The main tectonic events, depositional history, and the palaeogeography of the southern Urals during the Riphean-early Palaeozoic

Abstract

Investigation of former and new sections in the classic Riphean region of the Bashkirian Anticlinorium (BA) revealed that this Palaeozoic segment of the passive margin of the East European Platform (EEP), during its Proterozoic history was initially formed as an intracratonic aulacogen basin which developed as an inter-cratonic half-structure only after the Early Ordovician opening of the Uralian palaeo-ocean. The originally opposite half-structure of this Riphean basin may have been located either on the Afro-European part of Gondwana or along the margins of the Siberian Craton, or elsewhere, but definitely not along the conjugate Permian collision suture with the Kazakh Block or the Mugodzhary microcontinent. The ca. 1150 Ma (1635 to 488 Ma) depositional history of the Bashkirian Basin was punctuated several times due to major tectonic events (folding, downfaulting, uplift, magmatic intrusions, metamorphism) which may have caused extended periods of erosion and non-deposition. Furthermore, numerous cyclothems of unknown regional extent are incorporated into the enormous time span involved in the evolution of the ca. 12–15-km-thick Riphean sequence. The Riphean-Vendian sedimentary assemblages reflect alternating shallow-marine to peritidal and continental (fluviatile?) environments, the regional distribution of which is illustrated in sets of palaeogeographic maps. Angular unconformities are seen at the base of the Lower Riphean Ai Formation (dated at 1635 Ma), at the base of the Middle Riphean Mashak Formation (dated at 1340 Ma), at the base of the Upper Riphean Zilmerdak Formation (approx. 1000 Ma) and below the middle Ordovician quartzites containing shelly fossils which overlie steeply folded and slightly metamorphosed Proterozoic suites along the southeastern margin of the Bashkirian Anticlinorium. During the Vendian to Ordovician interval major eastern parts of the Bashkirian Anticlinorium as well as regions further north along the European margin of the Urals were subjected to strong diastrophism, probably related to the Cadomian or Baikalian Orogeny. Later on, during the Ordovician, the western half-part of the Bashkirian Basin turned into a marginal basin of the East European Platform and thus was modified by the early stages of the Uralian oceanic development.