Abstract

Lower Permian sediments of the Kosva River in the central Urals (Gubakha area) were deposited on the eastern part of the Russian Platform, near the margin of the Ural foreland basin. The late Gzhelian (Orenburgian) is characterized by Palaeoaplysina and fusulinid limestones. The base of the Asselian does not exhibit traces of emersion. The Asselian is subdivided into two substages: the Kholodnian and the Shikhanian. The Asselian deposits form a second-order sequence (7 m.y.) with a maximum flooding period (MFP) during the Kholodnian and a highstand system tract (HST) during the Shikhanian. The Kholodnian shows 3 third-order sequences (AS1 to AS3) and 19 shallowing-upward high-frequency sequences composed of small Palaeoaplysina buildup (thickness 1-6m)-fusulinacean (wackestone-packstone) limestones deposited on the mid-ramp. The Shikhanian deposits form a third-order sequence (AS4) with facies of mid-ramp in MFP, and inner ramp in the HST. The lower part of the Shikhanian is concealed on the Kholodny section, except for some outcrops that show packstone and wackestone limestones deposited on the mid-ramp. The upper part of the Shikhanian is known on the Belaya Gora section with 17 shallowing-upward high-frequency sequences consisting of small foraminiferal (wackestone-packstone-grainstone) limestones of inner ramp. A second-order sequence (13 m. y.), interpreted as tectonic origin, is observed during Sakmarian and Artinskian in this area, produced by the progression westwards of the Urals orogen and of the Urals foreland basin on the Russian platform. The Sakmarian, studied on the Belaya Gora section, is subdivided into two substages: the Tastubian and the Sterlitamakian. The Tastubian presents two third-order sequences SA1 and SA2 and the Sterlitamakian one third-order sequence SA3. The sequence SA1 exhibits 33 shallowing-upward high-frequency sequences composed of small foraminiferal (wackestone-grainstone or wackestone-packstone) limestones with colonial corals deposited on the inner ramp during the transgressive system tract (TST), Palaeoaplysina small buildup-fusulinacean (wackestone-packstone) limestones deposited on the mid-ramp during the MFP, fusulinacean (wackestone-packstone) limestones with colonial corals deposited on the mid-ramp during HST. The sequence SA2 presents 22 shallowing-upward high-frequency sequences composed of Palaeoaplysina small buildup-fusulinacean (wackestone-packstone) limestones deposited on the mid-ramp during the TST and the MFP. The sequence SA3 shows 20 shallowing-upward high-frequency sequences composed of Palaeoaplysina small buildup-fusulinacean wackestone (2-10 m) and silicified bryozoan (wackestone-packstone) limestones deposited on the mid-ramp during the TST and outer platform during the MFP. The Artinskian, studied on the Belaya Gora section, is subdivided into four substages: the Bursevkian, Irginian, Sarginian and Saraninian. The Artinskian presents one composite carbonate third-order sequence (AR1-2) corresponding to the Bursevkian and Irginian deposits and one composite turbiditic third-order sequence (AR3-4) corresponding to the Sarginian and Saraninian deposits. The Bursevkian, the Irginian and the lowermost Sarginian exhibit 16 shallowing-upward high-frequency sequences composed of silicified bryozoan (wackestone-packstone) limestones deposited on the outer ramp during the MFP. During Sakmarian and Artinskian, the paleoenvironments evolve from the mid-ramp to belt of buildups, outer ramp and turbiditic basin with a migration of facies belts westwards. In the Gubakha area, the control of sequences is both eustatic and tectonic during the early Permian. The comparison with southern Urals and North America shows that the number of third order sequences is different between all the sections: four or five during Asselian, one to five during Sakmarian and two or four during Artinskian.

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