572 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



the field to the Highwood Mountains and the alkalic province of central 

 Montana. 



Summary. The Yellowstone subprovince as a whole is one of olivine 

 basalt parentage and although it is in a Laramide belt of mild to appreci- 

 able deformation, it is definitely not of the orogenic andesite lineage; it 

 belongs to the nonorogenic calc-alkalic and alkalic provinces east of the 

 central Rockies. 



Crowsnest Volcanic Field. Still another Cretaceous volcanic field has 

 been described in the deformed belt (MacKenzie, 1956). It is known as 

 the Crowsnest volcanics and occurs about 30 miles north of the interna- 

 tional boundary ( see Figs. 20.2 and 20.6 ) . The volcanic deposit lies within 

 the mid-Cretaceous sediments; viz., the continental Blairmore (Dakota) 

 formation underlies it, and the marine Blackstone (Benton) overlies it. 

 At Coleman the lower unit consists of trachyte agglomerate beds. This is 

 overlain by ash beds with scattered large fragments of pyroclastics; this 

 in turn is overlain by water-laid ash beds rich in andesite, and this in turn 

 by more ash beds with varying amounts of coarse pyroclastics. Some of 

 the ash beds are hard resembling flows; but no actual flow rocks are 

 reported. Four main lithologies have been identified, namely, augite 

 trachyte breccia, tinguaite, andesite tuff, and phonolite tuff. The name 

 blairmorite has been suggested for certain analycite-rich fine-grained rocks 

 in the volcanics. 



The rock on account of its ultra-alkaline nature, will show numerous varia- 

 tions in texture and in proportions of component minerals . . . (MacKenzie, 

 1956). 



It is evident that the Crowsnest volcanics are alkalic and related to the 

 central Montana petrographic province rather than a field to the south in 

 the Foothill belt of Montana, the Dearborn River which is generally 

 andesitic. The Dearborn River field is described under a later heading. 



The age by stratigraphic position is Mid-Cretaceous, and by potassium- 

 argon dating (Folinsbee et al., 1957) is 96 m.y. The Crowsnest volcanics 

 are slightly older than the Dearborn River Volcanics, according to strati- 

 graphic position, and slightly younger than the main Nevadan batholithic 

 intrusions to the west. 



The Crowsnest volcanic field is estimated to have a maximum average 



thickness of about 1000 feet and to spread over 500-600 square miles. It 

 contains about 50 cubic miles of rock. 



Southern Colorado Plateau Basalt-Rhyolite Province 



Extent. Several volcanic fields along the southern margin of the 

 Colorado Plateau may be conveniently grouped together because of their 

 close proximity, but they hardly have enough common characteristics to 

 justify the grouping. The questionable province starts on the east with 

 the Jemez field in north-central New Mexico and includes the Mount 

 Taylor, the Datil, the San Francisco, and the Uinkaret fields. See map, Fig. 

 36.1. 



Cliaracteristics. The Jemez, Mt. Taylor, and San Francisco fields are 

 the result of large, central vent-type volcanoes. One large volcano or a 

 cluster of several with numerous, later, small cinder and lava cones make 

 up the fields. The rocks range from basalt to rhyolites and appear to repre- 

 sent the basalt-rhyolite differentiation suit. The source of the lavas in the 

 large Datil field is not recorded in the literature as far as the writer can 

 determine. The Uinkaret field consists of youthful small cones and basalt 

 flows. 



Description of Fields. The Mt. Taylor field is dominated by the Mount 

 Taylor volcano which erupted in late Miocene time, after folding and 

 faulting in the district. 



The volcano broke out in a syncline. The eruption, which occurred in a 

 fairly well defined compositional sequence, began with rhyolitic tuff. This 

 was followed by relatively quiet eruptions of porphyritic lavas in which two 

 and possibly three series are distinguishable on the basis of their content of 

 potash feldspar. The oldest of these is porphyritic trachyte, but the volume 

 is very small. The next eruption was a large volume of porphyritic latite, 

 interrupted, however, by at least one more flow of porphyritic trachyte. The 

 latite, in turn, was followed by a slightly smaller volume of porphyritic 

 andesite. 



The total volume of the tuffs and lavas is about 12.5 cubic miles, of which 

 about 5 cubic miles is rhyolitic tuff, 4 cubic miles is latite, and 3.6 cubic 

 miles is porphyritic andesite. 



The erosion surfaces that subsequendy were developed around the base of 

 the cone later became flooded with sheets of nonporphyritic basaltic and 

 andesitic lavas erupted from the scores of vents that comprise the volcanic 

 field. A few of the sheets were erupted prior to the latest eruptions on Mount 



