MESOZOIC

STRATIGRAPHY

Stratigraphy

For a long time, the stratigraphy of the Late Jurassic of the Ajoie district has only insufficiently been understood. The main reasons were the bad outcrop conditions due to the flat plateau morphology, where outcrops can only be found in quarries or street and railway outcrops (Gygi, 2000a). The construction of the Transjurane highway and associated quarrying at several localities is thus a unique possibility to study not only the palaeontology, but also the stratigraphy of the Late Jurassic of northwestern Switzerland.

Lithostratigraphy

The lithostratigraphy is mainly based on Gygi (1995, 2000a, b) but older synonyms are still partially used by geotechnical offices. At the moment, excavations are mainly carried out in the Middle to Late Oxfordian Vellerat Formation, the Early Kimmeridgian Courgenay Formation, and the Kimmeridgian Reuchenette Formation. Besides the Banné Member and the poorly defined Virgula Member, the Reuchenette Formation lacks any further official subdivision.

Meanwhile, the “Virgula Member” of Gygi (2000a, b) has also been named “northern Virgula Marls” (Jank et al., 2006a), “Virgula Marls” (Jank et al., 2006b), and “lower Virgula Member” (Colombié & Rameil, 2007). Until this member is clearly defined, we use the term “Marnes à virgula”. Moreover, Jank et al. (2005, 2006a, b) recently introduced several lithological terms such as “Nerinean limestones”, “Coral Limestones” or “Nautilidenschichten” in order to better characterize the Reuchenette formation. This inofficial terminology is currently also applied by the Palaeontology A16.

Nonetheless, the Palaeontology A16 aims to re-describe and re-define the lithostratigraphic framework, and to establish an official terminology for the Late Jurassic of northwestern Switzerland.

Prior to the establishment of the Palaeontology A16, almost no ammonites have been known from the Late Jurassic of the Ajoie district. The biostratigraphic frame has only been poorly known, and the sediments above the Marnes à virgula were thought to be Tithonian in age. However, since the excavations of the Palaeontology A16 started, many ammonites have been discovered. These ammonites allowed to establish a well-defined biostratigraphical frame for the upper Reuchenette Formation of northwestern Switzerland (Jank et al., 2005; Jank et al., 2006a, b). This also showed that no Tithonian (“Portlandian”) sediments crop out in the Ajoie, contrary to what was mentioned previously on the geological map of St. Ursanne (Diebold, 1960). The biostratigraphical frame is further confirmed with numerous ostracods, which are frequently encountered in screenwashed samples.

The ammonite zonation currently used are the one of Cariou et al. (1997) for the Oxfordian, and the one of Hantzperque et al. (1997) for the Kimmeridgian. The Kimmeridgian is marked through an increased provincialism with three domains: the “Domaine Téthysien”, the “Domaine Boréal” and the “Biome Franco-Germanique” which lies in between and comprises the study area (Fig. 1).

Further, Gygi (2000a, b, 2001, 2003) recently published important monographies dealing with the perisphinctacean ammonite biostratigraphy of the Late Jurassic of Northern Switzerland, and Jank et al. (2005, 2006a,b) provided a general litho- and biostratigraphical frame for the Reuchenette Formation.

Sequence- and cyclostratigraphic concepts allow high-resolution correlations of stratigraphic sections, once a bio- and/or chronostratigraphic framework is established (Strasser, 1994). Such correlations permit conclusions on the relative importance, interactions, and timing of tectonics, climate change, and sea-level change not only for the long term (millions of year), but also for time periods in the range of tens- to hundreds- of thousands of years (Hillgärtner, 1999). They are also prerequisite for analyzing small-scale lateral facies variations and depositional geometries with high precision (e.g., Samankassou et al., 2003).

A sedimentological, sequential, and cyclostratigraphic framework for the Late Jurassic carbonate platform of the Swiss Jura Mountains has been established by Gygi et al. (1998) and Hug (2003) for the Oxfordian, Colombié (2002), Colombié & Strasser (2005) and Jank et al. (2005, 2006a, b) for the Kimmeridgian, and by Rameil (2005) for the Late Kimmeridgian and Tithonian. Finally, this enabled to put forward a tethyan-to-boreal correlation in the Kimmeridgian (Jank et al. 2005, 2006a, Colombié & Rameil, 2007).