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The Aptian stage


, : The Aptian stage. Bulletin de l'Institut Royal des Sciences Naturelles de Belgique Sciences de la Terre 66(SUPPL ): 31-43

Detailed multidisciplinary analyses of several sections through the Barremian-Aptian interval resulted in the revision of the Aptian stage, including the Barremian-Aptian boundary. The Aptian Working Group (= AWG) established an integrated stratigraphy based on biostratigraphy (ammonites, belemnites, calcareous nannofossils, planktonic and benthic foraminifera, dinoflagellates, radiolarians), magnetostratigraphy, isotope stratigraphy (delta-13C and 86Sr/87Sr), event stratigraphy, sequence stratigraphy, and cyclostratigraphy. Historical precedence was considered and the preliminary proposals made at the Copenhagen Symposium in 1983 were considered as starting point. The new information highlighted the difficulty of using the first appearance of Prodeshayesites for the definition of the base of the Aptian, and revealed the importance of biostratigraphy based on other fossil groups, especially micro- and nannofossils, as well as physical and chemical tools such as magnetostratigraphy, chemostratigraphy, sequence stratigraphy and cyclostratigraphy in global correlations. Following the discussions at and after the Second International Symposium on Cretaceous Stage Boundaries (Brussels, 8-16 September 1995), the majority of the AWG selected the base of magnetic chron MO as the event for the definition of the base of the Aptian stage. It must be emphasized that magnetostratigraphy must always be integrated with biostratigraphy and magnetic chron MO must be identified in relation to palaeontological events. After accepting the base of magnetic chron MO as the base of the Aptian stage, the AWG identified the Gorgo a Cerbara section (Umbria-Marche Basin, Central Italy) as possible GSSP for the base of the Aptian Stage. This section represents an excellent exposure of Barremian to Aptian pelagic carbonates, and offers a wide range of available stratigraphies including magnetostratigraphy, calcareous nannofossil and planktonic foraminiferal biostratigraphy, radiolarian biostratigraphy, dinoflagellate biostratigraphy, chemostratigraphy (delta-13C) and cyclostratigraphy. Moreover, the Oceanic Anoxic Event 1a is represented by the black shales of the Selli level. The record of ammonites is not continuous; however, a few diagnostic layers have been detected. The Aptian substage subdivision was also discussed, but the AWG has not yet provided recommendations. While a two-fold subdivision is adopted for the Boreal Aptian, a three-fold subdivision is often applied to low-latitude sequences. The integrated stratigraphy elaborated by the AWG for the stage is still under revision and Aptian subdivisions will be the subject of further work.

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