Les craies à silex du Turonien supérieur au Santonien du Boulonnais (France) au regard de la stratigraphie événementielle. Comparaison avec le Kent (U.K.)

Abstract

In the previous edition of the 1:50,000-scale Marquise geological map, the Upper Cretaceous white flint-bearing chalk - accessible on the northern glacis of the Boulonnais - was mapped as an "Upper Turonian to Senonian" unit of uniform lithology. The distinction between Upper Turonian and Coniacian was based on presence of (a) Micraster "breviporus Agassiz" (= leskei Desmoulins) and Holaster planus Mantell, and (b) Micraster cor testudinarium Goldfuss associated with Inoceramus involutus J. de C. Sowerby (J.-P. Destombes in Hatrival coord., 1971). Although this distinction is still generally valid, recent research carried out on both sides of the Strait of Dover has revealed lithological and bio-ecological marker beds throughout the Turonian-Coniacian-Senonian chalk unit. Thus 26 marker beds have been described over a thickness of about 180 m from the basal Turonian to the upper part of the Santonian, with 21 of these having been recognized in the Boulonnais. Turonian to Santonian events Four formations were described by Robaszynski et al. (1980) in the Turonian to Santonian chalk of the Boulonnais, corresponding to successive facies within the chalk succession. These, from the base up, are (cf. Fig. 1): - Grand Blanc-Nez Formation (nodular chalk); - Mottelettes Formation (subnodular marly chalk); - Guet Formation (white chalk with rare flint); - Caffiers Formation (white chalk with abundant flint). Since 1983, however, studies carried out at first in Germany by Ernst et al. (1983), then in England by Mortimore (1983, 1986, 1987), Robinson (1986), Jenkyns et al. (1994) and Gale (1996), have revealed the wide geographic extent of specific lithological horizons such as marl beds and hardgrounds as well as bio-horizons characterized by the brief abundance of a certain number of fossils. The concept of lithological or ecological "events" (or "litho-ecoevents" as termed by the Anglo-Saxon authors) has been applied successfully by Mortimore and Pomerol (1987), Gale (1996), Amédro et al. (1997) and Amédro and Robaszynski (in press, b) to obtain precise correlations over large distances in the Turonian chalk of the Anglo-Paris Basin. Most of the marker beds reflect events that affected large areas of the Chalk Sea. Several marl beds, for example, corres-pond to the diagenesis of volcanic ash and may therefore be considered as reflecting "instant" deposition (Vanderaveroet et al., 2000). An additional advantage of this method is that one can divide the stratigraphic succession into a set of isochronous units from which it is possible to assess the possible diachronism of formations from one section to another. Thus eight units, T1 to T8, all bounded by marl beds, have been defined in the Turonian by Amédro and Robaszynski (in press, b). Upward from the base of the Turonian, the succession of the litho-ecoevents recognized in Kent is as follows (events not been identified in the Boulonnais, generally due to gaps in the observations, are indicated by square brackets). Lower Turonian - Meads Marl 4 (Mortimore, 1986), a centimetre-thick bed commonly hollowed out in the sections due to weathering, immediately overlain by hardground containing Sciponoceras from the Cenomanian; - a bed with abundant inoceramus of the group Mytiloides (Mortimore, 1986); thickness: 2 to 4 m; - Lulworth Marl (Gale, 1996), a thin dark marl bed overlain by a limonitic hardground. Middle Turonian - Round Down Marl (Robinson, 1986), a decimetre-thick dark marl susceptible to weathering; - New Pit Marls (Mortimore, 1983): two well-marked marl beds separated by 2 to 4 m of chalk with a massive appearance from afar; - a bed with abundant Inoceramus lamarcki Parkinson (Jenkyns et al., 1994); thickness: about 2 m; - Glynde Marls (Mortimore, 1983): a set of thin dark marl beds over a vertical distance of about 3 to 4 m; - Southerham Marls (Mortimore, 1983): a group of four marl beds equidistant from one another, with the first and last being the thicker and more visible (interval thickness: 2 to 3 m). In fresh sections, not yet weathered, the middle beds may not be apparent. Upper Turonian - Caburn Marl (Mortimore, 1983): dark layer, 0.15 m thick, very prone to weathering and consequently generally hollowed out, overlying a fairly consistent layer of flint; - Bridgewick Marls (Mortimore, 1983): a group of two or three marl beds occurring over a vertical distance of 1.5 to 2 m and associated with large nodular flints. Typically, at the regional scale, the Bridgewick Marls cap two composite hardgrounds in which we find the first Sternotaxis plana (Mantell) and that are themselves overlain by several highly fossiliferous hardgrounds with abundant echinids (S. plana, Micraster leskei Desmoulins) and ammonites (Sciponoceras bohemicum bohemicum [Fritsch], Scaphites geinitzii d'Orbigny, Hyphantoceras reussianum [d'Orbigny], Lewesiceras mantelli Wright and Wright, Subprionocyclus neptuni [Geinitz], etc.). The upper hardgrounds are separated by an interval containing many small flint ramifications corresponding to the silicification of Thalassinoides type bioturbations; - Lewes Marl (Mortimore, 1983), a decimetre-thick dark marl susceptible to erosion. It should be noted that the Lewes Marl is not present at Dover, where the succession is more condensed than in Sussex or in the Boulonnais (Gale, 1996). Coniacian - Navigation Hardground (Mortimore, 1983), 50-cm thick; the interval between the Lewes Marl and the Navigation Hardground corresponds to the total vertical distribution of Micraster normanniae Bucaille. From another standpoint, the disappearance of M. leskei and the appearance of M. cortestudinarium Goldfuss occurs in the upper third of the interval (Amédro and Robaszynski, in press b); - Hope Gap Hardground (Mortimore, 1983), several decimetres thick; - Beeding Hardground (Mortimore, 1983), well developed and several decimetres thick; - [East Cliff Marl] (Gale and Smith, 1982): marl layer 2 cm thick; - [Hope Point Marls] (Gale and Smith, 1982): set of five very thin marl layers over a vertical height of 1 m; - [East Cliff semitabular Flint] (Gale and Woodroof, 1981): bed of large flint nodules 15 cm thick which, taken individually, can be as much as 65 cm long. The upper surface of the flints is smooth and the lower surface wavy. Santonian - a first bed with abundant Cladoceramus (Bailey et al., 1984), actually formed by two beds containing abundant Cladoceramus undulatoplicatus (Roemer) and enclosing a layer of flint (Robinson, 1986); - Bedwell's columnar Flint (Rowe, 1900) associated with the second bed with abundant Cladoceramus (Bailey et al., 1984). This double marker bed, which is both lithological and bio-ecological is made up of a layer of nodular flint associated with rare paramoudras and many fragments of C. undulatoplicatus; - an interval with common Conulus albogalerus Leske (Bailey et al., 1983), 2 to 3 m thick; - Whitaker's three inch band (Whitaker, 1865): a continuous layer of nodular flint, 10 cm thick; - [Barrois'Sponge bed] (Barrois, 1876): limonitic hardground locally rich in sponges; - a [bed with abundant Conulus albogalerus] (Rowe, 1900), characterized by an abundance of Conulus over a thickness of about 30 cm; - an interval with common Actinocamax verus Miller (Bailey et al., 1983): 2 m thick, but the species is still present above the following marker bed; - Echinocorys Band (Rowe, 1900): continuous bed with many Echinocorys elevata (Brydone); - Bedwell Line (Rowe, 1900): layer with abundant nodular flint (0.20 to 0.30 m thick). In all, no less than 26 litho-ecoevents make it possible to locate oneself precisely within a chalk succession extending from the base of the Turonian to the Santonian-Campanian boundary in Kent. Even though the density of litho-ecoevents is not as high as in the Cenomanian, where 19 marker beds are recognized over 75 m of the chalk succession on both sides of the Strait of Dover (Amédro and Robaszynski, in press a), fine correlations can now be made from simple field observations. In a second phase, the use of micropaleontology provides useful additional information wherever a doubt exists as to the stratigraphic position. The stage boundaries The adopted stage boundaries respect as far as possible the recommendations put forward by the International Sub-Commission of Cretaceous Stratigraphy following the Brussels Symposium in 1995: - base of the Lower Turonian: appearance of Watinoceras devonense Wright and Kennedy (coincident with the disappearance of Sciponoceras gracile [Shumard] and S. bohemicum anterius Wright and Kennedy); - base of the Middle Turonian: appearance of Collignoniceras woollgari (Mantell); - base of the Upper Turonian: appearance of Romaniceras deverianum (d'Orbigny); - base of the Coniacian: appearance of Cremnoceramus deformis erectus (Meek); - base of the Santonian: appearance of Cladoceramus undulatoplicatus (Roemer). Marker beds in several key sections of the Boulonnais - Mimoyecques: three groups of marl beds in the Middle and Upper Turonian (Southerham, Caburn and Bridgewick marls); - Caffiers: eleven marker beds of the end Cenomanian to Coniacian and the eight lithological units T1 to T8; - Coquelles: two lithological marker beds in the Santonian: Bedwell's and Whitaker's; - St Tricat: three marker beds over 6.5 m of section: Echinocorys elevata (Brydone) Band, Bedwell Line and Actinocamax verus bed. Comparison between the Boulonnais and Kent - Most of the marker beds corresponding to litho-ecoevents defined in southeastern England are similar to those described in northern France. The few exceptions are mainly marker beds that have still to be identified on the French side due to gaps in the observations. - The sedimentary record is, however, thicker in the Boulonnais than in Kent in particular for the Upper Turonian (x 1.8). - The marl layers with bentonite (volcanogenic) are considered as isochrons and delimit eight units, T1 to T8. - The appearance of the first flints and the formation of the nodular facies in the Turonian were diachronous between the Boulonnais and Kent. - The interval between the Bridgewick and Lewes marls passes from 18 m thick at Caffiers to 4.50 m thick at Dover where the condensation is shown by the presence of four hardgrounds corresponding to the Chalk-Rock. - It is possible that certain marl beds in Kent (East Cliff Marl and Hope Point Marls) are lateral equivalents to silicified beds in the Boulonnais. - It would appear that the highest part of the Cretaceous known in the Boulonnais inlier is at St Tricat where the Bedwell Line has been recognized. Conclusions The composition of the "white flint-bearing chalk" of the Upper Turonian to top of the Santonian in the Boulonnais is fairly homogeneous and does not enable useful facies distinctions to be made in the field. In addition, as macrofossils are not very abundant from the Coniacian on, it is necessary to carry out micropaleontological studies of the benthic foraminifera in order to obtain precise stratigraphic information. However, the recognition of 18 litho-ecoevents in the Upper Turonian to Santonian chalk interval of western Europe, of which 13 have been found in the Boulonnais, now provides a precise idea of the stratigraphic position of a small outcrop if one or other of the marker beds can be recognized and characterized. Moreover, these marker beds support correlations over small to medium distances within a same basin such as the Anglo-Paris basin between the Boulonnais and Kent.

Key words: Chalk, Turonian, Coniacian, Santonian, Marker bed, Event stratigraphy, Stratigraphic correlation, Pas-de-Calais, Boulonnais, London Basin, Kent.