THE MAIN TECTONIC EVENTS, DEPOSITIONAL HISTORY, AND THE PALEOGEOGRAPHY OF THE SOUTHERN URALS DURING THE RIPHEAN-EARLY PALEOZOIC

Investigation of former and new sections in the classic Riphean region of the Bashkirian Anticlinorium (BA) revealed that this Palaeozoic se gment of the passive margin of the East European Platform (EEP), durin g its Proterozoic history was initially formed as an intracratonic aul acogen basin which developed as an inter-cratonic half-structure only after the Early Ordovician opening of the Uralian palaeo-ocean. The or iginally opposite half-structure of this Riphean basin may have been l ocated either on the Afro-European part of Gondwana or along the margi ns of the Siberian Craton, or elsewhere, but definitely not. along the conjugate Permian collision suture with the Kazakh Block or the Mugod zhary microcontinent. The ca. 1150 Ma (1635 to 488 Ma) depositional hi story of the Bashkirian Basin was punctuated several times due to majo r tectonic events (folding, downfaulting, uplift, magmatic intrusions, metamorphism) which may have caused extended periods of erosion and n on-deposition. Furthermore, numerous cyclothems of unknown regional ex tent are incorporated into the enormous time span involved in the evol ution of the ca. 12-15-km-thick Riphean sequence. The Riphean-Vendian sedimentary assemblages reflect alternating shallow-marine to peritida l and continental (fluviatile?) environments, the regional distributio n of which is illustrated in sets of palaeogeographic maps. Angular un conformities are seen at the base of the Lower Riphean Ai Formation (d ated at 1635 Ma), at the base of the Middle Riphean Mashak Formation ( dated at 1340 Ma), at the base of the Upper Riphean Zilmerdak Formatio n (approx. 1000 Ma) and below the middle Ordovician quartzites contain ing shelly fossils which overlie steeply folded and slightly metamorph osed Proterozoic suites along the southeastern margin of the Bashkiria n Anticlinorium. During the Vendian to Ordovician interval major easte rn parts of the Bashkirian Anticlinorium as well as regions further no rth along the European margin of the Urals were subjected to strong di astrophism, probably related to the Cadomian or Baikalian Orogeny. Lat er on, during the Ordovician, the western half-part of the Bashkirian Basin turned into a marginal basin of the East European Platform and t hus was modified by the early stages of the Uralian oceanic developmen t.