Jimmy F. Diehl

Paleomagnetic results from the Late Carboniferous/Early Permian Casper Formation: implications for northern Appalachian tectonics

Abstract Paleomagnetic samples were collected from 190 m of the Late Carboniferous/Early Permian Casper Formation in southeastern Wyoming. A total of 549 samples was drilled near the vicinity of Horse Creek Station at an average stratigraphic interval of 33 cm. All samples were reversely magnetized. Rock magnetic analyses indicate that the primary carrier of remanence in the formation is hematite. A selection criterion applied to the partial demagnetized data restricted the sample population to 233, resulting in a paleomagnetic North Pole located at 47.4°N, 127.4°E ( δ p =0.7 ; δ m =1.4 ). The Casper pole agrees well with other Late Carboniferous/Early Permian poles for cratonic North America. The tight clustering of these paleomagnetic poles suggests that little apparent polar motion with respect to North America occurred during this time. Comparing the stable North American poles with paleomagnetic poles from Late Carboniferous/Early Permian strata of the New England-Canadian Maritime region (Acadia) indicates that this region did not reach its present position relative to North America until at least the Early Permian.

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

The Late Cretaceous portion of North Americas 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.

Further paleomagnetic results for the San Juan volcanic field of southern Colorado

Abstract Combining paleomagnetic data for 17 new sites from the northwest portion of the (Oligocene) San Juan volcanic field of southern Colorado with data for 29 sites previously published yields a paleomagnetic pole at 85°N, 114°E (with a 95% confidence circle of 7.5° radius). A further combination of the San Juan data with the results of other studies on rocks of Oligocene age from tectonically stable parts of North America gives a mid-Tertiary reference pole located at 81°N, 132.5°E, with a confidence circle of approximately 4°. Mid-Tertiary paleomagnetic poles for the western edge of the continent diverge markedly from this reference pole.

40Ar/39Ar and paleomagnetic constraints on the evolution of Volcán de Santa María, Guatemala

40 Ar/ 39 Ar dating of 15 lava flows indicates that Volcan de Santa Maria grew episodically to 8 km 3 in size at an average rate of 0.12 km 3 /ka between 103 and 35 ka. The composite cone grew in four phases, including two periods of intense activity at ca. 72 ka and ca. 35 ka, during which 1.5 km 3 and 3 km 3 of basaltic to andesitic magmas were erupted. There is no evidence of further volcanism after ca. 35 ka until the great dacitic eruption in 1902. The average eruptive rate is 0.16 km 3 /ka, if products of the 1902 eruption and subsequent Santiaguito dome are included. Whereas the Mono Lake excursion is not clearly recorded at Volcan de Santa Maria, as had been inferred from earlier studies, virtual geomagnetic poles (VGPs) of the 35 ka cone-forming lavas exhibit high-amplitude paleosecular variation that may correspond in time to the Mono Lake excursion. Two older packages of lava flows are each associated with a distinctive cluster of VGPs, which supports the 40 Ar/ 39 Ar age model and the conclusion that cone building was episodic. During the final 60% of cone growth, lavas evolved from basaltic to andesitic (51%–57% SiO 2 ) with time, but with a regression to slightly less evolved compositions during the onset of the final cone-building phase. Despite the relatively small volume of Santa Maria, cone-growth processes and geochemical evolution through time mirror observations at other currently active volcanoes along the Central American volcanic arc, and may prove useful as an analogy in assessing long-term hazards posed by other predominantly basaltic-andesitic composite volcanoes.

Pre-Wisconsinan glacial stratigraphy, chronology, and paleomagnetics of west-central Wisconsin

Detailed evaluation of pedogenic, geologic, and paleomagnetic properties has enabled the identification of several pre-Wisconsinan stratigraphic units in west-central Wisconsin. The older unit is the Pierce Formation, consisting of the Hersey Member, which contains gray, calcareous, loam-textured till, and the Kinnickinnic Member, which contains rhythmically bedded lacustrine sediments deposited in a complex network of proglacial lakes. Paleomagnetic and pedogenic data from the Pierce Formation suggest that the Hersey Member was deposited during either the Emperor reversed event or the Matuyama reversed polarity epoch and that deposition of the Kinnickinnic Member spanned either the Emperor-Brunhes or the Matuyama-Brunhes boundary ∼460,000 or 730,000 B.P., during Pre-Illinoian time. The younger, pre-Wisconsinan unit is the River Falls Formation, which contains a reddish-brown, sandy-clay-loam-textured till and associated sand and gravel. Soil-profile development and regional correlation indicate that this unit is Illinoian in age. These results contradict previously published studies which suggested that tills of the Hersey Member and the River Falls Formation were deposited by the same Wisconsinan ice advance and that no pre-Wisconsinan sediment is present in western Wisconsin.

Paleomagnetic constraints on eruption patterns at the Pacaya composite volcano, Guatemala

Pacaya volcano is an active composite volcano located in the volcanic highlands of Guatemala about 40 km south of Guatemala City. Volcanism at Pacaya alternates between Strombolian and Vulcanian, and during the past five years there has been a marked increase in the violence of eruptions. The volcano is composed principally of basalt flows interbedded with thin scoria fall units, several pyroclastic surge beds, and at least one welded tuff. Between 400 and 2000 years BP the W-SW sector of the volcano collapsed producing a horseshoeshaped amphitheater (0.65 km3) and providing a window into the cones infrastructure. Lava flows and tephra exposed in the amphitheater are more then 200 m thick and when combined with flows erupted recently represent between 30 and 40% of the cones history. Pacaya is ideally suited for a paleomagnetic study into the timing and duration of eruption episodes at a large, composite volcano. We drilled 27 paleomagnetic sites (25 aa flows, 1 dike, and 1 welded tuff) from four lava-flow sequences with between 4 and 14 sites per sequence. The four sequences represent initial through historic activity at Pacaya. We resolved, what appear to be, 22 time-independent paleomagnetic sites by averaging together directions from successive sites where the sitemean directions were indistinguishable at the 95% level of confidence. However, mean-sequence directions of individual lava-flow sequences yielded unusually high Fisher precision parameters (k=44–224) and small circles of 63% confidence (a63=1.6–6.1°) suggesting as few as three or four time-independent sites were collected. This indicates that activity as Pacaya is strongly episodic and that episodes are characterized by voluminous outpouring of lavas. Modelling the data using Holocene PSV rates confirms this and shows that differences in within-sequence directions (6–11.5°) are consistent with emplacement of lava-flow sequences in less than 100 years to as many as 300 years. Relatively larger differences in directions (18–23°) between subjacent lava-flow sequences indicates that repose is at least 300–500 years and could be even longer.

Absolute geomagnetic paleointensity as recorded by ∼1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan)

Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ∼1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ∼375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×1022 Am2. The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.

Paleomagnetism of the ~1.1 Ga Coldwell Complex (Ontario, Canada): Implications for Proterozoic geomagnetic field morphology and plate velocities

We report new paleomagnetic data from the ~1108 Ma intrusive Coldwell Complex (Ontario, Canada) to investigate the apparent reversal asymmetry observed in some Midcontinent Rift (MCR) rocks. The rocks of eastern and central part of the complex are reversely magnetized with a group mean direction of D = 114.8°, I = −63.7° (α95 = 3.6°, N = 30). The corresponding paleomagnetic pole at Plat = 47.2°N, Plong = 206.5°E (A95 = 4.8°) is located close to the paleomagnetic poles from nearly coeval reversely magnetized rocks of the MCR system, including the lower lava flows at Mamainse Point. The rocks of western part of the complex are normally magnetized with a group mean direction (D = 298.0°, I = 56.9°, α95 = 5.8°, N = 10) that passes the reversal test with respect to the reversed group mean direction. Our results do not support the previous model in which the complex was emplaced during two periods of reversed geomagnetic field polarity separated by a period of normal polarity and hence encompasses two geomagnetic reversals. Instead, our new data indicate that the Coldwell Complex records only two polarity intervals separated by a symmetrical reversal at ~1102–1105 Ma. This reversal is likely equivalent to the lowermost reversal recorded at Mamainse Point and provides further evidence that the apparent reversal asymmetry reflects a plate motion rather than a persistent nondipole field geometry. Together with a high-quality data from the ~1098 Ma North Shore Volcanics, our data indicate a rapid velocity of Laurentia at ~25 ± 4 cm/yr. The fast plate motion may reflect a decreased mantle drag due to vigorous mantle indicated by widespread intraplate magmatism at ~1.1 Ga.

Preliminary study on the mineral magnetic properties of sediments from Kůlna Cave (Moravian Karst), Czech Republic

SummaryMagnetic property variations in marine, lacustrine and loess-paleosol sequences have proved to be useful proxies in climate change studies. However in order to correctly interpret the record of the magnetic property variations it is absolutely necessary to have a good understanding of the cause of the observed variations. Most of the ambiguity in loess-paleosol studies is in distinguishing the role of pedogenesis from other climatic factors. Studying the mineral magnetic properties of the protected cave sediments which have not undergone pedogenesis allows us to determine the degree to which detrital input is climatically driven. These results will help us better understand the variations observed in the surficial loess-paleosol sequences.This study reports mineral magnetic data collected from entrance facies sediments deposited during the early Wurmian glacial stage in the Kůlna Cave. The entrance facies sediments consist of loess-like silts with varying amount of talus. The magnetic susceptibility record from these sediments shows higher values in layers originating in colder climates which is different to that commonly observed in surficial loess deposits. Higher values of magnetic susceptibility in Kůlna sediments are probably due to higher concentrations of ferromagnetic minerals (magnetite and maghemite) and due to an increased proportion of superparamagnetic grains. The magnetic mineralogy and the grainsize distribution (grains larger than superparamagnetic) appear not to change throughout the studied profiles. Higher magnetic susceptibility accompanied by an increase in the superparamagnetic fraction observed in the sediments deposited during colder periods can be explained by an increased input from a pedogenic source when the vegetation cover was reduced and the erosion rate increased.