L'Aptien-Albien à Wissant et au tunnel sous la Manche à Sangatte (France) : la transgression éocrétacée au nord du bassin de Paris

Abstract

Introduction - The Lower Greensands and Gault at Wissant were considered by d'Orbigny (1842) as reference deposits for creation of the Albian stage, complementing the Aube type-region. The lithology and palaeontology of the Albian at Wissant have since been precisely described (see history and characteristics of the Gault and Greensands in Robaszynski and Amédro, 1986). This is not, however, the case for the beds beneath the Albian Greensands, i.e. the Aptian and Wealden facies. These beds were partially exposed after the 1971 and 1975 storms that removed the beach sands covering the foreshore substratum. A composite lithological succession of approximately 14 m was established for the Late Aptian (" Gargasian " and " Clansayesian ") alone, with the base of the Aptian marine formations and the contact with the Wealden continental facies remaining unknown. Road works along the D 940 road to the soutz of Wissant and a cored borehole at the site of the Channel-tunnel access shaft have provided new data concerning the Aptian and its transition with the Wealden facies. These data make it possible to specify the processes prevailing at the beginning of the Cretaceous transgression along the northern edge of the Boulonnais region. The Aptian-Albian in borehole F 86-7 W Sangatte Borehoie F 86-7, sited 57 m east of the Channel-tunnel access shaft at Sangatte, intersected Turonian chalk pro parte and Cenomanian rocks (cf. Amédro and Robaszynski, 1990), then Albian-Aptian and Wealden facies before finishing in the Palaeozoic at a depth of 115.30 m. The Saint-Pô Formation (or Gault) (n 6P), Middle-Late Albian in age, was intersected over 12.55 m between 87.55 m and 100.10 m. It is composed essentially of grey to black clay and contains four phosphatic interbeds: P6 at 90.90 m, P5 at 92.80 m, P4 at 94.48 m and P3 at 100.05 m. The Gardes Formation (or " Greensands ") (n 6G), Early Albian in age, was intersected over 2.45 m between 100.10 m and 102.55 m. It consists of sands that are gritty in places and contains the phosphatic beds P2 at 100.10 m and P1 at 101.07 m. The Wissant (n 5W) and Verlincthun (n 5V) formations and the Cat-Cornu (n 5C) Horizon, Aptian in age, were intersected for the first time in their entirety with respective thicknesses of 1.45 m, 12.10 m and 0.05 m. They consist of argillaceous-glauconitic sands and glauconitic deposits containing black gravel in the lower part. Beneath the Verlincthun Formation, the Cat-Cornu Horizon is made up of phosphatic nodules coated with glauconite. The underlying Wealden facies (1.10 m) are mainly represented by white sands and overlie Palaeozoic limestone. The Wealden facies - Aptian contact in the Fart quarry at Wissant The embankments cut during restructuring of the D 940 road provide a 6 m section where the contact between the Wealden continental facies and the Aptian marine formations is well exposed. The Verlincthun Formation is exposed over 4.5 m with arenaceous clay interbedded with glauconitic deposits. The underlying Cat-Cornu Horizon contains phosphatic nodules with Early Aptian ammonites. This horizon overlies a unit of plastic clay and sands representing the Wealden facies, which an auger borehole intersected for 26.50 m without reaching the base. Data from tise new Sangatte and Wissant sections 1. Palaeozoic basement - Borehole F 86-7 intersected the Palaeozoic basement at level -98 m NGF (French datum). This confirms the presence of a N-S-trending high in the Palaeozoic basement, extending at least as far as Marquise (Destombes and Destombes, 1963). 2. Wealden facies - The N-S-trending high of the Palaeozoic basement also corresponds to a decrease in thickness of the Weaiden facies: borehole F 86-7 only intersected 1.10 m whereas the borehole in the Fart quarry intersected at least 26 m. This data is in une with observations made at the beginning of the century by Olry (1904) who recorded 66.50 m ofWeaiden facies in a borehole near the church at Wissant. 3. Aptian - a. Definition of the Cat-Cornu Horizon (Early Aptian). Amédro and Mania (1976) defined the Cat-Cornu " Formation " in southern Boulonnais for a layer of phosphatic nodules with Early Aptian or " Bedoulian " ammonites. It seems to grade into the glauconitic sandstone intersected by he Wissant borehole in northern Boulonnais described by 0lry (1904). The new Wissant road section shows that it is in fact the same bed of nodules as in southern Boulonnais. It is thus more appropriate to name this layer the Cat-Cornu " Horizon " rather than " Formation " considering it is only a few centimetres thick. b. The age of oyster-bearing (Ostrea leymeriei) clay of the Verlincthun Formation is now known due to the discovery at the base, of the ammonite Parahoplites nutfieldiensis that dates the middle part as Late Aptian. c. The thickness of the Verlincthun Formation seems to be relatively homogeneous throughout the Boulonnais and varies between 8 and 15 m. From a lithology standpoint, the outcrop along the Wissant road is excellent for describing the lower part of the Verlincthun Formation. A sandstone interval above the oyster-bearing clay has also been shown to continue as far as Wissant bay. 4. Albian - The major feature is the absence of the Lottinghen Formation (end Albian = " Vraconian " auct.) in borehoie F 86-7. This could be the result of local erosion related to tise influence of the Quénocs structure, offshore Cap Blanc-Nez, the activity of which began at tise Albian-Cenomanian boundary (cf. Amédro, 1994). The " Boulonnais Gulf " during the Early Cretaceous Compared to previously published maps (Caulier, 1974; Amédro 1984, 1985; Amédro and Robaszynsks 1987), the new data make it possible to detail and modify several palaeogeographic boundaries (Fig. 5). 5A - Early Aptian: Cat-Cornu Horizon = n 5C This is the first evidence of the marine transgression reflecting the beginning of a 2nd-order positive eustatic event (Jacquin et al., in press). In the Boulonnais, this phase is only recognized at Sangatte and Wissant in the north and at Nesles in the south. 5B - Late Aptian: Verlincthun and Wissant formations = n 5V After a regression at the beginning of tise Late Aptian, tise sea extensively invaded the lower Boulonnais under coastal conditions, as indicated by the presence of sands and numerous oysterbearing layers. A short regressive period marked by white sands was followed, at the end Aptian, by a further transgressive pulse marked by the abundant ammonites in the Wissant Formation. 5C - Early Albian: Gardes Formation or " Greensands " = n 6G Compared to the previous situation, the sea extended slightly farther southwards (Desvres) and eastwards (Lottinghsen). 5D - Middle and Late Albian: Saint-Pô and Lottinghen Formations =n 6P Tise sea extended well beyond the Boulonnais Gulf and covered almost all of the N 110°-130°-trending Artois high. Conclusions - Tise new Sangatte and Wissant sections provide important data concerning the composition of the Aptian, including conclusive factors for reconstructing the palaeogeograpisic limits of the extension of Early and Late Aptian seas in the northwest of the Paris Basin.