EarthRef.org Reference Database (ERR) -- Algeo 1996

Phanerozoic geomagnetic polarity bias patterns have been reconstructed using polarity data from 278 stratigraphic formations of Cambrian-Jurassic age combined with data from an established geomagnetic polarity timescale for the Cretaceous-Recent. In addition to the well-known Cretaceous Normal Polarity Superchron and Kiaman Reversed Polarity Superchron, other first-order polarity features are recognized: (1) a Middle Cambrian-Middle Ordovician Burskan Reversed Polarity Bias Interval, (2) a Late Ordovician-Late Silurian Nepan Normal Polarity Bias Interval, (3) an Early Jurassic Normal Polarity Bias Interval, and, possibly, (4) a Middle Jurassic Normal Polarity Bias Interval. A combination of strong polarity bias and low reversal rates during the Ordovician may indicate the existence of a ''dual-polarity superchron'' containing a single major polarity transition, the Middle Ordovician Polarity Shift. Reconstruction of an accurate Phanerozoic polarity trend permits application of a ''polarity bias test'' to evaluate the primary character of magnetic remanences. A polarity bias test of British Siluro-Devonian remanences reveals that group ''A'' remanences (0¿--20¿ paleopole latitude; 30--100% normal polarity) exhibit polarity concordance with coeval non-British remanences, whereas group ''B'' remanences (25¿--50¿ paleopole latitude; 0--20% normal polarity) are strongly discordant, suggesting that the latter are largely of secondary origin. Analysis of groups of magnetic remanences also permits estimation of (1) characteristic timescales for formation polarity data and (2) evaluation of sources of age-dependent polarity-ratio variance. For the Cambrian-Jurassic polarity data set, formations exhibit a mean characteristic timescale of 1.0--1.5 m.y., and circa 50% of polarity-ratio variance is attributable to paleomagnetic sampling of a binomial variable (i.e., geomagnetic field polarity) and 50% to other factors (i.e., stochastic and systematic depositional and sampling biases, incorrect age estimates for stratigraphic formations and characteristic remanences, complex magnetizations, and low epochal reversal frequencies). ¿ American Geophysical Union 1996 Phanerozoic geomagnetic polarity bias patterns have been reconstructed using polarity data from 278 stratigraphic formations of Cambrian-Jurassic age combined with data from an established geomagnetic polarity timescale for the Cretaceous-Recent. In addition to the well-known Cretaceous Normal Polarity Superchron and Kiaman Reversed Polarity Superchron, other first-order polarity features are recognized: (1) a Middle Cambrian-Middle Ordovician Burskan Reversed Polarity Bias Interval, (2) a Late Ordovician-Late Silurian Nepan Normal Polarity Bias Interval, (3) an Early Jurassic Normal Polarity Bias Interval, and, possibly, (4) a Middle Jurassic Normal Polarity Bias Interval. A combination of strong polari