Study of a Silurian sequence of Dornes region (Central Iberian Zone, Portugal): The contribution of organic petrology and palynofacies

Abstract

A Wenlock (Silurian) sedimentary continuous sequence was sampled in the Dornes region (Central Iberian Zone, Portugal) to characterize the organic matter type using different microscopic analyses (organic petrology and palynofacies). Total organic carbon (TOC) was also determined. Samples are part of Foz da Sertã Formation and correspond to mudstone, punctually with sandstone, rich in organic matter (mean TOC value of 5.1%).Whole rock samples showed a dominance of non-granular graptolites (lath- and blocky-shape) and solid bitumen. Lath-shape graptolites display the characteristic segmented structure. In the blocky-shape graptolites, the fusellar layers of the graptolite wall are well marked and are an important distinctive feature. Foz da Sertã graptolites are optically anisotropic as well as the solid bitumen. Non-granular graptolite's maximum reflectance varies from 1.43 to 3.94% and solid bitumen maximum reflectance oscillates between 1.09 and 2.22%. Vitrinite reflectance equivalent was calculated using Bertrand and Malo's equation (values between 1.97 and 3.72%) denoting a post-mature stage for the organic matter. No relation was established with confidence between graptolite maximum reflectance and solid bitumen maximum reflectance.The palynofacies study confirmed not only the presence of graptolite and solid bitumen but also amorphous material derived from graptolite. Under transmitted white light, graptolite features were not noticed. To improve the palynofacies characterization, reflected white light was used on slides, thus exposing distinctive graptolite features. The different lights and preparation types used allowed a better characterization of the organic matter present in Foz da Sertã Formation samples, mainly the graptolites. The use of reflected white light, in the palynofacies study, proved to be extremely important in the description of the graptolite features.