On the age of the Surak and Inoceramic stages

Abstract

Chekanovskii, during his trip to Olenek in 1875, established for the Mesozoic deposits, developed according to Olenek and Lena, two divisions, which he named: the lower (dark clayey shales with Inoceramus retrorsus) - the Surak stage and the upper (light gray sandstones with I retrorsus and aucellami) - the Inoceramic Stage. The first mention of this is in the article by Lagusen, who described the fauna of these deposits and came to the conclusion that in age they can be comparable with the Volgian stages, and the Inoceramic Stage is apparently even younger than the upper Volgian Stage. Later, D.N. Sokolov, in connection with the question of the age of I retrorsus, recognized that the Surak stage corresponds to the tops of the upper Volgian stage or Berriasian; but he was inclined to consider the Inoceramian Stage to be just a facies change in the Surak Stage, differing from it in the co-occurrence of Inoceramians and Aucellae or even in the predominance of the latter. The idea of the position of both Chekanovskii stages at the Jurassic-Cretaceous boundary or at the very bottom of the Lower Cretaceous was accepted by all subsequent authors and transferred to summary works on the geology of Siberia.