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STRUCTURAL GEOLOGY OF NORTH AMERICA 



crystals, and has been identified megascopically as rhyolite. The cone is 

 much eroded. 



East Butte is made up of beds of trachyte, pumice, and obsidian, which 

 strike east-west and dip 30 degrees south. No vestige of a crater remains, 

 and it is possible that the butte is part of a tilted fault block. The third 

 butte, known as West or Middle Butte, lies 4 miles away. It is composed 

 entirely of basalt which dips 10 degrees south. If East and West Buttes 

 are both parts of the same tilted fault block, then interlayered trachyte 

 and basalt must be postulated. Whether a fault block or separate cones, 

 they were deeply dissected by erosion before the Snake River basalts were 

 spread around them. A thin section of the basalt of West Butte shows 

 "abundant feldspar, olivine, and pyroxene, with a little brown glass." 



A number of units in the Snake River plain are younger than the 

 rhyolites yet older than the basalts that cover most of the plain. They are 

 mostly basalts and associated lake beds. The extensive Pleistocene and 

 Recent basalts are said by Stearns to have come from about 400 vents 

 in the plain. He charted the position of about 300 of them. Except for the 

 cluster in the Craters of the Moon National Monument and the group 

 north of St. Anthony, they are rather evenly distributed and neither a rift 

 nor fault pattern is discernible, although here and there short rows of 

 cones occur. 



Near the north side of the Snake River Plain cinder cones 50 to 200 

 feet high predominate. However, over most of the plain the vents are 

 broad lava domes each usually about 100 feet high and the related flows 

 covering about 30 square miles. Only a suggestion of a crater or crater 

 rim is left generally when eruption ceases. The lava welled out quietly 

 and profusely and each vent had only one period of activity. With 

 activity over in one vent another one nearby seems to have formed and 

 poured out considerable lava. 



The geology of the western part of the Snake River volcanic field has 

 been summarized by Kirkham ( 1931 ) . He believes that the basal layer is 

 a Miocene basalt and that this is very widespread. He calls it the Colum- 

 bia River basalt, but describes it principally as an olivine basalt which 

 does not correlate with the tholeiitic basalts of the Columbia River basalt 

 field proper. This basal unit has been eroded irregularly and its existing 



thickness in outcrop ranges from 300 feet to over 1200 feet. The basal 

 "Columbia River basalt" occurs in three stratigraphic parts, namely, 

 lower and upper basalt flow units and intermediate lake beds containing 

 much tuff, the Payette formation. 



The Owyhee rhyolite, previously mentioned, rests on the basalt, at least 

 in the area of southwestern Idaho south of the Snake River. Kirkham 

 states that the rhyolite is actually a series, and is generally made up of 

 basalt and andesite flows at the bottom, and above by trachyte, latite, 

 and rhyolite flows interbedded with ash, fresh-water limestone, clay, 

 shale sandstone, and conglomerate layers. He correlates the series with 

 the Salt Lake formation south of the Snake River plain. The distribution 

 and stratigraphic and petrographic relations of the "Columbia River 

 basalt" and Owyhee "rhyolite" seem to need much more study before 

 the picture can be significantly summarized. 



Above the Owyhee rhyolite and Salt Lake beds is the widespread 

 Snake River basalt, so characteristic of the eastern part of the field pre- 

 viously described. The Snake River basalt flows give way to and are 

 covered by lake beds in western Idaho which are known as the Idaho 

 formation (Kirkham, 1931), but here as in the eastern part of the plain, 

 the Quaternary history was eventful with repeated, if scattered, construc- 

 tional volcanic activity, struggling against the destructional activity of the 

 Snake River for supremacy (Norman Anderson, personal communication). 



The Snake River volcanic field together with the Malheur and Colum- 

 bia field constitute a unique petrographic province from the tectonic 

 point of view. The western part of this great field covers the Nevadan 

 batholithic and orogenic complex, and the eastern arm lies across the 

 Laramide fold and thrust belt of the central Rockies (Chapter 22). We 

 are accustomed to a parallel arrangement of volcanic deposits with the 

 orogenic belt; even if discontinuous in extent, the volcanic fields do not 

 take a transcurrent direction. Here, however, the eastern arm of the 

 Snake River field extends almost at right angles over the underlying folds 

 and thrust sheets of southeastern Idaho and southwestern Montana. 



Malde (1959) reports a great fault zone along the northern boundary 

 of the Snake River Plain in the area west of Boise. Gravity, seismic, and 

 geologic studies indicate that at least 9000 feet of aggregate throw has 



