More about the Ordovician-Silurian transition beds at Mirny Creek, Omulev Mountains, NE Russia: Carbon isotopes and conodonts

摘要

Profound environmental and biodiversity changes take place in the Ordovician-Silurian boundary interval. The Mirny Creek and Neznakomka River bank sections discussed in this paper expose the upper Katian-lower Rhuddanian part of the boundary beds. The succession consists of carbonate rocks, partly with bioherms, alternating with argillaceous and siltstone packages that are well dated by graptolites. Microfossils are rare, especially in the Hirnantian, but conodonts provide some useful markers just below and above the Hirnantian stage boundaries. The Hirnantian δ~(13)C trend in the Mirny Creek section is the stratigraphically longest described so far and it has a highly specific shape. The trend commenced at the first appearance datum of Normalograptus extraordinarius or slightly below this level. The main peak occurs near the middle of the N. persculptus Biozone. Samples from the Neznakomka River suggest a somewhat wider peak interval than at Mirny Creek. Detailed comparison of the Mirny and Stirnas (Latvia) δ~(13)C curves shows a general similarity despite great specific features of both trends. Correlation of the δ~(13)C trends from China, Baltica and North America with that at Mirny Creek reveals a great variety of shapes of the carbon isotope curve. However, its rising limb commenced, if represented, everywhere close to the beginning of the N. extraordinarius Biozone or in terms of the Baltic succession, at the bottom of the Porkuni Regional Stage. Most likely a general shape of the HICE trend is pyramidal, which is peaking in the early N. persculptus Biochrone. Differences in the values and shape of an actual curve at different localities depend on local environmental conditions, sometimes modifying the global signal rather strongly.