PROBLEM OF THE INTERNATIONAL CHRONOSTRATIGRAPHIC CHART OF LOWER DEVONIAN AND STRATIGRAPHIC BOUNDARIES OF LOWER DEVONIAN STAGES IN SOUTHERN TIAN SHAN

Abstract

The boundaries of the International Chronostratigraphic Chart (ICC) are associated with the natural historical stages of the Earth's development, and their boundaries are usually determined by the palaeogeographical or palaeobiological event that is recognized in most regions of the world. Studies on the standardization of the Lower Devonian ICC stages duration have lasted more than 70 years. The first stages were Gedinnian, Siegenian and Emsian, but their duration was not accurately determined. The article shows the history of the establishment of the currently used Lochkovian, Pragian and Emsian stages. The key study method of the Lower Devonian stages deposits is biostratigraphic one, based on the use of archistratigraphic pelagic fossils, i.e. revealing of the zonal sequence over the conodonts, dacrioconarids and goniatites fossils and comparing them with the standard sequence adopted by the International Geological Congress (IGC). The article contains an analysis of the discussed problems related to the determination of the position of the global boundary stratotype section and point (GSSP) of the Lower Devonian stages. The use of the magnetostratigraphic method in comparison of the sections of the Emsian stage in Barrandian and Zarafshan range is also discussed. The Lower Devonian regional strata of the Zarafshan-Gissar and Turkestan-Altay mountain regions of the Southern Tian Shan are characterized. It is shown that the zonal sequences on conodonts and dacrioconarids presented in these regions almost completely coincide with the standard Lower Devonian sequences in the International Chronostratigraphic Chart. The characteristics of zones and volume of Bursykhirmanian, Sangitovarian, Khukarian, Kunjak, and Kitab horizons are given.