CENOZOIC

INTRODUCTION

Excavations and research along the future Transjurane motorway (A16, Jura/Switzerland)

Fig.1: Cenozoic sites location along the future Transjurane highway (A16). © SAP 2004.
 

Tab.1: Cenozoic sites listing along the future Transjurane motorway (A16). © SAP 2004.
 

The Molasse and the Molasse Basin

The literature attributes to de Saussure (1779-1796) the introduction of the term Molasse. Studer (1825) proposed a first stratigraphic outline in his “Monographie der Molasse” and Merian (1836, 1838) set up a first lithostratigraphic succession that is still used today (Fig.2).

Fig.2: Simplified stratigraphy of the Swiss Molasse Basin (modified from Keller, 1989 and Sommaruga, 1997).
 

Fig.4: Simplified transversal section of the Swiss Molasse Basin from Jura to Prealps (modified from Berger, 1989 and Sommaruga, 1997).  

Subalpine Molasse

Allochtonous series (its origin probably has to be located 50 to 100 km to the South), exclusively constituted by folded and faulted slices. It is adjacent to the Helvetian Zone and partially overthrusted by the Prealps (Homewood 1986), while its northwestern limit is characterized by an important inverse fault (Homewood et al. 1989). It includes the “Subalpine Flysch” and “North-Helvetian Flysch” (Berger 1996).

Plateau Molasse

Autochtonous, slightly deformed series (almost horizontal), including numerous vertical and transverse faults (Homewood et al. 1989). It is located between the frontal pressure of the subalpine zone and at the foot of the Jura range (Homewood 1986). Its real southern limit, diving under the Subalpine Molasse, is masked.

Jura Molasse

This series probably covered a major part of the Jura Mountains during Oligo-Miocene interval. The interest to study it is directly linked to the lack of knowledge on these deposits. The outcrops, due to the nature of the rocks (clays, marls and easily eroded soft sandstones), are very rare and often temporary preserved. They are only occasionally found, trapped in syncline valleys or protected in karstic fillings, most of the time covered with thick quaternary deposits. Furthermore, stratigraphic data continue to suffer from an obvious lack, as already mentioned by several authors (Aaubert 1975; Weidmann 1986).

Since the 90’s, numerous authors, Becker (2003), Becker et al. (2002, 2004), Berger (1989, 1992a, b, 1996), Bolliger (1994), Clément & Berger (1999), Engesser (1972, 1990), Kälin (1993, 1997a), Kälin & Engesser (2001), Kälin et al. (2001), Picot (2002), Picot et al. (1999, 2005), Reichenbacher et al. (1998) and Schumacher (2002), regularly published new contributions concerning the Jura Molasse. Today, thanks to these new data, we are able to set up a tectonic and stratigraphic frame in this geological field.

The depressions or grabens with Cenozoic fillings, located in tabular Jura (Ajoie, Liestal, Schaffhouse) and in the southern border of the Rhine Graben (Bonfol, Rodersdorf), are only few or not affected by the Mio-Pliocene shortening (folded Jura). The main part of their genesis is due to the repetitive changes of the stresses linked to the rifting of the Rhine Graben and in the reactivation of a complex set of pre-existent crustal discontinuities. These are directly inherited from the Hercynian orogenesis and from the Permo-Carboniferous post-orogenic transtension (Laubscher 2001; Picot et al. 2005 ; Schumacher 2002). The tectonic of the Rhine Graben shows a main phase of rifting between 35 and 28 million years, but continues to be active until the Quaternary.

Synclines with Cenozoic fillings located in folded Jura are also affected by the Mio-Pliocene folding of Jura (folds, overthrustings and transverse faults), in addition to the tectonic of the rhinish rifting. This folding, called alpine, is molded on the rhinish prestructures and linked to the push of the Alps, passed on through the Molasse Basin.

The stratigraphic interval of the Molasse deposits in the Jura area extends from 42 to 10 million years (Fig.5). The oldest locality is the karstic filling of Egerkingen, dated to Middle Eocene (MP14). The last deposits, which are also the youngest of all Swiss Cenozoic (except Quaternary deposits, the karst of “La Vue de Alpes” (MN15) and the “Deckenschotter” (MN17) of Irschel), are dated to the Mammal Zone MN9, with two outcrops in the Jura Mountains, Charmoille and Nebelberg.

The main lines of the Jura Molasse history

After the general uplift of the Jura region from Late Cretaceous to Paleocene, remains of weathered limestone, materialized by quartz or red clays minerals with iron pisolites (“Sidérolithique”), have been deposited during the Eocene in karstic pockets or along tectonic accidents.

Lacustrine environments (“Calcaires inférieurs & Terre jaune”) seem to be present from the Early Oligocene. They are absent in certain parts of the Jura area or still contain brackish episodes, and even coexist laterally with deltaic (“Gompholithes & Conglomerats”) or marine (“Meeressand & Calcaires marins, Septarienton”) environments. In fact, during this period, a marine transgression is attested in the south border of the Rhine Graben. Its coastline must have reached at least the Delémont Basin. Certain authors propose even a connection with the Perialpine Sea during the Early Oligocene (Reichenbacher 2000; Picot 2002). The Rhinish Sea is present until the early Late Oligocene. During the regression of this sea, a vast fluviatile system (“Molasse alsacienne”) took place, until the Late Oligocene, again replaced by lacustrine environments (“Calcaires delémontiens”) dated from the Late Chattian to the basal Aquitanian.

There is no more sedimentation in the Jura region during the main part of the Aquitanian stage, except the “Calcaires de La Chaux” and the “Grès et Marnes grises à gypse de Boudry”, probably because of an uplift of the south compartment of the Rhine Graben.

A marine perialpine episode (literature “Conglomérats & Grès de l’OMM”) invaded in two phases a pre-existent relief in the Jura area during the Burdigalian. This sea withdrew from the region at the end of the Burdigalian; it was replaced by coastal and brackish environments (“Marnes rouges & Gompholithes de l’OMM”) during the Early Langhian and with a vast fluvio-lacustrine system during the Late Langhian-Serravallian interval (“Calcaires & Marnes lacustres de l’OSM”). At the same period, a series of generally very coarse conglomerates (“Vogesenschüttung, Juranagelfluh & Glimmersandschüttung”), were supplied by a series of local alluvial fans in the south of the Vosges and Black Forest rift shoulders. The latest molassic sediments were deposited in Jura during the Early Tortonian, approximately 10 million years ago. Only occasional fluvio-lacustrine events are known (“Cailloutis & Argiles post-molassiques”) during the Pliocene.

The main localities studied

• “Oiselier” (POR-BBC, POR-OIS, POR-OIP, POR-OIC),
  "Gompholithes & Conglomérats" Group, Rupelian
  
  
• “La Beuchille” (DEL-BEU), "Molasse alsacienne" Group, Rupelian
  
  
• “La Beuchille Est” (DEL-BEE), "Septarienton" Group, Rupelian
  
  
• “Poillat” (CRD-POI); “Molasse alsacienne” Group, Rupelian
  
  
• “Solé” (CRD-SOL); “Molasse alsacienne” Group, Rupelian–Early Chattian;
  ”Rote Mergel des Mont Chaibeux” Formation, late Early–Middle Miocene