The Mont-des-Récollets section (N France): a key site for the Ypresian-Lutetian transition at mid-latitudes – reassessment of the boundary criterion for the base-Lutetian GSSP

ABSTRACT Re-excavation of the famous Mont-des-Récollets quarry in northern France and re-interpretation of the adjacent Cassel borehole have led to the reconstruction of one of the most complete Upper Ypresian and Lutetian stratigraphic successions of the southern North Sea Basin. It includes the entire suite of formations and members, from the top of the Hyon Sand Formation (top NP12) to the base of the Maldegem Formation (mid-NP15), originally defined in central Belgium, extending the lateral distribution of most of these units up to northern France. The similarity in lithofacies and calcareous nannofossil assemblages and the identification of in situ specimens of Nummulites laevigatus (Bruguière, 1792) and Campanile giganteum (Lamarck, 1804) at the Mont-des-Récollets bear witness to direct north-south connections between the Belgian Basin and the Paris Basin during Biochron NP14 and early Biochron NP15. However, direct connections have only been intermittent. During late Biochron NP12 and Biochron NP13, both basins became disconnected because of lowering of the global sea level in combination with uplift of the Paris Basin. The massively reworked large-sized N. laevigatus and the presence of N. laevigatus-bearing sandstone blocks and lignitic pebbles at the base of the Lede Formation indicate a major erosion phase at the NP14-NP15 Biochron transition in the Belgian Basin. This was probably due to uplift of the Brabant Massif. The new Mont-des-Récollets data, including the introduction of a new genus (Luminocanthus n. gen.) and ten new species (Blackites minusculus n. sp., Blackites praeinflatus n. sp., Luminocanthus eolutetiensis n. gen., n. sp., Luminocanthus plenilutetiensis n. gen., n. sp., Martiniaster cecellanoriae n. sp., Nannoturba joceliniae n. sp., Sphenolithus quadricornutus n. sp., Sphenolithus recolletensis n. sp., Trochoaster nodosus n. sp. and Trochastrites pyramidalis n. sp.) have allowed to substantially refine the calcareous nannofossil stratigraphy. The subdivision of Zone NP13 into 3 subzones (NP13-a to NP13-c) as well as the major nannofossil turnover at the base of Zone NP14 and the nine nannofossil-events within Zone NP14 have been recognised throughout the Belgian Basin. Several of these have been recorded in the Paris Basin, the Hampshire Basin and the Aquitaine Basin, highlighting their interbasinal correlation potential. The nannofossil data provide for the first time evidence that the base of the ‘Chaumont-en-Vexin sands' (term informally introduced herein to designate the base of the historical Lutetian stratotype) and the base of Unit A4 in the upper Aalter Sand Fomation are coeval or nearly coeval. This is also the case for the base of the overlying units, the ‘Glauconie Grossière s.s.' (as redefined in the Paris Basin by Blondeau in 1980) and the Brussel Sand Formation (‘Bruxellian' as originally defined in Belgium by Dumont in 1839). This investigation and its continuation, detailed in a forthcoming study of the Brussel Sand Formation, also revealed that the internationally accepted base-Lutetian boundary criterion (lowest occurrence or LO of Blackites inflatus) is difficult to apply in the North Sea Basin because of the extreme rarity of this marker species (e.g. not recorded at the Mont-des-Récollets). Worst of all, its strict application would imply that the major part of the Brussel Sand Formation and both the ‘Chaumont-en-Vexin sands’ and the lower part of the overlying ‘Glauconie Grossière s.s.’, base of the historical Lutetian stratotype, should be of Ypresian age, which is a contradictio in terminis. To resolve this contradiction, it is suggested to amend the original criterion (LO of B. inflatus), proved to be inadequate at middle and high latitudes and diachronous at low latitudes, and to replace it by the lowest occurrence (LO) of Discoaster sublodoensis. The latter is part of a major calcareous nannofossil turnover (= BALCAT-event), which has been identified at the base of the historical Lutetian stratotype. This new proposal would mean that the base of the Lutetian should be lowered down in the Gorrondatxe GSSP, probably by about 130 m (c. 1.3 Myr), to a level within the middle of Chron C22n, around 800 kyr earlier than the LO of Turborotalia frontosa. If this is the case, then the base would range in age between 49.11 Ma and 49.20 Ma, depending on the age model used.