Continued refinement of a global Geologic Timescale solely through increased precisionof biostratigraphic correlations philosophically suffers from the inherent lack of auniversal reference frame. Geomagnetic polarity reversals, which occur relatively rapidlyand simultaneously on a global scale, can provide the necessary universal reference frame,provided the polarity reversals are correlated within a well-defined biostratigraphic frameworkand occur with a fairly distinctive pattern.
Magnetostratigraphic correlations across the Cambrian-Ordovician boundary intervalindicate that normal polarity zones correlative to Late Cambrian conodont zones occurwithin sections from Texas, northern China, western Newfoundland, central Australia, andpossibly Kazakhstan. These correlations strongly suggest that temporal differences mayexist between sections in the absolute time value of key biostratigraphic horizons. Theremay also be very brief normal polarity zones correlative with Early Ordovician conodontand graptolite zonations, but those relationships have not yet been well-established.
Magnetostratigraphic correlations allow polarity to be unambiguously determinedfor the relevant continental unit, even in the absence of previous paleomagnetic investigation.Extension of this to Late Cambrian and Early Ordovician paleogeographic problemsindicate that North China, and probably also South China, underwent approximately 90°counterclockwise rotation during the Cambrian, and were most likely attached to or verynear the present northern margin of Australia during that time.
Paleomagnetic results from Upper Silurian through Middle Devonian carbonates ofthe Barrandian area, Czechoslovakia have at least three components of magnetization preservedwithin them. Two of the components appear to pass the fold test, indicating thatthey pre-date the deformation creating the basin, constrained to be not later than LateCarboniferous. Differences between the two components probably correspond to differenttimes of acquisition, and may record rapid plate motion of the Bohemian Massif during theMiddle Paleozoic.
Paleomagnetic results from Upper Ordovician to Lower Silurian carbonates fromAnticosti Island, Quebec are not reliable because of the extremely weak magnetization ofthese rocks. Sharp increases in intensity during thermal demagnetization experiments mayprovide insight into the chemical changes which occur within carbonate rocks during thermaldemagnetization, but at the present time those phenomenon are not well understood.
