Developing the Cambrian and Ordovician Magnetic Polarity Time Scale: Current Data and Attempt of Synthesis

Abstract

Abstract—The time scale of the geomagnetic polarity is the quintessence of our knowledge about the changes in the polarity of the Earth’s magnetic field in the geological past. The pattern of the Earth’s magnetic field polarity reversals is relatively well known for the Cenozoic, Mesozoic, and Late Paleozoic. However, our understanding of the behavior of geomagnetic polarity in the more ancient epochs is non-systematic and fragmentary. Obtaining the reliable high-quality information for the Lower and Middle Paleozoic should become the next important step.This paper compiles the main data yielded by recent studies on magnetic stratigraphy of the Lower Paleozoic (Cambrian and Ordovician) of the Siberian Platform and other regions conducted by our team and our colleagues. The synthesis of these data allows us to propose a working model of the Early Paleozoic magnetic polarity time scale which embodies the current knowledge about the character of changes in the geomagnetic field polarity on this interval of the geological time. The process of geomagnetic field polarity reversals in the Early Paleozoic is described by the following most important characteristics: (a) the existence of a superchron of reversed polarity during a significant part of the Early and Middle Ordovician (the third Phanerozoic superchron Moyero); (b) a high (probably peaking in the Phanerozoic) frequency of the geomagnetic reversals in the Middle Cambrian; (c) a decrease in the frequency of geomagnetic reversals in Late Cambrian and Tremadocian with the approach of the superchron; (d) the presence of anomalous period in the behavior of the geomagnetic field close to the Phanerozoic/Precambrian boundary; and (e) the absence of a double superchron in the Ordovician (Algeo, 1996).