IGNEOUS PROVINCES IN WESTERN UNITED STATES 



561 



after the deposition of the Rattlesnake formation. The Pleistocene in 

 central Oregon was mostly a time of erosion. 



The Rlue Mountains are flanked on the south by Mio-Pliocene volcanics 

 of the Payette and Owyhee formations and correlative beds. The pre- 

 Columbia River basalt formations are missing along the southeast side 

 of the Blue Mountains and the Payette, oldest in the area, is correlated 

 with the late Miocene Mascall on the north side. The High Lava Plains 

 (Fig. 29.15) south of the Blue Mountains are made up of relatively 

 undeformed young lava flows dotted in places by cinder cones and lava 

 buttes. The formations are dominantly Pliocene lavas, tuffs, and alluvium, 

 few of which have been formally named ( Baldwin, 1959 ) . 



Basalt Kindreds. H. A. Powers of the U.S. Geological Survey has com- 

 mented in a letter to the writer about the problem of basalt kindreds in 

 the northwestern states, and has charted the chemical analyses of about 

 65 characteristic basalts in regard to Si0 2 and MgO from the Columbia 

 Plateau, the Snake River downwarp, the Malheur Plateau and Hawaii. 

 He finds such a scatter of points that the concept of a clear-cut distinc- 

 tion of tholeiitic and olivine basalt seems to break down. The Columbia 

 River basalts of Miocene age run relatively high in Si0 2 and low in MgO; 

 the Hawaiian basalts classed as tholeiitic run slightly less in Si0 2 and 

 intermediate in MgO; Hawaiian rocks classed as olivine basalts are inter- 

 mediate to low in SiOo and low, intermediate and high in MgO; the 

 Snake River Pliocene and Recent basalts run generally low in SiO L > and 

 intermediate in MgO; the Steens Mountain basalts in the Malheur field 

 run intermediate to low in SiO L » and generally low in MgO. As a result 

 he says: 



In some provinces, there is a decided gap, or absence of rocks showing all 

 the intermediate stages. In such provinces there appears to be an impressive 



I difference between tholeiite and olivine basalt, in the chemical sense. My 

 feeling is that the concept of a fundamental distinction between two kindreds 

 of basalts has been developed from a concentration on such single provinces, 

 but that the concept breaks down and is not convincing when one considers 



! all the basalts that we know about from good comparable chemical analyses. 



I I have plotted in different ways about a thousand reasonably good analyses 

 of basalts trying to establish a natural division zone, and so far have succeeded 

 only in showing a complete gradation — a lot of crossbreeding if there are 

 really two kindred. 



On the other hand, he believes that perhaps a difference can be made 

 between flood eruptions and cinder cone or small lava dome eruptions, 

 and that this may reflect fundamental differences in the tectonic setting. 

 Such a distinction is based on the field characteristics and not on the 

 chemical compositions. In the Columbia River basalt field flood basalts 

 predominate and are presumed to have issued from fissures. Most of the 

 Pleistocene basalts in the Columbian River field are fissure flows also, 

 but some seem to be of cinder cone activity (Powers, personal communi- 

 cation). The Snake River and Malheur fields, on the other hand, are 

 mostly of the cinder cone and small lava dome type. 



Snake River Basalt Field. The eastern part of the Snake River lava 

 plain from King Hill and Twin Falls to Yellowstone Park, a distance 

 of 200 miles, has been studied in considerable detail by Stearns, et al., 

 (1938). They report that about 95 percent of the rock of the depression 

 or downwarp is the so-called Snake River basalt of Pliocene, Pleistocene, 

 and Recent age. Locally sedimentary lenses, closely related petrologicallv 

 to the flows, exist, and some of these are very fossiliferous such as the 

 Hagerman lake beds. In numerous places on the borders of the plain 

 rhyolitic flows and pyroclastics emerge from beneath the basalts. Per- 

 haps the rhyolites are younger and stratigraphically above the Challis 

 volcanics on the north border which are dominantly latite and andesite. 

 The Challis volcanics are regarded from fossil leaf beds as late Oligocene 

 or early Miocene, and ages up to early Pliocene have been assigned to the 

 rhyolites. At places rhyolites crop out within the basalt plain under the 

 basalt, and hence it is believed that the rhyolite volcanics extend widely 

 under the field and form the basal layer (Kirkham, 1931). 



The rhyolites have been loosely referred to as the Mount Bennett 

 rhyolite and Owyhee rhyolite, but much of the rock is quartz latite or 

 even possibly andesite similar to the Challis volcanics (Stearns et al., 

 1938). 



Three old cones are prominent landmarks in the area between Arco 

 and Blackfoot, and their building seems to predate the Snake River basalt. 

 Big Southern Butte, about 5 miles in diameter, rises nearly 2500 feet 

 above the plain and is composed of basaltic and rhyolitic flows. The 

 main mass is a light-colored porphyritic rock containing large quartz 



