Chapter 13 - Facies and Carbon Isotope Chemostratigraphy of Lower Jurassic Carbonate Deposits, Lusitanian Basin (Portugal): Implications and Limitations to the Application in Sequence Stratigraphic Studies

Abstract

The Lower Jurassic carbonate series of the Lusitanian Basin (Portugal), due to their remarkable exposure and high quality outcrops, are currently regarded as a pivotal point to the comprehension of the less-known offshore basins located along the Atlantic Margin. This study presents a detailed sedimentological and biostratigraphical framework and the carbon and oxygen isotopic record of one of the main organic-rich intervals of the Lusitanian Basin, the marly limestones with organic-rich facies member of the Vale das Fontes Formation (Pliensbachian). We report the occurrence of secular δ13C trend of the Lusitanian Basin concordant with the (second-order) Pliensbachian transgressive–regressive facies cycle. An overall increase in carbon isotopic record with transgression, reaching its maximum just above the maximum flooding interval and enhanced organic matter preservation is also observed. When compared with the defined third-order sequences, a transient δ13C increase during the onset of transgression is recognizable. Higher frequency variations in the carbon isotopic record can possibly be traced along the offshore studied sections and are interpreted to be the reflections of local controls which may eventually outpace the regional variation.