Geology of the southern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana

Klepper, M.R.; Weeks, R.A.; Ruppel, E.T.

doi:10.3133/pp292

Cited by 28 publications

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“…Here, Klepper et al (1957) interpreted that deposition of the Elkhorn Mountains volcanics fi lled a broad, deep, erosional valley that coincided with the axis of a syncline, with the axis of the syncline most likely controlling stream and paleovalley location. They further concluded that, subsequent to the intrusion of the Boulder batholith, the Elkhorn Mountains were subject to continuous erosion from Late Cretaceous to mid-Eocene time, that this erosion generated a mountainous, high-relief terrain that had the same basic confi guration as today, and that Paleogene and Neogene sediments later accumulated within this terrain.…”

Section: Intermontane Fluvial Incision In Basin Generationmentioning

confidence: 95%

Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

Schwartz

2013

Geological Society of America Bulletin

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The Paleogene Renova Formation is the earliest record of postcompressional sedimentation within and adjacent to the Helena Salient of the Cordilleran fold-and-thrust belt in southwestern Montana. Paleocurrent and compositional data from basin-margin facies document radiating paleodispersal away from high-relief (≥2 km) highlands coincident with modern mountainous areas. Source rocks within the paleohighlands included the same Archean metamorphic; Proterozoic, Paleozoic, and Mesozoic sedimentary; and Mesozoic plutonic and volcanic rocks as exposed in modern uplifts. Paleocurrent and compositional data from trunk fl uvial conglomerates and sandstones document the existence of an interbasinal drainage system that connected the Three Forks, western Gallatin, and Townsend Basins with headwaters farther to the west and southwest near the present-day Montana-Idaho border. Overall, the distribution of Paleogene mountainous areas and basins closely resembled modern geography, and the Paleogene drainage network was strikingly similar to the modern Missouri River headwater system. The Renova Formation records the early stages of decay of the Cordilleran orogenic belt, including the evolution of a complex intermontane basin network in southwestern Montana. High-energy Late Cretaceous to early Eocene fl uvial systems carved deep, large-scale paleovalleys into the orogenic wedge along zones of structural and stratigraphic weakness. At least a 5 km thickness of overburden was removed during this time. Incision was temporally correlative with early Cenozoic regional uplift and subtropical climatic conditions. Subsequent deposition of the Renova Formation was temporally correlative with the cessation of uplift, the initiation of crustal extension, and climatic cooling. However, extension is not interpreted to have played a major role in earliest basin development.

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Section: Intermontane Fluvial Incision In Basin Generationmentioning

confidence: 95%

Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

Schwartz

2013

Geological Society of America Bulletin

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“…The morainal forms are better preserved in these valleys, and the till appears slightly less weathered. The glacial deposits in the quadrangle are coextensive with those in the Basin quadrangle to the west, where recognized only one period of glaciation; he considered the single period to be almost certainly contemporaneous with the "Early" glacial period in th~ Elkhorn Mountains, which was possibly early Wisconsin in age (Klepper, Weeks, and Ruppel, 1957).…”

Section: Pleistocene Featuresmentioning

confidence: 99%

“…These older rocks are the welded tuffs and well-bedded sedimentary tuffs that constitute the upper two units of the Elkhorn Mountains . volcanics of Late Cretaceous age (Klepper, Weeks, and Ruppel, 1957). Both units are cut by dikes of rhyodacite porphyry.…”

Section: Rock Unitsmentioning

confidence: 99%

“…The oldest rocks exposed in the Jefferson City quadrangle are the Elkhorn Mountains volcanics, into which the Boulder batholith was intruded. East and southeast of the quadrangle, the Elkhorn Mountains volcan-• ics have been divided into three thick stratigraphic units (Klepper, Weeks and Ruppel, 1957): a lower unit-consisting predominantly of tuff, lapilli tuff, and tuff breccia; a middle unit of welded tuff and a few beds of nonwelded tuff; and an upper unit of well-bedded sedimentary tuff. In the Jefferson City quadrangle, only the middle and upper units are exposed.…”

Section: Elkhorn Mountains Volcanicsmentioning

confidence: 99%

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Geology and mineral deposits of the Jefferson City quadrangle, Jefferson and Lewis and Clark Counties, Montana

Becraft¹,

Picnkney²,

Rosenblum³

1963

Professional Paper

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Rock type m Ia_ ____________ • ___________ _ 16 Quartz monzonite adjacent to quartz veins_ Rock type md _________________________ _ 17 Mineralogical changes ______________ _ Rock type mda ________________________ _ 17 Chemical changes __________________ _ Rock type fl __________________________ _ 17 Alaskite adjacent to quartz veins ________ _ Rock type fla _________________________ _ 17 Granodiorite adjacent to chalcedony veins_ Rock type fib _________________________ _ 18 Elkhorn Mountains volcanics adjacent to Rock type fie _________________________ _ 18 quartz veins _________________________ _ Rock type fd ____. . : _____________________ _ 19 Comparison with Butte.district __________ _ Rock type fda _________________________ _

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“…The for tilt was applied to mean NRM directions outcrops are located in the southern Elkhorn associated with inclined volcanic strata. Mountains described by Klepper et al [1957], Because of the difficulty of determining exthe Townsend Valley described by Freeman et act orientations of fold axes in the field, all al. [1958], the Whitetail Creek Valley, Bone tilt corrections were applied by rotation about Bas;.n, the South Boulder Valley, and the Jeff-the present geologic strike; that is, a horizontal erson Valley near Lewis and Clark Cavern fold axis is assumed.…”

mentioning

confidence: 99%

Paleomagnetism of Upper Cretaceous volcanic rocks of southwestern Montana

Hanna¹

1967

J. Geophys. Res.

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Paleomagnetic studies of folded and flat-lying volcanic rocks in southwestern Montana indicate that most of the natural remanent magnetization (NRM) is primary and that secondary magnetization, where found, has been produced mainly by lightning. NRM directions of most rock units are grouped in either a normal or reversed sense after partial demagnetization, although directions of a few units of probable mudflow origin are highly dispersed. No evidence of self-reversed magnetization has been found; consequently, a field polarity sequence has been tentatively established in connection with Late Cretaceous and early Tertiary orogenic events. All folded rocks investigated belong to the Upper Cretaceous Elkhorn Mountains Volcanics; stratigraphic correlation between widely separated, structurally dissimilar outcrops of petrologically similar rocks is supported by NRM directions and polarities. Mean NRM directions of structurally restored, age-equivalent sites do not differ significantly from one another, which suggests that fold axes are nearly horizontal; however, they differ significantly from directions of nearby fiat-lying volcanic rocks of probable Tertiary age. The virtual geomagnetic pole calculated for the Elkhorn Mountains Volcanics (69øN, 171øW) agrees generally with Late Cretaceous poles previously calculated by others for North America, Madagascar, and the U.S.S.R., but it differs significantly from Early Cretaceous poles of Japan and Australia. •[NTRODUCTION The remanent magnetization of Upper Cretaceous volcanic rocks in southwestern Montana was studied as an aid to interpreting the geologic history. Rocks suitable for paleomagnetic investigations were sampled from the foothills of Bull Mountain and other localities within a 12,500-km •' area that is centered near 46øN, 112øW (Figure 1). Paleomagnetism of materials from Bull Mountain is of particular significance for two reasons--(1) the geologic age of the rocks there has been established and (2) the structural relations in the area are well known.

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Geology of the southern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana

Cited by 28 publications

References 0 publications

Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

Geology and mineral deposits of the Jefferson City quadrangle, Jefferson and Lewis and Clark Counties, Montana

Paleomagnetism of Upper Cretaceous volcanic rocks of southwestern Montana

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