Unveiling the organic-rich Upper Ordovician black shales of the Foz de Alge region (Central Iberian Zone, Portugal): Insights from geochemistry and organic petrography

Abstract

A sequence of Upper Ordovician black shales from the Casal de Alge area, Figueiró dos Vinhos, in the Central Iberian Zone, with unusually high organic matter content, is studied for the first time. These black shales rich in organic matter (31.21–41.29 %), also contain quartz (30–61 %) and illite (26–62 %), revealing the contribution of siliciclastic and detrital elements to their composition. The organic matter is composed of well-preserved graptolites and pyrobitumen. The abundance of graptolites supports a marine origin for the black shales, a conclusion further corroborated by the low δ¹³C_org values (−27.98 and − 27.66 ‰). High graptolite abundance indicates a warm climate and high primary productivity, likely enhanced by nutrient input from continental weathering, as evidenced by the presence of detrital elements in the black shale composition. The unusually high organic matter content in these Upper Ordovician black shales is likely attributable to the graptolite end-Ordovician mass extinction during the Hirnantian glaciation. Additionally, the high clay content in these shales may have further contributed to organic matter preservation, due to clay minerals' strong capacity to absorb.

The equivalent vitrinite reflectance (VR_eqo) ranges from 4.38 to 5.18 %, indicating that black shales reached the meta-anthracite stage. This high thermal maturation appears to be primarily associated with regional burial; however, circulation of hydrothermal fluids may also have contributed, potentially related to the Figueiró dos Vinhos and Bouçã granitic plutons. Moreover, these fluids likely played a role in the enrichment of K₂O of the black shales and induced the conversion of kaolinite, originally formed under warm and humid climatic conditions, into illite.