Magnetic Mineralogy and Paleomagnetic Record of the Nama Group, Namibia:
Implications for Large-Scale Remagnetization of West Gondwanaland and
Ediacaran Geomagnetic Instability
Abstract
The late Ediacaran to early Cambrian was marked by significant
biological and geochemical transformations, including the
diversification of animal life, and an enigmatic paleomagnetic record.
This study focuses on the Nama Group, a key geological unit for
understanding the Ediacaran-Cambrian transition, yet hampered by limited
paleogeographic constraints. Previous paleomagnetic studies identified
complex remagnetization patterns but lacked a detailed examination of
remanence carriers. To address this, we conducted field stability tests
and employed advanced rock magnetic techniques on unweathered borehole
core samples to complement paleomagnetic data. Thermal demagnetization
identified three magnetic components. C1 is a present-day field viscous
remanent magnetization (VRM) and was used for borehole core orientation.
C2, carried by single domain (SD) pyrrhotite and SD magnetite, aligns
with early Paleozoic remagnetization poles from West Gondwanaland and
was likely acquired through a thermoviscous magnetization (TVRM) in
magnetite and a thermoremanent magnetization (TRM) in pyrrhotite,
suggesting regional uplift and cooling between 500-480 Ma. This
quasi-synchronous remagnetization is supported by highly clustered
paleomagnetic poles and thermochronological data. C3 is an anomalous
component with significant inclination variation within the
stratigraphy, carried by large pseudo-single domain (PSD) magnetites,
possibly of primary, detrital origin. The pattern resembles findings
from other cratons, suggesting an unstable geomagnetic field that could
have persisted until ca. 540 Ma. This may represent one of the youngest
occurrences of such extreme geomagnetic instability in the Ediacaran and
provides the first evidence of this behavior in the Kalahari Craton,
adding new support for a largely unstable geomagnetic field during this
period.