BIOSTRATIGRAPHIC SUBDIVISION OF MESOZOIC DEPOSITS PENETRATED BY THE TYUMEN SUPERDEEP BOREHOLE

Abstract

The Tyumen superdeep borehole (TSB-6) drilled in north-western Siberia was the first that penetrated through Mesozoic deposits and probably entered the uppermost Paleozoic sediments. In the course of drilling, a clear succession of stratigraphic units, established formerly in separate Mesozoic sections of western Siberia, was revealed, The complete section of Triassic volcano-sedimentary deposits underlying the Jurassic sedimentary strata with net visible unconformity was recovered far the first time. Paleontological remains, which had recently been obtained from the borehole section, was used to substantiate biostratigraphic subdivision of the Mesozoic deposits penetrated by TSB-6, and to constrain the age of stratigraphic units and their stratigraphic ranges. Palynospectra characteristic of the Middle-Late Triassic, Jurassic, and earliest Cretaceous time allowed a wide chronostratigraphic correlation to be carried out within and beyond the Urengoi region. On the basis of macrofossil plant assemblages, the correlation of the TSB section with the reference borehole sections of the Urengoi area is suggested, Microfauna assemblages from coastal-marine interlayers occurring inside the Mesozoic continental sequence suggest the peculiar dynamic environments during the whole period of the Mesazoic sedimentation.