Magnetostratigraphic investigations of the lower paleozoic system boundaries, and associated paleogeographic implications
<p>Continued refinement of a global Geologic Timescale solely through increased precision of biostratigraphic correlations philosophically suffers from the inherent lack of a universal reference frame. Geomagnetic polarity reversals, which occur relatively rapidly and simultaneously on a gl...
| Summary: | <p>Continued refinement of a global Geologic Timescale solely through increased precision
of biostratigraphic correlations philosophically suffers from the inherent lack of a
universal reference frame. Geomagnetic polarity reversals, which occur relatively rapidly
and simultaneously on a global scale, can provide the necessary universal reference frame,
provided the polarity reversals are correlated within a well-defined biostratigraphic framework
and occur with a fairly distinctive pattern.</p>
<p>Magnetostratigraphic correlations across the Cambrian-Ordovician boundary interval
indicate that normal polarity zones correlative to Late Cambrian conodont zones occur
within sections from Texas, northern China, western Newfoundland, central Australia, and
possibly Kazakhstan. These correlations strongly suggest that temporal differences may
exist between sections in the absolute time value of key biostratigraphic horizons. There
may also be very brief normal polarity zones correlative with Early Ordovician conodont
and graptolite zonations, but those relationships have not yet been well-established.</p>
<p>Magnetostratigraphic correlations allow polarity to be unambiguously determined
for the relevant continental unit, even in the absence of previous paleomagnetic investigation.
Extension of this to Late Cambrian and Early Ordovician paleogeographic problems
indicate that North China, and probably also South China, underwent approximately 90°
counterclockwise rotation during the Cambrian, and were most likely attached to or very
near the present northern margin of Australia during that time.</p>
<p>Paleomagnetic results from Upper Silurian through Middle Devonian carbonates of
the Barrandian area, Czechoslovakia have at least three components of magnetization preserved
within them. Two of the components appear to pass the fold test, indicating that
they pre-date the deformation creating the basin, constrained to be not later than Late
Carboniferous. Differences between the two components probably correspond to different
times of acquisition, and may record rapid plate motion of the Bohemian Massif during the
Middle Paleozoic.</p>
<p>Paleomagnetic results from Upper Ordovician to Lower Silurian carbonates from
Anticosti Island, Quebec are not reliable because of the extremely weak magnetization of
these rocks. Sharp increases in intensity during thermal demagnetization experiments may
provide insight into the chemical changes which occur within carbonate rocks during thermal
demagnetization, but at the present time those phenomenon are not well understood.</p> |
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