Geology of the Three Forks quadrangle, Montana, with sections on petrography of igneous rocks

Abstract

The Three Forks quadrangle at the head of the Missouri River, has a long stratigraphic succession. The oldest rocks are Precambrian gneiss, derived from arkosic sandstone, and amphibolite, probably altered from mafic sills and lava flows.The unmetamorphosed sedimentary rocks range in age from late Precambrian to Recent, have an aggregate thickness of about 11,000 feet, and underlie about 85 percent of the quadrangle. The only systems not represented are Ordovician, Silurian, and Triassic. The oldest unmetamorphosed sedimy'1tary rocks are at least 4,000 feet of marine micaceous arkose and conglomerate, the North Boulder formation of the Belt series of late Precambrian age. The formation is a wedge arkose controlled by a buried east-west fault of pre-Belt or Belt age here named the Willow Creek fault. l Tonkinella cf. T. stephenensis Kobayashi Kootenia sp. (with 10 long marginal spines) Bolaspis sp. indet.

Geology and mineral deposits of the Miners Gulch area, Granite County, Montana

“…This amount of tilting is consistent with attitudes of Pliocene deposits described by Reynolds (1979) in nearby valleys characterized by Pliocene-Pleistocene block faulting. Similar fluvial terrace deposits in western Montana, considered to be of Pliocene to early Quaternary age, were described by Pardee (1951), Alden, (1953, and Robinson, (1963).…”

Journal of Research of the U. S. Geological Survey, 1973, volume 1, issue 2

“…They were identified by Adolf Knopf in 1947, were described by Barksdale (1951), and were stratigraphically reassigned from the Adel Mountain Volcanics of Lyons (1944, p. 449 and 459) to the western facies of the Two Medicine Formation (R. G. Schmidt, 1963;written commun., 1968) Klepper and others (1957), Smedes (1966), Robinson (1963), Lyons (1944), and Viele and Harris (1965). Rocks near Wolf Creek belong to the Two Medicine Formation and are correlative in part with the Elkhorn Mountains Volcanics (Schmidt, 1963;written commun., 1968).…”

Geology and mineral deposits, east flank of the Elkhorn Mountains, Broadwater County, Montana

“…The following geologic maps ~and reports have ibeen pu 1 blished recently of the ~areas that surround the present mapped area: The Canyon Ferry .quadrangle Ito the north mapped by Mertie, Fischer, and Hobbs (1951), the Elkih.orn MountJains to the west mapped by Klepper, Weeks, and Ruppel (1957), Smedes (1966), and Kno.pf (1963), the Three Forks quad! 'langle to the south mapped lby Robinson (1963), ~and the Townsend VraHey to the east mapped by Freeman, Ruppel, ~and , Lorenz and McMurtrey (1956), and Nelson ( 1963) . The ore deposits of the nor.th pant of the Pa:rk mining district have recently Jbeen discussed by Schell ( 1963) , and those of the Winston mining district, by Earll (1964).…”

“…Because the two formations are similar, thin, and generally poorly exposed, they were mapped as a single unit. The unit is wholly or partly equivalent to the Dry Creek 'Shale of earlier literature (Deiss, 1936;Emmons and Calkins, 1913;Lochman-Balk, 1950;Sloss and Laird, 1947;Hanson, 1952, Robinson, 1963.…”