0012-9402/06/020157-180012-9402/06/02■■■-■■ Eclogae geol. Helv. 99 (2006) 157–174 DOI 10.1007/s00015-006-1188-7 Birkhäuser Verlag, Basel, 2006 Biostratigraphy, sedimentology and sequence stratigraphy of the latest Hauterivian – Early Barremian drowning episode of the Northern Tethyan margin (Altmann Member, Helvetic nappes, Switzerland) STÉPHANE BODIN1*,ALEXIS GODET1,JEAN VERMEULEN2,PASCAL LINDER1 & KARL B. FÖLLMI1 Key words: Early Cretaceous, platform drowning, phosphatization, condensation, heterozoan, Northern Tethyan margin ABSTRACT RESUME During the Early Cretaceous, major palaeoceanographic changes are mirrored Durant le Crétacé inférieur, les changements paléoceanographiques sont enre- on the northern Tethyan carbonate platform by changes in the carbonate fac- gistrés sur la marge nord-Téthysienne par des fluctuations du mode de pro- tory and by platform drowning. The Altmann Member of the central Euro- duction des carbonates ainsi que des épisodes d’ennoiement de plate-forme. pean, northern Alpine Helvetic thrust and fold belt, contains the sedimentary Le Membre d’Altmann, qui affleure en Europe centrale, dans les nappes plis- record of one of these drowning events which occurred during the Late Hau- sées et charriées du domaine Helvétique, est le résultat d’un de ces ennoie- terivian – Early Barremian. It consists mainly of highly condensed beds, which ments de la plate-forme carbonatée se déroulant durant l’Hauterivien tardif et are rich in glaucony and phosphates. The Altmann Member was hitherto only le Barrémien précoce. Principalement constitué de couches fortement conden- poorly dated. New ammonite findings and a re-evaluation of existing am- sées riches en glauconie et phosphate, le Membre d’Altmann, qui peut être monite fauna allow to precisely date this drowning episode, starting in the relié à des changements paléocéanographiques majeurs durant le Crétacé infé- Pseudothurmannia seitzi biozone (latest Hauterivian) and lasting until the rieur, n’a pour l’instant été que faiblement daté. De nouvelles datations bios- Coronites darsi biozone (latest Early Barremian). These new age dates, cou- tratigraphiques basées sur des ammonites nouvellement trouvées, ainsi que pled with sequence stratigraphic interpretations allow to better understand the sur une réévaluation de celles précédemment publiées, permettent de précisé- unfolding of the drowning episode, which proceeded in two stages: The first ment dater cet épisode d’ennoiement de plate-forme. Celui-ci débute dans la stage consisted in an important phase of marine transgression during the latest zone d’ammonites à Pseudothurmannia seitzi (Hauterivien tardif) et se termi- Hauterivian, during which carbonate production was highly reduced; the sec- ne dans la zone à Coronites darsi (dernière zone du Barrémien inférieur). Ces ond stage is recorded during the latest Early Barremian by an important se- nouvelles datations, couplées avec des interprétations en stratigraphie séquen- quence boundary, which is associated with a phosphatized hardground, fol- tielle, permettent de mieux appréhender le déroulement de cet épisode d’en- lowed by rapid sea-level rise and the deposition of outer ramp sediment asso- noiement de plate-forme qui se développe en deux temps : une première étape ciated with the backstepping of the platform. Almost the whole early Barremi- correspond à une importante phase de transgression marine durant l’Hauteri- an is likely to be condensed in this phosphatized hardground, which is vien terminal, accompagnée par une réduction importante de la production associated to a second order sea-level lowstand. The onset of the drowning carbonatée. Une seconde étape est enregistrée durant la fin du Barrémien pré- event is linked to the Faraoni oceanic anoxic event, whereas during the Early coce par la formation d’un fond induré phosphaté associé à un bas niveau Barremian, phosphatization might be the result of important winnowing dur- marin, suivie par une remontée rapide du niveau marin et le dépôt de sédi- ing a period of highly eutrophic conditions. ments de rampe carbonatée externe associé à une forte rétrogradation de la plateforme. De plus, la majeure partie du Barrémien précoce est condensée dans ce fond induré phosphaté associé à un bas niveau marin de second ordre. Le début de cet épisode d’ennoiement de plate-forme semble être lié à l’évé- nement d’anoxie océanique du niveau Faraoni, tandis que durant le Barré- mien inférieur, la phosphatogenèse semble être le résultat d’un important van- nage des fonds océaniques durant une période d’eutrophie marine. 1. Introduction The Early Cretaceous record on the Helvetic platform is char- monites, belemnites, bivalves, brachiopods, sponges and echi- acterized by several drowning episodes expressed in highly noids) and have been the focus of several studies for nearly a condensed and phosphatized beds (Funk et al. 1993; Föllmi et century (e.g., Heim 1910–1916; Goldschmid 1927; Fichter 1934; al. 1994). These beds are relatively rich in macrofossils (am- Hantke 1961; Funk 1971; Briegel 1972; Wyssling 1986; Föllmi 1 Institut de Géologie, Université de Neuchâtel, Rue Emile Argand 11, CP 158, 2009 Neuchâtel, Switzerland 2 Grand Rue, 04330 Barrême, France * Corresponding author. E-mail address: [email protected] The Altmann Member drowning episode 157 Formations N (1) Schwälmis (678.675/195.525) Members Age (2) Nägeliberg (735.750/222.875) (3) Seerenwald (731.750/221.750) Upper SWITZERLAND Schrattenkalk middle Lower Aptian FRANCE (4) Morgenberghorn (626.800/163.025) (5) Pilatus (661.900/203.485) Lower Lower Aptian (6) Rawil (600.835/137.100) Het. Orbitolina Beds (7) Interlaken (631.500/170.900) (8) Oberlänggli (747.625/224.000) ITALY (9) Interrugg (741.875/223.750) Lower middle Upper Barremian - lowermost Aptian (10) Brisi (739.500/224.000) Schrattenkalk (11) Tierwis (742.970/234.730) Schrattenkalk Formation Vosges Black N Rhine Forest Drusberg lower Upper Barremian graben Fm Het. Basel Drusberg Altmann uppermost Hauterivian - Lower Barremian Zürich Upper (11) Upper Hauterivian (2) (9) Kieselkalk Luzern (3) Molasse basin (10)(8) Lidernen upper Lower to lower Upper Hauterivian Jura Helvetic Neuchâtel (5) nappes Bern (1) Chur Lower Lower Hauterivian Kieselkalk (7) Kieselkalk Formation Heteroz. Lausanne (4) Gemsmättli upper Lower Valanginian - lowermost Hauterivian Pré-Alpes(6) Alps Betliskalk Lower Valanginian Betlis Fm Sion Heterozoan Heteroz. Photozoan Photoz. Genève Carbonate production mode Lugano Fig. 2. Synthetic log of Helvetic platform deposits of Valanginian to Early 0 km 100 Po plain Aptian age. Fig. 1. Location of the studied sections and their geographical coordinates, expressed in the Swiss orthogonal coordinate system. The Helvetic realm is terivian; e.g., Cecca et al. 1994; Baudin et al. 1999; Baudin shown in dark grey. 2005) and a phase of enhanced phosphorus input during the Early Barremian (Bodin et al. 2006). Using improved bios- tratigraphy and sedimentology of the Altmann Mb, this study 1989; Kuhn 1996; van de Schootbrugge 2001) due to their im- aims at linking the initiation, unfolding and termination of the portance to date the intercalated sediments poor in biostrati- Altmann Mb drowning event to these major palaeoceano- graphically significant fossils. graphic events. This paper focuses on one of these condensed horizons, the Altmann Member (Mb), which separates the Kieselkalk For- 2. Geological setting mation (Fm; Hauterivian) from the rest of the Drusberg Fm (latest Early to early Late Barremian). Previously, the Alt- The studied sections are located in the Helvetic nappes of mann Mb has not been dated precisely although a latest Hau- Switzerland, situated in the northern part of the Alps and rep- terivian to earliest Barremian age was assumed (e.g., Funk resenting the northern Tethyan margin. This domain was 1969; Briegel 1972; Rick 1985; Wyssling 1986; Funk et al. 1993; thrusted, overthrusted and folded in a northward direction Föllmi et al. 1994). Thus, a first goal of this study is to provide during Alpine orogenesis (Fig. 1). With regards to its age, al- a high-resolution biostratigraphy for the Altmann Mb using most the entire Hauterivian is represented in this tectonic unit newly collected ammonites, as well as a reevaluation of those by the Kieselkalk Fm (Fig. 2), which is divided into three already collected during previous studies and stored in differ- members; the Lower Kieselkalk, the Lidernen and the Upper ent museums. The drowning horizons of the Lower Cretaceous Helvetic platform are of great interest since Föllmi et al. (1994) and Weissert et al. (1998) (see also Kuhn 1996; van de Schoot- Fig. 3. Examples of well-preserved representative ammonites from the Alt- brugge 2001) have postulated links between these episodes mann Mb. A-D: After the study of Wyssling 1986, re-identified, Vorarlberg re- and global palaeoceanographic events, as expressed by the car- gion, Austria (stored in the Museum of Natural history of Dornbirn, Austria); bon-isotope record. The Altmann Mb documents an episode E-F: Newly collected ammonites from the Tierwis section (all the newly col- for which no clear explanation has been yet formulated. On lected ammonites are stored at the Geological Institute of Neuchâtel). A: Balearites aff. mortilleti, specimen n° P9385. B: Kotetishvilia aff. nicklesi, spec- the other hand, major palaeoceanographic changes are present imen n° P9346. C: Metahoplites nodosus, specimen n° P11693. D: Avramidiscus in the Tethyan realm during the Late Hauterivian – Early Bar- cf. gastaldianus, specimen n° P6345. E: Torcapella gr. davydovi, bed Sa 33. F: remian, such as the Faraoni oceanic anoxic event (Late Hau- Emericiceras sp., bed Sa 33. G: Balearites sp. gr. mortilleti, bed Sa 16a. 158 S. Bodin et al. A B C D F 5 cm G 5 cm E 2 cm The Altmann Member drowning episode 159 Table 1. List of identified ammonites from the Location Sample Number Location Sample Number Altmann Mb. This study ETH Zürich Tierwis section Acrioceras sp. 4F 7102 Abrytusites sp. 1 Sa 16a Anahamulina davidsoni 1W 2273 Balearites sp. gr. mortilleti 1 Sa 16a Astieridiscus sp. 1R1 Emericiceras gr. koechlini 1 Sa 33 Avramidiscus aff. intermedius 1F 71007 Emericiceras sp.