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C. S. Grommé

Publications and source records attributed to C. S. Grommé.

14 recordsLinked to original sources
USGS2012

Paleozoic paleomagnetism and northward drift of the Alexander terrane, southeastern Alaska

Paleozoic limestone, graywacke, sandstone, milestone, red beds and volcanic rocks of the Alexander terrane, southeastern Alaska, have yielded six paleomagnetic pole positions after thermal and alternating-field demagnetization. These poles are from sample groups of late Middle Ordovician, Late Ordovician, Devonian, Late Devonian, and early and late Carboniferous age. To test various tectonic models for the structural development of this part of western North America, the paleomagnetic results are compared to those for the North American craton. It is found that the observed inclination and declination values deviate significantly from the values predicted for the present-day position of the Alexander terrane (55.5N, 133.5W). Better matching can be obtained for a paleoposition of the terrane at about 40N, 120W, in the present position of western Nevada and northeastern California. In addition, an in situ 25° clockwise rotation of the terrane is required to restore it to its original position.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition: 3. Its regional significance

Study of the variations of direction and intensity of the geomagnetic field as recorded by the Miocene lava flows on Steens Mountain, southeastern Oregon, has resulted in a detailed description of total field behavior during a reversal in polarity. In addition to information about the polarity reversal itself, the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15.5 Ma. In order to test the feasibility of using this record as a means of correlation in this part of the western United States, comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the Steens Basalt or rocks of equivalent age. Despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes, convincing similarities among some of the records are evident. The Steens Basalt paleomagnetic record does, indeed, have potential as a correlation tool during this time of widespread basaltic volcanism. Additionally, paleomagnetic data from flows that were sampled in detail yield a middle Miocene paleomagnetic pole at 88.3°N, 209.0° (α 95 = 6.3°) for the High Lava Plains of Oregon. This pole position is statistically indistinguishable from the earth's rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted. Data from selected sites within the coeval part of the Columbia River Basalt Group yield a paleomagnetic pole at 88.7°N, 171.6°E (α 95 = 4.0°). The Columbia River Basalt Group pole is statistically indistinguishable from either the rotational axis or from the High Lava Plains pole. These findings indicate no post‐20 Ma differential rotation between south‐eastern Washington and south‐central Oregon, in contrast to previous interpretations.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Early Cretaceous paleolatitude of the Yukon-Koyukuk province, Alaska

We report results from a paleomagnetic study of the Yukon-Koyukuk province, a key region for reconstructing the Cretaceous paleogeography of northern Alaska and northeastern Asia. The province lies between the displaced continental fragment of Arctic Alaska and the accreted terranes of southern Alaska. Although Lower Cretaceous volcanogenic rocks of the Yukon-Koyukuk province preserve primary remanent magnetizations, the primary component is masked by a pervasive overprint. The over-printing magnetization, which has dual polarity, was acquired after a Late Cretaceous orogeny left the region highly deformed. By fitting planes to tilt-corrected thermal demagnetization data and solving for the best intersection of the corresponding remagnetization circles, we separated the primary component, presumably a vestige of the original thermoremanent magnetization, from the overprinting component of the natural remanence. The primary magnetization corresponds to a mean paleopole of Early Cretaceous age at 60.2°N, 159°E. The mean paleolatitude is 69°N with the 95% confidence ellipse ranging from 60° to 75°N. This analysis was successful because the tilt corrections of the sampled strata had sufficient variation to ensure a strong intersection of the remagnetization circles. We infer that Lower Cretaceous rocks of the Brooks Range and the Yukon-Koyukuk province were once separated by a wide ocean basin that was closed by subduction, probably during the Cretaceous. The northern limb of the Angayucham terrane, which lies between the Brooks Range and the Yukon-Koyukuk province, may contain remnants of the now-closed ocean basin.

Journal of Geophysical Research Solid Earth↗
USGS2002

The oligocene Lund Tuff, Great Basin, USA: A very large volume monotonous intermediate

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (> 1000 km3) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite-dacite-andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff - one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province - provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km3. It was emplaced 29.02 ?? 0.04 Ma in and around the coeval White Rock caldera which has an unextended north-south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase > quartz ??? hornblende > biotite > Fe-Ti oxides ??? sanidine > titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63-71 wt% SiO2) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere. ?? 2002 Elsevier Science B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS1985

How the geomagnetic field vector reverses polarity

A highly detailed record of both the direction and intensity of the Earth's magnetic field as it reverses has been obtained from a Miocene volcanic sequence. The transitional field is low in intensity and is typically non-axisymmetric. Geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur, suggesting that liquid velocity within the Earth's core increases during geomagnetic reversals. ?? 1985 Nature Publishing Group.

Nature↗
USGS1982

Limits to northward drift of the Paleocene Cantwell Formation, central Alaska

Volcanic rocks of the Paleocene Cantwell Formation in central Alaska apparently originated at a paleolatitude of 83°N (α 95 = 9.7°), as indicated by paleomagnetic results. When compared with the Paleocene pole for the North American craton, the 95% confidence limits of the results suggest that terranes north of the Denali fault have moved no more than 550 km northward relative to the North American craton since Paleocene time.

Geology↗
USGS1979

Geologic interpretation of aeromagnetic map of the Seward and Blying Sound quadrangles, Alaska

An aeromagnetic survey of the Seward and northern part of the Blying Sound quadrangles was flown by Geometrics, Inc., during 1975-77 to provide magnetic data to aid in an appraisal of the mineral resources. Background details of the regional geology and major structures are described in a companion report by Tysdal and Case (1979), and interpretation of the gravity data are described in a report by Case and others (1979). Preliminary reports of a paleomagnetic investigation have been reported by Hillhouse and Gromme (1977), and a more complete analysis of the results is in preparation. The aeromagnetic survey was flown at a nominal height of 300 m above the surface and flight lines were spaced about 1.7 km apart. Total magnetic field was measured by a proton-precession magnetometer. The International Geomagnetic Reference Field (IGRF), updated to 1976, was removed, and the residual values were computer-contoured to produce the aeromagnetic map (sheet 1). Detailed aeromagnetic maps at scale 1:63,360 and a map at scale 1:250,000 have been released as Open File Reports 78-1080 through 1083 (U.S. Geological Survey, 1978). Magnetic anomalies in the Seward-Blying Sound region are somewhat less complex than in other parts of south-central Alaska, primarily because of the huge volume of relatively nonmagnetic flysch that constitutes the Valdez and Orca Groups. Several rather isolated groups of mafic-ultramafic rocks are significantly more magnetic than the flysch assemblages, and cause distinctive patterns of magnetic anomalies. A few of the granitic plutons are magnetic, but most appear to have little or no magnetic expression.

Alaska↗
USGS1972

Paleomagnetic correlations and Potassium-Argon dating of Middle Tertiary ash-flow sheets in the eastern Great Basin, Nevada and Utah

Directions of natural remanent magnetization are used to identify and correlate individual cooling units in the middle Tertiary ash-flow province in central and eastern Nevada and western Utah. Potassium-argon dating indicates that the minimum time between eruptions of individual but genetically related ash-flow cooling units is on the order of 0.8 m.y. As this interval is long in comparison with the secular variation of the direction of the geomagnetic field, in a given volcanic province the direction of natural thermoremanent magnetization is a unique characteristic of each cooling unit. Ash-flow sheets investigated include the Stone Cabin Formation, the tuff of Pancake Summit, the Windous Butte Formation, the Needles Range Formation, the Bates Mountain Tuff, and the tuff of Clipper Gap, of Oliogcene to early Miocene age. The original areas of individual ash-flow cooling units are as great as 8,000 km 2 ; the volumes, 1,300 km 3 . The paleomagnetic correlations, made over distances up to 200 km, confirm most of the previously made lithologic correlations and allow more accurate delineation of single cooling units. These correlations are particularly useful in those parts of the Basin and Range province where the Tertiary stratigraphic section consists mainly of ash-flow sheets, and the outcrops are confined to mountain ranges that are separated by alluvium-filled valleys.

Nevada, Utah↗
USGS1971

Geomagnetic polarity epochs: age and duration of the olduvai normal polarity event

New data show that the Olduvai normal geomagnetic polarity event is represented in Olduvai Gorge, Tanzania, by rocks covering a time span of roughly from 0.1 to 0.2 my and is no older than 2.0 my. Hence the long normal polarity event of this age that is seen in deep-sea sediment cores and in magnetic profiles over oceanic ridges should be called the Olduvai event. The lava from which the Gilsàevent was defined may have been erupted during the Olduvai event and, if so, the term Gilsàshould now be abandoned. Many dated lavas that were originally assigned to the Olduvai event represent one or two much shorter normal polarity events that preceded the Olduvai event; these are herein named the Réunion normal polarity events. This revision brings the geomagnetic reversal time scale into conformity with the one implied by assumptions of uniform sedimentation rates on the ocean floor and uniform rates of sea-floor spreading.

Earth and Planetary Science Letters↗
USGS1971

Palaeomagnetism and potassium-argon ages of volcanic rocks of Ngorongoro caldera, Tanzania

Natural remanent magnetization (NRM) has been measured in 21 lava flows and 1 tuff in the south-west wall of Ngorongoro caldera, Tanzania. The lowest three lavas are normally magnetized, the next two have intermediate directions, and the remainder are reversed; potassium-argon dating places the reversal at 2.45 My, corresponding to the Gauss Matuyama epoch boundary. Alternating-field demagnetization was effective in removing the ubiquitous lightning-produced secondary magnetizations, but partial thermal demagnetization was not. Curie temperatures of the lavas fell into two groups: 250°–350°C and 570°–585°C, probably corresponding to unoxidized and oxidized titanomagnetites respectively. NRM directions in the lavas occurred in several distinct and stratigraphically continuous groups, suggesting intermittent eruption of groups of lavas. No correspondence was found between NRM groups and other parameters, such as Curie temperatures. The dispersion of mean NRM directions is large in comparison with late Quaternary lavas from low latitudes in the Pacific basin, but probably is not large relative to the proportion of the geomagnetic field in Africa that is non-dipolar. The mean palaeomagnetic pole is offset 10.5° from the geographic pole, but because of the large dispersion, the difference is not considered significant.

Ngorongoro caldera↗
USGS1967

Pliocene geomagnetic polarity epochs

A paleomagnetic and K-Ar dating study of 44 upper Miocene and Pliocene volcanic units from the western United States suggests that the frequency of reversals of the earth's magnetic field during Pliocene time may have been comparable with that of the last 3.6 m.y. Although the data are too limited to permit the formal naming of any new polarity epochs or events, four polarity transitions have been identified: the W10 R/N boundary at 3.7 ± 0.1m.y. , the A12 N/R boundary at 4.9 ± 0.1m.y. , the W32 N/R boundary at 9.0 ± 0.2m.y. , and the W36 R/N boundary at 10.8 ± 0.3 − 1.0m.y . The loss of absolute resolution of K-Ar dating in older rocks indicates that the use of well defined stratigraphic successions to identify and date polarity transitions will be important in the study of Pliocene and older reversals.

Earth and Planetary Science Letters↗