Shallow-water carbonates of the Coimbra Formation, Lusitanian Basin (Portugal): contributions to the integrated stratigraphic analysis of the Sinemurian sedimentary successions in the western Iberian Margin

Abstract

An integrated stratigraphic analysis of the Coimbra Formation was performed in the S. Pedro de Moel outcrops of the Lusitanian Basin (Portugal). This unit is dated from the lower–upper (Oxynotum Chronozone) Sinemurian and is subdivided into eight informal subunits. Except for its base, consisting of dolostones and microbialites, much of the succession consists of bioclastic and bioturbated micritic centimetric–decimetric limestones (sometimes rich in benthic macrofauna) alternating with millimetric–centimetric marly layers, all deposited in shallow-water carbonate ramp environments. Organic-rich sediments occur throughout, with total organic carbon reaching up to 12 wt%. At a broader scale, the Coimbra Formation is transgressive and part of a long-lasting 2nd-order transgressive–regressive facies cycle ending around the Sinemurian–Pliensbachian boundary. The vertical variation of $\delta ^{13}\mathrm{C}$ determined in bulk carbonate is characterized by relatively normal marine values ($0$–$2.5‰$); however, several negative shifts are associated with the organic-rich sediments, with a maximum amplitude of ${\sim }8‰$ in the Obtusum Chronozone. These shifts in bulk carbonate $\delta ^{13}\mathrm{C}$ are interpreted to be of diagenetic origin and, therefore, of local significance. However, it cannot be discarded that a regional/global signal is imprinted on the observed trends.

Keywords

• Lusitanian Basin
• Sinemurian
• Shallow-water carbonates
• Organic-rich deposits
• Stratigraphy
• Stable carbon isotopes
• Lusitanian Basin