :
Total Pages : 308 pages
Book Rating : 4.:/5 (184 download)

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Download or read book Geology of the Picture Gorge Quadrangle, Oregon written by Willis Harkness White and published by . This book was released on 1964 with total page 308 pages. Available in PDF, EPUB and Kindle. Book excerpt: There are seven stratigraphic units exposed in the Picture Gorge quadrangle, a 204-square mile area in east-central Oregon. isolated exposures of pre-Cretaceous serpentinites, metalimestones, and other metamorphic rocks record marine deposition of sands silts, and limy sediments followed by burial, lithification, peridotite intrusion, and several periods of deformation. Intrusion and deformation were accompanied by serpentinization, silcification. and calcite veination. Cretaceous conglomerates and intercalated sandstones, which unconformably overlie the metamorphic rocks, are a result of fluvial deposition in a nearshore or continental environment, burial, lithification, uplift, and erosion. Pebbles were derived from plutonicmetamorphic highlands. The Eocene Clamo Formation unconformably overlies the Cretaceous rocks and is made up of basalt conglomerate, silicified conglomerate, andesite breccias, and andesite flows. The fossiliferous John Day Formation, Oligo-Miocene in age, lies unconformably on the Clamo Formation and is pyroclastic in character. Ash was wind-transported from trachytic-andesitic sources to the west and deposited on land, in lakes, and in swamps. The 2, 400-foot section of volcanic siltstones and waterlaid volcaniclastic rocks includes some basalt near the base and an ash-flow tuff near the middle. The John Day Formation was folded slightly after consolidation. The fissure-fed Middle Mjocene Columbia River Basalt flows, at least 2, 300 feet thick, blanket 124 square miles of the quadrangle. Modal analyses of thin sections from 22 basalt flows suggest that flows cannot be correlated on the basis of mineralogy. Correlation must involve a combination of criteria, such as color, texture, and nature of outcrop, together with mineralogy and chemical analyses. Interfiow breccias, tuffs, volcanic siltstones and sandstones, dikes, and an irregular intrusion are also present. The Columbia River Basalt unconformably overlies the John Day Formation. After emplacement, the basalt flow rocks were folded into the west-trending Sunflower anticline and John Day syncline. Normal faulting occurred on the shared flank of the two folds. The Upper Miocene Mascall Formation lies unconformably upon the Columbia River Basalt. About 400 feet of well-bedded, rhy-. olitic volcanic siltstones and intercalated sand and gravel lenses suggest eoliari and fluvial deposition of ash in lakes and swamps. Folding and faulting occurred along the same trends which were active during Columbia River Basalt time. 'lhe Plio-Pleistocene Rattlesnake Formation rests with pronounced angular unconformity on the Mascall Formation. About 10 feet of poorly consolidated fluvial gravels enclose a 38-foot-thick ash-flow tuff. Pleistocene and recent events include capture of the middle fork of the John Day River, slumping, development of pediments, formation of stone garlands, stone stripes, and talus-free mounds, and deposition of stream gravels. An andesitic, hornblende-bearing white volcanic ash was deposited by wind.