Bol’shoi Ilek as the Ilek Formation Stratotype of the Lower Cretaceous and a New Dinosaur and Mammoth Fauna Site in the Southeastern Western Siberia

Abstract

This paper clarifies the structure of the Bol’shoi Ilek section (height up to 83 m), which is composed by the deposits of Cretaceous and Quaternary systems, in the southeastern part of Western Siberia. The major volume of the section comprises the Lower Cretaceous rocks of the delta complex, reaching a thickness of 80 m and characterizing the Ilek Formation stratotype. The remains of fishes, lizards, turtles, crocodiliforms, dinosaurs, and mammals typical for the Barremian–Early Albian were found for the first time when screen-washing the sand fraction. The Mesozoic rocks are superposed by Quaternary eolian–deluvial and eluvial deposits with a thickness of 3–13.5 m or more. This sequence consists of three layers with mammoth fauna remains, the middle of which is dated at 14C ~ 22 500 years ago. In addition, according to the investigation data, Bol’shoi Ilek is a huge landslide with an area of more than 1 km2. All these facts make it possible to draw an analogy with the even-aged section of the Shestakovo high bank which also partially exposes the giant landslide. Such landslides are triggered by earthquakes with a magnitude of more than nine points, which is indicative of a powerful seismic event that occurred in the southeastern part of the study region at the very end of the Pleistocene.