A Palaeomagnetic Investigation of the Lundy Dyke Swarm

1957 ◽  
Vol 94 (3) ◽  
pp. 187-193 ◽  
Author(s):  
D. J. Blundell
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Lava Flows ◽  
Dyke Swarm ◽  
Upper Oligocene

AbstractThe directions of the natural remanent magnetization of samples collected from dykes on Lundy have been measured and related to those of Tertiary lava flows in Northern Britain. Evidence is given suggesting that the dykes are Tertiary and pre-Upper Oligocene in age, and that the geomagnetic field was reversed at the time of their intrusion.

IV. The natural remanent magnetization of certain stable rocks from Great Britain

1957 ◽  
Vol 250 (974) ◽  
pp. 111-129 ◽  
Keyword(s):  
Great Britain ◽  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Preceding Paper ◽  
Natural Remanent Magnetization ◽  
Red Sandstone ◽  
North West

Certain Permian lavas, Devonian, Cambrian and Pre-Cambrian sediments are shown to be permanently magnetized in directions different from that of the present geomagnetic field. All the Palaeozoic rocks examined possess southward natural remanent magnetizations. Whilst it is not suggested that the geomagnetic field did not reverse during the whole of the Palaeozoic, it is believed that the collection from the Lower and Upper Old Red Sandstone is sufficiently representative to make reversals during these times most improbable. The Wentnor Series of Pre-Cambrian rocks of the Longmynd shows reversal of magnetization about an axis which is not significantly different from that determined for the Torridonian series of north-west Scotland in a preceding paper. This is taken to support the view that they are roughly contemporaneous.

scholarly journals A Feasibility Study of Microbialites as Paleomagnetic Recorders

2021 ◽  
Vol 9 ◽  
Author(s):  
Ji Jung ◽  
Julie A. Bowles
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Inner Core ◽  
Salt Lake ◽  
Natural Remanent Magnetization ◽  
High Coercivity ◽  
Magnetic Carrier ◽  
Deep Time ◽  
Geologic Record ◽  
Magnetic Field Variations

Microbialites–layered, organosedimentary deposits–exist in the geologic record and extend back in deep time, including all estimated times of inner core nucleation. Microbialites may preserve magnetic field variations at high-resolution based on their estimated growth rates. Previous studies have shown that microbialites can have a stable magnetization. However, the timing and origin of microbialite magnetization were not well determined, and no study has attempted to evaluate whether actively growing microbialites record the geomagnetic field. Here, we present centimeter-scale magnetization and magnetic property variations within the structure of modern microbialites from Great Salt Lake (GSL), United States, and Laguna Bacalar, Mexico, Pleistocene microbialites from GSL, and a Cambrian microbialite from Mongolia. All samples record field directions close to the expected value. The dominant magnetic carrier has a coercivity of 35–50 mT and unblocking temperatures are consistent with magnetite. A small proportion of additional high coercivity minerals such as hematite are also present, but do not appear to appreciably contribute to the natural remanent magnetization (NRM). Magnetization is broadly consistent along microbialite layers, and directional variations correlate with the internal slope of the layers. These observations suggest that the documented NRM may be primarily detrital in origin and that the timing of magnetization acquisition can be close to that of sediment deposition.

MAGNETIC ORIENTATION OF BOREHOLE CORES

Geophysics ◽  
1969 ◽  
Vol 34 (5) ◽  
pp. 772-774 ◽  
Author(s):  
M. Fuller
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Standard Procedure ◽  
Natural Remanent Magnetization ◽  
Core Sample ◽  
The Core ◽  
Viscous Component ◽  
Late Tertiary ◽  
Geomagnetic Pole ◽  
Orientation Method

The use of natural remanent magnetization (NRM) to orient boreholes was reported at least thirty years ago (e.g., Lynton, 1938). The method depends upon determining the direction of remanent magnetization of the sample and relating it to the geomagnetic field in which the rock was presumed to have been magnetized. If the NRM faithfully records the relevant geomagnetic field and the field is known, the orientation of the core sample is available. Unfortunately, the ancient geomagnetic pole positions are not, in general, sufficiently well known to make this method particularly successful for rocks that are older than late Tertiary. Moreover, the presence of a weak, viscous component of magnetization parallel to the present geomagnetic field at a given site may produce erroneous results unless it is recognized and eliminated. However, the existence of this component provides another means of orienting the core sample. Isolation of this component might initially appear to be difficult, but it is actually a standard procedure of paleomagnetism; and, in fact, many studies have implicitly demonstrated that the direction of the present geomagnetic field at the sample site is recoverable (e.g., As and Zijderveld, 1958; Zijderveld, 1967). Indeed a number of people have recognized the possibility of using this component to orient borehole samples (e.g., Hargraves, 1969—private communication). The use of this soft viscous component has been advocated recently to distinguish between normal and reversely magnetized rocks in connection with tests of the sea floor spreading hypothesis (Irving and Roy, 1968). Nevertheless, no explicit demonstration of the technique of orienting borehole cores has been published. In the course of paleomagnetic surveys, our demagnetization studies have revealed a number of examples of behavior which makes the orientation method possible. This note describes such behavior and explains how the orientation might be recovered.

V. The remanent magnetization of unstable Keuper Marls

1957 ◽  
Vol 250 (974) ◽  
pp. 130-143 ◽  
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Laboratory Experiments ◽  
Natural Remanent Magnetization ◽  
Single Domain ◽  
Dipole Field ◽  
Main Field ◽  
Axial Dipole ◽  
Geocentric Axial Dipole ◽  
Three Components

Measurements of the directions and intensities of magnetization of Keuper Marls from Sidmouth are described. The natural remanent magnetization of these rocks is shown to be unstable in the geomagnetic field. Certain laboratory experiments are described which show the natural remanent magnetization to consist of three components, a primary component created on, or soon after, deposition, in the same direction as that of the natural remanent magnetization of Keuper Sandstones and Marls described by Clegg, Almond & Stubbs (1954); a secondary component in the direction of a geocentric axial dipole field in Britain acquired since the last reversal of the main field and a temporary component built up by the geomagnetic field between collection and measurement. The temporary and secondary components are believed to be isothermal remanent magnetizations and to be due to the red haematite cement. Application of Néel’s theory of the magnetization of small single-domain particles shows that haematite grains of less than 0·15 μ in diameter will be magnetically unstable. The temporary and secondary components of magnetization are explained in terms of Néel’s theory. A suggested test of stability is described.

Evidence for geomagnetic field excursions and secular variation from the Wrangell Volcanics of Alaska

1976 ◽  
Vol 13 (4) ◽  
pp. 547-554 ◽  
Author(s):  
D. K. Bingham ◽  
D. B. Stone
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Lava Flows ◽  
Dipole Field ◽  
Common Occurrence ◽  
Late Tertiary ◽  
Field Activity ◽  
Field Excursion ◽  
Geocentric Axial Dipole ◽  
Final Flow

Paleomagnetic studies have been made on 36 late Tertiary lava flows (3–4 m.y.) from the Wrangell Volcanics. Final flow mean remanent magnetization directions show excursions of the geomagnetic field away from a mean corresponding to a geocentric axial dipole field. They also point to the possibility that such excursions may have been a more common occurrence at the time of extrusion of these lavas than appears to have been the case in Quaternary times. These excursions may be due to increased non-dipole field activity. Calculation of the paleosecular variation including field excursion data leads to high values of PSV which do not agree with existing models. Exclusion of field excursion data gives a result that is consistent with current PSV models, but does not allow differentiation between them.

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