Environmental trends and taphonomic pitfalls in the palynology of the Productive Series and Akchagylian of Azerbaijan

The Caspian Sea is the largest endorheic lake in the world and has developed in a more or less isolated state since the beginning of the Pliocene. As such, its base level and sedimentation regime are highly sensitive to climatic variability, and understanding the main driving mechanisms controlling the palaeoenvironmental evolution is far from straightforward. During the Pliocene and earliest Pleistocene two of the primary elements of the South Caspian hydrocarbon system were developed: the thick reservoir sands of the Productive Series, and the laterally extensive cap-rock of the Akchagylian regional stage. A reliable, well-dated climate and palaeoenvironmental proxy record was obtained for the Lokbatan outcrop of the South Caspian Basin in eastern Azerbaijan, which comprises the upper part of the Productive Series and the Akchagylian. High-resolution sampling was carried out across the transition interval.

Radiometric dating (⁴⁰Ar/³⁹Ar on volcanic ashes) of the marine Akchagylian assigned the Lokbatan section to the Late Pliocene-Early Pleistocene. Terrestrial and marine palaeoenvironments were reconstructed using palynology (pollen and dinocysts) and supported by particle size analysis and geochemical analyses. Pollen data were used to interpret long-term vegetation successions at a catchment scale. The Akchagylian pollen record shows cyclic vegetation patterns, potentially related to obliquity-scale climate forcing. Akchagylian dinocyst records presented a marine succession linked to degree of fresh water input, base level variation and interbasinal connectivity. The behaviour of the dinocysts and geochemical records is more strongly influenced by connectivity between the Caspian Sea and adjacent basins than by climate.

New methodological approaches were explored to constrain known problems of reworking in dynamic depositional settings during the Pliocene (Productive Series). Distinction of comparatively old fossils in a relatively young geological setting is not the only aspect of identifying reworking, and it is more difficult to identify relatively contemporaneous reworking. As such, the reliability of the environmental signal was assessed using fluorescence microscopy, in order to constrain reworking not only from deep-time, but on the scale of orbitally-driven climate cycles. Serious issues relating to depositional environments and palynomorph taphonomy (i.e. transport, deposition / reworking) within the Productive Series were highlighted. This must be understood before the local/regional environment and stratigraphic framework can be determined reliably. Overall, this work provides a step forward in the understanding of climatic effects on the sedimentary architecture of South Caspian oil and gas reservoir succession.