Révision lithostratigraphique et biostratigraphique du Kimméridgien de la région havraise (Normandie)

Abstract

The outcrops along the coast at Le Havre have been known since the middle of the l8th century (Bocage de Bléville, 1753). Nevertheless, their study really began 70 years later (La Bèche, 1822; Phillips, 1830; Lesueur, 1843; Dollfuss, 1863 a, b; Lennier ; 1868), since when no other papers were written until the second half of the 20th century (Rioult, 1958, 1961; Ziegler, 1962; Guyader, 1968, 1973). Only a few works have been published during the last 20 years (Rioult in Mégnien, 1980; Rioult and Guyader in Mégnien, 1980; Hantzpergue, 1989), but as a result of a lot of field trips since 1986 it was obviously necessary to restudy the stratigraphic framework. In 1980, Rioult and Guyader (in Mégnien 1980) identifïed six formations in the Kimmeridgian of the Le Havre area. These are, from bottom to top: the Argiles Brunes de Criquebœuf (of the Upper Oxfordian; Guyader 1968), the Calcaires coquilliers (Lennier 1868), the Argiles à Deltoideum delta (Rioult, in Mégnien, 1980), the Bancs de Plomb (Lennier 1868), and the Marnes de Bléville (Rioult, in Mégnien, 1980) comprising lower, middle (so-called Calcaires à Harpagodes [Guyader 1968] and Upper members of the Lower Kimmeridgian and the Argiles d'Octeville (Rioult and Guyader, in Mégnien, 1980) of the Upper Kimmeridgian. The regional dip toward the north and the post-Jurassic erosion do not permit all the formations to be observed in a single outcrop. Eight sections precisely located between Le Havre and Ecqueville (west of Pays de Caux) were necessary to obtain a complete profile of the exposed series (fig. 1). At the Cap de La Hève, the strata are visible over 9 m (from Marnes brunes de Criquebœuf to Calcaires à Harpagodes: fig. 2) about 150 m northwest of the Promenade du Nice Havrais. One to two meters of the lower member of the Marnes de Bléville can be seen at La Corvée (fig. 2), 2 500 m south of the descent of the "Grenouille" and "Tortue" wrecks. At Saint-Andrieux, 750 m south of the same descent (fig. 2), a very discontinous section over 11 m shows: the Calcaires coquilliers, the Argiles à Deltoideum delta, the lower Bancs de Plomb, the upper member of the Marnes de Bléville and the base of the Argiles d'Octeville. At Le Croquet, four sections (base of the rocky beach between Le Croquet and "Grenouille" and "Tortue" for the first, and 350 m south, 150 m south and 150 m north of the Le Croquet descent for the others) show about 5 m from the Bancs de Plomb to the Calcaires à Harpagodes, and 6.50 m, 6.30 m, 3.60 m of the Argiles d'Octeville (fig. 3). At Ecqueville (erroneously called La Brière) (fig. 4), the Argiles d'Octeville constitute 17 to 18 m of the lower cliffs between 700 m and 1 000 m north of the Fond du Val descent. Moreover, the data (Guyader, 1968) in four holes drilled during the 1960s by the Port Autonome du Havre were reexamined. This allows us to determine the detailed lithostratigraphy of the Kimmeridgian in the Le Havre area: a synthetic section is proposed (fig. 5), each formation or member and bed being precisely numbered [fin Arabic numerals (I to XII) for the formations and in Latin numerals f or the members] and described. Here, the thickness of the Kimmeridgian is about 45 m, as apposed to 63 m to 106 m between 35 km and 75 km to the east, 170 m in the Pays de Bray (about 100 km to the northeast), 80 m in the Boulonnais (175 km to the north), 250 m in the Dorset (175 km to the northwest) and 150 m in the North-Aquitaine Basin (350 km to the south). Very clayey, on the whole, with some more limy levels near the base, the lithology is similar to that in the Dorset, even though the iron levels of the lower part of the succession are much more developed in the latter area. The absence of major, coarse detrital, oolithic or reef events differentiates this section from that of the Pays de Bray, Boulonnais and North-Aquitaine, Basin. Sampled, where possible, the ammonites (fig. 5), of marked subboreal affinity (Pictonia, Rasenia at the base of the stage, Sutneria eumela at the top), allow one to recognize four zones of the Kimmeridgian: Baylei Z. (Calcaires coquilliers, Argiles à Deltoideum delta), Cymodoce Z. (Bancs de Plomb, Marnes de Bléville), Mutabilis Z. and Eudoxus Z. (Argiles d'Octeville). The Autissiodorensis Zone, the last of the stage, is absent probably as a result of post-Jurassique erosion rather than due to a lack of deposition. Ten horizons are identified: Baylei H. (Baylei Z.), Cymodoce H., Pseudoeumela H. (Cymodoce Z.), Linealis H., Mutabilis H., Lallierianum H., Schilleri H. (Mutabilis Z.), Orthocera H., Caletanum H., Contejeani H. (Eudoxus Z). The thickness of the Argiles d'Octeville (Upper Kimmeridgian) is twice that previously estimated from the outcrops (c. 35 m instead of ~ 17 m). Therefore, in order to facilitate the understanding and avoid any misinterpretation of the sections, the Formation of Argiles d'Octeville has been re-defined and divided into five members: the lower member of the Argiles du Croquet in the lower part, the upper member of the Argiles du Croquet, the lower member of the Argiles d'Ecqueville, the middle member of the Argiles d'Ecqueville and the upper member of the Argiles d'Ecqueville in the upper part. Lastly, two marker beds were defined: the Banc noduleux à bivalves and the Lumachelles à nanogyres.