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Russian Geology and Geophysics

2001 year, number 4


K. V. Zverev and V. A. Kazanenkov
Keywords: Wedge-like complex, Achimov sequence, accumulation slope, underwater fan, slope apron, turbidites, debris flow, hemipelagites
Pages: 617-630


The bottom deposits of a wedge-like complex in the Northern Ob' region were subjected to sedimentation analysis. On the basis of core material, several lithofacies have been recognized within the Achimov sequence: 1) massive and stratified sandstones; 2) gradational stratified and finely cross-bedded sandstones; 3) massive poorly graded siltstones with clay intraclasts; 4) gradational siltstones and mudstones. Differences in composition and structure of the recognized lithofacies were governed by the specific dynamics of sedimentation. The massive and stratified sandstones (lithofacies 1) making up a considerable volume of sand members distinguished within the Achimov sequence are interpreted as deposits of high-density sandy turbidite flows. The gradational stratified and finely cross-bedded sandstones (lithofacies 2), whose structure corresponds to the Bouma cycle, are considered classic sandy turbidites that were formed by low-density sandy turdidite flows between channels on an underwater slope and at its base or in underwater channels at the final stage of their filling. The finely bedded gradational siltstones and mudstones (lithofacies 3) are interpreted as fine-grained turbidites and hemipelagites formed by low-density fine-grained turbidite flows and hemipelagic sedimentation. The structure of gradational siltstones and mudstones is well described in terms of the Stow sequence. The massive poorly graded siltstones with clay intraclasts (lithofacies 4) are deposits of fine-grained debris flows having developed within the slope apron and between channels on underwater fans.
Sedimentation analysis shows an important role of processes of gravitational transportation of sediments on accumulation of the Achimov sequence, which is in agreement with the idea that the lower part of the Neocomian sedimentary complex has a wedge-like structure.