Observations à la note de M. Bilotte : Mise au point et données nouvelles sur l'organisation séquentielle, la biostratigraphie et la grade-datation dans le Campanien-Maastrichtien sous-pyrénéen (Ariège, Haute-Garonne), par

Réponse aux observations de P. Souquet et M.J. Fondecave-Wallez par M. Bilotte
P. Souquet et M.J. Fondecave-Wallez
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Dans le Campanien - Maastrichtien du bassin d'avant-pays nord des Pyrénées centrales, les datations numériques obtenues par la méthode de grade-datation appliquée à la lignée de Rosita fornicata s'accordent avec celles qui peuvent être déduites des ammonites sur les sites précis de leurs gisements. Ces grade-datations sont aussi corrélables aux classiques zones de foraminifères planctoniques. Dans le cadre biochronostratigraphique régional ainsi défini des critères faciologiques argumentent un découpage en séquences de dépôt qui paraissent synchrones avec les cycles de la stratigraphie séquentielle débutant à 75, 71, 68 et 67 Ma.


In a recent paper focused on the proposition of a new model to explain the tectonic-sedimentation relationships in foreland basins (Déramond et al., 1993) we have been led to reinterpret the Campanian - Maastrichtian sedimentary successions from the subpyrenean foreland basin (Fig. 1) in terms of sequence stratigraphy and to publish numerical ages given by grade-dating, i. e. a method (Gourinard, 1984) based on time-correlation of the evolution curve of the Rosita fornicata lineage (Fondecave-Wallez, 1988). Our sequence and grade-dating synthesis (Fig. 2) has been recently questioned (Bilotte, 1994), but we set out to prove that this refutation is not justified because it is based on stratigraphic data incorrectly interpreted as evidenced by the review of its major arguments and by the exposition of misunderstood or new data. The ages deduced from the ammonites are not at variance with our sequence and chronostratigraphic schema (Fig. 2). In the western half of the studied area the three groups of ammonite-bearing strata observed (Fig. 2: F1, F2, F3) are emplaced in the deepest part of three of our sequences. That is why we have proposed a rough time-correlation with the three maximum floodings of the equivalent cycles from the global chart (Haq et al., 1987), being this reasoning in agreement with the grade-datings given by the encompassing beds (Fig. 2). For the same fossils other numerical ages have been recently proposed (Bilotte, 1994) by comparison with a chronometric-calibrated scale of ammonite zones (unpublished north-american chart by J.D. Obradovich). The two propositions (with indication of the guide fossils and localities) are as follows: F1 (Baculites leopoliensis; Crabé, Plagne anticline): near 73.5 Ma (us): older than 71.3 Ma (Bilotte, 1994). In the Crabé area the location of the ammonite found north of the Crabé farm, in the northern limb of the anticline, is in agreement with the grade-dating at 73.5 Ma (Fig. 2) given to the south, near La Tuilerie (Fig. 1), by older strata closer to the fold axis; F2 (Hoploscaphites pumilis; Paillon, Saint-Marcet anticline): near 69.5 Ma (us); perhaps older than 71.3 (Bilotte, 1994). The difference here noted can be explained by the stratal arrangement: the age 69.5 Ma refers to the "Paillon siliceous sponge bed" in agreement with two encompassing grade-datings (Fig. 2 and 3: 69.7 Ma below); 68.9 Ma above); 71.3 Ma refers to the ammonite which comes from underlying marlstones in the northern limb of the anticline (near the Larin farm, Fig. 1); F3 (Hoploscaphites constrictus; Saint-Loup, Gensac anticline); near 67.5 Ma (us); between 68 and 65.4 Ma (Bilotte, 1994). In the St-Loup quarry, the location of the ammonite (Lepicard, 1985, Fig. 74) is in agreement with the grade-datings measured near the fossiliferous beds (67.3 Ma) and below (67.7 Ma). If we take into account their location all the ages deduced from the ammonites are consistent with our sequence and grade-dating schema (Fig. 2). 2. In the eastern half of the area two other taxa, older than the previous one, have been mentioned (Bilotte, 1994): Didymoceras stevensoni (age about 75.3 Ma, Richou anticline); Hoplitoplacenticeras marroti (age about 77 - 78 Ma, eastern end of the Dreuilhe anticline). These interesting dates have been used to "raise doubts as to the validity of (our) ages (grade-datings)" and to emphazise "considerable diflerences" up to 6 Ma, but by means of two arguments which are unrealistic: - The age of the fossil-bearing beds F1 (Crabé, Plagne anticline, fig. 1), which agrees with the ammonites of the locality (between 75.3 and 71.3 Ma), is questioned thanks to a worthless comparison (Bilotte, 1994, Fig. 3A) with the ages of the two above mentioned taxa which come from localities situated in other .structural units far to the East. at 9 km (Didymoceras stevensoni, near 75.3 Ma) and at 7.5 km (Hoplitoplacenticeras marroti, near 77 - 78 Ma). - The maximum gap invoked (6 Ma) is a difference artificially calculated between the age (77 - 78 Ma; Bilotte, 1994, Fig. 3A) attributed to the ammonite coming from the eastern end of the Dreuilhe anticline (Fig. 1) - i. e. from series not studied and not reconstructed below our major unconformity at 68 Ma (Fig. 2) - and a grade-dating (72.8 Ma) chosen (Bilotte, 1994, Fig. 3B) in another structural unit located 14 km far to the west-south-west (Péreille beds unconformably overlying the North-pyrenean frontal thrust in the Pech de Foix anticline, Fig. 1 and 2; Déramond et al., 1993, Fig. 14). If the false arguments are ruled out the ages indicated by the ammonites in their localities do not allow to refute our sequence and grade-dating schema (Fig. 2). They can logically take place in our sequence diagram. The grade-datings agree with the planktonic foraminifera biozonation. All the micropaleontological analysis (25 samples) have given numerical ages (grade-datings) based on the Haq's scale (1987) and biozones referenced to the current scale (Robaszynski et al., 1984; Haq et al., 1987). A good correlation has been systematically noted (Déramond, 1993, Fig. 13) as illustrated in the Figure 3 (15 samples precisely localized in order to permit verifications). These combined investigations resulted in numerous constraints allowing biochronostratigraphic correlations in the system of prograding rock units. We point out the following notable and misunderstood data: 1. At the eastern end of the Plagne anticline the Plagne Marls (Plagne 1 wedge, Fig. 2) underlying the Labarre Sandstone gave grade-datings at 73.5 Ma (Crabé), 73.7 and 72.5 Ma (Plagne) in agreement with a microfauna from the Maastrichtian Falsostuarti zone (Fig. 3, samples 4, 5, 6). The same correlation has been noticed (fig. 2) further to the South (Bédeille: 73.7, 73.2, 71.3 Ma) and to the East in the Mas d Azil anticline (Terre Rouge: 73.3, 72.5, 71.2 Ma; Fig. 3, samples 1, 2, 3 - Labouiche: 71.9, 71.4 Ma), and in the Roquefort (l'Herm: 71.3 Ma) and the Pech de Foix (Péreille: 72.8, 72.5 Ma) anticlines. 2. At the western end of the Plagne anticline, the marls are younger (Plagne 2 wedge, Fig. 2) as indicated by grade-datings at 69.3 Ma (Ausseing road), 69.9 Ma (Roquefort quarry) and a microfauna from the Maastrichtian Gansseri zone (Fig. 3, samples 7, 8, 9). Then the overlying Nankin Limestones (Nankin 1 from Bilotte, 1994, Fig. 3A) cannot be considered as older than 71.3 Ma and attributed to the Campanian times (according to the ammonite zonation). These limestones are maastrichtian in age as the Nankin Limestones which, further to the West in the Saint-Marcet anticline (Nankin 2 from Bilotte, 1994, Fig. 3A), equally overlye maastrichtian marls (St-Martory Marls) yielding microfossils from the Gansseri zone and consistent grade-datings at 69.7, 68.9 and 68.6 Ma (Fig. 3, Le Paillon, samples 10, 11, 12). Then the distinction between the so-called Campanian Nankin 1 and Maastrichtian Nankin 2 Limestones is a mistake; and the so-called Ausseing and St-Marcet sequences based on this distinction are unrealistic. 3. In the more western Gensac anticline, the marls (St-Loup Fm.) and marlstones (Gensac Fm.) are even younger as confirmed by grade-datings at 67.4 Ma (Gensac), 67.7 and 67.3 Ma (St-Loup quarry) in agreement with microfossils from upper part of the Gansseri zone or the Late Maastrichtian Mayaroensis zone (Fig. 3, samples 13, 14, 15). The micropaleontological biostratigraphy agrees with the grade-datation and the two combined methods allowed regional biochronostratigraphic correlations which led us to refute the sequence organization schema defended by M. Bilotte (1994, Fig. 3A). Facies changes are the basic criteria used for defining the sequences. To claim that the definition of our sequences (Fig. 2) "was deduced from correlations with the table of eustatic cycles compiled by B.U. Haq et al. (1987) using data provided by grade-dating" (Bilotte, 1994, p. 73) denotes a lack of understanding of our investigations wich intented not to illustrate the current eustatic model but to appreciate the disturbances introduced by thrust-tectonics in order to complete our tectono-sedimentary model by the characterization of tectonic systems tracts (Dérumond et al., 1993, table 1). In the marly series (the only facies which can be grade-dated as the Plagne or St-Martory Fm.) the lowstand systems tracts are composed of offshore muds and storm or turbidite grainstones, with a mixture of clastics deriving from the continent (quartz, micas) and the shoreline (bioclasts). In the first sequence (base at 7.5 Ma, Fig. 2) they include lime olistoliths north and close to the North- pyrenean frontal thrust (Bédeille, Péreille, Fig. 2). Such deposits linked to the thrust propagation are the latest one in the series we have identified (Morenci megaturbidite and 6 block slumps episodes, Nalzen area, Fig. 1; Déramond et al., 1993, Fig. 11 and 13) in lime bodies previously interpreted as reefs in situ (Bilotte, 1985). The transgressive and the early highstand systems tracts are made up of interbedded shales and mudstones, locally with ammonites. In the Gansseri zone of the Roquefort quarry they are represented by glauconitic mudstones occurring between two grade-datings (Fig. 3, samples 8, 9) at 69.9 Ma (below) and 68.9 Ma (above) and then related to the maximum flooding dated at 69.5 Ma in the cycle beginning at 71 Ma in the global chart (Haq et al., 1987). In the Campanian - Maastrichtian from the northcentral pyrenean foreland basin, the numerical ages obtained by grade-dating method agrees with those derived from the ammonites if the comparison is made in the precise sites of the datum-points. The grade-datings are also correlatable with the current planktonic foraminifera zones. These biochronostratigraphic correlations and facies criteria support and complete the interpretation previously explained (Déramond et al., 1993; Pyrenean Synthesis, B.R.G.M., in press) concerning a set of four depositional sequences related to the cycles of the global chart (Haq et al., 1987) beginning at 75, 71, 68 and 67 Ma.

Dernière mise à jour le 28.07.2015