AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Ammonite-calcareous nannofossil biostratigraphy of the Toarcian/Aalenian of São Gião (Portugal)

Abstract

This study presents the results of the biostratigraphy of calcareous nannofossils from the São Gião section of the Sub-Mediterranean Province at the Lusitanian Basin that affords a continuous stratigraphic succession spanning from the Upper Toarcian (Mactra and Aalensis ammonite Subzones) to the Lower Aalenian (Opalinum and Comptum ammonite Subzones). São Gião section is characterized by an alternance of marls and limestones couplets which were previously dated by applying a detailed ammonite-based and benthic foraminiferal zonation schemes (Henriques, 1992, 2000; Henriques et al., 1996; Sandoval et al., 2001; Henriques and Canales, 2013). The aims of this work are to provide a calcareous nannofossil zonation in order to establish an integrated chronostratigraphic framework based on the calibration of calcareous nannofossils with the ammonite zonation. The sedimentary sequence is composed by a continuous and a diversified calcareous nannofossil assemblage of extraordinary well-preserved specimens in which age-diagnostic taxa (the First Occurrence – FO – of Watznaueria britannica) offer a worthy stratigraphic framework and allow detecting four calcareous nannofossil zones well-known for Tethys and Boreal realms. The Boreal NJ8b nannofossil Subzone and the Tethys NJT 8c nannofossil Subzone were detected and are correlated with the ammonite Mactra Subzone, Aalensis Zone, of Late Toarcian age. The base of the Boreal NJ9 and the Tethys NJT 9 nannofossil Zones was also marked at the FO of W. britannica correlating with the Aalensis ammonite Subzone, Aalensis Zone, of Late Toarcian age. NJ9 and NJT 9 also cover Opalinum and Comptum ammonite Subzones of the Opalinum ammonite Zone, of Early Aalenian age. The consistently presence of Retecapsa incompta, Watznaueria contracta and Cyclagelosphaera margerelii also characterizes the São Gião section. These three species are the standard biostratigraphic index-species of Boreal and Tethys affinities that mark the base of nannofossil subzones NJ8a, NJ8b (R. incompta and W. contracta respectively), and NJT 8a, NJT 8b and NJT 8c (R. incompta, W. contracta and C. margerelii respectively). Taking in account our data, we further consider those events as diachronous having place first in the Sub-Mediterranean Province before the Toarcian/Aalenian boundary, then in the Boreal Realm and slightly later in the Tethys Realm during the Middle Aalenian. This study enhances our understanding of Early/Middle Jurassic calcareous nannofossil distribution and its correlation with the Western Europe standard sequences.