Biostratigraphie et paléoenvironnement des couches de passage Kimméridgien/Tithonien du Boulonnais (Pas-de-Calais) : nouvelles données paléontologiques (ammonites), organisation séquentielle et contenu en matière organique

Abstract

A recent multiparametric approach based on biostratigraphy, analysis of organic matter and sequential interpretation was applied to the Upper Kimmeridgian/Tithonian deposits of the Boulonnais (northwestern France) (Herbin et al. , 1993a, 1994; Proust et al. , 1993b). The succession was studied along the 20-km long coastalcliff section between Boulogne-sur-Mer and Cap Gris-Nez - Pointe de Courte-Dune (fig. 1). The deposits are a mixed siliciclastic/carbonate system with typical offshore sand bodies (Grès de Châtillon, Grès de la Crèche). They represent the most proximal facies of Kimmeridgian/Tithonian age exposed in northwestern Europe and are related to the vicinity of the London-Brabant Massif. Biostiatigraphy. The biostratigraphy of the section has been revised thanks to new ammonite finds, to the review of earlier mentioned and/or described ammonites, and to recent progress in Kimmeridgian and Tithonian biozonation (Cope, 1967, 1980; Hantzpergue, 1989). The whole section ranges from (fig. 2) Upper Kimmeridgian [Mutabilis Zone (Lallierianum Subzone), Eudoxux Zone (Orthocera, Caletanum, Contejeani subzones) and Autissiodorensis Zone] to Tithonian (Gigas-Elegans Zone to Kerberus Zone). At the top of the succession, lacustrine and lagoonal deposits correspond to the so-called " Purbeck Beds " in England. Two sections in the Argiles de Châtillon Formation were investigated in detail (fig. 3), north of Audresselles (at Pointe du Nid de Corbet and at Cran aux Oeufs - Cran Barbier) (fig. 1). Previously, the Kimmeridgian/Tithonian boundary was not accurately located and thought to be at the top of the Argiles de Châtillon, or within this formation at an ill-defined level. The present study precisely locates the Kimmeridgian/Tithonian boundary in the middle of the formation (fig. 3). The Argiles de Châtillon may be divided in two parts : 1 -The Lower Member (8 m) is composed of massive, black, pyritic mudstone with biodetrital, decimetrescale intercalations. At their base is a disconformity, and above are few centimetres of mudstone enclosing a rich ammonite fauna with common Propectinatites sp. and rarer Aulacostephanus autissiodorensis (Cott.), Aulacostephanus volgensis (Vischn), Tolvericeras (T.) murogense Hantz., Aspidoceras catalaunicum (Loriol). This fauna indicates the Autissiodorensis Zone (Autissiodorensis Subzone). Subboreal Perisphinctids (Propectinatites), known until now only from the English Autissiodorensis Zone, coexist with southern West-European Platform species (Tolvericeras (T.) murogense, Aspidoceras). The predominance of ammonites with a subserpenticone morphology is noteworthy, e.g. Propectinatites (evolute coiling, slow whorl-growing in height and thickness) in this level, interpreted as a transgressive bed overlying a discontinuity through facies analysis. Two limestone beds (15-20 cm each) are present in the middle of this member, the lower one with common Aspidoceras catalaunicum (Loriol) showing a globulous morphology, very different from the subserpenticone ammonites of the base. At the top of the member, a sandy limestone bed (50 cm) contains Au!acostephanus autissiodorensis (Cott.), Gravesia lafauriana Hantz., Tolvericeras (T.) n. sp., Aspidoceras catalaunicum (Loriol) (Autissiodorensis Zone, Autissiodorensis Subzone). Ammonites with a globulous morphology, evolute and with thick whorls, are present whereas subserpenticone ammonites are missing. This lower member belongs to the Autissiodorensis Zone (Autissiodorensis Subzone). 2 - The Upper Member (11 to 15 m) consists of black shale with coquina beds. At its base is a disconformity, and above it are a few centimetres of phosphatic mudstone with common Pectinatites (Arkellites) sp., and Propectinatites sp. and rarer fragmentary Gravesia cf. gigas (Zieten). Ammonites with subserpenticone morphology predominate in this level interpreted as a transgressive bed. The shales of this upper member contains Pectinatites (Arkellites) bleicheri (Loriol) (subserpenticone morphology) whereas Gravesia gigas (globulous morphology) is common at the top of the Argiles de Châtillon. The Upper Member belongs to the Gigas-Elegans Zone, the basal zone of the Tithonian. The upper part of the Autissiodorensis Zone (Irius Subzone) is missing and perhaps also the basal part of the Gigas Zone (species such as Tolvericeras (Pseudogravesia) hahni Hantz. have not been found). The Kimmeridgian/Tithonian boundary is located within the Argiles de Châtillon at the boundary between the two members of this formation, and is represented by a hiatus and disconformity. Depositional environments and sequential organization. The Kimmeridgian/Tithonian rocks of the Boulonnais were deposited in a homoclinal ramp setting (Proust et al., 1993b). The main facies and inferred depositional palaeoenvironments from bottom to top are: Grès de Châtillon (uppermost part). Medium- to coarse-grained trough cross-bedded bioclastic sandstone with some mud couplets and bundles, interpreted as estuarine tidal sand waves. The transition toward the Argiles de Châtillon presents coarsening upward, bioturbated sandy shale and sandy mudstone-wackestone, with some drift wood, phosphate and glauconite. It is interpreted as deepening-upward inner- to middle-ramp deposits in a lower shoreface environment. A ravinement surface separates them from the lower coarse-grained sandstone. Argiles de Châtillon. Lower Member (beds 1 to 16). It begins with an ammonite-rich bed (n° 1). This Lower Member is composed of thickening and coarsening-upward decimetric alternations of dark-grey massive shale and hummocky cross-stratified (HCS) bioclastic mudstone-wackestone, truncated by a composite sandy limestone bed along a downward shift surface. The latter is reworked in its middle part by an erosion surface and at its top by a thoroughly bioturbated horizon, which indicates the maximum flooding surface hiatus. The Lower Member corresponds to middle-ramp deposits. Upper Member (beds 17 to 21). It also begins with an ammonite-rich bed (n° 17) corresponding to a maximum palaeodepth. It consists of thickening and coarsening upward microlaminated organic-rich shale and HCS bioclastic sandy limestone, evolving from lower offshore to upper offshore palaeoenvironments (outer ramp). Its upper part is sharply truncated by a thicker limestone bed that predates deeper marine conditions. Upper Member (beds 22 to 23). The rocks are very similar to those observed in the upper part of the Lower Member. Upper Member (bed 24). Same facies as in the Lower Member. The upper part is enriched with sand, and wave-induced lenticular and flaser bedding, indicative of a strongly shallowing upward trend from storm-dominated middle-ramp to inner-ramp fair-weather wave-dominated deposits. Grès de la Crèche (lower part). Thick bedded, trough cross-bedded, thoroughly bioturbated, medium- to coarse-grained sandstone. The sharp basal contact with bed 24 is interpreted as a downward shift surface formed by the basinward shift of the wave base razor. The palaeoenvironmental evolution through time shows two superposed progradational multi-event sequences each beginning with a transgressive event (beds 1, 17). Maximum flooding in the section is located at the base of the Upper Member and corresponds to outer ramp deposits. Organic-matter distribution. The distribution of organic-matter was analysed. Two main organic-rich intervals exist in the whole section. The lower and richer one is in the Argiles de Châtillon, and the upper one in the Argiles de la Crèche (Herbin et al., 1993a, 1994). In the Argiles de Châtillon, the organic matter content is varied through the section (fig. 4). At the base of the Autissiodorensis Zone, just above the disconformity, the TOC reaches 3 to 7% with hydrogen indices typical of type II origin (IH up to 600 mg HC/g TOC). But, this thin accumulation (20 cm) abruptly disappears upsection with the TOC content falling to 2% on average, after which the TOC content shows a progressive upward decrease to less than 1%. The Lower Member of the Argiles de Châtillon belonging to the Autissiodorensis Zone is consequently characterized by a general loss in quantity (TOC petroleum potential) and quality of organic matter. In the Upper Member of the Argiles de Châtillon, corresponding to the Gigas-Elegans Zone, the distribution of organic matter shows a gaussian distribution, with a positive trend up to maximum enrichment of about 9% TOC, extending over 5 m, then a large trend with a minimum toward the base of the Grès de la Crèche spreads over about 10 m. The environment of the Argiles de Châtillon generally corresponds to a shallow shelf below the storm-wave base, with low general energy conditions and evidence of high-energy environments corresponding to the shell beds (Fürsich and Oschmann, 1986). Rare and thin distal storm beds occur in the strata with maximum organic content. More common and thicker beds generally occur in the parts of the succession containing little organic matter. This organic-matter distribution may be compared with that of rock of the same age in Great Britain (basin zone). Similar enrichment occurs in the same stratigraphical interval, at the base of the Autissiodorensis Zone, in Dorset, Yorkshire (Herbin et al., 1991, 1993a,b), Southern England and the Wash area (Gallois, 1979). They correspond in Great Britain to the top of the second organic-rich belt (Herbin et al., 1993a, 1994), which occurs from the upper part of the Eudoxus Zone to the lower part of the Autissiodorensis Zone. In the Boulonnais, the top of the Eudoxus Zone corresponds to a proximal facies, too much oxygenazid and devoid of organic matter. As a result of the overal deepening, shown by the base of the Autissiodorensis Zone, the Boulonnais shelf was subjected to the same dysaerobic environment as Dorset and Yorkshire, and recovered the organic signal previously existing in the basin. However these last references to the second organic-rich belt very rapidly disappeared with time, both in the distal (Dorset, Yorkshire) and in the proximal areas (Boulonnais). Consequently, although the transgression was much greater during the Autissiodorensis Zone than during the Eudoxus Zone, - -the preservation of organic matter was less effective during the Autissiodorensis Zone, regardless of the palaeodepth in the basin as well as on the shelf. In .the Boulonnais, above the disconformity corresponding to the upper part of the Autissiodorensis Zone which is missing, the transgression still was a major event in the Gigas-Elegans Zone, with deposition of microlaminated organic shale corresponding to a deep ramp floor. This coincides with great accumulation of organic matter [third organic rich beit in Dorset and Yorkshire (Herbin et al., 1993a, 1994)]. This variation in time of organic-matter content shows that several parameters, such as physiography, palaeodepth and also climate controled organic-matter quality and primary-productivity variations (Herbin et al., 1993a, 1994). Influence of palaeodepth on ammonite distribution. Confrontation of shell morphology in ammonite populations with relative sea-level fluctuations has become more common frequent these last years (El Hariri et al., 1992; Hantzpergue, 1991, 1993; Marchand, 1993...). Ammonite populations in the Argiles de Châtillon shows a real correlation between palaeodepth and sheil morphology. Within this formation, two major transgressive phases have been deduced from sedimentological study and the distribution of organic matter. The first, at the base of Argiles de Châtillon, corresponds to the beginning of the Autissiodorensis Zone; the second one at the base of the second member, corresponds to the lower part of the Gigas-Elegans Zone. In both levels ammonites with subserpenticone shells dominate (Propectinatites then Propectinatites and Pectinatites) (fig. 3). They originated from subboreal seas, particularly from Dorset, into the basin axis where they are abundant and diversified. Above these transgressive levels, palaeodepth decreases and West-European platform ammonites with globulous morphology (Aspidoceras, Gravesia) replaced the former ones. Subboreal species with subserpenticone morphology invaded the West-European Platform during transgressive phases, but became rarer when palaeodepth conditions became unfavourable for them (Geyssant, 1993).