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Abstract

Outcrops of the Nichatka Formation are located in the southern centroclinal part of the marginal Berezovskaya basin of the Siberian craton. Deposits of the gently sloping southeastern limb of the asymmetrical basin are unaltered, whereas those of the steep, partly overturned western limb exhibit cleavage and are locally slightly metamorphosed. In the Late Precambrian the Berezovskaya basin was a shallow marine area of the passive margin of the Siberian craton. The Nichatka Formation is the basal unit of the Dal'nyaya Tayga Group of the Patom Supergroup. On the southeastern limb of the Berezovskaya Basin, the formation is composed of massive and bedded diamictites, conglomerates, sandstones and thin-bedded rocks with dropstones. The diamictites contain erratic blocks of different size and roundness. The formation thins out eastward. On the western limb, the formation is represented predominantly by graded-bedded and less frequently massive diamictites, sandstones and rare thin bedded members with dropstones. The diamictites contain clasts with glacial facets and striae. The formation unconformably lies on the Ballaganakh Group of the Patom Supergroup or older rocks and grades into a marker member of thin-bedded red dolomitic marls and dolomites. Overlying carbonate-terrigenous formations of the Patom Supergroup are characterized by two negative (down to –6 and –10.5‰) δ13C anomalies and one positive (up to 3.9‰) anomaly. 87Sr/86Sr ratios in the carbonates above the Nichatka Formation increase upsection from 0.70725 in the overlying deposits to 0.70837 in the Nemakit-Daldyn (Fortunian) horizon of the early Cambrian. The Nichatka Formation as well as the host deposits are assigned to the Lower Vendian (Late Cryogenian) by reliable geological and isotopic correlations to biostratigraphically better studied Upper Precambrian sequences of the Ura Uplift in the Lena River basin. Stratigraphically similar glacial units were traced along the margins of the Baikal-Patom Highland up to the southern end of Lake Baikal and further to the NW towards East Sayan. The Siberian Craton was the main source area for these glacial units, which points to the existence of the great inland glaciers at the southern part of this craton. Some older Upper Precambrian diamictites of unclear age and genesis are also recorded in Central Siberia.

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