EARLY CRETACEOUS ANOXIC SEDIMENTOGENESIS WITHIN THE CARPATHIAN SEGMENT OF MESO-TETHYS
Keywords:
sedimentogenesis, geochemistry, oceanic anoxic event, black shale strata, autochthon, flysch, oil and gas deposits, Early Cretaceous, Meso-Tethys, Carpathians.Abstract
Global Barremian–Albian oceanic anoxic event caused, in particular, a deposition of organic-rich sediments within the Carpathian segment of Tethys. A thick oxygen mini-mum layer, which was spread over the entire World Ocean, in the Carpathian part of Meso-Tethys evidently covered a wide range of depths, and the sediments of this age, including both platform (autochthon) and the deposits of all the present-day Carpathian structural-facial units, were formed under its control. This explains the occurrence of organic-rich sediments of Barremian-Albian age within the entire Carpathian basin, both along and across the Carpathian Arc. This region allows studying the features of the in-fluence of the oxygen minimum layer on sedimentogenesis, as within the Ukrainian part of the Carpathian continental margin of Tethys there is a possibility of tracing the spec-trum of facies from the shallow-water shelf deposits to the deep-water distal parts of submarine fans, and, correspondingly, investigating the spatial-temporal extent of the oxygen minimum layer and the related processes of anoxic sedimentogenesis in differ-ent parts of the ancient continental margin. A comparison of two coeval sedimentary successions, which deposition occurred in essentially different parts of the same continental margin, allows estimating the influ-ence of different sedimentological factors on formation of organic-rich facies. The plat-form sediments of the Carpathian autochthon have been deposited within the shelf, while the flysch sediments of the Spaska and Shypotska formations have been formed as a result of gravity flows, which have transported the terrigenous material to the base of the continental slope. Hence, the two sedimentary successions, which are the subject of our study, are essentially different by their nature. However, the two important common features, namely their age correlation and the enrichment with the dispersed organic matter, make their comparative study important for understanding of the depositional environments within the Carpathian continental margin of Tethys. Geochemical and petrographic investigations showed that the organic-rich rocks of the Carpathian autochthon, being 0.5 to 7.0 m thick, contain up to 1.6 % of total organic carbon (TOC) and kerogen is mainly of terrestrial origin. Lithological and geochemical data allow stating that the deposition of Barremian-Aptian organic-rich sediments of the Carpathian autochthon occurred in oxygen deficit environments in bottom water and sediments, which provided an efficient fossilization of organic matter in sediments. The greater part of the organic matter was derived from the adjacent dryland of the East-European platform. Oil-generative potential of Barremian-Aptian rocks, penetrated by the existing wells, is insignificant because the rocks are immature within the investi-gated depths range. The 150 to 250 m thick flysch deposits of Spaska and Shypotska formations with TOC content of up to 8 % contain mixed terrestrial and marine kerogen with a high generative potential. They occur within a wide depth range and are consid-ered to be one of the potential source rocks in the region. Paleooceanographic reconstruction shows that the main part of the Barremian-Albian succession spreads under the Carpathian overthrust up to the ancient shelf break. We expect in this direction the increase of thickness of the organic-rich rocks comparing to the shallow part of the paleo-shelf, increase of organic matter content due to the deeper occurrence if sediments within the oxygen-minimum layer, higher share of the marine kerogen due to the greater distance from the shore, which significantly increases its generative potential, and the increase of thermal maturity of rocks, reaching the oil or even the gas window as a result of the deep occurrence under the Carpatian overthrust. Based on these considerations we assume, that the Barremian-Albian rocks of the Carpathian autochthon, occurring deeper under the Carpathian overthrust, along with the main generating Menilite Beds of Carpathian flysch could be an additional source of hydrocarbons for both Lopushna oil field and possibly existing other similar petroleum accumulations, and this confirms the prospects of the further exploration works in this region.
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