Литосфера (Dec 2017)
Structure and sedimentation environment of the Silurian and Devonian boundary deposits in the west of Zeravshan-Gissar mountain region (Obi-Safit section, Kitab State Geological Reserve, Uzbekistan)
Abstract
Continuous sequence of the boundary carbonate sediments of the Silurian and Devonian in the Ludford - Lochkov interval (Kupruk and lower part of Madmon Formation) have been studied in the Zeravshan-Gissars mountainous region of southern Tian-Shan, Kitab State Geological Reserve, Uzbekistan. The data comes from the stratotype section and some additional small sections in the Obi-Safit Gorge.The Kupruk Formation (Ludford-Pridolian) is composed of limestone and irregularly spotted dolomitic limestone containing numerous dilapidated bioherms, intraformational breccias, dolomite interlayers, siliceous connections. The lower part of the Madmon Formation (Lochkov) is represented by massive micritic and micritic-clotted limestones with coral and amphiporous biorstroms and rare carbonate breccias. Despite the abundance of organic remains, in the section there are no finds of biostratigraphic frames established for the standard boundary of the Silurian and Devonian due to zonal species of graptolites and conodonts which absents here in the border interval. The Obi-Safit section was analyzed from sedimentary, cyclostratigraphic and event-paleogeographic positions. Structural-genetic types of rocks have been established, their typification has been carried out, and the features of the Kupruk and Madmon biota have been studied. The characteristic of paleogeographic environments of precipitation formation is given. According to the intensity of the hydrodynamic effect on bottom, sedimentation rate and salinity of the bottom waters, complexes corresponding to the energy zones of the M. Irwin profile are distinguished: Y (predominantly high-dynamic, shallow water) and Z (mostly low-dynamic shallow water). In the section of Obi-Safit there are 12 transgress-regressive cyclothems 10 to 50 m thick. For the Kupruk Formation, an ideal cyclotema consisting of seven structural-genetic types of layers was reconstructed and paleogeographic model of their distribution was made. For the first time determinations and images of foraminifera and algae from this section are given. The Kupruk strata were formed in a basin with increased magnesium content and a frequent change in the hydrodynamic regime. Sedimentation took place with pauses, which were accompanied by exposure and drying of the bottom or its partial dissolution. The Madmon sequences accumulated under more stable environment with a relatively higher sea level standing in conditions of shallow normal marine sedimentation environments located below the base of the wave destruction. The maximum of the transgression within one cycle is determined by the presence of organogenic buildings, and the regression maximum is due to the appearance of carbonate conglomerate-breccias, dolomites and dolomitic limestones. The currently used Silurian and Devonian boundary in the Obi-Safit section is lolocated near 335 meters from the base of the section, but it has no a biostratigraphic and facies evidence. Due to the lack of biostratigraphic standard for this boundary, it is proposed to use the paleogeographic event traced in the west of the Zeravshan Range, although its accuracy is not of high grade. It is caused by the disappearance of isolation of this territory, the subsequent penetration of waters of an open sea basin and fixed by the presence of the numerous crinoid fragments in the upper part of the IX cyclotema. The first appearance of abundant fragments of crinoid stems is located on the 295 meter of the section.
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