Features of sedimentation and volcanism of the Tastauskaya rift structure in Central Kazakhstan

Abstract

Research subject. A meridional Tastau rift structure located in the Famenian sub-latitude rift system of Central Kazakhstan was investigated, including the specific features of its constituent sediments and volcanism, as well as the stages of its development.Materials and methods. The study was based on data collected during a long period of fieldwork, including a detailed study of the sequence of sedimentation processes, large-scale geocarting, sampling for various types of precision analyses.Results. The typomorphic features of the main rock types were determined. It was shown that basaltoid rocks occurred in the form of pillow structures with a hyaloclastite matrix between the pillows. Siliceous shales are saturated with thin ash silicic acid material. A geological scheme of the Tastau structure and its specific features was presented in a series of cross-sections. The developmental stages of this structure were established. The structure was referred to the rift type.Conclusion. The material and structural features of the investigated Tastau section are shown to be typical of the entire rift system of Central Kazakhstan. The pre-rift (Frasnian) stage is characterized by continental volcanism in the form of highpotassium and ultra-potassium rhyolite ignimbrites composing an encialic island-arc structure. These volcanites belong to the shoshonite and high-potassium lime-alkaline petrochemical series. The rift stage itself began in the Early Famenian age with the formation of a system of narrow deep troughs, into which the sea from the neighbouring Zhongaro-Balkhash sea region started to ingress. The conglomerates are characterized by thin flattened isometric pebbles, whose well-polished surface is similar to that of pebbles in modern sea beaches. The sandstones exhibit a high level of lamination and are characterized by poorly pelletized clastic material. The clastic material is represented by the underlying Frasnian rhyolitic ignimbrites carried in from the sides of the trough. The mature rift stage is characterized by the accumulation (under marine conditions) of pillow basalts and hyaloclastites and thin-layered silicite-clay-carbonate shales with abundant ash. The postrift stage is responsible for the formation of layered limestones of the Lower Tournaisian, developed also widely beyond the specified rift trough.