Palynofacies and organic geochemistry of the Sinemurian carbonate deposits in the western Lusitanian Basin (Portugal): Coimbra and Água de Madeiros formations

Abstract

The Lower Jurassic in the Lusitanian Basin (Portugal) is mainly represented by hemipelagic carbonated deposits rich in organic matter (OM). In this study, supported by a high-resolution stratigraphic and sedimentological framework, the analysis of the OM content of the Coimbra and Água de Madeiros formations in the reference outcrop sections of S. Pedro de Moel and Peniche is presented. This is a multidisciplinary approach to the study of 31 samples in these series, supported by organic petrography [palynofacies, spore coloration index (SCI) and random vitrinite reflectance (%Rr)] and organic geochemistry [total organic carbon (TOC) and biomarkers].The palynofacies analysis provides evidence that the OM is mainly composed of particles belonging to the amorphous organic matter (AOM) group. The phytoclasts correspond mostly to the non-opaque (translucent) subgroup, generally degraded and oxidized, with significant representation in some stratigraphic levels of the Coimbra Formation and the Polvoeira Member at Peniche. The palynomorphs are mainly composed of Classopollis pollen grains, zygospores, Acritarchs and Prasinophyte phycomata. The TOC data confirm that most samples are OM-rich, with some stratigraphic horizons of the Coimbra Formation reaching 8.8wt.%. The predominance of marine OM was confirmed by the Pr/nC17 and Ph/nC18 ratios. The Pr/Fi and homohopane ratios (C35/C34) indicate that some stratigraphic horizons were deposited under dysoxic-anoxic conditions. Despite the organic-rich nature of these two units, the random vitrinite reflectance (%Rr=0.45), the spore coloration index (3.0–4 0) and the presence of ββ hopanes suggest that these rocks, in terms of OM maturity, are thermally immature.