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High-resolution calcareous nannofossil biostratigraphy from the Bathonian ASSP of the Cabo Mondego Section (Lusitanian Basin, Portugal)

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A high-resolution calcareous nannofossil biostratigraphy is presented, based upon study of 27 samples from the Bajocian/Bathonian boundary section at Cabo Mondego, Portugal. This section is the Auxiliary Stratotype Section and Point (ASSP) for the base of the Bathonian Stage and has very highly resolved ammonite biostratigraphy. Here, the calcareous nannofossils range from moderately to exceptionally well preserved, and form diverse assemblages. Two standard nannofossil Zones of the Tethyan realm (NJT 10b, NJT 11) and two of the Boreal realm (NJ 10 and NJ 11) are documented. Mediterranean Province bioevents previously established, such as the First Occurrence (FO) of Watznaueria aff. W. communis, were distinguished within the uppermost part of the nannofossil Zone NJT 10b, in the Upper Bajocian, Parkinsoni Chronozone, Bomfordi Subchronozone; likewise, the FO of Watznaueria barnesiae was identified for the first time in the Upper Bajocian, in the Parkinsoni Chronozone, Bomfordi Subchronozone, i.e.earlier than previously recorded in Tethys setting (Lower Bathonian), but later than in the Boreal realm (Lower Bajocian); and the Last Occurrence (LO) of Carinolithus magharensis was recorded in the Lower Bathonian, in the Zigzag Chronozone, Parvum Subchronozone, Mondegoense Horizon, disappearing at different chronostratigraphic levels between low, mid and high paleolatitudes. Owing to the absence of primary biostratigraphic markers from the Boreal setting, e. g. Pseudoconus enigma, the FO of second-order bioevents, e. g. Cyclagelosphaera tubulata, Tetrapodorhabdus shawensis have been advantageous to establish the base of Zone NJ 11. Besides, the FO of Stephanolithion speciosum octum and Hexapodorhabdus cuvillieri were also recognized above the base but within the Zone NJ 11, in the Parkinsoni Chronozone, Bomfordi Subchronozone for the former, and in the Zigzag Chronozone, Parvum Subchronozone, Diniensis Horizon, for the latter species. The stratigraphical position of these last two events in the Cabo Mondego Section is later than in other locations where it has been referred to the NJ 10/NJ 11 boundary in the Parkinsoni Chronozone. The proposed nannofossil bio stratigraphic framework is easily correlated with both the Tethyan nannofossil zones and subzones and the Boreal zones, although a shift in the assigned age results in new insights into the biogeographical control of their stratigraphical trends. A cooler water regime in the Sub-Mediterranean Province is proposed by rare and scattered incursions of the mentioned Boreal species. The accurate calibration of the finest resolution nannofossil biostratigraphy with the ammonite chronostratigraphic scheme represents an improvement of knowledge of the Bajocian/Bathonian boundary.
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Document Type: Research Article

Publication date: September 1, 2018

This article was made available online on May 8, 2018 as a Fast Track article with title: "High-resolution calcareous nannofossil biostratigraphy from the Bathonian ASSP of the Cabo Mondego Section (Lusitanian Basin, Portugal)".

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