EVOLUTION OF TRILOBITE BIOFACIES IN CAMBRIAN BASINS OF THE SIBERIAN PLATFORM

Abstract

Cambrian biotic zonation on the Siberian Platform reflects differentiation of the depositional environments (inner shelf, outer shelf and open basin). The combination of the chart of trilobite biofacies replacement and the curve of sea-level fluctuations shows that trilobite biofacies replacement occurs as a rule at times of sign reversal and distinct change in the rates of sea-level rise or fall. The boundaries of major Siberian platform Cambrian chronostratigraphic units, such a stages and series, frequently coincide with the boundaries of biofacies in stratigraphic succession related to sea-level fluctuations. If these fluctuations are gradual and restricted, then the boundaries of the Cambrian stages and series cannot be isochronous levels at a global scale. The known levels for intercontinental correlation on the Siberian Platform include boundaries of the adjacent Triplagnostus gibbus and Tomagnostus fissus Zones from the uppermost Amganian Stage (Middle Cambrian) and the Glyptagnostus stolidotus and Glyptagnostus reticulatus Zones of the lower Upper Cambrian. Both levels correspond to boundaries between highstands and lowstands on the Siberian Platform and appear to serve as boundaries of high rank. Evolution of the trilobite biofacies zonation is illustrated by genera typical for each of the various Cambrian paleogeographic environments on the Siberian Platform.