Résumé
Dans l'aire stratotypique de l'étage Albien, des affleurements nouveaux ont dévoilé 40 m de coupes dans des portions jusqu'à présent inconnues des Argiles tégulines et des Marnes de Brienne (Albien moyen pars et Albien supérieur). La limite entre les Argiles tégulines et les Marnes de Brienne est définie pour la première fois et placée au sommet d'un banc induré aisément repérable à la fois sur le terrain et dans les sondages ("hardground L'Etape"). Cinq faciès successifs sont définis pour l'ensemble de l'Albien-type, soit du bas vers le haut : (1) banc gréso-glauconieux, (2) argiles silteuses, (3) argiles marneuses et bancs calcaires, (4) marnes argileuses, (5) gaize. À une échelle plus fine, une rythmicité est démontrée dans la sédimentation. Elle se traduit dans les Argiles tégulines par des cycles pluri-décimétriques débutant par des niveaux silteux et limités au sommet par des surfaces perforées. Dans les Marnes de Brienne, les cycles, métriques (sauf dans la gaize où ils sont décimétriques), montrent des alternances d'argile gris foncé et de marne gris pâle annonçant les craies rythmées du Cénomanien. Au plan de la biostratigraphie, cinq zones d'ammonites sont mises en évidence dans l'ensemble des gisements : successivement à Dimorphoplites niobe, D. biplicatus, Mortoniceras (Mortoniceras) pricei, M. (M.) inflatum et M. (Durnovarites) perinflatum. Elles permettent une corrélation directe entre elles et les zones de foraminifères précédemment définies en sondages. Ces données nouvelles sont replacées dans une courte synthèse résumant l'état des connaissances sur l'Albien-type en 1994.
Abstract
In the type area of the Albian stage (Aube, France), seven new outcrops have revealed 40 m sections in hitherto unknown portions of the Argiles tégulines and Marnes de Brienne Formations (Middle Albian pars and Upper Albian). In the present state of knowledge of the Albian stratotype, nearly 60% of the stratotype succession is now documented precisely from outcrops (as opposed to 30% previously). Five successive facies are defined for the whole of the Albian stratotype, working upwards (1) glauconitic sandstone, (2) silty clays, (3) marly clays and limestone beds, (4) clayey marls, (5) gaize. At a finer scale, repetitive patterns can be identified in the sedimentation. This is reflected in the Argiles tégulines by pluri-decimetric cycles that start with silty levels and are bounded at the top by a bored surface. In the Marnes de Brienne, the metric cycles (except the gaize where they are decimetric) exhibit alternating dark grey clay and pale grey marl that herald the rhythmic chalks of the Cenomanian. This cyclic pattern of sedimentation, occurring throughout the 120-130 m of clay-marl facies of the Albian stratotype, is reported here for the first time. The boundary between the Argiles tégulines and the Marnes de Brienne is defined for the first time, on the basis of lithological and sedimentological arguments, and placed at the top of an indurated bed (cailed here "hardground L'Etape") that is readily identifiable both in the field and from borehole samples. The boundary between the Marnes de Brienne containing gaize facies and the Chalk, i.e. the Albian/Cenomanian boundary, is located to within metres. By contrast, the lower boundary of the Albian remains difficult to pinpoint in the Aube. The oldest ammonite-yielding strata correspond to the glauconitic sandstone facies (1), located at the top of the Sables verts de l'Aube Formation and dated to the Hypacanthoplites milletioides zone. However, the age of the greater part of the Sables verts de l'Aube remains uncertain. In terms of biostratigraphy, five ammonite zones (Interval-Zone = I-Z) have been recognized in the new out-crops: Dimorphoplites niobe I-Z, Dimorphoplites biplicatus I-Z, Mortoniceras (Mortoniceras) pricei I-Z, Mortoniceras (Mortoniceras) inflatum I-Z and Mortoniceras (Durnovarites) perinflatum I-Z. This is the first report of them in the Albian stratotype. All told, fifteen ammonite zones are now documented in the Albian type of the twenty defined in the whole of the Paris Basin (including the Boulonnais and Kent). Among the foraminifera zones, which were previously defined in the Aube area mainly from borehole samples, seven are recognized in the new outcrops: Citharinella chapmani zone, Citharinella karreri zone, Citharinella aff. pinnaeformis zone, Citharinella pinnaeforrnis zone, Planularia complanata gracilistriata zone, Valvulineria angulata zone, Rotalipora appenninica zone. The first six have been successfully correlated with the ammonite zones. In total, ten of the fifteen foraminifera zones currently accepted in the Albian stratotype are now directly correlated with ammonite zones. The boundary between the Argiles tégulines and the Marnes de Brienne Formations coincides with an easily identifiable biostratigraphic event: base of the Mortoniceras (Mortoniceras) inflatum I-Z for ammonites, base of the Valvulineria angulata zone for foraminifera. Finally, figure 6 resumes the state of knowledge of the Albian stratotype in 1994 (relative position of old boreholes, old and new sections; lithology; biostratigraphy.
Dernière mise à jour le 29.07.2015