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STRUCTURAL GEOLOGY OF NORTH AMERICA 



present thicknesses indicate. At least 30,000 cubic miles of welded tuffs 

 were erupted in this subprovince. 



They originated in fissure eruptions, and dike feeders are the rule. No 

 cones, central vents, radial dike swarms, or quaquaversal dips have been 

 noted. Dr. Williams' ideas about die age of the welded tuff accumulations 

 is as follows. The rapidity of accumulation is startling because a layer 

 as thick as 1000 feet may have accumulated in one day. Although there 

 were many eruptive centers the entire explosive activity occurred un- 

 doubtedly in a very short time geologically, say a few thousand years. 

 The major activity of a certain group of fissures may have taken place in 

 three or four davs. This is deduced because of the absence of erosional 



J 



breaks of any kind in the succession of welded tuff flows. Soil profiles be- 

 tween flows were sought but not found. 



A potassium-argon age determination yielded a date of 35 =■= 2 m.y., and 

 Professor Williams thinks this will prove to be the age of the main unit of 

 the thick assemblage of welded tuffs over the entire subprovince. There 

 are younger welded tuffs above and outside the subprovince, but these are 

 another matter. The age would then be early or mid-Oligocene according 

 to which absolute time scale is used. 



According to E. F. Cook (personal communication), who has studied 

 the welded tuffs extensively through Nevada and western Utah and who 

 has attempted to correlate many measured sections, breaks in the deposi- 

 tional sequences occur, with flows and sediments interlayered. He believes 

 the eruptions were intermittent and extended through the Oligocene into 

 the Miocene, and suggests that the period through which the welded tuffs 

 were erupted was several million years long. T. B. Nolan informed the 

 writer that the 35 (or 34) m.y. potassium-argon date appears to conflict 

 with Miocene fossils in the Eureka-Austin-Winnemucca area. Since the 

 interest of a number of geologists and geochemists is high on the problem, 

 our knowledge will undoubtedly be more precise in a short while. 



The surface at the time of the numerous and widespread outbreaks 

 seems to have been very flat, according to Dr. Williams, because wherever 

 the base of the series is exposed it is without relief, and since the indi- 

 vidual avalanche flows can be traced scores of miles, there must not have 

 been sizable topographic obstructions in their way. This is especially true 



for the upper flows of the welded tuff sequence, according to Dr. Cook, 

 but he believes the early flows filled basins of appreciable relief or closure. 

 The bulk of the material of the flows is approximately of quartz latite 

 composition, and it is mostly slightly potassic with some parts rather 

 potassic. Some inclusions occur and these confirm the suspicion of Pro- 

 fessor Williams that assimilation of the crystalline basement of the silicic 

 crust is involved in the origin of the welded tuffs. 



Eleven ignimbrites are widespread in southeastern Utah and have been 

 given formal stratigraphic names by Mackin (1960). He says: 



The fact that the oldest of them lies unconformably across the beveled edges 

 of thrusts and folds involving late Cretaceous strata indicates that the beginning 

 of volcanic activity post-dates the Laramide orogeny. As planar units which 

 provide a record of Tertiary crustal movements, the ignimbrites confirm the 

 Gilbert theory, based originally on physiographic evidence, that block faulting 

 has been the characteristic type of post-Laramide deformation in the Great 

 Basin. 



The volcanic field of southwestern Utah abounds in welded tuffs. These 

 and associated volcanics are described by Cook ( 1957 ) in probably the 

 best account of them so far. Indurated acidic pyroclastic rocks ranging 

 from welded tuffs to bedded tuff-breccias dominate the volcanic column 

 which is several thousand feet thick. The bedded tuff-breccia occurs in 

 beds 2 to 20 feet thick and fills depressions in a rough topography de- 

 veloped on folded and faulted volcanic rocks. Its tuffs are non welded but 

 in one locality appear to grade downward into welded tuff-breccia. Cook 

 concludes that the bedded tuff breccia was deposited by a series of 

 rapidly moving, widely spreading flows of gas ( possibly steam ) , hot water, 

 and solid particles, in which the temperature was below that required for 

 welding. 



Breccia and air-fall tuff form a minor portion of the volcanic rocks of 

 the area. Lava flows, conspicuous locally but also of minor amount, in- 

 clude dacite porphyry, locally porous and fluidal, latite (?) and latite 

 porphyry, olive-brown to black andesite, and dark gray to black basalt. 

 The higher part of the Pine Valley Mountains consist of latite ( or quartz 

 latite ) porphyry. Except for a chilled basal phase the latite is lithologically 

 uniform throughout its thickness of 2000 feet, and is similar mineralogi- 



