Palynomorph optical analyses for thermal maturity assessment of Upper Ordovician (Katian-Hirnantian) rocks from Southern Estonia

Application of integrated, multi-proxy optical analytical techniques for the assessment of thermal maturity of organic matter was tested in Katian to Hirnantian (Upper Ordovician) sediments from a subsurface succession in southern Estonia. The investigated Valga-10 drill core section consists of a biostratigraphically well-constrained succession deposited in the outer part of a carbonate shelf (Livonian basin). The diverse and well-preserved palynological assemblages permitted the analysis of thermally-induced modifications of optical properties of small and large acanthomorph acritarchs. Qualitative methods such as UV-fluorescence and Acritarch Alteration Index (AAI) were calibrated against the quantitative Palynomorph Darkness Index (PDI). The occurrence of vitrinite-like organoclasts allowed reflectance measurements in some of the investigated samples. The strong fluorescence and pale to yellow colour of acritarchs, combined with low PDI and organoclast reflectance values, suggest thermally immature kerogen and a limited burial diagenesis in the study area. Moreover, PDI showed a good correlation with UV-fluorescence, allowing a calibration of these two techniques and proving the reliability of PDI for thermal maturity estimation in sedimentary rocks unsuitable for Rock-eval pyrolysis and vitrinite reflectance. Furthermore, the applied methodologies, permitted the differentiation of reworked from in situ organic matter and highlighted the potential of the former for sequence stratigraphic investigations.