Introduction

Les nappes Helvétiques affleurent dans la partie nord des Alpes suisses (Fig. 1) et documentent les restes de la marge nord-Téthysienne. Dans ces unités tectoniques, des carbonates de plate-forme alternent avec des sédiments phosphatés et glauconieux (par exemple, [7,10]). Dans cette note, de nouvelles ammonites appartenant à un de ces intervalles glauconieux (le membre du Chopf) sont figurées ; elles sont attribuées au Barrémien supérieur moyen. Cette importante découverte permet, non seulement de dater l'installation des faciès urgoniens (formation du Schrattenkalk) dans cette partie de la Téthys, mais aussi de discuter la calibration des courbes téthysiennes de référence du δ13C.

Cadre géographique et géologique

Le Barrémien du domaine Helvétique est composé de deux formations : la formation du Drusberg et la formation du Schrattenkalk (Fig. 2). En domaine proximal, la formation du Drusberg peut être divisée en deux membres : le membre d'Altmann et le membre du Drusberg (par exemple, [8,9]). En domaine distal (région de l'Alvier), Briegel [5] adjoint deux autres membres à la formation du Drusberg : le membre du Chopf et le membre d'Hurst. D'après cet auteur, l'âge de ces deux derniers membres serait Barrémien moyen, sans plus de précision.

Sédimentologie et biostratigraphie

Le membre du Chopf a été étudié au lieu dit du Barbielergrat (voir [5] pour une localisation précise), où son épaisseur atteint environ 2 m ; il affleure sous forme de quatre bancs calcaires intercalés entre les marnes du membre du Drusberg et du membre d'Hurst (Fig. 3). La glauconie est présente dans les deux premiers bancs et abondante dans le troisième. Des traces de phosphatogenèse peuvent être également discernées dans les trois derniers bancs. À la base du troisième banc, la présence de nombreux nodules phosphatés et de nombreuses bélemnites est à noter.

Parmi ces nodules, quatre ammonites phosphatées ont été trouvées (Fig. 3). Deux d'entre elles se sont avérées être des Hemihoplitidae sp., tandis que les deux autres sont identifiées comme étant une Ezeiceras aff. janus et une Hemihoplites sp. aff. limentinus (Fig. 4). Cet assemblage d'ammonites est typique de la partie terminale de la zone à Heinzia sayni et de la première moitié de la zone à Gerhardtia sartousiana (Barrémien supérieur moyen [12,13]).

Discussion

De plus, Bodin et al. [3] ont observé que la SIM Ba3 correspond à la transition entre la formation du Drusberg et celle du Schrattenkalk en domaine plus proximal (coupe de Tierwis, région du Säntis). Ainsi, le membre du Chopf doit correspondre en temps à la limite Drusberg–Schrattenkalk en domaine proximal. Par conséquent, en situation distale, l'installation du membre du Schrattenkalk, qui est séparé du membre du Drusberg par le membre d'Hurst, doit être plus jeune qu'en situation proximale. Cette observation souligne la progradation de la plate-forme urgonienne durant le Barrémien supérieur dans le domaine Helvétique (Fig. 5 ; par exemple, [4]). Il est également important de noter que cet âge est cohérent avec l'installation des faciès urgoniens dans les autres parties de la Téthys occidentale, et particulièrement dans le Vercors (par exemple, [2]).

Par ailleurs, Wissler et al. [14] ont publié une courbe haute résolution du δ13C pour cette région, montrant deux excursions positives durant le Barrémien supérieur, le membre du Chopf étant situé exactement au milieu de ces deux excursions (Fig. 6). En considérant cette nouvelle datation, il apparaît que la première excursion est plus âgée que la zone à G. sartousiana. D'après la datation du sommet du membre de l'Altmann par Bodin et al. [3] et la datation conséquente de la base du membre du Drusberg (zone à C. darsi), cette première excursion est datée de la partie inférieure du Barrémien supérieur (zone à H. uhligi–H. sayni).

Conclusions

The Helvetic fold-and thrust belt is situated in the northern part of the central European Alps (Fig. 1) and includes a sedimentary succession which documents the evolution of the northern Tethyan margin during the Mesozoic and Early Tertiary. The Lower Cretaceous comprised in this tectonic zone is composed of an alternation of photozoan and heterozoan platform carbonates and highly condensed phosphatized and glauconitic sediments (e.g., [7,10]). These latter sediments are associated with repetitive drowning episodes, which interfered with the growth of the northern Tethyan carbonate platform. In this paper, we report the discovery of ammonites from one of these glauconitic beds (the Chopf Member), which dates from the middle Late Barremian. This new age date allows us to attribute a maximal age to the installation of the Schrattenkalk Formation (Fm.), which is the equivalent of the Urgonian-type carbonates known from other parts of the Tethyan realm (e.g., [2,12]). Finally, this new age date helps also to refine the dating of δ13C references curves from the Tethyan realm.

In the Helvetic realm, the Barremian stage is characterized by the deposition of two distinct formations: the Drusberg Fm. and the Schrattenkalk Fm. (Fig. 2). In proximal settings, the Drusberg Fm. is divided into two members (Mb.): the Altmann Mb. and the Drusberg Mb. (e.g., [8,9]). In the distal part of the Helvetic platform, and particularly in the Alvier region, Briegel [5] divided the Drusberg Mb. into four members by adding two further members to the top of the Drusberg Mb.: the Chopf Mb. and the Hurst Mb. (Fig. 2). The Chopf Mb. represents a thin and glauconite-rich interval, for which no equivalent exists in the more internal part of the Helvetic shelf, whereas the Hurst Mb. consists of a marly succession that Briegel [5] considered as a distal equivalent of the lower part of the Schrattenkalk Fm. According to this author, these two latter members are dated to the ‘Middle Barremian’, without any further precision.

The Chopf Mb. was logged and sampled at Barbielergrat, 1 km northeast of Alvier, eastern Switzerland (Swiss coordinates: 750.650/220.075; [5,14]). At this locality, this member is composed of four principal beds and reaches a total thickness of approximately 2 m; it is under- and overlain by marly beds corresponding to the Drusberg Mb. and to the Hurst Mb., respectively (Fig. 3). The base of the Chopf Mb. is well-defined and marked by the presence of up to 10-cm-thick Thalassinoides burrows. The basal bed is composed of a spicule-rich limestone, whereas the three following beds are characterized by carbonate rich in intraclasts, echinodermal debris, and isolated phosphatized particles. Glaucony is present in the first two beds, and abundant in the third bed. At the base of the third bed, both phosphatized pebbles and fossils, as well as numerous belemnites, are abundant.

Among these nodules, four well-preserved phosphatized ammonites have been found (Fig. 3). Two of them are identified as Hemihoplitidae sp., whereas the two others are identified as Ezeiceras aff. janus and Hemihoplites sp. aff. limentinus (Fig. 4). This assemblage allows the attribution of the Chopf Mb. to the middle Late Barremian; more precisely, from the latest Heinzia Sayni and the first half of the Gerhardtia sartousiana zones (e.g., [12,13]).

The newly obtained age of the Chopf Mb. allows us to discuss the chronostratigraphic evolution of Late Barremian sediments distribution in the Helvetic realm, and also to refine correlations between δ13C reference curves for this particular section, published by Wissler et al. [14], and other settings in the Tethyan realm.

The presence of glaucony and phosphate within the Chopf Mb. relates its origin to a period of condensation during high sea level, even if subsequent sediment reworking and transport by gravity flow cannot be excluded. This horizon may therefore correspond to a maximum flooding surface (mfs) deposit. In the western Tethyan realm, the presence of a major mfs has been identified in sediments corresponding in age to the middle of the G. sartousiana zone (the Ba3 mfs; [1,2]). Hence, we propose that this horizon corresponds in time to the Ba3 mfs.

Bodin et al. [3] have observed that the transition between the Drusberg Fm. and the Schrattenkalk Fm. in more proximal areas of the Helvetic shelf (Tierwis section, Säntis region) corresponds equally to the Ba3 mfs. Indeed, they dated the boundary between the Altmann Mb. and Drusberg Mb. as middle C. darsi zone and associated it to a mfs (mfs Ba2). The sediments of the Drusberg Mb. are characterized by a regressive trend followed by a transgressive trend, which are attributed to the Ba2 highstand and the Ba3 transgressive systems tract, respectively. On top of the Drusberg Fm., the basal part of the Lower Schrattenkalk Mb. is characterized by a regressive trend that can be associated with the Ba3 highstand systems tract. The Drusberg–Schrattenkalk boundary may thus correspond to the Ba3 mfs.

The Chopf Mb. may therefore represent a distal time-equivalent of the sediments associated with the transition between the Drusberg Fm. and Schrattenkalk Fm. of more proximal areas. Hence, in distal areas of the Helvetic shelf, the onset of the Lower Schrattenkalk Mb. – separated from the Chopf Mb. by the Hurst Mb. – is therefore younger than in more proximal areas, which is related to the time-transgressive progradation of the ‘Urgonian’ platform during the Latest Barremian in the Helvetic realm (Fig. 5; e.g., [4]).

The maximal age obtained here for the onset of the Schrattenkalk Fm. is in good agreement with the onset of the Urgonian facies in other parts of the western Tethyan realm, and especially in the Vercors area (e.g., [2]). This underlines the close relationships between the sedimentary histories of the areas of the northern Tethyan margin, which now are part of France and Switzerland.

Wissler et al. [14] published a high-resolution δ13C curve for the Alvier region, showing two positive shifts for the Late Barremian. The Chopf Mb. is situated exactly in the middle of these two positive shifts (Fig. 6). With the new precise age date for the Chopf Mb., it appears that the first positive shift is older than the G. sartousiana zone. According to the new age dates for the lower and upper boundaries of the Altmann Mb. by Bodin et al. [3] and the corresponding maximal age for the base of the Drusberg Mb. (C. darsi zone), this first shift may be dated as early Late Barremian (H. uhligi–H. sayni zone).

In the Italian Cismon APTICORE drilling, the Late Barremian is also characterized by two positive shifts [6]. In the case these two shifts are correlated with the two excursions observed by Wissler et al. [14] in the Alvier region (Fig. 6), the older shift in Cismon – where precise ammonite stratigraphy is lacking – would also correspond to the early Late Barremian.

The glaucony-rich and phosphate-bearing Chopf Mb. (Helvetic zone, eastern Switzerland) is dated from the Gerhardtia sartousiana ammonite zone (middle Late Barremian) and its formation may be linked to the maximum flooding surface Ba3. This new age date indicates that the overlying Schrattenkalk Fm. – equivalent of Urgonian carbonates in other parts of the Tethyan realm – is younger than the G. sartousiana zone in distal parts of the Helvetic shelf. In inner parts of the shelf, using sequence-stratigraphic arguments, the onset of the Schrattenkalk Fm. dates as G. sartousiana zone. This dichotomy underlines the progradation of the distal Helvetic platform margin during the Late Barremian.