Journal
INTERNATIONAL JOURNAL OF EARTH SCIENCES
Volume 98, Issue 8, Pages 1949-1970Publisher
SPRINGER
DOI: 10.1007/s00531-008-0359-3
Keywords
Oysters; Strontium isotopes; Stratigraphy; Upper Jurassic; Portugal
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Strontium isotope stratigraphy was performed on oyster shells from the Late Jurassic of the Lusitanian Basin (central Portugal). This represents the first approach to obtain numerical ages for these strata. The new chronostratigraphic data provide a more precise age determination of several units. After a basin-wide hiatus sedimentation in the Late Jurassic is proven in the Cabo Mondego and Cabacos formations to resume as early as the Middle Oxfordian. The Alcobaca formation can be placed in the latest Late Oxfordian to Late Kimmeridgian, while data from the upper part of the Abadia Formation indicate an Early to Late Kimmeridgian age. The Farta Pao formation ranges from the latest Kimmeridgian to the latest Tithonian. The largely synchronous Sobral, Arranho I, and Arranho II members are overlain by the late Early to Late Tithonian Freixial Member. The brief, local carbonate incursion of the Arranho I member marks the Kimmeridgian-Tithonian boundary. Oysters are shown once more to be suitable for strontium isotope studies. Their calcitic shells are often unaffected by diagenesis. In particular for marginal marine Jurassic and Cretaceous strata, where belemnites are usually absent, oysters may serve as a valuable tool for isotope stratigraphy.
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