Magnetic Stratigraphy of the Upper Cambrian Key Section of the Chopko River, the Northwestern Siberian Platform, and New Constraints on Geomagnetic Reversal Frequency with Approaching the Moyero Superchron

Abstract

© 2020, Pleiades Publishing, Ltd. Abstract: Various geodynamo models proposed recently in the different ways explain the processes of geomagnetic field generation and its evolution over the course of the geological history. The testing capabilities of these models are strongly limited, inter alia, by the scarcity of the reliable magnetostratigraphic data for the Paleozoic and more ancient eras of geologic time, especially for the time periods before the establishment of the Kiama and Moyero superchrons. In this work, we present the results of the magnetostratigraphic and geochemical studies of the top part of the Upper Cambrian section in the Chopko river valley, Norilsk region, which is one of the most complete Upper Cambrian reference sections in the Siberian Platform. Our studies have shown that in the Late Cambrian there has been an interval of the reversed magnetic polarity which lasted at least 1.5 million years. Together with the previous data obtained on the Upper Cambrian of the Kulumbe river section and on the Early Ordovician of the Kotuy river (Pavlov and Gallet, 1998; Pavlov et al., 2017), our result presented in this study means that between the mid-Cambrian epoch of the extremely frequent reversals (Gallet et al., 2019) and the Ordovician Moyero superchron of the reversed polarity (Pavlov et al., 2005), there intervened at least two magnetic polarity intervals (reversed and normal) whose duration was at least one million years. This conclusion quite definitely points to a low frequency of the reversals on the eve of the Ordovician superchron and supports the concept according to which a process preparing the onset of the superchron takes place at the core/mantle boundary. The data obtained in our study support the hypothesis of the existence of three geodynamo regimes with sharp, on the order of a few million years, transitions between them (Gallet and Pavlov, 2016). At the same time, overall, the quality and amount of the currently existing magnetostratigraphic data for the pre-Mesozoic and, in particular, for the Paleozoic are still insufficient for confident testing this hypothesis.