356 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



steeper than on the north. Cutting the anticlines are two principle sets of 

 faults, viz., faults parallel with the strike of the zone on the anticlinal 

 axes, and faults in en echelon arrangement diagonally across the zone. 

 There are also a number of feather faults that terminate at the longi- 

 tudinal faults, together with two downdropped fault blocks. It has also 

 been noted that the formations vary in thickness across the zone or 

 "lineament," as Wilson (1936) calls it. The Lance formation and Lebo 

 member of the Fort Union thicken abruptly south of the axis of the fold. 

 This is taken to mean the beginning of flexing or faulting during the 

 deposition of the uppermost Cretaceous and Paleocene beds. 



Centers of volcanism are also aligned with the Nye-Bowler lineament. 

 The laccoliths of Limestone Butte and Round Mountain were intruded 

 along the westward projection of the belt. Also, the intrusions of Green 

 Mountain and Squaw Peak in the McLeod area came up along the linea- 

 ment (Wilson, 1936). 



The assemblage and relation of all the structural features of the Nye- 

 Bowler lineament have led Wilson (1936) to conclude that they are the 

 surface expression of a single deep-seated fault, along which both vertical 

 and horizontal movement took place. The south block both sank and 

 moved eastward. This horizontal movement is in the same direction as 

 that of the blocks north and south of the Lake basin and Cat Creek fault 



zones. 



STAGES OF OROGENY 



The first perceptible stage of the Laramide orogeny in south-central 

 Montana was marked by slightly coarser sediments in the Judith River 

 formation and by the eruption of volcanoes that may have heralded the 

 early rise of the Beartooth block. The earliest movement on the basement 

 fault of the Nye-Bowler lineament also occurred, and dikes were intruded 

 and agglomerate (the lower part of the Livingston formation) piled up 

 along the fault trace. The second stage, according to Wilson, lasted 

 through the deposition of the Bearpaw, Lennep, Colgate, and Lance for- 

 mations and the Lebo member of the Fort Union. The Nye-Bowler flexure 

 first appeared in the area of deposition of the Upper Cretaceous sediments 



just mentioned, and they accumulated thinly over the crest and thickly 

 on the depressed area to the south. The en echelon faults of normal dis- 

 placement also were formed at this time. Volcanism continued from the 

 first stage all through the second. 



The third stage was principally that of uplift of the Beartooth Moun- 

 tains, and with the orogeny the Nye-Bowler monocline was compressed 

 and arched, with the formation of its individual domes. Its normal faults 

 in part became reverse ones in face of the compression; more horizontal 

 movement occurred, and the feather faults came into existence. Erosion 

 was actively attacking the rising Beartooths, and the waste products were 

 spread out to the east as the Tongue River sandstones and shales. By the 

 time of the next uplift of the Beartooth block, which would be the fourth 

 stage, the Precambrian crystalline rocks in it had been exposed and the 

 beds of flexure had been considerably truncated. The following wave 

 of debris from the Beartooths spread over the truncated structures of the 

 basin. All this deposit, the "Wasatch sandstones and conglomerates," has 

 subsequently been eroded away save for a downfaulted and protected 

 block. The block faulting marks the last and fifth stage of the Laramide 

 orogeny in the region. This last stage should possibly be considered post- 

 Laramide. 



According to a recent study by McMannis (1958) three major spread- 

 ings of andesitic debris into the Crazy Mountain basin from the south- 

 west occurred, each one reaching further than the other. These were in 

 Judith River, Lennep, and Lebo times. The Lennep and Lebo "pulses" 

 of McMannis make up the second stage of Wilson above reviewed. The 

 Lebo volcanism and uplift constituted the culmination of the Laramide 

 orogeny, according to McMannis, and this occurred in Paleocene time. 



IGNEOUS CENTERS 



Distribution and General Structure 



Six igneous mountain clusters of the Central Montana Rockies may be 

 recognized as follows, beginning on the northwest: the Sweetgrass Hills, 

 the Bearpaw Mountains, the Little Rocky Mountains, the Highwood 

 Mountains (Fig. 23.1), the Moccasin and Judith Mountains (Fig. 23.2), 



