| Stratigraphic correlation and lithofacies of Devonian deposits in transition zone from the continent to the southern Pechora Sea |
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KIREYEV G.I., PREOBRAZHENSKAYA E.N., ZONN, M.S., MENNER V.VL., SHUVALOVA G.A., ABUSHIK A.F., EVDOKIMOVA I.O., MODZALEVSKAYA T.L, YUDINA YU.A., MOSKALENKO M.N., VEVEL YA.A., OBUKHOVSKAYA Studies of the sections of offshore wells drilled in the southern Pechora Sea showed that the stratigraphic subdivision and correlation of Devonian strata can be conducted using regional stages, horizons and formations recognized in adjacent onshore Devonian sequences.Correlation of offshore well sections and onshore sequences, regional stages, stages, and zonal biostratigraphic schemes was conducted using the regional well log marker grid, new paleontological and palynological datings, and the specific features of lithofacies variations in the basin during its Devonian evolution. Devonian deposits consist predominantly of shallow shelf lithofacies; similar lithofacies variations have been identified over the whole transition zone. The Pragian, Emsian, and Middle Devonian deposits are of limited occurrence because of the pre-Timan scour corresponding to the Taganyik global event. The lower Frasnian mudstones with occasional sandstone and siltstone interbeds of the Timan and Sargayevo Rs (30-120 m) are widespread and rest unconformably upon various Devonian horizons. However, they lack guide fossils for biostratigraphic identification, and this impedes the correlation of well sections with the exact Rs volumes and leaves the presence of the Dzhyer Rs and the age of pay sandstones as open issues. In recent years, new paleontological and palynological data provided a possibility to reliably correlate the Middle Frasnian multifacies deposits of the Domanik Rs. In the western and northwestern areas, this interval is represented by shallow-water mudstones and carbonates of the Sercheyyu Formation with signatures of anoxic environment (Dresvyanka and Prirazlomnaya prospects, thickness 48-120 in). Farther south, In the Pakhancheskaya prospect the upper part of the sequence contains numerous coral-stromatolporate beds. In the Varknavtskaya and Medyn More prospects still farther south, the Domanik Rs occurs as an essentially carbonate sequence (up to 50 m) with brachiopods representing the fades of the upper part of a ramp slope, from shallow shelf to a starved deep-sea basin. Basinal "domanikoid" lithofacies of this horizon were penetrated by onshore well Toravey 25. The carbonate members of the slope and basinal lithofacies of the Domanik Rs are considered as a reliable correlation marker. Rdm. The Upper Frasnian sequence begins with the mudstone member of the Vetlasyan Rs (0-20 m). It is dated by spores in well Medyn More 1. The lower part of the next Sirachoy Rs is represented in the western areas by the Laya Formation (48-80 m) consisting of shallow-water biogenic carbonates with small coral-stromatoporate bioherms and biostromes. These grade southeastward into ramp lithofades (compact, sometimes muddy limestones) and then into thin members of basinal domanikoid lithofacies. The deposits of the Upper Sirachoy, Yevlanovo and Livny Rs of the Frasnian in the northern Pechora-Kolva aulacogen and Khoreyver depression have been recognized as shallow shelf facies the Inzyrey Formation (120-250 m). It is subdivided into members, which exhibit clearly traceable facies changes from stratified shallow-shelf deposits to reefal slope facies and then basinal. The Inzyrey formation in complete type sections in the northern Lay and Kolva arches and in well sections penetrated in the Dresvyanka, Pakhancheskaya and Prirazlomnaya prospects is 127-250 m in thickness and consists of 4 members. Based on spores and some other guide fossils, the lower part of Member I corresponds to the Sirachoy Rs: the upper part of Member I, Member II, and much of Member III corresponds to the Yevlanovo Rs; and Member IV, time the Livny RS. Argilliceous Member II and variegated Member IV (the "Ukhta marker" Ru) are correlation markers. Ru marks the boundary between the Frasnian and Famennian stages. Southeastward, along the boundary between the shallow shelf and starved deep-sea basin, the lower part of the Inzyrey Formation grades into fringing or barrier reef massifs. Based on the stratigraphic volume of the above-reef part of the Inzyrey Formation (members 1П and IV), the age of the reefs is Sirachoy-early Yevlanovo. The barrier reef zone (with a thickness up to 200 m) extends northeastward from the Varknavtskaya prospect in the Klioreyver depression to the Varandey prospect and further offshore, where it was penetrated by wells in the Medyn More 2 prospect. Then the barrier reef bends southward along the Medyn swell (Medyn, Toboy and Myadsey prospects). Well Medyn More 3 penetrated the sequence of an interreef channel separating the Medyn onshore and Medyn-More (offshore) barrier reefs. It lacks basinal facies or thick in fill sequences. The sequence of interbedded carbonate and mudstone intervals exhibits the equivalents of the Domanik, Vetlasyan, lower Sirachoy. Rs and all members of the Inzyrey Formation, hi addition, the total thickness of the Middle-Upper Frasnian sequence is approximately the same as in the sequences with reefs. Barrier reefs contain discovered oil pools. Seals are provided by the Upper Yevlanovo-Livny deposits. The deep-sea basin southeast of the barrier reef, which presently lies partially in the offshore extension of the Varandey-Adzva zone, accumulated mostly domanikoid lithofacies during the Domanik, Sirachoy. and early Yevlanovo times. During the Middle and Late Yevlanovo times, these basins were infilled by mudstone members: South Toravey (equivalent of Member II of Insyrey Formation) and East Olenya (equivalent of the mudstone member in Member III), and were accreted to shallow shelf. The shallow shelf facies of the Famennian Stage within the study area are not characterized by abrupt facies variations. Its lithostratigraphic units are clearly traceable in most of the studied onshore and offshore sections. The Frasnian-Famennian boundary is marked by a hiatus probably corresponding to the Volgograd Rs. The Lower Famennian Substage is represented by carbonates of the Lekkharyaga Formation (150-250 m in thickness). The low proportion of clay in its lower part help us distinguish on gamma logs lower Zadonsk and the upper Yelets Rs. The mudstone-carbonate deposits of the Middle Famennian Substage (Ust-Pechora Rs and formation of the same name) are clearly traceable in most areas. The sequence begins with a thin (5-15 m) carbonate-mudstone member Rel-up (Yelets-Ust-Pechora marker). Above it is the basal carbonate member (20-50 m) overlain by a carbonate-mudstone sequence considered as regionally significant marker Rup (Ust-Pechora) dated by the Middle Famennian brachiopods and conodonts and the Lebedyan and Opukhtov spore assemblages. The sequence of the Ust-Pechora Formation ends with a carbonate member (up to 40 m). The total thickness of the Ust-Pechora Formation is 150-180 m.
The Upper Famennian Substage is represented by a carbonate sequence (140-180 m) of the Dzhebol Super Rs (Zelenets and Nyumylga Rs). A carbonate-mudstone member correlated with the Upper Famennian marker Rim3 dated as the upper Zelenets Rs is in places detected on well logs in the middle of the sequence. The marker member Rnm-t (Nyumylga-Toumaisian, up to 25 m) between the Famennian and Toumaisian is more straightforward to detect on well logs. It is difficult to draw the boundary between the Middle and Upper Famennian on well log curves, and the upper carbonate member of the Ust-Pechora Horizon and the Upper Famennian carbonate sequence are therefore often classified into a Nyadeyyu Formation. The proportion of mudstones in the Famennian decreases northeastward, and the clay marker beds largely grade into carbonates. Surface outcrops of Devonian deposits in the northern Vashutka-Talota - Chernov Range, Dolgiy and Zelenets islands, the western Pay Khoy, and the southwestern Novaya Zemlya Archipelago suggest that these and the southeastern sector of the Pechora Sea were parts of a shallow marine carbonate platform during the Middle and Late Frasnian and Famennian times. |