MESOZOIC

INTRODUCTION

Geographical setting

The Jura Mountain Chain is located between the Molasse Basin (Swiss Midland), the Rhine and Bresse Valleys, and the Vosges and Black Forest Mountains. The Jura lies in the form of an arc between Chambéry (France), where it touches the Alps, and Schaffhausen (northeastern Switzerland), where it disappears underneath the Molasse. The Jura has a length of about 300 km and a maximal width of about 70 km. The southeasternmost chain is the highest one with the Mont Tendre (1679 m).

The main operation area of the Mesozoic group of the “Palaeontology A16” is in the northwestern part of the Jura Mountain Chain, more precisely in the Ajoie district, located in the northern part of the Canton Jura. The main city of the Ajoie district is Porrentruy, where our office and archives are located. To the south, the Ajoie borders to the Delémont and Franches-Montagnes districts (marked by the Folded Jura Chain), and in the north to France, whereas the latter border is not accentuated with any distinct morphological feature (Fig. 1). The Ajoie is an ample, slightly folded plateau with a mean elevation of about 500 meters. The dominant landscape-forming morphological features are the dry valleys (arroyos), which have been generated by strong karstification of the region. This karstification also caused an important subterranean, fluviatile network, and only three still active small fluviatile valleys (Allaine, Couvatte, Vendeline) pervade the plateau.

Fig.1 Location of Mesozoic sites along the future Transjurane highway. © Paléontologie A16
  

Tab.1: Table showing the Mesozoic sites along the Transjurane highway.
© Paléontologie A16 2006.  

Geological setting

The Jura is a small, young (mainly Late Miocene and Pliocene), arcuate chain, comprising the Plateau or Tabular Jura in the north and the Folded Jura in the south, whereas the Folded Jura forms the main mountain chains with its typical anticlines, synclines, box folds (“Kofferfalten”) and “Kluses”. The Jura Mountain Chain corresponds to a basement high, which originated in the Oligocene between the Tertiary basins of the Molasse Plateau to the southeast, the Rhine Graben to the north, and the Bresse Graben to the west. The Mesozoic cover has been detached from its substratum, along incompetent members of Triassic formations, and moved towards the north for 2 to 25 km respective to the basement (e.g., Laubscher, 1961, 1965, 1974). Thus, no pre-Triassic strata crop out in the Jura. To the southwest, the Jura folds cross the Molasse Basin and join up with the most westerly folds of the French Subalpine chains (Trümpy, 1980).

The Jura Mountains are subdivided in two distinct structural units, the Folded Jura in the south and the Plateau or Tabular Jura in the north. The Plateau or Tabular Jura of Northern Switzerland is part of the Mesozoic cover of the crystalline massifs Black Forest (Germany) and Vosges (France). It consists of rhomb-shaped slabs of unfolded, subhorizontal Mesozoic rocks, separated by narrow dislocation belts. Folding affected the Tabular Jura only near the front of the Folded Jura, where detached folds of the Folded Jura are overthrusted on the Tabular Jura.

The Ajoie belongs almost entirely to the Tabular Jura, and it is only affected by small, low-amplitude folds and Oligocene faults. In the Mont Terri chain, the nothernmost Jura anticline is overthrust on the Ajoie plateau (Fig.2) (see Tschopp, 1960 for geological sections of the Ajoie district).

In the Ajoie district the highway crops almost entirely through Late Jurassic (Middle Oxfordian to Late Kimmeridgian) strata (see also stratigraphy). Actually, excavations are mainly undertaken in Late Kimmeridgian shallow carbonate platform carbonates between the Banné Member (Gygi, 2000) and the Marnes à virgula. This is an interval, which is particularly rich in biolaminites with dinosaur footprints and marly marine layers yielding vertebrate remains. So far, 7 sites with dinosaur footprints on 30 different levels at three time intervals, and several layers of fossiliferous marine marls and limestones have been discovered, partially excavated, and will still be excavated over the next few years. Moreover, from 2005 to 2006, excavations were carried out in Middle to Late Oxfordian strata (St. Ursanne and Vellerat Formations) yielding a rich invertebrate (especially crinoids) fauna.

Methodology

Palaeontological and sedimentological prospection, as well as geological mapping around the future highway course determines the approximate position of the most fossiliferous beds on the highway course (Photo 1).

Photo 1: The study of reference quarries is important to locate fossiliferous sites on the highway course. This figure shows the Courtemaîche—Sur Monteni quarry, a reference section for the upper St. Ursanne and the lower Vellerat Formations (Middle to Late Oxfordian).

Afterwards the exact position is localised during a phase called “sondages” using a shovel excavator (Photo 2).

Photo 2: Archaeological and palaeontological prospecting along the future course of the highway near Bure.

If significant findings are made, an excavation can be envisaged. These excavations are carried out well before the construction of the highway starts. Nevertheless, important sites or discoveries are sometimes made during the construction of the highway. In this case a suitable solution for an emergency excavation has to be worked out with the highway engineers in charge.