Petrograficzno-sedymentologiczne uwagi o osadach najniższego kambru środkowego biegu rzeki Leny (platforma syberyjska)*

Abstract

PETROGRAPHIC AND SEDIMENTOLOGIC REMARKS CONCERNING THE LOWEST CAMBRIAN SEDIMENTS DEPOSITED IN THE MIDDLE CHANNEL OF THE LENA RIVER (SIBERIAN PLATFORM)Summary In 1970 R. Michniak carried out field observations on the parastratotypical profile of the lowest Cambrian of the Lena river valley in the western Siberia and collected rock samples which hitherto were not submitted to petrographical studies. Results of thin slide studies performed by M. Turnau-Morawska are presented in this paper. The analysed rock samples derive from the tommotian stage and from the lower zones of the atdabanian stage. The presented scheme of the investigated profiles are based upon data from the russian geologic literature and are supplied by own field observations of R. Michniak. The boundary between the tommotian and atdabanian stage was designated in result of faunistic determinations without taking in account the lithologic character of the sediments. One of the main purposes of the microscopic analyses of thin slides of these rocks was the verification of the formerly stated stratigraphic divisions and the introducing of eventual corrections. This analysys demonstrated that the sediments of the lower part of the Leptocyathus polyseptus zone of the atdabanian stage ought to be after their petrographic character considered as belonging still to the tommotian stage. What concerns the correlation problem of sediments in corresponding zones of the investigated neighbouring profiles: Malykan, Isit and Żurinski Mys - the petrographic analysis demonstrated a general analogy of the rocks with some differences produced by slight environmental changes in the sea. The characteristic glauconitic limestone occurring in the Isit profile in the lower part of the tommotian stage do not appear in the remaining analysed profiles. In the Malykan profile no admixture of terrigenous material was observed and the dolomitization process was stated as remarkably intensive. However in all three profiles a similar character of environmental conditions could be noted. The marine basin was shallow and in the tommotian stage and in sediments slightly surpassing the boundary at the atdabanian stage a detrital inflow from a land of a low relief could be perceived. In the upper part of the atdabanian stage the sedimentary conditions became more uniform, not disturbed by marine currents or terrigenous inflow. As to the origin of dolomites which in the analized profiles occur in series of remarkable thickness, the authors did not state any presence of primary dolomites, all investigated calcareous rocks belong here to dolomitized limestones. The dolomitization phenomena may be observed in a gradual change of organic and oolitic structures by replacement of calcite by dolomite. Characteristic is moreover the often appearing of dolomite rhombs in a finegrained calcite aggregate. The dolomitization process proceeded probably in several stages and before the emergence of the sediments from the marine basin.