High-resolution calcareous nannofossil and foraminiferal investigations of the Bidart and Loubieng outcrop sections allow to define a time-calibrated sequence of 47 bio-events within the Danian/Selandian (D/S) boundary interval (61.2-59.7 Ma) of Aquitaine (SW France). The D/S boundary, as originally defined in Denmark (start of clastic sedimentation at the base of the Lellinge Greensand Formation), is marked by the end of the acme of the nannofossil family braarudosphaeraceae. This bio-event, dated at 59.9 Ma, has also been identified at the lithological change from limestone-dominated (Lasseube Formation) to marly sedimentation (Latapy Member of the Pont-Labau Formation) in SW Aquitaine and at the base of the red marls of the Itzurun Formation at Zumaia (Spain), recently designated as Global Stratotype Section and Point (GSSP) for the D/S boundary. This implies contemporaneity of this lithological shift throughout Europe and a GSSP proposal, which is consistent with the original boundary definition. The braarudusphaeraceae-event is believed to be due to the interruption of freshwater influx, probably related to a sudden decrease in precipitation. It is located at the top of nannofossil zone NP4 and within planktonic foraminiferal zone P3b and bracketed between the lowest occurrence.(LO) of Morozovella velascoensis (below) and the LO of Fasciculithus tympaniformis (above). It is coincident with the LO of Bomolithus elegans, the LCsO (Cs = consistent) of Fasciculithus janii and the LO of Subbotina velascoensis. The D/S boundary as originally defined is 400 k.y. posterior to a major discontinuity, recorded throughout the Tethyan Realm (Tunisia, Egypt) and up to now erroneously considered to correspond to the D/S boundary. This break in sedimentation, dated at 60.3 Ma and coinciding with the P3a/P3b boundary, is due to a major sea-level fall. It is correlated with sedimentation changes in the Aquitaine-Zumaia area (start of development of marly interbeds) and in the North Sea Basin (transition Bryozoan limestone-Calcisiltite in Denmark; transition shallow marine Mons Formation-continental Hainin Formation in Belgium). The Loubieng section supplements the Zumaia section. Because of its rich and well-preserved fossil content and continuous sedimentation it constitutes an excellent auxiliary section for the D/S boundary. |
