LATE CRETACEOUS AND EARLY TERTIARY ROCKY MOUNTAIN SYSTEMS— THE LARAMIDE OROGENY 



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that Precambrian cores show extensively are marked by thrusts on the 

 steep flank (Fig. 19.2). Such structures are here interpreted as primary 

 upthrusts and gravity slide phenomena. 



Many of the thrust faults of the western division are low angle and 

 define sheets that have moved horizontally considerable distances. Some 

 of the thrust sheets are folded by later movements. The thrust sheets were 

 rather thin and escaped regional metamorphism. In fact, the rocks 

 involved in the Laramide orogeny are characterized by an absence of 

 metamorphism, except perhaps some of the deeper Proterozoic strata. 

 This distinguishes them from the rocks of the Nevadan orogeny. It will 

 be recalled that the Nevadan in the batholithic belt is characterized by 

 isoclinal, nearly vertical folds, as well as flow cleavage. Isoclinal folding 

 is rare in the Laramide Rockies. The Nevadan is characterized by great 

 batholiths. Aside from the Idaho batholith, the Laramide orogenic belt 

 has few plutons large enough to be called batholiths; its intrusions are 

 mostly stocks, but the stocks exist in considerable number. The Nevadan 

 developed in sediments of the eugeosyncline, the Laramide in sediments 

 of the miogeosyncline and shelf. 



Canadian and Montana Rockies 



The Canadian and Montana Rockies consist of a mainland assemblage 

 of geosynclinal sediments of late Proterozoic, Paleozoic, and Mesozoic 

 age, cast into a great imbricate series of thrust sheets. The Proterozoic 

 rocks of western Montana form an extraordinarily thick group of clastic 

 sediments, known as the Relt series. Originally clays, sands, and marls, 

 diey have been metamorphosed to argillites, quartzites, and impure sid- 

 eritic marbles and limestones. They are at least 50,000 feet thick near 

 Missoula. The Paleozoic rocks are dominantly limestones, and are nearly 

 7000 feet thick. The Madison limestone of Mississippian age is about 2000 

 feet thick, and forms steep cliffs and canyon walls in many of the ranges 

 southeast of Missoula. The Mesozoic rocks are about 7000 feet thick and 

 are dominantly shales, with some limestone, sandstone, and conglomerate. 

 Consult tectonic and geologic maps of the Permian, Triassic, Jurassic, and 

 Cretaceous, Plates 8 to 12 for information on the deposition and distribu- 

 tion of the various stratigraphic systems, and Chapters 5 and 6 for isopachs. 



The Reltian has not been reported to be as thick in Idaho and Utah as 

 in western Montana, and north of the border it also seems to be thinner. 

 There, it crops out almost entirely west of the Rocky Mountain trench and 

 leaves the main Canadian Rockies to be composed mostly of Paleozoic 

 strata. The Cambrian thickens to over 15,000 feet along the Alberta- 

 British Columbia boundary, and most of the scenic ranges there are 

 sculptured in it. 



Very few intrusions occur east of the Rocky Mountain trench north of 

 the Idaho and Boulder batholiths. Large sheets of diorite and gabbro 

 split the Beltian rocks in places, and one near the Canadian border is 

 identified as a lava flow and is called the Purcell lava. The sills and flows 

 have been deformed with the Beltian strata. 



The igneous intrusions are very abundant and voluminous in west- 

 central Montana, and are composed chiefly of quartz monzonite and 

 diorite. 



Central Rockies 



The central Rockies consist of folded and thrust-faulted Paleozoic- 

 strata in their western part and Proterozoic, Paleozoic, and Mesozoic 

 along their eastern margin. The Mesozoic sediments were especially thick 

 in places, and a number of episodes of compression occurred from mid 

 Cretaceous to early Oligocene. The structures of some of the episodes 

 trend discordantly to those of others. Thick, coarse conglomerates 

 mark the orogenies and, being deformed themselves, add to the com- 

 plexity. 



A review of the tectonic maps of the Permian, Triassic, Jurassic, and 

 Cretaceous, Plates 8 to 12, will impress one with the fact that the Laramide 

 trough zone of orogeny, especially in Utah, embraced parts of two major 

 elements, the Cordilleran intermontane geanticline and the Mesozoic 

 trough. The sinking of the Permian trough in Utah started a series of 

 subsidences that followed generally one on top of the odier until the 

 Laramide orogeny. The total accumulation of sediments of the Permian 

 and Mesozoic, therefore, has been isopached, and the basin is charted on a 

 map so as to compare with the Laramide deformational belt. The map, 

 Fig. 19.3, shows the extent to which the Laramide belt cut into the 



