8915500tostratigraphic investigations of the Lower Paleozoic system boundaries, and associated paleogeographic implications.

摘要

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.;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;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.;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.;Magnetostratigraphic correlations across the Cambrian-Ordovician boundary interval indicate the 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.