Strontium isotope stratigraphy – a major step towards a solid stratigraphic framework for the Late Jurassic of the Lusitanian Basin (central Portugal)

The Lusitanian Basin (central Portugal) is a syn-rift basin on the Atlantic margin, filled with sediments of latest Triassic to Holocene age. Late Jurassic sedimentation commenced in the Mid Oxfordian, following a short phase of erosion, and is continuous into the Cretaceous. At the base of this succession, the Cabaços Formation has been identified as a source interval. Stratigraphic subdivision of the Late Jurassic rocks is difficult. A marked southward palaeo-slope of the basin has resulted in its asymmetric infill and, consequently, the deposition of large-scale diachronous lithostratigraphic units. Many lithostratigraphic units are not formally established. Classic biostratigraphic markers are largely absent due to adverse palaeoenvironmental conditions. Palynology is often restricted by diagenesis and has produced conflicting results. Sequence stratigraphy is partly hampered by Late Jurassic diapirism, overprinting global signatures and resulting in the subdivision of the Lusitanian Basin into several sub-basins.

In these circumstances, strontium isotope stratigraphy, applied to biogenic calcite, has proven to be of great value. Calcitic fossils, mainly oyster shells, but also belemnite rostra, occur throughout the marginal marine to restricted marine strata of the Lusitanian Basin, and thus provide sound tie points for most lithostratigraphic units. Cathodoluminescence analysis and atomic absorption spectrometry have enabled the selection of pristine skeletal material from more than 70 fossil specimens to ensure the analysis of unaltered strontium isotope values.

Ages calculated from strontium isotope ratios range from the Mid Oxfordian to Late Tithonian (159.5-146.5 Ma). In particular, the following age ranges were obtained: Cabaços Formation – Late Oxfordian to Early Kimmeridgian (155.3-154.4 Ma); Cabo Mondego Formation – Mid Oxfordian; Abadia Formation – Early to Late Kimmeridgian (152.3-153.4 Ma); Alcobaça formation – latest Oxfordian to Late Kimmeridgian (155-151.6 Ma); Farta Pao formation – Late Kimmeridgian to Late Tithonian (152-146.5 Ma). Further data will be collected to enable accurate sub-basin correlation.