--> Ammonite-calcareous nannofossil biostratigraphy of the Bathonian ASSP (Portugal)

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Ammonite-calcareous nannofossil biostratigraphy of the Bathonian ASSP (Portugal)


A fantastic well-preserved calcareous nannofossil assemblage from the Bathonian ASSP of Cabo Mondego Section was examined in an attempt to analyze its stratigraphical distribution and to establish a chronostratigraphic scheme in which ammonites and nannofossils are integrated. The Bathonian ASSP of Cabo Mondego Section is located in the Lusitanian Basin (Portugal) in the Sub-Mediterranean Province. This succession consists of an alternance of marls and limestones couplets wherein a significant work achieved by Fernández-López et al. (2006a,b) has established the standard ammonite zonation for the Sub-Mediterranean Province used here to calibrate the calcareous nannofossil biostratigraphy. Standard nannofossil biozones NJ10 and NJ11 from the Boreal realm, and NJT 10b and NJT 11 from the Tethys were recognized and calibrated at ammonite Horizon and Subchronozon resolution. Therefore, the First Occurrence (FO) of Watznaueria aff. W. communis and Watznaueria barneseae were distinguished within the uppermost part of the Tethyan nannofossil Zone NJT 10b, in the Upper Bajocian, Parkinsoni Chronozone, Bomfordi Subchronozone. While, the FO of Watznaueria aff. W. communis in the Sub-Mediterranean Province appears coeval with this event in the Tethys Province, the FO of W. barneseae occurred earlier than previously recorded in the Tethys setting (Lower Bathonian), but later than in the Boreal realm (Lower Bajocian). The Last Occurrence (LO) of Carinolithus magharensis was recorded in the Lower Bathonian, in the Zigzag Chronozone, Parvum Subchronozone, Mondegoense Horizon, displaying a certain degree of diachrony between low, mid and high paleolatitudes. Although Pseudoconus enigma is missing, a substitute evidence such as the FO’s of Cyclagelosphaera tubulata, Tetrapodorhabdus shawensis has been considered to establish the base of the Boreal Zone NJ11. The FO’s of Stephanolithion speciosum octum and Hexapodorhabdus cuvillieri were also detected above the base but within the Zone NJ11 in the Parkinsoni Chronozone, Bomfordi Subchronozone for the former, and in the Zigzag Chronozone, Parvum Subchronozone, Diniensis Horizon, for the later species. These later two events in the Cabo Mondego Section are reported younger than in other locations where they have been dated as NJ10/NJ11 boundary in the Parkinsoni Chronozone. The available data reveals that the presence although scattered abundance of Boreal species is indicative of the influence of cold sea surface waters in Cabo Mondego, and therefore associated with a paleoenvironmental/paleogeographical control.