Reef episodes, anoxia and sea-level changes in the Frasnian of the southern Timan (NE Russian platform)

Abstract

Abstract The development of the Frasnian (Upper Devonian) reef complexes of the southern Timan and Pechora region of northern European Russia is described. Barrier reef complexes progressively prograded eastwards during the Frasnian but the carbonate complexes were interrupted many times by regressive events. Using new conodont and ammonoid biostratigraphical dating, the timing of reef building episodes has been established which enables international correlation with other similar Devonian areas. Basinal anoxic and hypoxic deposits associated with the reef complexes of the Domanik facies provide the major hydrocarbon source rocks of the region and the palaeoenvironmental interpretation of these is discussed. Initial transgressions appear to have been associated with the global Taghanic Onlap of the late Givetian. The new level for the base of the Frasnian and Upper Devonian lies in the Timan Formation, after the deposition of which marine conditions mostly prevailed in the area examined until the late Frasnian when a sharp regression occurred with no evidence of the typical Kellwasser facies of Western Europe and other areas. Transgressive pulses initiated ammonoid biofacies in the Regional Sargaev Stage and the widespread Timan Event was marked by the spread of Timanites faunas. A significant deepening event which initiated the Domanik facies correlates approximately with the Middlesex black shale of New York and the main development of the Domanik facies with the Rhinestreet black shale of New York. There are faunal and floral peculiarities of the area, shown by endemic genera and rather different ranges of cosmopolitan species than elsewhere, which complicates precise international correlation. Nevertheless, several of the main sea-level deepening pulses of the Frasnian, documented in North America, Western Europe, North Africa and Australia, are recognizable and these are thought to represent eustatic events.