Abstract

Altered ash tuff and tuffite layers in the Jurassic–Cretaceous boundary rocks (Bazhenovo Formation) were studied in detail. These layers represent an interval ranging from the upper part of the middle Volgian substage to the uppermost Ryazanian and cover a huge area (more than 400 000 km²). Despite small thickness (10^–3–10^–2 m), these layers are prominent in borehole cores and correlated confidently in interborehole spaces. Therefore, they can be used as marker horizons and isochronous stratigraphic levels in the Bazhenovo Formation. The composition of ash layers is quite consistent and marked by lateral zoning, probably, related to fractionation of the primary pyroclastic material during the eolian and hydrodynamic transport and deposition. The petrographic, mineralogical, and geochemical data suggest that the original ash corresponds to basaltic andesite in composition and is related to arc (suprasubduction) volcanism beyond the West Siberian Plate. The probable source of ash material was located most likely southwest of the study region in the Transcaucasus. The studied tuff layers are very important for the regional stratigraphy and global correlation, since they can potentially be traced in both Boreal and Tethyan successions, and zircons found in tuffs can be used for the U–Pb dating.

中文翻译：

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西西伯利亚侏罗纪-白垩纪（Volgian-Ryazanian）边界岩中的火山凝灰岩和凝灰岩

摘要

详细研究了侏罗纪-白垩纪边界岩石（巴真诺沃组）中的灰烬凝灰岩和凝灰岩层。这些层代表了从中伏阶次阶段的上部到最上层的梁赞阶的一个区间，并覆盖了一个很大的区域（超过40万km ²）。尽管厚度很小（10 ^–3 –10 ^–2m），这些层在井眼岩心中很明显，并且在井间空间中具有确定的相关性。因此，它们可以作为巴日诺沃组的标志层和等时地层。灰分层的组成是相当一致的，并以横向分区为标志，这可能与风成岩和水动力输送和沉积过程中主要火山碎屑物质的分馏有关。岩石学，矿物学和地球化学数据表明，原始灰烬的成分与玄武岩安山岩相对应，并且与西西伯利亚板块以外的弧（超俯冲）火山活动有关。灰烬的可能来源最有可能位于Transcaucasus研究区域的西南方。研究的凝灰岩层对于区域地层和全球相关性非常重要，