The PETM record revealed by a new integrated high-resolution dinoflagellate cyst and geochemical data from the "Sparnacian" sediments in the Paris and adjacent basins
Résumé
The Paris Basin represents an historical cradle of the Paleogene stratigraphy, where the Paleocene Epoch and the "Sparnacian Stage" have been erected (Schimper, 1874; Dollfus, 1880). As highlighted by Aubry et al. (2005), whereas the chronostratigraphic connotation of the Sparnacian Stage occurred to be controversial since its definition, modern studies of the Late Paleocene - Early Eocene interval have revealed that the so-called "Sparnacian" deposits encompass a remarkable and short (~170 kyr) episode in the Cenozoic history, the Paleocene-Eocene Thermal Maximum (PETM, ~55.8-55.6 Ma). However, due to a large development of diverse and laterally variable, predominantly lagoonal and non-marine facies, the Paris Basin Upper Paleocene-Lower Eocene succession is still poorly documented and needs an updated chronostratigraphic correlation with other Paleogene records in adjacent basins and worldwide. Since almost 45 years the dinoflagellate cyst stratigraphy has significantly contributed to correlations of the Thanetian-Ypresian deposits in the Paris and adjacent basins (Châteauneuf & Gruas-Cavagnetto, 1968, 1978; Gruas-Cavagnetto, 1974; Costa and Downie, 1976). Nevertheless, data published on dinoflagellate cysts distribution in the Paleogene sediments of those basins remain too scattered (see Aubry et al., 2005) and need to be calibrated to the most recent biozonations. With the aim at reconstructing the "Sparnacian" palaeoenvironments as well as ensuring correlation with the PETM (and its Carbon Isotopic Excursion, CIE) events and related processes, new or already well known "Sparnacian" Dieppe-Hampshire and Paris Basins key localities (Cap d'Ailly and Sotteville-sur-Mer sections, Therdonne and Sinceny cores) have been investigated in details palynologically and chemostratigraphically. According to our new high-resolution data, the CIE begins within the Mortemer Fm in terrestrial or coastal environments and continues until the top of the Soissonnais Fm (Quesnel et al, this meeting). Within the CIE, δ13Corg values fluctuate between -25 and -30 ‰, while above and below they fluctuate between -22 and -26 ‰. The CIE interval contained in the lagoonal and shallow marine units reveals an extremely pronounced (compared with other PETM records worldwide) Apectodinium-acme (70-98% of dinocyst assemblage), sometimes accompanied by Pediastrum-blooms (fresh water algae). As mentioned by Gruas-Cavagnetto (1974), dinoflagellate assemblages from the Dieppe-Hampshire and Paris Basins "Sparnacian" do not contain the key species Apectodinium augustum (nominate species of the A. augustum zone corresponding the PETM-interval worldwide, Crouch et al., 2001), whereas it is present in the northern Belgian Basin Tienen Fm (De Coninck, 1975, 1999) and is coeval there with the CIE and Apectodinium-acme interval (Steurbaut et al., 2000, 2003). However, our calibration of the Apectodinium-acme to the CIE in the Dieppe-Hampshire and Paris Basins suggests its attribution to the A. augustum zone. As it was previously noted from southern England (Powell et al., 1996), the absence of species A. augustum in the Anglo-Paris Basin may be explained by its restriction to more offshore conditions. In localities studied here dinocyst assemblages dominated by Apectodinium spp. are characterized by a significant number of longer (compared to the holotype) specimens of A. parvum, which could represent an ecological onshore substitute of species A. augustum during the PETM.