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The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian–lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform

ARTEM KOUCHINSKY^*,, STEFAN BENGTSON^*, YVES GALLET, IGOR KOROVNIKOV, VLADIMIR PAVLOV^¶, BRUCE RUNNEGAR^||, GRAHAM SHIELDS^#, JAN VEIZER^**, EDWARD YOUNG^|| and KAREN ZIEGLER^||

^* Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
Équipe de Paéomagnétisme, UMR CNRS 7154, Institut de Physique du Globe de Paris, France
Institute of Petroleum Geology and Geophysics, Siberian Branch of Russian Academy of Sciences, Academician Koptyug Avenue, 3, 630090 Novosibirsk, Russia
^¶ Institute of Physics of the Earth, Bol’shaya Gruzinskaya 10, Moscow 123995, Russia
^|| Department of Earth and Space Sciences, University of California Los Angeles, CA 90095-1567, USA
^# Geologisch-Paläontologisches Institut, Westfälische Wilhelms-Universität, Correnstr. 24, 48149 Münster, Germany
^** Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, ON K1N 6N5, Canada

Author for correspondence: artem.kouchinsky{at}nrm.se

An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and magnetostratigraphic study through the upper Middle Cambrian–lowermost Ordovician shallow-marine carbonates of the northwestern margin of the Siberian Platform is reported. The interval was analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary of the Enisej River. It comprises the upper Ust’-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the excursion, showing ¹³C positive values as high as +4.6 and least-altered ⁸⁷Sr/⁸⁶Sr ratios of 0.70909, is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position of the SPICE excursion does not support traditional correlation of the boundary between the Orakta and Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia, South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are known to immediately precede the SPICE excursion and span the Middle–Upper Cambrian boundary. The Cambrian–Ordovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk Formation, in which carbon isotope values show a local maximum below a decrease in the upper part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian times at 7–10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian on chemostratigraphic grounds contains only 10–11 magnetic intervals.

Key Words: Cambrian • Ordovician • carbon • strontium • palaeomagnetism • stratigraphy