Elkhorn Mountains Volcanics Research Articles

Influence of Late Cretaceous magmatism on the Sevier orogenic wedge, western Montana

Late Cretaceous large-volume pluton emplacement and accompanying volcanism within the evolving western Montana thrust wedge may have played important roles in determining the geometric and kinematic development of the thrust wedge, thereby influencing patterns of sediment dispersal and subsidence in the adjacent foreland basin. Intrusion of the Upper Cretaceous Boulder batholith and coeval eruption of the Elkhorn Mountains Volcanics were focused mainly at the trailing margin of the Lombard-Eldorado allochthon. The resultant thick (16–17 km) igneous culmination evolved over a relatively short interval (80–70 Ma), thickening the orogenic wedge to the point of supercritical taper and facilitating continued motion along the Lombard-Eldorado thrust system, thrust imbrication at the wedge toe, and forelandward translation of the Helena salient. Delivery of eroded volcaniclastic detritus from the thickened thrust wedge and accelerated basin subsidence due to thrust loading resulted in accumulation of a thick (4–5 km) sequence of Campanian-Maastrichtian volcaniclastic strata in the foreland basin (Livingston Group). The structural and sedimentological effects of this structural-magmatic culmination are similar to those of basement-cored culminations elsewhere in the Sevier orogen and to the Neogene central Andean orogenic wedge.

Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics

Abstract The taphonomy of a newly discovered petrified forest in the Two Medicine Formation, south of Choteau, Montana, provides new constraints on the pre-thrusting position of the Boulder batholith and Elkhorn Mountains Volcanics. The petrified forest consists of more than 200 charcoalified, subaligned trees encased in an ash flow tuff and an immediately overlying bentonite. The ash flow tuff has been dated at 80.002± 0.114Ma (39Ar/40Ar on plagioclase). The ash flow and bentonite are interpreted to record a single catastrophic eruptive event that toppled and entombed the forest. The mean azimuth of the prostrate trunks, which are interpreted as the direction of travel of the ash flow, is 037° (2σ = 35°). Based upon age, mineralogy, and the absence of other contemporaneous volcanic centers, it is concluded that the Late Cretaceous Elkhorn Mountains Volcanics were the most likely source of the tuff and bentonite that encase the trees. The mean azimuth of the tree trunks is consistent with palinspastic res...

Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics

Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics

The Elkhorn Mountains revisited: New data for the Late Cretaceous paleomagnetic field of North America

The Late Cretaceous portion of North America's apparent polar wander (APW) path is poorly known due to a dearth of suitable post‐85‐Ma reference poles. A paleomagnetic re‐investigation of Elkhorn Mountains Volcanics (80 Ma) was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of APW during this time interval correlates with known Late Cretaceous tectonic events. A total of 249 paleomagnetic samples (30 sites) were collected from three separate areas within the Elkhorn Mountains. Using both alternating field and thermal demagnetization, a characteristic direction of magnetization was isolate at 15 sites (a95 < 15°) which passes a fold test at the 95% confidence level. The mean pole calculated from site virtual geomagnetic poles is located at 80.3°N, 189.5°E (N = 15; A95 = 9.6°; k = 16.8). Interestingly, the Elkhorn Mountains pole is not concordant with the Cretaceous stillstand poles but is nearly coincident with the Paleocene reference pole and the recently published pole for the Adel Mountains dated at 76 Ma. These new data suggest the onset of APW which brought the pole from its Cretaceous position to near its early Tertiary position occurred prior to 80 Ma and the transition was exceedingly rapid. The onset of this rapid APW now appears to coincide more closely in time with many of the significant Late Cretaceous tectonic events, e.g., the Laramide orogeny. However, since the Elkhorn Mountains Volcanics are floored by a major east‐directed thrust and have undergone crustal scale deformation, the data should be viewed with caution until more results are available for this time interval.

Composition, radioisotopic ages, and potential significance of an altered volcanic ash (bentonite) from the Upper Cretaceous Judith River Formation, Dinosaur Provincial Park, southern Alberta, Canada

A detailed integrated stratigraphic, sedimentologic, petrographic, geochemical and radioisotopic study of a Judith River Formation bentonite (the Plateau Tuff [PT]) from Dinosaur Provincial Park, southeast Alberta, Canada was undertaken 1) to establish a precise radioisotopic age for the PT and associated vertebrate fossils, and determine the reproducibility of radioisotopic data from the deposit, 2) to elucidate the depositional and diagenetic history and possible provenance of the PT, and 3) to conduct a comparative analysis of the geochemical compositions of the PT and associated ashes to assess whether sufficient variation exists for individual bentonites to be identifiable on the basis of their geochemical ‘fingerprints’. The PT represents the diagenetically altered remains of a non-reworked, vitric-crystal ash deposited and preserved within a shallow, areally extensive floodplain lake. Two or more accumulation events extending over a brief time interval (hours to days?) resulted in its deposition. The original composition of the PT's parent magma was rhyodacitic to rhyolitic. Trace element discrimination indicates a volcanic-arc tectonic setting for its subaerial, Plinian eruptive center, which was most likely located within the Elkhorn Mountains Volcanics complex of southwestern Montana. Radioisotopic dating of biotite and sanidine phenocrysts was conducted using conventional bulk-fusion K-Ar (biotite) and laser-fusion 40Ar/39Ar (biotite and sanidine) methods. 40Ar/39Ar laser-fusion analysis of sanidine is considered more precise and yields a reference age of 76.11 ± 0.22 Ma for this ash. Although the 40Ar/39Ar biotite and conventional K-Ar biotite results are concordant to one another and to the 40Ar/39Ar sanidine results within ls analytical error, the biotites serve only as a check on the more reliable sanidine results. Geochemical ‘fingerprinting’ of the PT and related Judith River Formation bentonites within and adjacent to Dinosaur Provincial Park (using principal component and factor analysis of XRF data) shows that fresh PT samples and a sample of a possible laterally equivalent ash unit (CH3-83) are distinguishable from other stratigraphically discrete Dinosaur Provincial Park bentonites as well as from weathered and/or contaminated PT samples. Radioisotopic dates for the PT and other bentonites bracketing the fossiliferous exposures of the Judith River Formation in Dinosaur Provincial Park are up to four million years younger than those of ashes from portions of the formation in northern Montana. The PT is equivalent in age to a bentonite directly underlying a ‘new’ dinosaur egg and ‘baby’ locality in southwestern Alberta.

Tectonic origin of fractures for fissure vein emplacement in the Boulder Batholith and adjacent rocks, Montana

In the Boulder batholith and adjacent rocks, the dominant and most productive metalliferous fissure veins trend approximately east and dip steeply. Other steep veins, generally narrower, trend northwest, northeast, and rarely north. Fissure veins also occur locally in thrust faults in volcanic rocks along the west side of the batholith. Fractures along which these veins were emplaced were probably initiated by a regional compressional stress field during Laramide (Late Cretaceous-early Tertiary) time. The batholith was emplaced during the Late Cretaceous and is synorogenic with respect to the surrounding Laramide thrust and fold belt, which is characterized by east-west compression. Postorogenic rocks (Eocene and younger) do not host major fissure vein deposits.Fractures utilized by east-trending metalliferous veins are parallel to the axis of maximum compressional stress and thus appear to be extensional; this may be the reason these veins are widest and longest. Northeast- and northwest-trending veins were probably emplaced along conjugate shears; these veins are narrower than east-trending veins. Veins that trend north are less common and probably formed along release fractures perpendicular to the principal axis of compressional stress. These veins are narrower than many of those that trend east.Mechanical behavior of various rock units during deformation had an important effect on ore deposition. The longest, widest, and most productive fissure veins occur in competent, brittle rocks (Precambrian crystalline basement, batholith intrusions, and Elkhorn Mountains Volcanics). Ductile rocks (Mesozoic shales and Belt Supergroup argillites) do not commonly contain major fissure veins, except where these rocks have been contact metamorphosed and have become more brittle. Paleozoic strata, mainly carbonates, are of intermediate ductility and are hosts to fissure veins in only a few districts adjacent to the batholith.

A diatreme-hosted gold deposit at Montana Tunnels, Montana

A bulk-minable, gold-silver deposit at Montana Tunnels, southwestern Montana, is located in the central part of a diatreme. This diatreme was emplaced along the faulted contact between andesitic volcaniclastic rocks, assigned to the Late Cretaceous Elkhorn Mountains Volcanics, and a sequence of quartz latitic ignimbrites, which are part of the Lowland Creek Volcanics of middle Eocene age. A north-northeast-striking swarm of quartz latite porphyry dikes was emplaced during the waning stages of Lowland Creek volcanism. The two largest dikes, dated at 45 to 50 m.y. (middle Eocene), were intruded into the diatreme prior to the cessation of brecciation and mineralization.The diatreme underlies a 1-km 2 area and is known from drilling to extend steeply downward for at least 310 m. The principal rock type in the diatreme is a matrix-rich breccia. It is characterized by a sand-size tuffaceous matrix of quartz latitic composition containing fragments of contiguous volcanic wall rocks and intrusive rocks derived from the Late Cretaceous Boulder batholith. The batholith is inferred to underlie the Montana Tunnels area. Foundered blocks of volcanic wall rocks lie at various attitudes against the walls of the diatreme and within it. Those composed of Elkhorn Mountains Volcanics underwent hydraulic brecciation after they sank into the diatreme. Blocks interpreted to be composed of pyroclastic base surge deposits from a subaerial tuff ring and a piece of carbonized wood subsided into the diatreme from the palcosurface. The diatreme is separated into two unequal parts by a west-northwest-striking oblique slip fault.Sulfide minerals in the diatreme occur as disseminations in the breccia matrix and, in sub-ordinate amounts, as widely spaced, multidirectional veinlets and as the matrix to brecciated blocks of Elkhorn Mountains Volcanics. All forms of mineralization consist of pyrite, sphalerite, galena, minor chalcopyrite, and rare electrum accompanied by a gangue of manganocalcite, siderite, and minor quartz. Clastic grains and fragments of the sulfides and gangue minerals are common and attest to multiple alternating episodes of brecciation and mineralization. In the ore zone the introduction of abundant manganocalcite and siderite was accompanied by pervasive sericitic alteration but only weak kaolinization and silicification. Sericitization grades outward beyond the ore zone to chlorite-montmorillonite-carbonate alteration, an assemblage that also characterizes the interiors of the late mineral dikes in the diatreme. Gold occurs in electrum, with a fineness of about 550, as inclusions of less than 200 mu m in pyrite and sphalerite. Inclusions of electrum and galena were precipitated together. Silver is present mainly in solid solution in galena.The Montana Tunnels gold-silver deposit is characterized geochemically by anomalously high concentrations of zinc, lead, and manganese and, in comparison to many volcanic-hosted precious metal deposits, is low in arsenic, antimony, and mercury. This metal association and the low fineness of the gold are features shared by the Wau gold deposit in Papua New Guinea. It is suggested that the mineralized diatreme observable at Montana Tunnels was emplaced several hundred meters beneath the palcosurface, upon which a gold-bearing maar volcano, like that partly preserved at Wau, was constructed.

Hydrochemical Facies in a Tertiary Basin in the Milligan Canyon Area, Southwest Montana

ABSTRACTEquilibria studies of ground‐water chemistry in a synclinal basin at Three Forks, Montana indicate that most waters in the Tertiary sediments and Madison Group carbonates are calcium‐bicarbonate, or calcium‐sulfate waters which are supersaturated with respect to calcite. Calcium concentrations range from 13 to 426 mg/l, bicarbonate concentrations range from 112 to 328 mg/l and sulfate concentration varies between 29 to 114 mg/l. Sodium concentration is between 10 and 312 mg/l, producing sodium‐bicarbonate waters in some areas.Water‐level data were used to construct a potentiometric map that shows flow direction to the east. The water evolution along the flow path is from calcium‐bicarbonate in the recharge areas, to calcium‐sulfate in the center of the area, and finally sodium‐bicarbonate in the discharge area. This evolutionary sequence can be explained by calcite, dolomite and gypsum dissolution, ion exchange, sulfate reduction and dilution by recharging waters from the Elkhorn Mountains Volcanics.

Composition and Time Relations of Plutonic and Associated Volcanic Rocks, Boulder Batholith Region, Montana

Research Article| December 01, 1974 Composition and Time Relations of Plutonic and Associated Volcanic Rocks, Boulder Batholith Region, Montana ROBERT I. TILLING ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1974) 85 (12): 1925–1930. https://doi.org/10.1130/0016-7606(1974)85<1925:CATROP>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT I. TILLING; Composition and Time Relations of Plutonic and Associated Volcanic Rocks, Boulder Batholith Region, Montana. GSA Bulletin 1974;; 85 (12): 1925–1930. doi: https://doi.org/10.1130/0016-7606(1974)85<1925:CATROP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Comparison of areally weighted bulk compositions for the Boulder batholith and prebatholith volcanic rocks (Elkhorn Mountains Volcanics) shows a close match in terms of K2O-Na2O-CaO-SiO2 variations. Detailed examination of available chemical data suggests, however, that the constituent units of the volcanic rocks differ among themselves, as well as from many of the batholith units. Most sampled volcanic rocks are chemically and isotopically similar to the volumetrically minor mafic members of the putonic main series of Tilling (1973). The sodic-series plutonic rocks, which form about 15 percent of the exposed batholith, apparently have no compositionally similar counterparts in the Elkhorn Mountains Volcanics in terms of bulk composition.The Butte Quartz Monzonite, which makes up about three-quarters of the batholith in terms of individual samples and bulk composition, is chemically distinct from most volcanic rocks. This relation suggests that only small amounts of magma of Butte Quartz Monzonite composition erupted onto the surface. The voluminous silicic ash flows of the middle member of the volcanic rocks were probably derived from the same magma that mostly crystallized subsurface to form the Butte Quartz Monzonite. Field, chemical, K-Ar age, and paleomagnetic evidence, however, strongly suggest that a time gap separated the extrusion of the middle-member volcanic rocks from the intrusion of the Butte Quartz Monzonite and related younger silicic variants.Compositional and time relations between the plutonic and genetically associated volcanic rocks are generally compatible with the concept that the volcanic rocks are eruptive equivalents of a shallow evolving batholith but seem incompatible with some specific aspects of the “extrusive complex” or “floored sheet” hypothesis as applied to the Boulder batholith by Hamilton and Myers (1967, 1974). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Boulder Batholith, Montana: A Product of Two Contemporaneous but Chemically Distinct Magma Series

Research Article| December 01, 1973 Boulder Batholith, Montana: A Product of Two Contemporaneous but Chemically Distinct Magma Series ROBERT I. TILLING ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (12): 3879–3900. https://doi.org/10.1130/0016-7606(1973)84<3879:BBMAPO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT I. TILLING; Boulder Batholith, Montana: A Product of Two Contemporaneous but Chemically Distinct Magma Series. GSA Bulletin 1973;; 84 (12): 3879–3900. doi: https://doi.org/10.1130/0016-7606(1973)84<3879:BBMAPO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Rocks of the Late Cretaceous composite Boulder batholith, though successively emplaced in a relatively small segment of the Earth's crust within a very brief time span (78 to 68 m.y.), can be grouped chemically into two magma series: (1) the main series, defined principally by plutons in the central and northern parts of the batholith; and (2) the sodic series, defined mostly by plutons in the southern part. For any given SiO2 content, the rocks of the main series tend to be higher in K2O and lower in Na2O than rocks of the sodic series. The chemical distinction between the two series proposed is also expressed by variation patterns for U, Th, Rb, and Sr abundances, by lead isotope compositions, but not by strontium isotope compositions.The prebatholith Elkhorn Mountains Volcanics (Late Cretaceous), especially the mafic members, are chemically and isotopically similar to the rocks of the main series, confirming geologic evidence of the genetic association between them. The postbatholith Lowland Creek Volcanics (early Eocene), though chemically more closely related to the sodic series, isotopically are more akin to, but slightly more radiogenic than, the main series. Post–Lowland Creek volcanic rocks (Miocene or Pliocene) are compositionally similar to the sodic series rocks. Spatial distribution of the batholith and the volcanic rocks exhibits a very crude chemical zonation of the region: for a given silica content, relatively more potassic rocks (main series and prebatholith volcanic rocks) tend to occur mainly in the north and east, whereas relatively more sodic rocks (sodic series and postbatholith volcanic rocks) predominate in the south and west.Available field, chemical, and isotopic evidence collectively suggests that the observed compositional variations for the Boulder batholith are most reasonably interpreted in terms of a model involving two magma series derived from two or more magma sources within the lower crust or upper mantle. These source regions are interred to vary chemically and isotopically, either laterally or vertically; in view of the rather small areal extent of the Boulder batholith, however, a vertically zoned source region is more probable. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleomagnetism of Upper Cretaceous volcanic rocks of southwestern Montana

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 flat-lying volcanic rocks of probable Tertiary age. The virtual geomagnetic pole calculated for the Elkhorn Mountains Volcanics (69°N, 171°W) 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.

Intertongued Upper Cretaceous Volcanic and Nonvolcanic Rocks, Central-Western Montana

A thick sequence of intertongued volcanic and nonvolcanic rocks, exposed on the northeast limb of a syncline immediately west of Garrison, central-western Montana, is a western facies of the Elkhorn Mountains Volcanics of Late Cretaceous age. This sequence, here named the Golden Spike Formation, ranges from 4000 to 8000 feet in thickness. In the northwestern sixth of the 9-mile-long outcrop belt, nonvolcanic rocks predominate. Nonvolcanic detritus which forms this relatively thin fluviatile sequence was derived from tectonic uplifts a short distance to the west or northwest of Garrison. In the southeastern area, a relatively thick sequence comprises mostly lavas and volcaniclastic rocks. Most volcaniclastic rocks were probably deposited by epigene processes ( e.g. , weathering, erosion, and redeposition by mudflows, landslides, or running water) and were derived from centers of Elkhorn volcanism to the east. This depositional pattern resulted in intimate interfingering of volcanic and nonvolcanic beds; the authors observed only limited mixing of these two components in individual beds. Throughout the formation many volcanic tongues wedge out within a short distance to the northwest. The virtual absence of volcanic debris in the fluviatile rocks deposited to the northwest indicates either that explosive volcanic activity was minimal in the Elkhorn volcanic field during deposition or that prevailing westerly winds kept pyroclastic material out of the Golden Spike area.

Deduction of flow regime from bedding character in conglomerates and sandstones

Deduction of flow regime from bedding character in conglomerates and sandstones