Late Ordovician cooling event: Evidence from the Siberian Craton
Abstract The Upper Ordovician of the Tungus Basin on the Siberian Platform is represented by cool-water carbonates. Onset of cool-water conditions starts at the Darriwilian, and it is associated with widespread phosphatization, especially in the Volginian, Kirensko-Kudrinian and Chertovskian regional stages (mid-Darriwilian–lower Sandbian). The shift from tropical-type to temperate-type carbonates was preceded by destruction of the warm-water “carbonate factory” and a large input of siliciclastic material (Baykit Sandstone) during the Vikhorevian and Mukteian regional stages (lower to mid-Darriwilian). Wide distribution of temperate-type carbonates across the Siberian Platform, located in the low latitudes in the Ordovician time, can be explained by upwelling of cold oceanic waters and their penetration into epicontinental seas. The same situation was recorded for the Upper Ordovician of the North American Platform. Newly discovered K-bentonite beds in the uppermost Sandbian–Katian of the Tungus Basin are also associated with cool-water carbonates. The K-bentonite beds point to extensive volcanism on or near the western (in present-day orientation) margin of the Siberian Craton in the Late Ordovician time. Timing of the volcanism is surprisingly close to the period of volcanic activity of the Taconic Arc near the margin of Laurentia. Both upwelling and volcanism seem to be activated by the same plate-tectonic reorganization, and both these processes seem to play an important role in a global, later Ordovician cooling terminated by the Hirnantian glaciation.