Caledonian Cyclic Unit in the Vendian–Lower Paleozoic Succession of the Ishim–Karatau Lithostructural Zone (Kazakhstan)

Abstract

The lithofacial study of Vendian–Lower Paleozoic sedimentary sequences of the Greater Karatau and Baikonur synclinorium, which are the constituents of the Ishim–Karatau lithostructural zone, revealed that Vendian, Cambrian, and Ordovician rocks form a continuous succession composing the Caledonian cyclic unit. In duration (220–240 Ma), the latter corresponds to a megacycle. The cyclic unit consists of the transgressive and regressive parts. The upper part of the transgressive succession includes facies of the continental slope, slope foot, near-continental area, and transitional zone between the near-continental and pelagic areas of the abyssal plain. It is shown that an avalanche sedimentation of the second global level occurred twice during the transgressive and regressive phases. The formation of transgressive facies was related to the extension regime during the basin opening, whereas regressive facies accumulated under the compression regime. The Ishim–Karatau zone is located in the western, outer part of Caledoninian structures in Kazakhstan. The Caledonian cyclic unit formed on the Vendian–Early Paleozoic Atlantic-type margin of the Kazakhstan microcontinent. Specific features of the cyclic development of mobile areas are outlined.