ИСТИНА

During the early-middle Cambrian transgression onto the Siberian Platform, an extensive black limestone complex was accumulated on its open marine part coined the Yudoma-Olenek basin, covering over 750,000 km2. These complex includes the Kuonamka, Sinsk, Inikan, and Shumnoy formations characterized by slope conglomeratic debrites and pelagic organic-rich limy mudstones, argillaceous sapropelic shales, and subordinated cherts (commonly rich in sponge spicules) and phosphatic layers. The mudstones yield the Sinsk-type Fossil Lagerstätten accumulated during low density suspended mud flows. Elemental analyses of these formations point to their deposition under low oxic conditions. In turn, the in situ fauna − abundant and low diverse lingulate brachiopods, miomeroid and later agnostoid trilobites, and flat wide polymeroids − show features which indicate an adaptation to dysaerobic conditions. Also, trace fossils indicating animal activity within the sea bottom are extremely rare in these facies and restricted to scarce Diplocrateria and Planolites, hence the bioturbation ichnofabric index drops to its minimum or near to it. At the microfacies scale, the pelagic mudstone resembles lacustrine varvites and consists of a frequent, rhythmic, submillimetric alternation of dark clay-rich and light sparitic laminae. The sparitic laminae contain pellets, intraclasts and abundant monospecific acritarchs of the Micrhystridium genus group, while the clay-rich ones bear finely disseminated organic matter. Such sedimentary couplets can be interpreted as reflecting seasonal sedimentation of “spring-summer” carbonates coinciding with phytoplankton blooms and “autumn-winter” clays containing degraded and compacted organic matter. These blooms, probably, are expressed in a continuous positive δ13C excursion coinciding with the black limestone complex accumulation. Both the carbon isotope and microfacies analyses suggest a high productivity of the basin during the formation of this complex. An increased organic content (up to 7-12%, which is suitable for an oil-source rock) is noted and the study of bulk biomarkers revealed the presence of various hopanes and steranes among the organic compounds of the kerogen. If steranes could derive from acritarch phytoplankton and fleshy algae swept by mud flows from the photic zone, the source of hopanes were bacterioplankton and bacteria which degraded the abundant algal remains, thus maintaining low oxic conditions.