- The eastern Salmon River Mountains of east-central Idaho are composed primarily of Precambrian Y metasedimentary rocks over 10,000 meters thick. Correlation with the Belt Supergroup of western Montana and northern Idaho has at various times been proposed and denied. The outcome of this controversy has direct bearing on the nature of the western margin of the North American craton during the Proterozoic.
U-Th-Pb zircon analyses indicate the stratigraphically lowest unit of the east-central Idaho sequence, the Yellowjacket Formation, is cut by Precambrian Y porphyritic granites (1370 (+OR-) 10 m.y.), the Cambrian alkalic Deep Creek pluton (Pb-Pb ages of 541 and 521 m.y. for two size fractions), and the Ordovician alkalic Arnett Creek pluton (492 (+OR-) 10 m.y.). Similarly, two size fractions of zircons from the alkalic Beaverhead pluton near Leadore, Idaho, yield Pb-Pb ages of 481 and 484 m.y.b.p. Zircons from the Precambrian granites contain a component of inherited lead, suggesting reported zircon ages for similar rocks of central Idaho may not accurately reflect the time of granite emplacement.
Detailed mapping near Salmon, Idaho, indicates isoclinally folded feldspathic quartzites of the Proterozoic Y Big Creek Formation are thrust onto the broadly folded, biotitic, feldspathic quartzites of the Yellowjacket Formation. The time of folding of the Big Creek rocks is unknown, but the folds in the Yellowjacket rocks are cut by the 1370 m.y. granites. Also, the biotite-grade metamorphic assemblage of the Yellowjacket Formation is overprinted by a contact aureole developed around the Proterozoic granites and may have formed during the pre-1370 m.y. folding event.
Regional inferences derived from these findings are as follows: (1) the east-central Idaho section and the Belt Supergroup are most likely time equivalent deposits; (2) the influx of sediments from the southwest into the Belt Basin does not require a source older than rocks equivalent to the lower part of the Belt Supergroup; and consequently, (3) there is no need to postulate a major Proterozoic high exposing pre-Beltian rocks in central Idaho ("Salmon River Arch" of Armstrong, 1975), at least in the strict sense of the original definition.
- Dissertation Note:
- Ph.D. The Pennsylvania State University 1981.
- Source: Dissertation Abstracts International, Volume: 42-07, Section: B, page: 2745.
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