Aptian and Albian Phylloceratids (Ammonoidea) from the Vocontian Basin (SE France)

Bernard Joly (1)

1 Particilier - Particulier

More than 2,200 pyritized ammonites of the Superfamily Phylloceratatoidea have been collected in consonance with the scale of the ammonite zonation of the expanded Marnes Bleues Fm in the Vocontian Basin (SE France). This abundant material allows the description and the figuration (specimens and sutures) of 28 taxa of which 4 are new: • Phylloceras (Hypophylloceras) moriezense SAYN, 1920 subsp. tenuicostulata nov. Diagnosis: variant of P. (H.) moriezense distinguished by a subrectangular section of the whorl and a flattening of the sides less accentuated than on the typical form, with a very fine costulation that descends almost to the middle of the side (early late Aptian); • Phylloceras (Goretophylloceras) vocontium nov. sp. Diagnosis: thick shell, convex sides, rounded and compressed venter, maximum thickness of the whorl near the umbilicus, open umbilicus, constrictions at least at the beginning of the last whorl, oval whorl section, suture line of subgenus Goretophylloceras (mid middle Aptian to earliest Albian); • Salfeldiella (Gyrophyllites) falloti nov. sp. Diagnosis: shell with a finely costulate test, smooth internal mold, open umbilicus, nearly flat sides, rounded venter, subquadratic whorl section, proverse constrictions of the internal mold, radial constrictions in small specimens, a ventral sinus in the largest specimens, suture line like that of the genus Gyrophyllites (late Early Aptian to late Aptian); • Phyllopachyceras brehereti nov. sp. Diagnosis: thick shell, flat sides, venter flat or slightly convex, whorl quadratic in section, suture lines like those of the genus Phyllopachyceras with tetraphyllic spatulate elongated saddles (early mid-Aptian to early late Aptian). Owing to the abundance of the Phylloceratoidea and the variations of specific diversity in 9 fossiliferous levels ranging in age from the late Early Aptian to the end of the Albian it is possible to identify 3 peaks of abundance: • peak 1 during the early mid-Aptian (base of the Martini zone), • peak 2 near the Aptian and Albian boundary, a few meters below the anoxic PAQUIER level, • peak 3 during the latest Albian (Blancheti zone). Of these 3 peaks the first is the most important probably because the Vocontian Basin attained its maximum depth during this time. The peaks appear to be correlated with the maximum flooding intervals of depositional sequences. But high sea-levels were not always favourable for the Phylloceratoidea. The very fossiliferous PAQUIER level, although it took place during a high in sea-level, is characterised by the virtual absence of the deep-water ammonites, e.g. Phylloceratids, Tetragonids and Lytoceratids. This fact is explained by the presence of a deep-water anoxia. Another depletion of Phylloceratoidea in conjonction with a great abundance of Desmoceratid and Mortoniceratid ammonites and benthic fauna (Inoceramids Bivalves) occurred during the early Late Albian. At that time, the Vocontian area appears to have reached a minimal depth resulting from a pause in the rate of subsidence associated with an influx of siliciclastic material that attained the center of the basin. After a last episode during the Early Cenomanian of the Vocontian Basin when the Phylloceratoidea occurred in significant numbers, this group of ammonites shows a drastic decrease in abundance. This evolution is correlated with the filling of the basin from west to east, thus restricting the Phylloceratoidea and the other deep-water ammonites to a small eastern area (Leiostraca area of THOMEL, 1965, 1980).

Mots clés: Ammonites; Phylloceratoidea; Cretaceous; Aptian; Albian; France; Vocontian Basin; Crétacé; Aptien; Albien; bassin vocontien

Domaines: Paléontologie; Stratigraphie

Bernard Joly. Aptian and Albian Phylloceratids (Ammonoidea) from the Vocontian Basin (SE France). Carnets de Géologie / Notebooks on Geology, 2008, CG2008 (M04), pp.1-60. ⟨hal-00325312⟩