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Abstract Inclination is the angle of a magnetization vector from horizontal. Clastic sedimentary rocks often experience inclination shallowing whereby syn‐ to post‐depositional processes result in flattened detrital remanent magnetizations relative to local geomagnetic field inclinations. The deviation of recorded inclinations from true values presents challenges for reconstructing paleolatitudes. A widespread approach for estimating flattening factors (f) compares the shape of an assemblage of magnetization vectors to that derived from a paleosecular variation model (the elongation/inclination [E/I] method). Few studies exist that compare the results of this statistical approach with empirically determined flattening factors and none in the Proterozoic Eon. In this study, we evaluate inclination shallowing within 1.1 billion‐year‐old, hematite‐bearing red beds of the Cut Face Creek Sandstone that is bounded by lava flows of known inclination. Taking this inclination from the volcanics as the expected direction, we found that detrital hematite remanence is flattened withwhereas the pigmentary hematite magnetization shares a common mean with the volcanics. Using the pigmentary hematite direction as the expected inclination results in. These flattening factors are consistent with those estimated through the E/I methodsupporting its application in deep time. However, all methods have significant uncertainty associated with determining the flattening factor. This uncertainty can be incorporated into paleomagnetic poles with the resulting ellipse approximated with a Kent distribution. Rather than seeking to find “the flattening factor,” or assuming a single value, the inherent uncertainty in flattening factors should be recognized and incorporated into paleomagnetic syntheses.
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Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments
SUMMARY Quasi-linear field-dependence of remanence provides the foundation for sedimentary relative palaeointensity studies that have been widely used to understand past geomagnetic field behaviour and to date sedimentary sequences. Flocculation models are often called upon to explain this field dependence and the lower palaeomagnetic recording efficiency of sediments. Several recent studies have demonstrated that magnetic-mineral inclusions embedded within larger non-magnetic host silicates are abundant in sedimentary records, and that they can potentially provide another simple explanation for the quasi-linear field dependence. In order to understand how magnetic inclusion-rich detrital particles acquire sedimentary remanence, we carried out depositional remanent magnetization (DRM) experiments on controlled magnetic inclusion-bearing silicate particles (10–50 μm in size) prepared from gabbro and mid-ocean ridge basalt samples. Deposition experiments confirm that the studied large silicate host particles with magnetic mineral inclusions can acquire a DRM with accurate recording of declination. We observe a silicate size-dependent inclination shallowing, whereby larger silicate grains exhibit less inclination shallowing. The studied sized silicate samples do not have distinct populations of spherical or platy particles, so the observed size-dependence inclination shallowing could be explained by a ‘rolling ball’ model whereby larger silicate particles rotate less after depositional settling. We also observe non-linear field-dependent DRM acquisition in Earth-like magnetic fields with DRM behaviour depending strongly on silicate particle size, which could be explained by variable magnetic moments and silicate sizes. Our results provide direct evidence for a potentially widespread mechanism that could contribute to the observed variable recording efficiency and inclination shallowing of sedimentary remanences.
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- Award ID(s):
- 1827263
- PAR ID:
- 10267269
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 224
- Issue:
- 3
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- 2001 to 2015
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/gji/ggaa559
