Место издания: Florence , Год издания: 2004

Traces fossils in Callovian-Oxfordian shallow-sea Abalak, Vasyugan, Tochino, Sigovo Formations and mainly continental Naunak Formation of Western Siberia are considered. The Abalak Formation usually has homogeneous structure and silty-clay composition. Vasyugan Formation is subdivided into two subformations: lower, mainly silty-clay, and upper which includes a number of silty-sandy beds of oil-and-gas bearing horizon Ju1. In the southeastern and central areas of the West Siberian basin in the middle part of Upper Subformation, the Intercoal member or its analogues are distinguished. In the northeastern direction the Lower Vasyugan Subformation is replaced by Tochino Formation close to it by structure and stratigraphic range and the Upper Vasyugan Subformation is replaced by sandstones of the Sigovo Formation. In southern and southeast directions the Vasyugan Formation is replaced by the Naunak Formation. The characteristic feature of the Callovian-Oxfordian sediments in West Siberia is almost ubiquitous distribution at their bottom basal bed Ju2-0.In the most full sections in Upper Pur region, the Vasyugan Formation has a well-marked cyclic structure and consists of several progradational or aggradation-progradational members of highstand system tract. Their internal structure is also reflected in trace fossil fades. The bottom of the members is underlain by black massive argillites with abundant pyrite and absence of trace fossils. Above, with the increasing of silt admixture, deformative bioturbation occurs, along with which Chondrites and others trace fossils of the Cruziana ichnofacies appear. Higer in the section, there are separate traces of Skolithos ichnofacies, which consist of Ophiomorpha, Diplocraterion and Skolithos. In the upper part of members Skolithos prevail and/or the sandstones without trace fossils occur. In more southern and eastern areas the section also consists of similar members, but with the reduced bottom elements and more thicker upper ones. The Intercoal member, having continental genesis and associated with lowstand system tract, dose not always falls I this regularity. The Abalak Formation is also composed of similar members, but here their top parts drop out of the section. Thus, the distribution of trace fossils through the entire section agree with the general conception of the relative sea level fluctuations in the Callovian-Oxfordian and the lateral distribution of sedimentation paleoenvironments.