IDAHO BATHOLITH AND THE OSBURN FAULT ZONE 



321 



quartzites and slates. Still farther northeast in Montana the batholith is 

 believed to be bordered chiefly by Beltian rocks, although little is known 

 geologically of this region. On its west side, the bordering formations, in 

 addition to the extensive Tertiary volcanics of later origin, include pre- 

 Tertiary sedimentary and volcanic strata which are intruded by it. The 

 pre-Tertiary rocks along the part of the western boundary north of Sal- 

 mon River are mostly so metamorphosed as to make correlation doubtful, 

 but along Snake River there are considerable thicknesses of Permian strata 

 and some Triassic beds, both of which include volcanics. Small granite 

 masses, presumably satellites, cut the Permian strata. The Thatuna 

 batholith is one of these (see Fig. 17.13). 



In numerous places, Tertiary strata, mainly Miocene (?) volcanic rocks, 

 rest on the eroded surface of the batholith; and it is clear that much of the 

 eastern part of the batholith now exposed was laid bare by erosion prior 

 to the Tertiary volcanism. Some of the volcanic flows resting on the batho- 

 lith may be as old as Oligocene ( Ross, 1928 ) . 



Satellites (?) 



The numerous plutons east of the Idaho batholith in western Montana, 

 such as the Philipsburg (Calkins, 1915), Boulder (Knopf, 1913), and 

 Marysville (Barrell, 1907), intrude either Cretaceous formations or older 

 Mesozoic formations that were folded and thrust following the deposition 

 of the Upper Cretaceous beds. The intrusions are distinctly discordant 

 with the folds and thrusts and, as far as known, were all emplaced after 

 the Laramide thrusting. They constitute a middle or late phase of the 

 Laramide orogeny. 



If the assumption is correct that the main batholith was intruded at the 

 same time as its smaller eastern neighbors, then the great pluton must be 

 Laramide in age and not Early Cretaceous or Late Jurassic (Nevadan). 

 In further consideration of this line of evidence, it may be seen (Fig. 21.2 

 and Tectonic Map of the United States, 1944) that the Philipsburg thrust 

 is truncated by the main eastward-extending appendage of the Idaho 

 batholith. But this appendage is represented on the new Tectonic Map of 

 the United States as a separate intrusion of later age than the main igne- 

 ous mass. The representation comes of necessity when the main mass is 



shown as Nevadan. Details are not known, because the appendage has not 

 yet been described in print. 



The Casto intrusion is exposed along the axis of a broad anticline and 

 involves both Permian (?) and Miocene (?) strata (Ross, 1935). Injec- 

 tions of pink granite into the Miocene (?) beds indicate the age of the 

 pluton to be Miocene (?), according to Ross; but then, the exact age of 

 the Tertiary beds is not known. Ross mentions other pink granites in the 

 northwest corner of Idaho and in British Columbia that probably cut Mio- 

 cene (?) strata and are distinctly younger than the Nelson batholith. The 

 Nelson is believed to be earlier than Late Cretaceous because pebbles of 

 its granite are found in the Blairmore conglomerate of Late Cretaceous 

 age. The pink granites appear to be the youngest of the plutons, even con- 

 siderably younger than the Boulder batholith (Ross, 1928). 



Setting in Laramide Tectonic Plan 



Figure 21.1 has been prepared to show in a broad way the relation of 

 the Idaho batholith to the Nevadan and Laramide orogenic belts. In brief, 

 the batholith is located at the junction of two arcuate segments of the 

 Laramide belt, one extending from Canada into Montana on the north, 

 and the other extending from Utah through Wyoming and southwestern 

 Idaho on the south. A third major structural element, the zone of thrusting 

 of the shelf ranges, converges here also. The converging of the three large 

 elements of the Laramide orogeny at about the position of the Idaho 

 batholith may be genetically significant. 



The dominant trend of the fold axes and thrusts about the batholith, as 

 shown in Fig. 21.1, is a generalization of the detail shown in Fig. 21.2. The 

 latter map was compiled from the Tectonic Map of the United States. 

 with faults of post-Laramide age (as well as known) deleted and with 

 additional fold axes and also some fault detail from the new Geologic Map 

 of Montana (1945) added. The conclusion reached by inspection of the 

 detailed map is that the intrusions are markedly discordant locally, but 

 in a broad way the structures of the sedimentary rocks wrap concordantly 

 around the east and north end of the main batholith. This may mean either 

 that the batholith was already there and served as a buttress around 

 which the Laramide structures were wrapped, or that in the process of 



