CENTRAL MONTANA ROCKIES 



357 



and the Crazy Mountains. Those where local domes have been created, 

 presumably by laccolithic intrusions in the Cretaceous strata, are Sweet- 

 grass Hills, Rearpaw, Little Rocky, Moccasin and Judith Mountains. The 

 other two groups, the Highwood and Crazy Mountains, are character- 

 ized by remarkable radiate dike swarms and not by domal uplifts. The 

 Highwood Mountains are on the north flank of the Little Relt Mountains, 

 and the Crazy Mountains are directly in the lowest part of the Crazy 

 Mountains basin (Structure Contour Map of the Montana Plains). 



Bearpaw Mountains 



The Rearpaw Mountains are made up of two large volcanic fields with 

 a central strip, 2 to 8 miles wide, of deformed and metamorphosed sedi- 

 mentary rocks, known as the Rearpaw Mountains structural arch. It 

 trends N 60° to 80° E as does an accompanying swarm of thousands 

 of dikes (Pecora, 1957). The oldest formation involved is the Madison, 

 and the youngest the Judith River of Late Cretaceous age. 



The arch was first developed as a prevolcanic structure and continued 

 to develop throughout the magmatic history. Vertical uplift of 5000 to 

 7000 feet is demonstrable, with block faulting in prevolcanic time per- 

 j mitting a good part of the uplift in places. 



The great abundance of Precambrian basement inclusions in the rocks of 

 latitic composition represents transportation vertically of at least 2 miles 

 through the Paleozoic and younger formations and at least 4 miles if the 

 1 volcanic pile is also pierced (which is 10,000 to 15,000 feet in maximum thick- 

 j ness). The extensive distribution of the inclusion-bearing felsic rocks over 

 I 1600 square miles of the Bearpaw Mountain uplift area and the absence of 

 quartzite fragments representative of the Belt series are significant relation- 

 ships that may indicate either an angular uncomformity and the removal of 

 the late Precambrian rocks of the Belt series before deposition in the early 

 Paleozoic sea in this region or a development of the felsic magma very deep in 

 the basement itself (Pecora, 1957). 



The volcanic activity ran its course during middle and late Eocene, 

 and radiogenic ages of zircons in a syenite are reported to be about 40 

 to 60 m.y. Post-volcanic faulting and intrusions have disturbed the orig- 

 inal attitude of much of the layering of the volcanic pile. 



A great variety of mafic subsilicic-alkalic to felsic silicic-alkalic rocks 

 occur, with the mafic rocks exceeding the felsic in volume. 



An extensive skirt of small thrust faults flanks on the south the Bi 

 paw igneous centers and domal uplift, and is regarded by Reeves 

 as a gravity slide phenomenon down slope from the uplift. 



Little Rocky Mountains 



The Little Rocky Mountains are a singular structural type. They lie- 

 apart — somewhat north — of the other uplifts and domes of central Mon- 

 tana, and are erosional features of a subcircular dome, about 20 miles 

 in diameter, which embraces more than 50 faulted subordinate domes. 

 See Fig. 23.5. Alkalic igneous rocks of Tertiary age, mainly in the form 

 of sills, have intruded the Cambrian strata but are not known to have 

 intruded sedimentary rocks younger than Cambrian. All their contact-, 

 with post-Cambrian rocks appear to be fault contacts, and indicate that 

 the igneous rocks, after having consolidated at and near the base of the 

 Cambrian, were deformed, broken, and faulted by upward pressure, 

 probably due to an underlying, rising magma ( Knechtel, 1944 ) . 



The subordinate domes on the large subcircular dome of the Little 

 Rocky Mountains were formed by bodies of igneous rock which were 

 punched upward into the sedimentary rocks. They range in diameter 

 from 13i to 3M miles. Each is typically subcircular or subelliptical in 

 plan and normally includes a hinged block that has been raised like a 

 trap door (Knechtel, 1944). See cross section of Fig. 23.5. 



Relation of Igneous Activity to Tilted Fault Blocks 



The Little Rocky, Moccasin, and Judith Mountains, the domes of the 

 Rig Relt, and those southeast of the Little Relt Mountains, and possibly 

 the Porcupine dome are variations of laccolithic uplifts. The Highwood 

 and Crazy Mountains are the products largely of extrusive activity, but 

 stocks, numerous dikes, and laccoliths are present. 



In spite of the flexures and deep-seated faults beneath them, the sedi- 

 mentary beds were almost horizontal in most places at the time oi 

 igneous activity. The belt of vigorous Laramide deformation lay to the 

 west. The laccolithic intrusions especially domed up the beds, but in the 

 two mountain groups where extrusive rocks are most abundant, the 

 strata seem little deformed below the volcanics. 



