Sedimentological Causes of Some Problems in the Cambrian Stratigraphy of the Siberian Platform

Abstract

A comprehensive approach to the study of the sedimentary cover of the Siberian Platform, or the North Asian Craton, including analysis of drilling logs, seismic profiles, and natural outcrop data has enabled a widespread revision to be made of the understanding of the extensive shallow-water sedimentation that occurred in early Palaeozoic intra-pericratonic seas of this region. The model, indicating the development of giant carbonate platforms and starved basins during the Cambrian, solves numerous stratigraphic controversies. In particular, it helps in the problem of the paradoxical combination of structural unconformities with the biostratigraphic continuity of sections, revealed during the course of geological mapping. This is explained by the progradation of primary dipping carbonate slopes for many tens of kilometres and their overlapping by horizontal shelf sediments. The model also explains the reasons for the marked facies differences within this region as well as the evolution of the depositional environments through the Cambrian. In addition, it accounts for the different lithostratigraphic sequences and sediment thicknesses identified on the one hand in composite sections from outcrops along the direction of progradation and on the other hand in the stratigraphy intersected in wells drilled within the same territories. For stratigraphic data to be correctly reproduced, this model requires several modifications to be made in the traditional format of stratigraphic charts and the completeness of accompanying documentation, including the validation of biostratigraphic scales for separate environmental regions, while putting aside the unified scale for all facies, and introducing sedimentation reconstructions as an additional element.